403 research outputs found
Rayleigh Scattering in the Atmosphere of the Warm Exo-Neptune GJ 3470b
GJ 3470b is a warm Neptune-size planet transiting an M dwarf star. Like the handful of other small exoplanets for which transmission spectroscopy has been obtained, GJ 3470b exhibits a flat spectrum in the near- and mid-infrared. Recently, a tentative detection of Rayleigh scattering in its atmosphere has been reported. This signal manifests itself as an observed increase of the planetary radius as a function of decreasing wavelength in the visible. We set out to verify this detection and observed several transits of this planet with the LCOGT network and the Kuiper telescope in four different bands (Sloan g, Sloan i, Harris B, and Harris V). Our analysis reveals a strong Rayleigh scattering slope, thus confirming previous results. This makes GJ 3470b the smallest known exoplanet with a detection of Rayleigh scattering. We find that the most plausible scenario is a hydrogen/helium-dominated atmosphere covered by clouds which obscure absorption features in the infrared and hazes which give rise to scattering in the visible. Our results demonstrate the feasibility of exoplanet atmospheric characterization from the ground, even with meter-class telescopes
Unusual Isotopic Abundances in a Fully-Convective Stellar Binary
Low-mass M dwarfs represent the most common outcome of star formation, but
their complex emergent spectra hinder detailed studies of their composition and
initial formation. The measurement of isotopic ratios is a key tool that has
been used to unlock the formation of our Solar System, the Sun, and the nuclear
processes within more massive stars. We observed GJ 745AB, two M dwarfs
orbiting in a wide binary, with the IRTF/iSHELL spectrograph. Our spectroscopy
of CO in these stars at the 4.7 micron fundamental and 2.3 micron
first-overtone rovibrational bandheads reveals 12C16O, 13C16O, and 12C18O in
their photospheres. Since the stars are fully convective, the atomic
constituents of these isotopologues should be uniformly mixed throughout the
stars' interiors. We find that in these M dwarfs, both 12C/13C and 16O/18O
greatly exceed the Solar values. These measurements cannot be explained solely
by models of Galactic chemical evolution, but require that the stars formed
from an ISM significantly enriched by material ejected from an exploding
core-collape supernova. These isotopic measurements complement the elemental
abundances provided by large-scale spectroscopic surveys, and open a new window
onto studies of Galactic evolution, stellar populations, and individual
systems.Comment: 11 pages, 4 data files, 3 figures, 2 tables. ApJ in pres
High Resolution, Differential, Near-infrared Transmission Spectroscopy of GJ 1214b
The nearby star GJ 1214 hosts a planet intermediate in radius and mass
between Earth and Neptune, resulting in some uncertainty as to its nature. We
have observed this planet, GJ 1214b, during transit with the high-resolution,
near-infrared NIRSPEC spectrograph on the Keck II telescope, in order to
characterize the planet's atmosphere. By cross-correlating the spectral changes
through transit with a suite of theoretical atmosphere models, we search for
variations associated with absorption in the planet atmosphere. Our
observations are sufficient to rule out tested model atmospheres with
wavelength-dependent transit depth variations >5e-4 over the wavelength range
2.1 - 2.4 micron. Our sensitivity is limited by variable slit loss and telluric
transmission effects.
We find no positive signatures but successfully rule out a number of
plausible atmospheric models, including the default assumption of a gaseous,
H-dominated atmosphere in chemical equilibrium. Such an atmosphere can be made
consistent if the absorption due to methane is reduced. Clouds can also render
such an atmosphere consistent with our observations, but only if they lie
higher in the atmosphere than indicated by recent optical and infrared
measurements.
