We have studied the X-ray point-source population of the 30 Doradus (30 Dor) star-forming complex in the Large Magellanic Cloud using high spatial resolution X-ray images and spatially resolved spectra obtained with the Advanced CCD Imaging Spectrometer (ACIS) on board the Chandra X-Ray Observatory. Here we describe the X-ray sources in a 17'x17' field centered on R136, the massive star cluster at the center of the main 30 Dor nebula. We detect 20 of the 32 Wolf-Rayet stars in the ACIS field. The cluster R136 is resolved at the subarcsecond level into almost 100 X-ray sources, including many typical O3-O5 stars, as well as a few bright X-ray sources previously reported.
We investigate the nature of the X-ray point source population within the Young Massive Cluster Westerlund 1. Chandra observations of 18 ks and 42 ks were used to determine the X-ray properties of emitters within Wd 1, while a comprehensive multiwavelength dataset was employed to constrain their nature. We find X-ray emission from a multitude of different stellar sources within Wd 1, including both evolved high mass and low mass pre-MS stars.
Rich young stellar clusters produce HII regions whose expansion into the nearby molecular cloud is thought to trigger the formation of new stars. However, the importance of this mode of star formation is uncertain. This investigation seeks to quantify triggered star formation (TSF) in IC 1396A (aka the Elephant Trunk Nebula), a bright-rimmed cloud (BRC) on the periphery of the nearby giant HII region IC 1396 produced by the Trumpler 37 cluster. X-ray selection of young stars from Chandra X-ray Observatory data is combined with existing optical and infrared surveys to give a more complete census of the TSF population. Over 250 young stars in and around IC 1396A are identified; this doubles the previously known population. A spatio-temporal gradient of stars from the IC 1396A cloud towards the primary ionizing star HD 206267 is found.
The distribution of young stars found in the Chandra Carina Complex Project (CCCP) is examined for clustering structure. X-ray surveys are advantageous for identifying young stellar populations compared to optical and infrared surveys in suffering less contamination from nebular emission and Galactic field stars. The analysis is based on smoothed maps of a spatially complete subsample of ~3000 brighter X-ray sources classified as Carina members and ~10000 stars from the full CCCP sample. The principal known clusters are recovered, and some additional smaller groups are identified. No rich embedded clusters are present, although a number of sparse groups are found. The CCCP reveals considerable complexity in clustering properties.
Coronal X-ray emission of the Pleiades stars is investigated, and maximum likelihood, integral X-ray luminosity functions are computed for Pleiades members in selected color-index ranges. A detailed search is conducted for long-term variability in the X-ray emission of those stars observed more than once. An overall comparison of the survey results with those of previous surveys confirms the ubiquity of X-ray emission in the Pleiades cluster stars and its higher rate of emission with respect to older stars. It is found that the X-ray emission from dA and early dF stars cannot be proven to be dissimilar to that of Hyades and field stars of the same spectral type. The Pleiades cluster members show a real rise of the X-ray luminosity from dA stars to early dF stars. X-ray emission for the young, solar-like Pleiades stars is about two orders of magnitude more intense than for the nearby solar-like stars.
We have conducted a detailed study of the object NGC 2451, which actually consists of two different open clusters A and B along the same line of sight at 206pc and 370pc distance, respectively. Although belonging to the nearest clusters, they have not been much investigated until present due to strong contamination by field stars. ROSAT X-ray observations and optical UBVR photometry are used to identify cluster members by means of X-ray emission and colour-magnitude diagrams. The identified stars concentrate nicely around the expected main sequences in the colour-magnitude diagram at the distances derived from astrometric investigations. Altogether, 39 stars are identified as member candidates of the nearer cluster A, 49 stars as member candidates of the more distant cluster B, and 22 faint stars are probably members of either of the two clusters, but due to large errors it is not clear to which one they belong. Further 40 stars identified with X-ray sources are probably non-members. For the first time, the range of known probable cluster members of NGC 2451 A and B has been extended downwards the main sequence to stars of spectral class M. Isochrone fitting yields an age of 50 to 80Myrs for NGC 2451 A and =~50Myrs for NGC 2451 B, consistent with the X-ray luminosity distribution functions, which are comparable to other clusters in the same age range. Except from the occurence of four flares, the stars of both clusters do not show strong long-term X-ray variability exceeding a factor 5 over a time span of 1 to 3 years.
