514 research outputs found
UV Spectroscopy of AB Doradus with the Hubble Space Telescope. Impulsive flares and bimodal profiles of the CIV 1549 line in a young star
We observed AB Doradus, a young and active late type star (K0 - K2 IV-V, P=
0.514 d) with the Goddard High Resolution Spectrograph of the post-COSTAR
Hubble Space Telescope with the time and spectral resolutions of 27 s and 15
km, respectively. The wavelength band (1531 - 1565 A) included the strong CIV
doublet (1548.202 and 1550.774, formed in the transition region at 100 000 K).
The mean quiescent CIV flux state was close to the saturated value and 100
times the solar one. The line profile (after removing the rotational and
instrumental profiles) is bimodal consisting of two Gaussians, narrow (FWHM =
70 km/s) and broad (FWHM =330km/s). This bimodality is probably due to two
separate broadening mechanisms and velocity fields at the coronal base. It is
possible that TR transient events (random multiple velocities), with a large
surface coverage, give rise to the broadening of the narrow component,while
true microflaring is responsible for the broad one.
The transition region was observed to flare frequently on different time
scales and magnitudes. The largest impulsive flare seen in the CIV 1549
emission reached in less than one minute the peak differential emission measure
(10**51.2 cm-3) and returned exponentially in 5 minutes to the 7 times lower
quiescent level.The 3 min average line profile of the flare was blue-shifted
(-190 km/s) and broadened (FWHM = 800 km/s). This impulsive flare could have
been due to a chromospheric heating and subsequent evaporation by an electron
beam, accelerated (by reconnection) at the apex of a coronal loop.Comment: to be published in AJ (April 98), 3 tables and 7 figures as separate
PS-files, print Table 2 as a landscap
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
A CCD photometric study of the late type contact binary EK Comae Berenices
We present CCD photometric observations of the W UMa type contact binary EK
Comae Berenices using the 2 metre telescope of Girawali Observatory,
India. The star was classified as a W UMa type binary of subtype-W by
\citet{sam1996}. The new V band photometric observations of the star reveal
that shape of the light curve has changed significantly from the one observed
by \citet{sam1996}. A detailed analysis of the light curve obtained from the
high-precision CCD photometric observations of the star indicates that EK Comae
Berenices is not a W-type but an A-type totally eclipsing W UMa contact binary.
The photometric mass ratio is determined to be 0.349 0.005. A temperature
difference of K between the components and an orbital
inclination of were obtained for the binary
system. Absolute values of masses, radii and luminosities are estimated by
means of the standard mass-luminosity relation for zero age main-sequence
stars. The star shows O'Connell effect, asymmetries in the light curve shape
around the primary and secondary maximum. The observed O'Connell effect is
explained by the presence of a hot spot on the primary component.Comment: 26 pages, 9 figures, 5 tables. Accepted for publication in New
Astronom
Pulsation of EE Cam
EE Cam is a previously little studied Delta Scuti pulsator with amplitudes
between those of the HADS (High-Amplitude Delta Scuti stars) group and the
average low-amplitude pulsators. Since the size of stellar rotation determines
both which pulsation modes are selected by the star as well as their
amplitudes, the star offers a great opportunity to examine the astrophysical
connections. Extensive photometric measurements covering several months were
carried out. 15 significant pulsation frequencies were extracted. The dominant
mode at 4.934 cd was identified as a radial mode by examining the phase
shifts at different wavelengths. Medium-dispersion spectra yielded a
value of km s. This shows that EE Cam belongs to the
important transition region between the HADS and normal Delta Scuti stars.Comment: 13 pages, 3 figures, 3 table
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.
