The Astrophysical Journal, 692:546–555, 2009 February 10
c 2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A.


doi:10.1088/0004-637X/692/1/546

PROPERTIES OF NEWLY FORMED DUST BY SN 2006JC BASED ON NEAR- TO MID-INFRAREDOBSERVATION WITH AKARI
I. Sakon1 , T. Onaka1 , T. Wada2 , Y. Ohyama2,5 , H. Kaneda2,6 , D. Ishihara1 , T. Tanabé3 , T. Minezaki3 , Y. Yoshii3 ,N. Tominaga1,7 , K. Nomoto1,8 , T. Nozawa4,8 , T. Kozasa4 , M. Tanaka1,8 , T. Suzuki1 , H. Umeda1 , S. Ohyabu2 , F. Usui2 ,H. Matsuhara2 , T. Nakagawa2 , and H. Murakami2
1 Department of Astronomy, School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan; isakon@astron.s.u-tokyo.ac.jp.
2 Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan
3 Institute of Astronomy, School of Science, University of Tokyo, 2-21-1 Ohsawa, Mitaka, Tokyo 181-0015, Japan
4 Department of Cosmosciences, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan

Received 2007 December 6; accepted 2008 October 8; published 2009 February 19

ABSTRACT
We present our latest results on near- to mid-infrared (MIR) observation of supernova (SN) 2006jc at 200 days
after the discovery using the Infrared Camera (IRC) on board AKARI. The near-infrared (2–5 μm) spectrum of
SN 2006jc is obtained for the first time and is found to be well interpreted in terms of the thermal emission
from amorphous carbon of 800 ± 10 K with the mass of 6.9 ± 0.5 × 10−5 M that was formed in the SN
ejecta. This dust mass newly formed in the ejecta of SN 2006jc is in a range similar to those obtained for otherseveral dust-forming core-collapse supernovae based on recent observations (i.e., 10−3 –10−5 M ). MIR photometricdata with AKARI/IRC MIR-S/S7, S9W, and S11 bands have shown excess emission over the thermal emission
by hot amorphous carbon of 800 K. This MIR excess emission is likely to be accounted for by the emission−3from warm amorphous carbon dust of 320 ± 10 K with the mass of 2.7+0.7
M rather than by the band
−0.5 × 10
emission of astronomical silicate and/or silica grains. This warm amorphous carbon dust is expected to havebeen formed in the mass-loss wind associated with the Wolf–Rayet stellar activity before the SN explosion. Our
result suggests that a significant amount of dust is condensed in the mass-loss wind prior to the SN explosion.
Key words: dust, extinction – infrared: ISM – stars: Wolf-Rayet – supernovae: general – supernovae: individual(SN2006jc)(IRS) mapping observation (Rho et al. 2008) reports that the
total mass formed in Cas A should be at least 0.02 M . There
still remains a gap in the produced dust mass in core-collapse
SN ejecta between those observational results and the theoreticalprediction of 0.1–1 M (Nozawa et al. 2003).
SN 2006jc is a peculiar Type Ib SN and was discovered on
2006 October 9.75 (UT) (Nakano et al. 2006). It is believedthat the progenitor star had experienced a luminous outburst
similar to those of luminous blue variables (LBVs) two years
prior to the SN event (Pastorello et al. 2007; Foley et al. 2007).
Evidence of substantial interaction of the SN ejecta with a dense
He-rich circumstellar medium (CSM) ejected during the LBVlike eruption was reported based on Chandra X-ray observations(Immler et al. 2008).
Recent observational studies on this SN have shown that SN
2006jc is an interesting target for the study of dust-forming
massive star SNe. The near-infrared (NIR) rebrightening of
SN 2006jc was firstly reported by Arkharov et al. (2006)
from late 2006 November through early December (∼ 50 days
after the discovery) based on J-, H-, and K-band observations.
Smith et al. (2008) report that both the appearance of a strong
continuum emission at red/NIR wavelengths and the fadingof redshifted sides of the narrow He i emission lines occurred
simultaneously between 51 and 75 days after the brightness
peak. These characteristics are interpreted as the evidence fordust formation in SN 2006jc, although the timescale is much
shorter than the general dust formation timescale, at least a few
hundred days, typical of other dust-forming SNe. They foundthat graphite grains with T ∼ 1600 K or slightly hotter silicate
grains well fit the optical(red) spectrum of day 75, pointing outthat the dust is mainly carbon and not silicate because of its hightemperature.

1. INTRODUCTION
Study on the dust formation in the ejecta of core-collapse
supernovae (SNe) is an important topic in exploring the originof dust in the early universe. Besides the theoretical studies thatsuggest the dust condensation in the ejecta of core-collapse SNe(Kozasa et al. 1991; Todini & Ferrara 2001), several pieces of theobservational evidence for the dust formation in supernova (SN)ejecta have so far been reported. For example, up to 0.02 M
of dust formation, which is close to the value of 0.1 M needed
for core-collapse SNe to account for the dust content of highredshift galaxies (Morgan & Edmunds 2003), in the ejecta of theType II SN 2003gd has been reported by Sugerman et al. (2006).
As for the case of the Type II SN 1987A, however, even thehighest estimate of the condensed mass in the SN ejecta reaches
only up to 7.5 × 10−4 M (Ercolano et al. 2007). Moreover,there is an alternative estimate for the newly condensed dust
mass in the ejecta of SN 2003gd of 4 × 10−5 M by Meikle
et al. (2007) based on different modeling analysis from that ofSugerman et al. (2006). Spitzer Multiband Imaging Photometer(MIPS) observations of the Galactic core-collapse SN remnant
Cassiopeia A (Cas A) show that the dust mass associatedwith it is much smaller (∼ 0.003 M ) than previously thought(Hines et al. 2004), while a recent Spitzer Infrared Spectrograph
5 Current Address: Institute of Astronomy, and Astrophysics Academia
Sinica, P.O. Box 23-141, Taipei 10617, Taiwan.
6 Current Address: Department of Astrophysics, Nagoya University,
Chikusa-ku, Nagoya 464-8602, Japan.
7 Current Address: Optical and Infrared Astronomy Division, National
Astronomical Observatory, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan.
8 Current Address: Institute for the Physics and Mathematics for the Universe,
University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8568, Japan.

