Stellar X-ray sources in the Chandra COSMOS survey

We present an analysis of the X-ray properties of a sample of solar- and late-type field stars identified in the Chandra Cosmic Evolution Survey (COSMOS), a deep (160ks) and wide (0.9 deg2) extragalactic survey. The sample of 60 sources was identified using both morphological and photometric star/galaxy separation methods. We determine X-ray count rates, extract spectra and light curves and perform spectral fits to determine fluxes and plasma temperatures. Complementary optical and near-IR photometry is also presented and combined with spectroscopy for 48 of the sources to determine spectral types and distances for the sample. We find distances ranging from 30pc to ~12kpc, including a number of the most distant and highly active stellar X-ray sources ever detected. This stellar sample extends the known coverage of the L_X-distance plane to greater distances and higher luminosities, but we do not detect as many intrinsically faint X-ray sources compared to previous surveys. Overall the sample is typically more luminous than the active Sun, representing the high-luminosity end of the disk and halo X-ray luminosity functions. The halo population appears to include both low-activity spectrally hard sources that may be emitting through thermal bremsstrahlung, as well as a number of highly active sources in close binaries.Comment: 13 pages, 5 figures, accepted for publication in Ap

A deep XMM-Newton X-ray observation of the Chamaeleon I dark cloud

Methods. The northern-eastern fringe of the Chameleon I dark cloud was observed with XMM-Newton, revisiting a region observed with ROSAT 15 years ago. Centered on the extended X-ray source CHXR49 we are able to resolve it into three major contributing components and to analyse their spectral properties. Furthermore, the deep exposure allows not only the detection of numerous, previously unknown X-ray sources, but also the investigation of variability and the study of the X-ray properties for the brighter targets in the field. We use EPIC spectra, to determine X-ray brightness, coronal temperatures and emission measures for these sources, compare the properties of classical and weak-line T Tauri stars and make a comparison with results from the ROSAT observation. Results. X-ray properties of T Tauri stars in Cha I are presented. The XMM-Newton images resolve some previously blended X-ray sources, confirm several possible ones and detect many new X-ray targets, resulting in the most comprehensive list with 71 X-ray sources in the northern Cha I dark cloud. The analysis of medium resolution spectra shows an overlapping distribution of spectral properties for classical and weak-line T Tauri stars, with the X-ray brighter stars having hotter coronae and a higher L_X/L_bol ratio. X-ray luminosity correlates with bolometric luminosity, whereas the L_X/L_bol ratio is slightly lower for the classical T Tauri stars. Large flares as well as a low iron and a high neon abundance are found in both types of T Tauri stars. Abundance pattern, plasma temperatures and emission measure distributions during quiescent phases are attributed toa high level of magnetic activity as the dominant source of their X-ray emission.Comment: 10 pages, 11 figures, accepted by A&

Chandra/ACIS-I study of the X-ray properties of the NGC 6611 and M16 stellar population

Mechanisms regulating the origin of X-rays in YSOs and the correlation with their evolutionary stage are under debate. Studies of the X-ray properties in young clusters allow to understand these mechanisms. One ideal target for this analysis is the Eagle Nebula (M16), with its central cluster NGC6611. At 1750 pc from the Sun, it harbors 93 OB stars, together with a population of low-mass stars from embedded protostars to disk-less Class III objects, with age <= 3Myrs. We study an archival 78 ksec Chandra/ACIS-I observation of NGC6611, and two new 80ksec observations of the outer region of M16, one centered on the Column V, and one on a region of the molecular cloud with ongoing star-formation. We detect 1755 point sources, with 1183 candidate cluster members (219 disk-bearing and 964 disk-less). We study the global X-ray properties of M16 and compare them with those of the Orion Nebula Cluster. We also compare the level of X-ray emission of Class II and Class III stars, and analyze the X-ray spectral properties of OB stars. Our study supports the lower level of X-ray activity for the disk-bearing stars with respect to the disk-less members. The X-ray Luminosity Function (XLF) of M16 is similar to that of Orion, supporting the universality of the XLF in young clusters. 85% of the O stars of NGC6611 have been detected in X-rays. With only one possible exception, they show soft spectra with no hard component, indicating that mechanisms for the production of hard X-ray emission in O stars are not operating in NGC 6611.Comment: Accepted in Ap

