L. K. Townsley

Pennsylvania State University, University Park, Maryland, United States

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Publications (182)769.55 Total impact

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    ABSTRACT: (abridged) We used the Chandra observatory to perform a deep (70 ksec) X-ray observation of the Gum 31 region and detected 679 X-ray point sources. This extends and complements the X-ray survey of the central Carina nebula regions performed in the Chandra Carina Complex Project. Using deep near-infrared images from our recent VISTA survey of the Carina nebula complex, our Spitzer point-source catalog, and optical archive data, we identify counterparts for 75% of these X-ray sources. Their spatial distribution shows two major concentrations, the central cluster NGC 3324 and a partly embedded cluster in the southern rim of the HII region, but majority of X-ray sources constitute a rather homogeneously distributed population of young stars. Our color-magnitude diagram analysis suggests ages of ~1-2 Myr for the two clusters, whereas the distributed population shows a wider age range up to ~10 Myr. We also identify previously unknown companions to two of the three O-type members of NGC 3324 and detect diffuse X-ray emission in the region. Our results suggests that the observed region contains about 4000 young stars in total. The distributed population is probably part of the widely distributed population of ~ 1-10 Myr old stars, that was identified in the CCCP area. This implies that the global stellar configuration of the Carina nebula complex is a very extended stellar association, in which the (optically prominent) clusters contain only a minority of the stellar population.
    03/2014;
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    ABSTRACT: A major impediment to understanding star formation in massive star forming regions (MSFRs) is the absence of a reliable stellar chronometer to unravel their complex star formation histories. We present a new estimation of stellar ages using a new method that employs near-infrared (NIR) and X-ray photometry, AgeJX. Stellar masses are derived from X-ray luminosities using the Lx - Mass relation from the Taurus cloud. J-band luminosities are compared to mass-dependent pre-main-sequence evolutionary models to estimate ages. AgeJX is sensitive to a wide range of evolutionary stages, from disk-bearing stars embedded in a cloud to widely dispersed older pre-main sequence stars. The MYStIX (Massive Young Star-Forming Complex Study in Infrared and X-ray) project characterizes 20 OB-dominated MSFRs using X-ray, mid-infrared, and NIR catalogs. The AgeJX method has been applied to 5525 out of 31,784 MYStIX Probable Complex Members. We provide a homogeneous set of median ages for over a hundred subclusters in 15 MSFRs; median subcluster ages range between 0.5 Myr and 5 Myr. The important science result is the discovery of age gradients across MYStIX regions. The wide MSFR age distribution appears as spatially segregated structures with different ages. The AgeJX ages are youngest in obscured locations in molecular clouds, intermediate in revealed stellar clusters, and oldest in distributed populations. The NIR color index J-H, a surrogate measure of extinction, can serve as an approximate age predictor for young embedded clusters.
    The Astrophysical Journal 03/2014; 787(2). · 6.73 Impact Factor
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    ABSTRACT: We present the Massive Star-forming Regions (MSFRs) Omnibus X-ray Catalog (MOXC), a compendium of X-ray point sources from {\em Chandra}/ACIS observations of a selection of MSFRs across the Galaxy, plus 30 Doradus in the Large Magellanic Cloud. MOXC consists of 20,623 X-ray point sources from 12 MSFRs with distances ranging from 1.7 kpc to 50 kpc. Additionally, we show the morphology of the unresolved X-ray emission that remains after the catalogued X-ray point sources are excised from the ACIS data, in the context of \Spitzer\ and {\em WISE} observations that trace the bubbles, ionization fronts, and photon-dominated regions that characterize MSFRs. In previous work, we have found that this unresolved X-ray emission is dominated by hot plasma from massive star wind shocks. This diffuse X-ray emission is found in every MOXC MSFR, clearly demonstrating that massive star feedback (and the several-million-degree plasmas that it generates) is an integral component of MSFR physics.
    The Astrophysical Journal Supplement Series 03/2014; 213(1). · 16.24 Impact Factor
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    ABSTRACT: MYStIX (Massive Young Star-Forming Complex Study in Infrared and X-ray; Feigelson et al. 2013) is a recent survey of star-forming regions (d<4 kpc) in the X-ray and infrared, revealing >30,000 young stellar complex members. Overall, ~150 subclusters of young stars are found (1 to >10 per region) using a statistical cluster finding algorithm -- the finite mixture model. The spatial arrangments of clusters in different regions can be divided into four classes -- simple, isolated clusters, clusters with a core-halo structure, clumpy clusters, and linear chains of clusters. Clusters are often projected on or near molecular-cloud clumps or cores, particularly in the latter morphological case where subclusters are often located along cloud filamentary structures. Subluster size is negatively correlated with cluster central density (power-law with index slightly shallower than -3) and with gas/dust absorption; both of which may be explained as an effect of subcluster expansion. Overall, star formation appears to be non-coeval in the MYStIX regions, due to the wide range of subcluster properties within individual regions.
