2,157 research outputs found
Detection of Close-In Extrasolar Giant Planets Using the Fourier-Kelvin Stellar Interferometer
We evaluate the direct detection of extrasolar giant planets with a
two-aperture nulling infrared interferometer, working at angles
, and using a new `ratio-of-two-wavelengths' technique.
Simple arguments suggest that interferometric detection and characterization
should be quite possible for planets much closer than the conventional inner
working angle, or angular resolution limit. We show that the peak signal from a
nulling infrared interferometer of baseline ( meters) will often
occur `inside the null', and that the signal variations from path-difference
fluctuations will cancel to first order in the ratio of two wavelengths. Using
a new interferometer simulation code, we evaluate the detectability of all the
known extrasolar planets as observed using this two-color method with the
proposed {\it Fourier Kelvin Stellar Interferometer (FKSI)}. In its minimum
configuration {\it FKSI} uses two 0.5-meter apertures on a 12.5-meter baseline,
and a field-of-regard. We predict that known
extrasolar planets are directly detectable using {\it FKSI}, with
low-resolution spectroscopy () being possible in the most favorable
cases. Spaceborne direct detection of extrasolar giant planets is possible with
meter baselines, and does not require the much longer baselines
provided by formation flying.Comment: Accepted for publication in ApJ Letter
The Male Gender Empathy Gap: Time for psychology to take action.
Gender is not just an equality issue, but a diversity issue. Although differences are celebrated in every other field, in the social sciences, gender differences are denied or played down. We aren’t supposed to generalize about gender, because – in general – men and women are the same, supposedly. However as human beings, most people intuitively recognise that although men and women share many similarities, we are different in important ways
Exoplanet Atmospheres
At the dawn of the first discovery of exoplanets orbiting sun-like stars in
the mid-1990s, few believed that observations of exoplanet atmospheres would
ever be possible. After the 2002 Hubble Space Telescope detection of a
transiting exoplanet atmosphere, many skeptics discounted it as a one-object,
one-method success. Nevertheless, the field is now firmly established, with
over two dozen exoplanet atmospheres observed today. Hot Jupiters are the type
of exoplanet currently most amenable to study. Highlights include: detection of
molecular spectral features; observation of day-night temperature gradients;
and constraints on vertical atmospheric structure. Atmospheres of giant planets
far from their host stars are also being studied with direct imaging. The
ultimate exoplanet goal is to answer the enigmatic and ancient question, "Are
we alone?" via detection of atmospheric biosignatures. Two exciting prospects
are the immediate focus on transiting super Earths orbiting in the habitable
zone of M-dwarfs, and ultimately the spaceborne direct imaging of true Earth
analogs.Comment: Annual Reviews of Astronomy and Astrophysics, in press. 47 pages, 16
Figures. This article was completed in January 2010; more recent references
are not include
Photochemistry of Anoxic Abiotic Habitable Planet Atmospheres: Impact of New HO Cross-Sections
We present a study of the photochemistry of abiotic habitable planets with
anoxic CO-N atmospheres. Such worlds are representative of early Earth,
Mars and Venus, and analogous exoplanets. HO photodissociation controls the
atmospheric photochemistry of these worlds through production of reactive OH,
which dominates the removal of atmospheric trace gases. The near-UV (NUV;
nm) absorption cross-sections of HO play an outsized role in OH
production; these cross-sections were heretofore unmeasured at habitable
temperatures ( K). We present the first measurements of NUV HO
absorption at K, and show it to absorb orders of magnitude more than
previously assumed. To explore the implications of these new cross-sections, we
employ a photochemical model; we first intercompare it with two others and
resolve past literature disagreement. The enhanced OH production due to these
higher cross-sections leads to efficient recombination of CO and O,
suppressing both by orders of magnitude relative to past predictions and
eliminating the low-outgassing "false positive" scenario for O as a
biosignature around solar-type stars. Enhanced [OH] increases rainout of
reductants to the surface, relevant to prebiotic chemistry, and may also
suppress CH and H; the latter depends on whether burial of reductants
is inhibited on the underlying planet, as is argued for abiotic worlds. While
we focus on CO-rich worlds, our results are relevant to anoxic planets in
general. Overall, our work advances the state-of-the-art of photochemical
models by providing crucial new HO cross-sections and resolving past
disagreement in the literature, and suggests that detection of spectrally
active trace gases like CO in rocky exoplanet atmospheres may be more
challenging than previously considered.Comment: Manuscript (this version) accepted to ApJ. Cross-section data
available at https://github.com/sukritranjan/ranjanschwietermanharman2020.
