981 research outputs found
The Galactic disk mass-budget : II. Brown dwarf mass-function and density
In this paper, we extend the calculations conducted previously in the stellar
regime to determine the brown dwarf IMF in the Galactic disk. We perform Monte
Carlo calculations taking into account the brown dwarf formation rate, spatial
distribution and binary fraction. Comparison with existing surveys seems to
exclude a power-law MF as steep as the one determined in the stellar regime
below 1 \msol and tends to favor a more flatish behaviour. Comparison with
methane-dwarf detections tends to favor an eventually decreasing form like the
lognormal or the more general exponential distributions determined in the
previous paper. We calculate predicting brown dwarf counts in near-infrared
color diagrams and brown dwarf discovery functions. These calculations yield
the presently most accurate determination of the brown dwarf census in the
Galactic disk. The brown dwarf number density is comparable to the stellar one,
pc. The corresponding brown dwarf mass
density, however, represents only about 10% of the stellar contribution, i.e.
\rho_{BD}\simle 5.0\times 10^{-3} \mvol. Adding up the local stellar density
determined previously yields the density of star-like objects, stars and brown
dwarfs, in the solar neighborhood \rho_\odot \approx 5.0\times 10^{-2} \mvol.Comment: 39 pages, Latex file, uses aasms4.sty, to be published in ApJ,
corrected version with correct figure
Substellar Companions to Main Sequence Stars: No Brown Dwarf Desert at Wide Separations
We use three field L and T dwarfs which were discovered to be wide companions
to known stars by the Two Micron All-Sky Survey (2MASS) to derive a preliminary
brown dwarf companion frequency. Observed L and T dwarfs indicate that brown
dwarfs are not unusually rare as wide (Delta >1000 A.U.) systems to F-M0
main-sequence stars (M>0.5M_sun, M_V<9.5), even though they are rare at close
separation (Delta <3 A.U.), the ``brown dwarf desert.'' Stellar companions in
these separation ranges are equally frequent, but brown dwarfs are >~ 10 times
as frequent for wide than close separations. A brown dwarf wide-companion
frequency as low as the 0.5% seen in the brown dwarf desert is ruled out by
currently-available observations.Comment: ApJL, in pres
Woe from stones: commemoration, identity politics and Estonia's 'War of Monuments'
No abstract available
A Strategy for Identifying the Grid Stars for the Space Interferometry Mission (SIM)
We present a strategy to identify several thousand stars that are
astrometrically stable at the micro-arcsecond level for use in the SIM (Space
Interferometry Mission) astrometric grid. The requirements on the grid stars
make this a rather challenging task. Taking a variety of considerations into
account we argue for K giants as the best type of stars for the grid, mainly
because they can be located at much larger distances than any other type of
star due to their intrinsic brightness. We show that it is possible to identify
suitable candidate grid K giants from existing astrometric catalogs. However,
double stars have to be eliminated from these candidate grid samples, since
they generally produce much larger astrometric jitter than tolerable for the
grid. The most efficient way to achieve this is probably by means of a radial
velocity survey. To demonstrate the feasibility of this approach, we repeatedly
measured the radial velocities for a pre-selected sample of 86 nearby Hipparcos
K giants with precisions of 5-8 m/s. The distribution of the intrinsic radial
velocity variations for the bona-fide single K giants shows a maximum around 20
m/s, which is small enough not to severely affect the identification of stellar
companions around other K giants. We use the results of our observations as
input parameters for Monte-Carlo simulations on the possible design of a radial
velocity survey of all grid stars. Our favored scenario would result in a grid
which consists to 68% of true single stars and to 32% of double or multiple
stars with periods mostly larger than 200 years, but only 3.6% of all grid
stars would display astrometric jitter larger than 1 microarcsecond. This
contamination level is probably tolerable.Comment: LaTeX, 21 pages, 8 figures, accepted by PASP (February 2001 issue).
Also available at http://beehive.ucsd.edu/ftp/pub/grid/kgiants.htm
Optical vortices with starlight: Implications for ground-based stellar coronagraphy
Using an l = 1 blazed fork-hologram at the focal plane of the Asiago 122 cm
telescope, we obtained optical vortices from the stellar system Rasalgethi
(alpha Herculis) and from the single star Arcturus (alpha Bootis). We have
analyzed the structure of the optical vortices obtained from non-monochromatic
starlight under very poor seeing conditions using a fast CCD camera to obtain
speckle patterns and carry out the lucky imaging technique, alternative to
adaptive optics. With the insertion of a red filter and of a Lyot stop we
performed l = 1 optical vortex coronography the double star HD74010. The
results are in agreement with theory and numerical simulations. Our results
open the way to applications of optical vortices to ground based astronomical
observations, in particular for coronagraphy with l > 1 masks. No intrinsic
orbital angular momentum was detected in the starlight.Comment: 4 pages, 5 figures. Revised data analysi
Gamma-Ray Burst Sequences in Hardness Ratio-Peak Energy Plane
The narrowness of the distribution of the peak energy of
spectrum of gamma-ray bursts (GRBs) and the unification of GRB population are
great puzzles yet to be solved. We investigate the two puzzles based on the
global spectral behaviors of different GRB population in the
plane (HR the spectral hardness ratio) with BATSE and HETE-2 observations. It
is found that long GRBs and XRFs observed by HETE-2 seem to follow the same
sequence in the plane, with the XRFs at the low end of this
sequence. The long and short GRBs observed by BATSE follow significantly
different sequences in the plane, with most of the short GRBs
having a larger hardness ratio than the long GRBs at a given .
