146 research outputs found
Feedback in Clouds II: UV Photoionisation and the first supernova in a massive cloud
Molecular cloud structure is regulated by stellar feedback in various forms.
Two of the most important feedback processes are UV photoionisation and
supernovae from massive stars. However, the precise response of the cloud to
these processes, and the interaction between them, remains an open question. In
particular, we wish to know under which conditions the cloud can be dispersed
by feedback, which in turn can give us hints as to how feedback regulates the
star formation inside the cloud. We perform a suite of radiative
magnetohydrodynamic simulations of a 10^5 solar mass cloud with embedded
sources of ionising radiation and supernovae, including multiple supernovae and
a hypernova model. A UV source corresponding to 10% of the mass of the cloud is
required to disperse the cloud, suggesting that the star formation efficiency
should be on the order of 10%. A single supernova is unable to significantly
affect the evolution of the cloud. However, energetic hypernovae and multiple
supernovae are able to add significant quantities of momentum to the cloud,
approximately 10^{43} g cm/s of momentum per 10^{51} ergs of supernova energy.
This is on the lower range of estimates in other works, since dense gas clumps
that remain embedded inside the HII region cause rapid cooling in the supernova
blast. We argue that supernovae alone are unable to regulate star formation in
molecular clouds, and that strong pre-supernova feedback is required to allow
supernova blastwaves to propagate efficiently into the interstellar mediumComment: 15 pages, 10 figures, submitted to MNRA
The First Y Dwarf Data From JWST Show That Dynamic and Diabatic Processes Regulate Cold Brown Dwarf Atmospheres
The James Webb Space Telescope (JWST) is now observing Y dwarfs, the coldest
known brown dwarfs, with effective temperatures T_eff <= 475 K. The first
published observations provide important information: not only is the
atmospheric chemistry out of equilibrium, as previously known, but the
pressure-temperature profile is not in the standard adiabatic form. The rapid
rotation of these Jupiter-size, isolated, brown dwarfs dominates the
atmospheric dynamics, and thermal and compositional changes disrupt convection.
These processes produce a colder lower atmosphere, and a warmer upper
atmosphere, compared to a standard adiabatic profile. Leggett et al. (2021)
presented empirical models where the pressure-temperature profile was adjusted
so that synthetic spectra reproduced the 1 <= lambda um <= 20 spectral energy
distributions of brown dwarfs with 260 <= T_eff K <= 540. We show that spectra
generated by these models fit the first JWST Y dwarf spectrum better than
standard-adiabat models. Unexpectedly, there is no 4.3 um PH_3 feature in the
JWST spectrum and atmospheres without phosphorus better reproduce the 4 um flux
peak. Our analysis of new JWST photometry indicates that the recently
discovered faint secondary of the WISE J033605.05-014350AB system
(Calissendorff et al. 2023) has T_eff = 295 K, making it the first dwarf in the
significant luminosity gap between the 260 K WISE J085510.83-071442.5, and all
other known Y dwarfs. The adiabat-adjusted disequilibrium-chemistry models are
recommended for analyses of all brown dwarfs cooler than 600 K, and a grid is
publicly available. Photometric color transformations are provided in an
Appendix.Comment: Accepted for publication in ApJ on 25 September 202
A uniform analysis of HD209458b Spitzer/IRAC lightcurves with Gaussian process models
We present an analysis of Spitzer/IRAC primary transit and secondary eclipse
lightcurves measured for HD209458b, using Gaussian process models to
marginalise over the intrapixel sensitivity variations in the 3.6 micron and
4.5 micron channels and the ramp effect in the 5.8 micron and 8.0 micron
channels. The main advantage of this approach is that we can account for a
broad range of degeneracies between the planet signal and systematics without
actually having to specify a deterministic functional form for the latter. Our
results do not confirm a previous claim of water absorption in transmission.
Instead, our results are more consistent with a featureless transmission
spectrum, possibly due to a cloud deck obscuring molecular absorption bands.
For the emission data, our values are not consistent with the thermal inversion
in the dayside atmosphere that was originally inferred from these data.
