Mon. Not. R. Astron. Soc. 420, 3557–3573 (2012)

doi:10.1111/j.1365-2966.2011.20272.x

Dust in historical Galactic Type Ia supernova remnants with Herschel
H. L. Gomez,1 † C. J. R. Clark,1 T. Nozawa,2 O. Krause,3 E. L. Gomez,1,4 M. Matsuura,5
M. J. Barlow,5 M.-A. Besel,3 L. Dunne,6 W. K. Gear,1 P. Hargrave,1 Th. Henning,3
R. J. Ivison,7,8 B. Sibthorpe,7 B. M. Swinyard5,9 and R. Wesson10
1 School of Physics & Astronomy, Cardiff University, The Parade, Cardiff CF24 3AA
2 Institute for the Physics and Mathematics of the Universe, University of Tokyo, Kashiwa, Chiba 277-8583, Japan
3 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
4 Las cumbres Observatory Global Telescope Network, 6740 Cortona Drive Suite 102, Goleta, CA 93117, USA
5 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT
6 School of Physics & Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD
7 UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ
9 Space Science and Technology Department, Rutherford Appleton Laboratory, Oxfordshire OX11 0QX
10 European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Vitacura, Santiago 19, Chile

Accepted 2011 November 28. Received 2011 November 26; in original form 2011 November 8

ABSTRACTThe origin of interstellar dust in galaxies is poorly understood, particularly the relative contributions from supernovae and the cool stellar winds of low-intermediate-mass stars. Recently,
large masses of newly formed dust have been discovered in the ejecta of core-collapse supernovae. Here, we present Herschel Photodetector Array Camera and Spectrometer (PACS) andSpectral and Photometric Imaging Receiver (SPIRE) photometry at 70–500 µm of the historical, young supernova remnants: Kepler and Tycho, both thought to be the remnants of TypeIa explosion events. We detect a warm dust component in Kepler’s remnant with Td = 82+4
−6 K
−3M ; this is spatially coincident with thermal X-ray emission
and mass ∼(3.1+0.8−0.6 ) × 10
and optical knots and filaments, consistent with the warm dust originating in the circumstellar
material swept up by the primary blast wave of the remnant. Similarly for Tycho’s remnant, we+2.3−3M . Comparing the spatial distribution
detect warm dust at 90+5
−7 K with mass (8.6−1.8 )×10
of the warm dust with X-rays from the ejecta and swept-up medium, and Hα emission arisingfrom the post-shock edge, we show that the warm dust is swept up interstellar material. We
find no evidence of a cool (25–50 K) component of dust with mass ≥0.07 M as observed incore-collapse remnants of massive stars. Neither the warm or cold dust components detected
here are spatially coincident with supernova ejecta material. We compare the lack of observed
supernova dust with a theoretical model of dust formation in Type Ia remnants which predicts
dust masses of 88(17) × 10−3 M for ejecta expanding into ambient surrounding densities of
1(5) cm−3 . The model predicts that silicon- and carbon-rich dust grains will encounter, at most,the interior edge of the observed dust emission at ∼400 years, confirming that the majorityof the warm dust originates from swept-up circumstellar or interstellar grains (for Kepler andTycho, respectively). The lack of cold dust grains in the ejecta suggests that Type Ia remnants
do not produce substantial quantities of iron-rich dust grains and has important consequences
for the ‘missing’ iron mass observed in ejecta. Finally, although, we cannot completely ruleout a small mass of freshly formed supernova dust, the Herschel observations confirm thatsignificantly less dust forms in the ejecta of Type Ia supernovae than in the remnants ofcore-collapse explosions.
Key words: supernovae: individual: Kepler – supernovae: individual: Tycho – dust, extinction – galaxies: abundances – submillimetre: ISM.
 Herschel is an European Space Agency’s space observatory with science instruments provided by European-led Principal Investigator consortia and with
important participation from NASA.
†E-mail: haley.gomez@astro.cf.ac.uk
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8 Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ

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1 I N T RO D U C T I O N

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Dust in galaxies is thought to be produced by in the stellar winds ofboth low-intermediate-mass (LIM) asymptotic giant branch (AGB)
stars (e.g. Gehrz 1989; Whittet 2003; Sargent et al. 2010) and, toan unknown extent, by massive stars (Höfner 2009; Gomez et al.
2010; Andrews et al. 2011; Gall, Hjorth & Anderson 2011a). Massive stars may also produce dust in the expanding gas ejecta when
they explode as supernovae (SNe; e.g. Clayton et al. 2001; Todini &Ferrara 2001). In the Milky Way (see Zhukovska & Gail 2008), themajor dust source is presumed to be LIM stars, but when accounting
for dust destruction time-scales and the observed dust mass in the
Galaxy, the dust injection rate from stars required is an order ofmagnitude higher than observed. An alternative source of dust isrequired to make up the dust budget (e.g. Jones 2001). This shortfall in the dust mass estimated from the dust injection rates from
LIM stars is also observed in the interstellar medium (ISM) of theLarge Magellanic Cloud (Matsuura et al. 2009), in galaxies out toz = 0.4 (Dunne et al. 2011) and in dusty high-redshift galaxies (seeMorgan & Edmunds 2003; Dwek, Galliano & Jones 2007). Although recent work by Jones & Nuth (2011) suggest that the dustbudget problem is not as problematic as previous works suggest
given the uncertainties in mass-loss rates and the efficiencies ofdust formation in stellar winds, it is clear that significant dust production in supernova (SN) ejecta would alleviate this dust budgetary problem. The average dust yield per SN required to explain
dusty galaxies at both low and high redshifts is of order 0.5–1 M
(Michałowski, Watson & Hjorth 2010; Gall, Anderson & Hjorth
2011b).
Although SNe have long been proposed as a source of dust in
the ISM (Dwek & Scalo 1980; Clayton et al. 2001), observational
evidence has been scarce until recently. Wooden et al. (1993) firstdetected emission from 10−4 M of dust in SN1987A using theKuiper Airborne Observatory. Subsequent mid–far-infrared (FIR)
observations of Galactic and extragalactic core-collapse remnantswith the Spitzer Space Telescope detect small quantities of warm
dust [∼(1–50) × 10−3 M ] in young (t < 1000 d; Sugerman et al.
2006; Meikle et al. 2007, 2011; Kotak et al. 2009; Andrews et al.
2010, 2011; Fabbri et al. 2011; Szalai et al. 2011) and old remnants (Williams, Chu & Gruendl 2006; Rho et al. 2008). The evidence for dust production in Type Ia remnants is scarce (Borkowski
et al. 2006; Seok et al. 2008): mid-IR observations of the Kepler supernova remnant (SNR) with the Infrared Space Observatory (ISO; Douvion, Lagage & Pantin 2001) found thermal emission from warm dust (100 K) with mass 10−4 M . More recently,
Ishihara et al. (2010) detected 10−3 M of swept-up (warm) dustin the Tycho SNR with AKARI, with a tentative suggestion that10−4 M of dust seen in the north-east (NE) of the remnant may
have been formed in the ejecta. Outside of our Galaxy, Borkowski
et al. (2006) obtained mid-IR observations of four Ia remnants in
the Large Magellanic Cloud, finding <10−2 M of swept-up interstellar dust. The dust masses from the mid–far-IR studies areorders of magnitude lower than needed to solve the dust budget
issues.
In order to investigate if SNRs contain cold dust missed by previous IR observations, the Submillimetre Common-User Bolometer
Array (SCUBA) was used to observe Cassiopeia A (Cas A), a remnant of a Galactic core-collapse SNe (Krause et al. 2008a). The
cold dust seen at 450 and 850 µm was interpreted as emission fromdust associated with the remnant due to the spatial correlation with
the X-ray, radio and submillimetre (submm) emission (Dunne et al.
2003). Subsequently, a significant fraction of the submm emission

