The Astronomical Journal, 139:1993–2002, 2010 May C 2010.
doi:10.1088/0004-6256/139/5/1993
The American Astronomical Society. All rights reserved. Printed in the U.S.A.
THE DUSTY CIRCUMSTELLAR DISKS OF B[e] SUPERGIANTS IN THE MAGELLANIC CLOUDS Joel H. Kastner1 , Catherine Buchanan2 , Raghvendra Sahai3 , William J. Forrest4 , and Benjamin A. Sargent5 1
Center for Imaging Science, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester, NY 14623, USA;
[email protected] 2 The School of Physics, The University of Melbourne, Victoria, 3010, Australia;
[email protected] 3 NASA/JPL, 4800 Oak Grove Drive, Pasadena, CA 1109, USA 4 Department of Physics & Astronomy, University of Rochester, Bausch & Lomb Hall, P.O. Box 270171, Rochester, NY 14627-0171, USA 5 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA Received 2009 August 28; accepted 2010 March 6; published 2010 April 8
ABSTRACT To better ascertain the nature of the infrared excesses that are characteristic of B[e] supergiants, we obtained Spitzer IRS spectroscopy and IRAC/MIPS imaging for a sample of nine B[e] supergiant stars in the Magellanic Clouds. We find that all nine stars display mid- to far-IR spectral and spatial characteristics indicative of the presence of circumstellar dust disks. Several of the sample B[e] supergiants display crystalline silicate features in their IRS spectra, consistent with grain processing in long-lived (i.e., orbiting) disks. Although it is possible that these disks are primordial in origin, large shell structures (with size scales of tens of parsec) are associated with five of the nine B[e] supergiants, suggesting that mass loss has provided the circumstellar material now orbiting these stars. Hence—via analogy to the class of post-asymptotic giant branch stars with binary companions and dusty, circumbinary disks—we speculate that B[e] supergiant stars may be post-red supergiants in binary systems with orbiting, circumbinary disks that are derived from post-main-sequence mass loss. Key words: circumstellar matter – Magellanic Clouds – stars: early-type – stars: emission-line, Be – supergiants
despite the large distance to the MCs, the MC B[e] supergiant sample is relatively well characterized spectroscopically, thus providing excellent targets for further study. The IR excesses characteristic of MC B[e] supergiants are attributed to a combination of thermal bremsstrahlung (free–free) radiation in the near-IR and emission from warm dust grains in the mid- to far-IR (Allen & Glass 1976; Zickgraf et al. 1985, 1986, 1989; Stahl et al. 1984; Roche et al. 1993). Both of these IR excess sources are thought to arise in circumstellar disks around the B[e] supergiants, an inference supported by polarization measurements that are indicative of non-spherical circumstellar envelope geometries (Zickgraf 2003; Magalh˜aes et al. 2006; and references therein). Mass loss is commonly invoked to explain the origin of these B[e] supergiant disks. Specifically, Zickgraf et al. (1985, 1986) hypothesized that the equatorial density enhancements inferred for B[e] supergiants, like those of main-sequence Be stars, represent the “slow” component of a two-component wind model, wherein the “fast,” “normal” hot-star wind component is confined to the stars’ polar regions. However, models in which B[e] supergiant disks are in Keplerian rotation remain viable (more accurately, both the equatorial wind and Keplerian disk models have difficulties; Porter 2003 and references therein). The same IR emission sources that characterize B[e] supergiants also characterize main-sequence Be stars and intermediate-mass, pre-main-sequence (Herbig Ae/Be) stars; main-sequence Be stars display near-IR free–free radiation, whereas the Herbig Ae/Be display thermal dust emission. Many examples of Herbig Ae/Be systems are found in the solar neighborhood, and some of these have been directly imaged, confirming that their dusty circumstellar material resides in disks and (in a few cases) demonstrating that the circumstellar disks in fact orbit the stars (e.g., Panic et al. 2008 and references therein). In contrast—with a few exceptions (e.g., Domiciano de Souza et al. 2007)—the geometries (let alone kinematics) of the circumstellar dust disks around B[e] supergiants have yet to be ascertained via direct imaging, due to the large distances to the
