5,378 research outputs found
The blue stragglers formed via mass transfer in old open clusters
In this paper, we present the simulations for the primordial blue stragglers
in the old open cluster M67 based on detailed modelling of the evolutionary
processes. The principal aim is to discuss the contribution of mass transfer
between the components of close binaries to the blue straggler population in
M67. First, we followed the evolution of a binary of 1.4M+0.9M.
The synthetic evolutionary track of the binary system revealed that a
primordial blue straggler had a long lifetime in the observed blue straggler
region of color-magnitude diagram. Second, a grid of models for close binary
systems experiencing mass exchange were computed from 1Gyr to 6Gyr in order to
account for primordial blue-straggler formation in a time sequence. Based on
such a grid, Monte-Carlo simulations were applied for the old open cluster M67.
Adopting appropriate orbital parameters, 4 primordial blue stragglers were
predicted by our simulations. This was consistent with the observational fact
that only a few blue stragglers in M67 were binaries with short orbital
periods. An upper boundary of the primordial blue stragglers in the
color-magnitude diagram (CMD) was defined and could be used to distinguish blue
stragglers that were not formed via mass exchange. Using the grid of binary
models, the orbital periods of the primordial BSs could be predicted. Compared
with the observations, it is clear that the mechanism discussed in this work
alone cannot fully predict the blue straggler population in M67. There must be
several other processes also involved in the formation of the observed blue
stragglers in M67.Comment: 11 pages, 6 figures, A&A accepte
Inside-Out Evacuation of Transitional Protoplanetary Disks by the Magneto-Rotational Instability
How do T Tauri disks accrete? The magneto-rotational instability (MRI)
supplies one means, but protoplanetary disk gas is typically too poorly ionized
to be magnetically active. Here we show that the MRI can, in fact, explain
observed accretion rates for the sub-class of T Tauri disks known as
transitional systems. Transitional disks are swept clean of dust inside rim
radii of ~10 AU. Stellar coronal X-rays ionize material in the disk rim,
activating the MRI there. Gas flows from the rim to the star, at a rate limited
by the depth to which X-rays ionize the rim wall. The wider the rim, the larger
the surface area that the rim wall exposes to X-rays, and the greater the
accretion rate. Interior to the rim, the MRI continues to transport gas; the
MRI is sustained even at the disk midplane by super-keV X-rays that Compton
scatter down from the disk surface. Accretion is therefore steady inside the
rim. Blown out by radiation pressure, dust largely fails to accrete with gas.
Contrary to what is usually assumed, ambipolar diffusion, not Ohmic
dissipation, limits how much gas is MRI-active. We infer values for the
transport parameter alpha on the order of 0.01 for GM Aur, TW Hyd, and DM Tau.
Because the MRI can only afflict a finite radial column of gas at the rim, disk
properties inside the rim are insensitive to those outside. Thus our picture
provides one robust setting for planet-disk interaction: a protoplanet interior
to the rim will interact with gas whose density, temperature, and transport
properties are definite and decoupled from uncertain initial conditions. Our
study also supplies half the answer to how disks dissipate: the inner disk
drains from the inside out by the MRI, while the outer disk photoevaporates by
stellar ultraviolet radiation.Comment: Accepted to Nature Physics June 7, 2007. The manuscript for
publication is embargoed per Nature policy. This arxiv.org version contains
more technical details and discussion, and is distributed with permission
from the editors. 10 pages, 4 figure
A young double stellar cluster in a HII region, emerging from its parent molecular cloud
We report the properties of a new young double stellar cluster in the region
towards IRAS 07141-0920 contained in the HII region Sh2-294. High-resolution
optical UBVRI and Halpha images, near-infrared JHKs and H2 filter images were
used to make photometric and morphological studies of the point sources and the
nebula seen towards Sh2-294. The optical images reveal an emission nebula with
very rich morphological details, composed mainly of UV scattered light and of
Halpha emission. Contrasting with the bright parts of the nebula, opaque,
elongated patches are seen. Our optical photometry confirms that the
illuminator of the nebula is likely to be a B0.5V star located at a distance of
about 3.2 kpc. Our near-IR images reveal an embedded cluster, extending for
about 2 pc and exhibiting sub-clustering: a denser, more condensed, sub-cluster
surrounding the optical high-mass B0.5V illuminator star; and a more embedded,
optically invisible, sub-cluster located towards the eastern, dark part of the
nebula and including the luminous MSX source G224.1880+01.2407, a massive
protostellar candidate that could be the origin of jets and extended features
seen at 2.12 micron. The double cluster appears to be clearing the remaining
molecular material of the parent cloud, creating patches of lower extinction
and allowing some of the least reddened members to be detected in the optical
images. We find 12 MS and 143 PMS members using 3 different methods: comparison
with isochrones in optical colour-magnitude diagrams, detection of near-IR
excess, and presence of Halpha emission. The most massive star fits a 4 Myr
post-MS isochrone. The age of the optically selected PMS population is
estimated to be 7-8 Myr. The IR-excess population shows sub-clustering on
scales as small as 0.23 pc and is probably much younger.Comment: 15 pages, 16 figure
Non-gray rotating stellar models and the evolutionary history of the Orion Nebular Cluster
Rotational evolution in the pre-main sequence (PMS) is described with new
sets of PMS evolutionary tracks including rotation, non-gray boundary
conditions (BCs) and either low (LCE) or high convection efficiency (HCE).
