367 research outputs found
On the trapping of stars by a newborn stellar supercluster
Numerical experiments conducted by Fellhauer et al. (MNRAS, 372, 338, 2006)
suggest that a supercluster may capture up to about 40 per cent of its mass
from the galaxy where it belongs. Nevertheless, in those experiments the
cluster was created making appear its mass out of nothing, rather than from
mass already present in the galaxy. Here we use a thought experiment, plus a
few simple computations, to show that the difference between the dynamical
effects of these two scenarios (i.e., mass creation vs. mass concentration) is
actually very important. We also present the results of new numerical
experiments, simulating the formation of the cluster through mass
concentration, that show that trapping depends critically on the process of
cluster formation and that the amounts of gained mass are substantially smaller
than those obtained from mass creation.Comment: 6 pages, 3 figures. Submitted to MNRA
On the unique possibility to increase significantly the contrast of dark resonances on D1 line of Rb
We propose and study, theoretically and experimentally, a new scheme of
excitation of a coherent population trapping resonance for D1 line of alakli
atoms with nuclear spin by bichromatic linearly polarized light ({\em
lin}{\em lin} field) at the conditions of spectral resolution of the
excited state. The unique properties of this scheme result in a high contrast
of dark resonance for D1 line of Rb.Comment: 9 pages, 7 figures. This material has been partially presented on
ICONO-2005, 14 May 2005, St. Petersburg, Russia. v2 references added; text is
changed a bi
High-resolution study of a star-forming cluster in the Cep-A HW2 region
Due to its relatively small distance (725 pc), the Cepheus A East
star-forming region is an ideal laboratory to study massive star formation
processes. Based on its morphology, it has been suggested that the flattened
molecular gas distribution around the YSO HW2 may be a 350-AU-radius massive
protostellar disk. Goal of our work is to ascertain the nature of this
structure. We have employed the Plateau de Bure Interferometer to acquire
(sub-)arcsecond-resolution imaging of high-density and shock tracers, such as
methyl cyanide (CH3CN) and silicon monoxide (SiO), towards the HW2 position. On
the 1-arcsecond (about 725 AU) scale, the flattened distribution of molecular
gas around HW2 appears to be due to the projected superposition, on the plane
of the sky, of at least three protostellar objects, of which at least one is
powering a molecular outflow at a small angle with respect to the line of
sight. The presence of a protostellar disk around HW2 is not ruled out, but
such structure is likely to be detected on a smaller spatial scale, or using
different molecular tracers.Comment: 6 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
The Distribution of Stellar Mass in the Pleiades
As part of an effort to understand the origin of open clusters, we present a
statistical analysis of the currently observed Pleiades. Starting with a
photometric catalog of the cluster, we employ a maximum likelihood technique to
determine the mass distribution of its members, including single stars and both
components of binary systems. We find that the overall binary fraction for
unresolved pairs is 68%. Extrapolating to include resolved systems, this
fraction climbs to about 76%, significantly higher than the accepted field-star
result. Both figures are sensitive to the cluster age, for which we have used
the currently favored value of 125 Myr. The primary and secondary masses within
binaries are correlated, in the sense that their ratios are closer to unity
than under the hypothesis of random pairing. We map out the spatial variation
of the cluster's projected and three-dimensional mass and number densities.
