The Astrophysical Journal, 718:1456–1459, 2010 August 1

doi:10.1088/0004-637X/718/2/1456

C 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

OBSERVATIONS OF THE OPTICAL TRANSIENT IN NGC 300 WITH AKARI/IRC: POSSIBILITIES OF
ASYMMETRIC DUST FORMATION
R. Ohsawa1 , I. Sakon1 , T. Onaka1 , M. Tanaka2 , T. Moriya2,1,3 , T. Nozawa2 , K. Maeda2 , K. Nomoto2 , N. Tominaga2,4 ,F. Usui5 , H. Matsuhara5 , T. Nakagawa5 , and H. Murakami5
1 Department of Astronomy, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan; ohsawa@astron.s.u-tokyo.ac.jp
2 Institute for the Physics and Mathematics of the Universe, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa Chiba 277-8583, Japan
3 Research Center for the Early Universe, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan
4 Department of Physics, Faculty of Science and Engineering, University of Konan, 8-9-1 Okamoto, Higashinada-ku, Kobe, Hyogo 658-8501, Japan
5 Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan

Received 2010 April 14; accepted 2010 June 11; published 2010 July 15

ABSTRACT
We present the results of near-infrared (NIR) multi-epoch observations of the optical transient in the nearby
galaxy NGC 300 (NGC 300-OT) at 398 and 582 days after the discovery with the Infrared Camera (IRC)on board AKARI. NIR spectra (2–5 μm) of NGC 300-OT were obtained for the first time. They show no
prominent emission nor absorption features, but are dominated by continuum thermal emission from the dust
around NGC 300-OT. NIR images were taken in the 2.4, 3.2, and 4.1 μm bands. The spectral energy distributions(SEDs) of NGC 300-OT indicate the dust temperature of 810 ± 14 K at 398 days and 670 ± 12 K at 582 days.
We attribute the observed NIR emission to the thermal emission from dust grains formed in the ejecta of
NGC 300-OT. The multi-epoch observations enable us to estimate the dust optical depth as 12 at 398 daysand 6 at 582 days at 2.4 μm by assuming an isothermal dust cloud. The observed NIR emission must be opticallythick, unless the amount of dust grains increases with time. Little extinction at visible wavelengths reported inearlier observations suggests that the dust cloud around NGC 300-OT should be distributed inhomogeneously so
as to not screen the radiation from the ejecta gas and the central star. The present results suggest the dust grains
are not formed in a spherically symmetric geometry, but rather in a torus, a bipolar outflow, or clumpy cloudlets.
Key words: circumstellar matter – dust, extinction – stars: evolution – stars: variables: general – stars: winds,
outflows
Online-only material: color figuresphase with rapid mass-loss rates prior to some kind of explosion
event in the evolution of massive stars (Thompson et al. 2009;
Pumo et al. 2009).
NGC 300-OT was discovered on 2008 May 14 in the nearby
spiral galaxy NGC 300 (Monard 2008), the distance of whichis 1.9 Mpc. NGC 300-OT has shown an intermediate peak absolute magnitude of Mbol ∼ −11.8 mag with an optical spectrum well reproduced by an F-type supergiant photosphere withemission lines of hydrogen, Ca ii, and [Ca ii], similar to thetypical spectra of low-luminosity SN impostors (Bond et al.
2009; Berger et al. 2009). The progenitor of NGC 300-OT was
not detected in Hubble Space Telescope (HST) archive images(Bond et al. 2009), but was identified in Spitzer archival images (Prieto 2008), which suggest the dust-enshrouded natureof the progenitor. Several interpretations have been proposed forthe nature of NGC 300-OT: a heavily dust-enshrouded OH/IR
star of 10–15 M (Bond et al. 2009), a dust-enshrouded
star with a luminosity of about 6 × 104 L indicative of a
10–20 M (Berger et al. 2009), or a massive (M ∼ 6–10 M )carbon-rich asymptotic giant branch (AGB) star or a post-AGB
star (Prieto et al. 2009). The validity of those mass ranges isconfirmed by Gogarten et al. (2009) based on the method thatderives the star formation history of a transient’s host stellar
population. Prieto et al. (2009) also pointed out the presenceof newly formed dust of 1500 K in addition to the pre-existing
circumstellar dust of ∼3 × 10−4 M of 400 K based on the nearto mid-infrared spectral energy distribution (SED) at 93 days
after the discovery. Further near-infrared (NIR) observations atlater epochs are crucial to investigate the properties of newly
formed dust grains at the outburst.

