Design Considerations for a Ground-based Transit Search for Habitable Planets Orbiting M dwarfs

By targeting nearby M dwarfs, a transit search using modest equipment is capable of discovering planets as small as 2 Earth radii in the habitable zones of their host stars. The MEarth Project, a future transit search, aims to employ a network of ground-based robotic telescopes to monitor M dwarfs in the northern hemisphere with sufficient precision and cadence to detect such planets. Here we investigate the design requirements for the MEarth Project. We evaluate the optimal bandpass, and the necessary field of view, telescope aperture, and telescope time allocation on a star-by-star basis, as is possible for the well-characterized nearby M dwarfs. Through these considerations, 1,976 late M dwarfs (R < 0.33 Rsun) emerge as favorable targets for transit monitoring. Based on an observational cadence and on total telescope time allocation tailored to recover 90% of transit signals from planets in habitable zone orbits, we find that a network of ten 30 cm telescopes could survey these 1,976 M dwarfs in less than 3 years. A null result from this survey would set an upper limit (at 99% confidence) of 17% for the rate of occurrence of planets larger than 2 Earth radii in the habitable zones of late M dwarfs, and even stronger constraints for planets lying closer than the habitable zone. If the true occurrence rate of habitable planets is 10%, the expected yield would be 2.6 planets.Comment: accepted to PAS

Discovery of three nearby L dwarfs in the Southern Sky

We report the discovery of three L dwarfs in the solar vicinity within 30 parsecs. These objects were originally found as proper motion objects from a combination of R and I photographic plates measured as part of the SuperCOSMOS Sky Surveys. We subsequently identified these objects as bona fide brown dwarf candidates on the basis of their R-I colour, as first criterion, and subsequently their J-K colours when the infrared data were available from the 2MASS database. Spectroscopic observations in the optical with the ESO 3.6m/EFOSC2 and in the near-infrared with the NTT/SOFI led to the classification of their spectral types as early L dwarfs.Comment: 4 pages including 2 figures, accepted for publication in Astronomy and Astrophysics Letter

The Big Occulting Steerable Satellite (BOSS)

Natural (such as lunar) occultations have long been used to study sources on small angular scales, while coronographs have been used to study high contrast sources. We propose launching the Big Occulting Steerable Satellite (BOSS), a large steerable occulting satellite to combine both of these techniques. BOSS will have several advantages over standard occulting bodies. BOSS would block all but about 4e-5 of the light at 1 micron in the region of interest around the star for planet detections. Because the occultation occurs outside the telescope, scattering inside the telescope does not degrade this performance. BOSS could be combined with a space telescope at the Earth-Sun L2 point to yield very long integration times, in excess of 3000 seconds. If placed in Earth orbit, integration times of 160--1600 seconds can be achieved from most major telescope sites for objects in over 90% of the sky. Applications for BOSS include direct imaging of planets around nearby stars. Planets separated by as little as 0.1--0.25 arcseconds from the star they orbit could be seen down to a relative intensity as little as 1e-9 around a magnitude 8 (or brighter) star. Other applications include ultra-high resolution imaging of compound sources, such as microlensed stars and quasars, down to a resolution as little as 0.1 milliarcseconds.Comment: 25pages, 4 figures, uses aaspp4, rotate, and epsfig. Submitted to the Astrophysical Journal. For more details see http://erebus.phys.cwru.edu/~boss

The Galactic disk mass function: reconciliation of the HST and nearby determinations

We derive and parametrize the Galactic mass function (MF) below 1 \msol characteristic of both single objects and binary systems. We resolve the long standing discrepancy between the MFs derived from the HST and from the nearby luminosity functions, respectively. We show that this discrepancy stemmed from {\it two} cumulative effects, namely (i) incorrect color-magnitude determined distances, due a substantial fraction of M dwarfs in the HST sample belonging to the metal-depleted, thick-disk population, as corrected recently by Zheng et al. and (ii) unresolved binaries. We show that both the nearby and HST MF for unresolved systems are consistent with a fraction $\sim$50% of M-dwarf binaries, with the mass of both the primaries and the companions originating from the same underlying single MF. This implies that $\sim$30% of M dwarfs should have an M dwarf companion and $\sim$20% should have a brown dwarf companion, in agreement with recent determinations. The present calculations show that the so-called "brown-dwarf desert" should be reinterpreted as a lack of high mass-ratio (m_2/m_1\la 0.1) systems, and does not preclude a substantial fraction of brown dwarfs as companions of M dwarfs or for other brown dwarfs.Comment: 16 pages, Latex file, uses aasms4.sty, to appear in ApJ Letter

