arXiv subjects

Matthew S. Povich

Publications and source records attributed to Matthew S. Povich.

At least 19 recordsLinked to original sources

arXiv2026

Structure and Large-Scale Kinematics of Young Stellar Populations in the NGC 6357 and NGC 6334 Giant Molecular Cloud Complex

We map the three-dimensional structure and large-scale kinematics of the young stellar populations in the G352 giant molecular cloud (GMC) complex. In radio and infrared images, G352 appears as long filament extending ~$3^{\circ}$ (~150 pc) parallel to the Galactic midplane. It connects the NGC 6357 and NGC 6334 giant H II regions and the GM1-24 compact H II region. We identify 1727 stellar members of G352 via matching large catalogs of Chandra X-ray point sources and Spitzer mid-infrared excess sources to the Gaia DR3 astrometric catalog. Our catalog of 11,470 X-ray point sources ranks among the three largest contiguous X-ray survey datasets ever assembled for a massive star-forming complex. We revise the mean heliocentric distance of G352 to $1670\pm 80$ pc, with the median parallaxes of seven constituent groups exhibiting a trend toward increasing distance with decreasing Galactic longitude. We identify two foreground stellar groups superimposed on NGC 6357 that may belong to the Sag OB4 association. The three massive clusters in NGC 6357 exhibit peculiar velocities that trail Galactic circular motion by ${\sim}8$ km/s, while the stars associated with NGC 6334 are more consistent with a circular orbit. GM1-24 has a distinct proper motion and smaller parallax compared to NGC 6334. The steep pitch angle of the GMC filament into the sky appears inconsistent with a spiral arm. The various stellar groups are not gravitationally bound to each other, making G352 a proto-OB association.

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arXiv2025

Fundamental Parameters for Central Stars of 103 Infrared Bowshock Nebulae

Stellar bowshock nebulae are arcuate shock fronts formed by the interaction of radiation-driven stellar winds and the relative motion of the ambient interstellar material. Stellar bowshock nebulae provide a promising means to measure wind-driven mass loss, independent of other established methods. In this work, we characterize the stellar sources at the center of bowshock nebulae drawn from all-sky catalogs of 24 $μ$m-selected nebulae. We obtain new, low-resolution blue optical spectra for \numstars~stars and measure stellar parameters temperature \teff, surface gravity \logg, and projected rotational broadening \vsini. We perform additional photometric analysis to measure stellar radius \rstar, luminosity \lstar, and visual-band extinction $A_V$. All but one of our targets are O and early B stars, with temperatures ranging from $T$=16.5--46.8~k\kelvin, gravities $\log g=$2.57--4.60, and \vsini~from $<$100--400~\kms. With the exception of rapid rotator $ζ$ Oph, bowshock stars do not rotate at or near critical velocities. At least 60 of 103 (60\%) OB bowshock stars are binaries, consistent with the multiplicity fraction of other OB samples. The sample shows a runaway fraction of 23\%, with \numrunaway~stars having $v_{\text{2D}}\geq25$~\kms. Of the 19 runaways, at least 15 ($\geq$79\%) are binaries, favoring dynamical ejection over the binary supernova channel for producing runaways. We provide a comprehensive census of stellar parameters for bowshock stars, useful as a foundation for determining the mass-loss rates for OB-type stars -- one of the single most critical factors in stellar evolution governing the production of neutron stars and black holes.

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arXiv2025

The Milky Way Project MOBStIRS: Parametrizing Infrared Stellar-Wind Bow Shock Morphologies with Citizen Science

