581 research outputs found
The mass ratio and the orbital parameters of the sdOB binary AA Doradus
The time sequence of 105 spectra covering one full orbital period of AA Dor
has been analyzed. Direct determination of Vsini for the sdOB component from 97
spectra outside of the eclipse for the lines MgII 4481 A and SiIV 4089 A
clearly indicated a substantially smaller value than estimated before. Detailed
modelling of line profile variations for 8 spectra during the eclipse for the
MgII 4481 A line, combined with the out-of-eclipse fits, gave Vsini =
31.8+/-1.8 km/s. The previous determinations of Vsini, based on the HeII 4686 A
line, appear to be invalid because of the large natural broadening of the line.
With the assumption of the solid-body, synchronous rotation of the sdOB
primary, the measured values of the semi-amplitude K1 and Vsini lead to the
mass ratio q = 0.213+/-0.013 which in turn gives K2 and thus the masses and
radii of both components. The sdOB component appears to be less massive than
assumed before, M1 = 0.25+/-0.05 Msol, but the secondary has its mass-radius
parameters close to theoretically predicted for a brown dwarf, M2 =
0.054+/-0.010 Msol and R2 = 0.089+/-0.005 Rsol. Our results do not agree with
the recent determination of Vuckovic et al. 2008 based on a K2 estimate from
line-profile asymmetries.Comment: Accepted by MNRA
A search for the optical counterpart to the magnetar CXOU J010043.1-721134
After our tentative detection of an optical counterpart to CXOU
J010043.1-721134 from archival Hubble Space Telescope (HST) imaging, we have
followed up with further images in four bands. Unfortunately, the source
originally identified is not confirmed. We provide deep photometric limits in
four bands and accurate photometry of field stars around the location of the
magnetar.Comment: 9 pages, accepted by Ap
Constraint violation in free evolution schemes: comparing BSSNOK with a conformal decomposition of Z4
We compare numerical evolutions performed with the BSSNOK formulation and a
conformal decomposition of a Z4-like formulation of General Relativity. The
important difference between the two formulations is that the Z4 formulation
has a propagating Hamiltonian constraint, whereas BSSNOK has a zero-speed
characteristic variable in the constraint subsystem. In spherical symmetry we
evolve both puncture and neutron star initial data. We demonstrate that the
propagating nature of the Z4 constraints leads to results that compare
favorably with BSSNOK evolutions, especially when matter is present in the
spacetime. From the point of view of implementation the new system is a simple
modification of BSSNOK.Comment: Published in PR
Search for Variable Stars in the Globular Cluster M3
We describe here results of a photometric time-sequence survey of the
globular cluster M3 (NGC 5272), in a search for contact and detached eclipsing
binary stars. We have discovered only one likely eclipsing binary and one SX
Phe type star in spite of monitoring 4077 stars with and observing 25
blue stragglers. The newly identified SX Phe star, V237, shows a light curve
with a variable amplitude. Variable V238 shows variability either with a period
of 0.49 d or with a period of 0.25 d. On the cluster colour-magnitude diagram,
the variable occupies a position a few hundredths of magnitude to the blue of
the base of the red giant branch. V238 is a likely descendent of a binary blue
straggler. As a side result we obtained high quality data for 42 of the
previously known RR Lyrae variables, including 33 of Bailey type ab, 7 type c
and 2 double-mode pulsators. We used equations that relate the physical
properties of RRc stars to their pulsation periods and Fourier parameters to
derive masses, luminosities, temperatures and helium parameters for five of the
RRc stars. One of the RRd stars (V79) has switched modes. In previous studies,
it was classified as RRab, but our observations show that it is an RRd star
with the first overtone mode dominating. This indicates blueward evolution on
the horizontal branch.Comment: 21 pages including 14 figures, Latex, requires mn.sty, psfig.sty.
