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High- and Intermediate-Mass Young Stellar Objects in the Large Magellanic Cloud
Archival Spitzer Infrared Array Camera (IRAC) and MIPS observations ofthe Large Magellanic Cloud (LMC) have been used to search for youngstellar objects (YSOs). We have carried out independent aperturephotometry of these data and merged the results from different passbandsto produce a photometric catalog. To verify our methodology we have alsoanalyzed the data from the SAGE and SWIRE Legacy programs; ourphotometric measurements are in general agreement with the photometryreleased by these programs. A detailed completeness analysis for ourphotometric catalog of the LMC shows that the 90% completeness limitsare, on average, 16.0, 15.0, 14.3, 13.1, and 9.2 mag at 3.6, 4.5, 5.8,8.0, and 24 ?m, respectively. Using our mid-infrared photometriccatalogs and two simple selection criteria, [4.5]-[8.0]>2.0 toexclude normal and evolved stars and[8.0]>14-([4.5]-[8.0]) to exclude background galaxies, wehave identified a sample of 2910 sources in the LMC that couldpotentially be YSOs. We then used the Spitzer observations complementedby optical and near-infrared data to carefully assess the nature of eachsource. To do so we simultaneously considered multiwavelength images andphotometry to assess the source morphology, spectral energy distribution(SED) from the optical through the mid-infrared wavelengths, and thesurrounding interstellar environment to determine the most likely natureof each source. From this examination of the initial sample, we suggestthat 1172 sources are most likely YSOs. We have also identified 1075probable background galaxies, consistent with the expected numberestimated from the SWIRE survey. Spitzer IRS observations of 269 of thebrightest YSOs from our sample have confirmed that gsim95% are indeedYSOs. An examination of color-color and color-magnitude diagrams showsno simple criteria in color-magnitude space that can unambiguouslyseparate the LMC YSOs from all asymptotic giant branch (AGB)/post-AGBstars, planetary nebulae, and background galaxies. A comprehensivesearch for YSOs in the LMC has also been carried out by the SAGE teamand reported by Whitney et al. There are three major differencesbetween these two searches. (1) In the common region of color-magnitudespace, ~850 of our 1172 probable YSOs are missed in the SAGE YSO catalogbecause their conservative point-source identification criteria haveexcluded YSOs superposed on complex stellar and interstellarenvironments. (2) About 20%-30% of the YSOs identified by the SAGE teamare sources we classify as background galaxies. (3) The SAGE YSO catalogidentifies YSO in parts of color-magnitude space that we excluded andthus contains more evolved or fainter YSOs missed by our analysis. Theshortcomings and strengths of both these YSO catalogs should beconsidered prior to statistical studies of star formation in the LMC.Finally, the mid-infrared luminosity functions in the IRAC bands of ourmost likely YSO candidates in the LMC can be well described by N(L)vprop L -1, which is consistent with the Salpeterinitial mass function if a mass-luminosity relation of L vprop M2.4 is adopted.
| Spitzer Survey of the Large Magellanic Cloud: Surveying the Agents of a Galaxy's Evolution (SAGE). I. Overview and Initial Results
We are performing a uniform and unbiased imaging survey of the LargeMagellanic Cloud (LMC; ~7deg×7deg) using theIRAC (3.6, 4.5, 5.8, and 8 μm) and MIPS (24, 70, and 160 μm)instruments on board the Spitzer Space Telescope in the Surveying theAgents of a Galaxy's Evolution (SAGE) survey, these agents being theinterstellar medium (ISM) and stars in the LMC. This paper provides anoverview of the SAGE Legacy project, including observing strategy, dataprocessing, and initial results. Three key science goals determined thecoverage and depth of the survey. The detection of diffuse ISM withcolumn densities >1.2×1021 H cm-2 permits detailed studies of dust processes in the ISM. SAGE'spoint-source sensitivity enables a complete census of newly formed starswith masses >3 Msolar that will determine the current starformation rate in the LMC. SAGE's detection of evolved stars withmass-loss rates >1×10-8 Msolaryr-1 will quantify the rate at which evolved stars injectmass into the ISM of the LMC. The observing strategy includes two epochsin 2005, separated by 3 months, that both mitigate instrumentalartifacts and constrain source variability. The SAGE data arenonproprietary. The data processing includes IRAC and MIPS pipelines anda database for mining the point-source catalogs, which will be releasedto the community in support of Spitzer proposal cycles 4 and 5. Wepresent initial results on the epoch 1 data for a region near N79 andN83. The MIPS 70 and 160 μm images of the diffuse dust emission ofthe N79/N83 region reveal a similar distribution to the gas emissions,especially the H I 21 cm emission. The measured point-source sensitivityfor the epoch 1 data is consistent with expectations for the survey. Thepoint-source counts are highest for the IRAC 3.6 μm band and decreasedramatically toward longer wavelengths, consistent with the fact thatstars dominate the point-source catalogs and the dusty objects detectedat the longer wavelengths are rare in comparison. The SAGE epoch 1point-source catalog has ~4×106 sources, and more areanticipated when the epoch 1 and 2 data are combined. Using Milky Way(MW) templates as a guide, we adopt a simplified point-sourceclassification to identify three candidate groups-stars without dust,dusty evolved stars, and young stellar objects-that offer a startingpoint for this work. We outline a strategy for identifying foreground MWstars, which may comprise as much as 18% of the source list, andbackground galaxies, which may comprise ~12% of the source list.
