Marta Malgorzata Sewilo

Surveying the Agents of Galaxy Evolution in the Tidally Stripped, Low Metallicity Small Magellanic Cloud (SAGE-SMC). I. Overview

The Small Magellanic Cloud (SMC) provides a unique laboratory for the study of the lifecycle of dust given its low metallicity (~1/5 solar) and relative proximity (~60 kpc). This motivated the SAGE-SMC (Surveying the Agents of Galaxy Evolution in the Tidally Stripped, Low Metallicity Small Magellanic Cloud) Spitzer Legacy program with the specific goals of studying the amount and type of dust in the present interstellar medium, the sources of dust in the winds of evolved stars, and how much dust is consumed in star formation. This program mapped the full SMC (30 deg^2) including the body, wing, and tail in seven bands from 3.6 to 160 μm using IRAC and MIPS on the Spitzer Space Telescope. The data were reduced and mosaicked, and the point sources were measured using customized routines specific for large surveys. We have made the resulting mosaics and point-source catalogs available to the community. The infrared colors of the SMC are compared to those of other nearby galaxies and the 8 μm/24 μm ratio is somewhat lower than the average and the 70 μm/160 μm ratio is somewhat higher than the average. The global infrared spectral energy distribution (SED) shows that the SMC has approximately 1/3 the aromatic emission/polycyclic aromatic hydrocarbon abundance of most nearby galaxies. Infrared color-magnitude diagrams are given illustrating the distribution of different asymptotic giant branch stars and the locations of young stellar objects. Finally, the average SED of H II/star formation regions is compared to the equivalent Large Magellanic Cloud average H II/star formation region SED. These preliminary results will be expanded in detail in subsequent papers.

INTRINSICALLY RED SOURCES OBSERVED BY SPITZER IN THE GALACTIC MIDPLANE

We present a highly reliable flux-limited census of 18,949 point sources in the Galactic midplane that have intrinsically red mid-infrared colors. These sources were selected from the Spitzer Space Telescope Galactic Legacy Infrared Midplane Survey Extraordinaire (GLIMPSE) I and II surveys of 274 deg2 of the Galactic midplane, and consist mostly of high- and intermediate-mass young stellar objects (YSOs) and asymptotic giant branch (AGB) stars. The selection criteria were carefully chosen to minimize the effects of position-dependent sensitivity, saturation, and confusion. The distribution of sources on the sky and their location in the Infrared Array Camera and the Multiband Image Photometer for Spitzer 24 μm color-magnitude and color-color space are presented. Using this large sample, we find that YSOs and AGB stars can be mostly separated by simple color-magnitude selection criteria into approximately 50%-70% of YSOs and 30%-50% of AGB stars. Planetary nebulae and background galaxies together represent at most 2%-3% of all the red sources. 1004 red sources in the GLIMPSE II region, mostly AGB stars with high mass-loss rates, show significant (≥0.3 mag) variability at 4.5 and/or 8.0 μm. With over 11,000 likely YSOs and over 7000 likely AGB stars, this is to date the largest uniform census of AGB stars and high- and intermediate-mass YSOs in the Milky Way Galaxy.

HERschel Inventory of The Agents of Galaxy Evolution (HERITAGE): The Large Magellanic Cloud dust

The HERschel Inventory of The Agents of Galaxy Evolution (HERITAGE) of the Magellanic Clouds will use dust emission to investigate the life cycle of matter in both the Large and Small Magellanic Clouds (LMC and SMC). Using the Herschel Space Observatory’s PACS and SPIRE photometry cameras, we imaged a 2° × 8° strip through the LMC, at a position angle of ~22.5° as part of the science demonstration phase of the Herschel mission. We present the data in all 5 Herschel bands: PACS 100 and 160 μm and SPIRE 250, 350 and 500 μm. We present two dust models that both adequately fit the spectral energy distribution for the entire strip and both reveal that the SPIRE 500 μm emission is in excess of the models by ~6 to 17%. The SPIRE emission follows the distribution of the dust mass, which is derived from the model. The PAH-to-dust mass (f_(PAH)) image of the strip reveals a possible enhancement in the LMC bar in agreement with previous work. We compare the gas mass distribution derived from the HI 21 cm and CO J = 1−0 line emission maps to the dust mass map from the models and derive gas-to-dust mass ratios (GDRs). The dust model, which uses the standard graphite and silicate optical properties for Galactic dust, has a very low GDR = 65^(+15) _(−18) making it an unrealistic dust model for the LMC. Our second dust model, which uses amorphous carbon instead of graphite, has a flatter emissivity index in the submillimeter and results in a GDR = 287^_(+25)_(−42) that is more consistent with a GDR inferred from extinction.

