855 research outputs found
SDSS J125637-022452: a high proper motion L subdwarf
We report the discovery of a high proper motion L subdwarf (
=0.617arcsec/yr) in the Sloan Digital Sky Survey spectral database. The optical
spectrum from the star SDSS J125637-022452 has mixed spectral features of both
late-M spectral subtype (strong TiO and CaH at 7000A) and mid-L spectral
subtype (strong wings of KI at 7700A, CrH and FeH), which is interpreted as the
signature of a very low-mass, metal-poor star (ultra-cool subdwarf) of spectral
type sdL. The near infrared (NIR) (J-Ks) colors from 2MASS shows the object to
be significantly bluer compared to normal L dwarfs, which is probably due a
strong collision induced absorption (CIA) due to H2 molecule. This is
consistent with the idea that CIA from H2 is more pronounced at low
metallicities. Proper motion and radial velocity measurements also indicate
that the star is kinematically "hot" and probably associated with the Galactic
halo population.Comment: 13 pages, 2 figures. Accepted for ApJ
Trigonometric parallaxes of ten ultracool subdwarfs
We measured absolute trigonometric parallaxes and proper motions with respect
to many background galaxies for a sample of ten ultracool subdwarfs.
The observations were taken in the H-band with the OMEGA2000 camera at the
3.5m-telescope on Calar Alto, Spain during a time period of 3.5 years. For the
first time, the reduction of the astrometric measurements was carried out
directly with respect to background galaxies. We obtained absolute parallaxes
with mean errors ranging between 1 and 3 mas.
With six completely new parallaxes we more than doubled the number of
benchmark ultracool (>sdM7) subdwarfs. Six stars in the M_{K_s} vs. J-K_s
diagram fit perfectly to model subdwarf sequences from M7 to L4 with [M/H]
between -1.0 and -1.5, whereas 4 are consistent with a moderately low
metallicity ([M/H]=-0.5) from M7 to T6. All but one of our objects have large
tangential velocities between 200 and 320 km/s typical of the Galactic halo
population.
Our results are in good agreement with recent independent measurements for
three of our targets and confirm the previously measured parallax and absolute
magnitude M_{K_s} of the nearest and coolest (T-type) subdwarf 2MASS 0937+29
with higher accuracy.
For all targets, we also obtained infrared J,H,K_s photometry at a level of a
few milli-magnitudes relative to 2MASS standards.Comment: Letter 4 pages 1 figure. accepted by Astronomy and Astrophysic
Models for the 3-D axisymmetric gravitational potential of the Milky Way Galaxy - A detailed modelling of the Galactic disk
Aims. Galaxy mass models based on simple and analytical functions for the
density and potential pairs have been widely proposed in the literature. Disk
models constrained by kinematic data alone give information on the global disk
structure only very near the Galactic plane. We attempt to circumvent this
issue by constructing disk mass models whose three-dimensional structures are
constrained by a recent Galactic star counts model in the near-infrared and
also by observations of the hydrogen distribution in the disk. Our main aim is
to provide models for the gravitational potential of the Galaxy that are fully
analytical but also with a more realistic description of the density
distribution in the disk component. Methods. From the disk model directly based
on the observations (here divided into the thin and thick stellar disks and the
HI and H disks subcomponents), we produce fitted mass models by combining
three Miyamoto-Nagai disk profiles of any "model order" (1, 2, or 3) for each
disk subcomponent. The Miyamoto-Nagai disks are combined with models for the
bulge and "dark halo" components and the total set of parameters is adjusted by
observational kinematic constraints. A model which includes a ring density
structure in the disk, beyond the solar Galactic radius, is also investigated.
Results. The Galactic mass models return very good matches to the imposed
observational constraints. In particular, the model with the ring density
structure provides a greater contribution of the disk to the rotational support
inside the solar circle. The gravitational potential models and their
associated force-fields are described in analytically closed forms, and in
addition, they are also compatible with our best knowledge of the stellar and
gas distributions in the disk component. The gravitational potential models are
suited for investigations of orbits in the Galactic disk.Comment: 22 pages, 13 figures, 11 tables, accepted for publication in A&
A New Sample of Cool Subdwarfs from SDSS: Properties and Kinematics
We present a new sample of M subdwarfs compiled from the 7th data release of
the Sloan Digital Sky Survey. With 3517 new subdwarfs, this new sample
significantly increases the number of spectroscopically confirmed low-mass
subdwarfs. This catalog also includes 905 extreme and 534 ultra sudwarfs. We
present the entire catalog including observed and derived quantities, and
template spectra created from co-added subdwarf spectra. We show color-color
and reduced proper motion diagrams of the three metallicity classes, which are
shown to separate from the disk dwarf population. The extreme and ultra
subdwarfs are seen at larger values of reduced proper motion as expected for
more dynamically heated populations. We determine 3D kinematics for all of the
stars with proper motions. The color-magnitude diagrams show a clear separation
of the three metallicity classes with the ultra and extreme subdwarfs being
significantly closer to the main sequence than the ordinary subdwarfs. All
subdwarfs lie below (fainter) and to the left (bluer) of the main sequence.
