202 research outputs found
How drifting and evaporating pebbles shape giant planets III: The formation of WASP-77A b and Bo\"otis b
Atmospheric abundances are thought to constrain the planet formation pathway,
because different species evaporate at different temperatures leaving distinct
signatures in the accreted atmosphere. The planetary C/O ratio is thought to
constrain the planet formation pathway, because of the condensation sequence of
HO, CO, CH, and CO, resulting in an increase of the gas phase C/O
ratio with increasing distance. Here we use a disc evolution model including
pebble growth, drift and evaporation coupled with a planet formation model that
includes pebble and gas accretion as well as planet migration to compute the
atmospheric compositions of giant planets. We compare our results to the recent
observations of the hot Jupiters WASP-77A b and Bo\"otis b, which
feature sub-solar and super-solar C/H and O/H values, respectively. Our
simulations reproduce these measurements and show that giants like WASP-77A b
should start to form beyond the CO evaporation front, while giants like
Bo\"otis b should originate from beyond the HO line. Our model
allows the formation of sub- and super-solar atmospheric compositions. However
simulations without pebble evaporation can not reproduce the super-solar C/H
and O/H ratios of Bo\"otis b's atmosphere without additional accretion
of solids. We identify the viscosity parameter of the disc as a key
ingredient, because the viscosity drives the inward motion of volatile enriched
vapor, responsible for the accretion of gaseous C and O. Depending on the
planet's migration history order-of-magnitude differences in atmospheric C/H
and O/H are expected. Our simulations also predict super-solar N/H for
Bo\"otis b and solar N/H for WASP-77A b. We conclude that pebble evaporation is
a key ingredient to explain the variety of exoplanet atmospheres, because it
can explain both, sub- and super-solar atmospheric abundances.Comment: Accepted by A&A, 7 pages, 4 figure
Prospects for Characterizing the Atmosphere of Proxima Centauri B
The newly detected Earth-mass planet in the habitable zone of Proxima Centauri could potentially host life - if it has an atmosphere that supports surface liquid water. We show that thermal phase curve observations with the James Webb Space Telescope (JWST) from 5-12 microns can be used to test the existence of such an atmosphere. We predict the thermal variation for a bare rock versus a planet with 35% heat redistribution to the nightside and show that a JWST phase curve measurement can distinguish between these cases at 4σ confidence, assuming photon-limited precision. We also consider the case of an Earth-like atmosphere, and find that the ozone 9.8 micron band could be detected with longer integration times (a few months). We conclude that JWST observations have the potential to put the first constraints on the possibility of life around the nearest star to the Solar System.Astronom
Mass Measurements of Black Holes in X-Ray Transients: Is There a Mass Gap?
We explore possible systematic errors in the mass measurements of stellar
mass black holes. We find that significant errors can arise from the assumption
of zero or constant emission from the accretion flow, which is commonly used
when determining orbital inclination by modelling ellipsoidal variations. For
A0620-00, the system with the best available data, we show that typical data
sets and analysis procedures can lead to systematic underestimates of the
inclination by ten degrees or more. A careful examination of the available data
for the 15 other X-ray transients with low-mass donors suggests that this
effect may significantly reduce the black hole mass estimates in several other
cases, most notably that of GRO J0422+32. With these revisions, our analysis of
the black hole mass distribution in soft X-ray transients does not suggest any
"mass gap" between the low end of the distribution and the maximum theoretical
neutron star mass, as has been identified in previous studies. Nevertheless, we
find that the mass distribution retains other previously identified
characteristics, namely a peak around 8M\odot, a paucity of sources with masses
below 5M\odot, and a sharp drop-off above 10M\odot.Comment: 18 pages, 10 figures; submitted to Ap
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