was shown to originate from molecular clouds along the line of sight(Krause et al. 2004; Wilson & Batrla 2005). Recent observations
with the Balloon-borne Large Aperture Submillimetre Telescope(BLAST) and the Herschel Space Observatory showed it was difficult to distinguish between cold dust in the remnant and cold
dust from intervening interstellar clouds using photometry alone(Barlow et al. 2010; Sibthorpe et al. 2010). These studies revealed a
new cool component of dust in Cas A with mass 0.08 M at ∼35 K,
yet even if all of the dust survived the passage through the shock,the dust mass is about 1 order of magnitude lower than necessary
to solve the dust budget problem.
A possible method to distinguish between SN dust and contamination from interstellar material along the line of sight was proposedby Dunne et al. (2009), where they traced the alignment of the dust
towards Cas A with the SNR magnetic field using polarimetry at
850 µm. They find that a significant fraction of the submm emission towards Cas A is strongly aligned with the magnetic field of theremnant. The dust mass inferred from the polarimetry study of Cas
A agrees with the dust masses estimated from Herschel photometry
observations of the core-collapse SNe SN1987A (Matsuura et al.
2011); in the former 0.2–1.0 M of dust was detected dependingon the composition. The limit on the ejected heavy element mass inthese sources places a further constraint on the maximum physicaldust mass, finding 0.2 < M d < 0.5 M .
The evidence for dust formation in core-collapse SNRs is accumulating, though it is not yet understood whether the amount of SN
dust created is dependent on the progenitor mass, the metallicity
or the type of explosion. Neither is it clear how much dust willsurvive, though theoretical studies suggest a significant fraction ofdust grains formed in ejecta will be destroyed before injection into
the ISM (Jones 2001; Bianchi & Schneider 2007) particularly inthe ejecta of envelope-poor core-collapse SNe or Type Ia remnants(Nozawa et al. 2010; Nozawa et al. 2011). As yet, there is no clear
evidence for freshly formed dust in the remnants of Type Ia explosions, though theoretical models of dust formation in the remnantsof Type Ia events suggest that up to 0.2 M dust could form in theejecta (Nozawa et al. 2011).
Historical remnants are unique probes for understanding dust
production and indeed destruction following core-collapse Type II
SNe as well as low-mass binary Type I events. In the Herschelera, we are finally beginning to probe the contribution of SNe to
the dust budget, yet given the somewhat large uncertainties in our
understanding of the dust yield from stellar sources in other galaxies and in interpreting statistical descriptions of the dust content
in galaxies from wide-area surveys (e.g. Dunne et al. 2011), it isimportant to investigate dust production in those nearest, youngest
and resolved galactic remnants. One can also argue that understanding dust formation in SNe is important regardless of the explosion
mechanism: dust formation following Type II SNe might suggestthat Type Ia SNe would also be likely dust producers (Clayton
et al. 1997; Travaglio et al. 1999). Alternatively, detecting dust
from circumstellar material (CSM) swept up by the blast wave ofa Type Ia can be a useful probe of the progenitor and explosion
mechanism.
The resolution, wavelength coverage and sensitivity of Herschelallow us to investigate the origin and quantity of dust in historical
SNRs. Here we present IR and submm observations (Section 2)of the Tycho and Kepler remnants. We determine the dust massin and towards the remnants in Sections 3 and 4 and compare theIR emission with multiwavelength tracers in Sections 3.1 and 4.1.
The origin of the dust is discussed in Section 5 and we use thetheoretical model of Nozawa et al. (2011) to compare the expectedDust in the Kepler and Tycho remnants
dust masses with the observations. The results are summarized in
Section 6.

2 O B S E RVAT I O N S A N D DATA R E D U C T I O N
2.1 Herschel photometry

2.2 Multiwavelength data sets
Optical data for the Tycho and Kepler remnants were obtained fromthe Hubble Space Telescope (HST) archive via ALADIN (Bonnarel
et al. 2000). For Kepler’s remnant we used observations in thefilters 658, 660 and 502 nm where the emission originates fromHα + [N II], [N II] and [O III], respectively (e.g. Sankrit et al. 2008).
Bad pixels were masked by hand. For Tycho, we use data observed
at 656 nm (Lee et al. 2010), tracing Hα in the eastern limb of theremnant.
Spitzer Multiband Imaging Photometer (MIPS; Rieke et al. 2004)
Level 2 calibrated data were obtained via the archive3 (Kepler –PI: W. Blair; Tycho – PI: J. Rho) at 24 and 70 µm. For 100-µm
data, we use the Improved Reprocessing of the IRAS Survey (IRIS;
Milville-Deschênes & Lagache 2005) which have been calibrated
with respect to data taken with the Diffuse Infrared BackgroundExperiment (DIRBE). Wide field Infrared Survey Explorer (WISE;
Wright et al. 2010) data were also obtained at 22 µm.
The radio data at 18 cm for Tycho were obtained through the Very
Large Array (VLA) archive. The Kepler data at 6 cm were kindly
provided by T. DeLaney (DeLaney et al. 2002). The Chandra Xray data for Kepler (PI: S. Reynolds; Reynolds et al. 2008) andTycho (PI: J. Hughes; see Badenes et al. 2006; Katsuda et al. 2010)
were available online as calibrated files extracted into soft and hardregions of the X-ray spectrum.4 For Kepler’s remnant, the X-ray data
used here have ranges 0.30–0.72, 0.72–0.90 and 1.7–2.1 keV, andfor Tycho, we have 0.95–1.26, 1.63–2.26 and 4.1–6.1 keV. These
ranges show line emission from silicon and iron originating fromthe ejecta as well as soft and hard (with thermal and non-thermal
contributions) X-ray continuum originating from the blast wave.
To compare the submm data with the molecular emission towards
and surrounding the remnants, we use high-resolution 12 CO(J = 2–
1) and 13 CO(J = 2–1) observations for Kepler (see Gomez et al.
2009, for full details). We also include our SCUBA image of Kepler
at 850 µm (Morgan et al. 2003; Gomez et al. 2009). Low-resolution
carbon monoxide 12 CO(J = 1–0) maps for Tycho’s remnant weretaken from the Canadian Galactic Plane Survey (Taylor et al. 2003).
The data were integrated over −68 to −53 km s−1 to compare theremnant with molecular gas thought to be interacting with the SNR(see Section 3.1).
2.3 Zero levels
Due to the difficulty in finding a suitable baseline with which tosubtract the background in the Herschel SPIRE maps, we correlate
the pixel intensity from Herschel with the archival IRAS IRIS maps
(calibrated with respect to DIRBE) to estimate the zero level (seeFig. 1, Table 1). The Herschel data were convolved to the same angular resolution as the IRIS 100-µm map (258 arcsec) and regridded
to 90-arcsec pixels. A least-squares fit analysis was performed on
the Herschel versus IRIS data (whilst masking out the remnant) todetermine the flux offsets; this sets the zero level of the PACS andSPIRE data assuming that the IRIS data are true representation of
the sky. A further scaling factor needs to be accounted for due tothe expected colour correction between the bands when comparing to the 100-µm band (the so-called ‘gain’ factor). These were
estimated by interpolating a modified blackbody curve ν 2 B(ν, T d )(after a preliminary correction of the zero levels). The zero levelof the PACS and SPIRE maps are negative except for the PACS

1 http://herschel.esac.esa.int/twiki/pub/Public/PacsCalibrationWeb/

3 http://irsa.ipac.caltech.edu/data/SPITZER/docs/spitzerdataarchives/

2 http://herschel.esac.esa.int/Docs/SPIRE

4 http://chandra.harvard.edu/pwarmo/openFITS/

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The Kepler and Tycho SNRs were observed with the Herschel (Pilbratt et al. 2010) Photodetector Array Camera and Spectrometer(PACS; Poglitsch et al. 2010) and Spectral and Photometric Imaging Receiver (SPIRE; Griffin et al. 2010) at 70, 100, 160, 250, 350
and 500 µm as part of the Mass Loss from Evolved StarS survey(MESS; Groenewegen et al. 2011). Kepler was observed on 2010
September 10 and March 11; Tycho on 2010 March 20 and January 18 for PACS and SPIRE, respectively. The PACS data wereobtained in ‘scan-map’ mode with speed 20 arcsec s−1 including apair of orthogonal cross-scans over an area ∼22 × 22 arcmin2 . The
SPIRE maps were observed in ‘large map’ mode with scan length of
30 arcmin over ∼32 × 32 arcmin2 ; a cross-scan length is also taken,
with a repetition factor of three. The data were processed following
the detailed description given in Groenewegen et al. (2011).
The PACS maps were reduced with the Herschel Interactive Processing Environment (HIPE; Ott 2010) applying all low-level reduction steps (including deglitching) to Level 1. The SCANAMORPHOS
software (Roussel 2011) was then used to remove effects due tothe thermal drifts and the uncorrelated 1/f noise of the individual
bolometers. Final Level 2 map creation through data projection on
to a changeable spatial grid was performed with SCANAMORPHOS.
Since the HIPE reduction was based on the calibration file version
v5, interim calibration factors of 1.12, 1.15 and 1.17 were applied
to the PACS data at the respective wavelengths to be consistent
with the current calibration file v6. The full width at half-maximum(FWHM) at 70, 100 and 160 µm is 6, 8 and 12 arcsec, respectively.
The flux calibration uncertainty for PACS is currently estimated as
10 per cent for the 70- and 100-µm bands and 20 per cent at 160 µm(Poglitsch et al. 2010). Colour corrections for PACS are expected
to be small compared to the calibration errors. To correct for colourwith dust spectrum ν 1.0 and temperatures of 100 K, we divided by
1.00 and 1.031 at 70 and 100 µm. The 20 K interstellar dust component dominates the 160-µm image, and as such the correction factor
is 0.957.
For SPIRE, the standard photometer pipeline (HIPE v.5.0) wasused (Griffin et al. 2010) with an additional iterative baseline removal step (e.g. Bendo et al. 2010). The SPIRE maps were createdwith the standard pipeline NAÏVE mapper. We multiply the 350-µm
data product by 1.0067 to be in line with most recent calibration
pipeline (equivalent to HIPE v7 reduction). Since the beam areas increase as a function of pixel scale,2 the FWHM is 18.1, 24.9 and36.4 arcsec for pixel sizes of 6, 10 and 14 arcsec, at 250, 350 and
500 µm, respectively. The SPIRE calibration methods and accuracies are outlined by Swinyard et al. (2010) and are estimated to beapproximately 10 per cent. The pipeline produces monochromatic
flux densities for point sources with νSν ∝ ν 1.0 , but at the longer
wavelengths, colour corrections become significant. We multipliedby the colour corrections 0.999, 1.0045, 1.0302 as defined in the
Herschel SPIRE manual2 , appropriate for β = 1.5 and extended
sources.

