arXiv subjects

Matthew S. Povich

Publications and source records attributed to Matthew S. Povich.

51 records · Page 3Linked to original sources

arXiv2012

Beyond Strömgren Spheres and Wind-Blown Bubbles: An Observational Perspective on H II Region Feedback

Massive stars produce copious quantities of ultraviolet radiation beyond the Lyman limit, photoionizing the interstellar medium (ISM) and producing H II regions. As strong sources of recombination- and forbidden-line emission, infrared continuum, and thermal (free-free) radio continuum, H II regions serve as readily-observable beacons of massive star formation in the Milky Way and external galaxies. Along with supernovae, H II regions are dominant sources of feedback in star-forming galaxies, injecting radiative and mechanical luminosity into the ISM. H II regions may prove more important than supernovae as triggers of star formation through localized compression of cold cloud cores. In this review, I give a broad overview of the structure and time-evolution of H II regions, emphasizing complications to the theoretical picture revealed by multiwavelength observations. I discuss a recent controversy surrounding the dominant feedback mechanism in 30 Doradus, the most luminous H II region in the Local Group. I summarize the first results from the Milky Way Project (MWP), which has produced a new catalog of several thousand candidate Galactic H II regions by enlisting >35,000 "citizen scientists" to search Spitzer Space Telescope survey images for bubble-shaped structures. The MWP and similar large catalogs enable empirical studies of Galactic H II region evolution across the full range of luminosities and statistical studies of triggered star formation.

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arXiv2011

Toward a Unification of Star Formation Rate Determinations in the Milky Way and Other Galaxies

The star formation rate (SFR) of the Milky Way remains poorly known, with often-quoted values ranging from 1 to 10 solar masses per year. This situation persists despite the potential for the Milky Way to serve as the ultimate SFR calibrator for external galaxies. We show that various estimates for the Galactic SFR are consistent with one another once they have been normalized to the same initial mass function (IMF) and massive star models, converging to 1.9 +/- 0.4 M_sun/yr. However, standard SFR diagnostics are vulnerable to systematics founded in the use of indirect observational tracers sensitive only to high-mass stars. We find that absolute SFRs measured using resolved low/intermediate-mass stellar populations in Galactic H II regions are systematically higher by factors of ~2-3 as compared with calibrations for SFRs measured from mid-IR and radio emission. We discuss some potential explanations for this discrepancy and conclude that it could be allayed if (1) the power-law slope of the IMF for intermediate-mass (1.5 M_sun < m < 5 M_sun) stars were steeper than the Salpeter slope, or (2) a correction factor was applied to the extragalactic 24 micron SFR calibrations to account for the duration of star formation in individual mid-IR-bright H II regions relative to the lifetimes of O stars. Finally, we present some approaches for testing if a Galactic SFR of ~2 M_sun/yr is consistent with what we would measure if we could view the Milky Way as external observers. Using luminous radio supernova remnants and X-ray point sources, we find that the Milky Way deviates from expectations at the 1-3 sigma level, hinting that perhaps the Galactic SFR is overestimated or extragalactic SFRs need to be revised upwards.

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arXiv2011

A Pan-Carina YSO Catalog: Intermediate-Mass Young Stellar Objects in the Carina Nebula Identified Via Mid-Infrared Excess Emission

We present a catalog of 1439 young stellar objects (YSOs) spanning the 1.42 deg^2 field surveyed by the Chandra Carina Complex Project (CCCP), which includes the major ionizing clusters and the most active sites of ongoing star formation within the Great Nebula in Carina. Candidate YSOs were identified via infrared (IR) excess emission from dusty circumstellar disks and envelopes, using data from the Spitzer Space Telescope Vela--Carina survey and the Two-Micron All Sky Survey. We model the 1--24 /mu m IR spectral energy distributions of the YSOs to constrain physical properties. Our Pan-Carina YSO Catalog (PCYC) is dominated by intermediate-mass (2 Msun < m < 10 Msun) objects with disks, including Herbig Ae/Be stars and their less evolved progenitors. The PCYC provides a valuable complementary dataset to the CCCP X-ray source catalogs, identifying 1029 YSOs in Carina with no X-ray detection. We also catalog 410 YSOs with X-ray counterparts, including 62 candidate protostars. Candidate protostars with X-ray detections tend to be more evolved than those without. In most cases, X-ray emission apparently originating from intermediate-mass, disk-dominated YSOs is consistent with the presence of low-mass companions, but we also find that X-ray emission correlates with cooler stellar photospheres and higher disk masses. We suggest that intermediate-mass YSOs produce X-rays during their early pre-main sequence evolution, perhaps driven by magnetic dynamo activity during the convective atmosphere phase, but this emission dies off as the stars approach the main sequence. Extrapolating over the stellar initial mass function scaled to the PCYC population, we predict a total population of >2x10^4 YSOs and a present-day star formation rate (SFR) of >0.008 Msun/yr. The global SFR in the Carina Nebula, averaged over the past ~5 Myr, has been approximately constant.

