The Astrophysical Journal, 817:145 (23pp), 2016 February 1

doi:10.3847/0004-637X/817/2/145© 2016. The American Astronomical Society. All rights reserved.

CONCURRENT FORMATION OF CARBON AND SILICATE DUST IN NOVA V1280 SCO
Itsuki Sakon1, Shigeyuki Sako2, Takashi Onaka1, Takaya Nozawa3, Yuki Kimura4, Takuya Fujiyoshi5,
Takashi Shimonishi6, Fumihiko Usui1, Hidenori Takahashi2, Ryou Ohsawa2, Akira Arai7, Makoto Uemura8,
Takahiro Nagayama9,10, Bon-Chul Koo11, and Takashi Kozasa12
1

Department of Astronomy, Graduate Schools 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 Astronomy, University of Tokyo, 2-21-1 Ohsawa, Mitaka, Tokyo 181-0015, Japan
3
National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
4
Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan
5
Subaru Telescope, National Astronomical Observatory of Japan, 650 North A’ohoku Place, Hilo, HI 96720, USA
6
Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Aramaki aza Aoba 6-3, Aoba-ku, Sendai 980-8578, Japan
7
Koyama Astronomical Observatory, Kyoto Sangyo University, Motoyama, Kamigamo, Kita-ku, Kyoto, 603-8555, Japan
8
Hiroshima Astrophysical Science Center, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan
9
Department of Physics and Astronomy, Graduate School of Science and Engineering, Kagoshima University, 1-21-35 Korimoto, Kagoshima 890-0065, Japan10
Department of Astrophysics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan
11
Department of Physics and Astronomy, Seoul National University , 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, Korea
12
Department of Cosmosciences, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
Received 2015 July 31; accepted 2015 December 2; published 2016 January 28

ABSTRACT
We present infrared multi-epoch observations of the dust-forming nova V1280 Sco over ∼2000 days from theoutburst. The temporal evolution of the infrared spectral energy distributions at 1272, 1616, and 1947 days can be
explained by the emissions produced by amorphous carbon dust of mass (6.6–8.7)×10−8 Me with arepresentative grain size of 0.01μm and astronomical silicate dust of mass (3.4–4.3)×10−7 Me with arepresentative grain size of 0.3–0.5 μm. Both of these dust species travel farther away from the white dwarf
without apparent mass evolution throughout those later epochs. The dust formation scenario around V1280 Sco
suggested from our analyses is that the amorphous carbon dust is formed in the nova ejecta followed by theformation of silicate dust either in the expanding nova ejecta or as a result of the interaction between the nova wind
and the circumstellar medium.
Key words: dust, extinction – infrared: stars – ISM: lines and bands – novae, cataclysmic variables – stars:
mass-loss(Yamaoka et al. 2007). V1280 Sco is classiﬁed as a Fe II nova
from its early optical spectrum (Munari et al. 2007) and, thus, iscaused by an explosion on a carbon–oxygen (CO) white dwarf.
The onset of dust formation around V1280 Sco was reported tooccur 23 days after the discovery (Das et al. 2007; Rudy
et al. 2007). These characteristics are consistent with aclassiﬁcation of V1280 Sco as a CO nova (Das et al. 2007).
The peak of dust formation was reported at between 36 and 45days in the nova wind ejected predominantly on Day 10.5
(Chesneau et al. 2008) with a wind speed of 500 km s−1(Yamaoka et al. 2007). An expanding dust shell moving at
0.35±0.03 mas day−1 has been reported based on VLTI/AMBER and MIDI observations performed between Days 23
and 145 (Chesneau et al. 2008). In addition to the major massloss peak on Day 10.5, a rebrightening of V1280 Sco was
observed on Day 110, which may correspond to the secondary
minor mass loss peak (Das et al. 2007). The evolution of thelight curve of V1280 Sco is extremely slow and, therefore, themass of the white dwarf is estimated to be MWD=0.6 Me or
smaller (Hounsell et al. 2010; Naito et al. 2012). The distanceto V1280 Sco is still under debate. A distance D of 1.6 kpc isestimated by Chesneau et al. (2008), D=1.25 kpc by Das
et al. (2008), and 0.63 kpc by Hounsell et al. (2010). Naito et al.
(2012) have recently revised the velocity of the dust shell to be
350±160 km s−1 and estimated the distance to beD=1.1±0.5 kpc. In the following analyses, we adopt
D=1.1 kpc as the distance to dust around V1280 Sco.
This study investigates the temporal evolution of the infraredemission from the dusty nova V1280 Sco with the objectives of

1. INTRODUCTION
Understanding the formation process of circumstellar dust isa fundamental step in exploring the origin of interstellar dust.
From an observational point of view, however, we have limited
knowledge about how dust condenses in gaseous stellar ejecta
containing nucleosynthesized heavy elements and how thedusty circumstellar environment is consequently formed.
Classical novae provide unique opportunities to observe those
processes on a realistic timescale thanks to their relatively
frequent occurrence in the nearby universe (<a few kpc fromthe Sun). The composition of dust formed in the stellar ejecta
depends on the chemical properties of the stellar atmosphere;
carbonaceous dust is formed when the C/O ratio in theenvelope exceeds unity (carbon-rich), while silicate dust isformed when the C/O ratio is less than unity (oxygen-rich)(Waters 2004). Recent infrared observations of some dusty
novae, however, have detected the presence of emission fromboth carbonaceous and silicate dust: e.g., Nova V842 Cen(Smith et al. 1994); Nova QV Vul (Gehrz et al. 1992), Nova
V705 Cas (Evans et al. 2005). The origin of such bimodal dust
species is not yet fully understood. Infrared monitoring
observations of nearby dusty novae over several years fromtheir outbursts are crucial in investigating the temporal
evolution of the dust properties at different epochs and inuncovering the dust formation history in the circumstellar andinterstellar environments around the novae.
V1280 Sco was discovered on 2007 February 4.86 by
Japanese amateur astronomers Y. Nakamura and Y. Sakurai
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Table 1
Summary of the Imaging Observations
EpochaInstrument

Band

Wavelength (μm)

Band Width (μm)

Integration Time (s)Date

Day 150

Kanata/TRISPEC
Subaru/COMICS
Subaru/COMICS
Subaru/COMICS
Subaru/COMICSGAO/GIRCS
Gemini-S/T-ReCS
Gemini-S/T-ReCS
Gemini-S/T-ReCS
Gemini-S/T-ReCS
Gemini-S/T-ReCS
IRSF/SIRIUS
Gemini-S/T-ReCS
Gemini-S/T-ReCS
Gemini-S/T-ReCS
Gemini-S/T-ReCS
Gemini-S/T-ReCS
IRSF/SIRIUS
Gemini-S/T-ReCS
Gemini-S/T-ReCS
Gemini-S/T-ReCS
Gemini-S/T-ReCS
Gemini-S/T-ReCS

J/Ks
N8.8
N11.7
Q18.8
Q24.5
J/H/Ks
Si-1
Si-3
Si-5
Qa
Qb
J/H/Ks
Si-1
Si-3
Si-5
Qa
Qb
J/H/Ks
Si-1
Si-3
Si-5
Qa
Qb

1.25/2.15
8.8
11.7
18.75
24.56
1.25/1.65/2.15
7.73
9.69
11.66
18.30
24.56
1.22/1.65/2.16
7.73
9.69
11.66
18.30
24.56
1.22/1.65/2.16
7.73
9.69
11.66
18.30
24.56

0.26/0.32
0.8
1.0
0.90
0.75
0.26/0.29/0.32
0.69
0.93
1.13
1.51
1.92
0.26/0.29/0.32
0.69
0.93
1.13
1.51
1.92
0.26/0.29/0.32
0.69
0.93
1.13
1.51
1.92

0.5/0.1
20
20
60
60
360
120
120
120
120
600
64
60
60
60
120
120
480
60
60
60
120
120

2007 Jun 26
2007 Jul 7
2007 Jul 7
2007 Jul 7
2007 Jul 7
2010 Aug 26
2010 Aug 1
2010 Aug 1
2010 Aug 1
2010 Aug 1
2010 Aug 1
2011 Jul 27
2011 Jul 10
2011 Jul 10
2011 Jul 10
2011 Jul 10
2011 Jul 102012 Sep 26
2012 Jun 62012 Jun 62012 Jun 62012 Jun 62012 Jun 6

Day 1272

Day 1616

Day 1947

Note.
a
The epochs of the observations with Subaru/COMICS and Gemini-S/T-ReCS in days after the discovery.