1. INTRODUCTION
Massive stars are expected to play an important role in theinterstellar dust budget in young dwarf galaxies as well as ingalaxies in the early universe because of their relatively shortevolution lifetime (e.g., Dwek et al. 2007; Morgan & Edmunds2003). Recently, several eruptive objects with intermediate maximum absolute luminosities between classical nova eruptions
and supernova (SN) explosions have been discovered. They are
termed as SN impostors (Van Dyk et al. 2000) and suggested
to be a super-outburst of a luminous blue variable (LBV)-likeevent rather than a complete disruption of the progenitor. SN
impostors provide us with useful opportunities to investigate
the effect of an eruptive outburst on the yield of circumstellar dust around the evolved massive stars before core collapse(Sakon et al. 2009; Mattila et al. 2008). Among SN impostors,
SN 2008S in NGC 6946 (Arbour & Boles 2008) and the opticaltransient in NGC 300 (hereafter NGC 300-OT; Monard 2008)
are peculiar ones. One of their outstanding characteristics isthat their progenitors are deeply embedded in circumstellar dust
shells. Khan et al. (2010) carried out a systematic mid-infrared
photometric search for candidate objects which are analogous to
the progenitors of SN 2008S and NGC 300-OT in four nearby
galaxies M33, NGC 300, M81, and NGC 6946 with Spitzer/
IRAC. They found dozens of extreme asymptotic giant branch(EAGB) objects similar to SN 2008S and NGC 300-OT progenitors, but none of them are brighter than the NGC 300-OT
progenitor and redder than the SN 2008S progenitor. They pointout that the rarity of such extremely red and luminous progenitors can be interpreted as the short duration of the dust-obscured1456

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OBSERVATIONS OF THE OPTICAL TRANSIENT IN NGC 300 WITH AKARI/IRC
7
6

1457

Spectrum at t=398 (NP-mode)
Spectrum at t=582 (NG-mode)
Photometry at t=398
Photometry at t=582

Flux (mJy)

5
4
3
2
1
0
2.5

3.0

3.5
Wavelength ( µm)

4.0

4.5

5.0

Figure 1. NIR spectrum of NGC 300-OT. The thicker line with the pluses shows the spectrum at 398 days taken with the prism (λ/Δλ∼20), the line with the crosses
shows the spectrum at 582 days taken with the grism (λ/Δλ∼100), the squares show the fluxes obtained from the NIR images at 398 days, and the diamonds show thefluxes derived from the NIR images at 582 days.
(A color version of this figure is available in the online journal.)

In this paper, we present our results of NIR observationsof NGC 300-OT on the 398th and the 582nd days with theInfrared Camera (IRC) on board AKARI (Onaka et al. 2007).
NIR spectroscopy in 2–5 μm and imaging in the three filter
bands of N2 (2.4 μm), N3 (3.2 μm), and N4 (4.1 μm) werecarried out with the IRC. We focus on the time evolutionof the emission from newly formed dust and investigate thedust formation process around NGC 300-OT during its eruptive
outburst.
2. OBSERVATIONS AND DATA REDUCTION
The first-epoch data sets were obtained on 2009 June 16,
corresponding to the epoch of 398 days after the discovery,
as part of the Director’s Time (DT). They consist of imaging
observations in the three filter bands centered at 2.4 μm (N2),3.2 μm (N3), and 4.1 μm (N4) (Onaka et al. 2007) of pointing
ID 5200806.1 and 5200806.2, and spectroscopic observations
with the NIR prism (NP; Δλ/λ ∼ 20) for 2–5 μm (Ohyama
et al. 2007) of pointing ID 5200805.1 and 5200807.1. The
second-epoch data sets were obtained on 2009 December 17
corresponding to the epoch of 582 days after the discovery as
part of the Mission Program (MP) “Interstellar Medium in Our
Galaxy and Nearby Galaxies” (ISMGN; Kaneda et al. 2009).
They include imaging observations in the N2, N 3, and N4 bands(pointing ID 1422250.1 and 1422250.2) and spectroscopic
observations with the NIR grism (NG; Δλ/λ ∼ 100) for
2.5–5 μm (Ohyama et al. 2007; pointing ID 1422249.1 and
1422249.2).
The imaging data were reduced with the AKARI/IRC Imaging Toolkit for Phase 3 data version 200810156 and the flux
was measured with phot command in the apphot package of
IRAF.7 The spectroscopic data were reduced with the AKARI/
IRC spectroscopy toolkit for phase 3 data version 20090211.
The NIR spectra of NGC 300-OT were taken in the slitless
spectroscopy mode. Therefore, they are blended with the spectra of diffuse emission from the host galaxy NGC 300. We
carefully subtracted the contribution of diffuse emission fromNGC 300 by interpolating the signals around NGC 300-OT. The
67

http://www.ir.isas.jaxa.jp/ASTRO-F/Observation/DataReduction/IRC/
http://www.iraf.net/