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No. 1, 2009AKARI OBSERVATION OF SN2006jc

547

Figure 1. (a) RGB false color image of SN 2006jc obtained with the AKARI/IRC NIR/N3 (blue), MIR-S/S7 (green), and S11 (red) bands. The image size is 175 ×200 . (b) Slitless spectroscopic image with NIR/NP covered with 171 pixels by 137 pixels. The spatial scale of each pixel is 1. 46. The horizontal pixel position ofeach source in (a) corresponds to that of the direct light of each spectrum in (b). The dispersion direction to longer wavelengths is shown with the white arrow. The
pixel-to-wavelength relation in (b) is given by λ = 0.4799 × (X − Xd )0.5 + 1.5687, where λ is the wavelength in μm at the horizontal pixel position X, and Xd is thehorizontal pixel position of the direct light of the source in concern.

While most of the dust-forming SNe that have been observationally reported are Type II except for the case of Type Ib
SN 1990I (Elmhamdi et al. 2004), the Type Ib SN 2006jc gives
us a unique opportunity to investigate the dust formation not
only in the SN ejecta, but also in rich circumstellar materials
that have come from the mass-loss events prior to final corecollapse. NIR to mid-infrared (MIR) observations of SN 2006jc
in the early phase of dust condensation are essential to examine the composition and properties of the newly formed dustin the SN ejecta and the preexisting dust in the circumstellar
medium separately. In this paper, we present the new NIR spectrum and the MIR photometric data of SN 2006jc taken with theInfrared Camera (IRC) (Onaka et al. 2007) on board AKARI,
the Japanese infrared satellite (Murakami et al. 2007), on 2006
April 29 (200 days after the discovery or the epoch of 220 days
after the explosion). We derive the properties of the carriers ofrebrightened NIR emission and the MIR emission to understandthe dust formation in SN 2006jc.
2. OBSERVATION AND DATA REDUCTION
Two pointed observations of SN 2006jc were performed at
00:36:22(UT) and 02:15:47(UT) on 2007 April 29 as part of thedirector’s time of AKARI. The former observation (ID:5124071)
was performed with the AOT04 mode, in which the MIR spectroscopic data were taken with two grisms, SG1 (5.4–8.4 μm)
and SG2 (7.5–12.9 μm), the MIR imaging data were taken withthe broadband filter, S9W (6.7–11.6 μm) of the MIR-S channel, the NIR spectroscopic data were taken with the grism, NG(2.5–5.0 μm), and the NIR imaging data were taken with thebroadband filter, N3 (2.7–3.8 μm) of the NIR channel. The latter
observation (ID:512472) was performed with AOT02b mode, inwhich the MIR imaging data were taken with two medium-band
filters, S7 (5.9–8.4 μm) and S11 (8.5–11.3 μm) of the MIR-S
channel and simultaneously, the NIR spectroscopic data wastaken with the prism, NP (1.8–5.2 μm), and the NIR imaging
data were taken with the broadband filter, N3 (2.7–3.8 μm) of
the NIR channel. The target was very faint and the spectrum wasseriously affected by the blending with that of the host galaxy
UGC4904, which prevent us from obtaining useful spectral
information with NG, SG1, and SG2. In the following analysis,

we use the spectrum taken with NP as well as the imaging datawith N3, S7, S9W, and S11. The total exposure time was 206 s,
206 s, 224 s, 56 s, and 224 s for NP, N3, S7, S9W, and S11,
respectively.
Each of the imaging data reduction procedures, including thesubtraction of the detector dark current, corrections for the effectof the hit by high-energy ionizing particles and the scattered light(Sakon et al. 2007), the flat fielding, the distortion correction,and the shift and co-addition of the exposure frames for N3,
S7, S9W, and S11 bands follows those in the IRC Imaging
Pipeline Version 20070912. The unit conversion factors from
ADU per unit time to Jy are taken from AKARI IRC Data Users
Manual version 1.3. The pixel scale of the NIR channel is 1. 46
and that of the MIR-S channel is 2. 34. The FWHM of theimage size is 4. 0, 5. 1, 5. 5, and 4. 8 and the peak central pixel
flux is 5.9%, 12.0%, 11.5%, and 12.1% for NIR/N3, MIRS/S7, S9W, and S11, respectively (Onaka et al. 2007). Eachof the spectroscopic data reduction procedures, including thesubtraction of the detector dark current, correction for the highenergy ionizing particles effects, flat fielding, and the shift andco-addition of the exposure frames for NP data follows those in
the IRC Spectroscopy Toolkit Version 20070913 (Ohyama et al.
2007).
Figure 1(a) shows the RGB false color image of SN 2006jc
produced with AKARI/IRC NIR/N3(blue), MIR-S/S7(green),
and S11(red) bands, where the data taken with N3 and S11bands are degraded into the Gaussian beam with the FWHM
of 5. 1, so that they match with the image size of the S7band data. SN 2006jc is located close (∼ 10 ) to the nucleusof the host galaxy UGC4904, and the diffuse background or
foreground component of UGC4904 must be carefully removed
to obtain the flux solely from the SN 2006jc. The photometricdecomposition technique employed is described in Section 2.1.
The NIR spectral image of SN 2006jc is obtained by the slitless
spectroscopy with AKARI/IRC NP (see Figure 1(b)), and it
shows that the spectrum of SN 2006jc is seriously blended withthat of UGC4904 and a careful subtraction of the UGC4904
component is needed to derive the pure spectrum of SN 2006jc.
The spectroscopic decomposition technique employed is also
described in Section 2.2.

