427 research outputs found
Dynamical evolution of active detached binaries on log Jo - log M diagram and contact binary formation
Orbital angular momentum (Jo), systemic mass (M) and orbital period (P)
distributions of chromospherically active binaries (CAB) and W Ursae Majoris (W
UMa) systems were investigated. The diagrams of log Jo - log P, log M - log P
and log Jo-log M were formed from 119 CAB and 102 W UMa stars. The log Jo-log M
diagram is found to be most meaningful in demonstrating dynamical evolution of
binary star orbits. A slightly curved borderline (contact border) separating
the detached and the contact systems was discovered on the log Jo - log M
diagram. Since orbital size (a) and period (P) of binaries are determined by
their current Jo, M and mass ratio q, the rates of orbital angular momentum
loss (dlog Jo/dt) and mass loss (dlog M/dt) are primary parameters to determine
the direction and the speed of the dynamical evolution. A detached system
becomes a contact system if its own dynamical evolution enables it to pass the
contact border on the log Jo - log M diagram. Evolution of q for a mass loosing
detached system is unknown unless mass loss rate for each component is known.
Assuming q is constant in the first approximation and using the mean decreasing
rates of Jo and M from the kinematical ages of CAB stars, it has been predicted
that 11, 23 and 39 cent of current CAB stars would transform to W UMa systems
if their nuclear evolution permits them to live 2, 4 and 6 Gyrs respectively.Comment: 28 pages, including 6 figures and 2 tables, accepted for publication
in MNRA
Resolving the pulsations of subdwarf B stars: HS 0039+4302, HS 0444+0458, and an examination of the group properties of resolved pulsators
We continue our program of single-site observations of pulsating subdwarf B
(sdB) stars and present the results of extensive time series photometry of HS
0039+4302 and HS 0444+0458. Both were observed at MDM Observatory during the
fall of 2005. We extend the number of known frequencies for HS 0039+4302 from 4
to 14 and discover one additional frequency for HS 0444+0458, bringing the
total to three. We perform standard tests to search for multiplet structure,
measure amplitude variations, and examine the frequency density to constrain
the mode degree .
Including the two stars in this paper, 23 pulsating sdB stars have received
follow-up observations designed to decipher their pulsation spectra. It is
worth an examination of what has been detected. We compare and contrast the
frequency content in terms of richness and range and the amplitudes with
regards to variability and diversity. We use this information to examine
observational correlations with the proposed pulsation mechanism as
well as alternative theories.Comment: 32 pages, 18 figures, 7 tables. Accepted for publication in MNRA
A spectroscopy study of nearby late-type stars, possible members of stellar kinematic groups
Nearby late-type stars are excellent targets for seeking young objects in
stellar associations and moving groups. The origin of these structures is still
misunderstood, and lists of moving group members often change with time and
also from author to author. Most members of these groups have been identified
by means of kinematic criteria, leading to an important contamination of
previous lists by old field stars. We attempt to identify unambiguous moving
group members among a sample of nearby-late type stars by studying their
kinematics, lithium abundance, chromospheric activity, and other age-related
properties. High-resolution echelle spectra () of a sample of
nearby late-type stars are used to derive accurate radial velocities that are
combined with the precise Hipparcos parallaxes and proper motions to compute
galactic-spatial velocity components. Stars are classified as possible members
of the classical moving groups according to their kinematics. The spectra are
also used to study several age-related properties for young late-type stars,
i.e., the equivalent width of the lithium Li~{\sc i} \space 6707.8 \space \AA
\space line or the index. Additional information like X-ray
fluxes from the ROSAT All-Sky Survey or the presence of debris discs is also
taken into account. The different age estimators are compared and the moving
group membership of the kinematically selected candidates are discussed. From a
total list of 405 nearby stars, 102 have been classified as moving group
candidates according to their kinematics. i.e., only 25.2 \% of the
sample. The number reduces when age estimates are considered, and only 26
moving group candidates (25.5\% of the 102 candidates) have ages in agreement
with the star having the same age as an MG memberComment: 39 pages, 11 figures. Accepted for publication in Astronomy \&
Astrophysic
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
Orbital Eccentricity Distribution of Solar-Neighbour Halo Stars
We present theoretical calculations for the differential distribution of
stellar orbital eccentricity for a sample of solar-neighbour halo stars. Two
types of static, spherical gravitational potentials are adopted to define the
eccentricity e for given energy E and angular momentum L, such as an isochrone
potential and a Navarro-Frenk-White potential that can serve as two extreme
ends covering in-between any realistic potential of the Milky Way halo. The
solar-neighbour eccentricity distribution \Delta N(e) is then formulated, based
on a static distribution function of the form f(E,L) in which the velocity
anisotropy parameter \beta monotonically increases in the radial direction away
from the galaxy center, such that beta is below unity (near isotropic velocity
dispersion) in the central region and asymptotically approaches \sim 1
(radially anisotropic velocity dispersion) in the far distant region of the
halo. We find that \Delta N(e) sensitively depends upon the radial profile of
\beta, and this sensitivity is used to constrain such profile in comparison
with some observational properties of \Delta N_{obs}(e) recently reported by
Carollo et al. (2010). Especially, the linear e-distribution and the fraction
of higher-e stars for their sample of solar-neighbour inner-halo stars rule out
a constant profile of \beta, contrary to the opposite claim by Bond et al.
