775 research outputs found
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
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
LP 349-25: a new tight M8V binary
We present the discovery of a tight M8V binary, with a separation of only 1.2
astronomical units, obtained with the PUEO and NACO adaptive optics systems,
respectively at the CFHT and VLT telescopes. The estimated period of LP 349-25
is approximately 5 years, and this makes it an excellent candidate for a
precise mass measurement.Comment: Accepted by Astronomy and Astrophysics Letter
Differential Radial Velocities and Stellar Parameters of Nearby Young Stars
Radial velocity searches for substellar mass companions have focused
primarily on stars older than 1 Gyr. Increased levels of stellar activity in
young stars hinders the detection of solar system analogs and therefore there
has been a prejudice against inclusion of young stars in radial velocity
surveys until recently. Adaptive optics surveys of young stars have given us
insight into the multiplicity of young stars but only for massive, distant
companions. Understanding the limit of the radial velocity technique,
restricted to high-mass, close-orbiting planets and brown dwarfs, we began a
survey of young stars of various ages. While the number of stars needed to
carry out full analysis of the problems of planetary and brown dwarf population
and evolution is large, the beginning of such a sample is included here. We
report on 61 young stars ranging in age from beta Pic association (~12 Myr) to
the Ursa Majoris association (~300 Myr). This initial search resulted in no
stars showing evidence for companions greater than ~1-2 M_Jup in short period
orbits at the 3 sigma-level. Additionally, we present derived stellar
parameters, as most have unpublished values. The chemical homogeneity of a
cluster, and presumably of an association, may help to constrain true
membership. As such, we present [Fe/H] abundances for the stars in our sample.Comment: Accepted for publication in the PAS
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
Five new very low mass binaries
We report the discovery of companions to 5 nearby late M dwarfs (>M5),
LHS1901, LHS4009, LHS6167, LP869-26 and WT460, and we confirm that the recently
discovered mid-T brown dwarf companion to SCR1845-6357 is physically bound to
that star. These discoveries result from our adaptive optics survey of all M
dwarfs within 12 pc. The new companions have spectral types M5 to L1, and
orbital separations between 1 and 10 AU. They add significantly to the number
of late M dwarfs binaries in the immediate solar neighbourhood, and will
improve the multiplicity statistics of late M dwarfs. The expected periods
range from 3 to 130 years. Several pairs thus have good potential for accurate
mass determination in this poorly sampled mass range.Comment: 5 pages, 2 figures, submitted to Astronomy & Astrophysic
Resolved Spectroscopy of M Dwarf/L Dwarf Binaries. II. 2MASS J 17072343-0558249AB
We present IRTF SpeX observations of the M/L binary system 2MASS
J17072343-0558249. SpeX imaging resolves the system into a 1"01+/-0.17 visual
binary in which both components have red near infrared colors. Resolved
low-resolution (R~150) 0.8-2.5 micron spectroscopy reveals strong H2O, CO and
FeH bands and alkali lines in the spectra of both components, characteristic of
late-type M and L dwarfs. A comparison to a sample of late-type field dwarf
spectra indicates spectral types M9 and L3. Despite the small proper motion of
the system (0"100+/-0"009 yr^{-1}), imaging observations over 2.5 yr provide
strong evidence that the two components share common proper motion. Physical
association is also likely due to the small spatial volume occupied by the two
components (based on spectrophotometric distances estimates of 15+/-1 pc) as
compared to the relatively low spatial density of low mass field stars. The
projected separation of the system is 15+/-3 AU, similar to other late-type M
and L binaries. Assuming a system age of 0.5-5 Gyr, we estimate the masses of
the binary components to be 0.072-0.083 and 0.064-0.077 M_sun, with an orbital
period of roughly 150-300 yr. While this is nominally too long a baseline for
astrometric mass measurements, the proximity and relatively wide angular
separation of the 2MASS J1707-0558AB pair makes it an ideal system for studying
the M dwarf/L dwarf transition at a fixed age and metallicity
Effects of regional climate change on brown rust disease in winter wheat
Projected climate changes will affect wheat crop production both in the main processes of plant growth and development but also in the occurrences and severities of plant diseases. We assessed the potential infection periods of wheat leaf rust (WLR) at two climatologically different sites in Luxembourg. A threshold-based model, taking hourly values of air temperatures, relative humidity and precipitation during night-time into account, was used for calculating favourable WLR infection days during three periods throughout the cropping season. Field experiments were conducted during the 2003â2013 period at the selected sites. Projected climate data, from a multi model ensemble of regional climate models (spatial resolution 25 km) as well as an additional projection with a higher spatial resolution of 1.3 km, were used for investigating the potential WLR infection periods for two future time spans. Results showed that the infections of WLR were satisfactorily simulated during the development of wheat at both sites for the 2003â2013 period. The probabilities of WLR detection were close to 1 and the critical success index ranged from 0.80 to 0.94 (perfect score = 1 for both). Moreover, the highest proportions of favourable days of WLR infection were simulated during spring and summer at both sites. Regional climate projections showed an increase in temperatures by 1.6 K for 2041â2050 and by 3.7 K for 2091â2100 compared to the reference period 1991â2000. Positive trends in favourable WLR infection conditions occur at both sites more conducive than in the reference period due to projected climatic conditions
The M Dwarf GJ 436 and its Neptune-Mass Planet
We determine stellar parameters for the M dwarf GJ 436 that hosts a
Neptune-mass planet. We employ primarily spectral modeling at low and high
resolution, examining the agreement between model and observed optical spectra
of five comparison stars of type, M0-M3. Modeling high resolution optical
spectra suffers from uncertainties in TiO transitions, affecting the predicted
strengths of both atomic and molecular lines in M dwarfs. The determination of
Teff, gravity, and metallicity from optical spectra remains at ~10%. As
molecules provide opacity both in lines and as an effective continuum,
determing molecular transition parameters remains a challenge facing models
such as the PHOENIX series, best verified with high resolution and
spectrophotometric spectra. Our analysis of GJ 436 yields an effective
temperature of Teff = 3350 +/- 300 K and a mass of 0.44 Msun. New Doppler
measurements for GJ 436 with a precision of 3 m/s taken during 6 years improve
the Keplerian model of the planet, giving a minimum mass, M sin i = 0.0713 Mjup
= 22.6 Mearth, period, P = 2.6439 d, and e = 0.16 +/- 0.02. The noncircular
orbit contrasts with the tidally circularized orbits of all close-in
exoplanets, implying either ongoing pumping of eccentricity by a more distant
companion, or a higher Q value for this low-mass planet. The velocities indeed
reveal a long term trend, indicating a possible distant companion.Comment: 27 pages, 7 figures, accepted to PAS
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