Issue |
A&A
Volume 518, July-August 2010
Herschel: the first science highlights
|
|
---|---|---|
Article Number | L142 | |
Number of page(s) | 5 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/201014513 | |
Published online | 16 July 2010 |
Herschel: the first science highlights
LETTER TO THE EDITOR
Cold dust in three massive evolved stars
in the LMC
,![[*]](/icons/foot_motif.png)
M. L. Boyer1
- B. Sargent1 -
J. Th. van Loon2 -
S. Srinivasan3 -
G. C. Clayton4 -
F. Kemper5 -
L. J. Smith1 -
M. Matsuura6,7 -
Paul M. Woods5 -
M. Marengo8 - M. Meixner1, - C. Engelbracht9
- K. D. Gordon1 -
S. Hony10 - R. Indebetouw11
- K. Misselt 9 - K. Okumura10
- P. Panuzzo10 - D. Riebel12
- J. Roman-Duval1 - M. Sauvage10
- G. C. Sloan13
1 - Space Telescope Science Institute, 3700 San Martin Drive,
Baltimore, MD 21218, USA
2 - School of Physical & Geographical Sciences, Lennard-Jones
Laboratories, Keele University, Staffordshire ST5 5BG, UK
3 - Institut d'Astrophysique de Paris, CNRS UPR 341, 98bis, Boulevard
Arago, 75014 Paris, France
4 - Louisiana State University, Department of Physics &
Astronomy, 233-A Nicholson Hall, Tower Dr., Baton Rouge, LA 70803, USA
5 - Jodrell Bank Centre for Astrophysics, Alan Turing Building, School
of Physics and Astronomy, University of Manchester, Oxford Road,
Manchester M13 9PL, UK
6 - Department of Physics and Astronomy, University College London,
Gower Street, London WC1E 6BT, UK
7 - Mullard Space Science Laboratory, University College London,
Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
8 - Department of Physics and Astronomy, Iowa State University, Ames,
IA, 50011, USA
9 - Steward Observatory, University of Arizona, 933 North Cherry Ave.,
Tucson, AZ 85721, USA
10 - CEA, Laboratoire AIM, Irfu/SAp, Orme des Merisiers, 91191
Gif-sur-Yvette, France
11 - National Radio Astronomy Observatory, Department of Astronomy,
University of Virginia, PO Box 3818, Charlottesville, VA 22903 USA
12 - Johns Hopkins University, Department of Physics and Astronomy,
Homewood Campus, Baltimore, MD 21218, USA
13 - Department of Astronomy, Cornell University, Ithaca, NY 14853, USA
Received 26 March 2010 / Accepted 20 Mai 2010
Abstract
Massive evolved stars can produce large amounts of dust, and
far-infrared (IR) data are essential for determining the contribution
of cold dust to the total dust mass. Using Herschel,
we search for cold dust in three very dusty massive evolved stars in
the Large Magellanic Cloud: R71 is a luminous blue variable,
HD 36402 is a Wolf-Rayet triple system, and IRAS05280-6910 is
a red supergiant. We model the spectral energy distributions using
radiative transfer codes and find that these three stars have mass-loss
rates up to ,
suggesting that high-mass stars are important contributors to the
life-cycle of dust. We found far-IR excesses in two objects, but these
excesses appear to be associated with ISM and star-forming regions.
Cold dust (T<100 K) may thus not be
an important contributor to the dust masses of evolved stars.
Key words: Magellanic Clouds - circumstellar matter - stars: mass-loss - stars: massive - submillimeter: stars
1 Introduction
Intermediate-mass asymptotic giant branch (AGB) stars are potentially
the dominant dust source in the Galaxy (Gehrz
1989) and in
low-metallicity environments like the Large Magellanic Cloud
(LMC; Srinivasan
et al. 2009; Matsuura et al. 2009)
and other dwarf galaxies
(Boyer et al. 2009).
