568 research outputs found
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
Characterising & classifying the local population of ultracool dwarfs with Gaia DR2 and EDR3
Ultracool dwarfs (UCDs) are the lowest mass products of star formation and span the end of the
stellar main sequence from very-low mass, hydrogen-burning M stars to the coolest brown dwarfs.
In this thesis we characterise and classify the ultracool dwarf population in the solar neighbourhood using the accuracy and precision of data from the Gaia space observatory. Combining
astrometric (in particular parallax) and photometric data from Gaia DR2 and EDR3 with photometry from UKIDSS, SDSS and 2MASS, we prepare some of the largest and most accurate,
near-100% complete volume-limited populations of nearby, field late-M, L and T dwarfs. From
these samples we derive key population characteristics such as colour-absolute magnitude relationships, the stellar luminosity function, the binary fraction and the binary mass ratio. Our
statistical-based approach differs from much of the UCD literature to date which seeks to prepare
meta-catalogues from disparate surveys and individual spectroscopic observations with distance
determined by indirect methods. Our approach offers improvements in scale, completeness, and
distance accuracy. In particular we use Gaia to update the colour-magnitude relations and
derive the stellar luminosity functions in MJ and MG of the UCDs. We calculate the binary
fraction of the late-M and early-L dwarfs as a function of spectral type by carefully modelling the
over-luminous unresolved binary population and show that late-M dwarf binaries reside almost
exclusively in equal-mass pairs or twins.
Given the complex spectral features of UCDs, consistent and accurate classification is challenging. We investigate the current traditional methods of classification and evaluate a range of
alternative techniques including supervised and unsupervised machine learning.
In a separate study we use Gaia data to prepare a large, cylindrical sample of FGK main
sequence dwarf stars to calculate the structure of the vertical density distribution close to the
galactic plane, in fine detail, as a function of colour. Using our derived colour-dependent thin
disk scale height we directly determine the star formation history of the solar neighbourhood by
modelling the evolution of stellar populations using state-of-the-art PARSEC isochrones.Open Acces
An 800-million-solar-mass black hole in a significantly neutral Universe at redshift 7.5
Quasars are the most luminous non-transient objects known and as a result
they enable studies of the Universe at the earliest cosmic epochs. Despite
extensive efforts, however, the quasar ULAS J1120+0641 at z=7.09 has remained
the only one known at z>7 for more than half a decade. Here we report
observations of the quasar ULAS J134208.10+092838.61 (hereafter J1342+0928) at
redshift z=7.54. This quasar has a bolometric luminosity of 4e13 times the
luminosity of the Sun and a black hole mass of 8e8 solar masses. The existence
of this supermassive black hole when the Universe was only 690 million years
old---just five percent of its current age---reinforces models of early
black-hole growth that allow black holes with initial masses of more than about
1e4 solar masses or episodic hyper-Eddington accretion. We see strong evidence
of absorption of the spectrum of the quasar redwards of the Lyman alpha
emission line (the Gunn-Peterson damping wing), as would be expected if a
significant amount (more than 10 per cent) of the hydrogen in the intergalactic
medium surrounding J1342+0928 is neutral. We derive a significant fraction of
neutral hydrogen, although the exact fraction depends on the modelling.
However, even in our most conservative analysis we find a fraction of more than
0.33 (0.11) at 68 per cent (95 per cent) probability, indicating that we are
probing well within the reionization epoch of the Universe.Comment: Updated to match the final journal versio
The Flat Transmission Spectrum of the Super-Earth GJ1214b from Wide Field Camera 3 on the Hubble Space Telescope
Capitalizing on the observational advantage offered by its tiny M dwarf host,
we present HST/WFC3 grism measurements of the transmission spectrum of the
super-Earth exoplanet GJ1214b. These are the first published WFC3 observations
of a transiting exoplanet atmosphere. After correcting for a ramp-like
instrumental systematic, we achieve nearly photon-limited precision in these
observations, finding the transmission spectrum of GJ1214b to be flat between
1.1 and 1.7 microns. Inconsistent with a cloud-free solar composition
atmosphere at 8.2 sigma, the measured achromatic transit depth most likely
implies a large mean molecular weight for GJ1214b's outer envelope. A dense
atmosphere rules out bulk compositions for GJ1214b that explain its large
radius by the presence of a very low density gas layer surrounding the planet.
High-altitude clouds can alternatively explain the flat transmission spectrum,
but they would need to be optically thick up to 10 mbar or consist of particles
with a range of sizes approaching 1 micron in diameter.Comment: 17 pages, 12 figures, accepted for publication in Ap
The Flat Transmission Spectrum of the Super-Earth GJ1214b from Wide Field Camera 3 on the Hubble Space Telescope
Capitalizing on the observational advantage offered by its tiny M dwarf host,
we present HST/WFC3 grism measurements of the transmission spectrum of the
super-Earth exoplanet GJ1214b. These are the first published WFC3 observations
of a transiting exoplanet atmosphere. After correcting for a ramp-like
instrumental systematic, we achieve nearly photon-limited precision in these
observations, finding the transmission spectrum of GJ1214b to be flat between
1.1 and 1.7 microns. Inconsistent with a cloud-free solar composition
atmosphere at 8.2 sigma, the measured achromatic transit depth most likely
implies a large mean molecular weight for GJ1214b's outer envelope. A dense
atmosphere rules out bulk compositions for GJ1214b that explain its large
radius by the presence of a very low density gas layer surrounding the planet.
High-altitude clouds can alternatively explain the flat transmission spectrum,
but they would need to be optically thick up to 10 mbar or consist of particles
with a range of sizes approaching 1 micron in diameter.Comment: 17 pages, 12 figures, accepted for publication in Ap
Keck Laser Guide Star Adaptive Optics Monitoring of 2MASS J1534-2952AB: First Dynamical Mass Determination of a Binary T Dwarf
(Abridged) We present multi-epoch imaging of the T5.0+T5.5 binary 2MASS
J1534-2952AB obtained with the Keck laser guide star adaptive optics system.
Combined with an extensive (re-)analysis of archival HST imaging, we find a
total mass of 0.056+/-0.003 Msun (59+/-3 Mjup). This is the first field binary
for which both components are directly confirmed to be substellar. This is also
the coolest and lowest mass binary with a dynamical mass determination to date.
Using evolutionary models, we derive an age of 0.78+/-0.09 Gyr for the system,
and we find Teff = 1028+/-17 K and 978+/-17 K and masses of 0.0287+/-0.0016
Msun (30.1+/-1.7 Mjup) and 0.0269+/-0.0016 Msun (28.2+/-1.7 Mjup) for the
individual components. These precise measurements generally agree with previous
studies of T dwarfs and affirm the current theoretical models. However, (1) the
temperatures are about 100 K cooler than derived for similar objects and
suggest that the ages of field brown dwarfs may be overestimated. Also, (2) the
H-R diagram positions are discrepant with current predictions. While this may
arise from large errors in the evolutionary models, the likely cause is a
modest (~100 K) overestimate in temperatures determined from model atmospheres.
We discuss future tests of theory as the sample of substellar dynamical masses
increases. In particular, we suggest that low-mass field binaries with
dynamical masses ("mass benchmarks") can serve as reference points for Teff and
log(g) and thereby constrain ultracool atmosphere models, as good as or even
better than single brown dwarfs with age estimates ("age benchmarks").Comment: ApJ, in press. Typo in Eqn. 6 corrected in v
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