152 research outputs found
Correcting for Activity Effects on the Temperatures, Radii, and Estimated Masses of Low-Mass Stars and Brown Dwarfs
We present empirical relations for determining the amount by which the
effective temperatures and radii---and therefore the estimated masses---of
low-mass stars and brown dwarfs are altered due to chromospheric activity.
Accurate estimates of stellar radii are especially important in the context of
searches for transiting exoplanets, which rely upon the assumed stellar
radius/density to infer the planet radius/density. Our relations are based on a
large set of well studied low-mass stars in the field and on a set of benchmark
low-mass eclipsing binaries. The relations link the amount by which an active
object's temperature is suppressed, and its radius inflated, to the strength of
its Halpha emission. These relations are found to approximately preserve
bolometric luminosity. We apply these relations to the peculiar brown-dwarf
eclipsing binary 2M0535-05, in which the active, higher-mass brown dwarf has a
cooler temperature than its inactive, lower-mass companion. The relations
correctly reproduce the observed temperatures and radii of 2M0535-05 after
accounting for the Halpha emission; 2M0535-05 would be in precise agreement
with theoretical isochrones were it inactive. The relations that we present are
applicable to brown dwarfs and low-mass stars with masses below 0.8 Msun and
for which the activity, as measured by Halpha, is in the range -4.6 < log
Lha/Lbol < -3.3. We expect these relations to be most useful for correcting
radius and mass estimates of low-mass stars and brown dwarfs over their active
lifetimes (few Gyr). We also discuss the implications of this work for
determinations of young cluster IMFs.Comment: To appear in Cool Stars 17 proceeding
Discovery of a Visual T-Dwarf Triple System and Binarity at the L/T Transition
We present new high contrast imaging of 8 L/T transition brown dwarfs using
the NIRC2 camera on the Keck II telescope. One of our targets, the T3.5 dwarf
2MASS J08381155 + 1511155, was resolved into a hierarchal triple with projected
separations of 2.5+/-0.5 AU and 27+/-5 AU for the BC and A(BC) components
respectively. Resolved OSIRIS spectroscopy of the A(BC) components confirm that
all system members are T dwarfs. The system therefore constitutes the first
triple T-dwarf system ever reported. Using resolved photometry to model the
integrated-light spectrum, we infer spectral types of T3, T3, and T4.5 for the
A, B, and C components respectively. The uniformly brighter primary has a bluer
J-Ks color than the next faintest component, which may reflect a sensitive
dependence of the L/T transition temperature on gravity, or alternatively
divergent cloud properties amongst components. Relying on empirical trends and
evolutionary models we infer a total system mass of 0.034-0.104 Msun for the BC
components at ages of 0.3-3 Gyr, which would imply a period of 12-21 yr
assuming the system semi-major axis to be similar to its projection. We also
infer differences in effective temperatures and surface gravities between
components of no more than ~150 K and ~0.1 dex. Given the similar physical
properties of the components, the 2M0838+15 system provides a controlled sample
for constraining the relative roles of effective temperature, surface gravity,
and dust clouds in the poorly understood L/T transition regime. Combining our
imaging survey results with previous work we find an observed binary fraction
of 4/18 or 22_{-8}^{+10}% for unresolved spectral types of L9-T4 at separations
>~0.1 arcsec. This translates into a volume-corrected frequency of
13^{-6}_{+7}%, which is similar to values of ~9-12% reported outside the
transition. (ABRIDGED)Comment: Accepted for publication in the Astrophysical Journal. 23 pages, 12
figure
Dynamical Mass of the Substellar Benchmark Binary HD 130948BC
(Abridged) We present Keck, HST, and Gemini-North observations of the L4+L4
binary HD 130948BC which together span ~70% of the binary's orbital period. We
determine a total dynamical mass of 0.109+/-0.002 Msun (114+/-2 Mjup). The flux
ratio is near unity, so both components are unambiguously substellar for any
plausible mass ratio. An independent constraint on the age of the system is
available from the G2V primary HD 130948A. The available indicators suggest an