| MSX, 2MASS, and the LARGE MAGELLANIC CLOUD: A Combined Near- and Mid-Infrared View
The Large Magellanic Cloud (LMC) has been observed by the MidcourseSpace Experiment (MSX) in the mid-infrared and the Two Micron All SkySurvey (2MASS) in the near-infrared. We have performed across-correlation of the 1806 MSX catalog sources and nearly 1.4 million2MASS cataloged point and extended sources and find 1664 matches. Usingthe available color information, we identify a number of stellarpopulations and nebulae, including main-sequence stars, giant stars, redsupergiants, carbon- and oxygen-rich asymptotic giant branch (AGB)stars, planetary nebulae, H II regions, and other dusty objects likelyassociated with early-type stars. A total of 731 of these sources haveno previous identification. We compile a listing of all objects, whichincludes photometry and astrometry. The 8.3 μm MSX sensitivity is thelimiting factor for object detection: only the brighter red objects,specifically the red supergiants, AGB stars, planetary nebulae, and H IIregions, are detected in the LMC. The remaining objects are likely inthe Galactic foreground. The spatial distribution of the infrared LMCsources may contribute to understanding stellar formation and evolutionand the overall galactic evolution. We demonstrate that a combined mid-and near-infrared photometric baseline provides a powerful means ofidentifying new objects in the LMC for future ground-based andspace-based follow-up observations.
| Obscured AGB stars in the Magellanic Clouds. I. IRAS candidates
We have selected 198 IRAS sources in the Large Magellanic Cloud, and 11in the Small Magellanic Cloud, which are the best candidates to bemass--loosing AGB stars (or possibly post--AGB stars). We used thecatalogues of \cite[Schwering \& Israel (1990)]{ref42} and\cite[Reid et al. (1990)]{ref36}. They are based on the IRAS pointedobservations and have lower detection limits than the Point SourceCatalogue. We also made cross-identifications between IRAS sources andoptical catalogues. Our resulting catalogue is divided in 7 tables.Table \ref{tab1} lists optically known red supergiants and AGB stars forwhich we found an IRAS counterpart (7 and 52 stars in the SMC and LMC,respectively). Table \ref{tab2} lists ``obscured'' (or ``cocoon'') AGBstars or late-type supergiants which have been identified as such inprevious works through their IRAS counterpart and JHKLM photometry (2SMC and 34 LMC sources; no optical counterparts). Table \ref{tab3} listsknown planetary nebulae with an IRAS counterpart (4 SMC and 19 LMC PNe).Table \ref{tab4} lists unidentified IRAS sources that we believe to begood AGB or post--AGB or PNe candidates (11 SMC and 198 LMC sources).Table~\ref{tab5} lists unidentified IRAS sources which could be any typeof object (23 SMC and 121 LMC sources). Table \ref{tab6} lists IRASsources associated with foreground stars (29 SMC and 135 LMC stars).Table \ref{tab7} lists ruled out IRAS sources associated with HIIregions, hot stars, etc... We show that the sample of IRAS AGB stars inthe Magellanic Clouds is very incomplete. Only AGB stars more luminousthan typically 10^4 L_\odot and with a mass-loss rate larger thantypically 5 10^{-6} M_\odot/yr could be detected by the IRAS satellite.As a consequence, one expects to find very few carbon stars in the IRASsample. We also expect that most AGB stars with intermediate mass--lossrates have not been discovered yet, neither in optical surveys, nor inthe IRAS survey. Tables 1 to 8 are also available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Positional reference stars in the Magellanic Clouds
The equatorial coordinates are determined of 926 stars (mainly ofgalactic origin) in the direction of the Magellanic Clouds at the meanepoch T = 1978.4 with an overall accuracy characterized by the meanvalues of the O-C coordinates, Sa = 0.35 arcsec and Sd = 0.38 arcsec,calculated from the coordinates of the Perth reference stars. Thesevalues are larger than the accuracy expected for primary standard stars.They allow the new positions to be considered as those of reliablesecondary standard stars. The published positions correspond to anunquestionable improvement of the quality of the coordinates provided inthe current catalogs. This study represents an 'astrometric step' in thestarting of a 'Durchmusterung' of the Magellanic Clouds organized by deBoer (1988, 1989).
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Observation and Astrometry data
| Constellation: | Dorade |
| Right ascension: | 04h38m49.80s |
| Declination: | -69°27'14.8" |
| Apparent magnitude: | 8.132 |
| Distance: | 331.126 parsecs |
| Proper motion RA: | 13.3 |
| Proper motion Dec: | 25.1 |
| B-T magnitude: | 10.207 |
| V-T magnitude: | 8.304 |
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