L and T Dwarf Models and the L to T Transition

Using a model for refractory clouds, a novel algorithm for handling them, and the latest gas-phase molecular opacities, we have produced a new series of L and T dwarf spectral and atmosphere models as a function of gravity and metallicity, spanning the \teff range from 2200 K to 700 K. The correspondence with observed spectra and infrared colors for early- and mid-L dwarfs and for mid- to late-T dwarfs is good. We find that the width in infrared color-magnitude diagrams of both the T and L dwarf branches is naturally explained by reasonable variations in gravity and, therefore, that gravity is the "second parameter" of the L/T dwarf sequence. We investigate the dependence of theoretical dwarf spectra and color-magnitude diagrams upon various cloud properties, such as particle size and cloud spatial distribution. In the region of the L$\to$T transition, we find that no one cloud-particle-size and gravity combination can be made to fit all the observed data. Furthermore, we note that the new, lower solar oxygen abundances of Allende-Prieto, Lambert, & Asplund (2002) produce better fits to brown dwarf data than do the older values. Finally, we discuss various issues in cloud physics and modeling and speculate on how a better correspondence between theory and observation in the problematic L$\to$T transition region might be achieved.Comment: accepted to the Astrophysical Journal, 21 figures (20 in color); spectral models in electronic form available at http://zenith.as.arizona.edu/~burrow

Measuring Fundamental Parameters of Substellar Objects. II: Masses and Radii

We present mass and radius derivations for a sample of very young, mid- to late M, low-mass stellar and substellar objects in Upper Sco and Taurus. In a previous paper, we determined effective temperatures and surface gravities for these targets, from an analysis of their high-resolution optical spectra and comparisons to the latest synthetic spectra. We now derive extinctions, radii, masses and luminosities by combining our previous results with observed photometry, surface fluxes from the synthetic spectra and the known cluster distances. These are the first mass and radius estimates for young, very low mass bodies that are independent of theoretical evolutionary models (though our estimates do depend on spectral modeling). We find that for most of our sample, our derived mass-radius and mass-luminosity relationships are in very good agreement with the theoretical predictions. However, our results diverge from the evolutionary model values for the coolest, lowest-mass targets: our inferred radii and luminosities are significantly larger than predicted for these objects at the likely cluster ages, causing them to appear much younger than expected. We suggest that uncertainties in the evolutionary models - e.g., in the choice of initial conditions and/or treatment of interior convection - may be responsible for this discrepancy. Finally, two of our late-M objects (USco 128 and 130) appear to have masses close to the deuterium-fusion boundary (9--14 Jupiters, within a factor of 2). This conclusion is primarily a consequence of their considerable faintness compared to other targets with similar extinction, spectral type and temperature (difference of 1 mag). Our result suggests that the faintest young late-M or cooler objects may be significantly lower in mass than the current theoretical tracks indicate.Comment: 54 pages, incl. 5 figs, accepted Ap

A Dedicated M-Dwarf Planet Search Using The Hobby-Eberly Telescope

We present first results of our planet search program using the 9.2 meter Hobby-Eberly Telescope (HET) at McDonald Observatory to detect planets around M-type dwarf stars via high-precision radial velocity (RV) measurements. Although more than 100 extrasolar planets have been found around solar-type stars of spectral type F to K, there is only a single M-dwarf (GJ 876, Delfosse et al. 1998; Marcy et al. 1998; Marcy et al. 2001) known to harbor a planetary system. With the current incompleteness of Doppler surveys with respect to M-dwarfs, it is not yet possible to decide whether this is due to a fundamental difference in the formation history and overall frequency of planetary systems in the low-mass regime of the Hertzsprung-Russell diagram, or simply an observational bias. Our HET M-dwarf survey plans to survey 100 M-dwarfs in the next 3 to 4 years with the primary goal to answer this question. Here we present the results from the first year of the survey which show that our routine RV-precision for M-dwarfs is 6 m/s. We found that GJ 864 and GJ 913 are binary systems with yet undetermined periods, while 5 out of 39 M-dwarfs reveal a high RV-scatter and represent candidates for having short-periodic planetary companions. For one of them, GJ 436 (rms = 20.6 m/s), we have already obtained follow-up observations but no periodic signal is present in the RV-data.Comment: 12 pages, 14 figures, accepted for publication in the Astronomical Journa