Mass-loss influences stellar evolution, especially for massive stars with strong winds. Stellar wind bow shock nebulae driven by Galactic OB stars can be used to measure mass-loss rates ($\dot{M}$). The standoff distance ($R_{0}$) between the star and the bow shock is set by momentum flux balance between the stellar wind and the surrounding interstellar medium (ISM). We created the Milky Way Project: MOBStIRS (Mass-loss rates for OB Stars driving IR bow Shocks) using the online Zooniverse citizen science platform. We enlisted several hundred students to measure $R_0$ and two other projected shape parameters for 764 cataloged IR bow shocks. MOBStIRS incorporated 1528 JPEG cutout images produced from Spitzer GLIMPSE and MIPSGAL survey data. Measurements were aggregated to compute shape parameters for each bow shock image deemed high-quality by participants. The average statistical uncertainty on $R_0$ is $12.5\%$ but varies from ${<}5\%$ to ${\sim}40\%$ among individual bow shocks, contributing significantly to the total error budget of $\dot{M}$. The derived nebular morphologies agree well with (magneto)hydrodynamic simulations of bow shocks driven by the winds of OB stars moving at $V_a = 10-40~km~s^{-1}$ with respect to the ambient interstellar medium (ISM). A systematic correction to $R_0$ to account for viewing angle appears unnecessary for computing $\dot{M}$. Slightly more than half of MOBStIRS bow shocks are asymmetric, which could indicate anisotropic stellar winds, ISM clumping on sub-pc scales, time-dependent instabilities, and/or misalignments between the local ISM magnetic field and the star-bow shock axis.

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arXiv2024

The Tarantula -- Revealed by X-rays (T-ReX)

The Tarantula Nebula (30 Doradus) is the most important star-forming complex in the Local Group, offering a microscope on starburst astrophysics. At its heart lies the exceptionally rich young stellar cluster R136, containing the most massive stars known. Stellar winds and supernovae have carved 30 Dor into an amazing display of arcs, pillars, and bubbles. We present first results and advanced data processing products from the 2-Ms Chandra X-ray Visionary Project, "The Tarantula - Revealed by X-rays" (T-ReX). The 3615 point sources in the T-ReX catalog include massive stars, compact objects, binaries, bright pre-main-sequence stars and compact young stellar (sub)clusters in 30 Dor. After removing point sources and excluding the exceptionally bright supernova remnant N157B (30 Dor B), the global diffuse X-ray maps reveal hot plasma structures resolved at 1-10 pc scales, with an absorption-corrected total-band (0.5-7 keV) X-ray luminosity of $2.110\times 10^{37}$ erg s$^{-1}$. Spatially-resolved spectral modeling provides evidence for emission lines enhanced by charge-exchange processes at the interfaces. We identify a candidate for the oldest X-ray pulsar detected to date in 30 Dor, PSR J0538-6902, inside a newly-resolved arctuate X-ray wind nebula, the Manta Ray. The long time baseline of T-ReX monitored dozens of massive stars, several showing periodic variability tied to binary orbital periods, and captured strong flares from at least three low-mass Galactic foreground stars.

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arXiv2023

Radio survey of the stellar population in the infrared dark cloud G14.225-0.506

The IRDC G14.225-0.506 is associated with a network of filaments, which result in two different dense hubs, as well as with several signposts of star formation activity. The aim of this work is to study the cm continuum emission to characterize the stellar population in G14.2. We performed deep (~1.5-3 microJy) radio continuum observations at 6 and 3.6 cm using the VLA in the A configuration (~0.3''). We have also made use of observations taken during different days to study the presence of variability at short timescales. We detected a total of 66 sources, 32 in the northern region G14.2-N and 34 in the southern region G14.2-S. Ten of the sources are found to be variable. Based on their spectral index, the emission in G14.2-N is mainly dominated by non-thermal sources while G14.2-S contains more thermal emitters. Approximately 75% of the sources present a counterpart at other wavelengths. In the inner 0.4~pc region around the center of each hub, the number of IR sources in G14.2-N is larger than in G14.2-S by a factor of 4. We also studied the relation between the radio luminosity and the bolometric luminosity, finding that the thermal emission of the studied sources is compatible with thermal radio jets. For our sources with X-ray counterparts, the non-thermal emitters follow a Güdel-Benz relation with k = 0.03. We found similar levels of fragmentation between G14.2-N and G14.2-S, suggesting that both regions are most likely twin hubs. The non-thermal emission found in the less evolved objects suggests that G14.2-N may be composed of more massive YSOs as well as being in a more advanced evolutionary stage, consistent with the filament-halo gradient in age and mass from previous works. Our results confirm a wider evolutionary sequence starting in G14.2-S as the youngest part, followed by G14.2-N, and ending with the most evolved region M17.