1. INTRODUCTION The study of massive stars is central to a wide variety of fields in astrophysics, ranging from reionization of the early universe (Bromm & Larson 2004) to supernovae, nucleosynthesis, and galactic evolution (Maeder & Conti 1994). The evolution of very massive stars is poorly understood, however, as a consequence of their very short formation and nuclear-burning timescales. One of the most luminous yet least understood classes of massive star is that of the B[e] supergiants and hypergiants (Lamers et al. 1998; Kastner et al. 2006; and references therein). These enigmatic early-type supergiants are characterized by bright Balmer and forbidden line emission and large infrared excesses. They have typical luminosities of 3 × 104 –106 L , corresponding to a range of ∼15 to ∼70 M in initial mass (Zickgraf et al. 1986; Gummersbach et al. 1995). It is not clear, however, how (indeed, whether) B[e] supergiants fit into the evolutionary sequences established thus far for single, massive stars (e.g., Schaerer et al. 1993; Stothers & Chin 1996). The majority of spectroscopically confirmed B[e] supergiants are found in the Large and Small Magellanic Clouds (LMC and SMC), as a consequence of the relatively small extinction toward the MCs (relative to that along the Galactic disk) and the lack of ambiguity in determining distances to (and hence bolometric luminosities of) MC stars. A list of all 15 spectroscopically confirmed MC B[e] supergiants known as of the turn of the 20th century was compiled by Lamers et al. (1998; also see Zickgraf 2006), although there are likely many more such stars lurking in the MCs (e.g., Wisniewski et al. 2007; we also note the presence of many additional MC OB emission-line supergiants in the lists of, e.g., Sanduleak 1970 and Smith Neubig & Bruhweiler 1999). There are also undoubtedly a very large number of B[e] supergiants in the Galaxy, beyond the handful listed in Lamers et al. (1998), and the number of confirmed galactic B[e] supergiants is likely to grow substantially in coming years (e.g., ∼40 galactic B[e] supergiant candidates have been identified, via their peculiar IR colors, by Hadfield et al. 2007). However, 1993
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Table 1 Sample of Magellanic Cloud B[e] Supergiants Observed with IRS Object Name(s)
Sp. Type
HD
LHA
RMC
MSX LMC
38489 ··· 34664 ··· 37974 269217b ··· 268835 269599 ···
120-S 134 115-S 18 120-S 22 120-S 12 120-S 127 120-S 89 115-S 65 120-S 73 120-S 111 120-S 93
··· ··· ··· ··· 126 82 50 66 ··· ···
887 ··· 262 ··· 890 ··· ··· 1326 583 323
B0 B0 B0 B0.5 B0.5 B2-3 B2-3 B8I B8:Iab B9Ib–A0I
E(B − V )
Teff
log Lbol
(kK)
(L )
(mag)
26 25 23 23 22.5 19 17 12 ··· 10
5.9 5.6 5.9 5.3 6.1 5.4 5.7 5.5 ··· 4.7
0.2–0.25 0.26 0.25–0.3 0.2–0.25 0.25 0.2 0.15–0.2 0.12 ··· 0.20
Referencesa
Membership
1, 2 1, 3 1, 2 1, 2, 3 1, 2 1 1, 2 1, 2 1 1
LMC SMC LMC LMC LMC LMC SMC LMC LMC LMC
Notes. a References for stellar spectral types, bolometric luminosities, effective temperatures, and B−V color excess: (1) Lamers et al. 1998; (2) Zickgraf et al. 1986; (3) Zickgraf et al. 1989. b IRS observations obtained at a position later determined to be that of an IR source unrelated to the star (see the text).
MCs and the large line-of-sight extinction toward Galactic B[e] supergiants. In Spitzer Space Telescope Cycle 1, we used the Infrared Spectrograph (IRS; Houck et al. 2004) to obtain low-resolution 5–35 μm spectra of two B[e] supergiants in the LMC—RMC 126 and RMC 66 (Kastner et al. 2006)—as part of an IRS survey of the LMC’s most luminous 8 μm sources (Buchanan et al. 2006, 2009; Kastner et al. 2008). We determined that these two stars display mid-IR spectra that are quite unlike those of the mass-losing, late-type stars and compact H ii regions that dominate the LMC 8 μm sources. Specifically, these two B[e] supergiants are characterized by flat infrared spectral energy distributions (SEDs) with superimposed silicate dust emission features. Furthermore, the IRS spectrum of RMC 66 displays spectral evidence of a combination of polycyclic aromatic hydrocarbons (PAHs) and both crystalline and amorphous silicate grains. On this basis, we concluded that the dust in these two systems likely resides in long-lived circumstellar disks that orbit the stars. This result would appear to be at odds with the two-component wind model proposed by Zickgraf et al. (1985). To investigate both the frequency of appearance and the range of properties of B[e] supergiant star dust disks, and thereby probe the origin and evolution of these disks, we conducted a Spitzer IRS spectroscopic survey of thermal dust emission from a sample of LMC/SMC B[e] supergiants and hypergiants. We augmented these spectroscopic data with archival Spitzer IRAC/ MIPS imaging from the “SAGE” Legacy program (Meixner et al. 2006) and our own targeted MIPS 70 μm imaging. In the present paper, we describe the Spitzer survey results in terms of their implications for the prevalence and likely origins of the dusty circumstellar disks of B[e] supergiants, and for the nature of B[e] supergiants themselves. In a subsequent paper (R. Sahai et al. 2010, in preparation), we will present the results of fits of circumstellar dust disk models to the IRS spectra. 2. OBSERVATIONS 2.1. Sample The sample of LMC and SMC B[e] supergiants considered here is drawn from the Lamers et al. (1998) compilation of stars that are well established as B[e] supergiants on the basis of optical spectroscopy. In Table 1, we present this B[e] supergiant sample in order of decreasing effective temperature (based on spectral types listed in Lamers et al. 1998).
