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Title: CHARACTERIZING THE POPULATION OF BRIGHT INFRARED SOURCES IN THE SMALL MAGELLANIC CLOUD

Abstract

We have used the Infrared Spectrograph (IRS) on the Spitzer Space Telescope to observe stars in the Small Magellanic Cloud (SMC) selected from the Point Source Catalog of the Midcourse Space Experiment (MSX). We concentrate on the dust properties of the oxygen-rich evolved stars. The dust composition has smaller contributions from alumina compared to the Galaxy. This difference may arise from the lower metallicity in the SMC, but it could be a selection effect, as the SMC sample includes more stars that are brighter and thus more massive. The distribution of the SMC stars along the silicate sequence looks more like the Galactic sample of red supergiants than asymptotic giant branch stars (AGBs). While many of the SMC stars are definitively on the AGB, several also show evidence of hot bottom burning. Three of the supergiants show PAH emission at 11.3 μ m. Two other sources show mixed chemistry, with both carbon-rich and oxygen-rich spectral features. One, MSX SMC 134, may be the first confirmed silicate/carbon star in the SMC. The other, MSX SMC 049, is a candidate post-AGB star. MSX SMC 145, previously considered a candidate OH/IR star, is actually an AGB star with a background galaxy at zmore »  = 0.16 along the same line of sight. We consider the overall characteristics of all the MSX sources, the most infrared-bright objects in the SMC, in light of the higher sensitivity and resolution of Spitzer , and compare them with the object types expected from the original selection criteria. This population represents what will be seen in more distant galaxies by the upcoming James Webb Space Telescope ( JWST ). Color–color diagrams generated from the IRS spectra and the mid-infrared filters on JWST show how one can separate evolved stars from young stellar objects (YSOs) and distinguish among different classes of YSOs.« less

Authors:
 [1];  [2];  [3];  [4];  [5]
  1. Institute for Scientific Research, Boston College, 140 Commonwealth Avenue, Chestnut Hill, MA 02467 (United States)
  2. Center for Astrophysics and Planetary Science, Cornell University, Ithaca, NY 14853-6801 (United States)
  3. Research School of Astronomy and Astrophysics, Australian National University, Cotter Road, Weston Creek ACT 2611 (Australia)
  4. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States)
  5. National Geospatial Intelligence Agency, 7500 GEOINT Drive, Springfield, VA 22150 (United States)
Publication Date:
OSTI Identifier:
22661381
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 834; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ALUMINIUM OXIDES; ASYMPTOTIC SOLUTIONS; CARBON STARS; CATALOGS; COLOR; GIANT STARS; INFRARED SPECTRA; MAGELLANIC CLOUDS; METALLICITY; MILKY WAY; OXYGEN; POINT SOURCES; POLYCYCLIC AROMATIC HYDROCARBONS; SENSITIVITY; SILICATES; SPACE; STAR EVOLUTION; TELESCOPES; VISIBLE RADIATION

