arXiv subjects

Konstantin V. Getman

Publications and source records attributed to Konstantin V. Getman.

69 records · Page 4Linked to original sources

arXiv2010

A Chandra Observation of the Obscured Star-Forming Complex W40

The young stellar cluster illuminating the W40 H II region, one of the nearest massive star forming regions, has been observed with the ACIS detector on board the Chandra X-ray Observatory. Due to its high obscuration, this is a poorly-studied stellar cluster with only a handful of bright stars visible in the optical band, including three OB stars identified as primary excitation sources. We detect 225 X-ray sources, of which 85% are confidently identified as young stellar members of the region. Two potential distances of the cluster, 260 pc and 600 pc, are used in the paper. Supposing the X-ray luminosity function to be universal, it supports a 600 pc distance as a lower limit for W40 and a total population of at least 600 stars down to 0.1 Mo under the assumption of a coeval population with a uniform obscuration. In fact, there is strong spatial variation in Ks-band-excess disk fraction and non-uniform obscuration due to a dust lane that is identified in absorption in optical, infrared and X-ray. The dust lane is likely part of a ring of material which includes the molecular core within W40. In contrast to the likely ongoing star formation in the dust lane, the molecular core is inactive. The star cluster has a spherical morphology, an isothermal sphere density profile, and mass segregation down to 1.5 Mo. However, other cluster properties, including a \leq{1} Myr age estimate and ongoing star formation, indicate that the cluster is not dynamically relaxed. X-ray diffuse emission and a powerful flare from a young stellar object are also reported.

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arXiv2010

Innovations in the Analysis of Chandra-ACIS Observations

As members of the instrument team for the Advanced CCD Imaging Spectrometer (ACIS) on NASA's Chandra X-ray Observatory and as Chandra General Observers, we have developed a wide variety of data analysis methods that we believe are useful to the Chandra community, and have constructed a significant body of publicly-available software (the ACIS Extract package) addressing important ACIS data and science analysis tasks. This paper seeks to describe these data analysis methods for two purposes: to document the data analysis work performed in our own science projects, and to help other ACIS observers judge whether these methods may be useful in their own projects (regardless of what tools and procedures they choose to implement those methods). The ACIS data analysis recommendations we offer here address much of the workflow in a typical ACIS project, including data preparation, point source detection via both wavelet decomposition and image reconstruction, masking point sources, identification of diffuse structures, event extraction for both point and diffuse sources, merging extractions from multiple observations, nonparametric broad-band photometry, analysis of low-count spectra, and automation of these tasks. Many of the innovations presented here arise from several, often interwoven, complications that are 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.

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arXiv2009

Methods for Estimating Fluxes and Absorptions of Faint X-ray Sources

X-ray sources with very few counts can be identified with low-noise X-ray detectors such as ACIS onboard the Chandra X-ray Observatory. These sources are often too faint for parametric spectral modeling using well-established methods such as fitting with XSPEC. We discuss the estimation of apparent and intrinsic broad-band X-ray fluxes and soft X-ray absorption from gas along the line of sight to these sources, using nonparametric methods. Apparent flux is estimated from the ratio of the source count rate to the instrumental effective area averaged over the chosen band. Absorption, intrinsic flux, and errors on these quantities are estimated from comparison of source photometric quantities with those of high S/N spectra that were simulated using spectral models characteristic of the class of astrophysical sources under study. The concept of 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. Our nonparametric method is tested against the apparent spectra of 2000 faint sources in the Chandra observation of the rich young stellar cluster in the M17 HII region. We show that the intrinsic X-ray properties can be determined with little bias and reasonable accuracy using these observable photometric quantities without employing often uncertain and time-consuming methods of non-linear parametric spectral modeling. Our method is calibrated for thermal spectra characteristic of stars in young stellar clusters, but recalibration should be possible for some other classes of faint X-ray sources such as extragalactic AGN.