    01/2014;
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    ABSTRACT: The Massive Young star-forming complex Study in Infrared and X-rays (MYStIX) project, described by Feigelson et al. (2013ApJS..209...26F), seeks to identify and study samples of young stars in 20 nearby (0.4
    12/2013;
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    ABSTRACT: The Massive Young star-forming complex Study in Infrared and X-rays (MYStIX) project, described by Feigelson et al. (2013ApJS..209...26F), provides a comprehensive, parallel study of 20 Galactic massive star-forming regions (MSFRs; d=0.4-3.6kpc). The core data products of MYStIX are tables of "MYStIX probable complex members" (MPCMs) in each target MSFR, compiled by Broos et al. (2013ApJS..209...32B). MPCMs are identified using a combination of X-ray imaging data from the Chandra X-ray Observatory and infrared (IR) data from the United Kingdom Infrared Telescope (UKIRT), the Two-Micron All-Sky Survey (2MASS), and the Spitzer Space Telescope. The basic input data for MIRES were near-IR (NIR) and mid-IR (MIR) photometric catalogs. We also use NIR and MIR images and mosaics for visualizing the point-source populations with respect to various nebular structures. We provide high-level descriptions of each input catalog in section 2.(1 data file).
    12/2013;
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    ABSTRACT: MYStIX (Massive Young Star-Forming Complex Study in Infrared and Xray) seeks to characterize 20 OB-dominated young clusters and their environs at distances d < 4 kpc using imaging detectors on the Chandra X-ray Observatory, Spitzer Space Telescope, and the United Kingdom InfraRed Telescope. The observational goals are to construct catalogs of star-forming complex stellar members with well-defined criteria, and maps of nebular gas (particularly of hot X-ray emitting plasma) and dust. A catalog of MYStIX Probable Complex Members (MPCMs) with several hundred OB stars and > 30, 000 low mass premain sequence is assembled. This sample and related data products will be used to seek new empirical constraints on theoretical models of cluster formation and dynamics, mass segregation, OB star formation, star formation triggering on the periphery of HII regions, the survivability of protoplanetary disks in HII regions. This paper give an introduction and overview of the project, covering the data analysis methodology and application to two star forming regions, NGC 2264 and the Trifid Nebula.
    The Astrophysical Journal Supplement Series 09/2013; 209(2). · 16.24 Impact Factor
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    ABSTRACT: The Massive Young star-forming complex Study in Infrared and X-ray (MYStIX) uses data from the Chandra X-ray Observatory to identify and characterize the young stellar populations in twenty Galactic (d < 4 kpc) massive star-forming regions. Here, the X-ray analysis for Chandra ACIS-I observations of ten of the MYStIX ?elds is described, and a catalog of >10,000 X-ray sources is presented. In comparison to other published Chandra source lists for the same regions, the number of MYStIX detected faint X-ray sources in a region is often doubled. While the higher catalog sensitivity increases the chance of false detections, it also increases the number of matches to infrared stars. X-ray emitting contaminants include foreground stars, background stars, and extragalactic sources. The X-ray properties of sources in these classes are discussed.