The center wavelength, λc, and bandwidth, Δλ, are summarized in Table 1. The plate scale of the detector was 0 13 andthe ﬁeld of view (FOV) was 41″ × 31″. To cancel out the highbackground radiation, the secondary mirror was chopped at afrequency of ∼0.5 Hz with a 10″ throw in the north–south
direction. The chopped image was subtracted from the image
taken immediately before to eliminate sky emission. Since thechopping residual pattern was almost negligible for brightobjects, nodding was not employed. Images collected at both
chopping positions were used. The total on-source integration
times are summarized in Table 1. The data reduction was
performed in the standard manner for mid-infrared groundbased imaging observations, including dark subtractions andﬂat ﬁelding. The ﬂux calibration of V1280 Sco was achieved
using measurements of the two standard stars listed in Cohenet al. (1999): λ Sgr for N8.8, N11.7, and Q18.8, and μ Cep forQ24.5. The aperture size for the photometry was set to be 2″ inradius.
For the NL spectroscopy of V1280 Sco on Day 150, aslit width of 0 33 was used. As in the case of the imaging
observations, only the secondary mirror chopping was
performed at a frequency of ∼0.5 Hz with a 10″ throw in
the north–south direction. The position angle (PA) of theslit was set to 0° so that images at both chopping positions
would fall on the slit. Spectra collected at both chopping
positions were co-added, resulting in the total on-source
integration time of 20 s for NL spectroscopy (see Table 2).
The data reduction was performed in the standard manner
for mid-infrared ground-based spectroscopic observations,
including dark subtraction, ﬂat ﬁelding using domeﬂat,wavelength calibration using atmospheric lines, and distortion correction. Finally, a one-dimensional spectrum of
V1280 Sco was extracted. Atmospheric absorption in thedetermining the temporal evolution of dust properties andexamining how the dusty circumstellar environment is createdaround the nova on a timescale of several years following theoutburst. Details of our observations are described in Section 2.
The evolution of the mid-infrared images of V1280 Sco over2000 days following the outburst is presented in Section 3. The
near-infrared spectrum of V1280 Sco on Day 940 is shown in
Section 4. The properties and the origin of dust around V1280
Sco are discussed in Section 5. The dust and molecular bandfeatures in the near- and mid-infrared spectra of V1280 Sco are
discussed in Section 6.
2. OBSERVATIONS
N-band (8–13 μm) and Q-band (17–26 μm) imaging andN-band low-resolution (NL) spectroscopic data of V1280 Sco
on Day 150 were collected with the Cooled Mid-Infrared
Camera and Spectrometer (COMICS; Kataza et al. 2000)
on the Subaru Telescope and those on Days 1272, 1616, and1947 were collected with the Thermal-Region Camera
Spectrograph (T-ReCS; de Buizer et al. 2005, pp. 84–87) on
the Gemini-South telescope. In addition, a near-infrared(2.55–4.9 μm) spectrum on Day 940 was taken with the
AKARI/Infrared Camera (IRC; Onaka et al. 2007). Long-term
photometric monitoring observations in the optical and nearinfrared have been made continuously from the very initial
phase of the outburst using the Optical and Infrared Synergetic
Telescopes for Education and Research (OISTER). A summaryof the imaging observations is given in Table 1.
2.1. Subaru/COMICS data
Imaging observations of V1280 Sco on Day 150 were madewith four medium-band ﬁlters, N8.8, N11.7, Q18.8, and Q24.5.
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Table 2
Summary of Spectroscopic Observations
EpochaInstrument

Mode

Wavelength (μm)

Resolution

Integration Time (s)Date

Day 150
Day 940
Day 1272
Day 1616
Day 1947

Subaru/COMICSAKARI/IRC
Gemini-S/T-ReCS
Gemini-S/T-ReCS
Gemini-S/T-ReCS

NL
NG
NL
NL
NL

7.8–13.3
2.55–4.9
7.70–12.97
7.70–12.97
7.70–12.97

R∼160 near 10μm
R∼100 near 3μm
R∼100 near 10μm
R∼100 near 10μm
R∼100 near 10μm

20
40
600
60
600

2007 Jul 7
2009 Sep 82010 Aug 1
2011 Jul 102012 Jun 6

Note.
a
The epochs of the observations with Subaru/COMICS, Gemini-S/T-ReCS, and AKARI/IRC in days after the discovery.

scaling the ﬂux of the obtained spectra to match thephotometric data in the Si-1, Si-3, and Si-5 bands.

extracted spectrum was corrected by division by thespectrum of a standard star, λ Sgr, whose absolute
spectrum is provided in Cohen et al. (1999). The absolute
ﬂux calibration including the correction for slit efﬁciency
was performed by simply scaling the ﬂux level of theobtained spectrum to match the photometric data in the 8.8
and 11.7μm bands.

2.3. AKARI/IRC data
AKARI (Murakami et al. 2007) performed a warm mission
after the exhaustion of liquid helium in 2007 August until thetermination of the mission in 2011 November. A near-infrared(2.55–4.9 μm) spectrum of V1280 Sco on Day 940 was takenwith AKARI/IRC as part of the AKARI phase 3-II Open Time
Program “Spectral Evolution of Novae in the Near-IR Basedon AKARI Observations (SENNA)” (PI: I. Sakon). Pointing IDsof the data are 5201030.1 and 5201030.2. Each observation
was performed with the IRCZ4 AKARI Astronomical Observing Template (AOT) using the NIR grism disperser (NG;Ohyama et al. 2007). The target was placed in the Np aperture,a 1′×1′ FOV for the spectroscopy of a point source in the
NIR channel. Since the image size of the IRC/NIR channel has
a FWHM of ∼4 7, which is much larger than the size of
V1280 Sco even at the later epochs (see Section 3), V1280 Sco
on Day 940 can be regarded as a point source when observedwith IRC on AKARI.
A single pointed observation performed with this AOT
produces, sequentially, ﬁve dark frames, four spectra with NG,
a reference image with N3 (3.2 μm), ﬁve spectra with NG, andﬁve dark frames. Each exposure consists of one short exposure
and one long one. Since the long-exposure images weresaturated, we used only the short-exposure images in thepresent study. The ﬁnal dark frame was produced by medianﬁltering ten short-exposure dark frames to correct for any
effects of cosmic rays and it was then subtracted from each
short-exposure science frame.
The signal pattern originating from the sky background and/or foreground emission in the Np aperture was carefully
removed. This pattern was reproduced by convolving theforeground and/or background spectra collected at the Ns slit
over the whole area of the sky within the Np aperture,assuming that the sky brightness is uniform (Sakon et al. 2012).
In order to remove the cosmic-ray hits, we applied a 3σclipping of 18 dark-subtracted short-exposed NG spectra. In
this process, bad pixels identiﬁed in the dark image weremasked out and a shift in position due to the pointing
uncertainty during a pointed observation was estimated fromthe zeroth-order images of bright sources in the FOV. The
spectrum of V1280 Sco was extracted by using an extraction
box with a width of 5 pixels (7 3) in the spatial direction and alength of 300 pixels in the dispersion direction. The extracted
spectrum was divided by the NG system response curve toobtain the ﬁnal ﬂux-calibrated 2.5–5μm spectrum of V1280
Sco on Day 940.

2.2. Gemini-S/T-ReCS data
Mid-infrared imaging and spectroscopic observations of
V1280 Sco on Days 1272, 1616, and 1947 were carried out at
the Gemini-South observatory under the Subaru/Gemini Time
Exchange Programmes GS-2010B-C-7 on 2010 August 1, GS2011B-C-4 on 2011 July 10, and GS-2012A-C-5 on 2012 June
6. For the imaging portion, ﬁve medium-band ﬁlters, Si-1, Si-3,
Si-5, Qa, and Qb, were used. The center wavelength, λc,and bandwidth, Δλ, are summarized in Table 1. The pixel
scale of the T-ReCS detector was 0 09 and the FOV was
28 8×21 6. Both chopping and nodding were performed tocancel out the high background radiation and the chopping
residual pattern. The chopping was performed at a frequency of∼3 Hz with a 15″ throw in the north–south direction. The onsource integration time is summarized in Table 1. The
mireduce task included in the Gemini IRAF package(https://www.gemini.edu/node/10795) was used to produce
ﬁnal co-added, chop-subtracted, and nod-subtracted data fromthe raw data ﬁles. The ﬂux calibration was accomplished byobserving the Cohen standard star HD 151680 (Cohenet al. 1999). The aperture size for the photometry was set tobe 2″ in radius.
A slit width of 0 35 was used for the N-band lowresolution spectroscopy of V1280 Sco on Days 1272, 1616,
and 1946. As in the case of the imaging observations, both
chopping and nodding were employed. The chopping was
performed at a frequency of ∼2 Hz with a 15″ throw in thenorth–south direction. The instrument position angle was set
to 120° so that the slit would run approximately along thelong axis of V1280 Sco. The on-source integration time issummarized in Table 2. The msreduce task included in theGemini IRAF package was used to reduce the mid-infrared
spectrum of V1280 Sco at each epoch. The wavelength
calibration was carried out using telluric atmospheric lines.
Finally, a one-dimensional spectrum of V1280 Sco was
extracted. Atmospheric absorption in the extracted spectrum
was corrected by division by the spectrum of a standard star,
HD 151680, whose calibrated spectrum is provided in Cohenet al. (1999). The absolute ﬂux calibration including thecorrection for slit efﬁciency was performed by simply
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Sakon et al.

Figure 1. The mid-infrared images of V1280 Sco on Day 150 taken at N8.8 (8.8 μm), N11.7 (11.7 μm), Q18.8 (18.8 μm), and Q24.5 (24.5 μm) using Subaru/
COMICS. The image of a standard star (λ Sgr for N8.8 and N11.7, μ Cep for Q18.8 and Q24.5) is shown in the right upper corner of each panel for comparison. The
signal of each image is normalized by the total signal within 1″ radius aperture. The 1″ scale bar is shown in the ﬁrst panel. North is to the left and east is toward thebottom in each image.

Observatory. The center wavelength, λc, the bandwidth, Δλ,and the integration times for those observations are given in
Table 1.

