152 research outputs found
MALT90 molecular content on high-mass IR-dark clumps
High mass stars form in groups or clusters within massive cores in dense
molecular clumps with sizes of 1pc and masses of 200Msun which are important
laboratories for high-mass star formation in order to study the initial
conditions. We investigate the physical and chemical properties of high-mass
clumps in order to better understand the early evolutionary stages and find
targets that show star formation signs. We selected the high-mass clumps from
ATLASGAL survey that were identified as dark at 8/24m wavelengths and used
MALT90 data which provides a molecular line set to investigate the physical and
chemical conditions in early evolutionary stages. Eleven sources have
significant SiO detection (over 3) which usually indicates outflow
activities. Thirteen sources are found with blue profiles in both or either
HCO and/or HNC lines and clump mass infall rates are estimated to be in the
range of 0.2E+3 Msunyr 1.8E-2 Msunyr. The excitation
temperature is obtained as <24K for all sources. The column densities for
optically thin lines of HCO and HNC are in the range of
0.4-8.8(E+12) cm, and 0.9-11.9(E+12) cm, respectively, while it
is in the range of 0.1-7.5(E+14) cm for HCO and HNC lines. The
column densities for NH were ranging between 4.4-275.7(E+12)
cm as expected from cold dense regions. Large line widths of
NH might indicate turbulence and large line widths of HCO,
HNC, and SiO indicate outflow activities. Mean optical depths are 20.32, and
23.19 for optically thick HCO and HCN lines, and 0.39 and 0.45 for their
optically thin isotopologues HCO and HNC, respectively.
This study reveals the physical and chemical properties of 30 high-mass IR-dark
clumps and the interesting targets among them based on their emission line
morphology and kinematics.Comment: 59 pages, 11 figures, Accepted for publication in A &
The XMM-Newton Optical Monitor Survey of the Taurus Molecular Cloud
The Optical Monitor (OM) on-board XMM-Newton obtained optical/ultraviolet
data for the XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST),
simultaneously with the X-ray detectors. With the XEST OM data, we aim to study
the optical and ultraviolet properties of TMC members, and to do correlative
studies between the X-ray and OM light curves. In particular, we aim to
determine whether accretion plays a significant role in the optical/ultraviolet
and X-ray emissions. The Neupert effect in stellar flares is also investigated.
Coordinates, average count rates and magnitudes were extracted from OM images,
together with light curves with low time resolution (a few kiloseconds). For a
few sources, OM FAST mode data were also available, and we extracted OM light
curves with high time resolution. The OM data were correlated with Two Micron
All Sky Survey (2MASS) data and with the XEST catalogue in the X-rays. The XEST
OM catalogue contains 2,148 entries of which 1,893 have 2MASS counterparts.
However, only 98 entries have X-ray counterparts, of which 51 of them are known
TMC members and 12 additional are TMC candidates. The OM data indicate that
accreting stars are statistically brighter in the U band than non-accreting
stars after correction for extinction, and have U-band excesses, most likely
due to accretion. The OM emission of accreting stars is variable, probably due
to accretion spots, but it does not correlate with the X-ray light curve,
suggesting that accretion does not contribute significantly to the X-ray
emission of most accreting stars. In some cases, flares were detected in both
X-ray and OM light curves and followed a Neupert effect pattern, in which the
optical/ultraviolet emission precedes the X-ray emission of a flare, whereas
the X-ray flux is proportional to the integral of the optical flux.Comment: Accepted by A&A, to appear in a special section/issue dedicated to
the XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST). Version
with higher resolution figures available at this
http://www.issibern.ch/teams/Taurus/papers.htm
Accretion bursts in magnetized gas-dust protoplanetary disks
Aims and Methods. Accretion bursts triggered by the magnetorotational
instability (MRI) in the innermost disk regions were studied for protoplanetary
gas-dust disks formed from prestellar cores of various mass and
mass-to-magnetic flux ratio . Numerical magnetohydrodynamics
simulations in the thin-disk limit were employed to study the long-term (~Myr) evolution of protoplanetary disks with an adaptive turbulent
-parameter, which depends explicitly on the strength of the magnetic
field and ionization fraction in the disk. The numerical models also feature
the co-evolution of gas and dust, including the back-reaction of dust on gas
and dust growth. Results. Dead zone with a low ionization fraction and temperature on the order of several hundred Kelvin forms in the
inner disk soon after its formation, extending from several to several tens of
astronomical units depending on the model. The dead zone features pronounced
dust rings that are formed due to the concentration of grown dust particles in
the local pressure maxima. Thermal ionization of alkaline metals in the dead
zone trigger the MRI and associated accretion burst, which is characterized by
a sharp rise, small-scale variability in the active phase, and fast decline
once the inner MRI-active region is depleted of matter. The burst occurrence
frequency is highest in the initial stages of disk formation, and is driven by
gravitational instability (GI), but declines with diminishing disk mass-loading
from the infalling envelope. There is a causal link between the initial burst
activity and the strength of GI in the disk fueled by mass infall from the
envelope. Abridged.Comment: Accepted for publication in Astronomy & Astrophysic
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