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Start Over Subject open star clusters Publisher CDS
1361. X-ray classification of young star clusters
ID:
ivo://CDS.VizieR/J/ApJ/659/585
Title:
X-ray classification of young star clusters
Short Name:
J/ApJ/659/585
Date:
21 Oct 2021
Publisher:
CDS
Description:
A large volume of low signal-to-noise, multidimensional data is available from the CCD imaging spectrometers aboard the Chandra X-Ray Observatory and the X-Ray Multimirror Mission (XMM-Newton). To make progress analyzing this data, it is essential to develop methods to sort, classify, and characterize the vast library of X-ray spectra in a nonparametric fashion (complementary to current parametric model fits). We have developed a spectral classification algorithm that handles large volumes of data and operates independently of the requirement of spectral model fits. We use proven multivariate statistical techniques including principal component analysis and an ensemble classifier consisting of agglomerative hierarchical clustering and K-means clustering applied for the first time for spectral classification. The algorithm positions the sources in a multidimensional spectral sequence and then groups the ordered sources into clusters based on their spectra. These clusters appear more distinct for sources with harder observed spectra. The apparent diversity of source spectra is reduced to a three-dimensional locus in principal component space, with spectral outliers falling outside this locus. The algorithm was applied to a sample of 444 strong sources selected from the 1616 X-ray emitting sources detected in deep Chandra imaging spectroscopy of the Orion Nebula Cluster.
1362. X-ray detections in the {sigma} Ori cluster
ID:
ivo://CDS.VizieR/J/A+A/521/A45
Title:
X-ray detections in the {sigma} Ori cluster
Short Name:
J/A+A/521/A45
Date:
21 Oct 2021
Publisher:
CDS
Description:
We investigated the X-ray emission from young stars and brown dwarfs in the {sigma} Orionis cluster ({tau}=~3Ma, d=~385pc) and its relation to mass, the presence of circumstellar discs, and separation to the cluster centre by taking advantage of the superb spatial resolution of the Chandra X-ray Observatory.
1363. X-ray emission in Orion Nebula young stars
ID:
ivo://CDS.VizieR/J/ApJS/160/450
Title:
X-ray emission in Orion Nebula young stars
Short Name:
J/ApJS/160/450
Date:
21 Oct 2021
Publisher:
CDS
Description:
We investigate the spatial distribution of X-ray-emitting plasma in a sample of young Orion Nebula Cluster stars by modulation of their X-ray light curves due to stellar rotation. The study, part of the Chandra Orion Ultradeep Project (COUP, Cat. <J/ApJS/160/319>), is made possible by the exceptional length of the observation: 10 days of ACIS integration during a time span of 13 days, yielding a total of 1616 detected sources in the 17'x17' field of view. We here focus on a subsample of 233 X-ray-bright stars with known rotational periods. We search for X-ray modulation using the Lomb Normalized Periodogram method.
1364. X-ray emission in the Ursa Major stream.
ID:
ivo://CDS.VizieR/J/ApJ/351/492
Title:
X-ray emission in the Ursa Major stream.
Short Name:
J/ApJ/351/492
Date:
21 Oct 2021
Publisher:
CDS
Description:
Results are reported from a comprehensive survey of X-ray emission from stars known or suspected to be members of the UMa cluster and/or stream. Of the 42 UMa member stars surveyed, 18 were detected as X-ray sources, and spectral analysis was performed for 10 stars with sufficient X-ray counts. Consideration is given to relations between X-ray luminosity, color, and kinematics of the sample stars, and the X-ray spectra of the UMa stars are discussed in the context of the general problem of stellar X-ray temperatures. Also confirmed is the lack of X-ray-emitting A dwarfs among UMa members; among stars of later spectral type there is a rather large dispersion in X-ray luminosity. This dispersion cannot readily be explained by contamination with field star interlopers and appears rather to be a property of the UMa X-ray luminosity distribution function.
1365. X-ray-emitting young stars in the Orion nebula
ID:
ivo://CDS.VizieR/J/ApJ/574/258
Title:
X-ray-emitting young stars in the Orion nebula
Short Name:
J/ApJ/574/258
Date:
21 Oct 2021
Publisher:
CDS
Description:
The Orion Nebula Cluster (ONC) and the molecular cloud (OMC) in its vicinity have been observed with the ACIS-I detector on board the Chandra X-ray Observatory with 23hr exposure in two observations, on 1999 October 12 and 2000 April 1. We detect 1075 X-ray sources, most with sub-arcsecond positional accuracy. Ninety-one percent of the sources are spatially associated with known stellar members of the cluster, and an additional 7% are newly identified deeply embedded cloud members.
1366. X-ray flares in Orion young stars. I.
ID:
ivo://CDS.VizieR/J/ApJ/688/418
Title:
X-ray flares in Orion young stars. I.
Short Name:
J/ApJ/688/418
Date:
21 Oct 2021
Publisher:
CDS
Description:
Pre-main-sequence (PMS) stars are known to produce powerful X-ray flares, which resemble magnetic reconnection solar flares scaled by factors up to 104. However, there are numerous puzzles, including the structure of X-ray-emitting coronae and magnetospheres, the effects of protoplanetary disks, and the 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, Getman et al. Cat. J/ApJS/160/319). The current study (Paper I) examines the flare morphologies, and provides a general comparison of COUP flare characteristics with those of other active X-ray stars and the Sun.