2.4. Near-infrared Photometric Data Collected Through
OISTER
Almost simultaneously with the Subaru/COMICS observations on Day 150 (2007 July 7), near-infrared photometricobservations of V1280 Sco in the J and Ks bands were carriedout on 2007 June 26 using the Triple Range Imager andSpectrograph (TRISPEC; Watanabe et al. 2005) on the 1.5 m
KANATA telescope at Higashi-Hiroshima Observatory. Also
almost simultaneous with the Gemini-S/T-ReCS observation
on Day 1272 (2010 August 1), near-infrared photometricobservations of V1280 Sco in the J, H, and Ks bands werecarried out on 2010 August 26 using the Gunma Infrared
Camera and Spectrograph (GIRCS; Takahashi & Nishihara2007) on the 1.5 m telescope at Gunma Astronomical
Observatory (GAO; Hashimoto et al. 2002). Finally, almost
simultaneously with the Gemini-S/T-ReCS observations on
Days 1616 (2011 July 10) and 1947 (2012 June 6), nearinfrared photometric observations of V1280 Sco in the J, H,
and Ks bands were carried out on 2011 July 27 and 2012
September 26, respectively, using the Simultaneous three-color
Infrared Imager for Unbiased Surveys (SIRIUS; Nagayama
et al. 2001) mounted on the 1.4 m telescope of the IRSF
(Infrared Survey Facility) at the South African Astronomical

3. TEMPORAL EVOLUTION OF MID-INFRAREDIMAGES OF V1280 SCO
The mid-infrared images of V1280 Sco on Days 150, 1272,
1616, and 1947 are shown in Figures 1–4, respectively. On
Day 150, the size of the emitting region cannot be spatiallyresolved with the Subaru/COMICS at any wavelength. The
intrinsic size of the emitting region of V1280 Sco at eachwavelength λ is modeled by a circular Gaussian proﬁlegiven byGl (x , y) = Vltot (2ps l2 )- 2
⎧ 1 ⎛ (x - x )2 + ( y - y )2 ⎞ ⎫00´ exp ⎨ - ⎜
⎟ ⎬,
2
2
s
⎝
⎠⎭
⎩
l
1

⎪

⎪

⎪

⎪

(1 )

where (x0, y0) are the central coordinates of the emitting regionin pixels, Vltot is the ﬂux density of V1280 Sco at wavelength λ,and σλ is the size of the emitting region represented by theGaussian dispersion in pixels. The parameters in Equation (1)are determined by deconvolving the image of V1280 Sco in
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Figure 2. The mid-infrared images of V1280 Sco on Day 1272 taken at Si-1 (7.73 μm), Si-3 (9.69 μm), Si-5 (11.66 μm), Qa (18.3 μm), and Qb (24.56 μm) using
Gemini-S/T-ReCS. The image of standard star HD 151680 is shown in the right upper corner of each panel for comparison. The signal of each image is normalized by
the total signal within the 1″ radius aperture. The 1″ scale bar is shown in the ﬁrst panel. North is to the left and east is toward the bottom in each image.

each band with a point-spread function of the standard star at
the same wavelength. At all the wavelengths used, the obtained
values of σλ are withinthe range of positional uncertainty due
to the chopping stability in an exposure (∼1 pixel=0 13).
Speciﬁcally, the size obtained for the emitting region at thelongest wavelength, 24μm, on Day 150 was s24.5m m = 0.5
pixel, which corresponds to a distance of 0 065 from the whitedwarf. Because of the large positional uncertainty (∼1 pixel)
mentioned above, 0 065 is regarded as an upper limit. Nearand mid-infrared photometry of V1280 Sco on Day 150 issummarized in Table 3.
On Days 1272, 1616, and 1947, all the mid-infrared imagesof V1280 Sco exhibit elongation along PA=20°. In
particular, the morphology on Day 1947 is very similar to thatobserved by Chesneau et al. (2012) on Day 1589. Near- andmid-infrared photometric results for V1280 Sco on Days 1272,
1616, and 1947 are listed in Table 4.

characterized by strong red continuum emission with a small
unidentiﬁed infrared (UIR) emission feature at 3.3μm. Small
dips at 4.26 and 4.6μm may be the absorption features due to
CO2 gas and CO gas, respectively. The properties of thecarriers of the emission and/or absorption features are
discussed in Section 6.
5. TEMPORAL EVOLUTION OF INFRARED SPECTRAL
ENERGY DISTRIBUTIONS OF V1280 SCO
5.1. Temperature and Luminosity Evolution of the White Dwarf
Adopting MWD=0.6 Me as the mass of the white dwarf in
V1280 Sco (e.g., Hounsell et al. 2010; Naito et al. 2012), thetemperature and luminosity of the white dwarf on Day 150 after
the outburst are estimated to be TWD=2.5×104 K and
LWD=1.6×104Le assuming a chemical composition in theenvelope of X=0.35, Y=0.33, C+O=0.30, where X isthe fractional abundance of hydrogen, Y is that of helium, andC+O=0.30 is the combined abundance of carbon andoxygen (Kato & Hachisu 1994). When the nova evolves to thenebular phase, the temperature and luminosity of the whitedwarf reach TWD=1.0×105 K and LWD=1.8×104Le. In
the following analyses, we adopt these latter values for the

4. THE NEAR-INFRARED SPECTRUM OF V1280 SCO
AT DAY 940.
Figure 5 shows the near-infrared spectrum of V1280 Sco on
Day 940 obtained with AKARI/IRC. The spectrum is
5

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Sakon et al.

Figure 3. The same as Figure 2 but for the epoch of Day 1616.

(HAC), SiC, and silicates (Evans & Gehrz 2012). Taking into
account the relatively smooth spectral shape except for a small
absorption structure due to interstellar silicate, we expect that
amorphous carbon is responsible for the infrared continuum
emission observed in V1280 Sco on Day 150.

white dwarf on Days 1272, 1616, and 1947. We note that thoseluminosity values are signiﬁcantly larger than that estimatedfrom the relation between core mass and luminosity for whitedwarfs given by Paczynski (1971). However, the relationshipof Paczynski (1971) is applicable only to the case where thetemperature of the C+O core is much higher than in novae,
and therefore it is not appropriate for our analysis.

5.2.2. Can an Optically Thin Dust Emission Model Explain the
Observed SED on Day 150?

5.2. Infrared Spectral Energy Distribution of V1280 Sco on
Day 150

Assuming that spherical amorphous carbon dust grains witha uniform radius a and a total mass Mi are located at a distanceR from the observer, and that they give off optically thinthermal emission with an equilibrium temperature Ti, theobserved ﬂux density is

5.2.1. Analysis Based on Optically Thin Dust Model Fitting

Figure 6 shows the near- to mid-infrared spectral energy
distribution (SED) of V1280 Sco on Day 150. It is dominatedby the thermal dust emission with a small absorption structureat around 9.7μm due to silicate dust. Although the origin of thesilicate dust is not clear, it is likely to be predominantly
associated with interstellar silicate dust located along the line ofsight toward V1280 Sco since neither apparent emission
features from newly formed silicate nor infrared light echoesfrom the circumstellar silicate have been observed between
Days 23 and 145 (Chesneau et al. 2008). To our knowledge,
the dust components that have ever been identiﬁed in CO
novae are amorphous carbon, hydrogenated amorphous carbon⎛4⎞-1⎛ a ⎞2abs
fni (l ) = Mi ⎜ pra.car.a3⎟ pBn (l , Ti ) Qa.car.
(l ) ⎜ ⎟ ,
⎝D⎠
⎝3
⎠

(2 )

where ρa.car. is the density of an amorphous carbon dust particle
abs(l ) is the absorption(we use ρa.car. = 1.87 g cm−3) and Qa.car.
efﬁciency of amorphous carbon of radius a (ACAR sample;
Zubko 1996). The optically thin emission from multitemperature amorphous carbon components with foreground6

The Astrophysical Journal, 817:145 (23pp), 2016 February 1

Sakon et al.

Figure 4. The same as Figure 2 but for the epoch of Day 1947.

temperatures (i.e., N = 2), 350 and 900K, with a foregroundsilicate extinction with τ9.7=0.24±0.01 (see Figure 6). The
best-ﬁt parameters are summarized in Table 5.
AV=1.2 mag is inferred from Marshall et al. (2006) toward
the direction of V1280 Sco. Assuming the ratio of
AV t9.7 = 18.5  1.5 (Roche & Aitken 1984), the interstellar
extinction expected at 9.7μm toward the direction of V1280
Sco is τ9.7=0.065, which is smaller than the valueτ9.7=0.24 obtained from our SED analysis. This discrepancy
may be attributed to the uncommon composition of dust grainsintervening between V1280 Sco and us, given that the value ofAV/τ9.7 is sensitive to the abundance ratio of silicate dust to
carbon dust. Because no apparent light echoes containing
silicate emission have been reported between Days 23 and 145
by Chesneau et al. (2008), it is less likely that the silicate that
contributes in producing an absorption structure at 9.7μm on
Day 150 is associated with V1280 Sco. In our SED analysis on
Days 1272, 1616, and 1947 in Section 5.3.2, therefore, we
assume that the extinction of τ9.7=0.24 is fully associatedwith interstellar silicate dust located in the line of sight toward
V1280 Sco. We note that the results of the SED analysis in
Section 5.3.2 are not signiﬁcantly altered whatever value
between τ9.7=0.24 and τ9.7=0.065 we adopt as theforeground silicate extinction in the direction toward
V1280 Sco.