Spitzer SAGE Infrared Photometry of Massive Stars in the Large Magellanic Cloud

We present a catalog of 1750 massive stars in the Large Magellanic Cloud (LMC), with accurate spectral types compiled from the literature, and a photometric catalog for a subset of 1268 of these stars, with the goal of exploring their infrared properties. The photometric catalog consists of stars with infrared counterparts in the Spitzer SAGE survey database, for which we present uniform photometry from 0.3 to 24 μm in the UBVIJHKs +IRAC+MIPS24 bands. The resulting infrared color-magnitude diagrams illustrate that the supergiant B[e], red supergiant, and luminous blue variable (LBV) stars are among the brightest infrared point sources in the LMC, due to their intrinsic brightness, and at longer wavelengths, due to dust. We detect infrared excesses due to free-free emission among ~900 OB stars, which correlate with luminosity class. We confirm the presence of dust around 10 supergiant B[e] stars, finding the shape of their spectral energy distributions (SEDs) to be very similar, in contrast to the variety of SED shapes among the spectrally variable LBVs. The similar luminosities of B[e] supergiants (log L/L ☉ ≥ 4) and the rare, dusty progenitors of the new class of optical transients (e.g., SN 2008S and NGC 300 OT), plus the fact that dust is present in both types of objects, suggests a common origin for them. We find the infrared colors for Wolf-Rayet stars to be independent of spectral type and their SEDs to be flatter than what models predict. The results of this study provide the first comprehensive roadmap for interpreting luminous, massive, resolved stellar populations in nearby galaxies at infrared wavelengths.

Surveying the Agents of Galaxy Evolution in the Tidally Stripped, Low Metallicity Small Magellanic Cloud (SAGE-SMC). II. Cool Evolved Stars

We investigate the infrared (IR) properties of cool, evolved stars in the Small Magellanic Cloud (SMC), including the red giant branch (RGB) stars and the dust-producing red supergiant (RSG) and asymptotic giant branch (AGB) stars using observations from the Spitzer Space Telescope Legacy program entitled Surveying the Agents of Galaxy Evolution in the Tidally Stripped, Low Metallicity SMC, or SAGE-SMC. The survey includes, for the first time, full spatial coverage of the SMC bar, wing, and tail regions at IR wavelengths (3.6-160 μm). We identify evolved stars using a combination of near-IR and mid-IR photometry and point out a new feature in the mid-IR color-magnitude diagram that may be due to particularly dusty O-rich AGB stars. We find that the RSG and AGB stars each contribute 20% of the global SMC flux (extended + point-source) at 3.6 μm, which emphasizes the importance of both stellar types to the integrated flux of distant metal-poor galaxies. The equivalent SAGE survey of the higher-metallicity Large Magellanic Cloud (SAGE-LMC) allows us to explore the influence of metallicity on dust production. We find that the SMC RSG stars are less likely to produce a large amount of dust (as indicated by the [3.6] – [8] color). There is a higher fraction of carbon-rich stars in the SMC, and these stars appear to reach colors as red as their LMC counterparts, indicating that C-rich dust forms efficiently in both galaxies. A preliminary estimate of the dust production in AGB and RSG stars reveals that the extreme C-rich AGB stars dominate the dust input in both galaxies, and that the O-rich stars may play a larger role in the LMC than in the SMC.

Broad Radio Recombination Lines from Hypercompact H II Regions

ABSTRACTThe H92 recombination line was observed toward six massive star formation regions (MSFRs), and theH76 line was observed toward one MSFR. All seven MSFRs were suspected of harboring hypercompact (HC)H ii regions. The goal was to detect broad-line sources and to investigate their properties. The sources wereselected according to their small sizes, high brightness temperatures, and rising continuum spectra (typicalspectral index þ 1, S / ) at centimeter wavelengths. Two of the HC Hii candidates, G25.5+0.2 and NGC7538 (IRS 1), were previously known to have extremely broad lines (line widths of 160 and 180 km s 1 ,respectively). Sixteen separate, compact, radio continuum components were detected, fourteen of which weredetected in either the H92 or H76 line. Eight sources have line widths (FWHMs) greater than 40 km s 1 ;typical ultracompact (UC) H ii region line widths are 25–30 km s 1 . These broad lines may be produced by acombination of thermal, turbulent, and electron impact broadening, and large-scale motions (rotation, expansion,jets, shocks, inflow, disk, etc.). On the basis of one line and a relatively low spatial resolution, we are unable todetermine the relative contributions from each mechanism. All the MSFRs in the current sample are composed oftwo or more continuum components. The large projected separations between the continuum components withina given MSFR indicate that they are unlikely to be gravitationally bound massive protostars. Possible origins ofthe observed intermediate-sloped power-law spectral energy distributions (SEDs) are discussed. It is suggestedthat hierarchal clumping in HC H ii regions may produce the observed power-law SEDs.Subject headings: H ii regions — radio lines: ISM — stars: formation1. INTRODUCTIONThe formation and earliest evolution of massive stars is oneof astrophysics’ least understood problems. The observationalrecord is far too incomplete to piece together a coherent theoryof massive star formation and subsequent evolution. Theconventional model assumes that ultracompact (UC) H iiregions represent the earliest manifestation of massive starsafter forming via rapid accretion of ambient gas onto a pro-tostellar hydrostatic core. Norberg & Maeder (2000) andBehrend & Maeder (2001) proposed a ‘‘growing accretionrate’’ scenario, in which the accretion rate increases as themass of the protostar increases. However, the hypothesis offormation via accretion for massive stars (M > 8 10 M )hasbeen questioned by Bonnell et al. (1998), who have proposedan alternative formation mechanism based on mergers oflower mass protostars in dense young clusters. The accretionhypothesis predicts infall of molecular gas to form an equa-torial, Keplerian, accretion disk accompanied by bipolar out-flows along the protostar’s spin axis. Massive bipolar outflowshave been observed toward numerous massive star formationregions (MSFRs; Shepherd & Churchwell 1996; Ridge 2000;Beuther et al. 2002, and references therein). However, thereexist only a few massive protostars with candidate accretiondisks (see Garay & Lizano 1999; Churchwell 2002; Shepherdet al. 2002, and references therein). The small number ofsources is not surprising, because the observations are difficultwith current telescopes, and the phase of rapid accretion islikelytobeshortlived(<10

