511 research outputs found
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
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
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
VST processing facility: first astronomical applications
VST--Tube is a new software package designed to process optical astronomical
images. It is an automated pipeline to go from the raw exposures to fully
calibrated co-added images, and to extract catalogs with aperture and PSF
photometry. A set of tools allow the data administration and the quality check
of the intermediate and final products. VST-Tube comes with a Graphical User
Interface to facilitate the interaction between data and user. We outline here
the VST--Tube architecture and show some applications enlightening some of the
characteristics of the pipeline.Comment: Presented to the 54th Congress SAIt, 4-7 May 2010, Naples, Ital
KiDS0239-3211: A new gravitational quadruple lens candidate
We report the discovery of a candidate to quadrupole gravitationally lensed
system KiDS0239-3211 based on the public data release 3 of the KiDS survey and
machine learning techniques
X-Ray flares in Orion Young Stars. II. Flares, Magnetospheres, and Protoplanetary Disks
We study the properties of powerful X-ray flares from 161 pre-main sequence
(PMS) stars observed with the Chandra X-ray Observatory in the Orion Nebula
region. Relationships between flare properties, protoplanetary disks and
accretion are examined in detail to test models of star-disk interactions at
the inner edge of the accretion disks. Previous studies had found no
differences in flaring between diskfree and accreting systems other than a
small overall diminution of X-ray luminosity in accreting systems. The most
important finding is that X-ray coronal extents in fast-rotating diskfree stars
can significantly exceed the Keplerian corotation radius, whereas X-ray loop
sizes in disky and accreting systems do not exceed the corotation radius. This
is consistent with models of star-disk magnetic interaction where the inner
disk truncates and confines the PMS stellar magnetosphere. We also find two
differences between flares in accreting and diskfree PMS stars. First, a
subclass of super-hot flares with peak plasma temperatures exceeding 100 MK are
preferentially present in accreting systems. Second, we tentatively find that
accreting stars produce flares with shorter durations. Both results may be
consequences of the distortion and destabilization of the stellar magnetosphere
by the interacting disk. Finally, we find no evidence that any flare types,
even slow-rise flat-top flares are produced in star-disk magnetic loops. All
are consistent with enhanced solar long-duration events with both footprints
anchored in the stellar surface.Comment: Accepted for publication in ApJ (07/17/08); 46 pages, 14 figures, 2
table
X-shooter spectroscopy of young stellar objects in Lupus: Lithium, iron, and barium elemental abundances
With the purpose of performing a homogeneous determination of elemental
abundances for members of the Lupus T association, we analyzed three chemical
elements: lithium, iron, and barium. The aims were: to derive the Li abundance
for ~90% of known class II stars in the Lupus I, II, III, IV clouds; to perform
chemical tagging of a region where few Fe abundance measurements have been
obtained in the past, and no determination of the Ba content has been done up
to now. We also investigated possible Ba enhancement, as this element has
become increasingly interesting in the last years following the evidence of Ba
over-abundance in young clusters, the origin of which is still unknown. Using
X-shooter@VLT, we analyzed the spectra of 89 cluster members, both class II and
III stars. We measured the strength of the Li line and derived the abundance of
this element through equivalent width measurements and curves of growth. For
six class II stars we also measured the Fe and Ba abundances using the spectral
synthesis and the code MOOG. The veiling contribution was taken into account
for all three elements. We find a dispersion in the strength of the Li line at
low Teff and identify three targets with severe Li depletion. The nuclear age
inferred for these highly Li-depleted stars is around 15 Myr, which exceeds the
isochronal one. As in other star-forming regions, no metal-rich members are
found in Lupus, giving support to a recent hypothesis that the Fe abundance
distribution of most of the nearby young regions could be the result of a
common and widespread star formation episode involving the Galactic thin disk.
We find that Ba is over-abundant by ~0.7 dex with respect to the Sun. Since
current theoretical models cannot reproduce this Ba abundance pattern, we
investigated whether this unusually large Ba content might be related to
effects due to stellar parameters, stellar activity, and accretion.Comment: 15 pages, 14 figures, 3 tables; accepted for publication in A&A;
abstract shortene
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