122 research outputs found
An HST/STIS Optical Transmission Spectrum of Warm Neptune GJ 436b
GJ 436b is a prime target for understanding warm Neptune exoplanet
atmospheres and a target for multiple JWST GTO programs. Here, we report the
first space-based optical transmission spectrum of the planet using two
HST/STIS transit observations from 0.53-1.03 microns. We find no evidence for
alkali absorption features, nor evidence of a scattering slope longward of 0.53
microns. The spectrum is indicative of moderate to high metallicity (~100-1000x
solar) while moderate metallicity scenarios (~100x solar) require aerosol
opacity. The optical spectrum also rules out some highly scattering haze
models. We find an increase in transit depth around 0.8 microns in the
transmission spectra of 3 different sub-Jovian exoplanets (GJ 436b, HAT-P-26b,
and GJ 1214b). While most of the data come from STIS, data from three other
instruments may indicate this is not an instrumental effect. Only the transit
spectrum of GJ 1214b is well fit by a model with stellar plages on the
photosphere of the host star. Our photometric monitoring of the host star
reveals a stellar rotation rate of 44.1 days and an activity cycle of 7.4
years. Intriguingly, GJ 436 does not become redder as it gets dimmer, which is
expected if star spots were dominating the variability. These insights into the
nature of the GJ 436 system help refine our expectations for future
observations in the era of JWST, whose higher precision and broader wavelength
coverage will shed light on the composition and structure of GJ 436b's
atmosphere.Comment: 20 pages, 11 figures, 5 tables, Accepted to AJ. A full version of
table 1 is included as table1_mrt.tx
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
The Very Low Albedo of WASP-12b From Spectral Eclipse Observations with
We present an optical eclipse observation of the hot Jupiter WASP-12b using
the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope.
These spectra allow us to place an upper limit of (97.5%
confidence level) on the planet's white light geometric albedo across 290--570
nm. Using six wavelength bins across the same wavelength range also produces
stringent limits on the geometric albedo for all bins. However, our
uncertainties in eclipse depth are 40% greater than the Poisson limit and
may be limited by the intrinsic variability of the Sun-like host star --- the
solar luminosity is known to vary at the level on a timescale of
minutes. We use our eclipse depth limits to test two previously suggested
atmospheric models for this planet: Mie scattering from an aluminum-oxide haze
or cloud-free Rayleigh scattering. Our stringent nondetection rules out both
models and is consistent with thermal emission plus weak Rayleigh scattering
from atomic hydrogen and helium. Our results are in stark contrast with those
for the much cooler HD 189733b, the only other hot Jupiter with spectrally
resolved reflected light observations; those data showed an increase in albedo
with decreasing wavelength. The fact that the first two exoplanets with optical
albedo spectra exhibit significant differences demonstrates the importance of
spectrally resolved reflected light observations and highlights the great
diversity among hot Jupiters.Comment: 8 pages, 4 figures, 1 table, published in ApJL, in pres
Gemini/GMOS Optical Transmission Spectroscopy of WASP-121b: signs of variability in an ultra-hot Jupiter?
We present ground-based, spectroscopic observations of two transits of the
ultra-hot Jupiter WASP-121b covering the wavelength range 500 - 950 nm
using Gemini/GMOS. We use a Gaussian process framework to model instrumental
systematics in the light curves, and also demonstrate the use of the more
generalised Student's-T process to verify our results. We find that our
measured transmission spectrum, whilst showing overall agreement, is slightly
discrepant with results obtained using HST/STIS, particularly for wavelengths
shortward of 650 nm. In contrast to the STIS results, we find evidence
for an increasing blueward slope and little evidence for absorption from either
TiO or VO in our retrieval, in agreement with a number of recent studies
performed at high-resolution. We suggest that this might point to some other
absorbers, particularly some combination of recently detected atomic metals, in
addition to scattering by hazes, being responsible for the excess optical
absorption and observed vertical thermal inversion. Our results are also
broadly consistent with previous ground-based photometry and 3D GCM
predictions, however, these assumed different chemistry to our retrievals. In
addition, we show that the GMOS observations are repeatable over short periods
(days), similarly to the HST/STIS observations. Their difference over longer
periods (months) could well be the result of temporal variability in the
atmospheric properties (i.e. weather) as predicted by theoretical models of
ultra-hot Jupiters; however, more mundane explanations such as instrumental
systematics and stellar activity cannot be fully ruled out, and we encourage
future observations to explore this possibility.Comment: 17 pages, 10 Figures. Accepted for publication in MNRA
The Hubble PanCET program: The near-ultraviolet transmission spectrum of WASP-79b
We present Hubble Space Telescope (HST) transit observations of the
Hot-Jupiter WASP-79b acquired with the Space Telescope Imaging Spectrograph
(STIS) in the near ultraviolet (NUV). Two transit observations, part of the
PanCET program, are used to obtain the transmission spectra of the planet
between 2280 and 3070{\AA}. We correct for systematic effects in the raw data
using the jitter engineering parameters and polynomial modelling to fit the
white light curves of the two transits. We observe an increase in the
planet-to-star radius ratio at short wavelengths, but no spectrally resolved
absorption lines. The difference between the radius ratios at 2400 and
3000{\AA} reaches (4.5). Although the NUV
transmission spectrum does not show evidence of hydrodynamical escape, the
strong atmospheric features are likely due to species at very high altitudes.
