The First Direct Measurements of Magnetic Fields on Very Low-Mass Stars

We present the first direct magnetic field measurements on M dwarfs cooler than spectral class M4.5. Utilizing a new method based on the effects of a field on the FeH band near 1 micron, we obtain information on whether the integrated surface magnetic flux (Bf) is low (well under 1 kilogauss), intermediate (between 1 and about 2.5 kG), or strong (greater than about 3 kG) on a set of stars ranging from M2 down to M9. We also measure the rotational broadening (vsini) and Halpha emission for more than 20 stars. Our goal is to advance the understanding of how dynamo field production varies with stellar parameters for very low-mass stars, how the field and emission activity are related, and whether there is a connection between the rotation and magnetic flux. We find that fields are produced throughout the M-dwarfs. Among the early M stars we have too few targets to yield conclusive results. In the mid-M stars, there is a clear connection between slow rotation and weak fields. In the late-M stars, rotation is always measureable, and the strongest fields go with the most rapid rotators. These very cool rapid rotators have the largest magnetic flux in the whole sample. Halpha emission is found to be a good general proxy for magnetic fields. The drop-off in fractional emission near the bottom of the main sequence is not accompanied by a drop-off in magnetic flux, lending credence to the hypothesis that it is due to atmospheric coupling to the field rather than changes in the field itself. It is clear that the methodology we have developed can be further applied to discover more about the behavior of magnetic dynamos and magnetic activity in cool and fully convective objects.Comment: 33 pages, accepted by ApJ, abstract abbreviated for astro-p

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

On the Correlation between the Magnetic Activity Levels, the Metallicities and the Radii of Low-Mass Stars

The recent burst in the number of radii measurements of very low-mass stars from eclipsing binaries and interferometry of single stars has opened more questions about what can be causing the discrepancy between the observed radii and the ones predicted by the models. The two main explanations being proposed are a correlation between the radius of the stars and their activity levels or their metallicities. This paper presents a study of such correlations using all the data published to date. The study also investigates correlations between the radii deviation from the models and the masses of the stars. There is no clear correlation between activity level and radii for the single stars in the sample. Those single stars are slow rotators with typical velocities v_rot sini < 3.0 km s^-1. A clear correlation however exists in the case of the faster rotating members of binaries. This result is based on the of X-ray emission levels of the stars. There also appears to be an increase in the deviation of the radii of single stars from the models as a function of metallicity, as previously indicated by Berger et al. (2006). The stars in binaries do not seem to follow the same trend. Finally, the Baraffe et al. (1998) models reproduce well the radius observations below 0.30-0.35Msun, where the stars become fully convective, although this result is preliminary since almost all the sample stars in that mass range are slow rotators and metallicities have not been measured for most of them. The results in this paper indicate that stellar activity and metallicity play an important role on the determination of the radius of very low-mass stars, at least above 0.35Msun.Comment: 22 pages, 4 figures. Accepted for publication on Ap

Multiepoch Radial Velocity Observations of L Dwarfs

We report on the development of a technique for precise radial-velocity measurements of cool stars and brown dwarfs in the near infrared. Our technique is analogous to the Iodine (I2) absorption cell method that has proven so successful in the optical regime. We rely on telluric CH4 absorption features to serve as a wavelength reference, relative to which we measure Doppler shifts of the CO and H2O features in the spectra of our targets. We apply this technique to high-resolution (R~50,000) spectra near 2.3 micron of nine L dwarfs taken with the Phoenix instrument on Gemini-South and demonstrate a typical precision of 300 m/s. We conduct simulations to estimate our expected precision and show our performance is currently limited by the signal-to-noise of our data. We present estimates of the rotational velocities and systemic velocities of our targets. With our current data, we are sensitive to companions with M sin i > 2MJ in orbits with periods less than three days. We identify no companions in our current data set. Future observations with improved signal-to-noise should result in radial-velocity precision of 100 m/s for L dwarfs.Comment: Accepted for publication in ApJ, 24 pages, 7 figure

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

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

Discovery of an M4 Spectroscopic Binary in Upper Scorpius: A Calibration Point for Young Low-Mass Evolutionary Models

We report the discovery of a new low-mass spectroscopic (SB2) stellar binary system in the star-forming region of Upper Scorpius. This object, UScoCTIO5, was discovered by Ardila (2000), who assigned it a spectral class of M4. A KeckI HIRES spectrum revealed it to be double-lined, and we then carried out a program at several observatories to determine its orbit. The orbital period is 34 days, and the eccentricity is nearly 0.3. The importance of such a discovery is that it can be used to help calibrate evolutionary models at low masses and young ages. This is one of the outstanding problems in the study of formation mechanisms and initial mass functions at low masses. The orbit allows us to place a lower limit of 0.64 +- 0.02 M_sol on the total system mass. The components appear to be of almost equal mass. We are able to show that this mass is significantly higher than predicted by evolutionary models for an object of this luminosity and age, in agreement with other recent results. More precise determination of the temperature and surface gravity of the components would be helpful in further solidifying this conclusion.Comment: 17 pages, 4 figures, accepted for publication in Ap

Astrometric Discovery of GJ 802b: In the Brown Dwarf Oasis?

The Stellar Planet Survey is an ongoing astrometric search for giant planets and brown dwarfs around a sample of about 30 M-dwarfs. We have discovered several low-mass companions by measuring the motion of our target stars relative to their reference frames. The lowest mass discovery thus far is GJ 802b, a companion to the M5-dwarf GJ 802A. The orbital period is 3.14 +/- 0.03 y, the system mass is 0.214 +/- 0.045 Msolar, and the semi-major axis is 1.28 +/- 0.10 AU or 81 +/- 6 mas. Imaging observations indicate that GJ 802b is likely to be a brown dwarf with the astrometrically determined mass 0.058 +/- 0.021 Msolar (one sigma limits). The remaining uncertainty in the orbit is the eccentricity that is now loosely constrained. We discuss how the system age limits the mass and the prospects to further narrow the mass range when e is more precisely determined.Comment: 13 pages, 6 figures, accepted for publication in ApJ on May 9, 200