Submitted, MNRA
Constraint preserving boundary conditions for the Z4c formulation of general relativity
We discuss high order absorbing constraint preserving boundary conditions for
the Z4c formulation of general relativity coupled to the moving puncture family
of gauges. We are primarily concerned with the constraint preservation and
absorption properties of these conditions. In the frozen coefficient
approximation, with an appropriate first order pseudo-differential reduction,
we show that the constraint subsystem is boundary stable on a four dimensional
compact manifold. We analyze the remainder of the initial boundary value
problem for a spherical reduction of the Z4c formulation with a particular
choice of the puncture gauge. Numerical evidence for the efficacy of the
conditions is presented in spherical symmetry.Comment: 18 pages, 8 figure
A Search for Distant Galactic Cepheids Toward l=60
We present results of a survey of a 6-square-degree region near l=60, b=0 to
search for distant Milky Way Cepheids. Few MW Cepheids are known at distances
>~ R_0, limiting large-scale MW disk models derived from Cepheid kinematics;
this work was designed to find a sample of distant Cepheids for use in such
models. The survey was conducted in the V and I bands over 8 epochs, to a
limiting I~=18, with a total of ~ 5 million photometric observations of ~ 1
million stars. We present a catalog of 578 high-amplitude variables discovered
in this field. Cepheid candidates were selected from this catalog on the basis
of variability and color change, and observed again the following season. We
confirm 10 of these candidates as Cepheids with periods from 4 to 8 days, most
at distances > 3 kpc. Many of the Cepheids are heavily reddened by intervening
dust, some with implied extinction A_V > 10 mag. With a future addition of
infrared photometry and radial velocities, these stars alone can provide a
constraint on R_0 to 8%, and in conjunction with other known Cepheids should
provide good estimates of the global disk potential ellipticity.Comment: 18 pages, 4 tables, 13 figures (LaTeX / AASTeX
Evolution of Horizontal Branch Stars in Globular Clusters: The Interesting Case of V79 in M3
New observations of variable stars in the globular cluster M3 reveal that the
RR Lyrae variable V79 is a double-mode (RRd) variable with the first overtone
mode dominating. In all previous studies, V79 was found to be a fundamental
mode (RRab) pulsator with an irregular light curve. This is the first observed
mode switch for an RR Lyrae variable and it is direct observational evidence
for blueward evolution of horizontal branch stars in the Oosterhoff type I
cluster M3. It also demonstrates that there is a connection between the Blazhko
effect and pulsational mode mixing in RR Lyrae variables. These new
observations also show that the strength of the overtone oscillations in the
RRd star V68 in M3 may have increased in the last 70 years, thus indicating
blueward evolution for V68 as well. A survey of previously published
investigations of RRd stars in Oosterhoff type II systems indicates that there
is marginal evidence for an increase in the strength of fundamental mode
oscillations in two stars: V30 in M15 and AQ Leo. If these increases are
confirmed by future observations, it will indicate redward evolution for RRd
stars in type II systems.Comment: 10 pages including 3 figures, Latex, requires aaspp4.sty. Accepted by
ApJ
Radial Velocity Studies of Close Binary Stars.IV
Radial-velocity measurements and sine-curve fits to the orbital velocity
variations are presented for the fourth set of ten close binary systems: 44
Boo, FI Boo, V2150 Cyg, V899 Her, EX Leo, VZ Lib, SW Lyn, V2377 Oph, Anon Psc
(GSC 8-324), HT Vir. All systems are double-lined spectroscopic binaries with
only two of them not being contact systems (SW Lyn and GSC 8-324) and with five
(FI Boo, V2150 Cyg, V899 Her, EX Leo, V2377 Oph) being the recent photometric
discoveries of the Hipparcos satellite project. Five of the binaries are
triple-lined systems (44 Boo, V899 Her, VZ Lib, SW Lyn, HT Vir). Three (or
possibly four) companions in the triple-lined systems show radial-velocity
changes during the span of our observations suggesting that these are in fact
quadruple systems. Several of the studied systems are prime candidates for
combined light and radial-velocity synthesis solutions.Comment: aastex5.0, 5 figures in PS; submitted to Astron.
An empirical temperature calibration for the Delta a photometric system. II. The A-type and mid F-type star
With the Delta a photometric system, it is possible to study very distant
galactic and even extragalactic clusters with a high level of accuracy. This
can be done with a classical color-magnitude diagram and appropriate
isochrones. The new calibration presented in this paper is a powerful
extension. For open clusters, the reddening is straightforward for an
estimation via Isochrone fitting and is needed in order to calculate the
reddening-free, temperature sensitive, index (g1-y)0. As a last step, the
calibration can be applied to individual stars. Because no a-priori
reddening-free photometric parameters are available for the investigated
spectral range, we have applied the dereddening calibrations of the Stromgren
uvbybeta system and compared them with extinction models for the Milky Way. As
expected from the sample of bright stars, the extinction is negligible for
almost all objects. As a next step, already established calibrations within the
Stromgren uvbybeta, Geneva 7-color, and Johnson UBV systems were applied to a
sample of 282 normal stars to derive a polynomial fit of the third degree for
the averaged effective temperatures to the individual (g1-y)0 values with a
mean of the error for the whole sample of Delta T(eff) is 134K, which is lower
than the value in Paper I for hotter stars. No statistically significant effect
of the rotational velocity on the precision of the calibration was found.Comment: 5 pages, 2 figures, accepted by A&
H-alpha Emission Variability in the gamma-ray Binary LS I +61 303
LS I +61 303 is an exceptionally rare example of a high mass X-ray binary
(HMXB) that also exhibits MeV-TeV emission, making it one of only a handful of
"gamma-ray binaries". Here we present H-alpha spectra that show strong
variability during the 26.5 day orbital period and over decadal time scales. We
detect evidence of a spiral density wave in the Be circumstellar disk over part
of the orbit. The H-alpha line profile also exhibits a dramatic emission burst
shortly before apastron, observed as a redshifted shoulder in the line profile,
as the compact source moves almost directly away from the observer. We
investigate several possible origins for this red shoulder, including an
accretion disk, mass transfer stream, and a compact pulsar wind nebula that
forms via a shock between the Be star's wind and the relativistic pulsar wind.Comment: Accepted to Ap
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