However, dust production in high-mass stars
()
remains uncertain. It has been suggested that
supernovae (SNe) might be the dominant dust factory at high-z,
since
intermediate-mass stars have not yet had time to evolve into AGB stars
(Dwek
et al. 2009; Morgan & Edmunds 2003).
However, the amount of dust formed in SNe in
the local universe is much less than required to explain the dust seen
at high-z (e.g., Sugerman et al. 2006; Andrews
et al. 2010). It is also unclear
if the dust forms before or after the SN explosion.
Alternatively,
Sloan et al. (2009)
and Valiante et al. (2009)
show that AGB stars can
contribute dust at high redshifts. These studies point to a need to
measure the total dust mass from all types of stars to obtain a global
picture of dust evolution in galaxies.
Part of the LMC was observed with Herschel
(Pilbratt et al. 2010)
as part of the science demonstration program
(SDP) and the Legacy program entitled HERschel Inventory of The
Agents of Galaxy Evolution (HERITAGE; Meixner
et al. 2010). In this letter, we describe a first
look at Herschel Photodetector Array Camera and
Spectrometer (PACS; Poglitsch
et al. 2010) and Spectral and Photometric Imaging
REceiver (SPIRE; Griffin
et al. 2010) detections of 3
examples of dust-producing massive evolved stars in the
LMC: the
Wolf-Rayet (WR) system HD 36402, the luminous blue variable
(LBV)
HDE 269006 (or R71), and the red supergiant (RSG) OH/IR star
IRAS05280-6910. While such stars have been studied extensively in the
mid-IR (e.g., Crowther
2007; Bonanos
et al. 2009; van Loon et al. 2010; Morris
et al. 1999; Clark et al. 2003), this
study is among the first to probe them at
.
Table 1: Target information and Herschel flux densities.
![]() |
Figure 1: Herschel images of R71 (left panels), IRAS05280-6910 (middle panels), and HD 36402 (right panels, also see Fig. 3). Contours on a linear scale are included where it is difficult to see the detection. |
Open with DEXTER |
2 Mid- to far-infrared photometry
We obtained PACS (100 and 160 m) and SPIRE (250, 350 and
500
m)
fluxes using apertures roughly the size of the
source full-width at half-maximum and sky apertures
avoiding regions of high background (Table 1). We also
performed aperture
photometry on the Multiband Imaging Photometer for Spitzer
(MIPS; Rieke et al. 2004)
images from the Surveying The Agents of
Galaxy Evolution Spitzer Legacy program
(SAGE; Meixner et al. 2006).
In Sect. 3
we examine the optical to far-IR spectral energy
distributions (SEDs). Optical UBVI and near-IR JHK
photometry are
from the Magellanic Clouds Photometric Survey (Zaritsky
et al. 1997) and
the Two Micron All-Sky Survey (Skrutskie
et al. 2006), via the SAGE
catalog. J, K, and L'
photometry of IRAS05280-6910 are from
van Loon et al. (2005).
I-band photometry for HD 36402 is from the
Deep
Near-Infrared Southern Sky Survey (Epchtein
et al. 1997). InfraRed Array
Camera (IRAC; Fazio et al.
2004) 3.6-8.0 m photometry is
from the SAGE catalog. Spectra from the InfraRed Spectrograph
(IRS; 5.2-38
m;
Houck et al. 2004)
and MIPS spectra
(MIPS-SED; 52-97
m)
are also included (e.g.,
SAGE-Spec; van
Loon et al. 2010; Kemper et al. 2010). We
correct for extinction using
the extinction map from Schlegel
et al. (1998), assuming the stars lie
midway the LMC contribution:
mag
for HD 36402
and IRS05280-6910 and
mag
for R71.
3 The IR nature of detected sources
The vast majority of point-sources detected in the LMC SDP data appear to be young stellar objects (YSOs) or background galaxies (Sewio et al. 2010); AGB stars were not detected. Here, we describe the Herschel observations of three very dusty massive stars (Fig. 1).