Table 3Results of the Near- to Mid-infrared Photometory on Day 150
Instrument

Band

Wavelength (μm)

Flux Density (Jy)

KANATA/TRISPEC
KANATA/TRISPEC
Subaru/COMICS
Subaru/COMICS
Subaru/COMICS
Subaru/COMICS

J
Ks
N8.8
N11.7
Q18.8
Q24.5

1.25
2.15
8.8
11.7
18.8
24.5

0.5±0.03
10.6±0.20
131.5±2.0
125.5±2.0
66.1±2.5
24.5±2.5

silicate extinction is given byN⎧⎛ Q abs (l) ⎞ ⎫fnmodel (l) = å fni (l) exp ⎨ - t9.7 ⎜ abs a.sil.
⎟ ⎬,⎝ Qa.sil. (9.7 m m) ⎠ ⎭
⎩
i=1
⎪

⎪

⎪

⎪

(3 )

where N is the number of amorphous carbon components withabs(l ) is the absorpdifferent representative temperatures, Qa.sil.
tion efﬁciency of astronomical silicate of radius a(Draine 1985), and τ9.7 is the optical depth of foregroundsilicate dust at 9.7μm. The observed near- to mid-infrared
SED on Day 150 is reproduced well by the optically thinemission from amorphous carbon components at two different
7The Astrophysical Journal, 817:145 (23pp), 2016 February 1

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Table 4
Results of the Near- to Mid-infrared Photometry on Days 1272, 1616, and 1947
Epoch
Day 1272

Day 1616

Day 1947

Instrument

Band

Wavelength (μm)

Flux Density (Jy)GAO/GIRCS
GAO/GIRCS
GAO/GIRCS
Gemini-S/T-ReCS
Gemini-S/T-ReCS
Gemini-S/T-ReCS
Gemini-S/T-ReCS
Gemini-S/T-ReCS
IRSF/SIRIUS
IRSF/SIRIUS
IRSF/SIRIUS
Gemini-S/T-ReCS
Gemini-S/T-ReCS
Gemini-S/T-ReCS
Gemini-S/T-ReCS
Gemini-S/T-ReCS
IRSF/SIRIUS
IRSF/SIRIUS
IRSF/SIRIUS
Gemini-S/T-ReCS
Gemini-S/T-ReCS
Gemini-S/T-ReCS
Gemini-S/T-ReCS
Gemini-S/T-ReCS

J
H
Ks
Si-1
Si-3
Si-5
Qa
Qb
J
H
Ks
Si-1
Si-3
Si-5
Qa
Qb
J
H
Ks
Si-1
Si-3
Si-5
Qa
Qb

1.24
1.66
2.16
7.73
9.69
11.66
18.30
24.56
1.22
1.65
2.16
7.73
9.69
11.66
18.30
24.56
1.22
1.65
2.16
7.73
9.69
11.66
18.30
24.56

0.635±0.013
0.683±0.017
0.824±0.028
30.98±0.16
28.82±0.18
25.24±0.18
19.42±0.22
10.89±0.23
0.51±0.05
0.54±0.06
0.66±0.12
23.29±0.14
26.43±0.16
23.36±0.16
18.32±0.54
12.89±0.61
0.49±0.05
0.47±0.05
0.57±0.06
16.53±0.04
17.88±0.04
16.56±0.02
14.69±0.06
9.59±0.20

Figure 6. Near- to mid-infrared SED of V1280 Sco on Day 150 constructedfrom the photometric data in the near- to mid-infrared (solid squares) and the
N-band low-resolution spectroscopic data (gray pluses). Assuming opticallythin emission modeled with Equation (3), the best-ﬁt spectrum to reproduce theobserved near- to mid-infrared SED on Day 150 is shown by the thick blackline. The optically thin emission from the amorphous carbon components of
900K and 350K is shown with a gray line and dashed line, respectively,
taking account of the foreground silicate extinction of τ9.7=0.24.

Figure 5. The near-infrared spectrum of V1280 Sco on Day 940 obtained withAKARI/IRC observations.

Table 5
The Dust Properties in V1280 Sco on Day 150 Assuming an Optically Thin
Emission Model

The blackbody angular radius θbb in arcsecond for the
Ti=2=350 K component is given by⎛ ¥ i=2⎞1 2
q bb = 2.06 ´ 10 5 ⎜
fn (l) dn ⎟ (sTi4= 2)-1 2
⎝ 0
⎠

ò

(4 )

Composition
Temperature Ti
Mass Miwhere fni = 2 (l ) is the observed ﬂux density carried by the350K component and σ is the Stefan–Boltzmann constant. We
get θbb=0 054, which is just slightly below the radius of theemitting region of the 350K component (i.e., 0 065) obtained
from the deconvolved 24.5μm image on Day 150. Therefore,

Component 1 (i = 1)

Component 2 (i = 2)

Amorphous Carbon
900±10K
3.2×10−9 Me

Amorphous Carbon
350±10K
1.0×10−6 Me

the 350K dust shell must be both optically thick and spatiallyunresolved. In this case, the dust mass of amorphous carbon
becomes 2.7×10−6 Me by assuming that the Planck mean8

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Table 6
The Dust Properties in V1280 Sco on Days 1272, 1616, and 1947
Epoch (days)

1272

Amorphous Carbon
1616

1947

1272

Astronomical Silicate
1616

1947

Temperature (K)Mass (10−7 Me)

540±10+0.05
0.790.05500±10+0.05
0.790.04440±10+0.12
0.890.10220±10+2.8
8.41.0230±10+3.2
7.11.2210±10+3.8
7.52.0Figure 8. Plots of the temperature of amorphous carbon with different radiiof a=0.01, 0.1, and 1.0μm calculated as a function of the distance r fromthe white dwarf of MWD=0.6 Me modeled with (TWD, LWD ) =(1.0 ´ 10 5 K, 1.8 ´ 10 4L ). The data points obtained as a result of the SED
ﬁtting at each epoch on Days 1272, 1616, and 1947 are shown with red crosses,
where the radius of the semimajor axis of the deconvolved emitting region
modeled with the elliptic Gaussian for the Si-1 is adopted to indicate thelocation of the amorphous carbon component.

Figure 9. Plots of the temperature of astronomical silicate with different radiiof a=0.01, 0.1, and 1.0μm calculated as a function of the distance r from themodeled
with
white
dwarf
of
MWD=0.6 Me(TWD, LWD ) = (1.0 ´ 10 5 K, 1.8 ´ 10 4 Le). The broken red line indicates
the temperature of the astronomical silicate component obtained as a result of
the SED ﬁtting on Day 1947 assuming the optically thin dust emission model.

Figure 7. Near- to mid-infrared SED of V1280 Sco on Days 1272 (top), 1616
(middle), and 1947 (bottom). The photometric values in the near- to midinfrared are shown with black squares and the N-band low-resolution spectrum
is shown with gray pluses. The best-ﬁt model curve assuming optically thinemission from amorphous carbon and astronomical silicate given by
Equation (5) is shown with a solid black line in each panel. The gray line
and broken line in each panel indicate the spectra of emission carried byamorphous carbon and astronomical silicate, respectively.

emission cross section for a carbon grain of radius a is
proportional to aTi2= 2 (Gilman 1974; Gehrz et al. 1980) and thatthe density of amorphous carbon is ρa.car.=1.87 g cm−3.
9The Astrophysical Journal, 817:145 (23pp), 2016 February 1

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Figure 10. A schematic view of the dust geometry assumed in V1280 Sco on Days 1272, 1616, and 1947. Amorphous carbon and astronomical silicate are assumed tobe distributed in bipolar cones with a common opening angle f. Each component is conﬁned within the volume enclosed by the inner and outer walls, where thedensity distribution is assumed to be proportional to r−2. The viewing direction is from the surface to the back of the paper/screen.
Table 7The Best-ﬁt Parameters Obtained for aa.car. = aa.sil. = 0.01 m m
Epoch (days)
Opening angle; f (deg)iniAmor. carbon initial number density; na.car.
(cm−3)
inAmor. carbon inner radius; ra.car. (AU)out
Amor. carbon outer radius; ra.car.
(AU)
ini
Astro. silicate initial number density; na.sil.
(cm−3)
inAstro. silicate inner radius; ra.sil. (AU)out
Astro. silicate outer radius; ra.sil.
(AU)

1272

1616

1947

24.0±0.5−3+0.3
2.50.3 ×10+0.1
1.3-0.1×102
2+0.3
2.40.2 ×10−4+0.4
3.2-0.3 ×10
2+0.4
3.60.3 ×10+0.6
7.0-0.5 ×102

18.0±0.5−3+0.2
1.30.2 ×10+0.1
1.5-0.1×102
2+0.8
4.60.6 ×10−4+0.7
6.0-0.6 ×10
2+0.4
3.50.2 ×10+6.5
11.1-2.0 ×102

15.0±0.5−3+0.2
1.00.1 ×10+0.1
1.7-0.2 ×102
2+2.2
6.40.9 ×10−4+0.8
6.3-0.6 ×10
2+0.6
5.50.7 ×10
2+1.6
8.3-0.8 ×10

Table 8The Best-ﬁt Parameters Obtained for aa.car. = aa.sil. = 0.1 m m
Epoch (days)
Opening angle; f (deg)iniAmor. carbon initial number density; na.car.
(cm−3)
inAmor. carbon inner radius; ra.car. (AU)out
Amor. carbon outer radius; ra.car.
(AU)
ini
Astro. silicate initial number density; na.sil.
(cm−3)
inAstro. silicate inner radius; ra.sil. (AU)out
Astro. silicate outer radius; ra.sil.
(AU)

1272

1616

1947

26.0±0.5−5+0.3
2.50.3 ×10+0.2
5.1-0.6 ×101
2+0.1
0.90.1 ×10−6+0.2
1.3-0.2 ×10
2+0.3
2.00.2 ×10
2+0.5
4.8-0.4 ×10

19.5±0.5−5+0.1
0.90.1 ×10+0.2
5.4-0.4 ×101
2+0.4
1.80.2 ×10−6+0.2
1.1-0.2 ×10
2+0.2
2.40.2 ×10
2+0.5
5.5-0.3 ×10

15.0±0.5−5+0.1
0.80.1×10+0.6
6.1-0.2 ×101
2+1.2
3.90.8 ×10−6+0.2
1.6-0.2 ×10
2+0.2
3.20.3 ×10+0.4
5.6-0.2 ×102

10

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Figure 11. The data points used for the model ﬁtting were constructed from the continuum points, which were selected to exclude any possible dust emission featuresand emission lines, in the N-band low-resolution spectra in addition to the N-band and Q-band photometric data. The best-ﬁt curves obtained as a result of the SED
ﬁtting at each epoch of Days 1272, 1616, and 1947 assuming a single grain size of aa.car. = aa.sil. = 0.01 m m are shown in the right panels with a solid black line. The
gray line and broken line in each left panel indicate the spectra of emission carried by amorphous carbon and astronomical silicate, respectively. Plots of thetemperature of amorphous carbon and astronomical silicate as a function of the distance from the white dwarf obtained as a result of the SED ﬁtting at each epoch on
Days 1272, 1616, and 1947 are shown in the right panels.

