21 research outputs found
Ground-based optical transmission spectrum of the hot Jupiter HAT-P-1b
Time-series spectrophotometric studies of exoplanets during transit using
ground-based facilities are a promising approach to characterize their
atmospheric compositions. We aim to investigate the transit spectrum of the hot
Jupiter HAT-P-1b. We compare our results to those obtained at similar
wavelengths by previous space-based observations. We observed two transits of
HAT-P-1b with the Gemini Multi-Object Spectrograph (GMOS) instrument on the
Gemini North telescope using two instrument modes covering the 320 - 800 nm and
520 - 950 nm wavelength ranges. We used time-series spectrophotometry to
construct transit light curves in individual wavelength bins and measure the
transit depths in each bin. We accounted for systematic effects. We addressed
potential photometric variability due to magnetic spots in the planet's host
star with long-term photometric monitoring. We find that the resulting transit
spectrum is consistent with previous Hubble Space Telescope (HST) observations.
We compare our observations to transit spectroscopy models that marginally
favor a clear atmosphere. However, the observations are also consistent with a
flat spectrum, indicating high-altitude clouds. We do not detect the Na
resonance absorption line (589 nm), and our observations do not have sufficient
precision to study the resonance line of K at 770 nm. We show that even a
single Gemini/GMOS transit can provide constraining power on the properties of
the atmosphere of HAT-P-1b to a level comparable to that of HST transit studies
in the optical when the observing conditions and target and reference star
combination are suitable. Our 520 - 950 nm observations reach a precision
comparable to that of HST transit spectra in a similar wavelength range of the
same hot Jupiter, HAT-P-1b. However, our GMOS transit between 320 - 800 nm
suffers from strong systematic effects and yields larger uncertainties.Comment: A&A, accepted, 16 pages, 8 figures, 5 table
A new method to correct for host star variability in multi-epoch observations of exoplanet transmission spectra
Transmission spectra of exoplanets orbiting active stars suffer from
wavelength-dependent effects due to stellar photospheric heterogeneity.
WASP-19b, an ultra-hot Jupiter (T 2100 K), is one such strongly
irradiated gas-giant orbiting an active solar-type star. We present optical
(520-900 nm) transmission spectra of WASP-19b obtained across eight epochs
using the Gemini Multi-Object Spectrograph (GMOS) on the Gemini-South
telescope. We apply our recently developed Gaussian Processes regression based
method to model the transit light curve systematics and extract the
transmission spectrum at each epoch. We find that WASP-19b's transmission
spectrum is affected by stellar variability at individual epochs. We report an
observed anticorrelation between the relative slopes and offsets of the spectra
across all epochs. This anticorrelation is consistent with the predictions from
the forward transmission models, which account for the effect of unocculted
stellar spots and faculae measured previously for WASP-19. We introduce a new
method to correct for this stellar variability effect at each epoch by using
the observed correlation between the transmission spectral slopes and offsets.
We compare our stellar variability corrected GMOS transmission spectrum with
previous contradicting MOS measurements for WASP-19b and attempt to reconcile
them. We also measure the amplitude and timescale of broadband stellar
variability of WASP-19 from TESS photometry, which we find to be consistent
with the effect observed in GMOS spectroscopy and ground-based broadband
photometric long-term monitoring. Our results ultimately caution against
combining multi-epoch optical transmission spectra of exoplanets orbiting
active stars before correcting each epoch for stellar variability.Comment: Accepted for publication in MNRA
Contemporary medical television and crisis in the NHS
This article maps the terrain of contemporary UK medical television, paying particular attention to Call the Midwife as its centrepiece, and situating it in contextual relation to the current crisis in the NHS. It provides a historical overview of UK and US medical television, illustrating how medical television today has been shaped by noteworthy antecedents. It argues that crisis rhetoric surrounding healthcare leading up to the passing of the Health and Social Care Act 2012 has been accompanied by a renaissance in medical television. And that issues, strands and clusters have emerged in forms, registers and modes with noticeable regularity, especially around the value of affective labour, the cultural politics of nostalgia and the neoliberalisation of healthcare
A combined transmission spectrum of the Earth-sized exoplanets TRAPPIST-1 b and c
Three Earth-sized exoplanets were recently discovered close to the habitable
zone of the nearby ultracool dwarf star TRAPPIST-1. The nature of these planets
has yet to be determined, since their masses remain unmeasured and no
observational constraint is available for the planetary population surrounding
ultracool dwarfs, of which the TRAPPIST-1 planets are the first transiting
example. Theoretical predictions span the entire atmospheric range from
depleted to extended hydrogen-dominated atmospheres. Here, we report a
space-based measurement of the combined transmission spectrum of the two inner
planets made possible by a favorable alignment resulting in their simultaneous
transits on 04 May 2016. The lack of features in the combined spectrum rules
out cloud-free hydrogen-dominated atmospheres for each planet at 10-
levels; TRAPPIST-1 b and c are hence unlikely to harbor an extended gas
envelope as they lie in a region of parameter space where high-altitude
cloud/haze formation is not expected to be significant for hydrogen-dominated
