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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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| Open clusters with Hipparcos. I. Mean astrometric parameters
New memberships, mean parallaxes and proper motions of all 9 openclusters closer than 300 pc (except the Hyades) and 9rich clusters between 300 and 500 pc have been computed using Hipparcosdata. Precisions, ranging from 0.2 to 0.5 mas for parallaxes and 0.1 to0.5 mas/yr for proper motions, are of great interest for calibratingphotometric parallaxes as well as for kinematical studies. Carefulinvestigations of possible biases have been performed and no evidence ofsignificant systematic errors on the mean cluster parallaxes has beenfound. The distances and proper motions of 32 more distant clusters,which may be used statistically, are also indicated. Based onobservations made with the ESA Hipparcos astrometry satellite
| 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.
| 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.
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| Takýmyýldýz: | Yengec |
| Sað Açýklýk: | 08h34m59.64s |
| Yükselim: | +21°05'49.2" |
| Görünürdeki Parlaklýk: | 11.015 |
| özdevim Sað Açýklýk: | -33.6 |
| özdevim Yükselim: | -16.3 |
| B-T magnitude: | 11.583 |
| V-T magnitude: | 11.062 |
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