11

The Astrophysical Journal, 817:145 (23pp), 2016 February 1

Sakon et al.

Figure 12. The same as Figure 11 but for aa.car. = aa.sil. = 0.1 m m .

5.3. Infrared SEDs of V1280 Sco on Days 1272, 1616,
and 1947

emission, which were not present in the SED obtained on Day
150 (see Figure 6).
To investigate the temporal evolution of properties of
constituent dust, a simple SED ﬁtting was carried out at eachepoch based on a two-component (astronomical silicate andamorphous carbon) model; a single equilibrium temperature isassumed for each dust component. Assuming optically thin

5.3.1. Analysis Based on Optically Thin Dust Model Fitting

A notable point in the near- to mid-infrared SEDs of V1280
Sco on Days 1272, 1616, and 1947 (see Fig 7) is a conspicuous
appearance of the 9.7 and 18μm amorphous silicate features in
12

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Table 9The Best-ﬁt Parameters Obtained for aa.car. = 0.01 m m and aa.sil. = 0.1 m m
Epoch (days)
Opening angle; f (deg)iniAmor. carbon initial number density; na.car.
(cm−3)
inAmor. carbon inner radius; ra.car. (AU)out
Amor. carbon outer radius; ra.car.
(AU)
ini
Astro. silicate initial number density; na.sil.
(cm−3)
inAstro. silicate inner radius; ra.sil. (AU)out
Astro. silicate outer radius; ra.sil.
(AU)

1272

1616

1947

24.0±0.5−3+0.2
2.00.2 ×10+0.1
1.3-0.1×102
2+0.3
2.50.2 ×10
−6
+0.1
×10
1.30.12+0.2
2.00.2 ×10
2+1.0
5.0-0.5 ×10

18.0±0.5−3+0.2
1.30.2 ×10+0.1
1.5-0.1×102
2+0.5
4.60.5 ×10
−6
+0.2
×10
1.80.22+0.2
2.00.2 ×10+1.0
6.0-0.5 ×102

15.0±0.5−3+0.2
1.10.1 ×10+0.1
1.7-0.2 ×102
2+0.6
6.60.6 ×10
−6
+0.2
×10
1.50.22+0.2
2.70.2 ×10+1.0
6.7-0.5 ×102

geometry in the infrared, the observed ﬂux density is given bymodel curves for a=0.01 and 0.1 μm. Therefore, with our
two-component model that assumes a single equilibrium
temperature for each, the grain size of amorphous carbon dust
formed in the nova ejecta is estimated to be in the range froma=0.01 to 0.1μm. We note, however, that a temperature
distribution in each dust component is expected but not taken
into consideration in this simple model. A more accurate
estimate of the grain size of amorphous carbon assuming anappropriate dust geometry is discussed in Section 5.3.2.
The Qa and Qb images taken on Day 1947 (see Figure 4) are
dominated by the astronomical silicate emission, as we haveshown above (see Figure 7), and they exhibit an elongatedshape along PA∼20°. The similarity between the image
shapes taken in Qa and Qb bands and those in Si-1, Si-3, andSi-5 bands may indicate that both the carbon and silicate dust
components reside predominantly in a dusty bipolar nebula(Chesneau et al. 2012). The projected distance corresponding to
the semimajor axis of the deconvolved emitting region modeledwith an elliptic Gaussian is 0 40±0 03 in the Qa band and0 45±0 04 in the Qb band on Day 1947.
Figure 9 shows equilibrium temperatures of optically thinastronomical silicate as a function of distance from the whitedwarf calculated for different radii a=0.01, 0.1, and 1.0μm.
Astronomical silicate dust achieves an equilibrium dust
temperature of 210K at a distance of 1 1 from the whitedwarf for a=0.1 μm and 0 34 for a=1.0 μm. Comparing
these values with the semimajor axes of the mid-infrared
emitting region in the Qa and Qb bands derived above, we
conclude that the typical grain size of astronomical silicate
around V1280 Sco must be in the range from 0.1 to 1.0μm. In
the next section, the results of the SED analyses taking accountof the dust geometry are discussed to examine more detailed
physical properties of circumstellar dust around V1280 Sco.
Then, the origin of each dust species is discussed byinvestigating the temporal evolution of the resultant geometricand physical parameters of circumstellar dust around V1280
Sco between the three epochs in Section 5.4.

⎛4⎞-1
fnmodel (l) = Ma.car. ⎜ pra.car.a 3⎟⎝3⎠⎛ a ⎞2abs(l ) ⎜ ⎟´ pBn (l , Ta.car.) Qa.car.
⎝D⎠
⎛4
⎞-1
+ Ma.sil. ⎜ pra.sil.a 3⎟⎝3⎠⎛ a ⎞2abs(l ) ⎜ ⎟ ,´ pBn (l , Ta.sil.) Qa.sil.
⎝D⎠

(5 )

where Ta.car. and Ta.sil. are the equilibrium temperatures, Ma.car.
and Ma.sil. are the dust masses, ρa.car. (1.87 g cm−3), and ρa.sil.
(3.3 g cm−3) are the densities of an amorphous carbon (a.car.)and astronomical silicate (a.sil.) dust particle, respectively. We
note that our SED analysis requires the optical depth of ISM
silicates at 9.7μm to be τ9.7=0.24±0.01 to account for thesmall absorption structure in the spectrum of V1280 Sco on
Day 150. Assuming that this extinction is fully of interstellar
origin, the best-ﬁt parameters obtained for Days 1272, 1616,
and 1947 based on the two-component model are summarizedin Table 6. Here we assume that the dust size is smaller thanabsabs(l ) and Qa.sil.
(l ) are approximately
1μm and thus Qa.car.
proportional to a in the mid-infrared wavelength range. The
obtained dust masses Ma.car. and Ma.sil. are, therefore, almost
independent of a.
The temporal evolution of amorphous carbon properties ischaracterized by a decrement in the temperature without
signiﬁcant mass evolution at those three epochs. This suggests
the kinematic behavior of the amorphous carbon dust, which
travels farther from the white dwarf without being destroyed.
The projected distance corresponding to the semimajor axis of
the deconvolved emitting region modeled with an elliptic
Gaussian in the Si-1 band, which is supposed to trace theamorphous carbon component in our photometric dataset(Figure 7), is 0 27±0 02 on Day 1272, 0 32±0 02 on
Day 1616, and 0 40±0 04 on Day 1947. If the spread of
dust is in the shape of a bipolar nebulae and is viewed edge-on(e.g., Chesneau et al. 2012), the size of the emitting region
corresponds to the average distance to which the dust has
traveled from the white dwarf. Figure 8 shows temperatures ofamorphous carbon grains of different radii a=0.01 μm,
0.1μm, and 1.0μm as a function of distance from the whitedwarf. We base our calculations on the same WD properties as
before, i.e., MWD=0.6 Me, TWD=1.0×105 K, and
LWD=1.8×104Le, and further assume that these values all
remain constant on Days 1272, 1616, and 1947. Three data
points obtained by ﬁtting the SED are located between the

5.3.2. Dust SED Analysis

The elongated shape of V1280 Sco along PA∼20° seen inboth the N- and Q-band images on Day 1947 suggests that the
mid-infrared emission is dominated by carbon and silicate dust
grains in a bipolar nebula. This leads us to assume a simple
geometry, in which both the amorphous carbon and astronomical silicate components are distributed in bipolar cones with acommon opening angle f (see Figure 10) and are viewed
edge-on.
This geometry is consistent with the description of thestructure of the ejected shells given by Naito et al. (2013). We
assume the density distributions of amorphous carbon
13

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Figure 13. The same as Figure 11 but for aa.car. = 0.01 m m and aa.sil. = 0.1 m m .

[na.car.(r )] and astronomical silicate [na.sil.(r)] to bein⎧0(r < ra.car.
)
⎪
2
⎛ r ⎞
⎪
ini
in
out
na.car.(r ) = ⎨ na.car.
´ ⎜ in ⎟ (ra.car.
< r < ra.car.
)⎪⎝ ra.car. ⎠
⎪
out
< r)
(ra.car.
⎩0

and
in
⎧0
(r < ra.sil.
)
⎪
2
⎛ r ⎞
⎪
ini
in
out
na.sil.(r ) = ⎨ na.sil.
´ ⎜ in ⎟ (ra.sil.
< r < ra.sil.
),⎪⎝ ra.sil. ⎠
⎪
out
< r)
(ra.sil.
⎩0

(6 )

14

(7 )

The Astrophysical Journal, 817:145 (23pp), 2016 February 1

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Figure 14. The same as Figure 11 but for aa.car. = 0.01 m m and aa.sil. = 0.2 m m .

respectively. Analyses are made independently for two
different spherical grain radii aa.car. = aa.sil. = 0.1 m m
and aa.car. = aa.sil. = 0.01 m m .
For such a geometric conﬁguration and density distributions,the ﬂux density from the white dwarf, fnWD (l , z ), at r = z isrespectively, as a function of distance r from the white
in
in
out
and ra.sil.
are the inner wall radii, ra.car.
dwarf, where ra.car.
ini
ini
out
and ra.sil. are the outer wall radii, and na.car. and na.sil. are
the initial number densities at the position of the innerwall for amorphous carbon and astronomical silicate,

15

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Figure 15. The same as Figure 11 but for aa.car. = 0.01 m m and aa.sil. = 0.3 m m .