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arXiv2009

Protoplanetary Disk Evolution around the Triggered Star Forming Region Cepheus B

The Cepheus B (CepB) molecular cloud and a portion of the nearby CepOB3b OB association, one of the most active regions of star formation within 1 kpc, have been observed with the IRAC detector on board the Spitzer Space Telescope. The goals are to study protoplanetary disk evolution and processes of sequential triggered star formation in the region. Out of ~400 pre-main sequence (PMS) stars selected with an earlier Chandra X-ray Observatory observation, 95% are identified with mid-infrared sources and most of these are classified as diskless or disk-bearing stars. The discovery of the additional >200 IR-excess low-mass members gives a combined Chandra+Spitzer PMS sample complete down to 0.5 Mo outside of the cloud, and somewhat above 1 Mo in the cloud. Analyses of the nearly disk-unbiased combined Chandra+Spitzer selected stellar sample give several results. Our major finding is a spatio-temporal gradient of young stars from the hot molecular core towards the primary ionizing O star HD 217086. This strongly supports the radiation driven implosion (RDI) model of triggered star formation in the region. The empirical estimate for the shock velocity of 1 km/s is very similar to theoretical models of RDI in shocked molecular clouds...ABRIDGED... Other results include: 1. agreement of the disk fractions, their mass dependency, and fractions of transition disks with other clusters; 2. confirmation of the youthfulness of the embedded CepB cluster; 3. confirmation of the effect of suppression of time-integrated X-ray emission in disk-bearing versus diskless systems.

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arXiv2008

X-ray flares in Orion young stars. I. Flare characteristics

Pre-main sequence (PMS) stars are known to produce powerful X-ray flares which resemble magnetic reconnection solar flares scaled by factors up to 10^4. However, numerous puzzles are present including the structure of X-ray emitting coronae and magnetospheres, effects of protoplanetary disks, and effects of stellar rotation. To investigate these issues in detail, we examine 216 of the brightest flares from 161 PMS stars observed in the Chandra Orion Ultradeep Project (COUP). These constitute the largest homogeneous dataset of PMS, or indeed stellar flares at any stellar age, ever acquired. Our effort is based on a new flare spectral analysis technique that avoids nonlinear parametric modeling. It can be applied to much weaker flares and is more sensitive than standard methods. We provide a catalog with >30 derived flare properties and an electronic atlas for this unique collection of stellar X-ray flares. The current study (Paper I) examines the flare morphologies, and provides general comparison of COUP flare characteristics with those of other active X-ray stars and the Sun. Paper II will concentrate on relationships between flare behavior, protoplanetary disks, and other stellar properties. Several results are obtained. First, the COUP flares studied here are among the most powerful, longest, and hottest stellar X-ray flares ever studied. Second, no significant statistical differences in peak flare luminosity or temperature distributions are found among different morphological flare classes, suggesting a common underlying mechanism for all flares. Third, comparison with the general solar-scaling laws indicates that COUP flares may not fit adequately proposed power-temperature and duration-temperature solar-stellar fits. Fourth, COUP super-hot flares are found to be brighter but shorter than ... ABRIDGED

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arXiv2008

X-Ray flares in Orion Young Stars. II. Flares, Magnetospheres, and Protoplanetary Disks

We study the properties of powerful X-ray flares from 161 pre-main sequence (PMS) stars observed with the Chandra X-ray Observatory in the Orion Nebula region. Relationships between flare properties, protoplanetary disks and accretion are examined in detail to test models of star-disk interactions at the inner edge of the accretion disks. Previous studies had found no differences in flaring between diskfree and accreting systems other than a small overall diminution of X-ray luminosity in accreting systems. The most important finding is that X-ray coronal extents in fast-rotating diskfree stars can significantly exceed the Keplerian corotation radius, whereas X-ray loop sizes in disky and accreting systems do not exceed the corotation radius. This is consistent with models of star-disk magnetic interaction where the inner disk truncates and confines the PMS stellar magnetosphere. We also find two differences between flares in accreting and diskfree PMS stars. First, a subclass of super-hot flares with peak plasma temperatures exceeding 100 MK are preferentially present in accreting systems. Second, we tentatively find that accreting stars produce flares with shorter durations. Both results may be consequences of the distortion and destabilization of the stellar magnetosphere by the interacting disk. Finally, we find no evidence that any flare types, even slow-rise flat-top flares are produced in star-disk magnetic loops. All are consistent with enhanced solar long-duration events with both footprints anchored in the stellar surface.