Instead, we agree with another re-analysis of these same data, which concluded
a non-inverted atmosphere provides a better fit. We find that a solar-abundance
clear-atmosphere model without a thermal inversion underpredicts the measured
emission in the 4.5 micron channel, which may suggest the atmosphere is
depleted in carbon monoxide. An acceptable fit to the emission data can be
achieved by assuming that the planet radiates as an isothermal blackbody with a
temperature of K.Comment: 18 pages, 5 figures, 6 tables. Accepted by MNRA
The effect of a small amount of hydrogen in the atmosphere of ultrahot magma-ocean planets: atmospheric composition and escape
Here we investigate how small amounts of hydrogen (much smaller than the mass
of the exoplanet) above a magma ocean on a rocky exoplanet may modify the
atmospheric chemistry and atmospheric escape.We use a chemical model of a magma
ocean coupled to a gas equilibrium code. An energy-limited model is used to
compute atmospheric escape. The composition of the vapor above a magma ocean is
drastically modified by hydrogen, even for very modest amounts of H ( planetary mass). Hydrogen consumes much of the O(g), which, in
turn, promotes the evaporation of metals and metal oxides (SiO, Mg, Na, K, Fe)
from the magma ocean. Vast amounts of HO are produced by the same process.
At high hydrogen pressures, new hydrogenated species such as SiH form in
the atmosphere. In all cases, H, H, and HO are the dominant
nonmetal-bearing volatile species. Sodium is the dominant atmospheric
metal-bearing species at T 2000K and low H content, whereas Fe is dominant
at high H content and low temperature, while SiO predominates at T>3000 K. We
find that the atmospheric Mg/Fe, Mg/Si, and Na/Si ratios deviate from those in
the underlying planet and from the stellar composition. As such, their
determination may constrain the planet's mantle composition and H content. As
the presence of hydrogen promotes the evaporation of silicate mantles, it is
conceivable that some high-density, irradiated exoplanets may have started life
as hydrogen-bearing planets and that part of their silicate mantle evaporated
(up to a few of Si, O, and Fe) and was subsequently lost owing to the
reducing role of H. Even very small amounts of H can alter the atmospheric
composition and promote the evaporation to space of heavy species derived from
the molten silicate mantle of rocky planets.Comment: Accepted for publication in A&
An independent analysis of the Spitzer/IRAC phase curves of WASP43 b
We present here a reanalysis of the Spitzer Space Telescope phase curves of
the hot Jupiter WASP43 b, using the wavelet pixel-Independent Component
Analysis, a blind signal-source separation method. The data analyzed were
recorded with the InfraRed Array Camera and consisted of two visits at 3.6
m, and one visit at 4.5 m, each visit covering one transit and two
eclipse events. To test the robustness of our technique we repeated the
analysis on smaller portions of the phase curves, and by employing different
instrument ramp models. Our reanalysis presents significant updates of the
planetary parameters compared to those reported in the original phase curve
study of WASP43 b. In particular, we found (1) higher nightside temperatures,
(2) smaller hotspot offsets, (3) a greater consistency (1 )
between the two 3.6~m visits, and (4) a greater similarity with the
predictions of the atmospheric circulation models. Our parameter results are
consistent within 1 with those reported by a recent reanalysis of the
same data sets. For each visit we studied the variation of the retrieved
transit parameters as a function of various sets of stellar limb-darkening
coefficients, finding significant degeneracy between the limb-darkening models
and the analysis output. Furthermore, we performed the analysis of the single
transit and eclipse events, and we examined the differences between these
results with the ones obtained with the whole phase curve. Finally we provide a
formula useful to optimize the trade-off between precision and duration of
observations of transiting exoplanets.Comment: published on A
Accuracy tests of radiation schemes used in hot Jupiter global circulation models