16

The Astrophysical Journal, 817:145 (23pp), 2016 February 1

Sakon et al.

Figure 16. The same as Figure 11 but for aa.car. = 0.01 m m and aa.sil. = 0.5 m m .

17

The Astrophysical Journal, 817:145 (23pp), 2016 February 1

Sakon et al.

Table 10The Best-ﬁt Parameters Obtained for aa.car. = 0.01 m m and aa.sil. = 0.2 m m
Epoch (days)
Opening angle; f (deg)iniAmor. carbon initial number density; na.car.
(cm−3)
inAmor. carbon inner radius; ra.car. (AU)out
Amor. carbon outer radius; ra.car.
(AU)
ini
Astro. silicate initial number density; na.sil.
(cm−3)
inAstro. silicate inner radius; ra.sil. (AU)out
Astro. silicate outer radius; ra.sil.
(AU)

1272

1616

1947

24.0±0.5−3+0.2
2.10.2 ×10+0.1
1.3-0.1×102
2+0.3
2.50.2 ×10
−7
+0.1
×10
1.60.12+0.2
2.00.2 ×10+1.0
4.9-0.5 ×102

18.0±0.5−3+0.2
1.20.2 ×10+0.1
1.5-0.1×102
2+0.5
4.60.5 ×10
−7
+0.2
×10
1.80.22+0.2
2.00.2 ×10+2.0
6.0-1.0 ×102

15.0±0.5−3+0.2
1.30.2 ×10+0.1
1.7-0.2 ×102
2+0.6
6.60.6 ×10
−7
+0.2
×10
1.80.22+0.2
2.70.2 ×10+1.0
7.0-0.5 ×102

Table 11
The Best-ﬁt Parameters Obtained for aa.car. = 0.01 m m and aa.sil. = 0.3 m m
Epoch (days)
Opening angle; f (deg)ini(cm−3)
Amor. carbon initial number density; na.car.
inAmor. carbon inner radius; ra.car. (AU)out
Amor. carbon outer radius; ra.car.
(AU)
ini
Astro. silicate initial number density; na.sil.
(cm−3)
inAstro. silicate inner radius; ra.sil.
(AU)outAstro. silicate outer radius; ra.sil.
(AU)

1272

1616

1947

24.0±0.5−3+0.3
2.20.2 ×10+0.1
1.3-0.1×102
2+0.3
2.50.2 ×10−8+0.7
7.1-0.7 ×10
2+0.1
1.50.1 ×10+1.0
4.5-0.5 ×102

18.0±0.5−3+0.2
1.30.2 ×10+0.1
1.5-0.1×102
2+0.5
4.60.5 ×10−8+0.8
7.9-0.8 ×10
2+0.1
1.70.1×10+2.0
5.7-1.0 ×102

15.0±0.5−3+0.2
1.00.2 ×10+0.1
1.7-0.2 ×102
2+0.6
6.60.6 ×10−8+0.7
6.3-0.7 ×10
2+0.2
2.40.2 ×10+1.0
6.4-0.5 ×102

Table 12The Best-ﬁt Parameters Obtained for aa.car. = 0.01 m m and aa.sil. = 0.5 m m
Epoch (days)
Opening angle; f (deg)ini(cm−3)
Amor. Carbon initial number density; na.car.
inAmor. Carbon inner radius; ra.car. (AU)out
Amor. Carbon outer radius; ra.car.
(AU)
ini
Astro. silicate initial number density; na.sil.
(cm−3)
inAstro. silicate inner radius; ra.sil.
(AU)outAstro. silicate outer radius; ra.sil.
(AU)

1272
24.0±0.5−3+0.2
2.10.2 ×10+0.1
1.3-0.1×102
2+0.3
2.50.2 ×10
−8
+0.7
×10
7.10.72+0.1
1.30.1 ×10+1.0
4.3-0.5 ×102

1616
18.0±0.5−3+0.1
1.20.1×10+0.1
1.5-0.1×102
2+0.5
4.60.5 ×10
−8
+0.8
×10
7.90.82+0.1
1.30.1 ×10+2.0
6.3-1.0 ×102

1947
15.0±0.5−3+0.1
0.90.1 ×10+0.1
1.7-0.2 ×102
2+0.6
6.60.6 ×10
−8
+0.6
×10
6.30.62+0.2
1.90.2 ×10+1.0
6.9-0.5 ×102

Table 13
The Dust Mass Evolution among Days 1272, 1616 and 1947
Epoch (days)
1272
1616
1947

Amor. Carbon (Me)Astro. Silicate (Me)−8
+2.0
6.61.4 ×10−8+1.7
8.7-1.5 ×10−8+1.0
8.41.7 ×10−7+0.6
3.40.4 ×10−7+1.3
4.3-1.2 ×10−7+0.8
4.00.8 ×10

given by⎞⎛ L
1
fnWD (l, z )  ⎜ WD
⎟ pBn (TWD)
44pz 2⎝ sTWD ⎠
z
⎧
⎫
2
abs
´ exp ⎨-paa.car.
Qa.car.
(l )n a.car.(r ) dr ⎬
⎩
⎭
0
z
⎧
⎫
2
abs
´ exp ⎨-paa.sil.
Qa.sil.
(l )n a.sil.(r ) dr ⎬.
⎩
⎭
0

ò

(8 )
Figure 17. The plot of the inner wall radius of astronomical silicate on Day 1947
obtained for each grain radius of aa.sil.=0.1, 0.2, 0.3, and 0.5 μm. The broken
line indicates the semimajor axis length of the deconvolved Qb band image of
V1280 Sco modeled with an elliptical Gaussian proﬁle on Day 1947, i.e., 0 45.

ò(i.e., TWD = 1.0 ´ 10 5 K and LWD = 1.8 ´ 10 4L ). The
temperature of a dust grain, Tχ(ζ), where χ=a.car. or a.
sil. in our case, in local radiative equilibrium at r=ζ

We do not take into account backward scattering forsimpliﬁcation and assume the same stellar parameters as
before for the white dwarf on Days 1272, 1616, and 1947
18

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Sakon et al.

Figure 18. The near-infrared spectrum of V1280 Sco on Day 940 observed with AKARI/IRC. The spectrum is normalized to the pseudo-continuum modeled by asecond-order polynomial function through the continuum points at ∼3.0, ∼4.1, and ∼4.8 μm. The wavelength ranges of features arising from the aromatic C–Hstretches and aliphatic C–H stretches are indicated with black horizontal bars. The wavelength ranges of absorption features possibly carried by CO2 gas at
4.26 μm and CO gas at 4.6 μm are shown with gray horizontal bars. The small feature at 4.05 μm is attributed to Brα.

aa.car.=0.01 μm, and 5.1×101, 5.4×101, and 6.1×101AU
when aa.car.=0.1 μm. The dust expansion velocity of
0.35±0.03 mas day−1 reported by Chesneau et al. (2008)
corresponds to 0.19 AU day−1. If the expansion velocity stays
constant, the dust will reach distances of 2.4×102, 3.1×102,
and 3.7×102AU on Days 1272, 1616, and 1947, respectively. Moreover, the deconvolved images of V1280 Sco takenin the Si-1 band on Days 1272, 1616, and 1947 exhibit
elongated structures along PA∼20° and are modeled withelliptical Gaussian proﬁles with semimajor axis lengths of
0 27, 0 32, and 0 4, which correspond to 1.5×102,
1.8×102, and 2.2×102AU, respectively. Judging by thelocation of amorphous carbon dust in our model, therepresentative size of amorphous carbon dust produced around
V1280 Sco is more likely to be 0.01μm than 0.1 μm. We notethat the uncertainty in the adopted distance toward V1280 Sco
can directly affect the distance between the dust and the whitedwarf constrained from the observations. However, even if we
adopt the shortest distance of D=0.63 kpc toward V1280 Sco
estimated by Hounsell et al. (2010), the semimajor axis lengthsof the deconvolved image observed on Days 1272, 1616, and1947 correspond to 0.9×102, 1.0×102, and 1.3×102AU,
respectively, which are still much larger than the inner wall
radii of amorphous carbons obtained for aa.car.=0.1μm, andtherefore the representative size of amorphous carbon dust issigniﬁcantly smaller than 0.1 μm.
The best-ﬁt parameters obtained on Days 1272, 1616, and1947 calculated for the cases of aa.car.=0.01 μm,
aa.sil.=0.1 μm, 0.2 μm, 0.3 μm, and 0.5μm are summarizedin Tables 9–12, respectively. The model spectra calculated withthose best-ﬁt parameters are shown in the left panels of
Figures 13–16. Temperatures of amorphous carbon andastronomical silicate are shown as a function of distance fromthe white dwarf in the right panels of Figures 13–16. The bestinﬁt ra.sil.
ʼs on Day 1947 are shown in Figure 17 for differentinaa.sil. used in the SED analysis. Because ra.sil.
must be smaller
than the semimajor axis of the deconvolved Qb band image of
V1280 Sco on Day 1947 (i.e., 0 45), we conclude that the
typical size of silicate dust must be larger than 0.3μm (seesatisﬁes¥

