59 research outputs found
New neighbours VI. Spectroscopy of DENIS nearby stars candidates
We present spectra of 36 nearby star candidates and 3 red giant candidates,
identified in the DENIS database by Phan-Bao et al. (2003). 32 of the dwarf
candidates are indeed nearby red dwarfs, with spectral types from M5.5 to M8.5.
Out of 11 targets with low proper motion (mu < 0.1$ arc-sec/yr) but a Reduced
Proper Motion above an inclusive threshold, 9 are red dwarfs. The 4
contaminants are all reddened F-K main sequence stars, and they could have been
eliminated by checking for some well known high latitude molecular clouds.
These stars might be of interest as probes of interstellar absorption. For the
red dwarfs we derive spectral types and spectroscopic distances, using a new
calibration of the PC3 spectral index to absolute magnitudes in the I, J, H and
K photometric bands.
We confirm 2 new members of the 12 pc volume (2 new M8.5), and one M7.5 NLTT
object closer than 10pc. We also show that one quarter of the stars with
photometric distances under 30 pc have too small a proper motion for inclusion
in the NLTT catalog.Comment: Accepted by A&
Galactic S Stars: Investigations of Color, Motion, and Spectral Features
Known bright S stars, recognized as such by their enhanced s-process
abundances and C/O ratio, are typically members of the asymptotic giant branch
(AGB) or the red giant branch (RGB). Few modern digital spectra for these
objects have been published, from which intermediate resolution spectral
indices and classifications could be derived. For published S stars we find
accurate positions using the Two-Micron All Sky Survey (2MASS), and use the
FAST spectrograph of the Tillinghast reflector on Mt. Hopkins to obtain the
spectra of 57 objects. We make available a digital S star spectral atlas
consisting of 14 spectra of S stars with diverse spectral features. We define
and derive basic spectral indices that can help distinguish S stars from
late-type (M) giants and carbon stars. We convolve all our spectra with the
SDSS bandpasses, and employ the resulting gri magnitudes together with 2MASS
JHK mags to investigate S star colors. S stars have colors similar to carbon
and M stars, and are therefore difficult to distinguish by color alone. Using
near and mid-infrared colors from IRAS and AKARI, we identify some of the stars
as intrinsic (AGB) or extrinsic (with abundances enhanced by past
mass-transfer). We also use V band and 2MASS magnitudes to calculate a
temperature index for stars in the sample. We analyze the proper motions and
parallaxes of our sample stars to determine upper and lower limit absolute
magnitudes and distances, and confirm that most are probably giants.Comment: 11 pages. Accepted for publication in ApJS July 19, 2011. Spectra
available as http://hea-www.harvard.edu/~pgreen/SStarAtlas.ta
Universal Formulae for Percolation Thresholds
A power law is postulated for both site and bond percolation thresholds. The
formula writes , where is the space
dimension and the coordination number. All thresholds up to are found to belong to only three universality classes. For first two
classes for site dilution while for bond dilution. The last one
associated to high dimensions is characterized by for both sites and
bonds. Classes are defined by a set of value for . Deviations
from available numerical estimates at are within and
for high dimensional hypercubic expansions at . The
formula is found to be also valid for Ising critical temperatures.Comment: 11 pages, latex, 3 figures not include
Towards a fully consistent Milky Way disc model: Part 1 The local model based on kinematic and photometric data
We present a fully consistent evolutionary disc model of the solar cylinder.
The model is based on a sequence of stellar sub-populations described by the
star formation history (SFR) and the dynamical heating law (given by the
age-velocity dispersion relation AVR). The combination of kinematic data from
Hipparcos and the finite lifetimes of main sequence (MS) stars enables us to
determine the detailed vertical disc structure independent of individual
stellar ages and only weakly dependent on the IMF. The disc parameters are
determined by applying a sophisticated best fit algorithm to the MS star
velocity distribution functions in magnitude bins. We find that the AVR is well
constrained by the local kinematics, whereas for the SFR the allowed range is
larger. A simple chemical enrichment model is included in order to fit the
local metallicity distribution of G dwarfs. In our favoured model A the power
law index of the AVR is 0.375 with a minimum and maximum velocity dispersion of
5.1 km/s and 25.0 km/s, respectively. The SFR shows a maximum 10 Gyr ago and
declines by a factor of four to the present day value of 1.5 M_sun/pc^2/Gyr. A
best fit of the IMF leads to power-law indices of -1.46 below and -4.16 above
1.72 M_sun avoiding a kink at 1 M_sun. An isothermal thick disc component with
local density of ~6% of the stellar density is included. A thick disc
containing more than 10% of local stellar mass is inconsistent with the local
kinematics of K and M dwarfs.Comment: 20 pages, 18 figs., accepted by MNRA
Identification of red high proper-motion objects in Tycho-2 and 2MASS catalogues using Virtual Observatory tools
Aims: With available Virtual Observatory tools, we looked for new M dwarfs in
the solar neighbourhood and M giants with high tangential velocities. Methods:
From an all-sky cross-match between the optical Tycho-2 and the near-infrared
2MASS catalogues, we selected objects with proper motions >50mas/yr and very
red V-Ks colours. For the most interesting targets, we collected
multi-wavelength photometry, constructed spectral energy distributions,
estimated effective temperatures and surface gravities from fits to atmospheric
models, performed time-series analysis of ASAS V-band light curves, and
assigned spectral types from low-resolution spectroscopy obtained with CAFOS at
the 2.2m Calar Alto telescope. Results: We got a sample of 59 bright red high
proper-motion objects, including fifty red giants, four red dwarfs, and five
objects reported in this work for the first time. The five new stars have
magnitudes V~10.8-11.3mag, reduced proper motions midway between known dwarfs
and giants, near-infrared colours typical of giants, and effective temperatures
Teff~2900-3400K. From our time-series analysis, we discovered a long secondary
period in Ruber 4 and an extremely long primary period in Ruber 6. With the
CAFOS spectra, we confirmed the red giant nature of Ruber 7 and 8, the last of
which seems to be one of the brightest metal-poor M giants ever identified.Comment: Accepted in Astronomy & Astrophysic
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