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arXiv2023

XUE. Molecular inventory in the inner region of an extremely irradiated Protoplanetary Disk

We present the first results of the eXtreme UV Environments (XUE) James Webb Space Telescope (JWST) program, that focuses on the characterization of planet forming disks in massive star forming regions. These regions are likely representative of the environment in which most planetary systems formed. Understanding the impact of environment on planet formation is critical in order to gain insights into the diversity of the observed exoplanet populations. XUE targets 15 disks in three areas of NGC 6357, which hosts numerous massive OB stars, among which some of the most massive stars in our Galaxy. Thanks to JWST we can, for the first time, study the effect of external irradiation on the inner ($< 10$ au), terrestrial-planet forming regions of proto-planetary disks. In this study, we report on the detection of abundant water, CO, CO$_2$, HCN and C$_2$H$_2$ in the inner few au of XUE 1, a highly irradiated disk in NGC 6357. In addition, small, partially crystalline silicate dust is present at the disk surface. The derived column densities, the oxygen-dominated gas-phase chemistry, and the presence of silicate dust are surprisingly similar to those found in inner disks located in nearby, relatively isolated low-mass star-forming regions. Our findings imply that the inner regions of highly irradiated disks can retain similar physical and chemical conditions as disks in low-mass star-forming regions, thus broadening the range of environments with similar conditions for inner disk rocky planet formation to the most extreme star-forming regions in our Galaxy.

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arXiv2022

Spectroscopic Confirmation of a Population of Isolated, Intermediate-Mass YSOs

Wide-field searches for young stellar objects (YSOs) can place useful constraints on the prevalence of clustered versus distributed star formation. The Spitzer/IRAC Candidate YSO (SPICY) catalog is one of the largest compilations of such objects (~120,000 candidates in the Galactic midplane). Many SPICY candidates are spatially clustered, but, perhaps surprisingly, approximately half the candidates appear spatially distributed. To better characterize this unexpected population and confirm its nature, we obtained Palomar/DBSP spectroscopy for 26 of the optically-bright (G<15 mag) "isolated" YSO candidates. We confirm the YSO classifications of all 26 sources based on their positions on the Hertzsprung-Russell diagram, H and Ca II line-emission from over half the sample, and robust detection of infrared excesses. This implies a contamination rate of <10% for SPICY stars that meet our optical selection criteria. Spectral types range from B4 to K3, with A-type stars most common. Spectral energy distributions, diffuse interstellar bands, and Galactic extinction maps indicate moderate to high extinction. Stellar masses range from ~1 to 7 $M_\odot$, and the estimated accretion rates, ranging from $3\times10^{-8}$ to $3\times10^{-7}$ $M_\odot$ yr$^{-1}$, are typical for YSOs in this mass range. The 3D spatial distribution of these stars, based on Gaia astrometry, reveals that the "isolated" YSOs are not evenly distributed in the Solar neighborhood but are concentrated in kpc-scale dusty Galactic structures that also contain the majority of the SPICY YSO clusters. Thus, the processes that produce large Galactic star-forming structures may yield nearly as many distributed as clustered YSOs.

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arXiv2022

The Effect of Molecular Cloud Properties on the Kinematics of Stars Formed in the Trifid Region