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3 K E P L E R ’ S S U P E R N OVA R E M N A N T

Table 1. Zero level on the PACS and SPIRE
maps calculated using least-squares fit to Herschel data and IRAS IRIS data. Errors are
the 1σ estimated from bootstrapping the data
1000 times and refitting each set.
Wavelength
(µm)
70
100
160
250
350
500

Zero level (MJy sr−1 )KeplerTycho
4.7 ± 0.4
−3.1 ± 0.2−4.8 ± 0.3−37.4 ± 3.8−30.5 ± 2.0−25.2 ± 1.4−0.9 ± 0.1−2.5 ± 0.3−2.6 ± 0.2−30.8 ± 4.8−20.4 ± 2.1−7.7 ± 1.3

70-µm map of Kepler. The least-squares fit was then repeated (seee.g. Bracco et al. 2011). The errors on the fitting were estimatedusing the confidence intervals from the bootstrap method, where1000 fits were made to the bootstrapped data (Table 1). This error
is added in quadrature along with the calibration errors listed above
and introduces (at most) an error of ∼17 per cent (well within the
20 per cent calibration error used for PACS at 160 µm); note that
this does not account for any systematic errors.

3.1 The multiwavelength view
In Fig. 3, we present a multiwavelength view centred on Kepler’s
SNR, comparing the Herschel PACS and SPIRE photometric datawith synchrotron emission from the remnant at 6 cm, unrelated
molecular gas (traced by the CO emission) and 850-µm emission
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Figure 1. The Herschel SPIRE intensities for the Kepler observations versus
IRAS IRIS 100-µm in MJy sr−1 . The SPIRE maps are convolved to the same
resolution as the IRIS maps and regridded on to the same pixel scale. The
red line is the least-squares function, the grey circles are the Herschel data
divided by the gain factor (due to the colour correction between IRIS at
100 µm and the SPIRE wavelengths). The data are masked at the locationof the remnant. The grey shaded regions indicate the results from the 1000
bootstrap fits to the same data set.

The Kepler SN event was first seen by Johannes Kepler in 1604
and the remnant has a shell-like structure ∼3 arcmin in diameter.
Estimates of its distance, using H I absorption features, range from
3.9 to 6 kpc (Reynoso & Goss 1999; Sankrit et al. 2008). Its classification has been controversial (see the excellent reviews in Blairet al. 2007; Reynolds et al. 2008; Sankrit et al. 2008) with evidence
pointing towards both Type Ia – the thermonuclear explosion of alow-mass accreting star in a binary system – and Type Ib – the corecollapse of a massive star. The deepest X-ray analysis with Chandra(Reynolds et al. 2008), however, suggests that a Type Ia is the most
likely classification using both iron abundance constraints and thelack of a neutron star. The deep X-ray images also provide evidenceof the blast wave interacting with dense CSM. This is unexpected forType Ia events although observations of CSM are becoming morewidespread; the presence of such a dense CSM surrounding Kepler
led Reynolds to suggest the thermonuclear explosion of a single
massive star (M ∗ ∼ 8 M ) as an alternative to the canonical whitedwarf (WD) binary scenario of Type Ia explosions. Whether Kepler
was the result of a single progenitor or WD–AGB binary system
explosion, the mechanism was likely to be a deflagration explosion(Yang et al. 2009; see Woosley, Taam & Weaver 1986, for a review
on explosion models). In this scenario, the heating of the ejecta byradioactive elements is higher than in core-collapse remnants; theejecta densities will also decrease rapidly, though the ejecta is well
mixed since the reverse shock progresses quickly through the layers
in a deflagration explosion (see Section 5.1).
The warm dust emission detected in Kepler with ISO (Douvion
et al. 2001) follows the well-known north–south shell asymmetry
seen also at optical, X-ray and radio wavelengths. The correlationof the dust with the optical Hα suggested the dust was shock-heatedcircumstellar dust swept up by the shockwave of the blast. Blair et al.
(2007) used Spitzer to revise the dust mass to 5.4 × 10−4 M . They
also found that emission at 70 µm arises only in the brightest knotsof the shell seen at 24 µm. The 160-µm Spitzer data were of poor
quality due to its low resolution and strong background gradient.
Morgan et al. (2003) originally interpreted their longer wavelength450- and 850-µm SCUBA data as evidence for 0.3–1 M of cold
dust associated with the remnant. They compared the emission
from interstellar material towards Kepler using submm continuum
imaging and spectroscopic observations of atomic and moleculargas, via H I, 12 CO and 13 CO and detected weak CO emission fromdiffuse, optically thin gas at the locations of some of the submm
clumps. The contribution to the submm emission from foregroundmolecular and atomic clouds using these CO tracers was found tobe negligible, but these emission lines may not be tracing all of thegas in the ISM.
The three-colour Herschel PACS image of Kepler is displayed inFig. 2. The remnant is clearly seen in blue and follows the same
asymmetric structure as seen in the X-ray and radio (Section 3.1):
the emission is brightest in the north along an arc-like shell and someemission is seen towards the centre of the remnant. The low-level
red structures seen across the map (outside of the remnant) originate
from dusty interstellar clouds with temperatures ∼13–20 K.

Dust in the Kepler and Tycho remnantsfrom cold dust. The complete shell-like structure of SN material isclearly seen at radio wavelengths, tracing the forward and reverse
shocks (DeLaney et al. 2002). In Fig. 4 we compare the optical and
X-ray emission with the warm dust from Herschel-PACS at 70 µm.
The well-known north–south asymmetry seen at optical, radio and
X-ray wavelengths with the enhanced northern emission (thought tooriginate from the motion of Kepler through the surrounding interstellar material) is also seen in the PACS data. The ‘arm’ structure
across the middle (clearly seen in the radio, Fig. 3) originates froma projection effect of material in the front and back of the shell(Blair et al. 2007). Herschel detects faint IR emission arising fromthis component at 70 µm. The faint ‘ear’-like features seen along
the outermost mid-section of the expanding shell at radio and X-ray
wavelengths (Figs 3 and 4) are not seen in the IR. At the longer(160 µm) PACS and the SPIRE wavelengths, it is difficult to disentangle any SNR emission from the large-scale interstellar structures
extending across the region; the maps are increasingly dominated byunrelated dust clouds at temperatures of ∼16–20 K. The southern
cloud of dust which is outside the area of the blast wave and ejecta
regions is the same cloud originally detected in molecular emission
by Gomez et al. (2009) at 5 km s−1 . The cold dust cloud in the NE

Figure 3. Multiwavelength montage of Kepler’s SNR and its environs. PACS (top) and SPIRE (middle) bands centred at RA = 262.◦ 671, Dec. = −21.◦ 4914
(J2000.0, yellow cross); the region shown is 7.2 × 7.2 arcmin2 . The location of the forward shock (radius 103 arcsec) is indicated with the white circle (DeLaneyet al. 2002). Also shown (bottom) are the SCUBA 850 µm (Morgan et al. 2003), the CO(J = 2–1) map integrated from −200 < v < 200 km s−1 (Gomez et al.
2009) and the 6-cm VLA map (DeLaney et al. 2002).
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Figure 2. Three-colour FIR–submm image of Kepler’s SNR with PACS at
70 (blue), 100 (green) and 160 µm (red). The image has been smoothed.
The area shown is 16.5 × 16.6 arcmin2 .

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is coincident with the CO structure at 10 km s−1 (see Section 5.3 formore discussion).
Optical emission arising from dense, knotty regions in the circumstellar medium overrun by the blast wave is traced in [N II] and Hα.
Interestingly, the 70-µm emission correlates with the Hα + [N II]
features (Fig. 4) in both the northern limb [particularly in the northwest (NW) region] and across the projected shell along the middle.
Since the Hα emission originates from collisional heating of the thinregion behind the SN shock front (i.e. from swept-up surrounding
material), the spatial agreement with the PACS emission and theionized features suggest this dust component arises from the sweptup medium. This is further supported when comparing the FIR
with X-ray emission. There are three X-ray components in Kepler:
continuum X-rays from the blast wave sweeping up surrounding
circumstellar and interstellar material, X-ray line emission from thewarm ejecta, and continuum emission arising from the non-thermal
synchrotron emission in the north, south and eastern limbs. The
warm dust emission in the FIR is confined within the radius of
the outer blast wave and is seen only at the brightest points in the
optical, radio and X-ray (Fig. 4 – as noted in Blair et al. 2007)
with peaks at 70 µm seen where the soft and hard thermal X-rays
overlap. Therefore, the warm dust is associated with the densest
gas behind the forward shock rather than the inner ejecta material;
this confirms the dust originates from the outermost regions of theremnant i.e. from the swept-up shocked circumstellar and/or ISM(Section 5.2).