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We define the reference pixel row so that it goes across only
the host galaxy UGC4904, and obtain the signal count profile
P (X) in units of ADU pixel−1 along the reference pixel row (seeFigure 2(b)). The position of the reference pixel row is selected
so that P (X) has as sharp a profile as possible, and therefore we
use P (X) as the PSF. We obtain the signal count profile Pi (X)(i = 1, 2, . . . , n) in units of ADU pixel−1 along the ith pixelrow (see Figure 2(c)), where n is defined by the number of the
pixel rows that cover the SN 2006jc (see Figure 2(a)). Then we
assume the signal count profile of the UGC4904 component,
PiUGC4904 (X), in Pi (X) as a convolution of the PSF written as
PiUGC4904 (X) =∞ηi (k)P (X − k),

(1)

k=−∞

i

(b)

(c)

where the distribution function ηi (k) is assumed to have theGaussian form given by k − δi.
(2)
ηi (k) = hi exp −si
The best-fit parameters of hi , δi , and si (i = 1, 2, . . . , n)2are obtained, so that the sum of Pi (X) − PiUGC4904 (X) for
50  X  69 and 79  X  90 becomes minimum, where thedata of Pi (X) in 70  X  78 have contribution from the signalof SN 2006jc and have been excluded. The signal count profile
SN2006jc
of the SN 2006jc component Pi(X) is then calculated as
SN2006jc

Pi(d)Figure 2. Procedures for extracting the emission carried by SN 2006jc adopted
for the photometric data taken with AKARI/IRC bands with the emission fromthe host galaxy UGC4904 separated. (a) The areas of the reference pixel row,
the ith target pixel row (i = 1, 2, . . . , n), and the test pixel row (see text) are
shown over the NIR/N3 band image of SN 2006jc and UGC4904. The image
size is 180 ×200 and the pixel scale is 1. 46. (b) The signal count profile P (X)in units of ADU pixel−1 along the reference pixel. (c) The signal count profile
Pi (X) in units of ADU pixel−1 along the ith target pixel row is shown with thickline and the best-fit result of PiUGC4904 (X), the model signal count profile of
the UGC4904 component in Pi (X) with Equation (1), is shown with the grayline. (d) The signal count profile Ptest (X) in units of ADU pixel−1 along the test
UGC4904(X),
pixel row is shown with the thick line and the best-fit result of Ptest
the signal count profile of the UGC4904 component along the test pixel row
modeled with Equation (1), is shown with the gray line.

2.1. Photometric Decomposition Technique
In order to separate the emission carried by the SN 2006jc
from that by the host galaxy, the signal count profiles of the hostgalaxy component have to be modeled and reproduced. For thispurpose, we first derive the most appropriate point-spread function (PSF) in the image and then assume the galaxy component
as a convolution of the source with PSF. These procedures arecarried out both in the vertical and horizontal directions independently to check the reliability and consistency of the results.

(X) = Pi (X) − PiUGC4904 (X).

(3)

In order to evaluate how well the signal count profile of the
UGC4904 component is reproduced by a convolution of the
PSF modeled by the signal count profile P (X) obtained along
the reference pixel row, we define the test pixel row so thatit goes across only the galaxy UGC4904 (see Figure 2(d)),
apply the same methods described above to the the signal countprofile along the test pixel row Ptest (X), and obtain the bestUGC4904
fit signal count profile Ptest(X) modeled by Equation (1).
The standard deviation of the residual profile, Ptest (X) −UGC4904
Ptest(X), from the zero value is only 6.8 ADU pixel−1 for
NIR/N3 compared to the peak signal of ∼ 150 ADU pixel−1(Figure 2(d)), which confirms that the signal count profile of the
UGC4904 component is well reproduced by our method. The
value of 6.8 ADU pixel−1 is used to estimate the uncertainty in
the N3 band flux of SN 2006jc due to the subtraction procedureof the UGC4904 component.
Finally, the total N3 band flux estimated from the analysis
on the signal count profiles along the X direction, fνX (N 3), is
derived asfνX (N 3) = αN3n SN2006jc

Pi(X)dX,(4)i=1

where αN3 is the unit conversion factor from ADU to Jansky forN3 band (Tanabé et al. 2008). The uncertainty in the N3 band
UGC4904 2 0.5
BG 2
+ σN3
,
flux of SN 2006jc is calculated as σN3
BG
where σN3 is the 1σ noise-equivalent flux for NIR N3 band
data, taken with the AOT04 mode in units of mJy calculated
UGC4904
from the typical fluctuation of the sky counts, and σN3
isthe uncertainty which stems from the subtraction procedures of
the host-galaxy component given by 6.8 × αN3 π (n/2)2 . Here nAKARI OBSERVATION OF SN2006jc

No. 1, 2009

Table 1
Flux Density of SN 2006jc on 2007 April 29 (200 Days After the Discovery)
Band
NIR/N3MIR-S/S7
MIR-S/S9W
MIR-S/S11

fνX
(mJy)

fνY
(mJy)