The dynamical states of molecular clouds may affect the properties of the stars they form. In the vicinity of the Trifid Nebula ($d=1180\pm25$ pc), the main star cluster (Trifid Main) lies within an expanding section of the molecular cloud; however, ~0.3 deg to the north (Trifid North), the cloud's velocity structure is more tranquil. We acquired a Chandra X-ray observation to identify pre-main-sequence stars in Trifid North, complementing a previous observation of Trifid Main. In Trifid North, we identified 51 candidate pre-main-sequence stars, of which 13 are high-confidence Trifid members based on Gaia EDR3 parallaxes and proper motions. We also re-analyzed membership of Trifid Main and separated out multiple background stellar associations. Trifid North represents a stellar population ~10% as rich as Trifid Main that formed in a separate part of the cloud. The 1D stellar velocity dispersion in Trifid North ($0.6\pm0.2$ km/s) is three times lower than in Trifid Main ($1.9\pm0.2$ km/s). Furthermore, in Trifid Main, proper motions indicate that the portion of the star cluster superimposed on the optical nebula is expanding. Expansion of the HII region around the O-star HD 164492A, and the resulting gas expulsion, can explain both the motions of the stars and gas in Trifid Main. Contrary to previous studies, we find no evidence that a cloud-cloud collision triggered star formation in the region.

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arXiv2021

Characterizing the X-ray Emission of Intermediate-Mass Pre-Main-Sequence Stars

We use X-ray and infrared observations to study the properties of three classes of young stars in the Carina Nebula: intermediate-mass (2--5~M$_\odot$) pre-main sequence stars (IMPS; i.e. intermediate-mass T Tauri stars), late-B and A stars on the zero-age main sequence (AB), and lower-mass T Tauri stars (TTS). We divide our sources among these three sub-classifications and further identify disk-bearing young stellar objects versus diskless sources with no detectable infrared (IR) excess emission using IR (1--8 $μ$m) spectral energy distribution modeling. We then perform X-ray spectral fitting to determine the hydrogen absorbing column density ($N_{\rm H}$), absorption-corrected X-ray luminosity ($L_{\rm X}$), and coronal plasma temperature ($kT$) for each source. We find that the X-ray spectra of both IMPS and TTS are characterized by similar $kT$ and $N_{\rm H}$, and on average $L_{\rm X}$/$L_{\rm bol} \sim4\times10^{-4}$. IMPS are systematically more luminous in X-rays (by $\sim$0.3 dex) than all other sub-classifications, with median $L_{\rm X} = 2.5\times10^{31}$ erg s$^{-1}$, while AB stars of similar masses have X-ray emission consistent with TTS companions. These lines of evidence converge on a magneto-coronal flaring source for IMPS X-ray emission, a scaled-up version of the TTS emission mechanism. IMPS therefore provide powerful probes of isochronal ages for the first $\sim$10 Myr in the evolution of a massive stellar population, because their intrinsic, coronal X-ray emission decays rapidly after they commence evolving along radiative tracks. We suggest that the most luminous (in both X-rays and IR) IMPS could be used to place empirical constraints on the location of the intermediate-mass stellar birth line.

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arXiv2021

SPICY: The Spitzer/IRAC Candidate YSO Catalog for the Inner Galactic Midplane

We present ~120,000 Spitzer/IRAC candidate young stellar objects (YSOs) based on surveys of the Galactic midplane between l~255 deg and 110 deg, including the GLIMPSE I, II, and 3D, Vela-Carina, Cygnus X, and SMOG surveys (613 square degrees), augmented by near-infrared catalogs. We employed a classification scheme that uses the flexibility of a tailored statistical learning method and curated YSO datasets to take full advantage of IRAC's spatial resolution and sensitivity in the mid-infrared ~3-9 micron range. Multi-wavelength color/magnitude distributions provide intuition about how the classifier separates YSOs from other red IRAC sources and validate that the sample is consistent with expectations for disk/envelope-bearing pre-main-sequence stars. We also identify areas of IRAC color space associated with objects with strong silicate absorption or polycyclic aromatic hydrocarbon emission. Spatial distributions and variability properties help corroborate the youthful nature of our sample. Most of the candidates are in regions with mid-IR nebulosity, associated with star-forming clouds, but others appear distributed in the field. Using Gaia DR2 distance estimates, we find groups of YSO candidates associated with the Local Arm, the Sagittarius-Carina Arm, and the Scutum-Centaurus Arm. Candidate YSOs visible to the Zwicky Transient Facility tend to exhibit higher variability amplitudes than randomly selected field stars of the same magnitude, with many high-amplitude variables having light-curve morphologies characteristic of YSOs. Given that no current or planned instruments will significantly exceed IRAC's spatial resolution while possessing its wide-area mapping capabilities, Spitzer-based catalogs such as ours will remain the main resources for mid-infrared YSOs in the Galactic midplane for the near future.