    The Astrophysical Journal Supplement Series 09/2013; 209(2). · 16.24 Impact Factor
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    ABSTRACT: The Massive Young star-forming Complex Study in Infrared and X-rays (MYStIX) project requires samples of young stars that are likely members of 20 nearby Galactic massive star-forming regions. Membership is inferred from statistical classification of X-ray sources, from detection of a robust infrared excess that is best explained by circumstellar dust in a disk or infalling envelope, and from published spectral types that are unlikely to be found among field stars. We present the MYStIX membership lists here, and describe in detail the statistical classification of X-ray sources via a \Naive Bayes Classi
    The Astrophysical Journal Supplement Series 09/2013; 209(2). · 16.24 Impact Factor
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    ABSTRACT: The MYStIX project (Massive Young Star-Forming Complex Study in Infrared and X-rays) provides a comparative study of 20 Galactic massive star-forming complexes (d = 0.4 to 3.6 kpc). Probable stellar members in each target complex are identified using X-ray and/or infrared data via two pathways: (1) X-ray detections of young/massive stars with coronal activity/strong winds; or (2) infrared excess (IRE) selection of young stellar objects (YSOs) with circumstellar disks and/or protostellar envelopes. We present the methodology for the second pathway, using Spitzer/IRAC, 2MASS, and UKIRT imaging and photometry. Although IRE selection of YSOs is welltrodden territory, MYStIX presents unique challenges. We combine IR spectral energy distribution (SED) fitting with IR color cuts and spatial clustering analysis to identify IRE sources and isolate probable YSO members in each MYStIX target field from the myriad types of contaminating sources that can resemble YSOs: extragalactic sources, evolved stars, nebular knots, and even unassociated foreground/background YSOs. Applying our methodology consistently across 18 of the target complexes, we produce the MYStIX IRE Source (MIRES) Catalog comprising 20,719 sources, including 8686 probable stellar members of the MYStIX target complexes. We also classify the SEDs of 9365 IR counterparts to MYStIX X-ray sources to assist the first pathway, the identification of X-ray detected stellar members. The MIRES catalog provides a foundation for follow-up studies of diverse phenomena related to massive star cluster formation, including protostellar outflows, circumstellar disks, and sequential star formation triggered by massive star feedback processes.
    The Astrophysical Journal Supplement Series 09/2013; 209(2). · 16.24 Impact Factor
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    ABSTRACT: The MYStIX (Massive Young Star-Forming Complex Study in Infrared and X-ray; Feigelson et al. 2013) project seeks to characterize 20 OB-dominated young star forming regions (SFRs) at distances <4 kpc using photometric catalogs from the Chandra X-ray Observatory, Spitzer Space Telescope, and UKIRT and 2MASS NIR telescopes. A major impediment to understand star formation in the massive SFRs is the absence of a reliable stellar chronometer to unravel their complex star formation histories. We present estimation of stellar ages using a new method that employs NIR and X-ray photometry, t(JX). Stellar masses are directly derived from absorption-corrected X-ray luminosities using the Lx-Mass relation from the Taurus cloud. J-band magnitudes corrected for absorption and distance are compared to the mass-dependent pre-main-sequence evolutionary models of Siess et al. (2000) to estimate ages. Unlike some other age estimators, t(JX) is sensitive to all stages of evolution, from deeply embedded disky objects to widely dispersed older pre-main sequence stars. The method has been applied to >5500 out of >30000 MYStIX stars in 20 SFRs. As individual t(JX) values can be highly uncertain, we report median ages of samples within (sub)clusters defined by the companion study of Kuhn et al. (2013). Here a maximum likelihood model of the spatial distribution produces an objective assignment of each star to an isothermal ellipsoid or a distributed population. The MYStIX (sub)clusters show 0.5 < t(JX) < 5 Myr. The important science result of our study is the discovery of previously unknown age gradients across many different MYStIX regions and clusters. The t(JX) ages are often correlated with (sub)cluster extinction and location with respect to molecular cores and ionized pillars on the peripheries of HII regions. The NIR color J-H, a surrogate measure of extinction, can serve as an approximate age predictor for young embedded clusters.
    07/2013;
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    ABSTRACT: The MYStIX (Massive Young Star-Forming Complex Study in Infrared and X-ray; Feigelson et al. 2013) project provides improved censuses of young stars in 20 nearby OB-dominated star-forming regions that were observed by the Chandra X-ray observatory, the Spitzer Space Telescope, and the UKIRT/UKIDSS and 2MASS surveys. The sample of >33,000 members reveals new details about the structure of clusters in these regions. Clusters of young stars are identified using finite mixture models -\ the sums of isothermal ellipsoids used to model individual (sub)clusters. Maximum likelihood estimation is used to estimate the model parameters and the Akaike Information Criterion is used to detemine the number of subclusters. In the MYStIX star-forming regions, ˜150 subclusters are found (1 to >10 per region). The distribution of cluster core radii is log-normal, peaked at 0.18 pc (similar to the ONC) with a standard deviation of 0.4 dex. The locations of subclusters are often correlated with molecular-cloud clumps or cores. We also recover several well-known embedded subclusters such as the BN-KL region in Orion and the KW Object cluster in M 17. MYStIX star-forming regions typically have one dominant cluster surrounded by smaller subclusters and filamentary groups of young stars. Some clusters are well fit by the ellipsoid model (e.g. Flame Nebula), but others have lumpy structure and are poorly fit (e.g. M 17). A few clusters have a core-halo structure modeled with two overlapping ellipsoids (e.g. RCW 36). Clumpy and core-halo structures could originate in the merger of subclusters. There is a power-law relation between the fitted cluster central density and core radius (index slightly shallower than -3), which may be an effect of cluster expansion. There is also a statistically significant negative relation between median gas/dust absorption of a subcluster and the subcluster's size that can also be explained by cluster expansion if absorption acts as a proxy for age.