3.2 The spectral energy distribution
The IR–submm spectral energy distribution (SED) towards Kepler’s
SNR is shown in Fig. 5. The fluxes estimated from Spitzer, HerschelFigure 5. The IR–radio SED towards Kepler’s SNR. The previously published fluxes from the literature (Table 2) are shown with errors as verticalbars (Spitzer = purple triangles; IRAS = blue diamonds; SCUBA = orange
squares). The solid black line is the sum of the two-temperature thermal
dust components fitted to the IR (T w = 86 K and T c = 20 K) and submm
fluxes (dot–dashed and dashed lines, respectively). The triple-dot–dashedline shows the non-thermal SED expected from the synchrotron, Sν ∝ ν −0.71
scaled by the 6-cm fluxes. The red line shows the total SED from the nonthermal and thermal components. Note that the previous published integrated
fluxes use different aperture sizes to this work.

and SCUBA in this work within an aperture of radius 120 arcsec
centred on the remnant are plotted with their errors (estimated fromthe calibration error, the error in the zero level estimated in Section 2.3 and from the variations in the pixel rms within the aperture).
All the data were smoothed to the resolution of the SPIRE 500-µm
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Figure 4. Comparison of the warm dust emission traced by PACS 70-µm data (white contours) with optical and X-ray. Left: three-colour image of Kepler’s
SNR with the HST at 658 nm (red), 660 nm (green) and 502 nm (blue) indicating emission at Hα + [N II], [N II] and [O III], respectively. The optical emission
mostly arises from shocked circumstellar/interstellar material interacting with the primary blast wave. The north component indicated with the fiducial isthought to be ejecta material (Sankrit et al. 2008) which has overrun the primary shock boundary due to the contact discontinuity in the blast wave. Right:
three-colour X-ray image using Chandra archive data showing soft (0.3–0.72 keV: red), (0.72–0.9 keV: green) and hard X-rays (1.7–2.1 keV: blue). The images
are centred on α J2000 = 262.◦ 663 and δ J2000 = −21.◦ 485 and are 3 × 3 arcmin2 across.

Dust in the Kepler and Tycho remnants

3563

Table 2. Fluxes in Jy measured for Kepler’s remnant within the 120-arcsec aperture. Note that the previous published integrated
fluxes use different aperture sizes to this work. References: a Blair et al. (2007); b average values from Douvion et al. (2001),
Arendt (1989) and Saken, Fesen & Shull (1992) IRAS data; c Gomez et al. (2009); d 450 µm; e measured on Spitzer MIPS archivedata.
IR fluxes towards Kepler SNR (Jy)Published
This work
Non-thermal

24 µm

70 µm

100 µm160 µm

250 µm350 µm500 µm

850 µm

9.5 ± 1.0a
9.5 ± 1.0e
0.02

5.6 ± 1.4b
12.3 ± 2.7
0.05

2.9 ± 1.1b
11.2 ± 2.4
0.06

<0.9a
16.5 ± 2.9
0.09–
13.0 ± 2.8
0.12–
5.8 ± 1.20.16

3.0 ± 0.2c,d2.7 ± 0.6
0.20

1.0 ± 0.2c
0.7 ± 0.1
0.29

Sν = Nw × ν β B(ν, Tw ) + Nc × ν β B(ν, Tc ),(1)

where N w and N c represent the relative masses in the warm andcold component, B(ν, T) is the Planck function and β is the dust
emissivity index. The model was fitted to the SED (constrained by
the 12-µm flux) and the resulting parameters (N w , N c , T w , T c , β)which gave the minimal chi-squared were found. We also applieda bootstrap analysis to our SED-fitting to determine the errors on
the SED model. The photometry measurements were perturbed
according to the errors at each wavelength, the new fluxes werefitted in the same way, and the SED parameters recorded, with theprocedure repeated 1000 times.
The total dust mass is calculated using equation (2):


Nw
Nc
S500 D 2
+
×
,
(2)
Md =
κ500
B(500 µm, Tw ) B(500 µm, Tc )where κ 500 is the dust absorption coefficient and D is the distance(we choose the 500-µm flux since this is less sensitive to the temperature). The distance to the remnant is uncertain, with measurementsof H I absorption placing Kepler at ∼5 kpc (see Reynoso & Goss
1999, for a full discussion). Sankrit et al. (2008) have revised themeasurement to 3.9+1.9−1.4 kpc using HST proper motion data. We follow Blair et al. (2007) in adopting a distance of 4 kpc. To determine
the dust mass, we use κ 500 = 0.1 m2 kg−1 with κ ∝ λ−β , appropriate
for grains in the Milky Way and β ∼ 1–2 (Draine & Lee 1984). The
best-fitting dust mass within the aperture obtained from the SED

5 http://irsa.ipac.caltech.edu/data/SPITZER/docs/mips/C 2012 The Authors, MNRAS 420, 3557–3573
C 2012 RAS
Monthly Notices of the Royal Astronomical Society model (equations 1 and 2) is therefore M w = 0.0035 M and M c =2.2 M , with temperatures T w = 84 K, T c = 19 K and β = 1.5. The
temperature of the warm component is consistent with collisional
heating of dust in the shocked gas with temperature and electron
density of 1.2 keV and ne ∼ 20 cm−3 (Bouchet et al. 2006, theirfig. 15).
The median parameters and confidence intervals from the bootstrap method are listed in Table 4. The dust mass depends strongly
on the choice of κ, e.g. typical interstellar dust grains are described
well with κ = 6.7 m2 kg−1 at 70 µm (e.g. Draine & Li 2001). In thiscase, the warm dust mass would be reduced to M w = 1 × 10−3 M .
We note that the uncertainty in κ could be an order of magnitude.
Temperature maps of the dust emission towards Kepler werecreated using the convolved IR–submm maps from 24 to 500 µm,
and fitting a two-temperature modified blackbody SED to eachpixel. We extracted the warm and cold components for each pixel,
shown in Figs 6(a) and 6(b). In order to account for the non-thermal
component, the flux due to synchrotron radiation was subtracted
from the dust maps on a pixel-by-pixel basis before the greybody
fits were made. The synchrotron contribution in each pixel was
estimated using a spectral index map created from the 6- and 23-cm
VLA observations convolved to the same beam (see also DeLaneyet al. 2002). The somewhat flatter spectral index in the northernbright ring (α ∼ −0.68 compared to the average value of −0.71)
resulted in removing most of the cold dust in this region, though
there is residual emission associated with this component within the
SNR shell (Fig. 6b). This cold dust feature appears to be coincident
with a similar structure seen in the warm dust component, but thecold dust is further out from the centre by ∼10 arcsec compared to
the warm dust emission which is spatially coincident with the radio(Fig. 6a).
The cold dust structures in the north are also coincident withthe structures seen at 850 µm with the SCUBA camera (Gomez
et al. 2009; Fig. 3) originally attributed to SN dust due to the lackof correspondence between the CO gas from molecular interstellar
clouds towards Kepler (Fig. 3) and the 850-µm emission. The larger
Herschel map clearly shows large-scale and extended interstellar
dust structures right across the entire region towards Kepler andimplies that the argument in Gomez et al. was flawed. This could
simply be due to a non-constant ratio of molecular gas-to-dust, orthat the 12 CO and 13 CO lines are not effective tracers of the gas
in this region. Alternatively, it is possible that, towards Kepler, theinterstellar dust structures are associated with atomic rather thanmolecular gas.
If the cold dust in the north of the remnant is located at the
distance of the remnant, the location of the emission is suggestiveof dust swept up by the blast wave, though the mass of dust in
the northern shell is rather substantial at ∼1.0 M for T ∼ 20 K.
Elsewhere, the cold dust component does not spatially coincide withany of the SNR tracers including X-ray, radio and optical emission,

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data using a Gaussian kernel. The aperture is somewhat larger thanthe SNR boundary defined by the edge of the radio (103 arcsec) but
was chosen to encompass the SNR emission in the images smoothed
to the 500-µm beam. The Spitzer fluxes were colour-corrected using algorithms detailed in the MIPS handbook.5 We also compare
our fluxes with previously published fluxes in the literature (Braun
1987; Arendt 1989; Douvion et al. 2001; Morgan et al. 2003; Blairet al. 2007), although these were obtained using apertures with radius ranging from 100 to 120 arcsec and therefore differ from thefluxes in Table 2 accordingly. Note also that we measure the flux
encompassed in the aperture around Kepler, and therefore include
emission from the foreground cloud.
The contribution to the IR/submm fluxes from synchrotron emission is obtained by scaling the VLA radio map of Kepler using
a constant spectral index: Sν ∝ ν α , where the mean value of α is−0.71 but ranges from −0.6 to −0.85 (DeLaney et al. 2002). The
synchrotron emission at 500 µm is 0.20 ± 0.06 Jy (∼15 per centof the total flux). We fit a two-component modified blackbody tothe dust SED after the synchrotron component has been subtracted;
the model is the sum of two modified Planck functions each with acharacteristic temperature, T w and T c (equation 1):

3564

H.L Gomez et al.

and is unlikely to be related to the SN itself. We discuss the originof the cold dust further in Section 5.3.
Finally, we also tested for the presence of cool dust with temperatures in the range 30–40 K as found in Cas A (Barlow et al. 2010;
Sibthorpe et al. 2010): the flux contribution of the warm component(visible at 24 µm) was subtracted from the 70- and 100-µm maps,
however we find no evidence of a cool dust component.

