The Structure and the Distance of Collinder 121 from Hipparcos and Photometry: Resolving the Discrepancy

We present further arguments that the Hipparcos parallaxes for some of the clusters and associations represented in the Hipparcos catalog should be used with caution in the study of the Galactic structure. It has been already shown that the discrepancy between the Hipparcos and ground based parallaxes for several clusters including the Pleiades, Coma Ber and NGC 6231 can be resolved by recomputing the Hipparcos astrometric solutions with an improved algorithm diminishing correlated errors in the attitude parameters. Here we present new parallaxes obtained with this algorithm for another group of stars with discrepant data - the galactic cluster Cr 121. The original Hipparcos parallaxes led de Zeeuw et al. to conclude that Cr 121 and the surrounding association of OB stars form a relatively compact and coherent moving group at a distance of 550 -- 600 pc. Our corrected parallaxes reveal a different spatial distribution of young stellar populace in this area. Both the cluster Cr 121 and the extended OB association are considerably more distant (750 -- 1000 pc), and the latter has a large depth probably extending beyond 1 kpc. Therefore, not only are the recalculated parallaxes in complete agreement with the photometric uvbybeta parallaxes, but the structure of the field they reveal is no longer in discrepancy with that found by the photometric method.Comment: 7 pages, 1 figure, ApJL accepte

Open clusters with Hipparcos I. Mean astrometric parameters

New memberships, mean parallaxes and proper motions of all 9 open clusters closer than 300 pc (except the Hyades) and 9 rich clusters between 300 and 500 pc have been computed using Hipparcos data. Precisions, ranging from 0.2 to 0.5 mas for parallaxes and 0.1 to 0.5 mas/yr for proper motions, are of great interest for calibrating photometric parallaxes as well as for kinematical studies. Careful investigations of possible biases have been performed and no evidence of significant systematic errors on the mean cluster parallaxes has been found. The distances and proper motions of 32 more distant clusters, which may be used statistically, are also indicated.Comment: 15 pages, A&A in pres

Discovery of high proper motion ancient white dwarfs: nearby massive compact halo objects?

We present the discovery and spectroscopic identification of two very high proper motion ancient white dwarf stars, found in a systematic proper motion survey. Their kinematics and apparent magnitude clearly indicate that they are halo members, while their optical spectra are almost identical to the recently identified cool Halo white dwarf WD0346+246. Canonical stellar halo models predict a white dwarf volume density of two orders of magnitude less than the approx 7*10^{-4} Solar masses per pc^{-3} inferred from this survey. With the caveat that the sample size is very small, it appears that a significant fraction, about 10%, of the local dark matter halo is in the form of very old, cool, white dwarfs.Comment: 8 pages, 2 figures, accepted for publication in ApJL; references adde

On the Radii of Extrasolar Giant Planets

We have computed evolutionary models for extrasolar planets which range in mass from 0.1 to 3.0 Jovian Masses, and which range in equilibrium temperature from 113 K to 2000 K. We present four sequences of models, designed to show the structural effects of a solid core and of internal heating due to the conversion of kinetic to thermal energy at pressures of tens of bars. The model planetary radii are intended for comparisons with radii derived from observations of transiting extrasolar planets. To provide such comparisons, we expect that of order 10 transiting planets with orbital periods less than 200 days can be detected around bright (V<10) main-sequence stars for which accurate, well-sampled radial velocity measurements can be readily accumulated. Through these observations, structural properties of the planets will be derivable, particularly for low-mass, high-temperature planets. Implications regarding the transiting companion to OGLE-TR-56 recently announced by Konacki et al. are discussed. With regard to the confirmed transiting planet, HD 209458b, we find, in accordance with other recent calculations, that models without internal heating predict a radius that is 35 percent smaller than the observed radius. We explore the possibility that HD 209458b owes its large size to dissipation of energy arising from ongoing tidal circularization of the orbit. We show that residual scatter in the current radial velocity data set for HD 209458b is consistent with the presence of an as-of-yet undetected second companion, and that further radial velocity monitoring of the star is indicated.Comment: 23 pages, 3 figures, accepted by Astrophysical Journa

A New Nearby Candidate Star Cluster in Ophiuchus at d = 170 pc

The recent discoveries of nearby star clusters and associations within a few hundred pc of the Sun, as well as the order of magnitude difference in the formation rates of the embedded and open cluster populations, suggests that additional poor stellar groups are likely to be found at surprisingly close distances to the Sun. Here I describe a new nearby stellar aggregate found by virtue of the parallel proper motions, similar trigonometric parallaxes, and consistent color-magnitude distribution of its early-type members. The 120 Myr-old group lies in Ophiuchus at $d$ $\simeq$ 170 pc, with its most massive member being the 4th-magnitude post-MS B8II-III star $\mu$ Oph. The group may have escaped previous notice due to its non-negligible extinction ($A_V$ $\simeq$ 0.9 mag). If the group was born with a normal initial mass function, and the nine B- and A-type systems represent a complete system of intermediate-mass stars, then the original population was probably of order $\sim$200 systems. The age and space motion of the new cluster are very similar to those of the Pleiades, $\alpha$ Per cluster, and AB Dor Moving Group, suggesting that these aggregates may have formed in the same star-forming complex some $\sim10^8$ yr ago.Comment: 23 pages, 3 figs., to appear in Nov. 2006 A