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.Comment: 5 pages, 3 figures; accepted for publication in A&A Letter

A 1.3 cm wavelength radio flare from a deeply embedded source in the Orion BN/KL region

Aims: Our aim was to measure and characterize the short-wavelength radio emission from young stellar objects (YSOs) in the Orion Nebula Cluster and the BN/KL star-forming region. Methods: We used the NRAO Very Large Array at a wavelength of 1.3 cm and we studied archival X-ray, infrared, and radio data. Results: During our observation, a strong outburst (flux increasing >10 fold) occurred in one of the 16 sources detected at a wavelength of 1.3cm, while the others remained (nearly) constant. This source does not have an infrared counterpart, but has subsequently been observed to flare in X-rays. Curiously, a very weak variable double radio source was found at other epochs near this position, one of whose components is coincident with it. A very high extinction derived from modeling the X-ray emission and the absence of an infrared counterpart both suggest that this source is very deeply embedded.Comment: 7 pages, 5 figures, accepted for publication in A&

Simultaneous X-ray, radio, near-infrared, and optical monitoring of Young Stellar Objects in the Coronet cluster

Multi-wavelength (X-ray to radio) monitoring of Young Stellar Objects (YSOs) can provide important information about physical processes at the stellar surface, in the stellar corona, and/or in the inner circumstellar disk regions. While coronal processes should mainly cause variations in the X-ray and radio bands, accretion processes may be traced by time-correlated variability in the X-ray and optical/infrared bands. Several multi-wavelength studies have been successfully performed for field stars and approx. 1-10 Myr old T Tauri stars, but so far no such study succeeded in detecting simultaneous X-ray to radio variability in extremely young objects like class I and class 0 protostars. Here we present the first simultaneous X-ray, radio, near-infrared, and optical monitoring of YSOs, targeting the Coronet cluster in the Corona Australis star-forming region, which harbors at least one class 0 protostar, several class I objects, numerous T Tauri stars, and a few Herbig AeBe stars. [...] Seven objects are detected simultaneously in the X-ray, radio, and optical/infrared bands; they constitute our core sample. While most of these sources exhibit clear variability in the X-ray regime and several also display optical/infrared variability, none of them shows significant radio variability on the timescales probed. We also do not find any case of clearly time-correlated optical/infrared and X-ray variability. [...] The absence of time-correlated multi-wavelength variability suggests that there is no direct link between the X-ray and optical/infrared emission and supports the notion that accretion is not an important source for the X-ray emission of these YSOs. No significant radio variability was found on timescales of days.Comment: 11 pages, 11 figures, accepted for publication in A&A (06 Dec 2006

A Chandra ACIS Study of the Young Star Cluster Trumpler 15 in Carina and Correlation with Near-infrared Sources

Using the highest-resolution X-ray observation of the Trumpler 15 star cluster taken by the Chandra X-ray Observatory, we estimate the total size of its stellar population by comparing the X-ray luminosity function of the detected sources to a calibrator cluster, and identify for the first time a significant fraction (~14%) of its individual members. The highest-resolution near-IR observation of Trumpler 15 (taken by the HAWK-I instrument on the VLT) was found to detect most of our X-ray selected sample of cluster members, with a K-excess disk frequency of 3.8+-0.7%. The near-IR data, X-ray luminosity function, and published spectral types of the brightest members support a cluster age estimate (5-10 Myr) that is older than those for the nearby Trumpler 14 and Trumpler 16 clusters, and suggest that high-mass members may have already exploded as supernovae. The morphology of the inner ~0.7 pc core of the cluster is found to be spherical. However, the outer regions (beyond 2 pc) are elongated, forming an `envelope' of stars that, in projection, appears to connect Trumpler 15 to Trumpler 14; this morphology supports the view that these clusters are physically associated. Clear evidence of mass segregation is seen. This study appears in a Special Issue of the ApJS devoted to the Chandra Carina Complex Project (CCCP), a 1.42 square degree Chandra X-ray survey of the Great Nebula in Carina.Comment: Accepted for the ApJS Special Issue on the Chandra Carina Complex Project (CCCP), scheduled for publication in May 2011. All 16 CCCP Special Issue papers are available at http://cochise.astro.psu.edu/Carina_public/special_issue.html through 2011 at least. 30 pages; 8 figures; 3 table