0.386 ± 0.046
0.586 ± 0.063
0.694 ± 0.098
0.495 ± 0.061

0.376 ± 0.042
0.548 ± 0.070
0.735 ± 0.105
0.501 ± 0.098

is the number of the pixel rows to cover the whole region of SN
2006jc.
The same decomposition techniques are applied in the vertical direction to derive the total N3 band flux, fνY (N 3), estimated from the analysis on the signal count profiles along the
Y direction. The obtained N3 band fluxes, fνX (N 3) and fνY (N 3),
are listed in Table 1. These two values are in good agreement
within uncertainties. We adopted the weighted average of thetwo values taking account of the error as the final N3 band fluxof SN 2006jc.
We obtain the flux densities of SN 2006jc with the MIRS/S7, S9W, and S11 bands using the same decomposition
technique as that adopted for the NIR/N3 band, and the results
are summarized in Table 1. The flux densities estimated fromthe signal count profiles along the X and Y directions are ingood agreement within uncertainties for any of the MIR-S/S7,
S9W, and S11 bands. Finally, we also tried PSF photometry of
SN 2006jc using stars in the field and found that the results are
the same within the uncertainty. We note that the uncertainty
in the subtraction of the host-galaxy light dominates in thephotometry uncertainty for both cases, and that the apertures
for the photometry used in our methods are adequately large.
2.2. Spectral Decomposition Technique
The NIR spectrum of SN 2006jc is obtained by the slitless
spectroscopy with NIR/NP, in which the spectrum is dispersedin the X direction, and a spectrum of a certain source iscontaminated by the spectra of other sources aligning in thedispersion direction. Therefore, the spectrum of SN 2006jc
suffers severe blending with the light from UGC4904. We havedeveloped a spectral decomposition technique for the spectral
image of NIR/NP. We use the imaging data taken with NIR/
N3 to investigate the structure of UGC4904 and to reproduce
the spectral components of UGC4904 in the spectral image of
NIR/NP, and obtain a pure spectrum of SN 2006jc.
For the first step, the position in the Y direction of the NIR/N3
data is adjusted to that of the NIR/NP data with an accuracy of
one-tenth of the pixel.
We define the reference pixel row in the imaging data takenwith NIR/N3 covering from Y = 28 to Y = 29, so that itgoes across only UGC4904 and does not include SN 2006jc(see Figure 3(a)). The signal count profile along the reference
pixel row, IY[28:29] (X), is obtained by averaging the data fromY = 28 to Y = 29 (see Figure 3(b)). Then we define the target
pixel row covering from Y = 20 to Y = 25, so that it goesacross the major part of SN 2006jc (see Figure 3(a)), and thesignal count profile along the target pixel row, IY[20:25] (X), is
obtained by averaging the data from Y = 20 to Y = 25 (seeFigure 3(c)).
We model the signal count profile of the UGC4904
UGC4904
component, IY[20:25](X), contained in IY[20:25] (X) with

UGC4904(X) =IY[20:25]

549∞
ηY[20:25] (k)IY[28:29] (X − k),

(5)

k=−∞

where the free distribution function, ηY[20:25] (k), is determined
UGC4904so that the sum of |IY[20:25] (X)−IY[20:25](X)|2 for 50  X  69
and 79  X  100 becomes minimum, where the data of Pi (X)in 70  X  78 have contribution from the signal of SN 2006jc
and have been excluded (see Figure 3(c)). The obtained resultof ηY[20:25] (k) is shown in Figure 4(a).
Then we define the reference pixel row and the target pixel
row in the spectral image of NIR/NP, so that each has the same
Y range as that defined in the image of NIR/N3. The signal
count profiles along the reference pixel row, SY[28:29] (X) (seeFigure 3(f)), and along the target pixel row, SY[20:25] (X) (seeFigure 3(g)), in the spectral image of NIR/NP are obtained.
Using the signal count profile along the reference pixel
row, SY[28:29] (X), and the obtained distribution function
ηY[20:25] (k), the signal count profile of the UGC4904 component,
UGC4904
SY[20:25](X), contained in SY[20:25] (X) can be modeled with
UGC4904
SY[20:25]
(X) =

∞


ηY[20:25] (k)SY[28:29] (X − k),

(6)

k=−∞

which is shown with the gray line in Figure 3(g). The sigSN2006jc
nal count profile of SN 2006jc, SY[20:25] (X), contained in
SY[20:25] (X) is given by
SN2006jc

UGC4904(X),
SY[20:25] (X) = SY[20:25] (X) − SY[20:25](7)

and is shown with the thin black line in Figure 3(g).
In order to evaluate the accuracy in the estimate of the signal
count profile of the UGC4904 component in the spectral imageof NIR/NP by this spectral decomposition technique, we define
the test pixel row both in the image of NIR/N3 and NP coveringfrom Y = 35 to Y = 40, so that it goes across the host galaxy
UGC4904 (see Figures 3(a) and 3(e)) and applied the same
technique (see Figures 3(d) and 3(h)).
We model the signal count profile of the UGC4904 compoUGC4904
nent, IY[35:40](X), contained in IY[35:40] (X) with
UGC4904
IY[35:40]
(X) =

∞


ηY[35:40] (k)IY[28:29] (X − k),

(8)

k=−∞

where the free distribution function ηY[35:40] (k) is derived in thesame way as ηY[20:25] (k) (see Figure 3(d)). The obtained resultof ηY[35:40] (k) is shown in Figure 4(b).
The signal count profile of the UGC4904 component,
UGC4904
SY[35:40](X), is given by
UGC4904
SY[35:40]
(X) =

∞


ηY[35:40] (k)SY[28:29] (X − k).

(9)

k=−∞
UGC4904
The obtained results of SY[35:40](X) and the residual profile
UGC4904
given by SY[35:40] (X) − SY[35:40] (X) are shown with the grayline and the thin line in Figure 3(h), respectively. The standard
deviation of the residual profile from zero is 8.1 (ADU pixel−1 ),which confirms that the signal count profile of the UGC4904
component is properly estimated by our spectral decomposition

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SAKON ET AL.
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(e)

(b)

(f)

(c)

(g)

(d)

(h)

Figure 3. Procedures for extracting the emission carried by SN 2006jc adopted for the spectroscopic data taken with AKARI/IRC NIR/NP, with the emission carriedby the host galaxy UGC4904 separated. See text for details.