    07/2013;
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    ABSTRACT: The MYStIX project (Massive Young stellar clusters Study in Infrared and X-rays) is compiling comprehensive catalogs of the stellar membership in ~20 Galactic massive star-forming regions (d = 0.4 to 3.6 kpc). MYStIX is the first project of its kind to study such a large sample of Galactic regions in parallel, employing a homogeneous set of multiwavelength data analysis techniques. Probable stellar members in each target region are identified using X-ray and/or infrared photometry via two pathways: (1) X-ray detections of young/massive stars with coronal activity/strong winds or (2) Infrared (IR) excess selection of young stellar objects (YSOs) with circumstellar disks and/or protostellar envelopes. In this contribution, we present the methodology and initial results of pathway (2), using Spitzer/IRAC, 2MASS, and UKIDSS imaging and photometry. Although IR excess selection of YSOs is well-trodden territory, MYStIX presents unique challenges. The target regions run the gamut from relatively nearby, lower-mass clusters in uncrowded fields located toward the outer Galaxy (e.g. NGC 2264, the Flame Nebula), to massive clusters located at greater distances along complicated, inner Galaxy sightlines (e.g. NGC 6357, M17). We have developed a new procedure combining IR spectral energy distribution fitting with IR color cuts and spatial clustering analysis to separate probable YSO members in each MYStIX target field from the myriad types of contaminating objects that resemble YSOs: extragalactic sources, evolved stars, PAH nebular knots, and even unassociated foreground/background YSOs. Applying this technique consistently across our target regions, we have produced a catalog comprising several thousand YSOs that can serve as the basis for follow-up studies of diverse phenomena related to massive cluster formation, including protostellar outflows, circumstellar disks, and triggered star formation.
    01/2013;
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    ABSTRACT: XPHOT is an IDL implementation of a non-parametric method for estimating the apparent and intrinsic broad-band fluxes and absorbing X-ray column densities of weak X-ray sources. XPHOT is intended for faint sources with greater than ˜5-7 counts but fewer than 100-300 counts where parametric spectral fitting methods will be superior. This method is similar to the long-standing use of color-magnitude diagrams in optical and infrared astronomy, with X-ray median energy replacing color index and X-ray source counts replacing magnitude. Though XPHOT was calibrated for thermal spectra characteristic of stars in young stellar clusters, recalibration should be possible for some other classes of faint X-ray sources such as extragalactic active galactic nuclei.
    Astrophysics Source Code Library. 12/2012;
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    ABSTRACT: Observations of the spatial distributions of young stars in star-forming regions can be linked to the theory of clustered star formation using spatial statistical methods. The MYStIX project provides rich samples of young stars from the nearest high-mass star-forming regions. Maps of stellar surface density reveal diverse structure and subclustering. Young stellar clusters and subclusters are fit with isothermal spheres and ellipsoids using the Bayesian Information Criterion to estimate the number of subclusters. Clustering is also investigated using Cartwright and Whitworth's Q statistic and the inhomogeneous two-point correlation function. Mass segregation is detected in several cases, in both centrally concentrated and fractally structured star clusters, but a few clusters are not mass segregated.
    08/2012;
  • T. Montmerle, L. K. Townsley
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    ABSTRACT: As a result of feedback from massive stars, via their intense winds and/or supernova explosions, massive star-forming regions are entirely filled with hot, X-ray emitting plasmas, which escape into the ambient ISM. As shown recently by Townsley et al. for several ``extreme" cases (Carina, M17, NGC 3576, NGC 3603, 30 Dor), by way of large Chandra ACIS mosaics, extra, non-thermal emission lines are present on top of the standard lines emitted by hot plasmas. Some of them are very close to lines characteristic of charge-exchange reactions between the hot plasma and the cold surrounding material, suggesting that this mechanism operates on large spatial scales (several 10 pc) in star-forming regions in general. The connection with starburst galaxies is briefly mentioned, and it is pointed out that supernovae interacting with molecular clouds may also provide a good environment to look for charge exchange processes.