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arXiv2007

A Chandra Study of the Rosette Star-Forming Complex. I. The Stellar Population and Structure of the Young Open Cluster NGC 2244

We present the first high spatial resolution Chandra X-ray study of NGC 2244, the 2 Myr old stellar cluster immersed in the Rosette Nebula. Over 900 X-ray sources are detected; 77% have optical or FLAMINGOS near-infrared (NIR) stellar counterparts and are mostly previously uncatalogued young stellar cluster members. All known OB stars with spectral type earlier than B1 are detected and the X-ray selected stellar population is estimated to be nearly complete between 0.5 and 3 Msun. The X-ray luminosity function (XLF) ranges from 29.4<logLx<32 ergs/s in the hard (2-8keV) band. By comparing the NGC 2244 and Orion Nebula Cluster XLFs, we estimate a total population of 2000 stars in NGC 2244. A number of further results emerge from our analysis: The XLF and the associated K-band luminosity function indicate a normal Salpeter initial mass function (IMF) for NGC 2244. This is inconsistent with the top-heavy IMF reported from earlier optical studies that lacked a good census of <4Msun stars. The spatial distribution of X-ray stars is strongly concentrated around the central O5 star, HD 46150. The other early O star, HD 46223, has few companions. The cluster's stellar radial density profile shows two distinctive structures. This double structure, combined with the absence of mass segregation, indicates that this cluster is not in dynamical equilibrium. The spatial distribution of X-ray selected K-excess disk stars and embedded stars is asymmetric with an apparent deficit towards the north. The fraction of X-ray-selected cluster members with K-band excesses caused by inner protoplanetary disks is 6%, slightly lower than the 10% disk fraction estimated from the FLAMINGOS study based on the NIR-selected sample. This is due to the high efficiency of X-ray surveys in locating disk-free T Tauri stars.[Abridged]

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arXiv2006

The young stellar population in M17 revealed by Chandra

We report here results from a Chandra ACIS observation of the stellar populations in and around the Messier 17 H II region. The field reveals 886 sources, 771 of which have stellar counterparts in infrared images. In addition to comprehensive tables of X-ray source properties, several results are presented: * The X-ray Luminosity Function is calibrated to that of the Orion Nebula Cluster population to infer a total population of roughly 8000--10,000 stars * About 40% of the ACIS sources are heavily obscured with A_V > 10 mag. Some are concentrated around well-studied star-forming regions but most are distributed across the field. X-ray emission is detected from 64 of the hundreds of Class I protostar candidates that can be identified by near- and mid-infrared colors. These constitute the most likely protostar candidates known in M17. * The spatial distribution of X-ray stars is complex: we find a new embedded cluster, a 2 pc-long arc of young stars along the southwest edge of the M17 H II region, and 0.1 pc substructure within various populations. These structures may indicate that the populations are dynamically young. * All (14/14) of the known O stars but only about half (19/34) of the known B0--B3 stars in the M17 field are detected. Six of these stars exhibit very hard thermal plasma components (kT>4 keV) that may be due to colliding wind binaries. More than 100 candidate new OB stars are found, including 28 X-ray detected intermediate- and high-mass protostar candidates with infrared excesses. * Only a small fraction (perhaps 10%) of X-ray selected high- and intermediate- mass stars exhibit K-band emitting protoplanetary disks, providing further evidence that inner disks evolve very rapidly around more massive stars.

