8,190 research outputs found
An Age Constraint for the Very Low-Mass Stellar/Brown Dwarf Binary 2MASS J03202839-0446358AB
2MASS J03202839-0446358AB is a recently identified, late-type M dwarf/T dwarf
spectroscopic binary system for which both the radial velocity orbit for the
primary and spectral types for both components have been determined. By
combining these measurements with predictions from four different sets of
evolutionary models, we determine a minimum age of 2.0+/-0.3 Gyr for this
system, corresponding to minimum primary and secondary masses of 0.080 Msun and
0.053 Msun, respectively. We find broad agreement in the inferred age and mass
constraints between the evolutionary models, including those that incorporate
atmospheric condensate grain opacity; however, we are not able to independently
assess their accuracy. The inferred minimum age agrees with the kinematics and
absence of magnetic activity in this system, but not the rapid rotation of its
primary, further evidence of a breakdown in angular momentum evolution trends
amongst the lowest luminosity stars. Assuming a maximum age of 10 Gyr, we
constrain the orbital inclination of this system to i >~ 53 degrees. More
precise constraints on the orbital inclination and/or component masses of 2MASS
J0320-0446AB, through either measurement of the secondary radial velocity orbit
(optimally in the 1.2-1.3 micron band) or detection of an eclipse (only 0.3%
probability based on geometric constraints), would yield a bounded age estimate
for this system, and the opportunity to use it as an empirical test for brown
dwarf evolutionary models at late ages.Comment: 8 pages, 2 figures, accepted for publication to Astonomical Journa
The Close Binary Fraction of Dwarf M Stars
We describe a search for close spectroscopic dwarf M star binaries using data from the Sloan Digital Sky Survey to address the question of the rate of occurrence of multiplicity in M dwarfs. We use a template-fitting technique to measure radial velocities from 145,888 individual spectra obtained for a magnitude-limited sample of 39,543 M dwarfs. Typically, the three or four spectra observed for each star are separated in time by less than four hours, but for ~17% of the stars, the individual observations span more than two days. In these cases we are sensitive to large-amplitude radial velocity variations on timescales comparable to the separation between the observations. We use a control sample of objects having observations taken within a four-hour period to make an empirical estimate of the underlying radial velocity error distribution and simulate our detection efficiency for a wide range of binary star systems. We find the frequency of binaries among the dwarf M stars with a < 0.4 AU to be 3%-4%. Comparison with other samples of binary stars demonstrates that the close binary fraction, like the total binary fraction, is an increasing function of primary mass
The unusual distribution of molecular gas and star formation in Arp 140
We investigate the atomic and molecular interstellar medium and star
formation of NGC 275, the late-type spiral galaxy in Arp 140, which is
interacting with NGC 274, an early-type system. The atomic gas (HI)
observations reveal a tidal tail from NGC 275 which extends many optical radii
beyond the interacting pair. The HI morphology implies a prograde encounter
between the galaxy pair approximately 1.5 x 10**8 years ago. The Halpha
emission from NGC 275 indicates clumpy irregular star-formation, clumpiness
which is mirrored by the underlying mass distribution as traced by the Ks-band
emission. The molecular gas distribution is striking in its anti-correlation
with the {HII regions. Despite the evolved nature of NGC 275's interaction and
its barred potential, neither the molecular gas nor the star formation are
centrally concentrated. We suggest that this structure results from stochastic
star formation leading to preferential consumption of the gas in certain
regions of the galaxy. In contrast to the often assumed picture of interacting
galaxies, NGC 275, which appears to be close to merger, does not display
enhanced or centrally concentrated star formation. If the eventual merger is to
lead to a significant burst of star formation it must be preceded by a
significant conversion of atomic to molecular gas as at the current rate of
star formation all the molecular gas will be exhausted by the time the merger
is complete.Comment: 13 paper, accepted my Monthly Notices of the Royal Astronomical
Societ
The Rotation of M Dwarfs Observed by the Apache Point Galactic Evolution Experiment
We present the results of a spectroscopic analysis of rotational velocities
in 714 M dwarf stars observed by the SDSS III Apache Point Galactic Evolution
Experiment (APOGEE) survey. We use a template fitting technique to estimate
while simultaneously estimating , ,
and . We conservatively estimate that our detection limit is 8
km s. We compare our results to M dwarf rotation studies in the
literature based on both spectroscopic and photometric measurements. Like other
authors, we find an increase in the fraction of rapid rotators with decreasing
stellar temperature, exemplified by a sharp increase in rotation near the M
transition to fully convective stellar interiors, which is consistent with the
hypothesis that fully convective stars are unable to shed angular momentum as
efficiently as those with radiative cores. We compare a sample of targets
observed both by APOGEE and the MEarth transiting planet survey and find no
cases were the measured and rotation period are physically
inconsistent, requiring . We compare our spectroscopic results to
the fraction of rotators inferred from photometric surveys and find that while
the results are broadly consistent, the photometric surveys exhibit a smaller
fraction of rotators beyond the M transition by a factor of . We
discuss possible reasons for this discrepancy. Given our detection limit, our
results are consistent with a bi-modal distribution in rotation that is seen in
photometric surveys.Comment: 31 pages, 11 figures, 4 tables. Accepted for publication by A
The Orbit of the L dwarf + T dwarf Spectral Binary SDSS J080531.84+481233.0
[abridged] We report four years of radial velocity monitoring observations of
SDSS J080531.84+481233.0 that reveal significant and periodic variability,
confirming the binary nature of the source. We infer an orbital period of
2.020.03 yr, a semi-major axis of 0.76 AU, and an
eccentricity of 0.460.05, consistent with the amplitude of astrometric
variability and prior attempts to resolve the system. Folding in constraints
based on the spectral types of the components (L40.7 and T5.51.1),
corresponding effective temperatures, and brown dwarf evolutionary models, we
further constrain the orbital inclination of this system to be nearly edge-on
(9019), and deduce a large system mass ratio (M/M =
0.86), substellar components (M =
0.057 M, M = 0.048
M), and a relatively old system age (minimum age =
4.0 Gyr). The measured projected rotational velocity of the
primary ( = 34.10.7 km/s) implies that this inactive source is a
rapid rotator (period 3 hr) and a viable system for testing
spin-orbit alignment in very-low-mass multiples. The combination of
well-determined component atmospheric properties and masses near and/or below
the hydrogen minimum mass make SDSS J0805+4812AB an important system for future
tests of brown dwarf evolutionary models.Comment: 15 pages, 11 figures, accepted for publication to Ap
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