    Astronomische Nachrichten 04/2012; · 1.40 Impact Factor
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    ABSTRACT: ACIS Extract (AE), written in the IDL language, provides innovative and automated solutions to the varied challenges found in the analysis of X-ray data taken by the ACIS instrument on NASA's Chandra observatory. AE addresses complications found in many Chandra projects: large numbers of point sources (hundreds to several thousand), faint point sources, misaligned multiple observations of an astronomical field, point source crowding, and scientifically relevant diffuse emission. AE can perform virtually all the data processing and analysis tasks that lie between Level 2 ACIS data and publishable LaTeX tables of point-like and diffuse source properties and spectral models.
    Astrophysics Source Code Library. 03/2012;
  • Matthew S. Povich, L. K. Townsley, W. Orbin
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    ABSTRACT: We present first results from a 100-ks Chandra X-ray observation of the M17 SWex giant molecular cloud. M17 SWex is an infrared dark cloud (IRDC) resembling a flying dragon in Spitzer Space Telescope images. This IRDC is a very young but very active region of star formation, containing 200 intermediate-mass (2-8 Msun) young stellar objects (YSOs) plus a few massive YSOs and ultracompact H II regions. Our Chandra/ACIS image detected >850 X-ray point sources, spatially clustered along the IRDC filaments and many associated with embedded, intermediate-mass YSOs. This abundance of X-ray sources far exceeded expectations, given the extreme youth and high extinction (AV > 30 mag) of the region. Our preliminary results suggest that (1) M17 SWex is an even more active star-forming region than supposed, and/or (2) young, intermediate-mass, pre-main-sequence stars have surprisingly high X-ray luminosities. We have also found indications of diffuse, soft X-ray emission associated with the IRDC. The origins of this diffuse emission are mysterious. M.S.P. is supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST-0901646.
    01/2012;
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    ABSTRACT: This table contains 64 optical, infrared, and x-ray photometric and spectral parameters for the 200 O- and early-B stars with determined spectral types in the Chandra Carina Complex Project (Townsley+, 2011ApJS..194....1G). Some X-ray parameters are from Global X-ray properties of the O and B stars in Carina (Naze et al., 2011, Cat. J/ApJS/194/7). Other optical/infrared parameters are from Candidate X-ray-emitting OB stars in the Carina nebula identified via infrared spectral energy distributions (Povich+, 2011ApJS..194....6G). For a complete description of column label see A catalog of Chandra X-ray sources in the Carina nebula (Broos+, 2011, Cat. J/ApJS/194/2) and Carina OB stars: X-ray signatures of wind shocks and magnetic fields (This paper). (2 data files).
    VizieR Online Data Catalog. 05/2011;
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    ABSTRACT: The bulk of the X-ray data discussed here were taken prior to the CCCP, as part of a guaranteed time program (2006 August 31, PI: S. Murray, 88.4 ks, ObsID 6402). A comprehensive analysis of those data was provided by Albacete-Colombo et al. 2008 (AC08, Cat. J/A+A/490/1055). In this study, we limit our attention to the Trumpler 16 cluster and supplement the AC08 data with additional observations that partially overlap Tr 16 (ObsIDs 9482, 9483, 9488, in 2008 Aug-Sep and 6578 in 2006 Apr, and 4495 in 2004 Sep), which are described and mapped by Townsley et al. (2011ApJS..194....1T). We also invoke the new HAWK-I infrared observations (Preibisch et al. 2011, Cat. J/ApJS/194/10) for improved near-infrared counterpart information. (5 data files).
    VizieR Online Data Catalog. 05/2011;

Publication Stats

3k Citations
769.55 Total Impact Points

Institutions

  • 1998–2014
    • Pennsylvania State University
      • Department of Astronomy and Astrophysics
      University Park, Maryland, United States
  • 2011
    • Universität Potsdam
      • Institute of Physics and Astronomy
      Potsdam, Brandenburg, Germany
    • Universität Stuttgart
      Stuttgart, Baden-Württemberg, Germany
  • 2008
    • Vanderbilt University
      • Department of Physics and Astronomy
      Nashville, MI, United States
  • 2006
    • William Penn University
      Penn Hills, Pennsylvania, United States
  • 2005
    • Ruhr-Universität Bochum
      Bochum, North Rhine-Westphalia, Germany
  • 2004–2005
    • University of Leicester
      • Department of Physics and Astronomy
      Leiscester, England, United Kingdom
    • Massachusetts Institute of Technology
      • Kavli Institute for Astrophysics and Space Research
      Cambridge, Massachusetts, United States