These results indicate that the global spectral behaviors of the long GRB
sample and the XRF sample are similar, while that of short GRBs is different.
The short GRBs seem to be a unique subclass of GRBs, and they are not the
higher energy extension of the long GRBs (abridged).Comment: 9 pages, 3 figure
SB9: The Ninth Catalogue of Spectroscopic Binary Orbits
The Ninth Catalogue of Spectroscopic Binary Orbits
(http://sb9.astro.ulb.ac.be) continues the series of compilations of
spectroscopic orbits carried out over the past 35 years by Batten and
collaborators. As of 2004 May 1st, the new Catalogue holds orbits for 2,386
systems. Some essential differences between this catalogue and its predecessors
are outlined and three straightforward applications are presented: (1)
Completeness assessment: period distribution of SB1s and SB2s; (2) Shortest
periods across the H-R diagram; (3) Period-eccentricity relation.Comment: Accepte for publication in A&A, 6 pages, 6 figure
Keeping calm in the face of change: towards optimisation of FRP by reasoning about change
Functional Reactive Programming (FRP) is an approach to reactive programming where systems are structured as networks of functions operating on signals (time-varying values). FRP is based on the synchronous data-flow paradigm and supports both (an approximation to) continuous-time and discrete-time signals (hybrid systems).What sets FRP apart from most other languages for similar applications is its support for systems with dynamic structure and for higher-order reactive constructs. This paper contributes towards advancing the state of the art of FRP implementation by studying the notion of signal change and change propagation in a setting of structurally dynamic networks of n-ary signal functions operating on mixed continuous-time and discrete-time signals. We first define an ideal denotational semantics (time is truly continuous) for this kind of FRP, along with temporal properties, expressed in temporal logic, of signals and signal functions pertaining to change and change propagation. Using this framework, we then show how to reason about change; specifically, we identify and justify a number of possible optimisations, such as avoiding recomputation of unchanging values. Note that due to structural dynamism, and the fact that the output of a signal function may change because time is passing even if the input is unchanging, the problem is significantly more complex than standard change propagation in networks with static structure
NGC 5385, NGC 2664 and Collinder 21: three candidate Open Cluster Remnants
We present CCD UBVI photometric and medium/high resolution spectroscopic
observations obtained in the field of the previously unstudied dissolving open
cluster candidates NGC 5385, NGC 2664 and Collinder~21. Our analysis stands on
the discussion of star counts, photometry, radial velocity distribution, and
proper motions available from the Tycho~2 catalogue. All the three aggregates
clearly emerge from the mean Galactic field, but, regrettably, the close
scrutiny of proper motions and radial velocities reveals that we are not facing
any physical group. Instead, what we are looking at are just chance alignments
of a few bright unrelated stars. Our analysis casts some doubt on the Bica et
al. (2001) criterion to look for Possible Open Cluster Remnants. It seems
mandatory to define a better criterion to adopt for further investigations.Comment: 12 pages, 12 eps figures (7, 9 and 11 degraded in resolution),
accepted for publication in Astronomy & Astrophysic
How Dry is the Brown Dwarf Desert?: Quantifying the Relative Number of Planets, Brown Dwarfs and Stellar Companions around Nearby Sun-like Stars
Sun-like stars have stellar, brown dwarf and planetary companions. To help
constrain their formation and migration scenarios, we analyse the close
companions (orbital period < 5 years) of nearby Sun-like stars. By using the
same sample to extract the relative numbers of stellar, brown dwarf and
planetary companions, we verify the existence of a very dry brown dwarf desert
and describe it quantitatively. With decreasing mass, the companion mass
function drops by almost two orders of magnitude from 1 M_Sun stellar
companions to the brown dwarf desert and then rises by more than an order of
magnitude from brown dwarfs to Jupiter-mass planets. The slopes of the
planetary and stellar companion mass functions are of opposite sign and are
incompatible at the 3 sigma level, thus yielding a brown dwarf desert. The
minimum number of companions per unit interval in log mass (the driest part of
the desert) is at M = 31^{+25}_{-18} M_Jup. Approximately 16% of Sun-like stars
have close (P < 5 years) companions more massive than Jupiter: 11% +- 3% are
stellar, <1% are brown dwarf and 5% +- 2% are giant planets. The steep decline
in the number of companions in the brown dwarf regime, compared to the initial
mass function of individual stars and free-floating brown dwarfs, suggests
either a different spectrum of gravitational fragmentation in the formation
environment or post-formation migratory processes disinclined to leave brown
dwarfs in close orbits.Comment: Conforms to version accepted by ApJ. 13 pages formatted with
emulateapj.cl
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