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arXiv2019

The Massive Star-Forming Regions Omnibus X-Ray Catalog, Third Installment

We offer to the star formation community the third installment of the Massive Star-forming Regions (MSFRs) Omnibus X-ray Catalog (MOXC3), a compilation of X-ray point sources detected in 50 archival Chandra/ACIS observations of 14 Galactic MSFRs and surrounding fields. The MOXC3 MSFRs are NGC 2264, NGC 6193, RCW 108-IR, Aur OB1, DR15, NGC 6231, Berkeley 87, NGC 6357, AFGL 4029, h Per (NGC 869), NGC 281, Onsala 2S, G305, and RCW 49 (Wd 2); they have distances of 0.7 kpc to 4.2 kpc. Most exhibit clumped or clustered young stellar populations; several contain at least two distinct massive young stellar clusters. The total MOXC3 catalog includes 27,923 X-ray point sources. We take great care to identify even the faintest X-ray point sources across these fields. This allows us to remove this point source light, revealing diffuse X-ray structures that pervade and surround MSFRs, often generated by hot plasmas from massive star feedback. As we found in MOXC1 and MOXC2, diffuse X-ray emission is traceable in all MOXC3 MSFRs; here we perform spectral fitting to investigate the origins of selected diffuse regions. Once again, MOXC3 shows the value of high spatial resolution X-ray studies of MSFRs enabled by Chandra.

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arXiv2019

Mass-loss rates for O and early B stars powering bowshock nebulae: evidence for bi-stability behavior

Second only to initial mass, the rate of wind-driven mass loss determines the final mass of a massive star and the nature of its remnant. Motivated by the need to reconcile observational values and theory, we use a recently vetted technique to analyze the mass-loss rates in a sample of OB stars that generate bowshock nebulae. We measure peculiar velocities from new Gaia parallax and proper motion data and their spectral types from new optical and infrared spectroscopy. For our sample of 67 central stars in morphologically selected bowshocks nebulae, 67 are OB stars. The median peculiar velocity is 11 km/s, significantly smaller than classical `runaway star' velocities. Mass-loss rates for these O and early B stars agree with recently lowered theoretical predictions, ranging from ~10^-7 Msun/yr for mid-O dwarfs to 10^-9 Msun/yr for late-O dwarfs---a factor of about 2.7 lower than the often-used Vink et al. (2001) formulation. Our results provide the first observational mass-loss rates for B0--B3 dwarfs and giants---10^-9 to 10^-8 Msun/yr. We find evidence for an increase in the mass-loss rates below a critical effective temperature, consistent with predictions of the bi-stability phenomenon in the range Teff=19,000--27,000 K. The sample exhibits a correlation between modified wind momentum and luminosity, consistent in slope but lower by 0.43 dex in magnitude compared to canonical wind-luminosity relations. We identify a small subset of objects deviating most significantly from theoretical expectations as probable radiation-driven bow wave nebulae by virtue of their low stellar-to-nebular luminosity ratios. For these, the inferred mass-loss rates must be regarded as upper limits.