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arXiv2011

Carina OB Stars: X-ray Signatures of Wind Shocks and Magnetic Fields

The Chandra Carina Complex contains 200 known O- and B type stars. The Chandra survey detected 68 of the 70 O stars and 61 of 127 known B0-B3 stars. We have assembled a publicly available optical/X-ray database to identify OB stars that depart from the canonical Lx/Lbol relation, or whose average X-ray temperatures exceed 1 keV. Among the single O stars with high kT we identify two candidate magnetically confined wind shock sources: Tr16-22, O8.5 V, and LS 1865, O8.5 V((f)). The O4 III(fc) star HD 93250 exhibits strong, hard, variable X-rays, suggesting it may be a massive binary with a period of >30 days. The visual O2 If* binary HD 93129A shows soft 0.6 keV and hard 1.9 keV emission components, suggesting embedded wind shocks close to the O2 If* Aa primary, and colliding wind shocks between Aa and Ab. Of the 11 known O-type spectroscopic binaries, the long orbital-period systems HD 93343, HD 93403 and QZ Car have higher shock temperatures than short-period systems such as HD 93205 and FO 15. Although the X-rays from most B stars may be produced in the coronae of unseen, low-mass pre-main-sequence companions, a dozen B stars with high Lx cannot be explained by a distribution of unseen companions. One of these, SS73 24 in the Treasure Chest cluster, is a new candidate Herbig Be star.

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arXiv2011

X-ray Star Clusters in the Carina Complex

The distribution of young stars found in the Chandra Carina Complex Project (CCCP) is examined for clustering structure. X-ray surveys are advantageous for identifying young stellar populations compared to optical and infrared surveys in suffering less contamination from nebular emission and Galactic field stars. The analysis is based on smoothed maps of a spatially complete subsample of about 3000 brighter X-ray sources classified as Carina members, and about 10,000 stars from the full CCCP sample. The principal known clusters are recovered, and some additional smaller groups are identified. No rich embedded clusters are present, although a number of sparse groups are found. The CCCP reveals considerable complexity in clustering properties. The Trumpler 14 and 15 clusters have rich stellar populations in unimodal, centrally concentrated structures several parsecs across. Non-spherical internal structure is seen, and large-scale low surface density distributions surround these rich clusters. Trumpler 16, in contrast, is comprised of several smaller clusters within a circular boundary. Collinder 228 is a third type of cluster which extends over tens of parsecs with many sparse compact groups likely arising from triggered star formation processes. A widely dispersed, but highly populous, distribution of X-ray stars across the about 50 pc CCCP mosaic supports a model of past generations of star formation in the region. Collinder 234, a group of massive stars without an associated cluster of pre-main sequence stars, may be part of this dispersed population.

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arXiv2011

Candidate X-ray-Emitting OB Stars in the Carina Nebula Identified Via Infrared Spectral Energy Distributions

We report the results of a new survey of massive, OB stars throughout the Carina Nebula using the X-ray point source catalog provided by the Chandra Carina Complex Project (CCCP) in conjunction with infrared (IR) photometry from the Two Micron All-Sky Survey and the Spitzer Space Telescope Vela--Carina survey. Mid-IR photometry is relatively unaffected by extinction, hence it provides strong constraints on the luminosities of OB stars, assuming that their association with the Carina Nebula, and hence their distance, is confirmed. We fit model stellar atmospheres to the optical (UBV) and IR spectral energy distributions (SEDs) of 182 OB stars with known spectral types and measure the bolometric luminosity and extinction for each star. We find that the extinction law measured toward the OB stars has two components: Av=1--1.5 mag produced by foreground dust with a ratio of total-to-selective absorption Rv=3.1 plus a contribution from local dust with Rv>4.0 in the Carina molecular clouds that increases as Av increases. Using X-ray emission as a strong indicator of association with Carina, we identify 94 candidate OB stars with Lbol\geq10^4 Lsun by fitting their IR SEDs. If the candidate OB stars are eventually confirmed by follow-up spectroscopic observations, the number of cataloged OB stars in the Carina Nebula will increase by ~50%. Correcting for incompleteness due to OB stars falling below the Lbol cutoff or the CCCP detection limit, these results potentially double the size of the young massive stellar population.

