590 research outputs found
Protoplanetary Disk Evolution around the Triggered Star Forming Region Cepheus B
The Cepheus B (CepB) molecular cloud and a portion of the nearby CepOB3b OB
association, one of the most active regions of star formation within 1 kpc,
have been observed with the IRAC detector on board the Spitzer Space Telescope.
The goals are to study protoplanetary disk evolution and processes of
sequential triggered star formation in the region. Out of ~400 pre-main
sequence (PMS) stars selected with an earlier Chandra X-ray Observatory
observation, 95% are identified with mid-infrared sources and most of these are
classified as diskless or disk-bearing stars. The discovery of the additional
>200 IR-excess low-mass members gives a combined Chandra+Spitzer PMS sample
complete down to 0.5 Mo outside of the cloud, and somewhat above 1 Mo in the
cloud. Analyses of the nearly disk-unbiased combined Chandra+Spitzer selected
stellar sample give several results. Our major finding is a spatio-temporal
gradient of young stars from the hot molecular core towards the primary
ionizing O star HD 217086. This strongly supports the radiation driven
implosion (RDI) model of triggered star formation in the region. The empirical
estimate for the shock velocity of 1 km/s is very similar to theoretical models
of RDI in shocked molecular clouds...ABRIDGED... Other results include: 1.
agreement of the disk fractions, their mass dependency, and fractions of
transition disks with other clusters; 2. confirmation of the youthfulness of
the embedded CepB cluster; 3. confirmation of the effect of suppression of
time-integrated X-ray emission in disk-bearing versus diskless systems.Comment: Accepted for publication in The Astrophysical Journal. 48 pages, 14
figures. For a version with high-quality figures, see
http://www.astro.psu.edu/users/gkosta/RESEARCH/cepb_spitzer_chandra.pd
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 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
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
Research model robot-hexapod under static and dynamic loads
In the paper the stress-strain state of hexapod robot is considered in order to clarify its dynamical characteristics. Full-size model of hexapod robot is built in the SolidWorks program complex. The state of the robot was analyzed in an extremely dangerous location at static loading. Dynamic analysis was conducted to clarify oscillation of the support unit in the robot’s construction. The results of the survey show that such robot design cannot be used in the environments with the vibrating background below 5 Hz
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
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