3.1 R71
R71 (Fig. 1)
is an LBV with
(Lennon et al.
1993), and is currently experiencing an
unprecedented eruption that began in 2005. One month prior to the SDP
observations (Oct. 2009), it showed a
2 mag increase in visual
brightness and the V-band light curve was just
beginning to plateau
(Szczygiel et al. 2010).
Van Loon et al. (2010) find a lack of cold dust
in R71, as indicated by Spitzer data (
).
The Spitzer data were acquired just prior to the
current outburst, and the Herschel data presented
here were
obtained at near-maximum. However, the PACS and MIPS points appear
consistent with each other, such that a model fit to the optical to
MIPS data also agrees with the PACS points. This indicates that the
increased emission from the photosphere has not yet significantly
affected the 100-160
m
flux. Note that the MIPS
160
m
point is an upper limit.
IRS and ISO spectra of R71 show strong
PAH, crystalline forsterite and enstatite features
(Morris
et al. 2008; Buchanan et al. 2009; Voors
et al. 1999; Waters 2010). A 10.5 m amorphous
silicate feature indicates dominantly
oxygen-rich (O-rich) chemistry. Morris
et al. (2008) and Voors
et al. (1999)
speculate that the dust was formed during a prior RSG phase.
Figure 2
shows R71's SED. Ultraviolet spectra
(Blair et al. 2009)
are consistent with the stellar component included in
our model. We fit the SED with a 2-D UST
model
(online Table 2;
Ueta & Meixner 2003),
adopting spherical symmetry
and grain properties of the dominant species: amorphous
silicate. Three dust components are visible, including a previously
unknown excess visible in the SPIRE data at 250, 350 and 500 m
(8, 3.8, and 3
detections, respectively). The two warmer
components are modeled by the 2-D UST code
using two concentric
dust shells. We assume the gas-to-dust ratio for the LMC is
(cf. Meixner et al. 2010),
yielding
for the inner dust shell and
for the outer shell, assuming
10 km s-1, consistent with the
wind
speed of a shell ejected during an RSG phase. Stahl et al. (1986)
find
160 km s-1
from the H
line profile,
indicating the MLR for the inner shell could be an order of magnitude
higher. A DUSTY model (Nenkova et al. 1999)
fits the SED equally
well and estimates a MLR that is the 2-D UST
outer
shell value.
We fit the far-IR excess with a modified blackbody:
,
where
is the Planck
function at temperature
,
is the optical
depth, and
.
Here, we use
.
The resulting dust temperature is
K, which is
extremely low compared to the expected temperature of outer shells in
evolved stars (
30 K;
Speck et al. 2000),
and is instead
consistent with temperatures of dense ISM dust clouds (see
Sect. 4).
![]() |
Figure 2:
SEDs of R71 ( top), IRAS05280-6910 ( middle)
and HD 36402 ( bottom), fit to 2-D UST
models (orange lines). Far-IR excesses are fit to modified blackbodies
(green lines). The IRS and MIPS-SED spectra are shown in black
and the stellar components are the dotted lines. In the upper two
panels, the 24 |
Open with DEXTER |
3.2 IRAS05280-6910
IRAS05280-6910 is an RSG OH/IR star. It is detected in all Spitzer
and Herschel bands up to 350 m
(>3.5
;
Figs. 1
and 2).
The star is heavily extinguished in the optical and near-IR, and the 10
and 18
m
silicate features are seen in absorption in the IRS spectrum
(Kemper et al. 2010).
Van Loon et al. (2010) find no indication of dust
colder than
100 K
in the MIPS data.
