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The Stellar Populations of Praesepe and Coma Berenices
We present the results of a stellar membership survey of the nearby openclusters Praesepe and Coma Berenices. We have combined archival surveydata from the SDSS, 2MASS, USNOB1.0, and UCAC-2.0 surveys to compileproper motions and photometry for ~5 million sources over 300deg2. Of these sources, 1010 stars in Praesepe and 98 starsin Coma Ber are identified as candidate members with probability >80%442 and 61 are identified as high-probability candidates for the firsttime. We estimate that this survey is >90% complete across a widerange of spectral types (F0-M5 in Praesepe, F5-M6 in Coma Ber). We havealso investigated the stellar mass dependence of each cluster's mass andradius in order to quantify the role of mass segregation and tidalstripping in shaping the present-day mass function and spatialdistribution of stars. Praesepe shows clear evidence of mass segregationacross the full stellar mass range; Coma Ber does not show any cleartrend, but low number statistics would mask a trend of the samemagnitude as in Praesepe. The mass function for Praesepe (? ~ 600Myr; M ~ 500 Modot) follows a power law consistent with thatof the field present-day mass function, suggesting that anymass-dependent tidal stripping could have removed only the lowest massmembers (<0.15 Modot). Coma Ber, which is younger but muchless massive (? ~ 400 Myr; M ~ 100 Modot), follows asignificantly shallower power law. This suggests that some tidalstripping has occurred, but the low-mass stellar population has not beenstrongly depleted down to the survey completeness limit (~0.12Modot).
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
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| The formation and evolution of binary systems. III. Low-mass binaries in the Praesepe cluster
With the aim of investigating the binary population of the 700 Myr oldPraesepe cluster, we have observed 149 G and K-type cluster membersusing adaptive optics. We detected 26 binary systems with an angularseparation ranging from less than 0.08 to 3.3 arcsec (15-600 AU). Aftercorrecting for detection biases, we derive a binary frequency (BF) inthe log P (days) range from 4.4 to 6.9 of 25.3 +/- 5.4%, which issimilar to that of field G-type dwarfs (23.8%, Duquennoy & Mayor\cite{r20}). This result, complemented by similar ones obtained for the2 Myr old star forming cluster IC 348 (Paper II) and the 120 Myr oldPleiades open cluster (Paper I), indicates that the fraction oflong-period binaries does not significantly evolve over the lifetime ofgalactic open clusters. We compare the distribution of cluster binariesto the binary populations of star forming regions, most notably Orionand Taurus, to critically review current ideas regarding the binaryformation process. We conclude that it is still unclear whether thelower binary fraction observed in young clusters compared to Tassociations is purely the result of the early dynamical disruption ofprimordial binaries in dense clusters or whether it reflectsintrinsically different modes of star formation in clusters andassociations. We also note that if Taurus binaries result from thedynamical decay of small-N protostellar aggregates, one would predictthe existence of a yet to be found dispersed population of mostly singlesubstellar objects in the Taurus cloud. Based on observations obtainedat the Canada-France-Hawaii Telescope.
| Stellar Activity in Coeval Open Clusters: Praesepe and the Hyades
Randich & Schmitt found that the coronal activity of solar-type andlow-mass stars in Praesepe is significantly lower than that of stars inthe Hyades cluster. This result is quite surprising, since the Hyadesand Praesepe have approximately the same age and metallicity and areoften thought to have originated in the same giant molecular cloudcomplex. We have carried out several tests in order to find a possibleexplanation for this result. We have measured radial velocities of twogroups of Praesepe stars (a dF-dK sample and a dM sample) and havemeasured Hα as a chromospheric activity index for the dM sample.Based on analyses of these data, we conclude that the Praesepe catalogused in the X-ray analysis does not contain a significant number ofnonmembers and, thus, that membership problems do not seem to be thecause of the Randich & Schmitt result. The comparison of theHα equivalent widths for the M dwarfs in Praesepe with those inthe Hyades indicates that, at least for stars in this mass range, thePraesepe stars are as active or more active than their Hyadescounterparts. The similarity of chromospheric emission allows us toreject differences in the rotational velocity distribution as the originof the dissimilar L_x luminosity functions. We have also analyzed a fewROSAT PSPC pointings of Praesepe in order to obtain a new andindependent estimate of the X-ray luminosities and upper limits for asmall sample of Praesepe members. This analysis suggests that theprevious ROSAT PSPC analysis produced slightly optimistic X-ray upperlimits; however, the differences between the old and new upper limitsare not large enough to explain the dichotomy in the X-ray properties ofPraesepe and the Hyades. Therefore, our examination of the availabledata does not provide a clear reason to explain why the X-ray luminosityfunctions of the two clusters are different. Part of the explanationcould be found in the binaries. Speculatively, these clusters could havedifferent orbital period distributions, with more short-period binariesamong the Hyades, which would show larger coronal activity.
| Evolution of mass segregation in open clusters: some observational evidences
On the basis of the best available member list and duplicityinformation, we have studied the radial structure of Praesepe and of thevery young open cluster NGC 6231. We have found mass segregation amongthe cluster members and between binaries and single stars, which isexplained by the greater average mass of the multiple systems. However,the degree of mass segregation for stars between 1.5 and 2.3 M_sun isless pronounced in Praesepe than in the Pleiades. Furthermore, masssegregation is already present in the very young open cluster NGC 6231although this cluster is likely still not dynamically relaxed. Wediscuss the implications of these results and propose a qualitativescenario for the evolution of mass segregation in open clusters. InPraesepe the mass function of single stars and primaries appears to besignificantly different, like in the Pleiades. We observe an absence ofellipticity of the outer part of Praesepe. Tables 2 and 3 are availableonly in electronic form from the Strasbourg ftp server at 130.79.128.5
| The luminosity function of Praesepe. I - A proper motion and photometric search for candidate members
Proper motions have been measured for a color selected sample of starsin a square 4 x 4 deg region centered on the Praesepe open cluster. Themagnitude limit of the photometry is about 19 and of the proper motionsabout 18. A list is presented of 765 probable and possible members from9 to 18 based on the proper motions, magnitudes, and colors. The resultsare compared with other proper motion surveys, and a preliminaryluminosity function of M(v) = 11 is calculated.
| Investigation of the Praesepe cluster. I - Identification of halo members
Coravel radial velocities and UBVRI (Kron) photometry of 117 starsselected by their proper motion in the region surrounding Praesepe (NGC2632, C 0837 + 201) have permitted identification of 48 members of thecluster halo, up to a distance of 4 deg from the cluster center. The 43(F5-K0) new members represent 51 percent of the number of nucleusmembers known in this spectral interval. Ten spectroscopic binaries havebeen discovered among the F5-F0 members, resulting in a frequency of 23percent, which rises to 33 percent four probable binaries are taken intoaccount. Six orbits have been obtained, with periods ranging from 1.2 to143 d. Four additional binaries have been detected among the non-memberstars.
| UBVRI photometry of G, K, M HIPPARCOS stars
UBVRI data are presented for a set of 229 late-type stars, most of thembeing high proper motion stars. All these data are part of the InputCatalog planned observations for the Hipparcos mission.
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| Csillagkép: | Rák |
| Rektaszcenzió: | 08h45m27.94s |
| Deklináció: | +21°39'12.9" |
| Vizuális fényesség: | 10.531 |
| RA sajátmozgás: | -35.4 |
| Dec sajátmozgás: | -15.3 |
| B-T magnitude: | 11.306 |
| V-T magnitude: | 10.595 |
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