We quantify the detectability of stellar Milky Way satellites in the Sloan Digital Sky Survey (SDSS) Data Release 5. We show that the effective search volumes for the recently discovered SDSS--satellites depend strongly on their luminosity, with their maximum distance, $D_{max}$, substantially smaller than the Milky Way halo's virial radius. Calculating the maximum accessible volume, $V_{max}$, for all faint detected satellites, allows the calculation of the luminosity function for Milky Way satellite galaxies, accounting quantitatively for their detectability. We find that the number density of satellite galaxies continues to rise towards low luminosities, but may flatten at $M_V \sim -5$; within the uncertainties, the luminosity function can be described by a single power law $dN/dM_{V}= 10 \times 10^{0.1 (M_V+5)}$, spanning luminosities from $M_V=-2$ all the way to the luminosity of the Large Magellanic Cloud. Comparing these results to several semi-analytic galaxy formation models, we find that their predictions differ significantly from the data: either the shape of the luminosity function, or the surface brightness distributions of the models, do not match.Comment: accepted to Ap
Aims. According to some estimations, there are as many as 100 000 open clusters in the Galaxy, but less than 2000 of them have been discovered, measured, and cataloged. We plan to undertake data mining of multiwavelength surveys to find new star clusters. Methods. We have developed a new method to search automatically for star clusters in very large stellar catalogs, which is based on convolution with density functions. We have applied this method to a subset of the Two Micron All Sky Survey catalog toward the Galactic anticenter. We also developed a method to verify whether detected stellar groups are real star clusters, which tests whether the stars that form the spatial density peak also fall onto a single isochrone in the color-magnitude diagram. By fitting an isochrone to the data, we estimate at the same time the main physical parameters of a cluster: age, distance, color excess. Results. For the present paper, we carried out a detailed analysis of 88 overdensity peaks detected in a field of 16 × 16 degrees near the Galactic anticenter. From this analysis, 15 overdensities were confirmed to be new open clusters and the physical and structural parameters were determined for 12 of them; 10 of them were previously suspected to be open clusters by Kronberger (2006) and Froebrich (2007). The properties were also determined for 13 yet-unstudied known open clusters, thus almost tripling the sample of open clusters with studied parameters in the anticenter. The parameters determined with this method showed a good agreement with published data for a set of well-known clusters.
Automated search for star clusters in J, H, Ks data from 2MASS catalog has been performed using the method developed by Koposov et. al (2008). We have found and verified 153 new clusters in the interval of the galactic latitude −24 • < b < 24 • . Color excesses E(B − V ), distance moduli and ages were determined for 130 new and 14 yet-unstudied known clusters. In this paper, we publish a catalog of coordinates, diameters, and main parameters of all the clusters under study. A special web-site available at http://ocl.sai.msu.ru has been developed to facilitate dissemination and scientific usage of the results.
We use the currently most complete collection of reliable Cepheid positions (565 stars) out to ∼ 5 kpc based mostly on our photometric data to outline the spiral pattern of our Galaxy. We find the pitch-angle to be equal to 9-10 o with the most accurate estimate (i =9.5 ±0.1 o ) obtained assuming that the spiral pattern has a four-armed structure, and the solar phase angle in the spiral pattern to be χ ⊙ = 121 o ± 3 o . The pattern speed is found to be Ω sp = 25.2 ± 0.5 km/s/kpc based on a comparison of the positions of the spiral arms delineated by Cepheids and maser sources and the age difference between these objects.
Abstract. Radial-velocity and BV CCD observations of 29 red-giant candidates in the central part of the intermediate-age open cluster NGC 1817 have confirmed the membership of 21 stars. In addition, we have used radial velocities and new UBVR photoelectric photometry to investigate the membership of 59 new candidates located in an outer corona surrounding NGC 1817, identified from the POSS plates. We have confirmed 18 new members out to a distance of 27 from the centre, thus doubling the previous cluster radius. Ten definite spectroscopic-binary members have been discovered, corresponding to a binary frequency of 25.6% and four orbits have been determined so far. Three additional stars have been identified as possible spectroscopicbinary members. We derive a cluster mean velocity of +65.33 ± 0.09 (0.52 s.e.) km s −1 based on 29 members with no detected velocity variations plus the 3 binary members with orbital solutions. The radial distribution of the 39 red-giant members shows a strong concentration of spectroscopic binaries in the inner part of the cluster, similar to that observed in M 67, although NGC 1817 is much younger.Key words. open clusters and associations: individual: NGC 1817 -stars: binaries: spectroscopic -techniques: radial velocities -stars: AGB and post-AGB IntroductionThe physical limits of open clusters are difficult to define. The apparent diameters tabulated mainly by Lyngå (1987) are based on eye estimates, which depend on the density ratio of the cluster to the background field. In many cases, the true cluster extension can be significantly larger than the area investigated, and this has several important consequences for the determination of the luminosity function, the study of the radial structure, and on the distribution of stellar populations in the colour-magnitude diagram. The latter point is especially important for the study of red-giant evolution.The photometric investigation of NGC 1817 by Harris & Harris (1977) led us to believe that the area they studied was too small with respect to the real size of the cluster. Therefore the POSS charts were examined to identify red stars in the area surrounding NGC 1817 out to 45 from the centre, while the radius given by Lyngå (1987) is 10 . Fifty-nine stars were selected for further observations. We show in this paper that 18 of the 59 outer candidates have radial velocities and UBVR photometry consistent with cluster membership.The total number of red giants in NGC 1817 is now 39 (21 in the inner part and 18 in the outer part). Thus, this cluster becomes very interesting for comparing the radial distributions of the single and binary red giants, to extend the work of Raboud & Mermilliod (1994). It also provides another cluster for comparing the observed patterns of red giants in the colour-magnitude diagrams with those predicted by theoretical isochrones, in connection with the apparent mass spread among Article published by EDP Sciences and available at
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