363 research outputs found
Constraints to the Masses of Brown Dwarf Candidates from the Lithium Test
We present intermediate dispersion (0.7-2.2 \AA ~pix) optical
spectroscopic observations aimed at applying the ``Lithium Test'' to a sample
of ten brown dwarf candidates located in the general field, two in young open
clusters, and two in close binaries. We find evidence for strong Li depletion
in all of them, and thus infer lower mass limits of 0.065~M, depending
only slightly (0.005~M) on the interior models. None of the field
brown dwarf candidates in our sample appears to be a very young (age
~10~yr) substellar object. For one of the faintest proper motion
Pleiades members known (V=20.7) the Li test implies a mass greater than
0.08~M, and therefore it is not a brown dwarf. From our spectra
we estimate spectral types for some objects and present measurements of Halpha
emission strengths and radial velocities. Finally, we compare the positions in
the H-R diagram of our sample of brown dwarf candidates with the theoretical
region where Li is expected to be preserved (Substellar Lithium Region). We
find that certain combinations of temperature calibrations and evolutionary
tracks are consistent with the constraints imposed by the observed Li depletion
in brown dwarf candidates, while others are not.Comment: 20 pp.; 4 figs, available under request; plain LaTeX, ApJ in press,
OACatania-94-00
Brown Dwarfs in the Pleiades Cluster. III. A deep IZ survey
We present the results of a deep CCD-based IZ photometric survey of a ~1 sq.
deg area in the central region of the Pleiades Galactic open cluster. The
magnitude coverage of our survey (from I~17.5 down to 22) allows us to detect
substellar candidates with masses between 0.075 and 0.03 Msol. Details of the
photometric reduction and selection criteria are given. Finder charts prepared
from the I-band images are provided.Comment: 11 pages with 8 figures, 4 of them are finder charts given in gif
format. Accepted for publication in A&AS. Also available at
http://www.iac.es/publicaciones/preprints.htm
Integrated Trigger and Data Acquisition system for the NA62 experiment at CERN
The main goal of the NA62 experiment is to measure the branching ratio of the K+decay, collecting O(100) events in two years of data taking. Efficient online selection of interesting events and loss-less readout at high rate will be key issues for such experiment. An integrated trigger and data acquisition system has been designed. Only the very first trigger stage will be implemented in hardware, in order to reduce the total rate for the software levels on PC farms. Readout uniformity among different subdetectors and scalability were taken into account in the architecture design
A Test of Pre-Main Sequence Evolutionary Models Across the Stellar/Substellar Boundary Based on Spectra of the Young Quadruple GG Tau
We present spatially separated optical spectra of the components of the young
hierarchical quadruple GG Tau. Spectra of GG Tau Aa and Ab (separation 0".25 ~
35 AU) were obtained with the Faint Object Spectrograph aboard the Hubble Space
Telescope. Spectra of GG Tau Ba and Bb (separation 1".48 ~ 207 AU) were
obtained with both the HIRES and the LRIS spectrographs on the W. M. Keck
telescopes. The components of this mini-cluster, which span a wide range in
spectral type (K7 - M7), are used to test both evolutionary models and the
temperature scale for very young, low mass stars under the assumption of coeval
formation. Of the evolutionary models tested, those of Baraffe et al. (1998,
A&A, 337, 403) yield the most consistent ages when combined with a temperature
scale intermediate between that of dwarfs and giants. The version of the
Baraffe et al. models computed with a mixing length nearly twice the pressure
scale height is of particular interest as it predicts masses for GG Tau Aa and
Ab that are in agreement with their dynamical mass estimate.