Citation Formats

Kraemer, K. E., Sloan, G. C., Wood, P. R., Jones, O. C., and Egan, M. P., E-mail: kathleen.kraemer@bc.edu, E-mail: sloan@astro.cornell.edu, E-mail: wood@mso.anu.edu.au, E-mail: michael.p.egan@nga.mil. CHARACTERIZING THE POPULATION OF BRIGHT INFRARED SOURCES IN THE SMALL MAGELLANIC CLOUD. United States: N. p., 2017. Web. doi:10.3847/1538-4357/834/2/185.
Kraemer, K. E., Sloan, G. C., Wood, P. R., Jones, O. C., & Egan, M. P., E-mail: kathleen.kraemer@bc.edu, E-mail: sloan@astro.cornell.edu, E-mail: wood@mso.anu.edu.au, E-mail: michael.p.egan@nga.mil. CHARACTERIZING THE POPULATION OF BRIGHT INFRARED SOURCES IN THE SMALL MAGELLANIC CLOUD. United States. doi:10.3847/1538-4357/834/2/185.
Kraemer, K. E., Sloan, G. C., Wood, P. R., Jones, O. C., and Egan, M. P., E-mail: kathleen.kraemer@bc.edu, E-mail: sloan@astro.cornell.edu, E-mail: wood@mso.anu.edu.au, E-mail: michael.p.egan@nga.mil. Tue . "CHARACTERIZING THE POPULATION OF BRIGHT INFRARED SOURCES IN THE SMALL MAGELLANIC CLOUD". United States. doi:10.3847/1538-4357/834/2/185.
@article{osti_22661381,
title = {CHARACTERIZING THE POPULATION OF BRIGHT INFRARED SOURCES IN THE SMALL MAGELLANIC CLOUD},
author = {Kraemer, K. E. and Sloan, G. C. and Wood, P. R. and Jones, O. C. and Egan, M. P., E-mail: kathleen.kraemer@bc.edu, E-mail: sloan@astro.cornell.edu, E-mail: wood@mso.anu.edu.au, E-mail: michael.p.egan@nga.mil},
abstractNote = {We have used the Infrared Spectrograph (IRS) on the Spitzer Space Telescope to observe stars in the Small Magellanic Cloud (SMC) selected from the Point Source Catalog of the Midcourse Space Experiment (MSX). We concentrate on the dust properties of the oxygen-rich evolved stars. The dust composition has smaller contributions from alumina compared to the Galaxy. This difference may arise from the lower metallicity in the SMC, but it could be a selection effect, as the SMC sample includes more stars that are brighter and thus more massive. The distribution of the SMC stars along the silicate sequence looks more like the Galactic sample of red supergiants than asymptotic giant branch stars (AGBs). While many of the SMC stars are definitively on the AGB, several also show evidence of hot bottom burning. Three of the supergiants show PAH emission at 11.3 μ m. Two other sources show mixed chemistry, with both carbon-rich and oxygen-rich spectral features. One, MSX SMC 134, may be the first confirmed silicate/carbon star in the SMC. The other, MSX SMC 049, is a candidate post-AGB star. MSX SMC 145, previously considered a candidate OH/IR star, is actually an AGB star with a background galaxy at z  = 0.16 along the same line of sight. We consider the overall characteristics of all the MSX sources, the most infrared-bright objects in the SMC, in light of the higher sensitivity and resolution of Spitzer , and compare them with the object types expected from the original selection criteria. This population represents what will be seen in more distant galaxies by the upcoming James Webb Space Telescope ( JWST ). Color–color diagrams generated from the IRS spectra and the mid-infrared filters on JWST show how one can separate evolved stars from young stellar objects (YSOs) and distinguish among different classes of YSOs.},
doi = {10.3847/1538-4357/834/2/185},
journal = {Astrophysical Journal},
number = 2,
volume = 834,
place = {United States},
year = {Tue Jan 10 00:00:00 EST 2017},
month = {Tue Jan 10 00:00:00 EST 2017}
}
  • We present far-infrared spectra, {lambda} = 52-93 {mu}m, obtained with the Spitzer Space Telescope in the spectral energy distribution mode of its Multiband Imaging Photometer for Spitzer instrument, of a selection of luminous compact far-infrared sources in the Small Magellanic Cloud (SMC). These comprise nine young stellar objects (YSOs), the compact H II region N 81 and a similar object within N 84, and two red supergiants (RSGs). We use the spectra to constrain the presence and temperature of cool dust and the excitation conditions within the neutral and ionized gas, in the circumstellar environments and interfaces with the surroundingmore » interstellar medium. We compare these results with those obtained in the Large Magellanic Cloud (LMC). The spectra of the sources in N 81 (of which we also show the Infrared Space Observatory-Long-wavelength Spectrograph spectrum between 50 and 170 {mu}m) and N 84 both display strong [O I] {lambda}63 {mu}m and [O III] {lambda}88 {mu}m fine-structure line emission. We attribute these lines to strong shocks and photo-ionized gas, respectively, in a 'champagne flow' scenario. The nitrogen content of these two H II regions is very low, definitely N(N)/N(O) < 0.04 but possibly as low as N(N)/N(O) < 0.01. Overall, the oxygen lines and dust continuum are weaker in star-forming objects in the SMC than in the LMC. We attribute this to the lower metallicity of the SMC compared to that of the LMC. While the dust mass differs in proportion to metallicity, the oxygen mass differs less; both observations can be reconciled with higher densities inside star-forming cloud cores in the SMC than in the LMC. The dust in the YSOs in the SMC is warmer (37-51 K) than in comparable objects in the LMC (32-44 K). We attribute this to the reduced shielding and reduced cooling at the low metallicity of the SMC. On the other hand, the efficiency of the photo-electric effect to heat the gas is found to be indistinguishable to that measured in the same manner in the LMC, {approx}0.1%-0.3%. This may result from higher cloud-core densities, or smaller grains, in the SMC. The dust associated with the two RSGs in our SMC sample is cool, and we argue that it is swept-up interstellar dust, or formed (or grew) within the bow-shock, rather than dust produced in these metal-poor RSGs themselves. Strong emission from crystalline water-ice is detected in at least one YSO. The spectra constitute a valuable resource for the planning and interpretation of observations with the Herschel Space Observatory and the Stratospheric Observatory For Infrared Astronomy.« less