(2010). Our constraint of \beta \lesssim 0.5 at the galaxy center indicates
that the violent relaxation that has acted on the inner halo is effective
within a scale radius of \sim 10 kpc from the galaxy center. We discuss that
our result would help understand the formation and evolution of the Milky Way
halo.Comment: 13 pages, 6 figures, accepted for publication in MNRA
Contact Discontinuities in Models of Contact Binaries Undergoing Thermal Relaxation Oscillations
In this paper we pursue the suggestion by Shu, Lubow & Anderson (1979) and
Wang (1995) that contact discontinuity (DSC) may exist in the secondary in the
expansion TRO (thermal relaxation oscillation) state. It is demonstrated that
there is a mass exchange instability in some range of mass ratio for the two
components. We show that the assumption of {\it constant} volume of the
secondary should be relaxed in DSC model. For {\it all} mass ratio the
secondary alway satisfies the condition that no mass flow returns to the
primary through the inner Lagrangian point. The secondary will expand in order
to equilibrate the interaction between the common convective envelope and the
secondary. The contact discontinuity in contact binary undergoing thermal
relaxation does not violate the second law of thermodynamics. The maintaining
condition of contact discontinuity is derived in the time-dependent model. It
is desired to improve the TRO model with the advanced contact discontinuity
layer in future detailed calculations.Comment: 5 pages in emulateapj, 1 figur
Quantifying the contamination by old main-sequence stars in young moving groups: the case of the Local Association
The associations and moving groups of young stars are excellent laboratories
for investigating stellar formation in the solar neighborhood. Previous results
have confirmed that a non-negligible fraction of old main-sequence stars is
present in the lists of possible members of young stellar kinematic groups. A
detailed study of the properties of these samples is needed to separate the
young stars from old main-sequence stars with similar space motion, and
identify the origin of these structures. We used stars possible members of the
young (~ 10 - 650 Myr) moving groups from the literature. To determine the age
of the stars, we used several suitable age indicators for young main sequence
stars, i.e., X-ray fluxes and other photometric data. We also used
spectroscopic data, in particular the equivalent width of the lithium line Li I
and Halpha, to constrain the range of ages of the stars. By combining
photometric and spectroscopic data, we were able to separate the young stars
(10 - 650 Myr) from the old (> 1 Gyr) field ones. We found, in particular, that
the Local Association is contaminated by old field stars at the level of ~30%.
This value must be considered as the contamination for our particular sample,
and not of the entire Local Association. For other young moving groups, it is
more difficult to estimate the fraction of old stars among possible members.
However, the level of X-ray emission can, at least, help to separate two age
populations: stars with <200 Myr and stars older than this. Our results are
consistent with a scenario in which the moving groups contain both groups of
young stars formed in a recent star-formation episode and old field stars with
similar space motion. Only by combining X-ray and optical spectroscopic data is
it possible to distinguish between these two age populations.Comment: 7 pages, 7 figures. Accepted for publication in A&
The Solar Neighborhood. XXVI. AP Col: The Closest (8.4 pc) Pre-Main-Sequence Star
We present the results of a multi-technique investigation of the M4.5Ve flare
star AP Col, which we discover to be the nearest pre-main-sequence star. These
include astrometric data from the CTIO 0.9m, from which we derive a proper
motion of 342.0+/-0.5 mas yr^-1, a trigonometric parallax of 119.21+/-0.98 mas
(8.39+/-0.07 pc), and photometry and photometric variability at optical
wavelengths. We also provide spectroscopic data, including radial velocity
(22.4+/-0.3 km s^-1), lithium Equivalent Width (EW) (0.28+/-0.02 A), H-alpha EW
(-6.0 to -35 A), {\it vsini} (11+/-1 km s^-1), and gravity indicators from the
Siding Spring 2.3-m WiFeS, Lick 3-m Hamilton echelle, and Keck-I HIRES echelle
spectrographs. The combined observations demonstrate that AP Col is the closer
of only two known systems within 10 pc of the Sun younger than 100 Myr. Given
its space motion and apparent age of 12-50 Myr, AP Col is likely a member of
the recently proposed ~40 Myr old Argus/IC 2391 association.Comment: 31 pages, 11 figure
Chemical composition of the young open clusters IC2602 and IC2391
Galactic open clusters have been long recognized as one of the best tools to
investigate the chemical content of Galactic disk and its time evolution. In
the last decade, many efforts have been directed to chemically characterize the
old and intermediate age population; surprisingly, the chemical content of the
younger and close counterpart remains largely undetermined. In this paper we
present the abundance analysis of a sample of 15 G/K members of the young
pre-main sequence clusters IC 2602 and IC 2391. Along with IC 4665, these are
the first pre-main sequence clusters for which a detailed abundance
determination has been carried out so far. We analyzed high-resolution, high
S/N spectra acquired with different instruments (UVES and CASPEC at ESO, and
the echelle spectrograph at CTIO), using MOOG and equivalent width
measurements. Along with metallicity ([Fe/H]), we measured NaI, SiI, CaI, TiI
and TiII, and NiI abundances. Stars cooler than ~5500 show lower CaI, TiI, and
NaI than warmer stars. By determining TiII abundances, we show that, at least
for Ti, this effect is due to NLTE and over-ionization. We find average
metallicities [Fe/H] =0 and [Fe/H]=0.01 0.02 for IC 2602 and IC
2391, respectively. All the [X/Fe] ratios show a solar composition; the
accurate measurements allow us to exclude the presence of star-to-star scatter
among the members.Comment: 11 pages, accepted for publication in A&
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