624 research outputs found
The combination of ground-based astrometric compilation catalogues with the HIPPARCOS Catalogue. II. Long-term predictions and short-term predictions
The combination of ground-based astrometric compilation catalogues, such as
the FK5 or the GC, with the results of the ESA Astrometric Satellite HIPPARCOS
produces for many thousands of stars proper motions which are significantly
more accurate than the proper motions derived from the HIPPARCOS observations
alone. In Paper I (Wielen et al. 1999, A&A 347, 1046) we have presented a
method of combination for single stars (SI mode). The present Paper II derives
a combination method which is appropriate for an ensemble of 'apparently
single-stars' which contains undetected astrometric binaries. In this case the
quasi-instantaneously measured HIPPARCOS proper motions and positions are
affected by 'cosmic errors', caused by the orbital motions of the photo-centers
of the undetected binaries with respect to their center-of-mass. In contrast,
the ground-based data are 'mean values' obtained from a long period of
observation. We derive a linear 'long-term prediction' (LTP mode) for epochs
far from the HIPPARCOS epoch T_H ~ 1991.25, and a linear 'short-term
prediction' (STP mode) for epochs close to T_H. The most accurate prediction
for a position at an arbitrary epoch is provided by a smooth, non-linear
transition from the STP solution to the LTP solution. We present an example for
the application of our method, and we discuss the error budget of our method
for the FK6 (a combination of the FK5 with HIPPARCOS) and for the combination
catalog GC+HIP. For the basic fundamental stars, the accuracy of the FK6 proper
motions in the LTP mode is better than that of the HIPPARCOS proper motions
(taking here the cosmic errors into account) by a factor of more than 4.Comment: Slightly revised version. Accepted for publication in Astronomy and
Astrophysic
Absolute Proper Motions of Open Clusters: I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determined from
their member stars found in the Hipparcos Catalogue. 360 clusters were searched
for possible members, excluding nearby clusters with distances D < 200 pc.
Members were selected using ground based information (photometry, radial
velocity, proper motion, distance from the cluster centre) and information
provided by Hipparcos (proper motion, parallax). Altogether 630 certain and 100
possible members were found. A comparison of the Hipparcos parallaxes with
photometric distances of open clusters shows good agreement. The Hipparcos data
confirm or reject the membership of several Cepheids in the studied clusters.Comment: 25 pages, 4 figures, 3 tables. A&A Suppl. Ser. in pres
On the Nature of the Unique H-Emitting T Dwarf 2MASS J12373919+6526148
We explore and discount the hypothesis that the strong, continual
H-emitting T dwarf 2MASS J12373919+6526148 can be explained as a young,
low gravity, very low mass brown dwarf. The source is already known to have a
marginally-fainter absolute magnitude than similar T dwarfs with trigonometric
parallax measurements, and has a tangential velocity consistent with old disk
kinematics. Applying the technique of Burgasser, Burrows & Kirkpatrick on new
near infrared spectroscopy for this source, estimates of its {\teff},
and metallicity ([M/H]) are obtained. 2M 1237+6526 has a {\teff}
800-850 K. If [M/H] is solar, is as high as 5.5 (cgs) and this
source is older than 10 Gyr. We find a more plausible scenario to be a modestly
subsolar metallicity ([M/H] = -0.2) and moderate 5.0, implying
an age older than 2 Gyr and a mass greater than 0.035 M_{\sun}. The
alternative explanation of the unique emission of this source, involving an
interacting, close, double degenerate system, should be investigated further.
Indeed, there is some evidence of a {\teff} 500 K companion to 2M 1237+6526
on the basis of a possible [3.6]--[4.5] color excess. This
excess may, however, be caused by a subsolar metallicity.Comment: Astrophysical Journal, in press 15 pages, 5 figure
Galaxies with Spiral Structure up to z = 0.87 --Limits on M/L and the Stellar Velocity Dispersion
We consider seven distant galaxies with clearly evident spiral structure from
HST images. Three of these were chosen from Vogt et al. (1996) (VFP) and have
measured rotational velocities. Five were chosen from the Medium Deep Survey
and are studied in Sarajedini et al. 1996 (SGGR), and one galaxy is found in
both papers. We place upper limits on their mass-to-light ratios (M/L) by
computing M/L_B for a maximal disk. We find that these galaxies have maximal
disk mass-to-light ratios M/L_B = 1.5 - 3.5 M_sol/L_Bsol at the low end, but
within the range seen in nearby galaxies. The mass-to-light ratios are low
enough to suggest that the galaxies contain a young, rapidly formed stellar
population.
By using a Toomre stability criterion for formation of spiral structure, we
place constraints on the ratio of M/L to the stellar velocity dispersion. If
these galaxies have maximal disks they would have to be nearly unstable so as
to have small enough velocity dispersions that their disks are not
unrealistically thick. This suggests that there is a substantial amount of dark
matter present in the luminous regions of the galaxy.Comment: AAS Latex + PS Figure, accepted for publication in A
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