Table 1
Photometric Data of NGC 300-OT
λ(μm)
2.4
3.2
4.1

t = 398 days (mJy)t = 582 days (mJy)

0.75 ± 0.04
2.22 ± 0.07
3.50 ± 0.09

0.33 ± 0.02
1.45 ± 0.05
2.97 ± 0.08

value of a bad pixel was replaced by the median value of theadjoining pixels. The NG spectrum at 582 days was smoothedby a 3 pixel running mean to increase the signal-to-noise ratio
without degrading the spectral resolution.
3. RESULTS AND DISCUSSION
3.1. Spectral Data
Figure 1 shows the obtained NIR spectra of NGC 300-OT at
398 and 582 days. No significant contribution is recognized fromthe polycyclic aromatic hydrocarbon (PAH) features around3.3 μm, hydrogen recombination lines of Brackett α at 4.05 μm,
and Brackett β at 2.63 μm, nor forbidden lines from ionizedgas (e.g., [Mg iv] at 4.49 μm, [Ca iv] at 3.21 μm, and [Ca v]
at 4.15 μm) in the spectra. Therefore, the NIR spectra of
NGC 300-OT at both epochs are dominated by hot dust
continuum emission.
3.2. Imaging Data
The results of the photometry are listed in Table 1. Prieto et al.
(2010) have reported that the 3.6 μm flux at 587 days is 2.2 mJy,
which is in agreement with our results at 582 days. Comparedto the fluxes at 398 days, the fluxes at 582 days had declined by
56% in the N 2, 35% in the N 3, and 15% in the N 4 bands.
Since the noise level in the spectroscopic mode at 4.05 μm is
about 3 mJy (3σ ) and the estimated spectral resolution is 0.2 μm
for NP and 0.04 μm for NG, an upper limit of the intensityof Brackett α, which is expected to be the strongest among
the hydrogen recombination lines in 2–5 μm, is estimated as
5.1×10−17 W m−2 and 7.8×10−18 W m−2 for 398 and 582 days,
respectively. Thus, the contribution of Brackett α on the N 4 bandis supposed to be negligible.

1458

OHSAWA ET AL.

Vol. 718

16

log Emitting Radius in cm

t=398 days
t=582 days

15
-1

0

1
log Dust Opacity at 2.4 µ m

2

3Figure 2. Relation between the dust optical depth at 2.4 μm (τν ) and the emitting radius (R). The red solid line shows the relation at t = 398 days, while the blue
dotted line shows that at t = 582 days. The range of uncertainties is indicated by the thin lines. The dashed and dot-dashed lines are upper limits of the emitting radius,
respectively, at t = 398 days (R = 3 × 1015 cm) and at t = 582 days (R = 5 × 1015 cm), corresponding to a shell expansion velocity of 1000 km s−1 . Assumingthat the dust mass is constant and the dust cloud expands at a constant velocity, the τν –R line at t = 398 days is predicted to evolve to the gray shaded region at
t = 582 days (see the text).
(A color version of this figure is available in the online journal.)

