3,426 research outputs found
A photospheric metal line profile analysis of hot DA white dwarfs with circumstellar material
Some hot DA white dwarfs have circumstellar high ion absorption features in
their spectra, in addition to those originating in the photosphere. In many
cases, the line profiles of these absorbing components are unresolved. Given
the importance of the atmospheric composition of white dwarfs to studies of
stellar evolution, extra-solar planetary systems and the interstellar medium,
we examine the effect of including circumstellar line profiles in the abundance
estimates of photospheric metals in six DA stars. The photospheric C and Si
abundances are reduced in five cases where the circumstellar contamination is
strong, though the relative weakness of the circumstellar Si IV absorption
introduces minimal contamination, resulting in a small change in abundance. The
inability of previous, approximate models to reproduce the photospheric line
profiles here demonstrates the need for a technique that accounts for the
physical line profiles of both the circumstellar and photospheric lines when
modelling these blended absorption features.Comment: 7 pages, 5 figues, 3 tables, accepted for publication in MNRA
GALEX ultraviolet observations of stellar variability in the Hyades and Pleiades clusters
We present GALEX near ultraviolet (NUV:1750 - 2750A) and far ultraviolet
(FUV: 1350 - 1750A) imaging observations of two 1.2 degree diameter fields in
the Hyades and Pleiades open clusters in order to detect possible UV
variability of the member stars. We have performed a detailed software search
for short-term UV flux variability during these observations of the approx 400
sources detected in each of the Hyades and Pleiades fields to identify
flare-like (dMe) stellar objects. This search resulted in the detection of 16
UV variable sources, of which 13 can be directly associated with probable
M-type stars. The other UV sources are G-type stars and one newly discovered RR
Lyrae star, USNOB1.0 1069-0046050, of period 0.624 day and distance 4.5-7.0
kpc. Light curves of photon flux versus time are shown for 7 flare events
recorded on six probable dMe stars. UV energies for these flares span the range
2E27 to 5E29 erg, with a corresponding NUV variability change of 1.82 mag. Only
one of these flare events (on the star Cl* Melotte 25 LH129) can definitely be
associated with an origin on a member the Hyades cluster itself. Finally, many
of our M-type candidates show long periods of enhanced UV activity but without
the associated rapid increase in flux that is normally associated with a flare
event. However, the total UV energy output during such periods of increased
activity is greater than that of many short-term UV flares. These intervals of
enhanced low-level UV activity concur with the idea that, even in quiescence,
the UV emission from dMe stars may be related to a superposition of many small
flare events possessing a wide range of energies.Comment: PASP Submitte
Volatile-Rich Circumstellar Gas in the Unusual 49 Ceti Debris Disk
We present Hubble Space Telescope STIS far-UV spectra of the edge-on disk
around 49 Ceti, one of the very few debris disks showing sub-mm CO emission.
Many atomic absorption lines are present in the spectra, most of which arise
from circumstellar gas lying along the line-of-sight to the central star. We
determined the line-of-sight CI column density, estimated the total carbon
column density, and set limits on the OI column density. Surprisingly, no
line-of-sight CO absorption was seen. We discuss possible explanations for this
non-detection, and present preliminary estimates of the carbon abundances in
the line-of-sight gas. The C/Fe ratio is much greater than the solar value,
suggesting that 49 Cet harbors a volatile-rich gas disk similar to that of Beta
Pictoris.Comment: Accepted for publication in ApJ Letters. 5 pages, 4 figure
Monomer-dimer model in two-dimensional rectangular lattices with fixed dimer density
The classical monomer-dimer model in two-dimensional lattices has been shown
to belong to the \emph{``#P-complete''} class, which indicates the problem is
computationally ``intractable''. We use exact computational method to
investigate the number of ways to arrange dimers on
two-dimensional rectangular lattice strips with fixed dimer density . For
any dimer density , we find a logarithmic correction term in the
finite-size correction of the free energy per lattice site. The coefficient of
the logarithmic correction term is exactly -1/2. This logarithmic correction
term is explained by the newly developed asymptotic theory of Pemantle and
Wilson. The sequence of the free energy of lattice strips with cylinder
boundary condition converges so fast that very accurate free energy
for large lattices can be obtained. For example, for a half-filled lattice,
, while and . For , is accurate at least to 10 decimal
digits. The function reaches the maximum value at , with 11 correct digits. This is also
the \md constant for two-dimensional rectangular lattices. The asymptotic
expressions of free energy near close packing are investigated for finite and
infinite lattice widths. For lattices with finite width, dependence on the
parity of the lattice width is found. For infinite lattices, the data support
the functional form obtained previously through series expansions.Comment: 15 pages, 5 figures, 5 table
Spanning Trees on Lattices and Integration Identities
For a lattice with vertices and dimension equal or higher
than two, the number of spanning trees grows asymptotically
as in the thermodynamic limit. We present exact integral
expressions for the asymptotic growth constant for spanning trees
on several lattices. By taking different unit cells in the calculation, many
integration identities can be obtained. We also give on the
homeomorphic expansion of -regular lattices with vertices inserted on
each edge.Comment: 15 pages, 3 figures, 1 tabl
Local height probabilities in a composite Andrews-Baxter-Forrester model
We study the local height probabilities in a composite height model, derived
from the restricted solid-on-solid model introduced by Andrews, Baxter and
Forrester, and their connection with conformal field theory characters. The
obtained conformal field theories also describe the critical behavior of the
model at two different critical points. In addition, at criticality, the model
is equivalent to a one-dimensional chain of anyons, subject to competing two-
and three-body interactions. The anyonic-chain interpretation provided the
original motivation to introduce the composite height model, and by obtaining
the critical behaviour of the composite height model, the critical behaviour of
the anyonic chains is established as well. Depending on the overall sign of the
hamiltonian, this critical behaviour is described by a diagonal coset-model,
generalizing the minimal models for one sign, and by Fateev-Zamolodchikov
parafermions for the other.Comment: 34 pages, 5 figures; v2: expanded introduction, references added and
other minor change
Identification of a nearby stellar association in the Hipparcos catalog: implications for recent, local star formation
The TW Hydrae Association (~55 pc from Earth) is the nearest known region of
recent star formation. Based primarily on the Hipparcos catalog, we have now
identified a group of 9 or 10 co-moving star systems at a common distance (~45
pc) from Earth that appear to comprise another, somewhat older, association
(``the Tucanae Association''). Together with ages and motions recently
determined for some nearby field stars, the existence of the Tucanae and TW
Hydrae Associations suggests that the Sun is now close to a region that was the
site of substantial star formation only 10-40 million years ago. The TW Hydrae
Association represents a final chapter in the local star formation history.Comment: 5 pages incl figs and table
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