119 research outputs found
The shortest-period M-dwarf eclipsing system BW3 V38
The photometric data for a short-period (0.1984 day) eclipsing binary V38
discovered by the OGLE micro-lensing team in Baade's W indow field BW3 have
been analyzed. The de-reddened color (V-I_C)_0=2.3 and the light-curve
synthesis solution of the I-filter light curve suggest a pair of
strongly-distorted M-dwarfs, with parameters between those of YY Gem and CM
Dra, revolving on a tightest known orbit among binaries consisting of Main
Sequence stars. The primary, more massive and hotter, component maybe filling
its Roche lobe. The very small amount of angular momentum in the orbital motion
makes the system particularly important for studies of angular momentum loss at
the faint end of the Main Sequence. Spectroscopic observations of the orbital
radial velocity variations as well as of activity indicators are urgently
needed for a better understanding of the angular-momentum and
internal-structure evolutionary state of the system.Comment: latex aastex4.0, 16 pages, in that 4 figures (.ps inserted by
psfig.sty) and one table; submitted to PAS
CoRoT 102918586: a Gamma Dor pulsator in a short period eccentric eclipsing binary
Pulsating stars in eclipsing binary systems are powerful tools to test
stellar models. Binarity enables to constrain the pulsating component physical
parameters, whose knowledge drastically improves the input physics for
asteroseismic studies. The study of stellar oscillations allows us, in its
turn, to improve our understanding of stellar interiors and evolution. The
space mission CoRoT discovered several promising objects suitable for these
studies, which have been photometrically observed with unprecedented accuracy,
but needed spectroscopic follow-up. A promising target was the relatively
bright eclipsing system CoRoT 102918586, which turned out to be a double-lined
spectroscopic binary and showed, as well, clear evidence of Gamma Dor type
pulsations. We obtained phase resolved high-resolution spectroscopy with the
Sandiford spectrograph at the McDonald 2.1m telescope and the FEROS
spectrograph at the ESO 2.2m telescope. Spectroscopy yielded both the radial
velocity curves and, after spectra disentangling, the component effective
temperatures, metallicity and line-of-sight projected rotational velocities.
The CoRoT light curve was analyzed with an iterative procedure, devised to
disentangle eclipses from pulsations. We obtained an accurate determination of
the system parameters, and by comparison with evolutionary models strict
constraints on the system age. Finally, the residuals obtained after
subtraction of the best fitting eclipsing binary model were analyzed to
determine the pulsator properties. We achieved a quite complete and consistent
description of the system. The primary star pulsates with typical {\gamma} Dor
frequencies and shows a splitting in period which is consistent with high order
g-mode pulsations in a star of the corresponding physical parameters. The value
of the splitting, in particular, is consistent with pulsations in l = 1 modes.Comment: 12 pages, 10 figures. Accepted for publication in Astronomy and
Astrophysic
A new eclipsing binary system with a pulsating component detected by CoRoT
We report the discovery of CoRoT 102980178 (R.A.= 06:50:12.10, Dec.=
-02:41:21.8, J2000) an Algol-type eclipsing binary system with a pulsating
component (oEA). It was identified using a publicly available 55 day long
monochromatic lightcurve from the CoRoT initial run dataset (exoplanet field).
Eleven consecutive 1.26m deep total primary and the equal number of 0.25m deep
secondary eclipses (at phase 0.50) were observed. The following light elements
for the primary eclipse were derived: HJD_MinI= 2454139.0680 + 5.0548d x E. The
lightcurve modeling leads to a semidetached configuration with the photometric
mass ratio q=0.2 and orbital inclination i=85 deg. The out-of-eclipse
lightcurve shows ellipsoidal variability and positive O'Connell effect as well
as clear 0.01m pulsations with the dominating frequency of 2.75 c/d. The
pulsations disappear during the primary eclipses, which indicates the primary
(more massive) component to be the pulsating star. Careful frequency analysis
reveals the second independent pulsation frequency of 0.21 c/d and numerous
combinations of these frequencies with the binary orbital frequency and its
harmonics. On the basis of the CoRoT lightcurve and ground based multicolor
photometry, we favor classification of the pulsating component as a gamma
Doradus type variable, however, classification as an SPB star cannot be
excluded.Comment: 16 pages, 7 figures, 3 tables, accepted to Communications in
Asteroseismolog
Eclipsing Binaries in the OGLE Variable Star Catalog. IV. The Pre-Contact, Equal-Mass Systems
We used the database of eclipsing binaries detected by the OGLE microlensing
project in the pencil-beam search volume toward Baade's Window to define a
sample of 74 detached, equal-mass, main-sequence binary stars with short
orbital periods in the range 0.19<P<8 days. The logarithmic slope of the period
distribution, logN propto (-0.8 pm 0.2) logP, was used to infer the
angular-momentum-loss (AML) efficiency for the late, rapidly-rotating members
of close binaries. It is very likely that the main cause of the negative slope
