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 1.3 Gyr old and located 1.4 kpc from the Sun