atmospheres. Many denser atmospheres remain consistent with the featureless
transmission spectrum---from a cloud-free water vapour atmosphere to a
Venus-like atmosphere.Comment: Early release to inform further the upcoming review of HST's Cycle 24
proposal
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A continuum from clear to cloudy hot-Jupiter exoplanets without primordial water depletion
Thousands of transiting exoplanets have been discovered, but spectral analysis of their atmospheres has so far been dominated by a small number of exoplanets and data spanning relatively narrow wavelength ranges (such as 1.1–1.7 micrometres). Recent studies show that some hot-Jupiter exoplanets have much weaker water absorption features in their near-infrared spectra than predicted. The low amplitude of water signatures could be explained by very low water abundances, which may be a sign that water was depleted in the protoplanetary disk at the planet’s formation location, but it is unclear whether this level of depletion can actually occur. Alternatively, these weak signals could be the result of obscuration by clouds or hazes, as found in some optical spectra. Here we report results from a comparative study of ten hot Jupiters covering the wavelength range 0.3–5 micrometres, which allows us to resolve both the optical scattering and infrared molecular absorption spectroscopically. Our results reveal a diverse group of hot Jupiters that exhibit a continuum from clear to cloudy atmospheres. We find that the difference between the planetary radius measured at optical and infrared wavelengths is an effective metric for distinguishing different atmosphere types. The difference correlates with the spectral strength of water, so that strong water absorption lines are seen in clear-atmosphere planets and the weakest features are associated with clouds and hazes. This result strongly suggests that primordial water depletion during formation is unlikely and that clouds and hazes are the cause of weaker spectral signatures
Exoplanet Atmosphere Measurements from Transmission Spectroscopy and other Planet-Star Combined Light Observations
It is possible to learn a great deal about exoplanet atmospheres even when we
cannot spatially resolve the planets from their host stars. In this chapter, we
overview the basic techniques used to characterize transiting exoplanets -
transmission spectroscopy, emission and reflection spectroscopy, and full-orbit
phase curve observations. We discuss practical considerations, including
current and future observing facilities and best practices for measuring
precise spectra. We also highlight major observational results on the
chemistry, climate, and cloud properties of exoplanets.Comment: Accepted review chapter; Handbook of Exoplanets, eds. Hans J. Deeg
and Juan Antonio Belmonte (Springer-Verlag). 22 pages, 6 figure
A chemical model for the atmosphere of hot Jupiters
Our purpose is to release a chemical network, and the associated rate
coefficients, developed for the temperature and pressure range relevant to hot
Jupiters atmospheres. Using this network, we study the vertical atmospheric
composition of the two hot Jupiters (HD209458b, HD189733b) with a model that
includes photolyses and vertical mixing and we produce synthetic spectra. The
chemical scheme is derived from applied combustion models that have been
methodically validated over a range of temperatures and pressures typical of
the atmospheric layers influencing the observations of hot Jupiters. We compare
the predictions obtained from this scheme with equilibrium calculations, with
different schemes available in the literature that contain N-bearing species
and with previously published photochemical models. Compared to other chemical
schemes that were not subjected to the same systematic validation, we find
significant differences whenever non-equilibrium processes take place. The
deviations from the equilibrium, and thus the sensitivity to the network, are
more important for HD189733b, as we assume a cooler atmosphere than for
HD209458b. We found that the abundances of NH3 and HCN can vary by two orders
of magnitude depending on the network, demonstrating the importance of
comprehensive experimental validation. A spectral feature of NH3 at 10.5m
is sensitive to these abundance variations and thus to the chemical scheme. Due
to the influence of the kinetics, we recommend the use of a validated scheme to
model the chemistry of exoplanet atmospheres. Our network is robust for
temperatures within 300-2500K and pressures from 10mbar up to a few hundreds of
bars, for species made of C,H,O,N. It is validated for species up to 2 carbon
atoms and for the main nitrogen species.Comment: 20 pages, 10 figures. Accepted for publication in Astronomy &
Astrophysic
Helium in the eroding atmosphere of an exoplanet.
Helium is the second-most abundant element in the Universe after hydrogen and is one of the main constituents of gas-giant planets in our Solar System. Early theoretical models predicted helium to be among the most readily detectable species in the atmospheres of exoplanets, especially in extended and escaping atmospheres 1 . Searches for helium, however, have hitherto been unsuccessful 2 . Here we report observations of helium on an exoplanet, at a confidence level of 4.5 standard deviations. We measured the near-infrared transmission spectrum of the warm gas giant 3 WASP-107b and identified the narrow absorption feature of excited metastable helium at 10,833 angstroms. The amplitude of the feature, in transit depth, is 0.049 ± 0.011 per cent in a bandpass of 98 angstroms, which is more than five times greater than what could be caused by nominal stellar chromospheric activity. This large absorption signal suggests that WASP-107b has an extended atmosphere that is eroding at a total rate of 1010 to 3 × 1011 grams per second (0.1-4 per cent of its total mass per billion years), and may have a comet-like tail of gas shaped by radiation pressure