404 research outputs found
Aggregate Hazes in Exoplanet Atmospheres
Photochemical hazes have been frequently used to interpret exoplanet
transmission spectra that show an upward slope towards shorter wavelengths and
weak molecular features. While previous studies have only considered spherical
haze particles, photochemical hazes composed of hydrocarbon aggregate particles
are common throughout the solar system. We use an aerosol microphysics model to
investigate the effect of aggregate photochemical haze particles on
transmission spectra of warm exoplanets. We find that the wavelength dependence
of the optical depth of aggregate particle hazes is flatter than for spheres
since aggregates grow to larger radii. As a result, while spherical haze
opacity displays a scattering slope towards shorter wavelengths, aggregate haze
opacity can be gray in the optical and NIR, similar to those assumed for
condensate cloud decks. We further find that haze opacity increases with
increasing production rate, decreasing eddy diffusivity, and increasing monomer
size, though the magnitude of the latter effect is dependent on production rate
and the atmospheric pressure levels probed. We generate synthetic exoplanet
transmission spectra to investigate the effect of these hazes on spectral
features. For high haze opacity cases, aggregate hazes lead to flat, nearly
featureless spectra, while spherical hazes produce sloped spectra with clear
spectral features at long wavelengths. Finally, we generate synthetic
transmission spectra of GJ 1214b for aggregate and spherical hazes and compare
them to space-based observations. We find that aggregate hazes can reproduce
the data significantly better than spherical hazes, assuming a production rate
limited by delivery of methane to the upper atmosphere.Comment: 17 figures, accepted to Ap
Evaluation of a programme of transferable skills development within the PhD: views of late stage students
Recent years have seen an increasing emphasis placed upon the development of transferable skills within PhD degree programmes. This paper reports on steps taken to evaluate a programme of transferable skills development at a research intensive university in the UK, focussing on the views of late stage PhD students in the science, engineering and medical disciplines. It shows that most students report a positive impact from having taken part in transferable skills initiatives and that they have a positive attitude towards them. Participants report an enduring positive impact on their behaviour and consider that the training meets their perceived needs as they progress as researchers. However, amongst the population as a whole, there were differences in views. For example, it was found that females, overseas students and those mainly motivated to do the PhD by career-related reasons attach the greatest importance to such opportunities to develop transferable skills
Spectral Variability from the Patchy Atmospheres of T and Y Dwarfs
Brown dwarfs of a variety of spectral types have been observed to be
photometrically variable. Previous studies have focused on objects at the L/T
transition, where the iron and silicate clouds in L dwarfs break up or
dissipate. However, objects outside of this transitional effective temperature
regime also exhibit variability. Here, we present models for mid-late T dwarfs
and Y dwarfs. We present models that include patchy salt and sulfide clouds as
well as water clouds for the Y dwarfs. We find that for objects over 375 K,
patchy cloud opacity would generate the largest amplitude variability within
near-infrared spectral windows. For objects under 375 K, water clouds also
become important and generate larger amplitude variability in the mid-infrared.
We also present models in which we perturb the temperature structure at
different pressure levels of the atmosphere to simulate hot spots. These models
show the most variability in the absorption features between spectral windows.
The variability is strongest at wavelengths that probe pressure levels at which
the heating is the strongest. The most illustrative types of observations for
understanding the physical processes underlying brown dwarf variability are
simultaneous, multi-wavelength observations that probe both inside and outside
of molecular absorption features.Comment: 6 pages, 5 figures, Accepted for publication in ApJ Letter
Photolytic Hazes in the Atmosphere of 51 Eri b
We use a 1D model to address photochemistry and possible haze formation in
the irradiated warm Jupiter, 51 Eridani b. The intended focus was to be carbon,
but sulfur photochemistry turns out to be important. The case for organic
photochemical hazes is intriguing but falls short of being compelling. If
organic hazes form, they are likeliest to do so if vertical mixing in 51 Eri b
is weaker than in Jupiter, and they would be found below the altitudes where
methane and water are photolyzed. The more novel result is that photochemistry
turns HS into elemental sulfur, here treated as S. In the cooler
models, S is predicted to condense in optically thick clouds of solid
sulfur particles, whilst in the warmer models S remains a vapor along with
several other sulfur allotropes that are both visually striking and potentially
observable. For 51 Eri b, the division between models with and without
condensed sulfur is at an effective temperature of 700 K, which is within error
its actual effective temperature; the local temperature where sulfur condenses
is between 280 and 320 K. The sulfur photochemistry we have discussed is quite
general and ought to be found in a wide variety of worlds over a broad
temperature range, both colder and hotter than the 650-750 K range studied
here, and we show that products of sulfur photochemistry will be nearly as
abundant on planets where the UV irradiation is orders of magnitude weaker than
it is on 51 Eri b.Comment: 24 pages including 11 figures and a tabl
Detecting Water In the atmosphere of HR 8799 c with L-band High Dispersion Spectroscopy Aided By Adaptive Optics
High dispersion spectroscopy of brown dwarfs and exoplanets enables exciting
science cases, e.g., mapping surface inhomogeneity and measuring spin rate.
Here, we present band observations of HR 8799 c using Keck NIRSPEC
(R=15,000) in adaptive optics (AO) mode (NIRSPAO). We search for molecular
species (HO and CH) in the atmosphere of HR 8799 c with a template
matching method, which involves cross correlation between reduced spectrum and
a template spectrum. We detect HO but not CH, which suggests
disequilibrium chemistry in the atmosphere of HR 8799 c, and this is consistent
with previous findings. We conduct planet signal injection simulations to
estimate the sensitivity of our AO-aided high dispersion spectroscopy
observations. We conclude that contrast can be reached in band.
The sensitivity is mainly limited by the accuracy of line list used in modeling
spectra and detector noise. The latter will be alleviated by the NIRSPEC
upgrade.Comment: 14 pages, 5 figures, 5 tables, accepted for publication on AJ,
references update
Towards Robust Atmospheric Retrieval on Cloudy L Dwarfs: The Impact of Thermal and Abundance Profile Assumptions
Constraining L dwarf properties from their spectra is challenging.
Near-infrared spectra probe a limited range of pressures, while many species
condense within their photospheres. Condensation creates two complexities:
gas-phase species "rain out" (decreasing in abundances by many orders of
magnitude) and clouds form. We designed tests using synthetic data to determine
the best approach for retrieving L dwarf spectra, isolating the challenges in
the absence of cloud opacity. We conducted atmospheric retrievals on synthetic
cloud-free L dwarf spectra derived from the Sonora Bobcat models at SpeX
resolution using a variety of thermal and chemical abundance profile
parameterizations. For objects hotter than L5 (T ~ 1700 K), the limited
pressure layers probed in the near-IR are mostly convective; parameterized PT
profiles bias results and free, unsmoothed profiles should be used. Only when
many layers both above and below the radiative-convective boundary are probed
can parameterized profiles provide accurate results. Furthermore, a nonuniform
abundance profile for iron hydride (FeH) is needed to accurately retrieve bulk
properties of early- to mid- L dwarfs. Nonuniform prescriptions for other gases
in near-IR retrievals may also be warranted near the L/T transition (CH)
and early Y dwarfs (Na and K). We demonstrate the utility of using realistic
self-consistent models to benchmark retrievals and suggest how they can be used
in the future.Comment: 20 pages, 12 figures, 5 tables, submitted to Ap
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