Tides and the Evolution of Planetary Habitability

Tides raised on a planet by its host star's gravity can reduce a planet's orbital semi-major axis and eccentricity. This effect is only relevant for planets orbiting very close to their host stars. The habitable zones of low-mass stars are also close-in and tides can alter the orbits of planets in these locations. We calculate the tidal evolution of hypothetical terrestrial planets around low-mass stars and show that tides can evolve planets past the inner edge of the habitable zone, sometimes in less than 1 billion years. This migration requires large eccentricities (>0.5) and low-mass stars (<0.35 M_Sun). Such migration may have important implications for the evolution of the atmosphere, internal heating and the Gaia hypothesis. Similarly, a planet detected interior to the habitable zone could have been habitable in the past. We consider the past habitability of the recently-discovered, ~5 M_Earth planet, Gliese 581 c. We find that it could have been habitable for reasonable choices of orbital and physical properties as recently as 2 Gyr ago. However, when we include constraints derived from the additional companions, we see that most parameter choices that predict past habitability require the two inner planets of the system to have crossed their mutual 3:1 mean motion resonance. As this crossing would likely have resulted in resonance capture, which is not observed, we conclude that Gl 581 c was probably never habitable.Comment: 31 pages, 10 figures, accepted to Astrobiology. A version with full resolution figures is available at http://www.lpl.arizona.edu/~rory/publications/brjg07.pd

Multiplicity of Nearby Free-floating Ultra-cool Dwarfs: a HST-WFPC2 search for companions

We present HST/WFPC2 observations of a sample of 134 ultra-cool objects (spectral types later than M7) coming from the DENIS, 2MASS and SDSS surveys, with distances estimated to range from 7 pc to 105 pc. Fifteen new ultra-cool binary candidates are reported here. Eleven known binaries are confirmed and orbital motion is detected in some of them. We estimate that the closest binary systems in this sample have periods between 5 and 20 years, and thus dynamical masses will be derived in the near future. For the calculation of binary frequency we restrict ourselves to systems with distances less than 20 pc. After correction of the binaries bias, we find a ratio of visual binaries (at the HST limit of detection) of around 10%, and that ~15% of the 26 objects within 20 parsecs are binary systems with separations between 1 and 8 A.U. The observed frequency of ultra-cool binaries is similar than that of binaries with G-type primaries in the separation range from 2.1 A.U. to 140 A.U. There is also a clear deficit of ultra-cool binaries with separations greater than 15 A.U., and a possible tendency for the binaries to have mass ratios near unity. Most systems have indeed visual and near-infrared brightness ratios between 1 and 0.3. We discuss our results in the framework of current scenarios for the formation and evolution of free-floating brown dwarfs.Comment: 67 pages, 14 figures, Accepted for publication in AJ, September 2003. First submission to AJ: august 2002, 5 submission

An m sin i = 24 Earth Mass Planetary Companion To The Nearby M Dwarf GJ 176

We report the detection of a planetary companion with a minimum mass of m sin i = 0.0771 M_Jup = 24.5 M_Earth to the nearby (d = 9.4 pc) M2.5V star GJ 176. The star was observed as part of our M dwarf planet search at the Hobby-Eberly Telescope (HET). The detection is based on 5 years of high-precision differential radial velocity (RV) measurements using the High-Resolution-Spectrograph (HRS). The orbital period of the planet is 10.24 d. GJ 176 thus joins the small (but increasing) sample of M dwarfs hosting short-periodic planets with minimum masses in the Neptune-mass range. Low mass planets could be relatively common around M dwarfs and the current detections might represent the tip of a rocky planet population.Comment: 13 pages preprint, 3 figures, submitted to Ap