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arXiv2019

The Duration of Star Formation in Galactic Giant Molecular Clouds. I. The Great Nebula in Carina

We present a novel infrared spectral energy distribution (SED) modeling methodology that uses likelihood-based weighting of the model fitting results to construct probabilistic H-R diagrams (pHRD) for X-ray identified, intermediate-mass (2-8 $M_{\odot}$), pre-main sequence young stellar populations. This methodology is designed specifically for application to young stellar populations suffering strong, differential extinction ($ΔA_V > 10$ mag), typical of Galactic massive star-forming regions. We pilot this technique in the Carina Nebula Complex (CNC) by modeling the 1-8 $μ$m SEDs of 2269 likely stellar members that exhibit no excess emission from circumstellar dust disks at 4.5 $μ$m or shorter wavelengths. A subset of ${\sim}100$ intermediate-mass stars in the lightly-obscured Trumpler 14 and 16 clusters have available spectroscopic $T_{\rm eff}$, measured from the Gaia-ESO survey. We correctly identify the stellar temperature in 70% of cases, and the aggregate pHRD for all sources returns the same peak in the stellar age distribution as obtained using the spectroscopic $T_{\rm eff}$. The SED model parameter distributions of stellar mass and evolutionary age reveal significant variation in the duration of star formation among four large-scale stellar overdensities within the CNC and a large distributed stellar population. Star formation began ${\sim}10$ Myr ago and continues to the present day, with the star formation rate peaking ${<}3$ Myr ago when the massive Trumpler 14 and 16 clusters formed. We make public the set of 100,000 SED models generated from standard pre-main sequence evolutionary tracks and our custom software package for generating pHRDs and mass-age distributions from the SED fitting results.

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arXiv2019

The Milky Way Project Second Data Release: Bubbles and Bow Shocks

Citizen science has helped astronomers comb through large data sets to identify patterns and objects that are not easily found through automated processes. The Milky Way Project (MWP), a citizen science initiative on the Zooniverse platform, presents internet users with infrared (IR) images from Spitzer Space Telescope Galactic plane surveys. MWP volunteers make classification drawings on the images to identify targeted classes of astronomical objects. We present the MWP second data release (DR2) and an updated data reduction pipeline written in Python. We aggregate ${\sim}3$ million classifications made by MWP volunteers during the years 2012-2017 to produce the DR2 catalogue, which contains 2600 IR bubbles and 599 candidate bow-shock driving stars. The reliability of bubble identifications, as assessed by comparison to visual identifications by trained experts and scoring by a machine-learning algorithm, is found to be a significant improvement over DR1. We assess the reliability of IR bow shocks via comparison to expert identifications and the colours of candidate bow-shock driving stars in the 2MASS point-source catalogue. We hence identify highly-reliable subsets of 1394 DR2 bubbles and 453 bow-shock driving stars. Uncertainties on object coordinates and bubble size/shape parameters are included in the DR2 catalog. Compared with DR1, the DR2 bubbles catalogue provides more accurate shapes and sizes. The DR2 catalogue identifies 311 new bow shock driving star candidates, including three associated with the giant HII regions NGC 3603 and RCW 49.

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arXiv2019

Searching for Faint X-ray Emission from Galactic Stellar Wind Bow Shocks

We present a stacking analysis of 2.61 Msec of archival Chandra observations of stellar wind bow shocks. We place an upper limit on the X-ray luminosity of IR-detected bow shocks of $<2\times10^{29}$ erg s$^{-1}$, a more stringent constraint than has been found in previous archival studies and dedicated observing campaigns of nearby bow shocks. We compare the X-ray luminosities and $L_X/L_{\rm bol}$ ratios of bow shock driving stars to those of other OB stars within the Chandra field of view. Driving stars are, on average, of later spectral type than the "field of view" OB stars, and we do not observe any unambiguously high $L_X/L_{\rm bol}$ ratios indicative of magnetic stars in our sample. We additionally asses the feasibility of detecting X-rays from stellar wind bow shocks with the proposed Lynx X-ray Observatory. If the X-ray flux originating from the bow shocks is just below our Chandra detection limit, the nearest bow shock in our sample (at $\sim$0.4 kpc with an absorbing column of $\sim10^{21}$ cm$^{-2}$) should be observable with Lynx in exposure times on the order of $\sim$100 kiloseconds.