( )

( )

η

(b)

η

(a)

Figure 4. Obtained distribution functions (a) ηY[20:25] (k) for target pixel row and (b) ηY[35:40] (k) for test pixel row.

technique. The value of 8.1 (ADU pixel−1 ) is used to estimate
the systematic error in the spectral decomposition procedure.
We have made the wavelength calibration and the divisionby the system spectral response of NIR/NP, rNP (λ) in units of
SN2006jc
ADU mJy−1 , for the obtained signal count profile SY[20:25] (X)following the procedure in the IRC Spectroscopic toolkit(Ohyama et al. 2007). The absolute flux calibration for the NP
spectrum of SN 2006jc has been made by using the ratio of thetotal flux count of SN 2006jc in IY[20:25] (X) to that obtainedby the photometric decomposition technique in Section 2.1.
The 1σ error of the NP spectrum of SN 2006jc is obtained as
0.5
UGC4904
UGC4904
BGσNP(λ)2 + σNP(λ)2 , where σNP(λ) is the uncertainty in the subtraction of the UGC4904 component in units ofBG(λ) is the 1σ noisemJy, which is given by 8.1/rNP (λ), and σNP
equivalent flux for NIR/NP spectrum taken with the AOT04
mode in units of mJy quoted from Ohyama et al. (2007).

3. RESULTS
3.1. The Near- to Mid-Infrared Characteristics of SN 2006jc
The results of the photometry of SN 2006jc with AKARI/IRC N3, S7, S9W, and S11 bands are listed in Table 2. NIR
photometry of SN 2006jc with H and K bands was performed
simultaneously on and 2007 April 28 and 29 (Minezaki et al.
2007), using the multicolor imaging photometer (MIP) mounted
on the MAGNUM 2 m telescope at Haleakala Observatories inAKARI OBSERVATION OF SN2006jc

No. 1, 2009

Table 2
Results of the Photometry of SN 2006jc on 2007 April 29 (200 Days after the
Discovery)
Instrument

Bandλband (μm)

Flux Density (mJy)

MAGNUM/MIP
MAGNUM/MIP
AKARI/IRC
AKARI/IRC
AKARI/IRC
AKARI/IRC

H
K
N3
S7
S9W
S11

1.6
2.2
3.2
7.0
9.0
11.0

0.05 ± 0.02
0.14 ± 0.02
0.38 ± 0.04
0.57 ± 0.07
0.71 ± 0.10
0.50 ± 0.10

Hawaii (Kobayashi et al. 1998a, 1998b), and the results are
summarized also in Table 2.
The obtained NIR spectrum of SN 2006jc on day 200, together
with the photometric results are plotted in Figure 5. The NIR
spectrum of SN 2006jc is characterized by a continuum emission
peaking around at ∼ 4 μm, which can be attributed to thethermal emission from dust grains. Possible atomic hydrogen
recombination lines of Pf-ζ at 2.87 μm, Pf- at 3.04 μm, Pf-δat 3.30 μm, Pf-γ at 3.74 μm, Br-α at 4.07 μm, and Br-β at2.63 μm are recognized, though they are significant only within
2σ –3σ (see Figure 5). We also note a small dent around at
4.6 μm, possibly attributed to the CO absorption band.
3.2. Emission from the Newly Formed Dust in the NIR
Assuming that spherical dust grains of a certain kind X witha uniform particle radius aX and a total mass of MX are locatedat a distance of R from the observer, and that they emit opticallythin thermal radiation of the temperature of TX (K), the observed
flux density profile is given byfνX (λ) = MX4πρX aX33

−1
π Bν (λ, TX )Qabs
X (λ)

aX
R

2
,

(10)
where Qabs
X (λ) is the absorption efficiency and ρX is the densityof a dust particle of composition X. The NIR spectral energy
distribution (SED) of SN 2006jc on day 200 of H- and K-band
photometric data and the 2–5 μm spectroscopic data is fittedwith fνX (λ) for each case of the amorphous carbon (X = a.car.),and the astronomical silicate (X = a.sil.) as a composition of
dust. We assume the distance of R = 25.8 Mpc (Pastorello
et al. 2007) for SN 2006jc.
In the following analysis aX is set as 0.01 μm, taking accountof the fact that the average radius of the newly formed dust in
the expanding ejecta is less than 0.01 μm due to the low density
and the rapid cooling timescale of the ejecta gas (Nozawa et al.
2008).
As for the amorphous carbon case, the absorption efficiency,
Qabs
a.car. (λ), for a spherical amorphous carbon grain with a radiusof aa.car. = 0.01 μm is calculated from the optical constants of
Edo (1983). Assuming ρa.car. = 2.26 g cm−3 , the fit is carriedout with the equilibrium temperature Ta.car. and the total mass
Ma.car. being free parameters. The result of the fit is shown in
Figure 6(a), and the best-fit parameters of Ta.car. = 800 ± 10 K
and Ma.car. = 6.9 ± 0.5 × 10−5 M are obtained.
The flux density at each AKARI/IRC imaging band is calculated for the model spectrum taking account of the color
correction, which is given byfνa.car. (band) =∞a.car.
(ν)dν
0 Rband (ν)fν,∞ νband
R
(ν)dν
band
0
ν

(11)

