Publications (7)9.8 Total impact
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Article: The SAGE‐Spec Spitzer Legacy programme: the life‐cycle of dust and gas in the Large Magellanic Cloud – Point source classification I
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ABSTRACT: We present the classification of 197 point sources observed with the Infrared Spectrograph in the SAGE-Spec Legacy programme on the Spitzer Space Telescope. We introduce a decision-tree method of object classification based on infrared spectral features, continuum and spectral energy distribution shape, bolometric luminosity, cluster membership and variability information, which is used to classify the SAGE-Spec sample of point sources. The decision tree has a broad application to mid-infrared spectroscopic surveys, where supporting photometry and variability information are available. We use these classifications to make deductions about the stellar populations of the Large Magellanic Cloud and the success of photometric classification methods. We find 90 asymptotic giant branch (AGB) stars, 29 young stellar objects, 23 post-AGB objects, 19 red supergiants, eight stellar photospheres, seven background galaxies, seven planetary nebulae, two H ii regions and 12 other objects, seven of which remain unclassified.Monthly Notices of the Royal Astronomical Society 02/2011; 411(3):1597 - 1627. · 4.90 Impact Factor -
Article: The SAGE-Spec Spitzer Legacy program: The life-cycle of dust and gas in the Large Magellanic Cloud. Point source classification I
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ABSTRACT: We present the classification of 197 point sources observed with the Infrared Spectrograph in the SAGE-Spec Legacy program on the Spitzer Space Telescope. We introduce a decision-tree method of object classification based on infrared spectral features, continuum and spectral energy distribution shape, bolometric luminosity, cluster membership, and variability information, which is used to classify the SAGE-Spec sample of point sources. The decision tree has a broad application to mid-infrared spectroscopic surveys, where supporting photometry and variability information are available. We use these classifications to make deductions about the stellar populations of the Large Magellanic Cloud and the success of photometric classification methods. We find 90 asymptotic giant branch (AGB) stars, 29 young stellar objects, 23 post-AGB objects, 19 red supergiants, eight stellar photospheres, seven background galaxies, seven planetary nebulae, two HII regions and 12 other objects, seven of which remain unclassified. Comment: (43 pages, 21 figures, 4 tables including one large table out of order; to be published in MNRAS)09/2010; -
Article: Mid‐infrared diagnostics of metal‐rich H ii regions from VLT and Spitzer spectroscopy of young massive stars in W31★
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ABSTRACT: We present near-infrared Very Large Telescope/Infrared Spectrograph and Array Camera and mid-infrared (mid-IR) Spitzer/Infrared Spectrograph spectroscopy of the young massive cluster in the W31 star-forming region. H-band spectroscopy provides refined classifications for four cluster member O stars with respect to Blum et al. In addition, photospheric features are detected in the massive young stellar object (massive YSO) #26. Spectroscopy permits estimates of stellar temperatures and masses, from which a cluster age of ∼0.6 Myr and distance of 3.3 kpc are obtained, in excellent agreement with Blum et al. IRS spectroscopy reveals mid-IR fine structure line fluxes of [Ne ii–iii] and [S iii–iv] for four O stars and five massive YSOs. In common with previous studies, stellar temperatures of individual stars are severely underestimated from the observed ratios of fine-structure lines, despite the use of contemporary stellar atmosphere and photoionization models. We construct empirical temperature calibrations based upon the W31 cluster stars of known spectral type, supplemented by two inner Milky Way ultracompact (UC) H ii regions whose ionizing star properties are established. Calibrations involving [Ne iii] 15.5 μm/[Ne ii] 12.8 μm, [S iv] 10.5 μm/[Ne ii] 12.8 μm or [Ar iii] 9.0 μm/[Ne ii] 12.8 μm have application in deducing the spectral types of early to mid O stars for other inner Milky Way compact and UC H ii regions. Finally, evolutionary phases and time-scales for the massive stellar content in W31 are discussed, due to the presence of numerous young massive stars at different formation phases in a ‘coeval’ cluster.Monthly Notices of the Royal Astronomical Society 04/2010; 403(3):1433 - 1447. · 4.90 Impact Factor -
Article: The SAGE-Spec Spitzer Legacy program: The life-cycle of dust and gas in the Large Magellanic Cloud
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ABSTRACT: The SAGE-Spec Spitzer Legacy program is a spectroscopic follow-up to the SAGE-LMC photometric survey of the Large Magellanic Cloud carried out with the Spitzer Space Telescope. We present an overview of SAGE-Spec and some of its first results. The SAGE-Spec program aims to study the life cycle of gas and dust in the Large Magellanic Cloud, and to provide information essential to the classification of the point sources observed in the earlier SAGE-LMC photometric survey. We acquired 224.6 hours of observations using the InfraRed Spectrograph and the SED mode of the Multiband Imaging Photometer for Spitzer. The SAGE-Spec data, along with archival Spitzer spectroscopy of objects in the Large Magellanic Cloud, are reduced and delivered to the community. We discuss the observing strategy, the specific data reduction pipelines applied and the dissemination of data products to the scientific community. Initial science results include the first detection of an extragalactic "21 um" feature towards an evolved star and elucidation of the nature of disks around RV Tauri stars in the Large Magellanic Cloud. Towards some young stars, ice features are observed in absorption. We also serendipitously observed a background quasar, at a redshift of z~0.14, which appears to be host-less. Comment: 33 pages, 12 figures, 8 tables; accepted for publication by PASP04/2010; -
