455 research outputs found
Gaia Stellar Kinematics in the Head of the Orion A Cloud: Runaway Stellar Groups and Gravitational Infall
This work extends previous kinematic studies of young stars in the Head of
the Orion A cloud (OMC-1/2/3/4/5). It is based on large samples of infrared,
optical, and X-ray selected pre-main sequence stars with reliable radial
velocities and Gaia-derived parallaxes and proper motions. Stellar kinematic
groups are identified assuming they mimic the motion of their parental gas.
Several groups are found to have peculiar kinematics: the NGC 1977 cluster and
two stellar groups in the Extended Orion Nebula (EON) cavity are caught in the
act of departing their birthplaces. The abnormal motion of NGC 1977 may have
been caused by a global hierarchical cloud collapse, feedback by massive Ori
OB1ab stars, supersonic turbulence, cloud-cloud collision, and/or slingshot
effect; the former two models are favored by us. EON groups might have
inherited anomalous motions of their parental cloudlets due to small-scale
`rocket effects' from nearby OB stars. We also identify sparse stellar groups
to the east and west of Orion A that are drifting from the central region,
possibly a slowly expanding halo of the Orion Nebula Cluster. We confirm
previously reported findings of varying line-of-sight distances to different
parts of the cloud's Head with associated differences in gas velocity.
Three-dimensional movies of star kinematics show contraction of the groups of
stars in OMC-1 and global contraction of OMC-123 stars. Overall, the Head of
Orion A region exhibits complex motions consistent with theoretical models
involving hierarchical gravitational collapse in (possibly turbulent) clouds
with OB stellar feedback.Comment: Accepted for publication in MNRAS. 26 pages, 13 figures. The two 3-D
stellar kinematic movies, aimed as Supplementary Materials, can be found on
YouTube at: https://youtu.be/B4GHCVvCYfo (`restricted' sample) and
https://youtu.be/6fUu8sP0QFI (`full' sample
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
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-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
The Massive Star-forming Regions Omnibus X-ray Catalog
We present the Massive Star-forming Regions (MSFRs) Omnibus X-ray Catalog
(MOXC), a compendium of X-ray point sources from {\em Chandra}/ACIS
observations of a selection of MSFRs across the Galaxy, plus 30 Doradus in the
Large Magellanic Cloud. MOXC consists of 20,623 X-ray point sources from 12
MSFRs with distances ranging from 1.7 kpc to 50 kpc. Additionally, we show the
morphology of the unresolved X-ray emission that remains after the catalogued
X-ray point sources are excised from the ACIS data, in the context of \Spitzer\
and {\em WISE} observations that trace the bubbles, ionization fronts, and
photon-dominated regions that characterize MSFRs. In previous work, we have
found that this unresolved X-ray emission is dominated by hot plasma from
massive star wind shocks. This diffuse X-ray emission is found in every MOXC
MSFR, clearly demonstrating that massive star feedback (and the
several-million-degree plasmas that it generates) is an integral component of
MSFR physics.Comment: Accepted to ApJS, March 3, 2014. 51 pages, 25 figure
X-raying the Beating Heart of a Newborn Star: Rotational Modulation of High-energy Radiation from V1647 Ori
We report a periodicity of ~1 day in the highly elevated X-ray emission from
the protostar V1647 Ori during its two recent multiple-year outbursts of mass
accretion. This periodicity is indicative of protostellar rotation at
near-breakup speed. Modeling of the phased X-ray light curve indicates the
high-temperature (~50 MK), X-ray-emitting plasma, which is most likely heated
by accretion-induced magnetic reconnection, resides in dense (>~5e10 cm-3),
pancake-shaped magnetic footprints where the accretion stream feeds the newborn
star. The sustained X-ray periodicity of V1647 Ori demonstrates that such
protostellar magnetospheric accretion configurations can be stable over
timescales of years.Comment: 26 pages, 10 figure
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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