Finally, we revisit the issue of mass segregation in the Pleiades. We find
unambiguous evidence of segregation, and introduce a new method for quantifying
it.Comment: 41 pages, 14 figures To Be Published in The Astrophysical Journa
The Collimated Jet Source in IRAS 16547-4247: Time Variation, Possible Precession, and Upper Limits to the Proper Motions Along the Jet Axis
The triple radio source detected in association with the luminous infrared
source IRAS 16547-4247 has previously been studied with high angular resolution
and high sensitivity with the Very Large Array (VLA) at 3.6-cm wavelength. In
this paper, we present new 3.6 cm observations taken 2.68 years after the first
epoch that allow a search for variability and proper motions, as well as the
detection of additional faint sources in the region. We do not detect proper
motions along the axis of the outflow in the outer lobes of this source at a
4- upper limit of 160 km s. This suggests that these lobes
are probably working surfaces where the jet is interacting with a denser
medium. However, the brightest components of the lobes show evidence of
precession, at a rate of yr clockwise in the plane of
the sky. It may be possible to understand the distribution of almost all the
identified sources as the result of ejecta from a precessing jet. The core of
the thermal jet shows significant variations in flux density and morphology. We
compare this source with other jets in low and high mass young stars and
suggest that the former can be understood as a scaled-up version of the latter.Comment: 26 pages, 9 figure
Simulations of protostellar collapse using multigroup radiation hydrodynamics. I. The first collapse
Radiative transfer plays a major role in the process of star formation. Many
simulations of gravitational collapse of a cold gas cloud followed by the
formation of a protostellar core use a grey treatment of radiative transfer
coupled to the hydrodynamics. However, dust opacities which dominate extinction
show large variations as a function of frequency. In this paper, we used
frequency-dependent radiative transfer to investigate the influence of the
opacity variations on the properties of Larson's first core. We used a
multigroup M1 moment model in a 1D radiation hydrodynamics code to simulate the
spherically symmetric collapse of a 1 solar mass cloud core. Monochromatic dust
opacities for five different temperature ranges were used to compute Planck and
Rosseland means inside each frequency group. The results are very consistent
with previous studies and only small differences were observed between the grey
and multigroup simulations. For a same central density, the multigroup
simulations tend to produce first cores with a slightly higher radius and
central temperature. We also performed simulations of the collapse of a 10 and
0.1 solar mass cloud, which showed the properties of the first core to be
independent of the initial cloud mass, with again no major differences between
grey and multigroup models. For Larson's first collapse, where temperatures
remain below 2000 K, the vast majority of the radiation energy lies in the IR
regime and the system is optically thick. In this regime, the grey
approximation does a good job reproducing the correct opacities, as long as
there are no large opacity variations on scales much smaller than the width of
the Planck function. The multigroup method is however expected to yield more
important differences in the later stages of the collapse when high energy (UV
and X-ray) radiation is present and matter and radiation are strongly
decoupled.Comment: 9 pages, 5 figures, accepted for publication in A&
The Initial Mass Function of the Stellar Association NGC 602 in the Small Magellanic Cloud with Hubble Space Telescope ACS Observations
We present our photometric study of the stellar association NGC 602 in the
wing of the Small Magellanic Cloud (SMC). The data were taken in the filters
F555W and F814W using the Advanced Camera for Surveys (ACS) on-board the Hubble
Space Telescope (HST). Photometry was performed using the ACS module of the
stellar photometry package DOLPHOT. We detected more than 5,500 stars with a
magnitude range of 14 \lsim m_{555} \lsim 28 mag. Three prominent stellar
concentrations are identified with star counts in the observed field, the
association NGC 602 itself, and two clusters, one of them not being currently
in any known catalog. The Color-Magnitude Diagrams (CMDs) of both clusters show
features typical for young open clusters, while that of the association reveals
bright main sequence (MS) and faint pre-main sequence (PMS) stars as the
members of the system. We construct the initial mass spectrum (IMS) of the
association by applying an age-independent method of counting the PMS stars
within evolutionary tracks, while for the bright MS stars we transform their
magnitudes to masses with the use of mass-luminosity relations. The IMS of NGC
602 is found to be well represented by a single-power law, corresponding to an
Initial Mass Function (IMF) of slope \Gamma\approx -1.2 for 1 \lsim M/M{\solar}
\lsim 45. This indicates that the shape of the IMF of a star forming system in
the SMC for stars with masses higher than 1 M{\solar} seems to be quite similar
to the field IMF in the solar neighborhood.Comment: Accepted for publication in ApJ, 13 pages, 14 figures, emulateapj.cls
LaTeX style, full resolution version available on
http://www.astro.uni-bonn.de/~dgoulier/Science/NGC602/ms.pd
Trapped Protostellar Winds and their Breakout
Observations show that high-velocity jets stem from deeply embedded young
stars, which may still be experiencing infall from their parent cloud cores.