ò0 4pac Qc (l) pBn (Tc (z )) dn¥= ò pac2 Qcabs (l) fnWD (l , z ) dn .
0
2

abs

(9 )

The absorption coefﬁcients of astronomical silicate of radius a
at shorter wavelengths (λ < 0.1 μm) are calculated from Dwek
& Smith (1996) and those at longer wavelengths (λ > 0.1 μm)
are quoted from Draine (1985). The heating due to the energy
absorption of thermal infrared emission from other dust
particles is not taken into account.
Assuming that V1280 Sco is viewed edge-on, an infrared
spectrum produced by amorphous carbon and astronomicalsilicate in bipolar lobes of V1280 Sco is calculated for ageometric conﬁguration characterized by seven free paraout
out
ini
ini
in
in
, na.sil.
, ra.car.
, ra.sil.
, ra.car.
, ra.sil.
, and f. The opticalmeters: na.car.
depth at 9.7 μm, t9.7 = 0.24  0.01, due to interstellar silicate
toward V1280 Sco is ﬁxed. Least-squares ﬁtting is carried out
to obtain a set of seven best-ﬁt parameters on Days 1272, 1616,
and 1947. Data points used for the model ﬁtting were selected
to exclude any possible dust emission features and emission
lines, in the N-band low-resolution spectra in addition to the Nand Q-band photometric data.
The best-ﬁt parameters obtained on Days 1272, 1616, and1947 calculated for the two cases aa.car. = aa.sil. = 0.01 m m
and aa.car. = aa.sil. = 0.1 m m are summarized in Table 7 andTable 8, respectively. The model spectra calculated with thosebest-ﬁt parameters are shown in the left panels of Figures 11
and 12. Temperatures of amorphous carbon and astronomicalsilicate are shown as a function of distance from the whitedwarf in the right panels of Figures 11 and 12. In both cases,
the temporal evolution of the infrared SED of V1280 Sco atthese three epochs can be explained by a simple scenario in
which amorphous carbon and astronomical silicate dust grains
are moving away from the white dwarf.
inon Days 1272, 1616, and 1947 is,We ﬁnd that ra.car.
respectively, 1.3×102, 1.5×102, and 1.7×102AU when
19

The Astrophysical Journal, 817:145 (23pp), 2016 February 1

Sakon et al.

carbon (a.car.) and astronomical silicate (a.sil.) dust particle,
respectively. These values suggest that no signiﬁcant evolution
in dust mass occurred from Day 1272 through 1947. Thus,
the masses of amorphous carbon and astronomical silicate
around V1280 Sco are ∼8×10−8 Me and ∼4×10−7 Me,
respectively.
5.4. Dust Formation History Around V1280 Sco
The results of our analyses have shown that the infrared SED
evolution of V1280 Sco on Days 1272, 1616, and 1947 after
the outburst is well reproduced by emission from a mixture ofamorphous carbon and astronomical silicate that travel awayfrom the white dwarf without any apparent mass evolution atsuch later epochs. Although the absolute values depend on thegeometry, our dust model described in Section 5.3.2 has foundthe typical size of amorphous carbon dust as aa.car.= 0.01μm
and its mass as Ma.car.∼8×10−8 Me, and the corresponding
values for astronomical silicate as 0.3 μm < aa.sil<0.5 μm
and Ma.sil.∼4 ×10−7 Me. We note that the masses ofamorphous carbon dust obtained on Days 1272, 1616, and1947 are much smaller than that of optically thick amorphous
carbon on Day 150, i.e., 2.7×10−6 Me (see Section 5.2.2).
Moreover, the typical grain size of amorphous carbon dust, i.e.,aa.car.=0.01 μm, obtained on Days 1272, 1616, and 1947 ismuch smaller than that of carbon grains that have formed inother CO novae (e.g., ∼0.3 μm for Nova Serpentis 1978 on
Day 70; Gehrz et al. 1980). We suggest, therefore, thatsputtering as a result of the interaction with the circumstellar
medium as well as evaporation may have occurred foramorphous carbons during the period from Day 150 to
Day 1272.
All the images taken on Day 1947, at shorter wavelengths(e.g., Si-1) which are dominated by amorphous carbon
emission, or at longer wavelengths (e.g., Qa and Qb) which
are dominated by silicate dust, exhibit an elongated structure
along PA = 20°. The radius of the carbon-emitting region was
0 40±0 03 and that of the silicate-emitting region was
0 45±0 04 on Day 1947. Assuming that the nova
ejecta travel with a constant velocity v, the projected distanced to which the nova ejecta with a velocity range of
320 km s−1 < v < 620km s−1 (Naito et al. 2012) can travelin 1947 days is 0 33 < d < 0 64, which clearly covers both
amorphous carbon and silicate dust-emitting regions observedon Day 1947. In addition, no infrared echoes from the preexisting silicate dust were detected in our mid-infrared
spectrum taken on Day 150. It is therefore more likely thatthe silicate emission observed on Days 1272, 1616, and 1947 is
produced either by newly formed dust grains in the expanding
nova ejecta or as a result of interaction between the ejecta andthe circumstellar medium, rather than by pre-existing circumstellar dust grains.
The dust formation scenario around V1280 Sco suggested by
our analyses is that the amorphous carbon dust is formed in thenova ejecta followed by the silicate dust formation. Such dual
dust chemistry has been reported in a number of other novae
such as QV Vul (Gehrz et al. 1992) and V705 Cas (Evans et al.
1997; Mason et al. 1998). The dust formation sequence inV1280 Sco is consistent with those novae, where the carbonrich dust is identiﬁed early in the outburst and oxygen-rich dust
later in the outburst (Gehrz et al. 1998). Such chemical
multiplicity of the dust formation requires a chemical gradientand/or heterogeneity in the nova ejecta. In the case of V1280

Figure 19. The mid-infrared spectra of V1280 Sco on Days 1272, 1616, and1947 observed with Gemini/T-ReCS. The best-ﬁt model spectrum obtained asa result of the SED ﬁtting (see Section 5.3.2) at each epoch is used to deﬁne thecontinuum and is subtracted. The wavelength ranges of the UIR band features
recognized in our observations are shown with black horizontal bars. Peakpositions of 9.2, 9.8, 10.7, and 11.4μm features of crystalline silicates (e.g.,silica, forsterite, enstatite, and diopside) reported in Molster et al. (2002) arealso shown with gray horizontal error bars as a reference.

Figure 17). Moreover, the semimajor axis of the deconvolved
emitting region measured in the Qb band, which is dominatedby astronomical silicate, is systematically larger than thatmeasured in the Si-1 band, which is dominated by amorphous
carbon, on Days 1272, 1616, and 1947. This implies thatastronomical silicate is located farther from the white dwarf
inis always
than amorphous carbon. We have found that ra.sil.
inlarger than ra.car. when aa.sil. < 0.5 m m (aa.car. = 0.01 m m ; see
Tables 9–12). We therefore conclude that the typical grain sizeof amorphous carbon is aa.car. = 0.01 m m and that of
astronomical silicate is 0.3 m m < aa.sil. < 0.5 m m. A relatively
large grain size found for silicate dust in our analyses is
consistent with the general understanding that the grain size indust shells of novae is substantially larger than that in theinterstellar medium (Gehrz et al. 1998; Witt et al. 2001).
The dust masses of amorphous carbon and astronomicalsilicate calculated for aa.car.=0.01μm and aa.sil.=0.3μm aregiven in Table 13, where ρa.car. (1.87 g cm−3) and ρa.sil.
(3.3 g cm−3) are adopted as the densities of an amorphous
20

The Astrophysical Journal, 817:145 (23pp), 2016 February 1

Sakon et al.

Sco, such conditions may have been produced by multiple
ejection events of nova winds that have been observed, at least,on Days 10.5 and ∼110 (Chesneau et al. 2008) and that may becomposed of different materials dredged up from the underlying CO white dwarf core.
Another possibility is that dust is formed not just in the nova
ejecta but also in the pre-existing circumstellar medium as aresult of its interaction with shocks generated by the ejecta. If
the chemical properties in the nova ejecta and the pre-existing
circumstellar medium are quite different, the amorphous carbon
dust is formed in the carbon-rich nova ejecta and the silicate
dust in the oxygen-rich circumstellar medium. The presence ofmany cool clumpy gas clouds produced via interactions
between the pre-existing cool circumstellar gas and highvelocity gas ejected in the nova explosion has been proposedby Sadakane et al. (2010). The formation of silicate dust could
have occurred in those cool clumpy gas clouds if the cool
circumstellar material is oxygen-rich.
Optical and ultraviolet studies, however, indicate that thecomposition of matter ejected by classical novae is typicallyoxygen-rich (Gehrz et al. 1998; Starrﬁeld et al. 1998). If this isalso the case in V1280 Sco, the formation mechanism ofamorphous carbon dust has to be carefully addressed. Clayton
et al. (1999) suggest that carbon dust can form even in oxygenrich (C/O < 1) wind if free carbon exists abundantly and if
carbon nucleation occurs despite rapid oxidation. If this is thecase in the nova ejecta of V1280 Sco, the amorphous carbon
dust may have been produced in a region closer to the whitedwarf, where CO formation is disturbed by harsh radiation
and/or by Compton electrons, than the CO-forming region.
Then, the optically thick amorphous carbon dust may haveblocked the ultraviolet radiation from the WD, thus providing
an environment for the ejecta gas, which no longer contains
abundant free carbon atoms behind the optically thickamorphous carbon dust, to cool quickly. This situation may
have led silicate dust to condense in the gas at a relatively highdensity, which resulted in it attaining a relatively large grain
size of 0.3 μm < aa.sil.<0.5 μm.