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arXiv2011

Source Contamination in X-ray Studies of Star-Forming Regions: Application to the Chandra Carina Complex Project

We describe detailed simulations of X-ray-emitting populations to evaluate the levels of contamination by both Galactic and extragalactic X-ray sources unrelated to a star-forming region under study. For Galactic contaminations, we consider contribution from main-sequence stars and giants (not including cataclysmic variables and other classes of accretion-driven X-ray binary systems) as they make the dominant contribution at the position of the Carina Nebula. The simulations take into consideration a variety of technical factors involving a Galactic population synthesis model, stellar X-ray luminosity functions, Chandra telescope response, source detection methodology, and possible spatial variations in the X-ray background and absorption through molecular clouds. When applied to the 1.42 square-degree field of the Chandra Carina Complex Project (CCCP), the simulations predict ~5000 contaminating sources (1 source per square arcminute of the survey), evenly distributed across the field. The results of the simulations are further employed in a companion CCCP study to assign membership probabilities to individual sources.

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arXiv2011

A Naive Bayes Source Classifier for X-ray Sources

The Chandra Carina Complex Project (CCCP) provides a sensitive X-ray survey of a nearby starburst region over >1 square degree in extent. Thousands of faint X-ray sources are found, many concentrated into rich young stellar clusters. However, significant contamination from unrelated Galactic and extragalactic sources is present in the X-ray catalog. We describe the use of a naive Bayes classifier to assign membership probabilities to individual sources, based on source location, X-ray properties, and visual/infrared properties. For the particular membership decision rule adopted, 75% of CCCP sources are classified as members, 11% are classified as contaminants, and 14% remain unclassified. The resulting sample of stars likely to be Carina members is used in several other studies, which appear in a Special Issue of the ApJS devoted to the CCCP.

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arXiv2011

An Introduction to the Chandra Carina Complex Project

The Great Nebula in Carina provides an exceptional view into the violent massive star formation and feedback that typifies giant HII regions and starburst galaxies. We have mapped the Carina star-forming complex in X-rays, using archival Chandra data and a mosaic of 20 new 60ks pointings using the Chandra X-ray Observatory's Advanced CCD Imaging Spectrometer, as a testbed for understanding recent and ongoing star formation and to probe Carina's regions of bright diffuse X-ray emission. This study has yielded a catalog of properties of >14,000 X-ray point sources; >9800 of them have multiwavelength counterparts. Using Chandra's unsurpassed X-ray spatial resolution, we have separated these point sources from the extensive, spatially-complex diffuse emission that pervades the region; X-ray properties of this diffuse emission suggest that it traces feedback from Carina's massive stars. In this introductory paper, we motivate the survey design, describe the Chandra observations, and present some simple results, providing a foundation for the 15 papers that follow in this Special Issue and that present detailed catalogs, methods, and science results.

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arXiv2010

Spitzer Space Telescope observations of the Carina Nebula: The steady march of feedback-driven star formation

We report the first results of imaging the Carina Nebula with Spitzer/IRAC, providing a catalog of point sources and YSOs based on SED fits. We discuss several aspects of the extended emission, including dust pillars that result when a clumpy molecular cloud is shredded by massive star feedback. There are few "extended green objects" (EGOs) normally taken as signposts of outflow activity, and none of the HH jets detected optically are seen as EGOs. A population of "extended red objects" tends to be found around OB stars, some with clear bow-shocks. These are dusty shocks where stellar winds collide with flows off nearby clouds. Finally, the relative distributions of O stars and subclusters of YSOs as compared to dust pillars shows that while some YSOs are located within pillars, many more stars and YSOs reside just outside pillar heads. We suggest that pillars are transient phenomena, part of a continuous outwardly propagating wave of star formation driven by massive star feedback. As pillars are destroyed, they leave newly formed stars in their wake, which are then subsumed into the young OB association. Altogether, the current generation of YSOs shows no strong deviation from a normal IMF. The number of YSOs suggests a roughly constant star-formation rate over the past 3Myr, implying that star formation in pillars constitutes an important mechanism to construct unbound OB associations. Accelerated pillars may give birth to O-type stars that, after several Myr, could appear to have formed in isolation.

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arXiv2010

Evidence for Delayed Massive Star Formation in the M17 Proto-OB Association

Through analysis of archival images and photometry from the Spitzer GLIMPSE and MIPSGAL surveys combined with 2MASS and MSX data, we have identified 488 candidate young stellar objects (YSOs) in the giant molecular cloud M17 SWex, which extends ~50 pc southwest from the prominent Galactic H II region M17. Our sample includes >200 YSOs with masses >3 Msun that will become B-type stars on the main sequence. Extrapolating over the stellar initial mass function (IMF), we find that M17 SWex contains >1.3x10^4 young stars, representing a proto-OB association. The YSO mass function is significantly steeper than the Salpeter IMF, and early O stars are conspicuously absent from M17 SWex. Assuming M17 SWex will form an OB association with a Salpeter IMF, these results reveal the combined effects of (1) more rapid circumstellar disk evolution in more massive YSOs and (2) delayed onset of massive star formation.