is a discovery selection bias that progressively increases with the orbital
period length. Assuming a power-law dependence for the correction for the bias:
bias propto -C logP (with C ge 0), the AML braking-efficiency exponent alpha in
dH/dt = P^-alpha can take any value alpha = (-1.1 pm 0.2) + C. Very simple
considerations of discovery biases suggest C simeq 4/3, which would give an AML
braking law very close to the "saturated" one, with no dependence on the
period. However, except for plausibility arguments, we have no firm data to
support this estimate of C, so that alpha remains poorly constrained. The
results signal the utmost importance of the detection bias evaluation for
variable star databases used in analyses similar to the one presented in this
study.Comment: accepted by AJ, October 1999. AASTEX-4. 9 PS figures and 3 table
GU Boo: A New 0.6 Msun Detached Eclipsing Binary
We have found a new low-mass, double-lined, detached eclipsing binary, GU
Boo, among a sample of new variables from the ROTSE-I database. The binary has
an orbital period of 0.488728 +/- 0.000002 days, and estimated apparent
magnitudes Vrotse = 13.7 and I = 11.8. Our analysis of the light and radial
velocity curves of the system yields individual masses and radii of M1= 0.610
+/- 0.007 Msun, M2 = 0.599 +/- 0.006 Msun, R1= 0.623 +/- 0.016 Rsun, R2= 0.620
+/- 0.020 Rsun. The stars in GU Boo are therefore very similar to the
components of the eclipsing binary YY Gem. For this study we have adopted a
mean effective temperature for the binary of Teff = 3870 +/- 130 K. Based on
its space velocities we suggest that GU Boo is a main sequence binary, possibly
with an age of several Gyr. The metallicity of the binary is not well
constrained at this point but we speculate that it should not be very different
from solar. We have compared the physical parameters of GU Boo with current
low-mass stellar models, where we accounted for uncertainties in age and
metallicity by considering a wide range of values for those parameters. Our
comparisons reveal that all the models underestimate the radii of the
components of GU Boo by at least 10-15%. This result is in agreement with the
recent studies of YY Gem and CU Cnc.Comment: 41 pages, 10 figures, 11 tables; accepted by Ap
New pulsational properties of eight "anomalous" RR Lyrae variables
CCD photometry in the V band is presented for 7 field RR Lyrae stars selected
from a sample of eight variables which, according to data collected in the
literature, are expected to be {\it ab}-type pulsators, to have short periods
and hence high metallicity, and to be located at high {\it z} from the galactic
plane. New periods and epochs are derived for them. The new periods are only
slightly shorter than the values published on the last edition of the General
Catalog of Variable Stars (GCVS4). Instead, in six cases our amplitude of the
light variation is significantly smaller than that published on the GCVS4, and
in at least three cases the actual pulsation appears to be in the first
harmonic rather than in the fundamental mode. All the suggested {\it c}-type
pulsators show variations in the amplitude and/or quite scattered light curves.
Possible explanations are given. From a spectro-photometric analysis of the
sample, only DL Com is confirmed to pulsate in the fundamental mode, to have
short period, and to be located at relatively high {\it z}. A single object
cannot be taken as evidence for a significant metal rich population at large
distance from the galactic plane.Comment: 28 pages including text and tables, plain tex. Figures available
through anonymous ftp at ftp://astbo3.bo.astro.it/pub/bap/files/ (get
bap95-12-fig1.ps and bap95-12-figures.ps
Eclipsing Binaries in the OGLE Variable Star Catalogs.V. Long-Period Beta Lyrae-type Systems in the Small Magellanic Cloud and the PLC-beta Relation
Thirty eight long-period (P>10 days), apparently contact binary stars
discovered by the OGLE-II project in the SMC appear to be Beta Lyrae-type
systems with ellipsoidal variations of the cool components dominating over
eclipse effects in the systemic light variations and in the total luminosity. A
new period-luminosity- color (PLC) relation has been established for these
systems; we call it the PLC-beta relation, to distinguish it from the Cepheid
relation. Two versions of the PLC-beta relation - based on the (B-V)0 or (V-I)0
color indices - have been calibrated for 33 systems with (V-I)0>0.25 spanning
the orbital period range of 11 to 181 days. The relations can provide
maximum-light, absolute-magnitude estimates accurate to epsilon-M_V~0.35 mag.
within the approximate range -3<M_V<+1. In terms of their number in the SMC,
the long-period Beta Lyrae-type binaries are about 50 times less common than
the Cepheids. Nevertheless, their large luminosities coupled with continuous
light variations make these binaries very easy to spot in nearby galaxies, so
that the PLC-beta relation can offer an auxiliary and entirely independent
method of distance determination to nearby stellar systems rich in massive
stars. The sample of the long-period Beta Lyrae systems in the SMC analyzed in
this paper is currently the best defined and uniform known sequence of such
binaries.Comment: submitted for publication in Astronomical Journal; 8 PS figures, 2
table
- âŠ