Using this evolutionary model and a coeval (at 1.5 Myrs) temperature scale,
we find that the coldest component of the GG Tau system, GG Tau Bb, is
substellar with a mass of 0.044 +/- 0.006 Msun. This brown dwarf companion is
especially intriguing as it shows signatures of accretion, although this
accretion is not likely to alter its mass significantly. GG Tau Bb is currently
the lowest mass, spectroscopically confirmed companion to a T Tauri star, and
is one of the coldest, lowest mass T Tauri objects in the Taurus-Auriga star
forming region.Comment: 25 pages, 6 figures, accepted for publication in The Astrophysical
Journa
Discovery of "isolated" comoving T Tauri stars in Cepheus
During the course of a large spectroscopic survey of X-ray active late-type
stars in the solar neighbourhood, we discovered four lithium-rich stars packed
within just a few degrees on the sky. These very young stars are projected
several degrees away from the Cepheus-Cassiopea clouds, in front of an area
void of interstellar matter. As such, they are very good "isolated" T Tauri
star candidates. We acquired high-resolution optical spectra as well as
photometric data allowing us to investigate in detail their nature and physical
parameters with the aim of testing the "runaway" and "in-situ" formation
scenarios. We derive accurate radial and rotational velocities and perform an
automatic spectral classification. The spectral subtraction technique is used
to infer chromospheric activity level in the H-alpha line core and clean the
spectra of photospheric lines before measuring the equivalent width of the
lithium absorption line. Both physical (lithium content, magnetic activity) and
kinematical indicators show that all stars are very young (ages in the range
10-30 Myr). In particular, the spectral energy distribution of TYC4496-780-1
displays a strong near- and far-infrared excess, typical of T Tauri stars still
surrounded by an accretion disc. They also share the same Galactic motion,
proving that they form a homogeneous moving group of stars with the same
origin. The most plausible explanation of how these "isolated" T Tauri stars
formed is the "in-situ" model, although accurate distances are needed to
clarify their connection with the Cepheus-Cassiopeia complex
Spectroscopic binaries in a sample of ROSAT X-ray sources south of the Taurus molecular clouds
We report the results of our radial-velocity monitoring of spectroscopic
binary systems in a sample of X-ray sources from the ROSAT All Sky Survey south
of the Taurus-Auriga star-forming region. The original sample of approximately
120 sources by Neuhaeuser et al. was selected on the basis of their X-ray
properties and the visual magnitude of the nearest optical counterpart, in such
a way as to promote the inclusion of young objects. Roughly 20% of those
sources have previously been confirmed to be very young. We focus here on the
subset of the original sample that shows variable radial velocities (43
objects), a few of which have also been flagged previously as being young. New
spectroscopic orbits are presented for 42 of those systems. Two of the
binaries, RXJ0528.9+1046 and RXJ0529.3+1210, are indeed weak-lined T Tauri
stars likely to be associated with the Lambda Orionis region. Most of the other
binaries are active objects of the RS CVn-type, including several W UMa and
Algol systems. We detect a strong excess of short-period binaries compared to
the field, and an unusually large fraction of double-lined systems, as well as
an overall high frequency of binaries out of the original sample. These results
can be understood as selection effects. A short description of the physical
properties of each binary is provided, and a comparison with evolutionary
tracks is made using the stellar density as a distance-independent measure of
evolution (abridged).Comment: 36 pages, 10 figures, 7 tables, to appear in The Astronomical
Journal, March 200
Lithium Depletion in Fully Convective Pre-Main Sequence Stars
We present an analytic calculation of the thermonuclear depletion of lithium
in contracting, fully convective, pre-main sequence stars of mass M < 0.5
M_sun. Previous numerical work relies on still-uncertain physics (atmospheric
opacities and convection, in particular) to calculate the effective temperature
as a unique function of stellar mass. We assume that the star's effective
temperature, T_eff, is fixed during Hayashi contraction and allow its actual
value to be a free parameter constrained by observation. Using this
approximation, we compute lithium burning analytically and explore the
dependence of lithium depletion on T_eff, M, and composition. Our calculations
yield the radius, age, and luminosity of a pre-main sequence star as a function
of lithium depletion. This allows for more direct comparisons to observations
of lithium depleted stars. Our results agree with those numerical calculations
that explicitly determine stellar structure during Hayashi contraction. In
agreement with Basri, Marcy, and Graham (1996), we show that the absence of
lithium in the Pleiades star HHJ 3 implies that it is older than 100 Myr. We
also suggest a generalized method for dating galactic clusters younger than 100
Myr (i.e., those with contracting stars of M > 0.08 M_sun) and for constraining
the masses of lithium depleted stars.Comment: 13 pages, LaTex with 2 postscript figures, uses aaspp4.sty and
epsfig.sty, to appear in the Astrophysical Journa
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