2.2. IRS Spectroscopy Spitzer IRS spectra extending from 5 μm to 38 μm were obtained at the cataloged positions of the Table 1 objects (see Section 2.2.1). All IRS spectra were obtained with both lowresolution (resolving power 64–128) modules, SL and LL, using a standard on-source/off-source telescope nod technique (Buchanan et al. 2006). These two modules cover the spectral ranges 5.2–14 μm and 14–38 μm with slit widths of 3. 6 and 10. 6, respectively. Most of these IRS spectra were obtained during Spitzer Cycle 3 as part of program ID (PID) 30869; some have been published previously. Two spectra (for the LMC stars RMC 126 and RMC 66) were obtained in Cycle 1 and were presented and discussed in detail in Kastner et al. (2006); spectra for another three Table 1 objects (HD 34664, 38489, and LHA 120-S 93) were obtained in Cycle 3, and were included in the Buchanan et al. (2009) classification study of luminous midIR sources in the LMC. Cycle 3 IRS spectra of the remaining Table 1 objects, including two SMC stars (LHA 115-S 18 and RMC 50), are presented here for the first time (Section 3). In addition to the B[e] supergiants in Table 1, we also obtained IRS spectra at the positions of three luminous, early-type LMC stars (LMC 1-289, HD 32364, and HD 269211) as part of PID 30869. We had previously established (Kastner et al. 2008) that these stars—like several of the B[e] supergiants in Table 1 (HD 34664, RMC 126, RMC 66, and LHA 120-S 93)—are associated with compact 8 μm sources that are among the most luminous in the LMC, and that they resemble B[e] supergiants in terms of their near- to mid-IR colors. However, on the basis of the steeply rising SEDs and strong mid-IR emission lines displayed in their IRS spectra—as well as the presence of extensive IR nebulosity and lack of associated point-like counterparts in IRAC/MIPS imaging—we conclude all three stars are luminous, young O/B stars associated with compact H ii regions. We do not consider these stars further in this paper. 2.2.1. Target Positions
Many of the SIMBAD positions for Table 1 stars that were available prior to our Spitzer Cycle 3 observations were obtained from Sanduleak’s (1970) compilation of LMC members, but predated the comprehensive re-examination of Sanduleak catalog positions by B. Skiff and M. Morel (see Sanduleak 2008). Hence, the cataloged (SIMBAD) positions of many of the Table 1 stars were unreliable when these IRS data were
No. 5, 2010
DUSTY CIRCUMSTELLAR DISKS OF B[e] SUPERGIANTS IN MCs
1995
Table 2 Spitzer IRAC and MIPS Photometry of Magellanic Cloud B[e] Supergiantsa Object
3.6 μm
4.5 μm
5.8 μm
8.0 μm
24 μm
70 μm
HD 38489 LHA 115-S 18 HD 34664 LHA 120-S 12 RMC 126 RMC 50 RMC 66 HD 269599 LHA 120-S 93
661 (18) ··· 852 (31) 79 (3) 525 (26) ··· 364 (19) 314 (10) 139 (4)
807 (30) ··· 1047 (41) 81 (3) 636 (24) ··· 483 (15) 435 (13) 179 (5)
907 (32) ··· 1193 (40) 75 (2) 748 (20) ··· 563 (17) 526 (17) 213 (5)
965 (29) ··· 1135 (40) 62 (2) 810 (26) ··· 751 (24) 648 (17) 255 (7)
469 (3) ··· 416 (3) 17 (0.3) 1097 (6) ··· 862 (5) 1164 (6) 184 (1)
