529 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
A new method for estimating the pattern speed of spiral structure in the Milky Way
In the last few decades many efforts have been made to understand the effect
of spiral arms on the gas and stellar dynamics in the Milky Way disc. One of
the fundamental parameters of the spiral structure is its angular velocity, or
pattern speed , which determines the location of resonances in the
disc and the spirals' radial extent. The most direct method for estimating the
pattern speed relies on backward integration techniques, trying to locate the
stellar birthplace of open clusters. Here we propose a new method based on the
interaction between the spiral arms and the stars in the disc. Using a sample
of around 500 open clusters from the {\it New Catalogue of Optically Visible
Open Clusters and Candidates}, and a sample of 500 giant stars observed by
APOGEE, we find km s kpc, for a local
standard of rest rotation ~km s and solar radius ~kpc.
Exploring a range in and within the acceptable values, 200-240 km
s and 7.5-8.5 kpc, respectively, results only in a small change in our
estimate of , that is within the error. Our result is in close
agreement with a number of studies which suggest values in the range 20-25 km
s kpc. An advantage of our method is that we do not need
knowledge of the stellar age, unlike in the case of the birthplace method,
which allows us to use data from large Galactic surveys. The precision of our
method will be improved once larger samples of disk stars with spectroscopic
information will become available thanks to future surveys such as 4MOST.Comment: 10 pages, 6 figures, 4 tables, accepted for publication in MNRA
A new model for gravitational potential perturbations in disks of spiral galaxies. An application to our Galaxy
We propose a new, more realistic, description of the perturbed gravitational
potential of spiral galaxies, with spiral arms having Gaussian-shaped groove
profiles. We investigate the stable stellar orbits in galactic disks, using the
new perturbed potential. The influence of the bulge mass on the stellar orbits
in the inner regions of a disk is also investigated. The new description offers
the advantage of easy control of the parameters of the Gaussian profile of its
potential. We find a range of values for the perturbation amplitude from 400 to
800 km^2 s^{-2} kpc^{-1} which implies a maximum ratio of the tangential force
to the axisymmetric force between 3% and 6%, approximately. Good
self-consistency of arm shapes is obtained between the Inner Lindblad resonance
(ILR) and the 4:1 resonance. Near the 4:1 resonance the response density starts
to deviate from the imposed logarithmic spiral form. This creates bifurcations
that appear as short arms. Therefore the deviation from a perfect logarithmic
spiral in galaxies can be understood as a natural effect of the 4:1 resonance.
Beyond the 4:1 resonance we find closed orbits which have similarities with the
arms observed in our Galaxy. In regions near the center, in the presence of a
massive bulge, elongated stellar orbits appear naturally, without imposing any
bar-shaped potential, but only extending the spiral perturbation a little
inward of the ILR. This suggests that a bar is formed with a half-size around 3
kpc by a mechanism similar to that of the spiral arms. The potential energy
perturbation that we adopted represents an important step in the direction of
self-consistency, compared to previous sine function descriptions of the
potential. Our model produces a realistic description of the spiral structure,
able to explain several details that were not yet understood.Comment: 12 pag., 11 fig. Accepted for publication in A&A, 2012 December 1
UCAC3 Proper Motion Survey. I. Discovery of New Proper Motion Stars in UCAC3 with 0.40 "/yr > mu >= 0.18 "/yr between Declinations -90 deg and -47 deg
Presented here are 442 new proper motion stellar systems in the southern sky
between declinations -90\degr and -47\degr with 0\farcs40 yr
0\farcs18 yr. These systems constitute a 25.3% increase in
new systems for the same region of the sky covered by previous SuperCOSMOS
RECONS (SCR) searches that used Schmidt plates as the primary source of
discovery. Among the new systems are 25 multiples, plus an additional seven new
common proper motion companions found to previously known primaries. All stars
have been discovered using the third U.S. Naval Observatory (USNO) CCD
Astrograph Catalog (UCAC3). A comparison of the UCAC3 proper motions to those
from the Hipparcos, Tycho-2, Southern Proper Motion (SPM4), and SuperCOSMOS
efforts is presented, and shows that UCAC3 provides similar values and
precision to the first three surveys. The comparison between UCAC3 and
SuperCOSMOS indicates that proper motions in RA are systematically shifted in
the SuperCOSMOS data but are consistent in DEC data, while overall showing a
significantly higher scatter. Distance estimates are derived for stars having
SuperCOSMOS Sky Survey (SSS) , , and plate magnitudes
and Two-Micron All Sky Survey (2MASS) infrared photometry. We find 15 systems
estimated to be within 25 pc, including UPM 1710-5300 our closest new discovery
estimated at 13.5 pc. Such new discoveries suggest that more nearby stars are
yet to be found in these slower proper motion regimes, indicating that more
work is needed to develop a complete map of the solar neighborhood.Comment: 24 pages, 7 figures, 4 tables, accepted to the Astronomical Journal
July 07, 201
On the Importance of the Interclump Medium for Superionization: O VI Formation in the Wind of Zeta Pup
We have studied superionization and X-ray line formation in the spectra of
Zeta Pup using our new stellar atmosphere code (XCMFGEN) that can be used to
simultaneously analyze optical, UV, and X-ray observations. Here, we present
results on the formation of the O VI ll1032, 1038 doublet. Our simulations,
supported by simple theoretical calculations, show that clumped wind models
that assume void in the interclump space cannot reproduce the observed O VI
profiles. However, enough O VI can be produced if the voids are filled by a low
density gas. The recombination of O VI is very efficient in the dense material
but in the tenuous interclump region an observable amount of O VI can be
maintained. We also find that different UV resonance lines are sensitive to
different density regimes in Zeta Pup : C IV is almost exclusively formed
within the densest regions, while the majority of O VI resides between clumps.