We present the first high spatial resolution X-ray study of the massive star-forming region NGC 6357, obtained in a 38ks 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; 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.
NGC 6231 is a massive young star cluster, near the center of the Sco OB1 association. While its OB members are well studied, its low-mass population has received little attention. We present high-spatial resolution Chandra ACIS-I X-ray data, where we detect 1613 point X-ray sources. Our main aim is to clarify global properties of NGC 6231 down to low masses through a detailed membership assessment, and to study the cluster stars' spatial distribution, the origin of their X-ray emission, the cluster age and formation history, and initial mass function. We use X-ray data, complemented by optical and IR data, to establish cluster membership. The spatial distribution of different stellar subgroups also provides highly significant constraints on cluster membership, as does the distribution of X-ray hardness. We perform spectral modelling of group-stacked X-ray source spectra.
NGC 2516 has been observed by Chandra several times in order to correct the plate scale of the spacecraft's focal plane instruments. Because of this, Chandra has observed NGC 2516 with all four imaging arrangements available. In addition, NGC 2516 has been observed as part of the High Resolution Camera (HRC) guaranteed time program and is scheduled for return plate scale calibration visits. This makes it the best cluster to study for long-term variability. NGC 2516 is about 140Myr old and less than 400pc away. In our first paper, Harnden et al., 2001ApJ...547L.141H, we discussed the detection of 150 X-ray sources (42% of which are identified as cluster members) in the calibration data taken during the orbital activation phase of the Chandra mission. In our second paper, Damiani et al., 2003, Cat. <J/ApJ/588/1009>, we combined all the extant data sets and detected 284 sources, more than half of which are considered likely cluster members. In this our third paper, we further explore techniques of combining Advanced CCD Imaging Spectrometer (ACIS) and HRC Chandra data for timing analysis. We have been able to combine almost 70ks of observation time, spread over five epochs, to study variability in this cluster on multiple timescales. We find that while stochastic variability rates are about the same for all objects in the sample, the timescale for detecting variability is shorter for late-type stars. Both stochastic and flare variability rates seen in NGC 2516 are similar to those seen in younger clusters IC 348, NGC 1333, and M42.
IC348 is a nearby (~310pc), young (~2-3Myr) open cluster with >300 members identified from optical and infrared observations. We study the properties of the coronae of the young low-mass stars in IC348 combining X-ray and optical/infrared data. The four existing Chandra observations of IC348 are merged, thus providing a deeper and spatially more complete X-ray view than previous X-ray studies of the cluster. We have compiled a comprehensive catalog of IC348 members taking into account recent updates to the cluster census. Our data collection comprises fundamental stellar parameters, infrared excess indicating the presence of disks, Halpha emission as a tracer of chromospheric emission or accretion and mass accretion rates. We have detected 290 X-ray sources in four merged Chandra exposures, of which 187 are associated with known cluster members corresponding to a detection rate of ~60% for the cluster members of IC348 identified in optical/infrared studies. According to the most recent spectral classification of IC348 members only four of the X-ray sources are brown dwarfs (spectral type M6 and later). The detection rate is highest for diskless Class III stars and increases with stellar mass. This may be explained with higher X-ray luminosities for higher mass and later evolutionary stage that is evident in the X-ray luminosity functions. In particular, we find that for the lowest examined masses (0.1-0.25 Msun) there is a difference between the X-ray luminosity functions of accreting and non-accreting stars (classified on the basis of their Halpha emission strength) as well as those of disk-bearing and diskless stars (classified on the basis of the slope of the spectral energy distribution). These differences disappear for higher masses. This is related to our finding that the Lx/Lbol ratio is non-constant across the mass/luminosity sequence of IC348 with a decrease towards lower luminosity stars. Our analysis of an analogous stellar sample in the Orion Nebula Cluster suggests that the decline of Lx/Lbol for young stars at the low-mass end of the stellar sequence is likely universal.