Yet theory predicts that, early in this buildup, any outgoing wind is trapped
by incoming material of low angular momentum. As collapse continues and brings
in more rapidly rotating gas, the wind can eventually break out. Here we model
this transition by following the motion of the shocked shell created by impact
of the wind and a rotating, collapsing envelope. We first demonstrate, both
analytically and numerically, that our previous, quasi-static solutions are
dynamically unstable. Our present, fully time-dependent calculations include
cases both where the wind is driven back by infall to the stellar surface, and
where it erupts as a true outflow. For the latter, we find that the time of
breakout is sim 50,000 yr for wind speeds of 200 km/s. The reason for the delay
is that the shocked material, including the swept-up infall, must be able to
climb out of the star's gravitational potential well.
We explore the critical wind speed necessary for breakout as a function of
the mass transport rates in the wind and infall, as well as the cloud rotation
rate Omega0 and time since the start of infall. Breakout does occur for
realistic parameter choices. The actual breakout times would change if we
relaxed the assumption of perfect mixing between the wind and infall material.
Our expanding shells do not exhibit the collimation of observed jets, but
continue to expand laterally. To halt this expansion, the density in the
envelope must fall off less steeply than in our model.Comment: 44 pages, 10 figures, accepted to Ap
Star Formation in the Milky Way and Nearby Galaxies
We review progress over the past decade in observations of large-scale star
formation, with a focus on the interface between extragalactic and Galactic
studies. Methods of measuring gas contents and star formation rates are
discussed, and updated prescriptions for calculating star formation rates are
provided. We review relations between star formation and gas on scales ranging
from entire galaxies to individual molecular clouds.Comment: 55 pages, 15 figures, in press for Annual Reviews of Astronomy and
Astrophysics; Updated with corrected equation 5, improved references, and
other minor change
The near-infrared reflected spectrum of source I in Orion-KL
Source I in the Orion-KL nebula is believed to be the nearest example of a
massive star still in the main accretion phase. It is thus one of the best
cases for studying the properties of massive protostars to constrain high-mass
star formation theories. Near-infrared radiation from source I escapes through
the cavity opened by the OMC1 outflow and is scattered by dust towards our line
of sight. The reflected spectrum offers a unique possibility of observing the
emission from the innermost regions of the system and probing the nature of
source I and its immediate surroundings. We obtained moderately high
spectral-resolution (R~9000) observations of the near-infrared diffuse emission
in several locations around source I/Orion-KL. We observed a widespread rich
absorption line spectrum that we compare with cool stellar photospheres and
protostellar accretion disk models. The spectrum is broadly similar to strongly
veiled, cool, low-gravity stellar photospheres in the range Teff~3500-4500 K,
luminosity class I-III. An exact match explaining all features has not been
found, and a plausible explanation is that a range of different temperatures
contribute to the observed absorption spectrum. The 1D velocity dispersions
implied by the absorption spectra, sigma~30 km/s, can be explained by the
emission from a disk around a massive, mstar~10 Msun, protostar that is
accreting at a high rate, mdot~3x10^{-3} Msun/yr. Our observations suggest that
the near-infrared reflection spectrum observed in the Orion-KL region is
produced close to source I and scattered to our line of sight in the OMC1
outflow cavity. The spectrum allows us to exclude source I being a very large,
massive protostar rotating at breakup speed. We suggest that the absorption
spectrum is produced in a disk surrounding a ~10 Msun protostar, accreting from
its disk at a high rate of a few 10^{-3} Msun/yr.Comment: Accepted for publication on A&
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