The HARPS search for southern extra-solar planets. XX. Planets around the active star BD-08:2823

We report the detection of a planetary system around BD-08:2823, that includes at least one Uranus-mass planet and one Saturn-mass planet. This discovery serendipitously originates from a search for planetary transits in the Hipparcos photometry database. This program preferentially selected active stars and did not allow the detection of new transiting planets. It allowed however the identification of the K3V star BD-08:2823 as a target harboring a multiplanet system, that we secured and characterized thanks to an intensive monitoring with the HARPS spectrograph at the 3.6-m ESO telescope in La Silla. The stellar activity level of BD-08:2823 complicates the analysis but does not prohibit the detection of two planets around this star. BD-08:2823b has a minimum mass of 14.4+/-2.1 M_Earth and an orbital period of 5.60 days, whereas BD-08:2823c has a minimum mass of 0.33+/-0.03 M_Jup and an orbital period of 237.6 days. This new system strengthens the fact that low-mass planets are preferentially found in multiplanetary systems, but not around high-metallicity stars as this is the case for massive planets. It also supports the belief that active stars should not be neglected in exoplanet searches, even when searching for low-mass planets.Comment: 10 pages, 8 figures, 3 tables, accepted for publication in A&

Debris Disks in NGC 2547

We have surveyed the 30 Myr-old cluster NGC 2547 for planetary debris disks using Spitzer. At 4.5-8 um we are sensitive to the photospheric level down to mid-M stars (0.2 Msol) and at 24 um to early-G stars (1.2 Msol). We find only two to four stars with excesses at 8 um out of ~400-500 cluster members, resulting in an excess fraction <~1 percent at this wavelength. By contrast, the excess fraction at 24 um is ~40 percent (for B-F types). Out of four late-type stars with excesses at 8 um two marginal ones are consistent with asteroid-like debris disks. Among stars with strong 8 um excesses one is possibly from a transitional disk, while another one can be a result of a catastrophic collision. Our survey demonstrates that the inner 0.1-1 AU parts of disks around solar-type stars clear out very thoroughly by 30 Myrs of age. Comparing with the much slower decay of excesses at 24 and 70 um, disks clear from the inside out, of order 10 Myr for the inner zones probed at 8 um compared with a hundred or more Myr for those probed with the two longer wavelengths.Comment: Accepted to ApJ, 29 pages, 13 figs. A Note in Proof concerning cluster's age was added in the original submission of 2007 July 19. Full Tables 1 and 2 in the electronic form together with the article with full resolution figures are available at http://www.astro.ufl.edu/~ngorlova/disksNGC2547

HD 209458: Physical Parameters of the Parent Star and the Transiting Planet

The Sun-like star HD209458 harbors a close-in giant planet which transits across the star's disk, and thus allows an unprecedented access to the basic parameters of the planet, given a certain knowledge of the basic parameters of the star, namely its mass and radius. We present theoretical stellar evolution model calculations for HD 209458 and discuss the uncertainties involved in deriving the stellar mass and radius. We derive the mass, M=1.06 Msun, radius, R=1.18 Rsun,and age, t=5.2 Gyr of the star with uncertainties of 10% or more. The dominant sources of uncertainty remain to be the helium abundance estimate and the treatment of convection, even after an optimistic estimate for the effective temperature of the star. However, we find that in deriving the radius of the planet, R_p, the relevant stellar model input is the M/R relation, which runs orthogonal to a degeneracy in the transit light curve solution and greatly improves the estimate of R_p. Theoretically the M/R relation has a lower uncertainty than the M and R separately. We estimate the planet radius and mass to be R_p =1.42 +0.10/-0.13 R_J and M_p =0.69 +/- 0.02 M_J.Comment: 24 pages, 5 figures, accepted by the Astrophysical Journal; added correct version of figure 5 and fixed reference

A Hubble Space Telescope ACS Search for Brown Dwarf Binaries in the Pleiades Open Cluster

We present the results of a high-resolution imaging survey for brown dwarf binaries in the Pleiades open cluster. The observations were carried out with the Advance Camera for Surveys onboard the Hubble Space Telescope. Our sample consists of 15 bona-fide brown dwarfs. We confirm 2 binaries and detect their orbital motion, but we did not resolve any new binary candidates in the separation range between 5.4AU and 1700AU and masses in the range 0.035--0.065~Msun. Together with the results of our previous study (Martin et al., 2003), we can derive a visual binary frequency of 13.3$^{+13.7}_{-4.3}$\% for separations greater than 7~AU masses between 0.055--0.065~M_{\sun} and mass ratios between 0.45--0.9$<q<$1.0. The other observed properties of Pleiades brown dwarf binaries (distributions of separation and mass ratio) appear to be similar to their older counterparts in the field.Comment: 29 pages, 7 figures, 6 tables, accepted for publication in Ap