551

where Rband (ν) is the relative system spectral response ofeach AKARI/IRC band, and νband = c/λband corresponds to
the reference frequency of each AKARI/IRC band definedwith λN3 = 3.2 μm, λS7 = 7.0 μm, λS9W = 9.0 μm, andλS11 = 11.0 μm (Onaka et al. 2007). The model values thatwere converted into the AKARI/IRC calibration system offνa.car. (N 3), fνa.car. (S7), fνa.car. (S9W ), and fνa.car. (S11) are plottedby crosses in Figure 6(a). The observed data at S7, S9W, andS11 bands have excess emission over fνa.car. (S7), fνa.car. (S9W ),and fνa.car. (S11), respectively. The interpretation for the excess
component is discussed in Section 4.
As for the silicate case, the absorption efficiency, Qabs
a.sil. (λ),
for a spherical astronomical silicate grain with a radius ofaa.car. = 0.01 μm is taken from the values in Draine (1985),
and ρa.sil. = 3.3 g cm−3 is assumed. The fit was carried out inthe same way as for the amorphous carbon grains. The resultof the fit is shown in Figure 6(b), and the best-fit parametersof Ta.sil. = 920 ± 10 K and Ma.sil. = 4.2 ± 0.3 × 10−4 M
are obtained. The flux density at N3, S7, S9W, and S11 bands
is calculated for the best-fit model spectrum, taking accountof the color correction and is shown in Figure 6(b). The model
values of fνa.sil. (S7), fνa.sil. (S9W ), and fνa.sil. (S11) largely exceed
the observed flux density at S7, S9W, and S11 bands. The
disagreement in the MIR flux shows that astronomical silicate
is not a likely carrier of the continuum in the 2–5 μm region on
day 200.
We conclude that amorphous carbon is a likely carrier of the
NIR continuum on day 200. The spectrum is well accounted forby the amorphous carbon dust with the temperature of 800 ±10 K and with the total mass of Ma.car. = 6.9 ± 0.5 × 10−5 M .
This result is consistent with the suggestion made by Smith et al.
(2008) that the rebrightened NIR emission on 79 days is likely to
be carried by carbonaceous dust with an equilibrium temperatureof 1600 K, but not by silicate dust. The equilibrium temperatureof 800 K on day 200 derived in our analysis is consistent withthe scenario that this component is newly formed dust in thefree-expanding ejecta of the SN (Nozawa et al. 2008), in which
the illuminating radiation is assumed to be the thermal emission
from the gas ejecta heated by γ -rays and positrons produced
via the decay of radioactive elements such as 56 Ni and 56 Co(Tominaga et al. 2008).
4. THE CARRIERS OF THE MIR EXCESS COMPONENTThe MIR excess component is seen over the model spectrumof the amorphous carbon of 800 K at the S7, S9W, and S11bands. One possible candidate for the excess component is thedust with an emission band in the MIR region carried by silicate
and/or amorphous silica dust, which is discussed in Section 4.1.
Another possible candidate for the excess component is the thermal emission from the dust with temperature lower than 800 K.
The properties of the lower temperature dust are discussed in
Section 4.2. In Section 5.1, we discuss which one of these twocandidates is more likely.
In the following analysis, we model the near- to mid-infrared
spectrum as a combination of N kinds of dust components(Xi ; i = 1, . . . , N ), including the amorphous carbon discussed
in Section 3.2, and then the model spectrum is calculated asfνmodel (λ) =


−1


N

4
aXi 2
MXi πρXi aX3 i πBν λ, TXi Qabs
(λ)
,
Xi
3
R
i=1
(12)

SAKON ET AL.

Vol. 692

σ

β

ζ
ε
δ
γ
α

552

μ
Figure 5. NIR spectrum of SN 2006jc on day 200 after the discovery with AKARI/IRC NIR/NP. Photometric data taken with MAGNUM/MIP H, K bands (opensquare), and with AKARI/IRC NIR/N3, MIR-S/S7, S9W, S11 bands (solid square) are plotted together. The 1σ error level in the NP spectrum are also shown.

where the fit parameters MXi and TXi are the mass and thetemperature of the dust component Xi , and Qabs
Xi (λ), aXi , and ρXi
are the absorption efficiency, grain radius, and the density of
dust Xi , respectively.

(a)
ν

ν

4.1. Silicate and/or Amorphous SiO2 Model

ν

(b)

ν

Figure 6. Near- to mid-infrared SED of SN 2006jc on day 200 constructed by
the H- and K-band photometric data (open square), the 2–5 μm spectroscopic
data (thin cross), and the MIR photometric data (solid square). The results ofthe best-fit spectrum modeled with Equation (10) to reproduce the NIR data areshown for the case of (a) amorphous carbon (X = a.car.) and (b) astronomicalsilicate (X = a.sil.). The flux density at AKARI/IRC NIR/N3, MIR-S/S7, S9W,
and S11 bands simulated for the model spectrum taking account of the color
correction is shown with the thick crosses.

One possible candidate for the excess emission in S7, S9W,
and S11 bands is the dust band emission of the silicate and/or amorphous silica dust. In Figure 7, the absorption-efficiency
profiles Qabs
X (λ) of amorphous carbon (X = a.car.) from Edo(1983), astronomical silicate (X = a.sil.) from Draine (1985),
and amorphous SiO2 (X = silica) from Philipp (1985) are
shown together with the system spectral response curve of
the AKARI/IRC NIR/N3, MIR-S/S7, S9W, and S11 bands.
While Qabs
a.car. (λ) shows a smooth featureless profile without any
absband structures, Qabs
a.sil. (λ) and Qsilica (λ) show the band structure
peaking at ∼ 9.5–10.0 μm and ∼ 8.5–9.0 μm, respectively.
Therefore, these band structures intrinsic to the silicate-related
dust could contribute to the excess emission in S7, S9W, andS11 bands.
Firstly, a two-component (N = 2) model of the amorphous
carbon (X1 = a.car.) and the astronomical silicate (X2 = a.sil.)is employed. The near- to mid-infrared data of SN 2006jc on
day 200 are fitted with Equation (12), where Ta.car. = 800 K
and Ma.car. = 6.9 × 10−5 M are fixed, and Ta.sil. and Ma.sil.
are taken as the free parameters. The grain radii of bothcomponents are set as 0.01 μm. The best-fit model spectrum
is shown in Figure 8(a), and we obtain Ta.sil. = 710 ± 10 K and
Ma.sil. = 1.12 ± 0.02 × 10−4 M .
However, the flux densities in S7, S9W, and S11 bands predicted for this two-component (amorphous carbon + astronomical silicate) model taking account of the color correction cannot
well reproduce the observed MIR SED characterized by theenhanced S9W flux density (see Figure 8(a)). For one thing,
the band structure of astronomical silicate peaking at ∼ 9.5–
10.0 μm is almost fully included not only in S9W, but also
sometimes more efficiently depending on the dust temperaturein S11. For the other, the predicted model spectrum is too low inAKARI OBSERVATION OF SN2006jc

No. 1, 2009

553

(a)

a

μ

(b)

a

μ

(c)

a

μ

(d)

Figure 7. Absorption-efficiency profile Qabs
X (λ) of (a) amorphous carbon (X = a.car.), (b) astronomical silicate (X = a.sil.), and (c) amorphous SiO2 (X = silica).
(d) The system spectral response curve of AKARI/IRC NIR/N3, MIR-S/S7, S9W, and S11 bands.