age comparable to the Hyades, with the most precise age being 0.79 Gyr based on
gyrochronology. Therefore, HD 130948BC is now a unique benchmark among field L
and T dwarfs, with a well-determined mass, luminosity, and age. We find that
substellar theoretical models disagree with our observations. Both components
of HD 130948BC appear to be overluminous by a factor of ~2-3x compared to
evolutionary models. The age of the system would have to be notably younger
than the gyro age to ameliorate the luminosity disagreement. However,
regardless of the adopted age, evolutionary and atmospheric models give
inconsistent results, indicating systematic errors in at least one class of
models, possibly both. The masses of HD 130948BC happen to be very near the
theoretical mass limit for lithium burning, and thus measuring the differential
lithium depletion between B and C will provide a uniquely discriminating test
of theoretical models. The potential underestimate of luminosities by
evolutionary models would have wide-ranging implications; therefore, a more
refined age estimate for HD 130948A is critically needed.Comment: ApJ, accepted. Note that astro-ph posting date coincides with the
periastron passage for this binar
Limits on the Mass and Initial Entropy of 51 Eri b from Gaia EDR3 Astrometry
51 Eri b is one of the only young planets consistent with a wide range of
possible initial entropy states, including the cold-start scenario associated
with some models of planet formation by core accretion. The most direct way to
constrain the initial entropy of a planet is by measuring its luminosity and
mass at a sufficiently young age that the initial conditions still matter. We
present the tightest upper limit on 51 Eri b's mass yet (M < 11 Mjup at
2) using a cross-calibration of Hipparcos and Gaia EDR3 astrometry and
the orbit-fitting code orvara. We also reassess its luminosity using a direct,
photometric approach, finding log(Lbol/Lsun) = -5.50.2 dex. Combining this
luminosity with the 243 Myr age of the Pic moving group, of which
51 Eri is a member, we derive mass distributions from a grid of evolutionary
models that spans a wide range of initial entropies. We find that 51 Eri b is
inconsistent with the coldest-start scenarios, requiring an initial entropy of
>8 /baryon at 97% confidence. This result represents the first
observational constraint on the initial entropy of a potentially cold-start
planet, and it continues the trend of dynamical masses for directly imaged
planets pointing to warm- or hot-start formation scenarios.Comment: Accepted for publication in MNRAS (9 pages, 6 figures
Precise Dynamical Masses of Directly Imaged Companions from Relative Astrometry, Radial Velocities, and Hipparcos-Gaia DR2 Accelerations
We measure dynamical masses for five objects--three ultracool dwarfs, one
low-mass star, and one white dwarf--by fitting orbits to a combination of the
Hipparcos-Gaia Catalog of Accelerations, literature radial velocities, and
relative astrometry. Our approach provides precise masses without any
assumptions about the primary star, even though the observations typically
cover only a small fraction of an orbit. We also perform a uniform re-analysis
of the host stars' ages. Two of our objects, HD 4747B and HR 7672B, already
have precise dynamical masses near the stellar/substellar boundary and are used
to validate our approach. For Gl 758B, we obtain a mass of
, the most precise mass measurement of this
companion to date. Gl 758B is the coldest brown dwarf with a dynamical mass,
and the combination of our low mass and slightly older host-star age resolves
its previously noted discrepancy with substellar evolutionary models. HD
68017B, a late-M dwarf, has a mass of , consistent
with stellar theory and previous empirical estimates based on its absolute
magnitude. The progenitor of the white dwarf Gl 86B has been debated in the
literature, and our dynamical measurement of is
consistent with a higher progenitor mass and younger age for this
planet-hosting binary system. Overall, these case studies represent only five
of the thousands of accelerating systems identified by combining Hipparcos and
Gaia. Our analysis could be repeated for many of them to build a large sample
of companions with dynamical masses.Comment: 33 pages, 24 figures, 9 tables, AJ accepted with minor revision
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