  • We present our study on the infrared variability of point sources in the Small Magellanic Cloud (SMC). We use the data from the Spitzer Space Telescope Legacy Program “Surveying the Agents of Galaxy Evolution in the Tidally Stripped, Low Metallicity Small Magellanic Cloud” (SAGE-SMC) and the “Spitzer Survey of the Small Magellanic Cloud” (S{sup 3}MC) survey, over three different epochs, separated by several months to 3 years. Variability in the thermal infrared is identified using a combination of Spitzer’s InfraRed Array Camera 3.6, 4.5, 5.8, and 8.0 μm bands, and the Multiband Imaging Photometer for Spitzer 24 μm band. Anmore » error-weighted flux difference between each pair of three epochs (“variability index”) is used to assess the variability of each source. A visual source inspection is used to validate the photometry and image quality. Out of ∼2 million sources in the SAGE-SMC catalog, 814 meet our variability criteria. We matched the list of variable star candidates to the catalogs of SMC sources classified with other methods, available in the literature. Carbon-rich Asymptotic Giant Branch (AGB) stars make up the majority (61%) of our variable sources, with about a third of all of our sources being classified as extreme AGB stars. We find a small, but significant population of oxygen-rich (O-rich) AGB (8.6%), Red Supergiant (2.8%), and Red Giant Branch (<1%) stars. Other matches to the literature include Cepheid variable stars (8.6%), early type stars (2.8%), Young-stellar objects (5.8%), and background galaxies (1.2%). We found a candidate OH maser star, SSTISAGE1C J005212.88-730852.8, which is a variable O-rich AGB star, and would be the first OH/IR star in the SMC, if confirmed. We measured the infrared variability of a rare RV Tau variable (a post-AGB star) that has recently left the AGB phase. 59 variable stars from our list remain unclassified.« less
  • We present an analysis of the stellar kinematics of the Large Magellanic Cloud (LMC) based on {approx}5900 new and existing velocities of massive red supergiants, oxygen-rich and carbon-rich asymptotic giant branch (AGB) stars, and other giants. After correcting the line-of-sight velocities for the LMC's space motion and accounting for asymmetric drift in the AGB population, we derive a rotation curve that is consistent with all of the tracers used, as well as that of published H I data. The amplitude of the rotation curve is v{sub 0} = 87 {+-} 5 km s{sup -1} beyond a radius R{sub 0} =more » 2.4 {+-} 0.1 kpc and has a position angle of the kinematic line of nodes of {theta} = 142 deg. {+-} 5 deg. By examining the outliers from our fits, we identify a population of 376 stars, or {approx}>5% of our sample, that have line-of-sight velocities that apparently oppose the sense of rotation of the LMC disk. We find that these kinematically distinct stars are either counter-rotating in a plane closely aligned with the LMC disk, or rotating in the same sense as the LMC disk, but in a plane that is inclined by 54 deg. {+-} 2 deg. to the LMC. Their kinematics clearly link them to two known H I arms, which have previously been interpreted as being pulled out from the LMC. We measure metallicities from the Ca triplet lines of {approx}1000 LMC field stars and 30 stars in the kinematically distinct population. For the LMC field, we find a median [Fe/H] = -0.56 {+-} 0.02 with dispersion of 0.5 dex, while for the kinematically distinct stars the median [Fe/H] is -1.25 {+-} 0.13 with a dispersion of 0.7 dex. The metallicity differences provide strong evidence that the kinematically distinct population originated in the Small Magellanic Cloud. This interpretation has the consequence that the H I arms kinematically associated with the stars are likely falling into the LMC, instead of being pulled out.« less
  • We present far-infrared spectra, {lambda} = 52-93 {mu}m, obtained with the Spitzer Space Telescope in the spectral energy distribution mode of its MIPS instrument, of a representative sample of the most luminous compact far-infrared sources in the Large Magellanic Cloud (LMC). These include carbon stars, OH/IR asymptotic giant branch (AGB) stars, post-AGB objects and planetary nebulae, the R CrB-type star HV 2671, the OH/IR red supergiants (RSGs) WOH G064 and IRAS 05280 - 6910, the three B[e] stars IRAS 04530 - 6916, R 66 and R 126, the Wolf-Rayet star Brey 3a, the luminous blue variable (LBV) R 71, themore » supernova remnant N 49, a large number of young stellar objects (YSOs), compact H II regions and molecular cores, and a background galaxy at a redshift z {approx_equal} 0.175. We use the spectra to constrain the presence and temperature of cold dust and the excitation conditions and shocks within the neutral and ionized gas, in the circumstellar environments and interfaces with the surrounding interstellar medium (ISM). First, we introduce a spectral classification scheme. Then, we measure line strengths, dust temperatures, and IR luminosities. Objects associated with star formation are readily distinguished from evolved stars by their cold dust and/or fine-structure lines. Evolved stars, including the LBV R 71, lack cold dust except in some cases where we argue that this is swept-up ISM. This leads to an estimate of the duration of the prolific dust-producing phase ('superwind') of several thousand years for both RSGs and massive AGB stars, with a similar fractional mass loss experienced despite the different masses. We tentatively detect line emission from neutral oxygen in the extreme RSG WOH G064, which suggests a large dust-free cavity with implications for wind driving. In N 49, the shock between the supernova ejecta and ISM is revealed in spectacular fashion by its strong [O I] {lambda}63 {mu}m emission and possibly water vapor; we estimate that 0.2 M {sub sun} of ISM dust was swept up. On the other hand, some of the compact H II regions display pronounced [O III] {lambda}88 {mu}m emission. The efficiency of photoelectric heating in the interfaces of ionized gas and molecular clouds is estimated at 0.1%-0.3%. We confirm earlier indications of a low nitrogen content in the LMC. Evidence for solid state emission features is found in both young and evolved objects, but the carriers of these features remain elusive; some of the YSOs are found to contain crystalline water ice. The spectra constitute a valuable resource for the planning and interpretation of observations with the Herschel Space Observatory and the Stratospheric Observatory For Infrared Astronomy.« less