The near-infrared spectrum of V1280 Sco on Day 940 also
shows the absorption features due to CO gas at 4.6μm and,possibly, to CO2 gas at 4.26 μm. The continuum emission at
this wavelength is predominantly produced by amorphous
carbon as indicated by our SED analyses described in
Section 5.3.2. The presence of CO gas in absorption may
support the dust formation scenario discussed in Section 5.4, in
which the amorphous carbon dust has been formed in a region
closer to the white dwarf than the CO-forming region.
Figure 19 shows the continuum-subtracted N-band lowresolution spectra of V1280 Sco on Days 1272, 1616, and 1947
obtained with Gemini-S/T-ReCS. At Day 1272, the spectrum
is characterized by the presence of emission features at
8.1μm and 11.3μm. Silicon monoxide (SiO) is one of thecandidates for the carrier of the 8.1μm feature. The
8.1μm feature recognized on Day 1272 has become signiﬁcantly weaker by Day 1616. This can be interpreted as adecrease in the temperature of ejecta gas where SiO molecules
are no longer excited. SiO molecules are expected to beabundant in the chemical pathway leading to the formation ofsilicate dust (Kozasa et al. 1989). There exists observational
evidence for ongoing silicate dust formation in the ejecta ofType II-P supernova 2004et for the period of days 300–464
accompanied by a decline in mass of SiO (Kotak et al. 2009).
In the case of V1280 Sco, however, while the decline of the
8.1μm feature has been observed between Days 1272 and
1616, it was not apparent whether any signiﬁcant mass
evolution of silicate dust had taken place on Days 1272,
1616, and 1947 (see Section 5.3.2). This fact will not support
the interpretation that SiO is the carrier of the 8.1μm feature.
Alternatively, taking into account the similarity between theclass C UIR bands (Peeters et al. 2002) and the emission
features at 8.1 and 11.3μm in the spectrum of V1280 Sco on
Day 1272, HACs (Borghesi et al. 1987; Duley & Williams 1988; Sloan et al. 2007) and/or mixed aromatic–aliphaticorganic nanoparticles (Kwok & Zhang 2011) can be responsible for the emission features at 8.1 and 11.3μm. These
interpretations are consistent with the results obtained from the
AKARI/IRC spectroscopy on Day 940.
Finally, although astronomical silicate is required toreproduce the continuum emission, none of the crystalline
silicate features (e.g., 9.2 μm for silica, 9.8 μm for forsterite andenstatite, 10.7 μm for enstatite, and 11.4 μm for forsterite anddiopside; Molster et al. 2002) was detected in the continuumsubtracted N-band spectra of V1280 Sco at any epoch. Thissuggests that nucleation of silicate around V1280 Sco may
have happened at relatively low temperatures (e.g., <800 K;
Molster & Kemper 2005) and that no further crystallization
processes such as annealing by UV photons (Nuth &Hecht 1990) or partial crystallization by electron irradiation(Carrez et al. 2001) in the harsh environment around the whitedwarf have occurred for silicate dust around V1280 Sco at least
on a timescale of 2000 days from the outburst.

6. SPECTRAL FEATURES OVER THE CONTINUUM
EMISSION
Notable in the near-infrared spectrum of V1280 Sco on Day
940 obtained with the AKARI infrared camera (IRC) is thepresence of the UIR band at 3.3μm (see Figure 18). The shapeof this feature is characterized by unusually strong broad redwings at 3.4–3.6 μm. The prominent 3.4–3.6μm wing in thenear-infrared spectrum of V1280 Sco on Day 940 is consistent
with the spectral characteristics of the UIR bands observed inother novae such as V705 Cas (Evans et al. 1997) and V842
Cen (Hyland & McGregor 1989). The 3.3μm feature is usually
attributed to an aromatic C–H stretching mode, and the
3.4–3.6μm wing to aliphatic C–H stretching modes (Allamandola et al. 1989). We therefore suggest that a higher
aliphatic-to-aromatic ratio exists in the carrier of the UIR bands
in V1280 Sco on Day 940 than in other Galactic objects. As
discussed by Evans et al. (1997), HACs (Borghesi et al. 1987;
Duley & Williams 1988) rather than polycyclic aromatichydrocarbons (PAHs) could be a plausible candidate for the
carrier of UIR bands observed in V1280 Sco. Alternatively,
overabundance of nitrogen in the nova wind may also be
responsible for the relatively stronger 3.4–3.6μm to 3.3μm
ratio (Roche et al. 1996; Andreae & Drechsel 1990, p. 204).

7. SUMMARY
We present the results of the late-time multi-epoch infraredobservations of the dust-forming nova V1280 Sco. The
temperature and mass of dust formed in the nova wind are
examined based on the spectral decomposition of the infrared
SED obtained at each epoch. The SED on Day 150 suggests thepresence of optically thick amorphous carbon dust of mass
Ma.car. ~ 2.7 ´ 10-6 Me. The SED evolution in a period from21

The Astrophysical Journal, 817:145 (23pp), 2016 February 1

Sakon et al.

members of the Subaru telescope and Gemini-South, particularly
J. Rodomzki and P. Gomez for their support during theobservations with Gemini-S/T-ReCS. The Gemini Observatoryis operated by the Association of Universities for Research in
Astronomy, Inc., under a cooperative agreement with the NSF
on behalf of the Gemini partnership: the National Science
Foundation (United States), the National Research Council(Canada), CONICYT (Chile), the Australian Research Council(Australia), Ministério da Ciência, Tecnologia e Inovação
(Brazil), and Ministerio de Ciencia, Tecnologa e Innovación
Productiva (Argentina). This research is based in part onobservations with AKARI, a JAXA project with the participationof ESA. The authors thank all the members of the AKARI project
for their continuous support. Some of the data in this paper wereobtained by the Optical & Near-Infrared Astronomy InterUniversity Cooperation Program, supported by the MEXT of
Japan. The authors would like to thank T. Yoshikawa, K.
Tsumura, and M. Shirahata for their help in performing theobservation at IRSF. The authors are grateful to M. Kato and I.
Hachisu for providing the parameters of the white dwarf of
V1280 Sco based on the theoretical calculations. The authors are
grateful to M. Matsuura for providing useful comments.

Day 1272 to Day 1947 can be explained by emission from
amorphous carbon dust of mass Ma.car.∼8×10−8 Me with arepresentative size of aa.car=0.01μm, combined with emission from astronomical silicate of mass Ma.sil.∼4×10−7 Mewith a representative size of 0.3μm < aa.sil. < 0.5 μm. Theyboth traveled farther away from the white dwarf without
apparent mass evolution from Day 1272 to Day 1947. The
decrease in the mass of amorphous carbon in the period fromDay 150 to Day 1272 and the extraordinarily small grain size ofamorphous carbon (aa.car=0.01 μm) on Days 1272, 1616, and1947 compared with other CO novae may suggest thatsputtering and/or evaporation may have occurred for amorphous carbons during the period from Day 150 to Day 1272.
The dust formation scenario that emerges from ourobservations is that amorphous carbon dust is initially
produced in a region closer to the white dwarf, which thenblocks the ultraviolet radiation from the WD, to provide anenvironment for the O-rich gas to cool quickly and to condense
into silicate dust from the gas at a relatively high density. Coolclumpy clouds produced via interactions between the preexisting circumstellar gas and high-velocity nova ejecta may
have provided favorable conditions for the silicate dust to attaina large grain size of 0.3 μm < aa.sil.<0.5 μm.
The near- to mid-infrared spectra obtained on Days 940 and
1272 have exhibited signs of the UIR emission bands, which
are more plausibly expected to be carried by HACs and/or
mixed aromatic–aliphatic organic nanoparticles rather than
PAHs. The absence of those features in the mid-infrared spectraof V1280 Sco on Days 1616 and 1947 suggests that the carriersof the UIR emission bands observed on Day 1272 cannot
survive the harsh circumstellar environment produced by thewhite dwarf almost reaching the nebular phase.
Further observations of dust-forming novae with sufﬁcient
time resolution and monitoring duration will be indispensablein understanding how the dusty environment characterized byboth carbon-rich and oxygen-rich chemistry can be achieved
around novae. Mid-infrared studies with extremely high spatial
resolution (∼0 1) with an integral ﬁeld unit, which will beachieved by the mid-infrared instrument with an adaptive
optics system on board 30 m class telescopes (e.g., TMT/MICHI+MIRAO, Chun et al. 2006; Packham et al. 2012), willplay a crucial role in demonstrating the geometries of newly
formed dust in the nova wind and the pre-existing circumstellar
dust. In terms of the time resolution, in particular during thephase of silicate dust formation, TAO/MIMIZUKU (Kamizuka
et al. 2014) will play an important role in determining thephysical parameters that control the nucleation of silicate dust.
The wider wavelength coverage of SOFIA/FORCAST (Adams
et al. 2012) will be important for investigating the dust
properties (crystallization and/or amorphization) of silicatesfrom the mineralogical point of view. Finally, the spectroscopic
capability of SPICA (Nakagawa et al. 2014) in both the midand far-infrared with the highest sensitivity ever (Kataza
et al. 2015) will demonstrate the longer-term evolution of dust
from the epoch of its formation in the circumstellar environment until it becomes interstellar dust.

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