694 research outputs found
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
The Chamaeleon II low-mass star-forming region: radial velocities, elemental abundances, and accretion properties
Radial velocities, elemental abundances, and accretion properties of members
of star-forming regions (SFRs) are important for understanding star and planet
formation. While infrared observations reveal the evolutionary status of the
disk, optical spectroscopy is fundamental to acquire information on the
properties of the central star and on the accretion characteristics. 2MASS
archive data and the Spitzer c2d survey of the Chamaeleon II dark cloud have
provided disk properties of a large number of young stars. We complement these
data with spectroscopy with the aim of providing physical stellar parameters
and accretion properties. We use FLAMES/UVES+GIRAFFE observations of 40 members
of Cha II to measure radial velocities through cross-correlation technique, Li
abundances by means of curves of growth, and for a suitable star elemental
abundances of Fe, Al, Si, Ca, Ti, and Ni using the code MOOG. From the
equivalent widths of the Halpha, Hbeta, and the HeI-5876, 6678, 7065 Angstrom
emission lines, we estimate the mass accretion rates, dMacc/dt, for all the
objects. We derive a radial velocity distribution for the Cha II stars
(=11.4+-2.0 km/s). We find dMacc/dt prop. to Mstar^1.3 and to Age^(-0.82)
in the 0.1-1.0 Msun mass regime, and a mean dMacc/dt for Cha II of ~7*10^(-10)
Msun/yr. We also establish a relationship between the HeI-7065 Angstrom line
emission and the accretion luminosity. The radial velocity distributions of
stars and gas in Cha II are consistent. The spread in dMacc/dt at a given
stellar mass is about one order of magnitude and can not be ascribed entirely
to short timescale variability. Analyzing the relation between dMacc/dt and the
colors in Spitzer and 2MASS bands, we find indications that the inner disk
changes from optically thick to optically thin at dMacc/dt~10^(-10) Msun/yr.
Finally, the disk fraction is consistent with the age of Cha II.Comment: 21 Pages, 15 Figures, 7 Tables. Accepted for publication in Astronomy
and Astrophysics. Abstract shortene
New distance measures for classifying X-ray astronomy data into stellar classes
The classification of the X-ray sources into classes (such as extragalactic
sources, background stars, ...) is an essential task in astronomy. Typically,
one of the classes corresponds to extragalactic radiation, whose photon
emission behaviour is well characterized by a homogeneous Poisson process. We
propose to use normalized versions of the Wasserstein and Zolotarev distances
to quantify the deviation of the distribution of photon interarrival times from
the exponential class. Our main motivation is the analysis of a massive dataset
from X-ray astronomy obtained by the Chandra Orion Ultradeep Project (COUP).
This project yielded a large catalog of 1616 X-ray cosmic sources in the Orion
Nebula region, with their series of photon arrival times and associated
energies. We consider the plug-in estimators of these metrics, determine their
asymptotic distributions, and illustrate their finite-sample performance with a
Monte Carlo study. We estimate these metrics for each COUP source from three
different classes. We conclude that our proposal provides a striking amount of
information on the nature of the photon emitting sources. Further, these
variables have the ability to identify X-ray sources wrongly catalogued before.
As an appealing conclusion, we show that some sources, previously classified as
extragalactic emissions, have a much higher probability of being young stars in
Orion Nebula.Comment: 29 page
The Massive Star Forming Region Cygnus OB2. I. Chandra catalogue of association members
We present a catalogue of 1696 X-ray sources detected in the massive star
forming region (SFR) Cygnus OB2 and extracted from two archival Chandra
observations of the center of the region. A deep source extraction routine,
exploiting the low background rates of Chandra observations was employed to
maximize the number of sources extracted. Observations at other wavelengths
were used to identify low count-rate sources and remove likely spurious
sources. Monte Carlo simulations were also used to assess the authenticity of
these sources. We used a Bayesian technique to identify optical or near-IR
counterparts for 1501 (89%) of our sources, using deep observations from the
INT Photometric H-alpha Survey, the Two Micron All Sky Survey, and the UKIRT
Infrared Deep Sky Survey-Galactic plane Survey. 755 (45%) of these objects have
six-band r', H-alpha, i', J, H, and K optical and near-IR photometry. From an
analysis of the Poisson false-source probabilities for each source we estimate
that our X-ray catalogue includes < 1% of false sources, and an even lower
fraction when only sources with optical or near-IR associations are considered.
A Monte Carlo simulation of the Bayesian matching scheme allows this method to
be compared to more simplified matching techniques and enables the various
sources of error to be quantified. The catalogue of 1696 objects presented here
includes X-ray broad band fluxes, model fits, and optical and near-IR
photometry in what is one of the largest X-ray catalogue of a single SFR to
date. The high number of stellar X-ray sources detected from relatively shallow
observations confirms the status and importance of Cygnus OB2 as one of our
Galaxy's most massive SFRs.Comment: Accepted for publication in ApJS. 39 pages, 5 figures, 5 tables (full
tables available in the published version or on request to the author
On the accretion properties of young stellar objects in the L1615/L1616 cometary cloud
We present the results of FLAMES/UVES and FLAMES/GIRAFFE spectroscopic
observations of 23 low-mass stars in the L1615/L1616 cometary cloud,
complemented with FORS2 and VIMOS spectroscopy of 31 additional stars in the
same cloud. L1615/L1616 is a cometary cloud where the star formation was
triggered by the impact of the massive stars in the Orion OB association. From
the measurements of the lithium abundance and radial velocity, we confirm the
membership of our sample to the cloud. We use the equivalent widths of the
H, H, and the HeI 5876, 6678, 7065
\AAemission lines to calculate the accretion luminosities, ,
and the mass accretion rates, . We find in L1615/L1616 a
fraction of accreting objects (), which is consistent with the
typical fraction of accretors in T associations of similar age ( Myr).