4. DISCUSSION

Table 2
Results of SED Fit

4.1. Contributions from Two Dust Components
According to Prieto et al. (2009), the SED of NGC 300-OT
at 96 days is well explained by three blackbody components oftemperatures of ∼4000 K, ∼1500 K, and ∼480 K. The component of ∼4000 K is assumed to come from the photosphere. The
component of ∼480 K (hereafter the warm dust) is attributed tothe dust produced in the AGB/post-AGB phase. While the originof the component of ∼1500 K (hereafter the hot dust) remains
to be identified, it is most likely to be attributable to the newly
formed dust in the outburst ejecta (Prieto et al. 2009, 2010). If
these grains are heated by the radiation from the central star andare in radiative equilibrium, the temperature of the dust should
1follow the relation of Td ∝ L 4 , where Td is the dust temperatureand L is the luminosity of the central star. According to the light
curve by Bond et al. (2009), the luminosity of NGC 300-OT in
the V band had fallen by about 3 mag from 96 to ∼200 days. The
dust temperatures at ∼200 days should be about a half of that at
96 days, or the warm dust temperature at ∼200 days should be∼240 K. We estimate that the contribution to the NIR emission
from the warm dust decreases from that at 200 days and should
be at most 1% of the observed flux at 398 days, which is lower
than the uncertainties in the photometry. Thus, we can attributethe observed change of the SED entirely to the hot dust, whichcomes from dust grains that were formed in the ejecta.
4.2. Optical Depth of the Dust Cloud
To investigate the change in the properties of the dust,
we assume a simple model of an isothermal dust cloud andfit the−τSED derived from the NIR imaging data by fν ∝1 − e ν Bν (Td ), where τν is the dust optical depth, and Bν (Td )is the Planck function. We assume that τν is proportional to 1/λ.
We derive dust temperatures for both optically thin (τν 1)
and thick (τν 1) case. The dust temperatures and the totalluminosities are listed in Table 2. Based on this simple model,
the dust-emitting radius R can be given by

D 2 fν
R=
π (1 − e−τν ) Bν (Td )

 12
,

(1)

398 days

582 days

813 ± 144.0 ± 0.4668 ± 123.4 ± 0.3

678 ± 104.0 ± 0.5573 ± 123.3 ± 0.4

For τ  1
Td (K)
Ld (105 L )For τ  1
Td (K)
Ld (105 L )where D is the distance to NGC 300-OT from the observer andfν is the observed flux. In Figure 2, R is plotted against τν at
2.4 μm. According to Berger et al. (2009), an upper limit of
the expansion velocity of the ejecta is about 1000 km s−1 . If
the ejecta expands with this velocity, R must be smaller than
∼3 × 1015 cm (the dashed line in Figure 2) at 398 days and then
∼5 × 1015 cm (the dot-dashed line) at 582 days. Assuming thatthe dust mass did not change between 398 and 582 days, the dust
opacity should be proportional to R −2 . If the dust cloud expands
at a constant velocity, the ratio of the radius at 582 days to thatat 398 days must be equal to 582/398 ∼ 1.5. Therefore, theτν –R line at 398 days is predicted to evolve to the gray shaded
region in Figure 2. Only the region where the predicted line
overlaps with the observed line at 582 days gives consistentmodels both at 398 and 582 days. From Figure 2, a lower
limit of τν (2.4 μm) is estimated to be ∼12 at 398 days and∼6 at 582 days. Therefore, the dust cloud around NGC 300OT is optically thick at both epochs. It is consistent with thefeatureless spectra shown in Figure 1. Assuming the constantdensity, we can estimate a lower limit of the dust mass as
Md = (4π/3)R 2 τν κν −1 ∼ 10−5 M , where κν is the dust mass
absorption coefficient at 2.4 μm. The absorption coefficient κνis estimated as (3/4)2π λ−1 ρ −1 ∼ 104 cm2 g−1 , where ρ is themass density of the dust grains, ∼ 2 g cm−3 , assuming carbon
dust (Prieto et al. 2009). In case the density varies as r −2 , we needto assume the inner shell radius for the dust mass estimation.
If the inner shell radius is within a half or a quarter of R, theestimated dust mass will not change more than ±40 % of thatin the constant density case.

No. 2, 2010

OBSERVATIONS OF THE OPTICAL TRANSIENT IN NGC 300 WITH AKARI/IRC

If the dust cloud is optically thick, there must be a temperature
gradient. Thus, an isothermal model is a crude approximation,
particularly for the optically thick case. To examine how secure
the conclusion of the optically thick emission is, we consider
an optically thin cloud in some more detail. The total dust masscan be estimated for an optically thin case using Equation (4) in
Dwek et al. (1983):−6

Md  2 × 10

Td1000 K−5 

Ld
106 L


M ,

(2)