Article: Spitzer Spectra of 2MASS/MSX Selected Sources in the Small Magellanic Cloud
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ABSTRACT: We present results from our Spitzer Space Telescope study of late type stars in the Small Magellanic Cloud (SMC). The source selection and tentative identification of spectral type had been made using color-color data derived fromTwo-micron All-Sky Survey (2MASS) and Midcourse Space Experiment (MSX) measurements and the source identification model previously developed by Egan, van Dyk, and Price (2001) for the Large Magellanic Cloud (LMC). Results of the Spitzer SMC spectral survey show surprises in that many stars expected to be oxygen-rich, high-luminosity asymptotic giants turned out to have carbon-rich envelopes. Overpopulations in the Magellanic Cloud carbon star population, relative to Milky Way samples, have been noted by other research teams. Similar results for a 2MASS/MSX selected sample in the LMC have been found by the Spitzer team led by J. Kastner (Buchanan et al., in press). These results imply that the lower metallicity in the SMC (and LMC) allows much easier evolution to carbon-rich envelope chemistries across all mass ranges for AGB stars than is seen in the Milky Way. This fact has implications for the future use of color-color diagrams for classifying stars in other galaxies. In order to distinguish between C-rich and O-rich AGB stars by color, correct spectral bands must be chosen, and the metallicity and color relationship well understood. We will present the spectra from the Spitzer SMC sample in this paper, and compare 2MASS/MSX colors for the SMC with similar Milky Way samples. We will also examine the theoretical implications of the dearth of OH/IR stars found in the SMC versus previous expectations.11/2005; 37:1381. -
Article: The SAGE-Spec Spitzer Legacy programme: the life-cycle of dust and gas in the Large Magellanic Cloud – Point source classification I
[show abstract] [hide abstract]
ABSTRACT: We present the classification of 197 point sources observed with the Infrared Spectrograph in the SAGE-Spec Legacy programme on the Spitzer Space Telescope. We introduce a decision-tree method of object classification based on infrared spectral features, continuum and spectral energy distribution shape, bolometric luminosity, cluster membership and variability information, which is used to classify the SAGE-Spec sample of point sources. The decision tree has a broad application to mid-infrared spectroscopic surveys, where supporting photometry and variability information are available. We use these classifications to make deductions about the stellar populations of the Large Magellanic Cloud and the success of photometric classification methods. We find 90 asymptotic giant branch (AGB) stars, 29 young stellar objects, 23 post-AGB objects, 19 red supergiants, eight stellar photospheres, seven background galaxies, seven planetary nebulae, two H_(II) regions and 12 other objects, seven of which remain unclassified. -
Article: Spitzer Sage Survey of the Large Magellanic Cloud. III. Star Formation and ~1000 New Candidate Young Stellar Objects
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ABSTRACT: We present ~1000 new candidate Young Stellar Objects (YSOs) in the Large Magellanic Cloud selected from Spitzer Space Telescope data, as part of the Surveying the Agents of a Galaxy's Evolution (SAGE) Legacy program. The YSOs, detected by their excess infrared (IR) emission, represent early stages of evolution, still surrounded by disks and/or infalling envelopes. Previously, fewer than 20 such YSOs were known. The candidate YSOs were selected from the SAGE Point Source Catalog from regions of color-magnitude space least confused with other IR-bright populations. The YSOs are biased toward intermediate- to high-mass and young evolutionary stages, because these overlap less with galaxies and evolved stars in color-magnitude space. The YSOs are highly correlated spatially with atomic and molecular gas, and are preferentially located in the shells and bubbles created by massive stars inside. They are more clustered than generic point sources, as expected if star formation occurs in filamentary clouds or shells. We applied a more stringent color-magnitude selection to produce a subset of "high-probability" YSO candidates. We fitted the spectral-energy distributions (SEDs) of this subset and derived physical properties for those that were well fitted. The total mass of these well-fitted YSOs is ~2900 M_☉ and the total luminosity is ~2.1 × 10^6 L_☉ . By extrapolating the mass function with a standard initial mass function and integrating, we calculate a current star-formation rate of ~0.06 M_☉ yr^(–1), which is at the low end of estimates based on total ultraviolet and IR flux from the galaxy (~0.05 – 0.25 M_☉ yr^(–1)), consistent with the expectation that our current YSO list is incomplete. Follow-up spectroscopy and further data mining will better separate the different IR-bright populations and likely increase the estimated number of YSOs. The full YSO list is available as electronic tables, and the SEDs are available as an electronic figure for further use by the scientific community.
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2011
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Imperial College London
London, ENG, United Kingdom
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