4 T Y C H O ’ S S U P E R N OVA R E M N A N T
The Tycho SN event was observed by Tycho Brahe in 1572. It
lies at a distance of 1.5–3.8 kpc (Reynoso et al. 1997; Krause
et al. 2008b) and has a thin shell-like morphology with diameter∼8 arcmin across. Here, we use the distance given in D = 3.8 kpc


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Figure 6. Dust temperature maps derived from fitting a modified greybody
to the 24–500 µm maps towards Kepler’s SNR showing (a) warm dust and(b) cold dust. The colour bar shows the temperature in K. The solid black
circle indicates the photometry aperture used with radius 120 arcsec and theblack dashed circle is the average radius of the outer shock front 103 arcsec.

as this is consistent with the historical records of the explosion, thenon-detection of the remnant and the distance to the proposed binary companion (Lu et al. 2011) (see Krause et al. 2008b, for moredetails). The outer blast wave extends to an average distance of
251 arcsec from the centre, with the contact discontinuity between
the reverse-shock material and swept-up ISM at angular distancesof 183, 241 and 260 arcsec depending on the position angle (Warren
et al. 2005). Tycho’s light echo spectrum has revealed it to have beena Type Ia event (Krause et al. 2008b); it is the remnant of a canonicalsingle-degenerate binary explosion (Lu et al. 2011) as evident from
X-ray ejecta abundances (Decourchelle et al. 2001). This is further
supported by the lack of similarity in Tycho’s optical light curvewith typical subluminous (van den Bergh 1993) and overluminous(Ruiz-Lapuente 2004) Type Ia explosions (Krause et al. 2008b).
Badenes et al. (2006) performed detailed comparison between the
X-ray spectra and the results from hydrodynamical modelling of the
SN shocks, and found that the delayed detonation explosion model
was the best fit to the data. In this scenario, the SNR is expanding
into an ambient density of ∼0.6–3 cm−3 , with an ejecta mass of
1.3 M .
Many authors have suggested that Tycho is interacting withmolecular clouds (Reynoso & Goss 1999; Lee, Koo & Tatematsu
2004; Cai, Yang & Lu 2009; Xu, Wang & Miller 2011) particularly
with a large CO structure seen in the north and NE at velocities
between −63.5 and −61.5 km s−1 ; this structure is believed to be
responsible for the asymmetry seen in the X-ray and radio (seeFig. 8). Cai et al. (2009) showed that interaction with the ISM occurs
along the entire boundary of the remnant with interstellar structuresdetected at velocities from −69 to −58 km s−1 ; the total gas massof clouds interacting with Tycho is 103 M (Xu et al. 2010). Conversely, Tian & Leahy (2011) propose that the CO clouds are notinteracting with the remnant since the densities of the molecular gas
clouds in the north are ∼200 cm−3 , compared to the X-ray ejecta
parameters which suggest the remnant is ploughing into material ofdensity 0.2–3 cm−3 (Decourchelle et al. 2001; Badenes et al. 2006).
The difference in the density estimated for the surrounding material
from CO/X-ray abundances is a factor of 60 (rather than the 3 ordersof magnitude quoted in Tian & Leahy 2011). However, the densityof the H I cloud they detect at −43 to 57 km s−1 is comparable tothe density obtained from X-ray observations and could therefore
be tracing gas interacting with the remnant. Using a flat galacticrotation model (with standard constants) places this H I cloud at
d = 3.4 kpc which is consistent with the distance of Tycho’s SNR.
Ishihara et al. (2010) detect warm dust features in the north of
the remnant with AKARI and suggest these are linked to the CO
structures in the NE. They also find enhanced dust emission in the
NW region of the shell, and since they find no corresponding COfeature, they suggest this component could originate from SN dust.
Without the long wavelength data, they were unable to constrain
the amount of cold dust arising from SN material within the blastwave.
The three-colour Herschel PACS image is presented in Fig. 7. As
in Kepler, the remnant is clearly visible in blue, and the interstellar
structures become clearer at the longer PACS and SPIRE wavelengths (Fig. 8). Cold, dusty clouds peaking at 250 µm are seen to
the immediate east and west of the remnant and have temperatures
typical of the ISM. The bright SPIRE interstellar structure to theimmediate north of the remnant in Fig. 8 is highly suggestive ofemission originating from gas and dust swept up by the expanding
SNR. The structure is similar in appearance to the submm cavities
blown out by the winds from warm OB stars (e.g. Schneider et al.
2010) with filamentary fingers pointing inwards to the centre of theDust in the Kepler and Tycho remnants

3565

ionizing radiation (in this case the SNR). In the higher resolution
PACS data (Fig. 7), we see three small, warm sources embedded
within the SPIRE clouds (see Section 4.2, for further discussion).
The compact blue source to the north (Fig. 7) is the Cepheid Variable
star, V∗ AS Cas.
4.1 The multiwavelength view

Figure 8. Multiwavelength montage of Tycho’s SNR. From left to right – top: 12 CO(J = 1–0) data integrated from −68 < v < −53 km s−1 , 18-cm radio data
and Spitzer MIPS 24 µm. Middle: Herschel PACS. Bottom: SPIRE. The images are centred at RA = 6.◦ 3308, Dec. = 64.◦ 1372 (J2000.0; yellow cross) withsize 9 × 9 arcmin2 . The location of the forward shock (radius 251 arcsec) is indicated with the white circle.
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Figure 7. Three-colour FIR image of Tycho’s SNR with PACS data at 70,
100 and 160 µm. The area shown is 23.2 × 27.2 arcmin2 .

The emission from warm dust at 70 µm in the north has two brightcomponents in the NE and NW (Fig. 8); seen also in the AKARI
data at wavelengths out to 160 µm. This corresponds spatially to
peaks in 24-µm emission as well as the X-ray continuum which
traces the forward blast wave. The longer wavelength HerschelSPIRE data do not spatially correlate with radio or X-ray emission
from the remnant. In Fig. 9, we compare the PACS 70-µm emission
with a three-colour X-ray image (0.95–1.26, 1.63–2.26 and 4.1–
6.2 keV). The emission from warm dust at 70 µm is bounded within
the forward and reverse shocks of the SNR at 251 and 183 arcsec
respectively (Warren et al. 2005), and we do not see FIR emission
towards the interior of the remnant. The 250-µm emission (Fig. 8),
however, extends far outside the shock boundary and spatially correlates with the low-level structures and the peaks seen in the CO

3566

H.L Gomez et al.

map (Fig. 10). There does not appear to be any agreement between
the peaks at 70 and 250 µm though we see emission at both FIR
wavelengths where there is also CO emission. Comparing with the
CO molecular gas, the interstellar clouds seen by Herschel close
to the remnant in the north (labelled NW and NE) and to the east,
match structures seen at −61 and −65 km s−1 . In the south-east andsouth-west (SW), two faint CO clouds are seen in the same range
of velocity, both with associated 70- and 250-µm emission.
The correlation between the 70-µm emission and the X-rays
emitted at the outer edges of the SNR is striking. The hard Xray emission (4.1–6.2 keV) is filamentary in structure with thinrims extending outside of the circular edge both in the north andsouth-east side of the remnant. The peaks in the warm dust trace
the peaks in the hard X-rays. The FIR emission does not spatiallycoincide with the soft X-rays (Fig. 9) with peaks in the soft X-ray
lagging behind the 70-µm peaks by 27 arcsec in the NW, though
the soft emission at the edges of the outer blast wave do correlatewith the FIR (regions ‘A’ and ‘B’ in Fig. 9). The peaks in theradio emission also lag by ∼16 arcsec. The ejecta material does
not spatially correlate with the IR emission, with the peak in the
NW lagging behind the FIR by 23 arcsec. The correlation of warm
dust with the hard X-ray emission at the edges of the remnant isslightly surprising due to the large fraction of non-thermal emission
expected from shocks in this energy regime. However, dust is seen
where soft X-rays (from thermal emission due to the blast wave
sweeping up interstellar material) and hard X-rays overlap. Lee
et al. (2010) showed that the hard X-rays also contain a significant
fraction of thermal emission, we therefore conclude that the warm
dust is spatially correlated with the swept-up material. The lack of
70-µm emission in the SW region of the remnant, where the hard
X-rays are bright, may suggest that the CO (Fig. 10) and 250- µm
structure in this region originate from a foreground cloud which
is not associated with the remnant; this would mean very little
interstellar dust is being swept up on this side (in agreement withthe faster expansion rate of the SNR observed in this region). The
hard X-rays in the west–SW region are produced by non-thermal

Figure 10. Comparison of the warm and cold dust with Herschel (contours)
and molecular gas (inverted colourscale). Black contours trace 70-µm PACS
emission from warm dust (0.4, 5, 6, 7, 9, 10, 13, 16, 20 mJy pixel−1 ). White
contours trace the cold dust emitting at 250 µm (20, 50, 100, 150, 200,
250, 300, 400 mJy beam−1 ). The red solid circle traces the forward shock
wave at radius 251 arcsec, the red dashed circle traces the reverse-shockboundary at radius 183 arcsec (Warren et al. 2005). The forward shock is
at radius 251 arcsec. Following Ishihara et al. (2010), the major interstellar
CO clouds to the north of the remnant are labelled NW and NE. Note thatthe peaks in the 250-µm emission from cold dust are spatially coincident
with the unrelated molecular material and not the warm dust in the SNR.
The contribution to the 250-µm emission from synchrotron emission is<6 per cent.