Based on the average velocities and their dispersions, the extreme
and ultra subdwarfs likely belong to the Galactic halo, while the ordinary
subdwarfs are likely part of the old Galactic (or thick) disk. An extensive
activity analysis of subdwarfs is performed using H emission and 208
active subdwarfs are found. We show that while the activity fraction of
subdwarfs rises with spectral class and levels off at the latest spectral
classes, consistent with the behavior of M dwarfs, the extreme and ultra
subdwarfs are basically flat.Comment: 66 pages, 23 figures, accepted in Ap
Modelling resonances and orbital chaos in disk galaxies. Application to a Milky Way spiral model
Context: Resonances in the stellar orbital motion under perturbations from
spiral arms structure play an important role in the evolution of the disks of
spiral galaxies. The epicyclic approximation allows the determination of the
corresponding resonant radii on the equatorial plane (for nearly circular
orbits), but is not suitable in general.
Aims: We expand the study of resonant orbits by analysing stellar motions
perturbed by spiral arms with Gaussian-shaped profiles without any restriction
on the stellar orbital configurations, and we expand the concept of Lindblad
(epicyclic) resonances for orbits with large radial excursions.
Methods: We define a representative plane of initial conditions, which covers
the whole phase space of the system. Dynamical maps on representative planes
are constructed numerically, in order to characterize the phase-space structure
and identify the precise location of resonances. The study is complemented by
the construction of dynamical power spectra, which provide the identification
of fundamental oscillatory patterns in the stellar motion.
Results: Our approach allows a precise description of the resonance chains in
the whole phase space, giving a broader view of the dynamics of the system when
compared to the classical epicyclic approach, even for objects in retrograde
motion. The analysis of the solar neighbourhood shows that, depending on the
current azimuthal phase of the Sun with respect to the spiral arms, a star with
solar kinematic parameters may evolve either inside the stable co-rotation
resonance or in a chaotic zone.
Conclusions: Our approach contributes to quantifying the domains of resonant
orbits and the degree of chaos in the whole Galactic phase-space structure. It
may serve as a starting point to apply these techniques to the investigation of
clumps in the distribution of stars in the Galaxy, such as kinematic moving
groups.Comment: 17 pages, 15 figures. Matches accepted version in A&
The possibility of determining open-cluster parameters from BVRI photometry
In the last decades we witnessed an increase in studies of open clusters of
the Galaxy, especially because of the good determination for a wide range of
values of parameters such as age, distance, reddening, and proper motion. The
reliable determination of the parameters strongly depends on the photometry
available and especially on the U filter, which is used to obtain the color
excess E(B-V) through the color-color diagram (U-B) by (B-V) by fitting a zero
age main-sequence. Owing to the difficulty of performing photometry in the U
band, many authors have tried to obtain E(B-V) without the filter. But because
of the near linearity of the color-color diagrams that use the other bands,
combined with the fact that most fitting procedures are highly subjective (many
done "by eye") the reliability of those results has always been questioned. Our
group has recently developed, a tool that performs isochrone fitting in
open-cluster photometric data with a global optimization algorithm, which
removes the need to visually perform the fits and thus removes most of the
related subjectivity. Here we apply our method to a set of synthetic clusters
and two observed open clusters (Trumpler 1 and Melotte 105) using only
photometry for the BVRI bands. Our results show that, considering the cluster
structural variance caused only by photometric and Poisson sampling errors, our
method is able to recover the synthetic cluster parameters with errors of less
than 10% for a wide range of ages, distances, and reddening, which clearly
demonstrates its potential. The results obtained for Trumpler 1 and Melotte 105
also agree well with previous literature values.Comment: 5 pages, 5 figures, accepted for publication in
Astronomy&Astrophysic
Oxygen, -element and iron abundance distributions in the inner part of the Galactic thin disc. II
We have derived the abundances of 36 chemical elements in one Cepheid star,
ASAS 181024--2049.6, located R kpc from the Galactic center.
This star falls within a region of the inner thin disc poorly sampled in
Cepheids. Our spectral analysis shows that iron, magnesium, silicon, calcium
and titanium LTE abundances in that star support the presence of a plateau-like
abundance distribution in the thin disc within 5 kpc of the Galactic center, as
previously suggested by \cite{Maret15}. If confirmed, the flattening of the
abundance gradient within that region could be the result of a decrease in the
star formation rate due to dynamic effects, possibly from the central Galactic
bar.Comment: 5 pages, 3 figure
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