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arXiv2018

A Multiwavelength Look at Galactic Massive Star Forming Regions

We present a multiwavelength study of 28 Galactic massive star-forming H II regions. For 17 of these regions, we present new distance measurements based on Gaia DR2 parallaxes. By fitting a multicomponent dust, blackbody, and power-law continuum model to the 3.6 $μ$m through 10 mm spectral energy distributions, we find that ${\sim}34$% of Lyman continuum photons emitted by massive stars are absorbed by dust before contributing to the ionization of H II regions, while ${\sim}68$% of the stellar bolometric luminosity is absorbed and reprocessed by dust in the H II regions and surrounding photodissociation regions. The most luminous, infrared-bright regions that fully sample the upper stellar initial mass function (ionizing photon rates $N_C \ge 10^{50}~{\rm s}^{-1}$ and dust-processed $L_{\rm TIR}\ge 10^{6.8}$ L$_{\odot}$) have on average higher percentages of absorbed Lyman continuum photons ($\sim$51%) and reprocessed starlight ($\sim$82%) compared to less luminous regions. Luminous H II regions show lower average PAH fractions than less luminous regions, implying that the strong radiation fields from early-type massive stars are efficient at destroying PAH molecules. On average, the monochromatic luminosities at 8, 24, and 70 $μ$m combined carry 94% of the dust-reprocessed $L_{\rm TIR}$. $L_{70}$ captures ${\sim}52$% of $L_{\rm TIR}$, and is therefore the preferred choice to infer the bolometric luminosity of dusty star-forming regions. We calibrate SFRs based on $L_{24}$ and $L_{70}$ against the Lyman continuum photon rates of the massive stars in each region. Standard extragalactic calibrations of monochromatic SFRs based on population synthesis models are generally consistent with our values.

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arXiv2018

Circumstellar Disk Lifetimes In Numerous Galactic Young Stellar Clusters

Photometric detections of dust circumstellar disks around pre-main sequence (PMS) stars, coupled with estimates of stellar ages, provide constraints on the time available for planet formation. Most previous studies on disk longevity, starting with Haisch, Lada & Lada (2001), use star samples from PMS clusters but do not consider datasets with homogeneous photometric sensitivities and/or ages placed on a uniform timescale. Here we conduct the largest study to date of the longevity of inner dust disks using X-ray and 1--8 micrometre infrared photometry from the MYStIX and SFiNCs projects for 69 young clusters in 32 nearby star-forming regions with ages t<=5 Myr. Cluster ages are derived by combining the empirical AgeJX method with PMS evolutionary models, which treat dynamo-generated magnetic fields in different ways. Leveraging X-ray data to identify disk-free objects, we impose similar stellar mass sensitivity limits for disk-bearing and disk-free YSOs while extending the analysis to stellar masses as low as M~0.1 Mo. We find that the disk longevity estimates are strongly affected by the choice of PMS evolutionary model. Assuming a disk fraction of 100% at zero age, the inferred disk half-life changes significantly, from t1/2 ~ 1.3--2 Myr to t1/2 ~ 3.5 Myr when switching from non-magnetic to magnetic PMS models. In addition, we find no statistically significant evidence that disk fraction varies with stellar mass within the first few Myr of life for stars with masses <2 Mo, but our samples may not be complete for more massive stars. The effects of initial disk fraction and star-forming environment are also explored.

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arXiv2018

Stellar Parameters and Radial Velocities of Hot Stars in the Carina Nebula

The Carina Nebula is an active star forming region in the southern sky that is of particular interest due to the presence of a large number of massive stars in a wide array of evolutionary stages. Here we present the results of the spectroscopic analysis of 82 B-type stars and 33 O-type stars that were observed in 2013 and 2014. For 82 B-type stars without line blending, we fit model spectra from the Tlusty BSTAR2006 grid to the observed profiles of Hγ and He λλ 4026, 4388, and 4471 to measure the effective temperatures, surface gravities, and projected rotational velocities. We also measure the masses, ages, radii, bolometric luminosities, and distances of these stars. From the radial velocities measured in our sample, we find 31 single lined spectroscopic binary candidates. We find a high dispersion of radial velocities among our sample stars, and we argue that the Carina Nebula stellar population has not yet relaxed and become virialized.

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