Feedback continues to be solicite
BIOSIGNATURE GASES IN Hâ‚‚-DOMINATED ATMOSPHERES ON ROCKY EXOPLANETS
Super-Earth exoplanets are being discovered with increasing frequency and some will be able to retain stable H2-dominated atmospheres. We study biosignature gases on exoplanets with thin H2 atmospheres and habitable surface temperatures, using a model atmosphere with photochemistry and a biomass estimate framework for evaluating the plausibility of a range of biosignature gas candidates. We find that photochemically produced H atoms are the most abundant reactive species in H2 atmospheres. In atmospheres with high CO2 levels, atomic O is the major destructive species for some molecules. In Sun-Earth-like UV radiation environments, H (and in some cases O) will rapidly destroy nearly all biosignature gases of interest. The lower UV fluxes from UV-quiet M stars would produce a lower concentration of H (or O) for the same scenario, enabling some biosignature gases to accumulate. The favorability of low-UV radiation environments to accumulate detectable biosignature gases in an H2 atmosphere is closely analogous to the case of oxidized atmospheres, where photochemically produced OH is the major destructive species. Most potential biosignature gases, such as dimethylsulfide and CH3Cl, are therefore more favorable in low-UV, as compared with solar-like UV, environments. A few promising biosignature gas candidates, including NH3 and N2O, are favorable even in solar-like UV environments, as these gases are destroyed directly by photolysis and not by H (or O). A more subtle finding is that most gases produced by life that are fully hydrogenated forms of an element, such as CH4 and H2S, are not effective signs of life in an H2-rich atmosphere because the dominant atmospheric chemistry will generate such gases abiologically, through photochemistry or geochemistry. Suitable biosignature gases in H2-rich atmospheres for super-Earth exoplanets transiting M stars could potentially be detected in transmission spectra with the James Webb Space Telescope
Spectral distortions to the Cosmic Microwave Background from the recombination of hydrogen and helium
The recombination of hydrogen and helium at z~1000-7000 gives unavoidable
distortions to the Cosmic Microwave Background (CMB) spectrum. We present a
detailed calculation of the line intensities arising from the Ly-alpha (2p-1s)
and two-photon (2s-1s) transitions for the recombination of hydrogen, as well
as the corresponding lines from helium. We give an approximate formula for the
strength of the main recombination line distortion on the CMB in different
cosmologies, this peak occurring at about 170 microns. We also find a
previously undescribed long wavelength peak (which we call the
pre-recombination peak) from the lines of the 2p-1s transitions, which are
formed before significant recombination of the corresponding atoms occurred.
Detailed calculations of the two-photon emission line shapes are presented here
for the first time. The frequencies of the photons emitted from the two-photon
transition have a wide spectrum and this causes the location of the peak of the
two-photon line of hydrogen to be located almost at the same wavelength as the
main Ly-alpha peak. The helium lines also give distortions at similar
wavelengths, so that the combined distortion has a complex shape. The detection
of this distortion would provide direct supporting evidence that the Universe
was indeed once a plasma. Moreover, the distortions are a sensitive probe of
physics during the time of recombination. Although the spectral distortion is
overwhelmed by dust emission from the Galaxy, and is maximum at wavelengths
roughly where the cosmic far-infrared background peaks, it may be able to
tailor an experiment to detect its non-trivial shape.Comment: 12 pages, 12 figures; Minor corrections in text and references; MNRAS
in pres
Boomerang returns unexpectedly
Experimental study of the anisotropy in the cosmic microwave background (CMB)
is gathering momentum. The eagerly awaited Boomerang results have lived up to
expectations. They provide convincing evidence in favor of the standard
paradigm: the Universe is close to flat and with primordial fluctuations which
are redolent of inflation. Further scrutiny reveals something even more
exciting however -- two hints that there may be some unforeseen physical
effects. Firstly the primary acoustic peak appears at slightly larger scales
than expected. Although this may be explicable through a combination of mundane
effects, we suggest it is also prudent to consider the possibility that the
Universe might be marginally closed. The other hint is provided by a second
peak which appears less prominent than expected. This may indicate one of a
number of possibilities, including increased damping length or tilted initial
conditions, but also breaking of coherence or features in the initial power
spectrum. Further data should test whether the current concordance model needs
only to be tweaked, or to be enhanced in some fundamental way.Comment: 11 pages, 3 figures, final version accepted by Ap
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Dynamical Structure of Extreme Floods in the U.S. Midwest and the United Kingdom
Twenty extreme spring floods that occurred in the Ohio basin between 1901 and 2008, identified from daily river discharge data, are investigated and compared to the April 2011 Ohio River flood event. Composites of synoptic fields for the flood events show that all of these floods are associated with a similar pattern of sustained advection of low-level moisture and warm air from the tropical Atlantic Ocean and the Gulf of Mexico. The typical flow conditions are governed by an anomalous semistationary ridge, situated east of the U.S. East Coast, that steers the moisture and converges it into the Ohio River valley. Significantly, the moisture path common to all of the 20 cases studied here as well as the case of April 2011 is distinctly different from the normal path of Atlantic moisture during spring, which occurs farther west. It is shown further that the Ohio basin moisture convergence responsible for the floods is caused primarily by the atmospheric circulation anomaly advecting the climatological mean moisture field. Transport and related convergence due to the covariance between moisture anomalies and circulation anomalies are of secondary but nonnegligible importance. The importance of atmospheric circulation anomalies to floods is confirmed by conducting a similar analysis for a series of winter floods on the river Eden in northwest England
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