Using observational data and our theoretical predictions, we aim at
constraining 1) the differences obtained for the rotational evolution of stars
within the ONC by means of these different sets of models; 2) the initial
angular momentum of low mass stars, by means of their templates in the ONC. We
discuss the reliability of current stellar models for the PMS. While the 2D
radiation hydrodynamic simulations predict HCE in PMS, semi-empirical
calibrations either seem to require that convection is less efficient in PMS
than in the following MS phase or are still contradictory. We derive stellar
masses and ages for the ONC by using both LCE and HCE. The resulting mass
distribution for the bulk of the ONC population is in the range 0.20.3
{\msun} for our non-gray models and in the range 0.10.3{\msun} for models
having gray BCs. In agreement with Herbst et al. (2002) we find that a large
percentage (70%) of low-mass stars (M\simlt 0.5{\msun} for LCE;
M\simlt0.35{\msun} for HCE) in the ONC appears to be fast rotators (P4days).
Three possibilities are open: 1) 70% of the ONC low mass stars lose their
disk at early evolutionary phases; 2)their locking period is shorter; 3) the
period evolution is linked to a different morphology of the magnetic fields of
the two groups of stars. We also estimate the range of initial angular momentum
consistent with the observed periods. The comparisons made indicate that a
second parameter is needed to describe convection in the PMS, possibly related
to the structural effect of a dynamo magnetic field.Comment: 17 pages, 11 figure
Recommended from our members
Leveraging population admixture to characterize the heritability of complex traits.
Despite recent progress on estimating the heritability explained by genotyped SNPs (h(2)g), a large gap between h(2)g and estimates of total narrow-sense heritability (h(2)) remains. Explanations for this gap include rare variants or upward bias in family-based estimates of h(2) due to shared environment or epistasis. We estimate h(2) from unrelated individuals in admixed populations by first estimating the heritability explained by local ancestry (h(2)γ). We show that h(2)γ = 2FSTCθ(1 - θ)h(2), where FSTC measures frequency differences between populations at causal loci and θ is the genome-wide ancestry proportion. Our approach is not susceptible to biases caused by epistasis or shared environment. We applied this approach to the analysis of 13 phenotypes in 21,497 African-American individuals from 3 cohorts. For height and body mass index (BMI), we obtained h(2) estimates of 0.55 ± 0.09 and 0.23 ± 0.06, respectively, which are larger than estimates of h(2)g in these and other data but smaller than family-based estimates of h(2)
The XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST)
(abridged:) The XMM-Newton Extended Survey of the Taurus Molecular Cloud
(XEST) surveys the most populated ~5 square degrees of the Taurus star
formation region, using the XMM-Newton X-ray observatory to study the thermal
structure, variability, and long-term evolution of hot plasma, to investigate
the magnetic dynamo, and to search for new potential members of the
association. Many targets are also studied in the optical, and high-resolution
X-ray grating spectroscopy has been obtained for selected bright sources. The
X-ray spectra have been coherently analyzed with two different thermal models
(2-component thermal model, and a continuous emission measure distribution
model). We present overall correlations with fundamental stellar parameters
that were derived from the previous literature. A few detections from Chandra
observations have been added. The present overview paper introduces the project
and provides the basic results from the X-ray analysis of all sources detected
in the XEST survey.Comprehensive tables summarize the stellar properties of all
targets surveyed. The survey goes deeper than previous X-ray surveys of Taurus
by about an order of magnitude and for the first time systematically accesses
very faint and strongly absorbed TMC objects. We find a detection rate of 85%
and 98% for classical and weak-line T Tau stars (CTTS resp. WTTS), and identify
about half of the surveyed protostars and brown dwarfs. Overall, 136 out of 169
surveyed stellar systems are detected. We describe an X-ray luminosity vs. mass
correlation, discuss the distribution of X-ray-to-bolometric luminosity ratios,
and show evidence for lower X-ray luminosities in CTTS compared to WTTS.