synchrotron emission revealed by the presence of X-ray ‘stripes’ due
to tangled magnetic fields on small scales (Eriksen et al. 2011). Herethen the hard X-rays are dominated by non-thermal processes andthe lack of warm dust in this region further supports the hypothesisthat we only see dust in the shocked, swept-up ISM where we findthermal continuum X-rays and Hα emission – this is highlighted inFig. 11.
There is no correlation between the FIR and X-rays for the outermost ejecta material which has broken through the reverse-shock
region out to the very edge of the SNR (as labelled in Fig. 9). We
conclude that there is no evidence to suggest that the warm dust is
associated with the ejecta.
4.2 The spectral energy distribution
The IR–submm SED towards Tycho’s SNR is shown in Fig. 12.
The fluxes estimated from Spitzer and Herschel within an apertureof radius 251 arcsec centred on the remnant are plotted with theirerrors (Table 3; the zero-level offsets were applied to the maps before
fluxes were measured – Section 2.3). As the synchrotron emission
in Tycho has been steadily decreasing with a decay rate of 0.5 percent yr−1 (Klein et al. 1979), the radio fluxes were corrected for thisdecay rate to represent the same epoch as the Herschel observations.
The radio spectral slope exponent in the literature ranges from α =−0.52 (Katz-Stone et al. 2000) to −0.61 (Green 2001); for α =−0.61 the contribution of synchrotron to the 500-µm flux is 4 Jy
(17 per cent), for α = −0.52 this becomes 5 Jy. We use the former
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Figure 9. Three-colour X-ray image of Tycho’s SNR using Chandra
archive data showing soft thermal X-rays from ejecta and continuum (0.95–
1.26 keV: red) and hard X-rays tracing the both thermal and non-thermal
continuum components (4.1–6.1 keV: blue). The green X-rays trace theejecta emission (1.63–2.26 keV). White contours are 70-µm PACS emission (7, 9, 10, 13 mJy pixel−1 ). Two regions where the ejecta are protruding
out of the reverse shock out to the edge of the forward shock are labelled.
Labels ‘A’ and ‘B’ indicate regions where FIR emission is coincident with
soft and hard X-rays.

Dust in the Kepler and Tycho remnants

3567

the 24-µm emission and the outermost radio emission. The cold dust
map highlights a dust structure on the eastern edge which appears
to overlap with the warm dust (Fig. 13b) and appears to be swept-up
material. If this structure is at the distance of Tycho with T d ∼ 21 K
and absorption properties similar to interstellar dust, the dust mass
would be ∼2 M , well above the mass of heavy elements available
to form dust.
As with Kepler, we find no evidence for a separate cool dust
component (at T d ∼ 30–40 K) in the SNR when subtracting the hot
dust traced by emission at 24 µm from the 70- and 100-µm maps.

4.3 An aside on triggered star formation

Figure 12. The IR–radio SED of Tycho’s SNR. The previously publishedfluxes from the literature (Table 2) are shown with errors as vertical bars
[IRAS = blue diamonds; WISE = blue triangles; AKARI = purple triangles
from Ishihara et al. (2010)]. (Note that the latter study used an apertureof diameter 525 arcsec compared to the 504 arcsec in this work.) The solid
black curve is the sum of the two-temperature thermal dust components fitted
to the IR and submm fluxes (dot–dashed and dashed lines, respectively) attemperatures T w = 90 K and T c = 21 K. The red curve shows the total SED
from the non-thermal and thermal components. The synchrotron power-law
fit to the radio data is shown by the triple-dot–dashed line.

fluxes (Fig. 12). The submm flux within the aperture defined by theouter blast wave is therefore well above the expected synchrotron
emission.
As described in Section 3.2, we fit the IR–radio SED with twotemperature modified blackbodies and a power-law spectrum. The
best-fitting parameters from the SED are T w = 90 K, T c = 21 K, β =1.5 and dust mass M w = 8.6 × 10−3 and M c = 4.0 M . The medianparameters and confidence intervals from the bootstrap method arelisted in Table 4. The temperature maps are shown in Fig. 13. The
warm dust is clearly seen within the remnant at the same location asC 2012 The Authors, MNRAS 420, 3557–3573
C 2012 RAS
Monthly Notices of the Royal Astronomical Society Returning to the three FIR-bright, small clumps seen in Fig. 8, one
may jump to the conclusion that the remnant has triggered starformation in the compression of material as the blast wave moves
outwards. Triggered star formation has long been suggested as a viable star-forming mechanism (e.g. Elmegreen & Lada 1977), where
an expanding H II region compresses gas between the ionization
front and the shock front, causing fragmentation into gravitationally unstable, star-forming cores. SN-triggered star formation has
been inferred through evidence of SN-shocked molecular gas (e.g.
Reach & Rho 1999; Reynoso & Magnum 2001). The FIR-brightclumps seen by Herschel are detected at 24 µm, suggesting theseare star-forming cores rather than protostars. The temperatures weobtain from fitting their SEDs from 24 to 500 µm suggest each core
has a warm and cold dust temperature component with 53–57 K and∼12 K, with dust masses ranging from 1.4 to 2.8 M for the two
smaller cores and 14 M for the largest and brightest core. These
masses were estimated using standard interstellar κ values and (assuming they are at the distance of Tycho) indicate the formation ofintermediate- and high-mass stars.
The star-forming clumps lie outside of the average shock front
boundary of 251 arcsec (4.6 pc) from the centre but the nearest,
faintest clump (Fig. 8) is consistent with the distance reached by
the outermost blast wave in this region at 260 arcsec (as traced by
the hard X-rays). Indeed, the travel time for the shock front to reach

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Figure 11. Zoomed-in region on the eastern limb of Tycho’s remnant: (left) hard X-Ray; (middle) Hα emission and (right) 70-µm emission. The blast wave
is at radius 251 arcsec and extends inwards to 222 arcsec (enclosed by solid red circles). The reverse shock at 183 arcsec is indicated by the red dashed line.
In general, the hard X-ray emission is a combination of non-thermal and thermal emission from the interaction of the SN and the surrounding material. The
spatial agreement with the Hα emission suggests that the hard X-ray emission is dominated by thermal radiation in this region. The image is centred at RA =6.◦ 4718 and Dec. = 64.◦ 1372 with radius 72 arcmin × 72 arcsec.

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Table 3. Fluxes in Jy measured towards Tycho’s remnant within the 251-arcsec aperture defined in Fig. 8. References: a Douvion et al. (2001); b Ishihara et al.
(2010); AKARI fluxes measured at wavelengths: c 65 µm; d 22.9 µm; e 90 µm; f measured on WISE data at 22 µm.
IR fluxes towards Tycho SNR (Jy)Published
This work
Non-thermal

12 µm

24 µm

70 µm

100 µm140 µm160 µm

250 µm350 µm500 µm

1.8 ± 0.8a
–
–
–
–

–
28.3 ± 3.2b,d20.1 ± 3.0
18.9 ± 3.8f
0.65

38.6 ± 7.7b,c–44.8 ± 10.8–
40.6 ± 8.1b,e41.1 ± 8.8

46.7 ± 14.0b––

44.5 ± 13.4b–59.0 ± 14.1––
42.7 ± 8.6––
32.8 ± 6.5––
17.6 ± 3.5

1.28

1.58

1.98

2.09

2.73

3.34

4.14

β

Parameters
Tw
(K)

Tc
(K)

Dust mass
M d,w
M d,c
(10−3 M )(M )Kepler1.5
1.6 ± 0.2

84
82+4
−6

19
18+2
−1

3.5
3.1+0.8−0.6

2.2
2.1+0.8
−0.5

Tycho

1.5 ± 0.2
1.5 ±

90
90+5
−7

21
21+2
−1

8.6
8.6+2.3−1.8

3.7
5.1+0.9−0.6

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Table 4. The best-fitting parameters (top row) for dust towards and within
Kepler and Tycho’s SNRs and the median parameters (bottom row) estimatedfrom bootstrapping the fluxes within their errors. We quote the median fromthe distribution of values obtained from the bootstrap analysis with the68 per cent per cent confidence interval quoted for the error. The dust mass
absorption coefficient, κ, used to estimate the warm dust mass in Kepler’s−1and Tycho’s SNR is κ70 µm = 2 m2 kg . For the cold dust component−1
towards the SNRs, we use κ500 µm = 0.1 m2 kg .

these star-forming cores at radius r ∼ 5–6 pc from the centre is given
by t = 2r/5v (assuming the SNR is in the Sedov phase, where v is the
expansion velocity: v ∼ 4700 km s−1 ; Hayato et al. 2010). The shock
front would therefore be expected to reach the observed star-forming
regions at t = 425–510 yr. However, a simple time-scale argument
rules out any triggered star formation by Tycho’s SN since thegravitational collapse of a cloud requires time-scales of 105 yr, withthe proceeding protostellar phase needing approximately 106 yr. It
is possible that the stellar wind of the progenitor or the binary
companion was responsible for creating a low-density bubble due
to its stellar wind. Such a scenario was suggested by Badenes et al.
(2007) to explain RCW86 (confirmed observationally by Williams
et al. 2011) and the star-forming loop around the SNR G54.1+0.3(Koo et al. 2008).
5 DISCUSSION
5.1 A theoretical model of ejecta dust formation
Nozawa et al. (2011) have developed a theoretical model to follow the dust formation in a carbon deflagration event (Nomoto,Thielemann & Yokoi 1984) with ejecta mass of 1.38 M . In thismodel, dust grains form early-on, but are almost completely destroyed over a time-scale of 106 yr due to the passage through thereverse shock. The total dust masses formed at 100–300 d range
from 3 × 10−4 to 0.2 M , depending on the sticking probabilityof the grains and/or whether CO or SiO molecules are formed in
the ejecta (since this will affect the composition and mass of grainsthat condense out of the ejecta gas – Nozawa et al. 2003). Here, we
use this model to compare the dust masses observed with Herscheland the mass expected to have formed and survived in the ejecta
at approximately 400 years after the explosion event. (We refer the