When taken in concert with constraints from other groups, our results support
a consensus model in which the atmosphere of GJ 1214b contains significant H
and He, but where methane is depleted. If this depletion is the result of
photochemical processes, it may also produce a haze that suppresses spectral
features in the optical.Comment: 32 pages, 15 figures, preprint, accepted to ApJ, responded to
referee's comments. Comments welcom
K2-136: A Hyades Binary Star with a Neptune-sized Planet
We report the discovery of a Neptune-size planet (Rp = 3.0 Re) in the Hyades
Cluster. The host star is in a binary system, comprising a K5V star and M7/8V
star with a projected separation of 40 AU. The planet orbits the primary star
with an orbital period of 17.3 days and a transit duration of 3 hours. The host
star is bright (V=11.2, J=9.1) and so may be a good target for precise radial
velocity measurements. K2-136A c is the first Neptune-sized planet to be found
orbiting in a binary system within an open cluster. The Hyades is the nearest
star cluster to the Sun, has an age of 625-750 Myr, and forms one of the
fundamental rungs in the distance ladder; understanding the planet population
in such a well-studied cluster can help us understand and set constraints on
the formation and evolution of planetary systems.Comment: 13 pages, 8 figures, 2 tables. Accepted to AAS Journal
A global cloud map of the nearest known brown dwarf
Brown dwarfs -- substellar bodies more massive than planets but not massive
enough to initiate the sustained hydrogen fusion that powers self-luminous
stars -- are born hot and slowly cool as they age. As they cool below about
2,300 K, liquid or crystalline particles composed of calcium aluminates,
silicates and iron condense into atmospheric 'dust', which disappears at still
cooler temperatures (around 1,300 K). Models to explain this dust dispersal
include both an abrupt sinking of the entire cloud deck into the deep, unob-
servable atmosphere and breakup of the cloud into scattered patches (as seen on
Jupiter and Saturn). Thus far, observations of brown dwarfs have been limited
to globally integrated measurements, which can reveal surface inhomogeneities
but cannot unambiguously resolve surface features. Here we report a
two-dimensional map of a brown dwarf's surface that allows identification of
large-scale bright and dark features, indicative of patchy clouds.Comment: 17 pages, 8 figures. Spectra and map available upon reques
ACME Stellar Spectra. I. Absolutely Calibrated, Mostly Empirical Flux Densities of 55 Cancri and its Transiting Planet 55 Cancri e
The ACME Spectra project provides absolutely calibrated, mostly empirical
spectra of exoplanet host stars for use in analysis of the stars and their
planets. Spectra are obtained from ground-based telescopes and are tied
directly to calibrated ground- and space-based photometry. The spectra remain
only "mostly" empirical because of telluric absorption, but interpolation of
stellar models over the gaps in wavelength coverage provides continuous stellar
spectra. Among other uses, the spectra are suitable for precisely converting
observed secondary eclipses (occultations) into absolute flux units with
minimal recourse to models. In this letter I introduce ACME's methods and
present a calibrated spectrum of the nearby, super-Earth hosting star 55 Cancri
that spans the range from 0.81-5.05 micron. This spectrum is well-suited for
interpreting near- and thermal-infrared eclipse observations. With this
spectrum I show that the brightness temperature of the small, low-mass
transiting planet 55 Cnc e is 1950 +260/-190 K at 4.5 micron (cooler than
previously reported), which corresponds to a planetary flux of 0.44 +0.12/-0.08
mJy. This result suggests the planet has some combination of a nonzero albedo,
a moderately efficient redistribution of absorbed stellar irradiation, and/or
an optically thick atmosphere, but more precise eclipse measurements are
required to distinguish between these scenarii.Comment: Accepted to A&A. 6 pages, 1 figure, 1 electronic table. See
http://irtfweb.ifa.hawaii.edu/~spex/IRTF_Spectral_Library/ for an alternative
spectru
Characterizing Exoplanets in the Visible and Infrared: A Spectrometer Concept for the EChO Space Mission
Transit-spectroscopy of exoplanets is one of the key observational techniques
to characterize the extrasolar planet and its atmosphere. The observational
challenges of these measurements require dedicated instrumentation and only the
space environment allows an undisturbed access to earth-like atmospheric
features such as water or carbon-dioxide. Therefore, several exoplanet-specific
space missions are currently being studied. One of them is EChO, the Exoplanet
Characterization Observatory, which is part of ESA's Cosmic Vision 2015-2025
program, and which is one of four candidates for the M3 launch slot in 2024. In
this paper we present the results of our assessment study of the EChO
spectrometer, the only science instrument onboard this spacecraft. The
instrument is a multi-channel all-reflective dispersive spectrometer, covering
the wavelength range from 400 nm to 16 microns simultaneously with a moderately
low spectral resolution. We illustrate how the key technical challenge of the
EChO mission - the high photometric stability - influences the choice of
spectrometer concept and drives fundamentally the instrument design. First
performance evaluations underline the fitness of the elaborated design solution
for the needs of the EChO mission.Comment: 20 pages, 8 figures, accepted for publication in the Journal of
Astronomical Instrumentatio
Exoplanet phase curves: observations and theory
Phase curves are the best technique to probe the three dimensional structure
of exoplanets' atmospheres. In this chapter we first review current exoplanets
phase curve observations and the particular challenges they face. We then
describe the different physical mechanisms shaping the atmospheric phase curves
of highly irradiated tidally locked exoplanets. Finally, we discuss the
potential for future missions to further advance our understanding of these new
worlds.Comment: Fig.5 has been updated. Table 1 and corresponding figures have been
updated with new values for WASP-103b and WASP-18b. Contains a table
sumarizing phase curve observation
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