We fit the SED of IRAS05280-6910 using 2-D UST
(flattened geometry) and DUSTY (spherical
symmetry). Both models fit the SED reasonably well (only the 2-D UST
model is shown in Fig. 2),
but predict stronger absorption in the 10 m silicate
feature than is seen in the IRS spectrum, even when including grains as
large as 1
m
(increasing a0 suppresses
the silicate feature). This discrepancy may be due to the inclusion in
the slit of a nearby RSG (WOH G347), which is too faint to
contribute to the SED at other wavelengths, but shows emission near
10
m
with enough flux to veil the silicate absorption by the necessary
amount (van Loon et al. 2005).
Based on both model fits and assuming an outflow velocity of
20 km s-1
(as measured from maser emission; Marshall et al.
2004), the mass-loss rate (MLR) was
when the dust was produced (online Table 2). There is no
evidence of excess emission at
m in the SDP
data, implying there is no significant contribution from cold (
100 K)
and/or large (
m
- mm size) grains.
![]() |
Figure 3:
Composite HST/WFPC2 image of HD 36402 in H |
Open with DEXTER |
3.3 HD 36402
HD 36402 (Fig. 3)
is a WR star that is part of a triple
system (Moffat et al. 1990).
It is the reddest LMC WR star studied by
Bonanos et al. (2009)
in IRAC, potentially due to dust formed in colliding
stellar winds (Crowther 2007).
HD 36402 is detected by PACS
(Fig. 3b), which might indicate the presence of some cool
dust. However, the system is almost totally unresolved from the
stronger
far-IR emission immediately to the west (Figs. 1 and 3b), which appears
to originate from a nearby molecular
cloud, visible in H
(Fig. 3;
Chu
et al. 2005; Dopita et al. 1994). We
refer to this emission as HD 36402 IR1.
The IRS spectrum of HD 36402 from SAGE-Spec is
featureless and
reminiscent of R Coronae Borealis stars, which are C-rich (no silicate
dust) and hydrogen-deficient (no PAHs). The continuum emission is
thought to be due to amorphous carbon dust
(e.g., Kraemer et al. 2005).
The SED (Fig. 2)
shows a stellar component and a component from a detached, dusty shell
(2-24
m). The
far-IR SED is dominated by IR1,
so an accurate fit to the WR system in the far-IR is not
possible with this dataset. In the lowest resolution images (MIPS 160,
SPIRE 350 and 500
m),
IR1 is also unresolved from N51-YSO1
(upper limits in Fig. 2),
further
complicating the SED. A 2-D UST model
(Fig. 2,
orange line) gives
.
What is the nature of IR1? Based on its IRAC colors, Chu et al. (2005)
identify it as a YSO (N51-YSO2). We have attempted to fit a
two-component modified blackbody (
)
to the IR1 SED to
check if it instead originates from dust heated directly by the
radiation emanating from HD 36402. These fits yield a dust
temperature of 64 K, with 15 K dust in its wake,
consistent with this
scenario. However, a temperature of 64 K places the molecular
cloud
pc from the WR
system, assuming typical
luminosities of a WC4 star and two O star companions. The
high-resolution H
image (Fig. 3)
puts the
molecular cloud at least 1.2 pc away from the WR system,
assuming a
distance of 50 kpc to the LMC (Schaefer
2008). It thus seems
likely that the far-IR emission of IR1 is indeed related to a YSO
embedded in the molecular cloud rather than to direct heating from
HD 36402. In the HST image (Fig. 3), the cloud
appears brightest in the region adjacent to HD 36402, which
may
indicate some interaction between the two. If this
is indeed the
case, it is plausible that the formation of YSO2 was triggered by
HD 36402.
4 Implications
The new far-IR Herschel data have allowed us to take the first steps in assessing the contribution of cold dust to the total dust mass in 3 massive stars. For the RSG IRAS05280-6910, the models give a total dust mass of

The LBV R71 shows three dust components. A component emitting
at
resembles
RSG dust. A second component
dominates at
.
Its temperature (
K)
suggests it formed <50 yrs ago, assuming
and T0
= 1000 K, perhaps during the 1970s outburst
(Wolf & Zickgraf 1986).