(a)

ν

ν

(b)

ν

ν

6 < λ < 8.5 μm, where the S11 band does not have its sensitivity, but the S9W band does. Therefore, we need to have another
dust component that has a band structure in λ < 8.5 μm in its
absorption-efficiency profile. Amorphous SiO2 (silica) is one of
the candidates for such component (see Figure 7(c)). Moreover,silica should be able to be produced in SN ejecta (Nozawa et al.
2003, 2007). Its presence in the Cas An SN remnant has beenreported by a MIR spectroscopic observation with the Short
Wavelength Spectrometer (SWS) on board the Infrared Space
Observatory (ISO; Douvion et al. (2001) and also by a recent
Spitzer observation (Rho et al. 2008).
We assume a two-component (N = 2) model consisting of hot
amorphous carbon (X1 = a.car.) and amorphous silica (X2 =silica), where Ta.car. = 800 K and Ma.car. = 6.9 × 10−5 M are
fixed, and Tsilica and Msilica are taken as the free parameters. The
grain radii of both components are set to be 0.01 μm, and weassume the density of amorphous SiO2 ρsilica = 2.62 g cm−3 .
The best-fit model spectrum is shown in Figure 8(b), and we
obtain Tsilica = 790 ± 10 K and Msilica = 6.5 ± 0.1 × 10−5 M .
This time, the predicted flux densities are in good agreementwith the MIR SED characterized by the enhanced S9W flux
density.
4.2. Two-Temperature Amorphous Carbon Dust Model
Another possible candidate for the excess component is thethermal emission from the featureless dust with temperature
lower than 800 K. Our NIR to MIR data of SN 2006jc on day
200 are fitted with Equation (12), assuming a two-component(N = 2) model of hot amorphous carbon (X1 = h.a.car.) andwarm amorphous carbon (X2 = w.a.car.). The temperature andthe integrated mass of the former component are fixed to thevalues obtained in Section 3.2 and those of the latter component,
Tw.a.car. and Mw.a.car. , are taken as the free parameters. The
same properties of the absorption-efficiency profile, the grainradius and the density are assumed both for the warm and hot
components. The fitting is made, so that the flux densities of themodel spectrum at S7, S9W, and S11 bands that were converted
into the AKARI/IRC calibration system should best reproduce
the observed flux density at S7, S9W, and S11 bands. The

Figure 8. Best-fit model spectra with Equation (12) for the case of twocomponent (N = 2) models made of (a) amorphous carbon (X1 = a.car.) andastronomical silicate (X2 = a.sil.), and (b) amorphous carbon (X1 = a.car.)and amorphous SiO2 (X2 = silica). Symbols are the same as in Figure 6.

best-fit model spectrum is shown in Figure 9, and we obtain−3
Tw.a.car. = 320 ± 10 K and Mw.a.car. = 2.7+0.7
M−0.5 × 10−3
10 M .

554

SAKON ET AL.

νν

Figure 9. Results of the best-fit spectrum with Equation (12) are shown for thecase of two-component (N = 2) models made of two-temperature amorphous
carbons (X1 = h.a.car. and X2 = w.a.car.). Symbols are the same as in Figure 6.

We note that the observed NIR to MIR SED has doublepeaked characteristics, with one peak being located in the NIR
and the other peak in the MIR, and that the model fit with twotemperature amorphous carbon dust results in a considerable
temperature gap between the two. This temperature gap canbe explained if we assume the situation such that only the
hot component with the temperature of 800 K is the newly
formed dust in the ejecta of SN 2006jc, and that the 320 K
warm component corresponds to thermal emission from thepreexisting circumstellar dust farther away from the newly
formed dust with a dust-depleted region in between. The dustdepleted region may have been created by the dust evaporation
due to the shock breakout in the initial phase of the SN explosion.
5. DISCUSSION
5.1. Compositions of Dust Around the SN 2006jc
Observational evidence for the presence of silicate and/or
silica grains formed in SN ejecta has been reported for the
Cas A SN remnant (Douvion et al. 2001; Rho et al. 2008, andreferences therein) and SN 2003gd (Sugerman et al. (2006), andthe presence of silicate dust in the CSM is confirmed for SN
1987A (Bouchet et al. 2006). Ercolano et al. (2007) have shownthat no more than 15% by mass of dust in the SN 1987A has
been in the form of silicates, whilst Sugerman et al. (2006) havereported that the dust that condensed around SN 2003gd has
been predominantly silicate.
The progenitor of Type Ib SN 2006jc is considered to be a
Wolf–Rayet star, suffering the mass loss during its late evolution
and forming a dense He-rich CSM (Foley et al. 2007). The Helayer of core-collapse SNe is suggested to contain more carbon
than oxygen (Nomoto & Hashimoto 1988) and, especially, theWolf–Rayet star with strong mass loss leads to thick C-rich
CSM and envelope (Limongi & Chieffi 2006; Tominaga et al.
2008). Therefore, the formation of silicate and/or silica grainsin the C-rich CSM of SN 2006jc is less likely (e.g., Nozawa
et al. 2003).
Even if the silicate and/or silica dust is newly formedin the ejecta of SN 2006jc, their temperature reaches only
up to 100 K taking account of the optical absorption
and the collision with ejecta gas with 1000 K on day∼ 200 after the discovery (Nozawa et al. 2008). AKARI/IRC