GSC 2314-0530: the shortest-period eclipsing system with dMe components
CCD photometric observations in VRI colors and spectroscopic observations of
the newly discovered eclipsing binary GSC 2314-0530 (NSVS 6550671) with dMe
components and very short period of P=0.192636 days are presented. The
simultaneous light-curve solution and radial velocity solution allows to
determine the global parameters of GSC 2314-0530: T_{1}=3735 K; T_{2}=3106 K;
M_{1}=0.51 M_sun; M_{2}=0.26 M_sun; R_{1}=0.55 R_sun; R_{2}=0.29 R_sun;
L_{1}=0.053 L_sun; L_{2}=0.007 L_sun; i=72.5 degr; a=1.28 R_sun; d=59 pc. The
chromospheric activity of its components is revealed by strong emission in the
H_alpha line (with mean EW=5 A) and observed several flares. Empirical
relations mass-M_{bol}, mass-radius and mass-temperature are derived on the
basis of the parameters of known binaries with low-mass dM components.Comment: 10 pages, 11 figures, MNRAS (accepted
VW LMi: tightest quadruple system known. Light-time effect and possible secular changes of orbits
Tightest known quadruple systems VW LMi consists of contact eclipsing binary
with P_12 = 0.477551 days and detached binary with P_34 = 7.93063 days
revolving in rather tight, 355.0-days orbit. This paper presents new
photometric and spectroscopic observations yielding 69 times of minima and 36
disentangled radial velocities for the component stars. All available radial
velocities and minima times are combined to better characterize the orbits and
to derive absolute parameters of components. The total mass of the quadruple
system was estimated at 4.56 M_sun. The detached, non-eclipsing binary with
orbital period P = 7.93 days is found to show apsidal motion with U
approximately 80 years. Precession period in this binary, caused by the
gravitational perturbation of the contact binary, is estimated to be about 120
years. The wide mutual orbit and orbit of the non-eclipsing pair are found to
be close to coplanarity, preventing any changes of the inclination angle of the
non-eclipsing orbit and excluding occurrence of the second system of eclipses
in future. Possibilities of astrometric solution and direct resolving of the
wide, mutual orbit are discussed. Nearby star, HD95606, was found to form loose
binary with quadruple system VW LMi.Comment: 4 figures. accepted to MNRAS on July 31, 200
A Surprising Reversal of Temperatures in the Brown-Dwarf Eclipsing Binary 2MASS J05352184-0546085
The newly discovered brown-dwarf eclipsing binary 2MASS J05352184-0546085
provides a unique laboratory for testing the predictions of theoretical models
of brown-dwarf formation and evolution. The finding that the lower-mass brown
dwarf in this system is hotter than its higher-mass companion represents a
challenge to brown-dwarf evolutionary models, none of which predict this
behavior. Here we present updated determinations of the basic physical
properties of 2M0535-05, bolstering the surprising reversal of temperatures
with mass in this system. We compare these measurements with widely used
brown-dwarf evolutionary tracks, and find that the temperature reversal can be
explained by some models if the components of 2M0535-05 are mildly non-coeval,
possibly consistent with dynamical simulations of brown-dwarf formation.
Alternatively, a strong magnetic field on the higher-mass brown dwarf might
explain its anomalously low surface temperature, consistent with emerging
evidence that convection is suppressed in magnetically active, low-mass stars.
Finally, we discuss future observational and theoretical work needed to further
characterize and understand this benchmark system.Comment: 31 pages, 7 figures, accepted by Ap
A unified solution for the orbit and light-time effect in the V505 Sgr system
The multiple system V505 Sagittarii is composed of at least three stars: a
compact eclipsing pair and a distant component, which orbit is measured
directly using speckle interferometry. In order to explain the observed orbit
of the third body in V505 Sagittarii and also other observable quantities,
namely the minima timings of the eclipsing binary and two different radial
velocities in the spectrum, we thoroughly test a fourth-body hypothesis - a
perturbation by a dim, yet-unobserved object. We use an N-body numerical
integrator to simulate future and past orbital evolution of 3 or 4 components
in this system. We construct a suitable chi^2 metric from all available
speckle-interferometry, minima-timings and radial-velocity data and we scan a
part of a parameter space to get at least some of allowed solutions. In
principle, we are able to explain all observable quantities by a presence of a
fourth body, but the resulting likelihood of this hypothesis is very low. We
also discuss other theoretical explanations of the minima timings variations.
Further observations of the minima timings during the next decade or
high-resolution spectroscopic data can significantly constrain the model
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