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arXiv2006

X-ray Study of Triggered Star Formation and Protostars in IC 1396N

The IC 1396N cometary globule within the large nearby HII region IC 1396 has been observed with the ACIS detector on board the Chandra X-ray Observatory. We detect 117 X-ray sources, of which ~50-60 are likely members of the young open cluster Trumpler~37 dispersed throughout the HII region, and 25 are associated with young stars formed within the globule. Spitzer/2MASS photometry shows the X-ray population is very young: 3 older Class III stars, 16 classical T Tauri stars, 6 protostars including a Class 0/I system. We infer a total T Tauri population of ~30 stars in the globule, including the undetected population, with a star formation efficiency of 1-4%. An elongated source spatial distribution with an age gradient oriented towards the exciting star is discovered in the X-ray population of IC 1396N, supporting similar findings in other cometary globules. The geometric and age distribution is consistent with the RDI model for triggered star formation in CGs by HII region shocks. The inferred velocity of the shock front propagating into the globule is ~0.6km/s. The large number of X-ray-luminous protostars in the globule suggests either an unusually high ratio of Class I/0 vs. Class II/III stars, or a non-standard IMF favoring higher mass stars by the triggering process. The Chandra source associated with the luminous Class 0/I protostar IRAS 21391+5802 is one of the youngest stars ever detected in the X-ray band. We also establish for the first time that the X-ray absorption in protostars arises from the local infalling envelopes rather than ambient molecular cloud material.

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arXiv2006

An X-ray Census of Young Stars in the Massive Southern Star-Forming Complex NGC 6357

We present the first high spatial resolution X-ray study of the massive star forming region NGC 6357, obtained in a 38 ks Chandra/ACIS observation. Inside the brightest constituent of this large HII region complex is the massive open cluster Pismis 24. It contains two of the brightest and bluest stars known, yet remains poorly studied; only a handful of optically bright stellar members have been identified. We investigate the cluster extent and Initial Mass Function and detect ~800 X-ray sources with a limiting sensitivity of 10^{30} ergs s^{-1}; this provides the first reliable probe of the rich intermediate-mass and low-mass population of this massive cluster, increasing the number of known members from optical study by a factor of ~50. The high luminosity end (log L_h[2-8 keV]\ge 30.3 ergs s^{-1}) of the observed X-ray luminosity function in NGC 6357 is clearly consistent with a power law relation as seen in the Orion Nebula Cluster and Cepheus B, yielding the first estimate of NGC 6357's total cluster population, a few times the known Orion population. We investigate the structure of the cluster, finding small-scale substructures superposed on a spherical cluster with 6 pc extent, and discuss its relationship to the nebular morphology. The long-standing Lx - 10^{-7}L_{bol} correlation for O stars is confirmed. Twenty-four candidate O stars and one possible new obscured massive YSO or Wolf-Rayet star are presented. Many cluster members are estimated to be intermediate-mass stars from available infrared photometry (assuming an age of 1 Myr), but only a few exhibit K-band excess. We report the first detection of X-ray emission from an Evaporating Gaseous Globule at the tip of a molecular pillar; this source is likely a B0-B2 protostar.

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arXiv2006

X-rays from HH210 in the Orion nebula

We report the detection during the Chandra Orion Ultradeep Project (COUP) of two soft, constant, and faint X-ray sources associated with the Herbig-Haro object HH210. HH210 is located at the tip of the NNE finger of the emission line system bursting out of the BN-KL complex, northwest of the Trapezium cluster in the OMC-1 molecular cloud. Using a recent Halpha image obtained with the ACS imager on board HST, and taking into account the known proper motions of HH210 emission knots, we show that the position of the brightest X-ray source, COUP703, coincides with the emission knot 154-040a of HH210, which is the emission knot of HH210 having the highest tangential velocity (425 km/s). The second X-ray source, COUP704, is located on the complicated emission tail of HH210 close to an emission line filament and has no obvious optical/infrared counterpart. Spectral fitting indicates for both sources a plasma temperature of ~0.8 MK and absorption-corrected X-ray luminosities of about 1E30 erg/s (0.5-2.0 keV). These X-ray sources are well explained by a model invoking a fast-moving, radiative bow shock in a neutral medium with a density of ~12000 cm^{-3}. The X-ray detection of COUP704 therefore reveals, in the complicated HH210 region, an energetic shock not yet identified at other wavelengths.

