794 research outputs found
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 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 30% of M dwarfs
should have an M dwarf companion and 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
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