We performed a 1D simulation of the temperature and composition of WASP-79b
using Exo-REM. The temperature pressure profile crosses condensation curves of
radiatively active clouds, particularly MnS, MgSiO, Fe, and
AlO. Still, none of these species produces the level of observed
absorption at short wavelengths and can explain the observed increase in the
planet's radius. WASP-79b's transit depth reaches 23 scale height, making it
one of the largest spectral features observed in an exoplanet at this
temperature (1700 K). The comparison of WASP-79b's transmission spectrum
with three warmer hot Jupiters shows a similar level of absorption to WASP-178b
and WASP-121b between 0.2 and 0.3m, while HAT-P-41b's spectrum is flat.
The features could be explained by SiO absorption.Comment: Accepted for publication January 31, 2023 in the Journal Astronomy &
Astrophysic
A JWST NIRSpec Phase Curve for WASP-121b: Dayside Emission Strongest Eastward of the Substellar Point and Nightside Conditions Conducive to Cloud Formation
We present the first exoplanet phase curve measurement made with the JWST
NIRSpec instrument, highlighting the exceptional stability of this
newly-commissioned observatory for exoplanet climate studies. The target,
WASP-121b, is an ultrahot Jupiter with an orbital period of 30.6 hr. We analyze
two broadband light curves generated for the NRS1 and NRS2 detectors, covering
wavelength ranges of 2.70-3.72 micron and 3.82-5.15 micron, respectively. Both
light curves exhibit minimal systematics, with approximately linear drifts in
the baseline flux level of 30 ppm/hr (NRS1) and 10 ppm/hr (NRS2). Assuming a
simple brightness map for the planet described by a low-order spherical
harmonic dipole, our light curve fits suggest that the phase curve peaks
coincide with orbital phases deg (NRS1) and deg
(NRS2) prior to mid-eclipse. This is consistent with the strongest dayside
emission emanating from eastward of the substellar point. We measure
planet-to-star emission ratios of ppm (NRS1) and
ppm (NRS2) for the dayside hemisphere, and ppm (NRS1) and ppm (NRS2) for the nightside hemisphere. The latter nightside emission
ratios translate to planetary brightness temperatures of K (NRS1)
and K (NRS2), which are low enough for a wide range of
refractory condensates to form, including enstatite and forsterite. A nightside
cloud deck may be blocking emission from deeper, hotter layers of the
atmosphere, potentially helping to explain why cloud-free 3D general
circulation model simulations systematically over-predict the nightside
emission for WASP-121b.Comment: Accepted for publication in Astrophysical Journal Letters on December
29, 202
The UV-SCOPE mission: ultraviolet spectroscopic characterization of planets and their environments
UV-SCOPE is a mission concept to determine the causes of atmospheric mass loss in exoplanets, investigate the mechanisms driving aerosol formation in hot Jupiters, and study the influence of the stellar environment on atmospheric evolution and habitability. As part of these investigations, the mission will generate a broad-purpose legacy database of time-domain ultraviolet (UV) spectra for nearly 200 stars and planets. The observatory consists of a 60 cm, f/10 telescope paired to a long-slit spectrograph, yielding simultaneous, almost continuous coverage between 1203 Å and 4000 Å, with resolutions ranging from 6000 to 240. The efficient instrument provides throughputs < 4% (far-UV; FUV) and < 15% (near-UV; NUV), comparable to HST/COS and much better than HST/STIS, over the same spectral range. A key design feature is the LiF prism, which serves as a dispersive element and provides high throughput even after accounting for radiation degradation. The use of two delta-doped Electron-Multiplying CCD detectors with UV-optimized, single-layer anti-reflection coatings provides high quantum efficiency and low detector noise. From the Earth-Sun second Lagrangian point, UV-SCOPE will continuously observe planetary transits and stellar variability in the full FUV-to-NUV range, with negligible astrophysical background. All these features make UV-SCOPE the ideal instrument to study exoplanetary atmospheres and the impact of host stars on their planets. UV-SCOPE was proposed to NASA as a Medium Explorer (MidEx) mission for the 2021 Announcement of Opportunity. If approved, the observatory will be developed over a 5-year period. Its primary science mission takes 34 months to complete. The spacecraft carries enough fuel for 6 years of operations
Transiting Exoplanet Studies and Community Targets for JWST's Early Release Science Program
The James Webb Space Telescope will revolutionize transiting exoplanet
atmospheric science due to its capability for continuous, long-duration
observations and its larger collecting area, spectral coverage, and spectral
resolution compared to existing space-based facilities. However, it is unclear
precisely how well JWST will perform and which of its myriad instruments and
observing modes will be best suited for transiting exoplanet studies. In this
article, we describe a prefatory JWST Early Release Science (ERS) program that
focuses on testing specific observing modes to quickly give the community the
data and experience it needs to plan more efficient and successful future
transiting exoplanet characterization programs. We propose a multi-pronged
approach wherein one aspect of the program focuses on observing transits of a
single target with all of the recommended observing modes to identify and
understand potential systematics, compare transmission spectra at overlapping
and neighboring wavelength regions, confirm throughputs, and determine overall
performances. In our search for transiting exoplanets that are well suited to
achieving these goals, we identify 12 objects (dubbed "community targets") that
meet our defined criteria. Currently, the most favorable target is WASP-62b
because of its large predicted signal size, relatively bright host star, and
location in JWST's continuous viewing zone. Since most of the community targets
do not have well-characterized atmospheres, we recommend initiating preparatory
observing programs to determine the presence of obscuring clouds/hazes within
their atmospheres. Measurable spectroscopic features are needed to establish
the optimal resolution and wavelength regions for exoplanet characterization.
Other initiatives from our proposed ERS program include testing the instrument
brightness limits and performing phase-curve observations.(Abridged)Comment: This is a white paper that originated from an open discussion at the
Enabling Transiting Exoplanet Science with JWST workshop held November 16 -
18, 2015 at STScI (http://www.stsci.edu/jwst/science/exoplanets). Accepted
for publication in PAS
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