310 research outputs found
Galactic structure towards the Open Clusters NGC 188 and NGC 3680
We present the first comparisons of a newly developed Galactic Structure and
Kinematic Model to magnitude and color counts, as well as relative proper
motions, in the fields of the open clusters NGC 188 [(l, b)= (122.8 deg, +22.4
deg)] and NGC 3680 [(l,b)= (286.8 deg, +16.9 deg)]. In addition to determining
the reddening toward these two clusters, it is shown that starcounts at
intermediate Galactic latitudes in the range 11< V< 17 allow us to constrain
the model scale-height for disk subgiants. We obtain a mean value of 250 +/- 32
pc, in agreement with previous determinations of the scale-height for
red-giants. We are also able to constrain the scale-height of main-sequence
stars, and the distance of the sun from the Galactic plane, ruling out the
possibility of a value of +40 pc, in favor of a smaller value. Comparisons with
the observed proper-motion histograms indicate that the velocity dispersion of
disk main-sequence stars must increase with distance from the Galactic plane in
order to match the observed proper-motion dispersion. The required increase is
consistent with the values predicted by dynamical models, and provides a clear
observational evidence in favor of such gradients. The shape of the observed
proper-motion distribution is well fitted within the Poisson uncertainties.
This implies that corrections to absolute proper motion (and, therefore, space
velocities) for open clusters may be obtained using our model when no inertial
reference frame is available. Using this approach, the derived tangential
motions for NGC 188 and NGC 3680 are presented.Comment: Tex type, 29 pages, 9 postscript figures. Accepted for publication in
The Astronomical Journa
No evidence for a dark matter disk within 4 kpc from the Galactic plane
We estimated the dynamical surface mass density (Sigma) at the solar
Galactocentric distance between 2 and 4 kpc from the Galactic plane, as
inferred from the observed kinematics of the thick disk. We find Sigma(z=2
kpc)=57.6+-5.8 Mo pc^-2, and it shows only a tiny increase in the z-range
considered by our investigation. We compared our results with the expectations
for the visible mass, adopting the most recent estimates in the literature for
contributions of the Galactic stellar disk and interstellar medium, and
proposed models of the dark matter distribution. Our results match the
expectation for the visible mass alone, never differing from it by more than
0.8 $Mo pc^-2 at any z, and thus we find little evidence for any dark
component. We assume that the dark halo could be undetectable with our method,
but the dark disk, recently proposed as a natural expectation of the LambdaCDM
models, should be detected. Given the good agreement with the visible mass
alone, models including a dark disk are less likely, but within errors its
existence cannot be excluded. In any case, these results put constraints on its
properties: thinner models (scale height lower than 4 kpc) reconcile better
with our results and, for any scale height, the lower-density models are
preferred. We believe that successfully predicting the stellar thick disk
properties and a dark disk in agreement with our observations could be a
challenging theoretical task.Comment: Accepted for publication in ApJ Letter
Central Proper-Motion Kinematics of NGC 6752
We present proper motions derived from WFPC2 imaging for stars in the core of
the peculiar globular cluster NGC 6752. The central velocity dispersion in both
components of the proper motion is 12 km/s. We discuss the implications of this
result as well as the intrinsic difficulties in making such measurements. We
also give an alternative correction for the 34-row problem in the WFPC2 CCDs.Comment: 25 pages, 7 figures, 1 table included. Accepted for publication in A
Probing the nature of possible open cluster remnants with the Southern Prope r Motion Program
We discuss the nature of eleven Possible Open Cluster Remnants (POCRs)
by using absolute proper motions from the Southern Proper Motion (SPM)
Program 3 (Girard et al. [CITE], AJ, 127, 3060) combined with near infrared photometry from 2MASS.
The analysis is done by considering the distribution of stars
in the Color–Magnitude and the Vector Point diagrams. We successfully probed
the capabilities of the SPM catalog to detect a physical group by looking at the open
cluster Blanco 1. However,
within the uncertainties of the SPM3 catalog and basing
on 2MASS photometry we conclude that only one – ESO 282SC26 – out of eleven objects turns out to be a
probable physical group. We suggest it is an open cluster 1.3Â Gyr old and located 1.4Â kpc
from the Sun
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