Figure 13. Dust temperature maps derived from fitting a modified greybody
to the 24–500 µm maps for the area surrounding Tycho’s SNR with SED
components separated into (a) warm dust and (b) cold dust. The colour bar
shows the temperature in K. The solid black circle indicates the forwardshock radius at 251 arcsec.

reader to the full description in Nozawa et al. 2011, for details of thesimulation).
Fig. 14(a) shows the time evolution of the total dust mass of SN
grains for a Type Ia using the dust formation calculation with asticking probability equal to one. We consider three ambient gas
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densities in which the ejecta are expanding into, where nH = 1, 3
and 5 cm−3 since this will affect the amount of destruction of thedust grains we expect; the figure clearly shows that dust destruction
is more efficient for higher ambient densities. Using the relative
ages of Kepler and Tycho at 410(440) years and assuming that theejecta are expanding into an ambient gas density of 1 cm−3 , themodel predicts a surviving dust mass of ∼88(84) × 10−3 M : anorder of magnitude more than the warm dust mass of 3.1(8.6) ×
10−3 M estimated from the Herschel observations (Table 4). If theremnant is expanding into denser material, as certainly suggested
for both the Kepler and Tycho remnants, the surviving dust mass isconsequently reduced with 26(23) and 17(14) × 10−3 M of dustC 2012 The Authors, MNRAS 420, 3557–3573
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Monthly Notices of the Royal Astronomical Society surviving at 410(440) years for ambient densities of 3 and 5 cm−3
respectively; the surviving dust mass is still too high to be consistent
with the observations.
The comparison between the dust mass estimated from simulations and that observed with Herschel implies that even if all thewarm dust observed here was produced in the ejecta, the model
still overestimates the amount of dust expected in the ejecta. One
possible explanation for this discrepancy is that the formation ofdust grains in the ejecta is less efficient than the model, e.g. thesticking coefficient needed to form the grains is lower, or that theradioactive heating from nickel and cobalt is higher, therefore inhibiting the formation of dust (Nozawa et al. 2011). It is also possiblethat dust destruction via sputtering may be more efficient than themodel, for example due to smaller grain sizes formed and/or higher
ambient densities; the Herschel observations would then suggestthat dust destruction occurs efficiently on time-scales of ∼400 yr inType Ia ejecta. Finally, using a lower mass absorption coefficient inour estimate of the observed dust masses would also alleviate thisdiscrepancy.
Similarly, we see no observational evidence for cool or cold dust
associated with ejecta material as predicted using the dust formation
model. The lack of cold ejecta dust has important consequences on
our understanding of the formation of iron-rich dust grains. TypeIa ejecta are prime candidates for producing iron-rich dust as they
synthesize an order of magnitude more 56 Ni (which eventually decays into Fe) than Type II SNe. Furthermore, there is a well-known
problem in reconciling the predicted ejecta abundances of iron using
theoretical models (e.g. Thielemann, Nomoto & Hashimoto 1996)
with observed abundances which can differ by more than an order ofmagnitude. It has long been suggested that one way to reconcile thisissue is to ‘hide’ the missing iron in dust grains. The Nozawa et al.
(2011) model predicts that any newly formed iron-rich dust grains(which would form in the innermost regions of the ejecta) would
not yet be swept by the reverse shock at 400 days and would still
be cold. Since Herschel does not detect any significant (∼subsolar)
mass of cool dust associated with the ejecta, this strongly suggeststhat neither Kepler nor Tycho produced significant amounts of irongrains. These observations suggest that the formation of dust cannot explain the discrepancy between the observed and expected ironabundances in Type Ia SN ejecta (see also Hamilton & Fesen 1988).
5.2 The origin of the warm dust component
Given that we observe less dust than expected to be formed in theejecta itself, could a non-SN source of dust explain the IR emission?
One can estimate the mass of dust (Table 4) we would expect tobe swept up from circumstellar and interstellar material by the
SN blast wave using simple arguments. Kepler is not yet in the
Sedov phase (Decourchelle & Ballet 1994; Kinugasa & Tsunemi
1999; Cassam-Chenaı́ et al. 2004), which suggests that the remnant
is still dominated by the dynamics of the ejecta and has swept
up a mass similar or less than that ejected in the explosion. Theinterstellar density at the location of Kepler’s remnant (∼470 pc outof the galactic plane) is expected to be low at nH ∼ 10−3 –0.5 cm−3
(Vink 2008). Estimating the volume encompassed by the SN blast
wave, the swept-up interstellar gas mass would be ∼10−2 M andhence the swept-up dust mass is <10−4 M (with gas-to-dust ratiosof 100–150; e.g. Devereux & Young 1990). This is 2 orders ofmagnitude lower than the dust mass observed in Kepler.
As suggested by Blair et al. (2007) and Douvion et al. (2001), theorigin for the IR emission in Kepler could be swept-up circumstellar
dust. X-ray emission arises from this shocked dense circumstellar

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Figure 14. (a) The variation of dust mass with time since the SN event ina Type Ia remnant based on the model of Nozawa et al. (2011). The dust
masses with age are compared for different ambient densities of n = 1 cm−3
(black), 3 cm−3 (blue) and 5 cm−3 (red). The vertical lines indicate the timesince explosion for Kepler and Tycho. (b) The distance reached by ejecta
dust with time since explosion for the Kepler SNR. The location of theforward shock (FS) and reverse shock (RS) are indicated in red with the FS
matched to the observed location at 110–120 arcsec. The distances travelledby the dust grains are shown in blue (C) and black (Si). The grey vertical line
marks 410 years and the horizontal line marks a distance of 5.3 × 1018 cm.
Note that the Si grains are destroyed just after 400 years.

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H.L Gomez et al.

6 Since some of the circumstellar/interstellar dust swept up by the SN shock

wave will also be destroyed via sputtering, the masses estimated from these
simple swept-up volume arguments are therefore upper limits on the actual
swept-up dust mass we would expect.

This figure suggests that silicon and carbon dust grains formed in
the ejecta will encounter the reverse shock after 50 years, and at
400 years (due to deceleration from the gas drag) will be trapped at
5.3 × 1018 cm from the centre; for nH = 10 cm−3 , the dust would
be trapped at 4.5 × 1018 cm. The model thus predicts that any SN
dust in Kepler would lie at an angular distance of 78–91 arcsec.
The latter distance is consistent with the interior edge of the dust
emission seen with Herschel (Fig. 3) but not consistent with theemission seen out to 120 arcsec. In Tycho, the observed warm dust
is confined between a thin region at 220–250 arcsec from the centre(Figs 10 and 11). The model suggests that for a forward shock of∼250 arcsec, the SN dust would be located at ∼1 × 1019 cm, i.e. atan angular distance of 200 arcsec, again consistent with the interioredge of the observed warm dust emission in Tycho.
If the warm dust was formed in the SN ejecta, we would expect
it to be distributed within the forward and reverse shocks across theentire remnant. However, in Kepler’s SNR, the warm dust exhibitsa north–south asymmetry, and in Tycho, there is a north–NE asymmetry and we therefore only observe warm dust emission where
there is also plenty of surrounding material. For Kepler, this is the
CSM in the north which the remnant is ploughing into as it moves
at high velocities, and in Tycho, the dust originates from regions
where we see dense molecular clouds at the edge of the remnant.
This indicates that the majority of the warm dust is therefore swept
up. In summary, we cannot absolutely rule out that a small fractionof the warm dust in both remnants is formed in the SN ejecta, but we
propose that, from the spatial distribution, the warm dust in Tycho
originates from swept-up interstellar dust, and in Kepler, the warm
dust originates from swept-up circumstellar dust.