The treatment of radiation transport in global circulation models (GCMs) is crucial to correctly describe Earth and exoplanet atmospheric dynamics processes. The two-stream approximation and correlated-k method are currently state-of-the-art approximations applied in both Earth and hot Jupiter GCM radiation schemes to facilitate rapid calculation of fluxes and heating rates. Their accuracy have been tested extensively for Earth-like conditions, but verification of the methods' applicability to hot Jupiter-like conditions is lacking in the literature. We are adapting the UK Met Office GCM, the Unified Model (UM), for the study of hot Jupiters, and present in this work the adaptation of the Edwards-Slingo radiation scheme based on the two-stream approximation and the correlated-k method. We discuss the calculation of absorption coefficients from high temperature line lists and highlight the large uncertainty in the pressure-broadened line widths. We compare fluxes and heating rates obtained with our adapted scheme to more accurate discrete ordinate (DO) line-by-line (LbL) calculations ignoring scattering effects. We find that, in most cases, errors stay below 10 % for both heating rates and fluxes using ~ 10 k-coefficients in each band and a diffusivity factor D = 1.66. The two-stream approximation and the correlated-k method both contribute non-negligibly to the total error. We also find that using band-averaged absorption coefficients, which have previously been used in radiative-hydrodynamical simulations of a hot Jupiter, may yield errors of ~ 100 %, and should thus be used with caution.European Community’s Seventh Framework Programme FP7/2007-2013Science & Technology Facilities Council (STFC)Royal Societ
Observable signatures of wind-driven chemistry with a fully consistent three dimensional radiative hydrodynamics model of HD 209458b (dataset)
rt_u-as329 - tracer experimentrt_u-as361 - transmission - equilibriumrt_u-as298 - transmission -relaxationrt_u-ar698 - emission - equilibriumrt_u-ar697 - emission - relaxationrt_u-ar586 - relaxationrt_u-ar412 - equilibriumtf_u-ar475 - start from spun up windstf_u-ar354 - resolution 96X60X33 start from spun up windstf_u-ar333 - resolution 72X45X33 start from spun up windstf_u-aq931 - timescale x 1e-8tf_u-aq930 - timescale x 1e-4tf_u-aq815 - resolution 72X45X33tf_u-aq814 - resolution 96X60X33tf-u-aq801 - chemical equilibriumtf_u-aq557 - standard Cooper and Showman 2006tf_u-aq800 - Initialise all carbon in COThe data contained in this submission is associated with the publication Drummond et al, ApJL, 2018.The article associated with this dataset is located in ORE at: http://hdl.handle.net/10871/31897We present a study of the effect of wind-driven advection on the chemical composition of hot Jupiter atmospheres using a fully-consistent 3D hydrodynamics, chemistry and radiative transfer code, the Met Office Unified Model (UM). Chemical modelling of exoplanet atmospheres has primarily been restricted to 1D models that cannot account for 3D dynamical processes. In this work we couple a chemical relaxation scheme to the UM to account for the chemical interconversion of methane and carbon monoxide. This is done consistently with the radiative transfer meaning that departures from chemical equilibrium are included in the heating rates (and emission) and hence complete the feedback between the dynamics, thermal structure and chemical composition. In this letter we simulate the well studied atmosphere of HD 209458b. We find that the combined effect of horizontal and vertical advection leads to an increase in the methane abundance by several orders of magnitude; directly opposite to the trend found in previous works. Our results demonstrate the need to include 3D effects when considering the chemistry of hot Jupiter atmospheres. We calculate transmission and emission spectra, as well as the emission phase curve, from our simulations. We conclude that gas-phase non-equilibrium chemistry is unlikely to explain the model–observation discrepancy in the 4.5 μm Spitzer/IRAC channel. However, we highlight other spectral regions, observable with the James Webb Space Telescope, where signatures of wind-driven chemistry are more prominant.BD and DKS acknowledge funding from the European Research Council (ERC) under the European Unions Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement no. 336792. NJM is part funded by a Leverhulme Trust Research Project Grant. JM and IAB acknowledge the support of a Met Office Academic Partnership secondment. ALC is funded by an STFC studentship. DSA acknowledges support from the NASA Astrobiology Program through the Nexus for Exoplanet System Science. This work used the DiRAC Complexity system, operated by the University of Leicester IT Services, which forms part of the STFC DiRAC HPC Facility. This equipment is funded by BIS National E-Infrastructure capital grant ST/K000373/1 and STFC DiRAC Operations grant ST/K0003259/1. DiRAC is part of the National E-Infrastructure
Measuring and Replicating the 1-20 µm Energy Distributions of the Coldest Brown Dwarfs::Rotating, Turbulent and Non-Adiabatic Atmospheres
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