748 research outputs found
Hubble Space Telescope Near-IR Transmission Spectroscopy of the Super-Earth HD 97658b
Recent results from the Kepler mission indicate that super-Earths (planets
with masses between 1-10 times that of the Earth) are the most common kind of
planet around nearby Sun-like stars. These planets have no direct solar system
analogue, and are currently one of the least well-understood classes of
extrasolar planets. Many super-Earths have average densities that are
consistent with a broad range of bulk compositions, including both
water-dominated worlds and rocky planets covered by a thick hydrogen and helium
atmosphere. Measurements of the transmission spectra of these planets offer the
opportunity to resolve this degeneracy by directly constraining the scale
heights and corresponding mean molecular weights of their atmospheres. We
present Hubble Space Telescope near-infrared spectroscopy of two transits of
the newly discovered transiting super-Earth HD 97658b. We use the Wide Field
Camera 3's scanning mode to measure the wavelength-dependent transit depth in
thirty individual bandpasses. Our averaged differential transmission spectrum
has a median 1 sigma uncertainty of 23 ppm in individual bins, making this the
most precise observation of an exoplanetary transmission spectrum obtained with
WFC3 to date. Our data are inconsistent with a cloud-free solar metallicity
atmosphere at the 10 sigma level. They are consistent at the 0.4 sigma level
with a flat line model, as well as effectively flat models corresponding to a
metal-rich atmosphere or a solar metallicity atmosphere with a cloud or haze
layer located at pressures of 10 mbar or higher.Comment: ApJ in press; revised version includes an updated orbital ephemeris
for the plane
More Evidence for Variable Helium Absorption from HD 189733b
We present a new Keck/NIRSPEC observation of metastable helium absorption
from the upper atmosphere of HD 189733b, a hot Jupiter orbiting a nearby
moderately active star. We measure an average helium transit depth of % integrated over the [-20, 20] km/s velocity range. Comparing this
measurement to eight previously published transit observations with different
instruments, we find that our depth is 32% (9) lower than the average
of the three CARMENES transits, but only 16% (4.4) lower than the
average of the five GIANO transits. We perform 1D hydrodynamical simulations of
the outflow, and find that XUV variability on the order of 33%--common for this
star--can change the helium absorption depth by 60%. We conclude that changes
in stellar XUV flux can explain the observational variability in helium
absorption. 3D models are necessary to explore other sources of variability,
such as shear instability and changing stellar wind conditions.Comment: Published by A
Detection of Helium in the Atmosphere of the Exo-Neptune HAT-P-11b
The helium absorption triplet at a wavelength of 10,833 \AA\ has been
proposed as a way to probe the escaping atmospheres of exoplanets. Recently
this feature was detected for the first time using Hubble Space Telescope (HST)
WFC3 observations of the hot Jupiter WASP-107b. We use similar HST/WFC3
observations to detect helium in the atmosphere of the hot Neptune HAT-P-11b at
the confidence level. We compare our observations to a grid of 1D
models of hydrodynamic escape to constrain the thermospheric temperatures and
mass loss rate. We find that our data are best fit by models with high mass
loss rates of - g s. Although we do
not detect the planetary wind directly, our data are consistent with the
prediction that HAT-P-11b is experiencing hydrodynamic atmospheric escape.
Nevertheless, the mass loss rate is low enough that the planet has only lost up
to a few percent of its mass over its history, leaving its bulk composition
largely unaffected. This matches the expectation from population statistics,
which indicate that close-in planets with radii greater than 2 R
form and retain H/He-dominated atmospheres. We also confirm the independent
detection of helium in HAT-P-11b obtained with the CARMENES instrument, making
this the first exoplanet with the detection of the same signature of
photoevaporation from both ground- and space-based facilities.Comment: 12 pages, 9 figures, accepted for publication in ApJ
A Precise Water Abundance Measurement for the Hot Jupiter WASP-43b
The water abundance in a planetary atmosphere provides a key constraint on
the planet's primordial origins because water ice is expected to play an
important role in the core accretion model of planet formation. However, the
water content of the Solar System giant planets is not well known because water
is sequestered in clouds deep in their atmospheres. By contrast, short-period
exoplanets have such high temperatures that their atmospheres have water in the
gas phase, making it possible to measure the water abundance for these objects.
We present a precise determination of the water abundance in the atmosphere of
the 2 short-period exoplanet WASP-43b based on thermal
emission and transmission spectroscopy measurements obtained with the Hubble
Space Telescope. We find the water content is consistent with the value
expected in a solar composition gas at planetary temperatures (0.4-3.5x solar
at 1 confidence). The metallicity of WASP-43b's atmosphere suggested
by this result extends the trend observed in the Solar System of lower metal
enrichment for higher planet masses.Comment: Accepted to ApJL; this version contains three supplemental figures
that are not included in the published paper. See also our companion paper
"Thermal structure of an exoplanet atmosphere from phase-resolved emission
spectroscopy" by Stevenson et a
Missing Black Holes Unveil The Supernova Explosion Mechanism
It is firmly established that the stellar mass distribution is smooth,
covering the range 0.1-100 Msun. It is to be expected that the masses of the
ensuing compact remnants correlate with the masses of their progenitor stars,
and thus it is generally thought that the remnant masses should be smoothly
distributed from the lightest white dwarfs to the heaviest black holes.
However, this intuitive prediction is not borne out by observed data. In the
rapidly growing population of remnants with observationally determined masses,
a striking mass gap has emerged at the boundary between neutron stars and black
holes. The heaviest neutron stars reach a maximum of two solar masses, while
the lightest black holes are at least five solar masses. Over a decade after
the discovery, the gap has become a significant challenge to our understanding
of compact object formation. We offer new insights into the physical processes
that bifurcate the formation of remnants into lower mass neutron stars and
heavier black holes. Combining the results of stellar modeling with
hydrodynamic simulations of supernovae, we both explain the existence of the
gap, and also put stringent constraints on the inner workings of the supernova
explosion mechanism. In particular, we show that core-collapse supernovae are
launched within 100-200 milliseconds of the initial stellar collapse, implying
that the explosions are driven by instabilities with a rapid (10-20 ms) growth
time. Alternatively, if future observations fill in the gap, this will be an
indication that these instabilities develop over a longer (>200 milliseconds)
timescale.Comment: ApJ, accepted: comments added on recent Ugliano et al. and Kreidberg
et al. studie
Clouds in the atmosphere of the super-Earth exoplanet GJ 1214b
Recent surveys have revealed that planets intermediate in size between Earth and Neptune (‘super-Earths’) are among the most common planets in the Galaxy. Atmospheric studies are the next step towards developing a comprehensive understanding of this new class of object. Much effort has been focused on using transmission spectroscopy to characterize the atmosphere of the super-Earth archetype GJ 1214b, but previous observations did not have sufficient precision to distinguish between two interpretations for the atmosphere. The planet’s atmosphere could be dominated by relatively heavy molecules, such as water (for example, a 100 per cent water vapour composition), or it could contain high-altitude clouds that obscure its lower layers. Here we report a measurement of the transmission spectrum of GJ 1214b at near-infrared wavelengths that definitively resolves this ambiguity. The data, obtained with the Hubble Space Telescope, are sufficiently precise to detect absorption features from a high mean-molecular-mass atmosphere. The observed spectrum, however, is featureless. We rule out cloud-free atmospheric models with compositions dominated by water, methane, carbon monoxide, nitrogen or carbon dioxide at greater than 5σ confidence. The planet’s atmosphere must contain clouds to be consistent with the data
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