510 research outputs found
Stellar Hydrodynamics in Radiative Regions
We present an analysis of the response of a radiative region to waves
generated by a convective region of the star; this wave treatment of the
classical problem of ``overshooting'' gives extra mixing relative to the
treatment traditionally used in stellar evolutionary codes. The interface
between convectively stable and unstable regions is dynamic and nonspherical,
so that the nonturbulent material is driven into motion, even in the absence of
``penetrative overshoot.'' These motions may be described by the theory of
nonspherical stellar pulsations, and are related to motion measured by
helioseismology. Multi-dimensional numerical simulations of convective flow
show puzzling features which we explain by this simplified physical model.
Gravity waves generated at the interface are dissipated, resulting in slow
circulation and mixing seen outside the formal convection zone. The approach
may be extended to deal with rotation and composition gradients. Tests of this
description in the stellar evolution code TYCHO produce carbon stars on the
asymptotic giant branch (AGB), an isochrone age for the Hyades and three young
clusters with lithium depletion ages from brown dwarfs, and lithium and
beryllium depletion consistent with observations of the Hyades and Pleiades,
all without tuning parameters. The insight into the different contributions of
rotational and hydrodynamic mixing processes could have important implications
for realistic simulation of supernovae and other questions in stellar
evolution.Comment: 27 pages, 5 figures, accepted to the Astrophysical Journa
Impact of granulation effects on the use of Balmer lines as temperature indicators
Balmer lines serve as important indicators of stellar effective temperatures
in late-type stellar spectra. One of their modelling uncertainties is the
influence of convective flows on their shape. We aim to characterize the
influence of convection on the wings of Balmer lines. We perform a differential
comparison of synthetic Balmer line profiles obtained from 3D hydrodynamical
model atmospheres and 1D hydrostatic standard ones. The model parameters are
appropriate for F,G,K dwarf and subgiant stars of metallicity ranging from
solar to 1/1000 solar. The shape of the Balmer lines predicted by 3D models can
never be exactly reproduced by a 1D model, irrespective of its effective
temperature. We introduce the concept of a 3D temperature correction, as the
effective temperature difference between a 3D model and a 1D model which
provides the closest match to the 3D profile. The temperature correction is
different for the different members of the Balmer series and depends on the
adopted mixing-length parameter in the 1D model. Among the investigated models,
the 3D correction ranges from -300K to +300K. Horizontal temperature
fluctuations tend to reduce the 3D correction. Accurate effective temperatures
cannot be derived from the wings of Balmer lines, unless the effects of
convection are properly accounted for. The 3D models offer a physically well
justified way of doing so. The use of 1D models treating convection with the
mixing-length theory do not appear to be suitable for this purpose. In
particular, there are indications that it is not possible to determine a single
value of the mixing-length parameter which will optimally reproduce the Balmer
lines for any choice of atmospheric parameters.Comment: 6 pages, 3 figures, accepted for publication in A&
XHIP-II: Clusters and associations
Context. In the absence of complete kinematic data it has not previously been
possible to furnish accurate lists of member stars for all moving groups. There
has been an unresolved dispute concerning the apparent inconsistency of the
Hipparcos parallax distance to the Pleiades.
Aims. To find improved candidate lists for clusters and associations
represented among Hipparcos stars, to establish distances, and to cast light on
the Pleiades distance anomaly.
Methods. We use a six dimensional fitting procedure to identify candidates,
and plot CMDs for 20 of the nearest groups. We calculate the mean parallax
distance for all groups.
Results. We identify lists of candidates and calculated parallax distances
for 42 clusters and 45 associations represented within the Hipparcos catalogue.