N V is an intermediate case, with contributions from both the tenuous gas and
clumps.Comment: Accepted for publication in ApJL, 4 pages with 3 figure
The spiral structure of the Galaxy revealed by CS sources and evidence for the 4:1 resonance
We present a map of the spiral structure of the Galaxy, as traced by
molecular CS emission associated with IRAS sources which are believed to be
compact HII regions. The CS line velocities are used to determine the kinematic
distances of the sources, in order to investigate their distribution in the
galactic plane. This allows us to use 870 objects to trace the arms, a number
larger than that of previous studies based on classical HII regions. The
distance ambiguity of the kinematic distances, when it exists, is solved by
different procedures, including the latitude distribution and an analysis of
the longitude-velocity diagram. The well defined spiral arms are seen to be
confined inside the co-rotation radius, as is often the case in spiral
galaxies. We identify a square-shaped sub-structure in the CS map with that
predicted by stellar orbits at the 4:1 resonance (4 epicycle oscillations in
one turn around the galactic center). The sub-structure is found at the
expected radius, based on the known pattern rotation speed and epicycle
frequency curve. An inner arm presents an end with strong inward curvature and
intense star formation that we tentatively associate with the region where this
arm surrounds the extremity of the bar, as seen in many barred galaxies.
Finally, a new arm with concave curvature is found in the Sagitta to Cepheus
region of the sky
CARACTERISATION D'UN LASER Nd : YAG CONTINU A MODES COUPLES
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The Solar Neighborhood XXV: Discovery of New Proper Motion Stars with 0.40 "/yr > mu > 0.18 "/yr between Declinations -47 degrees and 00 degrees
We present 2817 new southern proper motion systems with 0.40 "/yr > mu > 0.18
"/yr and declination between -47 degrees and 00 degrees. This is a continuation
of the SuperCOSMOS-RECONS (SCR) proper motion searches of the southern sky. We
use the same photometric relations as previous searches to provide distance
estimates based on the assumption that the objects are single main sequence
stars. We find 79 new red dwarf systems predicted to be within 25 pc, including
a few new components of previously known systems. Two systems - SCR 1731-2452
at 9.5 pc and SCR 1746-3214 at 9.9 pc - are anticipated to be within 10 pc. We
also find 23 new white dwarf candidates with distance estimates of 15-66 pc, as
well as 360 new red subdwarf candidates. With this search, we complete the SCR
sweep of the southern sky for stars with mu > 0.18 "/yr and R_59F < 16.5,
resulting in a total of 5042 objects in 4724 previously unreported proper
motion systems. Here we provide selected comprehensive lists from our SCR
proper motion search to date, including 152 red dwarf systems estimated to be
within 25 pc (nine within 10 pc), 46 white dwarfs (ten within 25 pc), and 598
subdwarf candidates. The results of this search suggest that there are more
nearby systems to be found at fainter magnitudes and lower proper motion limits
than those probed so far.Comment: 47 pages, 16 of text. 7 figure
M dwarf stars in the light of (future) exoplanet searches
We present a brief overview of a splinter session on M dwarf stars as planet
hosts that was organized as part of the Cool Stars 17 conference. The session
was devoted to reviewing our current knowledge of M dwarf stars and exoplanets
in order to prepare for current and future exoplanet searches focusing in low
mass stars. We review the observational and theoretical challenges to
characterize M dwarf stars and the importance of accurate fundamental
parameters for the proper characterization of their exoplanets and our
understanding on planet formation.Comment: 4 pages, 1 figure. Summary of the splinter session "M dwarf stars in
the light of (future) exoplanet searches" held at the 17th Cambridge Workshop
on Cool Stars, Stellar Systems, and the Sun, June 28th 2012, Barcelona,
Spain. Submitted for publication in Astronomische Nachrichten - Astronomical
Notes (AN) 334, Issue 1-2, Eds Klaus Strassmeier and Mercedes L\'opez-Morale
Influence of interfaces reflectivity for central thickness measurement of a contact lens by low coherence interferometry
This paper evaluates quantitatively the relative interfaces reflectivity of a soft contact lens set in air or in water by a low coherence interferometric method. The sensitivity of the set-up is then measured and the minimum reflectivity is estimated to be as low as 0.02%. Measurement of the central thickness of the lens, even when it is immersed in water, is then possible using a new supercontinuum source
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