The third, more tentative dust component is visible
at
m. If this
feature corresponds to
cold circumstellar dust, the implied dust mass is
,
following Evans et al. (2003)
and assuming the
absorption coefficient,
,
is
(Ossenkopf & Henning 1994).
This is far too much dust for a
star of this mass; together with its very cold temperature
(9 K), this
high mass suggests that the far-IR emission is pre-existing ISM dust
swept up by stellar winds and/or is ISM dust along the
line-of-sight. Indeed, the contours in Fig. 1
appear to show diffuse emission at the position of R71 at the longest
wavelengths. If the far-IR emission instead originates from the
circumstellar envelope, then very large grains similar to the cm-sized
grains in the Egg Nebula (Jura
et al. 2000) might explain the far-IR
emission without requiring the implied large dust mass and cold
temperature. Follow-up spectroscopy or deeper SPIRE imaging may help
uncover the nature of the far-IR emission.
The WR system, HD 36402, seems to be forming dust in
colliding winds,
which is too warm to emit much at far-IR wavelengths. The apparent
far-IR emission from HD 36402 (Fig. 3b) is
unfortunately
totally overwhelmed by far-IR emission originating from a nearby
molecular cloud. Assuming
and
(Ossenkopf & Henning 1994),
we find
and
of dust in the molecular cloud.
Stellar evolution models, while still uncertain, show that
massive
stars like these eventually explode as SNe. Due to its proximity to a
molecular cloud, the HD 36402 remnant may resemble the SN
remnant
N49, which has swept up 0.2
of dust from the ISM
(van Loon
et al. 2010; Otsuka et al. 2010). The
absence of strong
evidence for very large grains in R71 and IRAS05280-6910 does not raise
the
prospects of RSG dust surviving a SN blast. Regardless of their fates,
the dust masses in these 3 stars are quite large, compared
with the
dust mass found in a typical AGB star, showing that high-mass stars
are important contributors to the life-cycle of dust even in
low-metallicity environments like the LMC. However, we emphasize
that we do not find strong evidence for cold dust and/or large grains
in any of the three objects discussed here, except where it is certain
(HD 36402) or likely (R71) to be of interstellar origin and
not
synthesized by the object itself. These observations indicate that
far-IR data of a much larger sample of luminous evolved stars in both
Magellanic Clouds will be obtained in the full HERITAGE dataset, from
which we expect clearer patterns to emerge.
We thank the referee for his or her helpful comments. This publication includes observations made with the NASA/ESA HST, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA). We acknowledge financial support from the NASA Herschel Science Center, JPL contract # 1381522. We thank the contributions and support from the European Space Agency (ESA), the PACS and SPIREteams, the Herschel Science Center and the NASA Herschel Science Center (esp. A. Barbar and K. Xu) and the PACS/SPIRE instrument control center at CEA-Saclay, which made this work possible.
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Online Material
Table 2: SED fit parameters and results.
Footnotes
- ... LMC
- Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
- ...
- Table 2 is only available in electronic form at http://www.aanda.org
- ...
- Visiting Scientist at Smithsonian Astrophysical Observatory, Harvard-CfA, 60 Garden St., Cambridge, MA, 02138.
All Tables
Table 1: Target information and Herschel flux densities.
Table 2: SED fit parameters and results.
All Figures
![]() |
Figure 1: Herschel images of R71 (left panels), IRAS05280-6910 (middle panels), and HD 36402 (right panels, also see Fig. 3). Contours on a linear scale are included where it is difficult to see the detection. |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
SEDs of R71 ( top), IRAS05280-6910 ( middle)
and HD 36402 ( bottom), fit to 2-D UST
models (orange lines). Far-IR excesses are fit to modified blackbodies
(green lines). The IRS and MIPS-SED spectra are shown in black
and the stellar components are the dotted lines. In the upper two
panels, the 24 |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Composite HST/WFPC2 image of HD 36402 in H |
Open with DEXTER | |
In the text |
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