1,171 research outputs found
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
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