5.3 The origin of the cold dust component
Approximately 2–5 M of cold dust is found in the apertures chosen
to encompass the SNR, for Kepler and Tycho, respectively (Table 4).
The substantial amount rules out a swept-up CSM or ISM origin of
dust, but could this massive cold component be freshly formed dustin the SN ejecta?
As well as using the temperature maps (Figs 6 and 13), we also
searched for an excess of submm emission in the SNR comparedto the unrelated molecular gas. The CO and submm fluxes (using
the 250 µm) in each pixel were compared and a best-fitting relation
determined, producing, for each pixel, an estimate of the 250-µm
flux that corresponds to a given CO flux. The scatter in the CO and250-µm fluxes for Kepler’s remnant is considerable, and when thesubmm flux associated with the CO is subtracted from the 250-µm
map, residual submm emission remains in the northern shell (in thesame location as the structure seen in the cold temperature map;
Fig. 6b); the dust mass in this feature is ∼0.6 M for T d ∼ 20 K.
The cold dust does not spatially coincide with any of the ejecta
tracers, and therefore we conclude that the submm emission is notdust formed in the SN ejecta.
There is no obvious CO structure at the location of the cold
dust shell seen in Kepler, though the difference between the flux at
250 µm in this region and the interstellar cloud to the east of theremnant is a factor of 5; the expected CO emission associated withthis faint cold dust feature is therefore at the noise level of the CO
map. There is submm ‘excess’ over the molecular gas both insideand outside the remnant, suggesting that these dust clouds in general
are not always well traced by the 12 CO(J = 2–1) emission; thismethod is therefore not able to distinguish between dust associatedwith the foreground dust component and the remnant.
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component with mass ∼1 M (Kinugasa & Tsunemi 1999; Blairet al. 2007; Chiotellis, Schure & Vink 2011). Given standard gas-todust ratios, this mass could easily explain the warm dust masses seenin the IR. In this scenario, the explosion is single-degenerate andthe progenitor has a relatively massive evolved companion, indeed
Chiotellis et al. (2011) suggest the companion was a 4–5 M AGB
star. Is the dust mass we observe here consistent with the expectedmass loss from an evolved companion star? Given the typical timescale for an AGB mass-loss phase of ∼106 yr and typical dust massloss rates of Ṁd ∼ 10−10 –10−8 M yr−1 (Meixner et al., 2004;
Groenewegen et al. 2007; Lagadec et al. 2008), we would expect≤0.01 M ejected during this phase.6 This is consistent with themass of warm IR-emitting dust in Kepler and supports either the
explosion of an intermediate-mass WD–AGB binary system (e.g.
Chiotellis et al. 2011) or the single progenitor 8 M AGB-starthermonuclear explosion suggested by Reynolds et al. (2008).
In Tycho’s remnant, it is unlikely that the swept-up dust originatesfrom the CSM as the companion star was inferred to be a low-mass
subgiant (Lu et al. 2011). Some of the dust could have been createdduring the prior AGB phase of the WD star; using the argument
above, we could again expect to see 0.01 M of dust ejected in thisphase which is consistent with the measured dust mass for Tycho.
However, there is no evidence for the presence of a dense CSM
surrounding Tycho. The ambient interstellar density as measured
from X-ray and radio evolution is often quoted as nH < 0.2 cm−3
(Dwarkadas & Chevalier 1998; Cassam-Chenaı́ et al. 2004; Katsuda
et al. 2010), but recent simulations suggest 3 cm−3 (Badenes et al.
2007). We would therefore expect a swept-up dust mass6 of up to
0.013–0.2 M for the range of ambient gas densities (with gas-todust ratio of 120). This is consistent with the warm dust component
we measure in Tycho’s remnant.
Recently, Tian & Leahy (2011), using H I absorption lines,
showed that a number of the molecular clouds thought to be interacting with Tycho are not at the location of the remnant. This
would imply there is not enough ISM surrounding Tycho for theremnant to sweep up the dust we observe. However, they identify
one H I cloud which they rule out as interacting with the remnant
due to its density of nH = 10 cm−3 , yet we include this here since
this is comparable to the density of the surrounding material as estimated from hydrodynamical modelling (Badenes et al. 2006). The
mass of gas in this cloud is ∼10 M . Furthermore, we identify afaint cloud overlapping with the warm dust in the NW of Tycho at−55 km s−1 (with gas mass ∼1 M using a CO–H2 factor of 3 ×
10−20 cm−2 (K km s−1 )−1 ) which is massive enough to explain theorigin of the warm dust feature detected here6 and in Ishihara et al.
(2011).
The warm dust we observe in these remnants is therefore coincident with the outermost shock fronts, indicative of swept-up
circumstellar or interstellar material. We can test this by investigating the location of the dust grains expected to form from the ejecta
gas using the model of Nozawa et al. (Section 5.1) with respect tothe forward and reverse shocks. In Fig. 14(b), we show the distancethe freshly formed ejecta grains will have reached in 1000 years
with ejecta ploughing into an ambient medium of density nH =5 cm−3 (chosen so that the model forward-shock location for Kepler agrees with the observed shock radius at ∼110–120 arcsec).

Dust in the Kepler and Tycho remnants

We compare the dust masses with the recent theoretical dust
formation model of Nozawa et al. (2011), where approximately
10−2 M of dust is expected to form in Kepler and Tycho. The Si
and C grains formed are expected to have traversed the reverse shock
approximately 50 years after the explosion and are then trapped at
400 years at a distance consistent with the interior edge of the warm
dust emission detected by Herschel. The observed dust mass of
the warm component is lower than the model predicts, suggestingthat dust formation in the ejecta is inhibited or destroyed more
efficiently. We do not detect a cool dust component in the innermost
regions of unshocked ejecta, suggesting that Type Ia ejecta are
not producing substantial masses of iron and are not responsible
for the ‘missing’ iron mass problem in the ejecta. The mass ofdust in these remnants is lower than that measured in core-collapse
SNRs, though there is a possibility that a fraction of the cold dust
observed towards both Tycho and Kepler could be hidden amongst
the dust components along the line of sight. Resolved observationsof younger Type Ia remnants with low ambient densities (and little
foreground contamination) are needed to investigate the evolutionof SN dust mass further.

6 CONCLUSIONS
Herschel observations of the remnants of the core-collapse SNe
Cas A and SN1987A show evidence for a significant mass of dust
spatially coincident with SNe ejecta. Using Herschel observationsof Kepler and the Type Ia SNR, Tycho, we do not find evidence forfreshly formed SN dust in these historical remnants.
We confirm a previously known warm dust component in Kepler’s remnant with Td = 82+4
−6 K, somewhat cooler than the dust
previously detected by Spitzer and ISO, and revise the dust mass to−3
M . The warm dust is spatially coincident with∼(3.1+0.8−0.6 ) × 10
thermal X-ray emission and optical knots and filaments on the outeredges of the shock front, confirming that this component originatesin the material swept up by the primary blast wave. Given the massand the density of the surrounding ISM, the warm dust could nothave originated from swept-up interstellar material, but is consistent
with swept-up circumstellar dust, confirming the presence of CSM
surrounding Kepler. The Herschel SPIRE images are dominated byemission from cold foreground interstellar clouds (∼20 K) which
overlap with the small dust clumps detected by previous groundbased SCUBA measurements. These were originally attributed to alarge mass of dust either formed in the ejecta and/or swept up by thecircumstellar medium. The resolution of Herschel does not allow us
to distinguish between cold SN dust or intervening interstellar material, however using dust temperature maps, and a comparison withmolecular gas (traced by CO), we find that most of the cold dust
originates from foreground, unrelated ISM. There is no evidence
that any of the cold dust within the aperture is spatially coincident
with ejecta material.
Similarly for Tycho’s remnant, we detect warm dust at 90+5
−7 K
−3
M . Comparing the spatial distribution
with mass (8.6+2.3−1.8 ) × 10of the warm dust with thermal and non-thermal X-rays from theejecta and swept-up medium, and with collisionally heated Hαemission arising from the thin post-shock edge, we suggest the warm
dust is swept-up interstellar material. This is further supported by
the significant amount of molecular gas surrounding the regions
where warm dust is observed and the large mass of interstellar
gas swept up by the remnant. We find no evidence for cool SN
dust at temperatures of 20–30 K as detected in the core-collapse
remnants Cas A and SN1987A; the lack of freshly formed dust in
both remnants supports the view that Kepler was the result of a
Type Ia explosion.
C 2012 The Authors, MNRAS 420, 3557–3573
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Monthly Notices of the Royal Astronomical Society 

AC K N OW L E D G M E N T S
We thank the referee for their helpful comments on the manuscript.
The images were produced with the software package APLPY(http://aplpy.github.com/); thanks to Eli Bressart and Thomas Robotaille for this. We thank Jason Kirk, Tracey DeLaney and Chris
North for useful discussions. HLG would like to acknowledge thesupport of Las Cumbres Observatory Global Telescope Network.
PACS has been developed by a consortium of institutes led byMPE (Germany) and including UVIE (Austria); KU Leuven, CSL,IMEC (Belgium); CEA, LAM (France); MPIA (Germany); INAFIFSI/OAA/OAP/OAT, LENS, SISSA (Italy); IAC (Spain). This development has been supported by the funding agencies BMVIT(Austria), ESA-PRODEX (Belgium), CEA/CNES (France), DLR(Germany), ASI/INAF (Italy) and CICYT/MCYT (Spain). SPIRE
has been developed by a consortium of institutes led by Cardiff
University (UK) and including University of Lethbridge (Canada);
NAOC (China); CEA, OAMP (France); IFSI, University of Padua(Italy); IAC (Spain); Stockholm Observatory (Sweden); Imperial
College London, RAL, UCL-MSSL, UKATC, University of Sussex (UK) and Caltech/JPL, IPAC, University of Colorado (USA).
This development has been supported by national funding agencies:
CSA (Canada); NAOC (China); CEA, CNES, CNRS (France); ASI(Italy); MCINN (Spain); Stockholm Observatory (Sweden); STFC,UKSA (UK) and NASA (USA). This work is based (in part) on observations made with the Spitzer Space Telescope, which is operated
by the Jet Propulsion Laboratory, California Institute of Technology
under a contract with NASA.

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C 2012 The Authors, MNRAS 420, 3557–3573
C 2012 RAS
Monthly Notices of the Royal Astronomical Society 