Detailed analysis (e.g., variability, rotation-activity relations, influence of
accretion on X-rays) will be discussed in a series of accompanying papers.Comment: 75 pg, 77 figs. Accepted by A&A, to appear in a special section/issue
dedicated to the XMM-Newton Extended Survey of the Taurus Molecular Cloud
(XEST). V2: ASCII Table 14 added. Version with higher resolution figures at
http://www.issibern.ch/teams/Taurus/papers.html or
http://www.astro.phys.ethz.ch/papers/guedel/guedel_p_nf.htm
The young stellar population in the Serpens Cloud Core: An ISOCAM survey
We present results from an ISOCAM survey in the two broad band filters LW2
(5-8.5 mu) and LW3 (12-18 mu) of a 0.13 square degree coverage of the Serpens
Main Cloud Core. A total of 392 sources were detected in the 6.7 mu band and
139 in the 14.3 mu band to a limiting sensitivity of ~ 2 mJy. Only about 50% of
the mid-IR excess sources show excesses in the near-IR J-H/H-K diagram. In the
central Cloud Core the Class I/Class II number ratio is 19/18, i.e. about 10
times larger than in other young embedded clusters such as rho Ophiuchi or
Chamaeleon. The mid-IR fluxes of the Class I and flat-spectrum sources are
found to be on the average larger than those of Class II sources. Stellar
luminosities are estimated for the Class II sample, and its luminosity function
is compatible with a coeval population of about 2 Myr which follows a three
segment power-law IMF. For this age about 20% of the Class IIs are found to be
young brown dwarf candidates. The YSOs are in general strongly clustered, the
Class I sources more than the Class II sources, and there is an indication of
sub-clustering. The sub-clustering of the protostar candidates has a spatial
scale of 0.12 pc. These sub-clusters are found along the NW-SE oriented ridge
and in very good agreement with the location of dense cores traced by
millimeter data. The smallest clustering scale for the Class II sources is
about 0.25 pc, similar to what was found for rho Ophiuchi. Our data show
evidence that star formation in Serpens has proceeded in several phases, and
that a ``microburst'' of star formation has taken place very recently, probably
within the last 10^5 yrs.Comment: 25 pages, 14 figures, accepted by A&A March 18th, see also
http://www.not.iac.es/~amanda
A Study of 3CR Radio Galaxies from z = 0.15 to 0.65. II. Evidence for an Evolving Radio Structure
Radio structure parameters were measured from the highest quality radio maps
available for a sample of 3CR radio galaxies in the redshift range 0.15 < z <
0.65. Combined with similar data for quasars in the same redshift range, these
morphology data are used in conjunction with a quantification of the richness
of the cluster environment around these objects (the amplitude of the
galaxy-galaxy spatial covariance function, Bgg) to search for indirect evidence
of a dense intracluster medium (ICM). This is done by searching for confinement
and distortions of the radio structure that are correlated with Bgg.
Correlations between physical size and hot spot placement with Bgg show
evidence for an ICM only at z 0.4,
suggesting an epoch of z ~ 0.4 for the formation of an ICM in these Abell
richness class 0-1, FR2-selected clusters. X-ray selected clusters at
comparable redshifts, which contain FR1 type sources exclusively, are
demonstrably richer than the FR2-selected clusters found in this study. The
majority of the radio sources with high Bgg values at z < 0.4 can be described
as ``fat doubles'' or intermediate FR2/FR1s. The lack of correlation between
Bgg and bending angle or Bgg and lobe length asymmetry suggests that these
types of radio source distortion are caused by something other than interaction
with a dense ICM. Thus, a large bending angle cannot be used as an unambiguous
indicator of a rich cluster around powerful radio sources. These results
support the hypothesis made in Paper 1 that cluster quasars fade to become
FR2s, then FR1s, on a timescale of 0.9 Gyrs (for H0 = 50 km s^-1 Mpc^-1).Comment: 44 pages, 8 figures, 2 tables; to be published in the September 2002
issue of The Astronomical Journa
Circumstellar disks and planets. Science cases for next-generation optical/infrared long-baseline interferometers
We present a review of the interplay between the evolution of circumstellar
disks and the formation of planets, both from the perspective of theoretical
models and dedicated observations. Based on this, we identify and discuss
fundamental questions concerning the formation and evolution of circumstellar
disks and planets which can be addressed in the near future with optical and
infrared long-baseline interferometers. Furthermore, the importance of
complementary observations with long-baseline (sub)millimeter interferometers
and high-sensitivity infrared observatories is outlined.Comment: 83 pages; Accepted for publication in "Astronomy and Astrophysics
Review"; The final publication is available at http://www.springerlink.co
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