We find agreement between parallax distance and photometric distances for the
most important clusters. For single stars in the Pleiades we find mean parallax
distance 125.6 \pm 4.2 pc and photometric distance 132 \pm 3 pc calibrated to
nearby groups of similar in age and composition. This gives no reason to doubt
either the Hipparcos database or stellar evolutionary theory.Comment: Accepted for publication in Astronomy Letters, 10 pages, 2 fig
Global Magellan-image map of Venus at full resolution
During its first 243-day mapping cycle, the Magellan spacecraft succeeded in imaging 84 percent of the surface of Venus at resolutions on the order of 100 meters; subsequent cycles have increased the total coverage to over 97 percent and provided redundant coverage of much of the planet with differing viewing geometries. Unfortunately, this full-resolution global dataset is in the form of thousands of individual orbit tracks (F-BIDR's) whose length-to-width ratio of nearly 1000:1 makes them minimally useful unless mosaicked. The Magellan project produced full-resolution mosaics (F-MIDR's) only for selected regions on the planet, whereas a global set of mosaics was made only at threefold degraded resolution (C1-MIDR's). Furthermore, although the F-MIDR's, which are approximately equidimensional, are much better suited for scientific interpretation than the F-BIDR's, they are still an unwieldy dataset: over 1500 quadrangles, each showing a region only about 600 km on a side, would be required to cover the entire planet. The USGS has therefore undertaken to produce and distribute a global, full resolution set of mosaics of the Magellan image data in a format that will be efficient for both hardcopy and digital use. The initial motivation was that it would provide an efficient means of verifying the integrity of the F-BIDR's to be archived on computer-compatible tape at the USGS Flagstaff facility. However, the resulting product, known as the FMAP, should also serve as an important resource for future scientific interpretation. It will offer several advantages beyond global coverage at full resolution. The first, alluded to above, is its division of the planet's surface to minimize the number of quadrangles and maximize their area, subject to the limits on the number of pixels imposed by state-of-the-art digital recording media and hardcopy output devices. The second, the use of improved 'cosmetic' processing techniques, will greatly reduce tonal discontinuities between component F-BIDR's in the FMAP compared to the standard Magellan mosaic products. Finally, wherever possible, the FMAP will incorporate data that were unavailable (e.g., because of processing delays) when the standard MIDR products were created, as well as data that were reprocessed to improve their radiometric or geometric quality
Direct detection of a substellar companion to the young nearby star PZ Telescopii
Aims: We study the formation of substellar objects (exoplanets and brown
dwarfs) as companions to young nearby stars. Methods: With high contrast AO
imaging obtained with NACO at ESO's VLT we search for faint
companion-candidates around our targets, whose companionship can be confirmed
with astrometry. Results: In the course of our imaging campaign we found a
faint substellar companion of the nearby pre-main sequence star PZ Tel, a
member of the beta Pic moving group. The companion is 5-6 mag fainter than its
host star in JHK and is located at a separation of only 0.3 arcsec (or 15 AU of
projected separation) north-east of PZ Tel. Within three NACO observing epochs
we could confirm common proper motion (>39 sigma) and detected orbital motion
of PZ Tel B around its primary (>37 sigma). The photometry of the newly found
companion is consistent with a brown dwarf with a mass of 24 to 40 MJup, at the
distance (50 pc) and age (8-20 Myr) of PZ Tel. The effective temperature of the
companion, derived from its photometry, ranges between 2500 and 2700 K, which
corresponds to a spectral type between M6 and M8. After beta Pic b, PZ Tel B is
the second closest substellar companion imaged directly around a young star.Comment: accepted for publication in A&A Letter
Rotational velocities of nearby young stars
Stellar rotation is a crucial parameter driving stellar magnetism, activity
and mixing of chemical elements. Furthermore, the evolution of stellar rotation
is coupled to the evolution of circumstellar disks. Disk-braking mechanisms are
believed to be responsible for rotational deceleration during the accretion
phase, and rotational spin-up during the contraction phase after decoupling
from the disk for fast rotators arriving at the ZAMS. We investigate the
projected rotational velocities vsini of a sample of young stars with respect
to the stellar mass and disk evolutionary state to search for possible
indications of disk-braking mechanisms. We analyse the stellar spectra of 220
nearby (mostly <100pc) young (2-600 Myr) stars for their vsini, stellar age,
Halpha emission, and accretion rates. The stars have been observed with FEROS
and HARPS in La Silla, Chile. The spectra have been cross-correlated with
appropriate theoretical templates. We build a new calibration to be able to
derive vsini values from the cross-correlated spectra. Stellar ages are
estimated from the LiI equivalent width at 6708 Ang. The equivalent width and
width at 10% height of the Halpha emission are measured to identify accretors
and used to estimate accretion rates. The vsini is then analysed with respect
to the evolutionary state of the circumstellar disks to search for indications
of disk-braking mechanisms in accretors. We find that the broad vsini
distribution of our targets extends to rotation velocities of up to more than
100 km/s and peaks at a value of 7.8+-1.2 km/s, and that ~70% of our stars show
vsini<30 km/s. Furthermore, we can find indications for disk-braking in
accretors and rotational spin-up of stars which are decoupled from their disks.
In addition, we show that a number of young stars are suitable for precise
radial-velocity measurements for planet-search surveys.Comment: 16 pages, 6 figures, accepted for publication in A&
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