1367. X-ray imaging of RCW 49
ID:
ivo://CDS.VizieR/J/ApJ/665/719
Title:
X-ray imaging of RCW 49
Short Name:
J/ApJ/665/719
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present the results of a high-resolution X-ray imaging study of the stellar population in the Galactic massive star-forming region RCW 49 and its central OB association Westerlund 2. We obtained a ~40ks X-ray image of a ~17'x17' field using the Chandra X-Ray Observatory and deep NIR images using the Infrared Survey Facility in a concentric ~8.3'x8.3' region. We detected 468 X-ray sources and identified optical, NIR, and Spitzer MIR counterparts for 379 of them.
1368. X-ray Luminosities in Blanco 1
ID:
ivo://CDS.VizieR/J/A+A/399/919
Title:
X-ray Luminosities in Blanco 1
Short Name:
J/A+A/399/919
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present X-ray Luminosity Distributions (XLDs) of late-type members (dF, dG, dK, dM) of the Blanco 1 cluster, based on ROSAT-HRI data and new astrometric-photometric membership obtained from the GSC-II project. For the first time we present the XLD of dM stars of this cluster. The high metallicity of Blanco 1 allows us to investigate the role of chemical composition on the coronal emission of late-type stars. Comparison between X-ray Luminosity Distributions of Blanco 1 and Pleiades, NGC 2516 and {alpha} Per suggests a possible metallicity effect in dM stars.
1369. X-ray luminosity functions of M37 low-mass stars
ID:
ivo://CDS.VizieR/J/ApJ/830/44
Title:
X-ray luminosity functions of M37 low-mass stars
Short Name:
J/ApJ/830/44
Date:
21 Oct 2021
Publisher:
CDS
Description:
We use a 440.5ks Chandra observation of the ~500Myr old open cluster M37 to derive the X-ray luminosity functions of its <=1.2M_{sun}_ stars. Combining detections of 162 M37 members with upper limits for 160 non-detections, we find that its G, K, and M stars have a similar median (0.5-7keV) X-ray luminosity L_X_=10^29.0^erg/s, whereas the L_X_-to-bolometric-luminosity ratio (L_X_/L_bol_) indicates that M stars are more active than G and K stars by ~1 order of magnitude at 500Myr. To characterize the evolution of magnetic activity in low-mass stars over their first ~600Myr, we consolidate X-ray and optical data from the literature for stars in six other open clusters: from youngest to oldest they are, the Orion Nebula Cluster (ONC), NGC 2547, NGC 2516, the Pleiades, NGC 6475, and the Hyades. For these, we homogenize the conversion of instrumental count rates to L_X_ by applying the same one-temperature emission model as for M37, and obtain masses using the same empirical mass-absolute magnitude relation (except for the ONC). We find that for G and K stars X-ray activity decreases ~2 orders of magnitude over their first 600Myr, and for M stars, ~1.5. The decay rate of the median L_X_ follows the relation L_X_{propto}t^b^, where b=-0.61+/-0.12 for G stars, -0.82+/-0.16 for K stars, and -0.40+/-0.17 for M stars. In L_X_/L_bol_ space, the slopes are -0.68+/-0.12, -0.81+/-0.19, and -0.61+/-0.12, respectively. These results suggest that for low-mass stars the age-activity relation steepens after ~625Myr, consistent with the faster decay in activity observed in solar analogs at t>1Gyr.
1370. X-ray obs. and membership probabilities of M37
ID:
ivo://CDS.VizieR/J/ApJ/809/161
Title:
X-ray obs. and membership probabilities of M37
Short Name:
J/ApJ/809/161
Date:
21 Oct 2021
Publisher:
CDS
Description:
Empirical calibrations of the stellar age-rotation-activity relation (ARAR) rely on observations of the co-eval populations of stars in open clusters. We used the Chandra X-ray Observatory to study M37, a 500-Myr-old open cluster that has been extensively surveyed for rotation periods (P_rot_). M37 was observed almost continuously for five days, for a total of 440.5ks, to measure stellar X-ray luminosities (L_X_), a proxy for coronal activity, across a wide range of masses. The cluster's membership catalog was revisited to calculate updated membership probabilities from photometric data and each star's distance to the cluster center. The result is a comprehensive sample of 1699 M37 members: 426 with P_rot_, 278 with X-ray detections, and 76 with both. We calculate Rossby numbers, R_0_=P_rot_/{tau}, where {tau} is the convective turnover time, and ratios of the X-ray-to-bolometric luminosity, L_X_/L_bol_, to minimize mass dependencies in our characterization of the rotation-coronal activity relation at 500Myr. We find that fast rotators, for which R_0_<0.09+/-0.01, show saturated levels of activity, with log(L_X_/L_bol_)=-3.06+/-0.04. For R_0_>=0.09+/-0.01, activity is unsaturated and follows a power law of the form R_0_^{beta}^, where {beta}=-2.03^+0.17^_-1.14_. This is the largest sample available for analyzing the dependence of coronal emission on rotation for a single-aged population, covering stellar masses in the range 0.4-1.3M_{sun}_, P_rot_ in the range 0.4-12.8 days, and L_X_ in the range 10^28.4-30.5^erg/s. Our results make M37 a new benchmark open cluster for calibrating the ARAR at ages of ~500Myr.

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