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arXiv2006

Chandra Study of the Cepheus B Star Forming Region: Stellar Populations and the Initial Mass Function

Cepheus B (Cep B) molecular cloud and a portion of the nearby Cep OB3b OB association, one of the most active regions of star formation within 1 kpc, has been observed with the ACIS detector on board the Chandra X-ray Observatory. We detect 431 X-ray sources, of which 89% are confidently identified as clustered pre-main sequence stars. Two main results are obtained. First, we provide the best census to date for the stellar population of the region. We identify many members of two rich stellar clusters: the lightly obscured Cep OB3b association, and the deeply embedded cluster in Cep B whose existence was previously traced only by a handful of radio sources and T Tauri stars. Second, we find a discrepancy between the X-ray Luminosity Functions of the Cep OB3b and the Orion Nebula Cluster. This may be due to different Initial Mass Functions of two regions (excess of ~0.3 solar mass stars), or different age distributions. Several other results are obtained. A diffuse X-ray component seen in the field is attributed to the integrated emission of unresolved low mass PMS stars. The X-ray emission from HD 217086 (O7n), the principle ionizing source of the region, follows the standard model involving many small shocks in an unmagnetized radiatively accelerated wind. The X-ray source #294 joins a number of similar superflare PMS stars where long magnetic structures may connect the protoplanetary disk to the stellar surface.

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arXiv2005

X-rays and young clusters: Membership, IMFs and distances

Sensitive imaging X-ray observations of young stellar clusters (YSCs, ages less than about 10 Myr) are valuable tools for the acquisition of an unbiased census of cluster members needed for Initial Mass Function (IMF) studies. Several dozen YSCs, both nearby and across the Galactic disk, have been observed with the Chandra and XMM-Newton satellites, detecting >10,000 low-mass cluster members. Many of these samples should be nearly complete down to about 1 Mo. An important additional benefit is that the YSC X-ray luminosity function appears to be universal with a lognormal shape, providing a new standard candle for measurement of YSC distances.

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arXiv2004

Chandra/ACIS Subpixel Event Repositioning. II. Further Refinements and Comparison between Backside and Front-side Illuminated X-ray CCDs

We further investigate subpixel event repositioning (SER) algorithms in application to Chandra X-ray Observatory (CXO) CCD imaging. SER algorithms have been applied to backside illuminated (BI) Advanced CCD Imaging Spectrometer (ACIS) devices, and demonstrate spatial resolution improvements in Chandra/ACIS observations. Here a new SER algorithm that is charge split dependent is added to the SER family. We describe the application of SER algorithms to frontside illuminated (FI) ACIS devices. The results of SER for FI CCDs are compared with those obtained from SER techniques applied to BI CCD event data. Both simulated data and Chandra/ACIS observations of the Orion Nebular Cluster were used to test and evaluate the achievement of the various SER techniques.

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arXiv2002

Chandra Study of Young Stellar Objects in the NGC 1333 Star-forming Cloud

NGC 1333, a highly active star formation region within the Perseus molecular cloud complex, has been observed with the ACIS-I detector on board the Chandra X-ray Observatory. In our image with a sensitivity limit of ~ 1e28 erg/s, we detect 127 X-ray sources, of which, 95 are known cluster members. The X-ray luminosity function of the discovered YSO population spans a range of 1e28-3.2e31 erg/s in the 0.5-8 keV band. Comparison with K-band source counts indicates that we detect all of the known cluster members with K < 12, and about half of members with K > 12. We detect seven X-ray emitting YSOs which drive optically visible jets as well as one deeply embedded object that has not been detected in near-IR observations. The presence or absence of an outflows does not appear to produce any difference in X-ray properties of YSOs. We also find no systematic differences in X-ray luminosity distributions between two complete subsamples of CTTs and WTTs. That suggests that there is no difference in the astrophysical mechanism of WTT and CTT X-ray emission production. Additional results include: the X-ray emission from two late-B stars which illuminate the reflection nebula originates from unresolved late-type companions; two T Tauri stars are discovered as previously unknown components of visual binaries; and the X-ray counterpart of SVS 16 has the column density much lower than that expected from near-IR photometry and thus its X-ray luminosity is not anomalously high, as has been previously suggested.

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