Vol. 692

S7/S11 band colors calculatedfor astronomical silicate and silica with 100
K are fνa.sil (S7) fνa.sil. (S11) = 5.2 × 10−3 and silica
silicafν (S7) fν (S11) = 4.1 × 10−2 , respectively, which are
by far smaller than the S7/S11 band color of ∼ 0.5 ± 0.2 obtained for the observed MIR excess component. Therefore, boththe astronomical silicate and amorphous silica with 100 K cannot reproduce the flux density at S7, S9W, and S11 bands ofthe excess component and should not be the major carriers of
the excess. Consequently, the MIR excess component over themodel spectrum of the amorphous carbon of 800 K is likely to
be IR light echo by amorphous carbon with 320 ± 10 K in the
CSM discussed in Section 4.2, and we conclude that amorphous
carbon of 800 ± 10 K of 6.9 ± 0.4 × 10−5 M is newly formedin the ejecta of SN 2006jc on day 200.
The present analysis assumes a single grain size of
0.01 μm. As far as the size is small compared to the wavelength in question, the derived dust mass does not depend on thegrain size. The grain temperature is a slow function of the grainsize, and thus the introduction of the size distribution will notaffect the mass estimate. In particular, the nucleation model suggests the formation of very small grains in the very rapid coolinggas of SN 2006jc (Nozawa et al. 2008). In the present analysis,
we did not apply a thorough radiative transfer and employed asingle-temperature approximation because the uncertainties in
the observational results are too large to warrant detailed modeling. As far as the dust-forming layer is thin and does not have asignificant temperature gradient, a single temperature provides
a reasonable approximation for the emission from the shell. The
major aim of the present analysis is to discuss the nature andorigin of the infrared emission detected by the present observations, and is not intended to derive an accurate estimate of thedust mass.
On the other hand, Mattila et al. (2008) have also presented
the Spitzer IRAC photometric data obtained at the similar epoch
to ours and explained their data in terms of the dust formationin the cool dense shell (CDS) produced by the interaction of
the ejecta onward shock with a dense shell of the circumstellar
material ejected by the progenitor. They obtained 3 × 10−4 Mof amorphous carbon as the mass of newly formed dust.
In any cases, the newly formed dust mass around the SN
2006jc is more than 3 orders of magnitude smaller than theamount needed for core-collapse SNe to contribute efficiently
to the early-universe dust budget (Morgan & Edmunds 2003).
Recent observational studies report the evidence of dust formation in the ejecta of several core-collapse SNe, but the produced
dust mass in the SN ejecta is generally found to be much smaller
than theoretically predicted values of 0.1–1 M (Nozawa
et al. 2003), i.e., no more than 7.5 × 10−4 M for SN 1987A(Ercolano et al. 2007), ∼ 10−4 M for SN 1999em (Elmhamdi
et al. 2003), and 4×10−5 M for SN 2003gd (Meikle et al. 2007).
The obtained amorphous carbon dust mass for SN 2006jc on day
200 in our study is consistent with those relatively small values
obtained for other several dust-forming core-collapse SNe, although our data is almost insensitive to the newly formed silicateof ∼ 100 K.
In addition to the dust formed in the ejecta, on the other
hand, our MIR photometric data suggest the presence of another−3
amorphous carbon dust of 320 ± 10 K of 2.7+0.7
M−0.5 × 10
as the circumstellar component. This component is expected
to be formed in the mass-loss wind associated with the Wolf–
Rayet stellar activity (e.g., Williams et al. 1992; Waters et al.
1997; Molster et al. 1999; Voors et al. 2000). It follows thatthe dust condensation not only in the SN ejecta itself, but also

AKARI OBSERVATION OF SN2006jc

No. 1, 2009

in the mass-loss wind associated with the prior events to the
SN explosion could make a significant contribution to the dust
formation by a massive star in its whole evolutional history.
6. SUMMARY
We present our latest results on NIR to MIR observations of
SN 2006jc at 200 days after the discovery using the IRC on
board AKARI. The NIR (2–5 μm) spectrum of SN 2006jc is
obtained for the first time, and is found to be well interpreted
in terms of the thermal emission from amorphous carbon of
800 ± 10 K of 6.9 ± 0.5 × 10−5 M that was formed in the SN
ejecta. This newly formed dust mass in the SN ejecta is in a range
similar to those obtained for other several dust-forming corecollapse SNe based on recent observations. MIR photometricdata with AKARI/IRC MIR-S/S7, S9W, and S11 bands haveshown excess emission over the thermal emission of amorphous
carbon of 800 K. This MIR excess emission is likely to beaccounted for by the emission from circumstellar amorphous−3
carbon dust of 320 ± 10 K of 2.7+0.7
M rather than by−0.5 × 10
the band emission of astronomical silicate and/or silica grains.
Since this circumstellar amorphous carbon dust of 320 K is
expected to be formed in the mass-loss wind associated with theWolf–Rayet stellar activity, our result suggests that a significant
amount of dust is condensed in the mass-loss wind associatedwith the prior events to the SN explosion. MIR spectroscopy ofearly-time SNe of similar type is highly important for further anddetailed analysis on the properties of newly formed dust in the
SN ejecta, distinguishing from the circumstellar or interstellar
dust component.
This work is based on observation of AKARI, a JAXA projectwith the participation of ESA. We thank all the members of
the AKARI project, particularly those who have engaged in
the observation planning and the satellite operation during theperformance verification phase, for their continuous help andsupport. We would also express our gratitude to the AKARI
data reduction team for their extensive work in developing data
analysis pipelines. This work is supported in part by a grant-inaid for Scientific Research on Priority Areas from the Ministryof Education, Culture, Sports, Science, and Technology of Japanand grants-in-aid for Scientific Research from the JSPS.

555
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