where Ld is the total dust luminosity in L . Using the temperature and the luminosity in Table 2 for τ  1, we obtain
Md ∼ 5.1 × 10−6 M at 398 days and Md ∼ 1.0 × 10−5 M at
582 days. Thus, if the emission is optically thin, the dust mass
must be increased by a factor of 2. Theoretical investigations
suggest that the nucleation and grain growth will cease in a relatively short time right after the onset of nucleation (Yamamoto& Hasegawa 1977; Draine & Salpeter 1977). Therefore, we
conclude that optically thin models cannot account for the observations.
Since the ratio of the opacity at the V band to that at 2.4 μm
is about 5, the optically thick dust cloud predicts strong extinction at visible wavelengths. Berger et al. (2009) reported
E(B−V ) = 0.05 ± 0.05 mag at 121 days, which suggests thatthe extinction by the dust cloud is very small or negligible.
Prieto et al. (2010) reported that the R-band magnitude has significantly faded to 23.9 ± 0.2 mag at 585 days, and they surmisethat NGC 300-OT is likely becoming self-enshrouded. An upper limit of the extinction at the R band at 585 days can be
estimated as 6 mag, if we simply assume the R-band magnitude at 585 days is the same as that at 120 days. Although it
may be true that part of the stellar radiation is attenuated, it is
too small to account for the extinction caused by the opticallythick dust cloud, more than ∼20 mag, in the preceding discussion. The apparent contradiction suggests that the dust aroundNGC 300-OT is not uniformly distributed, but rather is distributed in a spherically asymmetrical geometry, such as a torus,
bipolar, or clumpy form, so that the dust cloud does not completely screen the radiation from the ejecta gas and the central
star. The existence of a significantly asymmetrical dust cloud
was also proposed by Patat et al. (2010) and Berger et al. (2009).
The present results also support the existence of an asymmetricaldust cloud.
5. SUMMARY
We present the results of the NIR observations on NGC 300OT at 398 and 582 days after the discovery with the IRC on
board AKARI. NIR spectra (2–5 μm) are obtained for the firsttime. They show no prominent emission or absorption features
but are dominated by continuum radiation of the dust around1459

NGC 300-OT. NIR photometric data were taken in N 2, N 3,
and N 4 bands. The SED of NGC 300-OT indicates a dust
temperature of 810 ± 14 K at 398 days and 670 ± 12 K at
582 days. We estimate the dust optical depth as 12 on
day 398 and 6 on day 582, assuming an isothermal dust
cloud. Although the present model is very crude, optically thinmodels cannot account for the observations at days 398 and582 consistently. The large optical depths at NIR suggest alarge extinction at visible wavelengths, while little extinction isreported based on earlier observations (E(B − V ) = 0.05 ±0.05). Those circumstances suggest that the dust cloud aroundNGC 300-OT should be distributed asymmetrically so as to notscreen the radiation from the ejecta gas and the central star.
The present results suggest that the dust formation occurs in
NGC 300-OT in the form of a torus, a bipolar outflow, or clumpy
cloudlets.
This work is based on observations of AKARI, a JAXA
project with the participation of ESA. We thank all the membersof 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
Grants-in-Aid for Scientific Research from the JSPS and inpart by World Premier International Research Center Initiative(WPI Initiative), MEXT, Japan.
Facilities: Akari (IRC)
REFERENCES
Arbour, R., & Boles, T. 2008, CBET, 1234, 1
Berger, E., et al. 2009, ApJ, 699, 1850
Bond, H. E., et al. 2009, ApJ, 695, L154
Draine, B. T., & Salpeter, E. E. 1977, J. Chem. Phys., 67, 2230
Dwek, E., Galliano, F., & Jones, A. P. 2007, ApJ, 662, 927
Dwek, E., et al. 1983, ApJ, 274, 168
Gogarten, S. M., et al. 2009, ApJ, 703, 300
Kaneda, H., Koo, B. C., Onaka, T., & Takahashi, H. 2009, Adv. Space Res., 44,1038
Khan, R., et al. 2010, ApJ, 715, 1094
Mattila, S., et al. 2008, MNRAS, 389, 141
Monard, L. A. G. 2008, IAU Circ., 8946, 1
Morgan, H. L., & Edmunds, M. G. 2003, MNRAS, 343, 427
Ohyama, et al. 2007, PASJ, 59, S411
Onaka, T., et al. 2007, PASJ, 59, S401
Patat, F., et al. 2010, A&A, 510, 108
Prieto, J. L. 2008, ATel, 1550, 1
Prieto, J. L., et al. 2009, ApJ, 705, 1425
Prieto, J. L., et al. 2010, ATel, 2406, 1
Pumo, M. L., et al. 2009, ApJ, 705, 138
Sakon, I., et al. 2009, ApJ, 692, 546
Thompson, T. A., et al. 2009, ApJ, 705, 1364
Van Dyk, S. D., et al. 2000, PASP, 112, 1532
Yamamoto, T., & Hasegawa, H. 1977, Prog. Theor. Phys., 58, 816