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Register a new userA High Angular Resolution Multiplicity Survey of the Open Clusters alpha Persei and Praesepe
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Two hundred and forty-two members of the Praesepe and alpha Persei clusters ranging from B (~5Msun) to early-M (~0.5 Msun) have been surveyed with high angular resolution imaging. The 39 binary and 1 quadruple systems detected encompass separations from 0.053 to 7.28; 28 of the systems are new detections and there are 9 candidate substellar companions. The main results from the survey are: * Over the projected separation range of 26-581 AU and magnitude differences of DeltaK < 4.0 mag, the companion star fraction (CSF) for alpha Persei is 0.09 +/- 0.03 and for Praesepe is 0.10 +/- 0.03. This fraction is consistent with the field G-dwarf value, implying that there is not a systematic decline in multiplicity with age on timescales of at 4 +1/-1.5 AU, significantly smaller value than both the field G-dwarf and the nearby T Tauri distributions. A simple population synthesis model suggests that the G-dwarf binary population is a combination of ~30% dark cloud and ~70% giant molecular cloud systems. * An exploration of the binary star properties reveals a cluster CSF that increases with decreasing target mass and a cluster mass ratio distribution that rises more sharply for higher mass stars, but is independent of binary separation. These observational trends are consistent with several models of capture in small clusters and simulations of accretion following fragmentation in a cluster environment. * Among the cluster A stars, there is a higher fraction of binaries in the subset with X-ray detections, consistent with the hypothesis that lower mass companions are the true source of X-ray emission. * In alpha Persei, the rotational velocities for solar-type binaries with separations less that 60 AU the rotational evolution of young stars.
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We present the results of an all-sky survey made with the Fine Guidance Sensor on Hubble Space Telescope to search for angularly resolved binary systems among the massive stars. The sample of 224 stars is comprised mainly of Galactic O- and B-type stars and Luminous Blue Variables, plus a few luminous stars in the Large Magellanic Cloud. The FGS TRANS mode observations are sensitive to detection of companions with an angular separation between 0.01 and 1.0 and brighter than $triangle m = 5$. The FGS observations resolved 52 binary and 6 triple star systems and detected partially resolved binaries in 7 additional targets (43 of these are new detections). These numbers yield a companion detection frequency of 29% for the FGS survey. We also gathered literature results on the numbers of close spectroscopic binaries and wider astrometric binaries among the sample, and we present estimates of the frequency of multiple systems and the companion frequency for subsets of stars residing in clusters and associations, field stars, and runaway stars. These results confirm the high multiplicity fraction, especially among massive stars in clusters and associations. We show that the period distribution is approximately flat in increments of log P. We identify a number of systems of potential interest for long term orbital determinations, and we note the importance of some of these companions for the interpretation of the radial velocities and light curves of close binaries that have third companions.
Aims: Our scientific goal is to provide revised membership lists of the Alpha Per, Pleiades, and Praesepe clusters exploiting the second data release of Gaia and produce five-dimensional maps (ra, dec, parallax, pmRA, pmDEC) of these clusters. Methods: We implemented the kinematic method combined with the statistical treatment of parallaxes and proper motions to identify astrometric member candidates of three of the most nearby and best studied open clusters in the sky. Results: We cross-correlated the Gaia catalogue with large-scale public surveys to complement the astrometry of Gaia with multi-band photometry from the optical to the mid-infrared. We identified 517, 1248, and 721 bona-fide astrometric member candidates inside the tidal radius of the Alpha Per, the Pleiades, and Praesepe, respectively. We cross-matched our final samples with catalogues from previous surveys to address the level of completeness. We update the main physical properties of the clusters, including mean distance and velocity as well as core, half-mass, and tidal radii. We infer updated ages from the white dwarf members of the Pleiades and Praesepe. We derive the luminosity and mass functions of the three clusters and compare them to the field mass function. We compute the positions in space of all member candidates in the three regions to investigate their distribution in space. Conclusions: We provide updated distances and kinematics for the three clusters. We identify a list of members in the Alpha Per, Pleiades, and Praesepe clusters from the most massive stars all the way down into the hydrogen-burning limit with a higher confidence and better astrometry than previous studies. We produce complete 5D maps of stellar and substellar bona-fide members in these three regions. ABRIDGED
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D. Barrado y Navascues n MEC/Fulbright Fellow at the Smithsonian Astrophysical Observatory
1998
Randich and Schmitt [1995, A&A 298, 115] found that the coronal activity of solar-type and low mass stars in Praesepe is significantly lower than that of stars in the Hyades cluster. We have carried out several tests in order to find a possible explanation for this result. We have measured radial velocities of two groups of Praesepe stars (a dF-dK sample and a dM sample) and have measured H$alpha$ as a chromospheric activity index for the dM sample. We conclude that the Praesepe catalog used in the X-ray analysis does not contain a significant number of non-members. The comparison of the H$alpha$ equivalent widths for the M dwarfs in Praesepe with those in the Hyades indicates that, at least for stars in this mass range, the Praesepe stars are as active or more active than their Hyades counterparts. We have also analyzed a few ROSAT PSPC pointings of Praesepe in order to obtain a new and independent estimate of the X-ray luminosities and upper limits for a small sample of Praesepe members concluding that the small differences between the old and new upper limits are not large enough to explain the dichotomy in the X-ray properties of Praesepe and the Hyades. Therefore, our examination of the available data does not provide a clear reason to explain why the X-ray luminosity functions of the two clusters are different. Part of the explanation could be found in the binaries. Speculatively, these clusters could have different orbital period distributions, with more short period binaries among the Hyades, which would show larger coronal activity.
We analysed the wide-field near-infrared survey of the Praesepe cluster carried out by the UKIRT Infrared Deep Sky Survey (UKIDSS) Galactic Clusters Survey (GCS) and released by the Data Release 9 (DR9). We compare our Praesepe mass function (MF) with the ones of the Pleiades, alpha Per, and the Hyades. We also present preliminary results of a spectroscopic follow-up for the low mass members (M=<0.1Msol) in Praesepe, alpha Per and Pleiades using the Optical System for Imaging and low Resolution Integrated Spectroscopy (OSIRIS) mounted on the 10.4m Gran Telescopio Canarias (GTC). We also present the optical spectrum of the first L dwarf in Praesepe.
The present paper provides a general overview of the asteroseismic potential of delta Scuti stars in clusters, in particular focusing on convection diagnostics. We give a summarise of the last results obtained by the authors for the Praesepe cluster of which five delta Scuti stars are analysed. In that work, linear analysis is confronted with observations, using refined descriptions for the effects of rotation on the determination of the global stellar parameters and on the adiabatic oscillation frequency computations. A single, complete, and coherent solution for all the selected stars is found, which lead the authors to find important restrictions to the convection description for a certain range of effective temperatures. Furthermore, the method used allowed to give an estimate of the global parameters of the selected stars and constrain the cluster.
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