Resolution of Distance Ambiguities of Inner Galaxy Massive Star Formation Regions. I.

Fifty-four ultracompact (UC) H ii regions in the GLIMPSE survey region (jbj < 1 � and 30 � < l < 70 � ) were observed in H2CO and H110� using the 305 m Arecibo telescope. By analyzing H2CO absorption against the UC H ii region continuum emission, we resolve the distance ambiguity toward 44 sources. This determination is critical to measure global physical properties of UC H ii regions (e.g., luminosity, size, mass) and properties of the Galaxy (e.g., spiral structure, abundance gradients). We find that the distribution of UC H ii regions in this survey is consistent with a ‘‘ local spur,’’ the Perseus, Sagittarius, and Scutum arms as delineated by Taylor & Cordes. However, departures from model velocities produce distance uncertainties only slightly smaller than the proposed arm separations. Subject headings: Galaxy: disk — radio lines: general — stars: formation On-line material: machine-readable tables

The SAGE‐Spec Spitzer Legacy programme: the life‐cycle of dust and gas in the Large Magellanic Cloud – Point source classification I

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.

RESOLUTION OF DISTANCE AMBIGUITIES OF INNER GALAXY MASSIVE STAR FORMATION REGIONS. II.

We report simultaneous H110α and H2CO line observations with the NRAO Green Bank Telescope toward 72 H II regions in the Spitzer Space Telescope GLIMPSE survey area (|l| = 10°-65° and |b| ≤ 1°). We used the H110α line to establish the velocity of the H II regions and H2CO absorption lines to distinguish between near and far distances. Accurate distances are crucial for the determination of physical properties of massive star formation regions. We resolved the distance ambiguity of 44 H II regions. We detected multiple H II regions along 18 lines of sight located in the longitude interval 12°-31°, primarily a result of the relatively large telescope beam width. We could not resolve distance ambiguities for lines of sight with multiple H II regions, since we could not determine which H2CO lines were being absorbed against which H II region. We examined the projected location of H II regions whose distance ambiguities have been resolved (in this work and other similar studies) in the Galactic plane and in a longitude-velocity diagram for a recognizable spiral arm pattern. Although the highest density of points in the position-position plot approximately follows the spiral arms proposed by Taylor & Cordes, the dispersion is still about as large as the separation between their proposed arms. The longitude-velocity plot shows an increase in the density of sources at the points where the spiral arm loci proposed by Taylor & Cordes are approaching the locus of tangent point velocities and a lower density between the arm loci. However, it is not possible to trace spiral arms over significant segments of Galactic longitude in the longitude-velocity plot. We conclude that a very large number of H II regions in combination with more sophisticated Galactic rotation models will be required to obtain a more continuous spiral pattern from kinematic studies of H II regions than from fully sampled surveys of H I or CO.

Dust and Gas in the Magellanic Clouds from the HERITAGE Herschel Key Project. I. Dust Properties and Insights into the Origin of the Submillimeter Excess Emission

The dust properties in the Large and Small Magellanic clouds (LMC/SMC) are studied using the HERITAGE Herschel Key Project photometric data in five bands from 100 to 500 μm. Three simple models of dust emission were fit to the observations: a single temperature blackbody modified by a power-law emissivity (SMBB), a single temperature blackbody modified by a broken power-law emissivity (BEMBB), and two blackbodies with different temperatures, both modified by the same power-law emissivity (TTMBB). Using these models, we investigate the origin of the submillimeter excess, defined as the submillimeter emission above that expected from SMBB models fit to observations <200 μm. We find that the BEMBB model produces the lowest fit residuals with pixel-averaged 500 μm submillimeter excesses of 27% and 43% for the LMC and SMC, respectively. Adopting gas masses from previous works, the gas-to-dust ratios calculated from our fitting results show that the TTMBB fits require significantly more dust than are available even if all the metals present in the interstellar medium (ISM) were condensed into dust. This indicates that the submillimeter excess is more likely to be due to emissivity variations than a second population of colder dust. We derive integrated dust masses of (7.3 ± 1.7) × 105 and (8.3 ± 2.1) × 104 M ☉ for the LMC and SMC, respectively. We find significant correlations between the submillimeter excess and other dust properties; further work is needed to determine the relative contributions of fitting noise and ISM physics to the correlations.