We report the discovery of two giant planets orbiting stars in Praesepe (also known as the Beehive Cluster). These are the first known hot Jupiters in an open cluster and the only planets known to orbit Sun-like, main-sequence stars in a cluster. The planets are detected from Doppler shifted radial velocities; line bisector spans and activity indices show no correlation with orbital phase, confirming the variations are caused by planetary companions. Pr0201b orbits a V = 10.52 late F dwarf with a period of 4.4264 ± 0.0070 days and has a minimum mass of 0.540 ± 0.039 M Jup , and Pr0211b orbits a V = 12.06 late G dwarf with a period of 2.1451 ± 0.0012 days and has a minimum mass of 1.844 ± 0.064 M Jup . The detection of 2 planets among 53 single members surveyed establishes a lower limit on the hot Jupiter frequency of 3.8 +5.0 −2.4 % in this metal-rich open cluster. Given the precisely known age of the cluster, this discovery also demonstrates that, in at least 2 cases, giant planet migration occurred within 600 Myr after formation. As we endeavor to learn more about the frequency and formation history of planets, environments with well-determined properties -such as open clusters like Praesepe -may provide essential clues to this end.

We present angular diameters of the Hyades giants, γ, δ 1 , ǫ, and θ 1 Tau from interferometric measurements with the CHARA Array. Our errors in the limb-darkened angular diameters for these stars are all less than 2%, and in combination with additional observable quantities, we determine the effective temperatures, linear radii and absolute luminosities for each of these stars. Additionally, stellar masses are inferred from model isochrones to determine the surface gravities. These data show that a new calibration of effective temperatures with errors well under 100K is now possible from interferometric angular diameters of stars.

We present a carefully vetted equatorial (˘30˝Decl.) sample of all known single (within 4 2 ) mid M-dwarfs (M2.5V-M8.0V) extending out to 10 pc; their proximity and low masses make them ideal targets for planet searches. For this sample of 58 stars, we provide V J , R KC , I KC photometry, new low dispersion optical (6000´9000Å) spectra from which uniform spectral types are determined, multi-epoch Hα equivalent widths, and gravity sensitive N a I indices. For 12 of these 58 stars, strict limits are placed on the presence of stellar and sub-stellar companions, based on a pioneering program described here that utilizes precise infrared radial velocities and optical astrometric measurements in an effort to search for Jupiter-mass, brown dwarf and stellar-mass companions. Our infrared radial velocity precision using CSHELL at NASA's IRTF is "90 m s´1 over timescales from 13 days to 5 years. With our spectroscopic results the mean companion masses that we rule out of existence are 1.5 M JUP or greater in 10 day orbital periods and 7 M JUP or greater in 100 day orbital periods. We use these spectra to determine rotational velocities and absolute radial velocities of these twelve stars. Our mean astrometric precision using RECONS 6 (Research Consortium on Nearby Stars) data from 0.9-m telescope at Cerro Tololo Inter-American Observatory is "3 milli-arcseconds over baselines ranging from 9 to 13 years. With our astrometric results the mean companion masses that we rule out of existence are greater than 11.5 M JUP with an orbital period of 4 years and greater than 7.5 M JUP with an orbital period of 8 years. Although we do not detect companions around our sub-sample of 12 stars, we demonstrate that our two techniques probe a regime that is commonly missed in other companion searches of late type stars.

We use the sample of known stars and brown dwarfs within 5 pc of the Sun, supplemented with AFGK stars within 10 pc, to determine which stellar spectral types provide the most habitable real estate -defined to be locations where liquid water could be present on Earth-like planets. Stellar temperatures and radii are determined by fitting model spectra to spatially resolved broad-band photometric energy distributions for stars in the sample. Using these values, the locations of the habitable zones are calculated using an empirical formula for planetary surface temperature and assuming the condition of liquid water, called here the empirical habitable zone, or EHZ. Systems that have dynamically disruptive companions, assuming a 5:1 separation ratios for primary/secondary pairs and either object and a planet, are considered not habitable. We use the results of these calculations to derive a simple formula to predict the location of the EHZ for main sequence stars based on V − K color. We consider EHZ widths as more useful measures of the habitable real estate around stars than areas because multiple planets are not expected to orbit stars at identical stellar distances. This EHZ provides a qualitative guide on where to expect the largest population of planets in the habitable zone of main sequence stars. Because of their large numbers and lower frequency of short-period companions, M stars provide more EHZ real estate than other spectral types, possessing 36.5% of the habitable real estate en masse. K stars are second with 21.5%, while A, F, and G stars offer 18.5%, 6.9% and 16.6%, respectively. Our calculations show that three M dwarfs within 10 pc harbor planets in their EHZs -GJ 581 may have two planets (d with msini = 6.1 M ⊕ ; g with msini = 3.1 M ⊕ ), GJ 667 C has one (c with msini = 4.5 M ⊕ ), and GJ 876 has two (b with msini = 1.89 M Jup and c with msini = 0.56 M Jup ). If Earth-like planets are as common around low mass

We report new infrared radial velocity measurements obtained with CSHELL at NASA's Infrared Telescope Facility that reveal the M3.5 dwarf GJ 867B to be a single-lined spectroscopic binary with a period of 1.795 ± 0.017 days. Its velocity semi-amplitude of 21.4 ± 0.5 kms −1 corresponds to a minimum mass of 61 ± 7 M JUP ; the new companion, which we call GJ 867D, could be a brown dwarf. Stable astrometric measurements of GJ 867BD obtained with CTIO's 0.9-m telescope over the last decade exclude the presence of any massive planetary companions (7-18 M JUP ) with longer orbital periods (2-10 years) for the majority of orientations. These complementary observations are also used to determine the trigonometric distance and proper motion of GJ 867BD; the measurements are consistent with the HIPPARCOS measured values of the M2 dwarf GJ 867AC, which is itself a 4.1 day double-lined spectroscopic binary at a projected separation of 24. ′′ 5 (216 AU) from GJ 867BD. These new measurements strengthen the case that GJ 867AC and GJ 867BD are physically associated, making the GJ 867 system one of only four quadruple systems within 10 pc of the Sun (d= 8.82 ±0.08 pc) and the only among these with all M-dwarf (or cooler) components.