The mass accretion rate for these stars shows a trend with the mass of the
central object similar to that found for other star-forming regions, with a
spread at a given mass which depends on the evolutionary model used to derive
the stellar mass. Moreover, the behavior of the colors with indicates that strong accretors with dex show large excesses in the bands, as in previous
studies. We also conclude that the accretion properties of the L1615/L1616
members are similar to those of young stellar objects in T associations, like
Lupus.Comment: Accepted by Astronomy and Astrophysics. 17 pages, 11 figures, 6
table
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
The nature of the fluorescent iron line in V 1486 Ori
The fluorescent 6.4 keV iron line provides information on cool material in
the vicinity of hard X-ray sources as well as on the characteristics of the
X-ray sources themselves. First discovered in the X-ray spectra of the flaring
Sun, X-ray binaries and active galactic nuclei (AGN), the fluorescent line was
also observed in a number of stellar X-ray sources. The young stellar object
(YSO) V1486 Ori was observed in the framework of the Chandra Ultra Deep Project
(COUP) as the source COUP 331. We investigate its spectrum, with emphasis on
the strength and time variability of the fluorescent iron K-alpha line, derive
and analyze the light curve of COUP 331 and proceed with a time-resolved
spectral analysis of the observation. The light curve of V 1486 Ori shows two
major flares, the first one lasting for (approx) 20 ks with a peak X-ray
luminosity of 2.6*10^{32} erg/s (dereddened in the 1-10 keV band) and the
second one -- only partially observed -- for >60 ks with an average X-ray
luminosity of 2.4*10^{31} erg/s (dereddened). The spectrum of the first flare
is very well described by an absorbed thermal model at high temperature, with a
pronounced 6.7 keV iron line complex, but without any fluorescent K-alpha line.
The X-ray spectrum of the second flare is characterized by even higher
temperatures (>= 10 keV) without any detectable 6.7 keV Fe XXV feature, but
with a very strong fluorescent iron K-alpha line appearing predominantly in the
20 ks rise phase of the flare. Preliminary model calculations indicate that
photoionization is unlikely to account for the entire fluorescent emission
during the rise phase.Comment: 4 pages, letter, accepted for publication in A&
Multi-wavelength observing of a forming solar-like star
V2129 Oph is a 1.35 solar mass classical T Tauri star, known to possess a
strong and complex magnetic field. By extrapolating from an observationally
derived magnetic surface map, obtained through Zeeman-Doppler imaging, models
of V2129 Oph's corona have been constructed, and used to make predictions
regarding the global X-ray emission measure, the amount of modulation of X-ray
emission, and the density of accretion shocks. In late June 2009 we will under
take an ambitious multi-wavelength, multi-observing site, and near
contemporaneous campaign, combining spectroscopic optical, nIR, UV, X-ray,
spectropolarimetric and photometric monitoring. This will allow the validity of
the 3D field topologies derived via field extrapolation to be determined.Comment: 4 pages, proceedings of the 3rd MSSL workshop on High Resolution
X-ray Spectroscopy: towards IX
Variable X-ray emission from the accretion shock in the classical T Tauri star V2129 Ophiuchi
Context. The soft X-ray emission from high density plasma observed in several CTTS is usually associated with the accretion process. However, it is still unclear whether this high density âcoolâ plasma is heated in the accretion shock, or if it is coronal plasma fed or modified by the accretion process.
Aims. We conducted a coordinated quasi-simultaneous optical and X-ray observing campaign of the CTTS V2129 Oph. In this paper, we analyze Chandra grating spectrometer data and attempt to correlate the observed X-ray emitting plasma components with the characteristics of the accretion process and the stellar magnetic field constrained by simultaneous optical observations.
Methods. We analyze a 200 ks Chandra/HETGS observation, subdivided into two 100 ks segments, of the CTTS V2129 Oph. For the two observing segments corresponding to two different phases within one stellar rotation, we measure the density of the cool plasma component and the emission measure distribution.
Results. The X-ray emitting plasma covers a wide range of temperatures: from 2 up to 34 MK. The cool plasma component of V2129 Oph (T â 3â4 MK) varies between the two segments of the Chandra observation: during the first observing segment high density plasma (log N_c = 12.1_(-1.1)^(+0.6)) with high EM at ~3â4 MK is present, whereas, during the second segment, this plasma component has lower EM and lower density (logâN_e 3 R_â).
Conclusions. Our observation provides additional confirmation that the dense cool plasma at a few MK in CTTS is material heated in the accretion shock. The variability of this cool plasma component on V2129 Oph may be explained in terms of X-rays emitted in the accretion shock and seen with different viewing angles at the two rotational phases probed by our observation. In particular, during the first time interval a direct view of the shock region is possible, while, during the second, the accretion funnel itself intersects the line of sight to the shock region, preventing us from observing the accretion-driven X-rays
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