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arXiv2009

Embedded Young Stellar Object Candidates in the Active Star Forming Complex W51: Mass Function and Spatial Distribution

We present 737 candidate Young Stellar Objects (YSOs) near the W51 Giant Molecular Cloud (GMC) over an area of 1.25 deg x 1.00 deg selected from Spitzer Space Telescope data. We use spectral energy distribution (SED) fits to identify YSOs and distinguish them from main-sequence or red giant stars, asymptotic giant branch stars, and background galaxies. Based on extinction of each YSO, we separate a total of 437 YSOs associated with the W51 region from the possible foreground sources. We identify 69 highly embedded Stage 0/I candidate YSOs in our field with masses > 5 Msun (corresponding to mid-to early-B main-sequence spectral types) 46 of which are located in the central active star forming regions of W51A and W51B. From the YSOs associated with W51, we find evidence for mass segregation showing that the most massive YSOs are concentrated on the W51 HII region complex. We find a variation in the spatial distribution of the mass function (MF) of YSOs in the mass range between 5 Msun and 18 Msun. The derived slopes of the MF are -1.26 and -2.36 in the active star-forming region and the outer region, respectively. The variation of the MF for YSOs embedded in the molecular cloud implies that the distribution of stellar masses in clusters depends on the local conditions in the parent molecular cloud.

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arXiv2009

The Extended Environment of M17: A Star Formation History

M17 is one of the youngest and most massive nearby star-formation regions in the Galaxy. It features a bright H II region erupting as a blister from the side of a giant molecular cloud (GMC). Combining photometry from the Spitzer GLIMPSE survey with complementary infrared (IR) surveys, we identify candidate young stellar objects (YSOs) throughout a 1.5 deg x 1 deg field that includes the M17 complex. The long sightline through the Galaxy behind M17 creates significant contamination in our YSO sample from unassociated sources with similar IR colors. Removing contaminants, we produce a highly-reliable catalog of 96 candidate YSOs with a high probability of association with the M17 complex. We fit model spectral energy distributions to these sources and constrain their physical properties. Extrapolating the mass function of 62 intermediate-mass YSOs (M >3 Msun), we estimate that >1000 stars are in the process of forming in the extended outer regions of M17. From IR survey images from IRAS and GLIMPSE, we find that M17 lies on the rim of a large shell structure ~0.5 deg in diameter (~20 pc at 2.1 kpc). We present new maps of CO and 13CO (J=2-1) emission, which show that the shell is a coherent, kinematic structure associated with M17 at v = 19 km/s. The shell is an extended bubble outlining the photodissociation region of a faint, diffuse H II region several Myr old. We provide evidence that massive star formation has been triggered by the expansion of the bubble. The formation of the massive cluster ionizing the M17 H II region itself may have been similarly triggered. We conclude that the star formation history in the extended environment of M17 has been punctuated by successive waves of massive star formation propagating through a GMC complex.

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arXiv2008

Intrinsically Red Sources observed by Spitzer in the Galactic Mid-Plane

We present a highly reliable flux-limited census of 18,949 point sources in the Galactic mid-plane that have intrinsically red mid-infrared colors. These sources were selected from the Spitzer Space Telescope GLIMPSE I and II surveys of 274 deg^2 of the Galactic mid-plane, 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 IRAC and MIPS 24 microns 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. 1,004 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 microns. With over 11,000 likely YSOs and over 7,000 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.

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arXiv2008

Interstellar Weather Vanes: GLIMPSE Mid-Infrared Stellar-Wind Bowshocks in M17 and RCW49

We report the discovery of six infrared stellar-wind bowshocks in the Galactic massive star formation regions M17 and RCW49 from Spitzer GLIMPSE (Galactic Legacy Infrared Mid-Plane Survey Extraordinaire) images. The InfraRed Array Camera (IRAC) on the Spitzer Space Telescope clearly resolves the arc-shaped emission produced by the bowshocks. We combine Two Micron All-Sky Survey (2MASS), Spitzer, MSX, and IRAS observations to obtain the spectral energy distributions (SEDs) of the bowshocks and their individual driving stars. We use the stellar SEDs to estimate the spectral types of the three newly-identified O stars in RCW49 and one previously undiscovered O star in M17. One of the bowshocks in RCW49 reveals the presence of a large-scale flow of gas escaping the H II region at a few 10^2 km/s. Radiation-transfer modeling of the steep rise in the SED of this bowshock toward longer mid-infrared wavelengths indicates that the emission is coming principally from dust heated by the star driving the shock. The other 5 bowshocks occur where the stellar winds of O stars sweep up dust in the expanding H II regions.

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