We present an atmospheric retrieval analysis of the Y0 brown dwarf WISE J035934.06\(-\)540154.6 using the low-resolution 0.96--12 \(\mu\)m JWST spectrum presented in \citet{Beiler_2023}. We obtain ...volume number mixing ratios of the major gas-phase absorbers (H\(_2\)O, CH\(_4\), CO, CO\(_2\), PH\(_3\), and H\(_2\)S) that are 3--5\(\times\) more precise than previous work that used HST spectra. We also find an order-of-magnitude improvement in the precision of the retrieved thermal profile, a direct result of the broad wavelength coverage of the JWST data. We used the retrieved thermal profile and surface gravity to generate a grid of chemical forward models with varying metallicity, (C/O)\(_\textrm{atm}\), and strengths of vertical mixing as encapsulated by the eddy diffusion coefficient \(K_\textrm{zz}\). Comparison of the retrieved abundances with this grid of models suggests that the deep atmosphere of WISE 0359\(-\)54 shows signs of vigorous vertical mixing with \(K_\textrm{zz}=10^9\) cm\(^{2}\) s\(^{-1}\). To test the sensitivity of these results to our 5-knot spline thermal profile model, we performed a second retrieval using the \citet{Madhusudhan_2009} thermal profile model. While the results of the two retrievals generally agree well, we do find differences between the retrieved values of mass and volume number mixing ratio of H\(_2\)S with fractional differences of the median values of \(-\)0.64 and \(-\)0.10, respectively. In addition, the 5-knot thermal profile is consistently warmer at pressure between 1 and 70 bar. Nevertheless, our results underscore the power that the broad-wavelength infrared spectra obtainable with the James Webb Space Telescope have to characterize the atmospheres of cool brown dwarfs.
The power spectral density (PSD) of the X-ray emission variability from the accretion disc-corona region of black hole X-ray binaries and active galactic nuclei has a broken power law shape with a ...characteristic break time-scale. If the disc and the jet are connected, the jet variability may also contain a characteristic time-scale related to that of the disc-corona. Recent observations of the blazar Mrk 421 have confirmed the broken power law shape of the PSD of its jet X-ray variability. We model the time variability of a blazar, in which emitting particles are assumed to be accelerated by successive shock waves flowing down the jet with a varying inter-shock time-scale. We investigate the possible relation between the characteristic time-scales in the disc and jet variability based on the above model, along with mathematically and physically simulated disc variability. We find that both the PSD of the jet and disc variability may have a broken power law shape but the break time-scales are not related in general except only in systems with a small range of BH mass. The break in the jet and the disc PSD are connected to the interval between large amplitude outbursts in the jet (inter-shock time-scale) and to the viscous time-scale in the disc, respectively. In frequency bands where multiple emission processes are involved or emission is from lower energy particles, the break in the PSD may not be prominent enough for detection.
Upcoming James Webb Space Telescope (JWST) observations will allow us to study exoplanet and brown dwarf atmospheres in great detail. The physical interpretation of these upcoming high ...signal-to-noise observations requires precise atmospheric models of exoplanets and brown dwarfs. While several one-dimensional and three-dimensional atmospheric models have been developed in the past three decades, these models have often relied on simplified assumptions like chemical equilibrium and are also often not open-source, which limits their usage and development by the wider community. We present a python-based one-dimensional atmospheric radiative-convective equilibrium model. This model has heritage from the Fortran-based code (Marley et al.,1996} which has been widely used to model the atmospheres of Solar System objects, brown dwarfs, and exoplanets. In short, the basic capability of the original model is to compute the atmospheric state of the object under radiative-convective equilibrium given its effective or internal temperature, gravity, and host--star properties (if relevant). In the new model, which has been included within the well-utilized code-base PICASO, we have added these original features as well as the new capability of self-consistently treating disequilibrium chemistry. This code is widely applicable to Hydrogen-dominated atmospheres (e.g., brown dwarfs and giant planets).
Disequilibrium chemistry due to vertical mixing in the atmospheres of many brown dwarfs and giant exoplanets is well-established. Atmosphere models for these objects typically parameterize mixing ...with the highly uncertain \(K_{\rm zz}\) diffusion parameter. The role of mixing in altering the abundances of C-N-O-bearing molecules has mostly been explored for solar composition atmospheres. However, atmospheric metallicity and the C/O ratio also impact atmospheric chemistry. Therefore, we present the \texttt{Sonora Elf Owl} grid of self-consistent cloud-free 1D radiative-convective equilibrium model atmospheres for JWST observations, which includes a variation of \(K_{\rm zz}\) across several orders of magnitude and also encompasses sub-solar to super-solar metallicities and C/O ratios. We find that the impact of \(K_{\rm zz}\) on the \(T(P)\) profile and spectra is a strong function of both \(T_{\rm eff}\) and metallicity. For metal-poor objects \(K_{\rm zz}\) has large impacts on the atmosphere at significantly higher \(T_{\rm eff}\) compared to metal-rich atmospheres where the impact of \(K_{\rm zz}\) is seen to occur at lower \(T_{\rm eff}\). We identify significant spectral degeneracies between varying \(K_{\rm zz}\) and metallicity in multiple wavelength windows, in particular at 3-5 \(\mu\)m. We use the \texttt{Sonora Elf Owl} atmospheric grid to fit the observed spectra of a sample of 9 early to late T- type objects from \(T_{\rm eff}=550-1150\) K. We find evidence for very inefficient vertical mixing in these objects with inferred \(K_{\rm zz}\) values lying in the range between \(\sim\) 10\(^1\)-10\(^4\) cm\(^2\)s\(^{-1}\). Using self-consistent models, we find that this slow vertical mixing is due to the observations probing mixing in the deep detached radiative zone in these atmospheres.
Variable stars in stellar clusters can offer key constraints on stellar evolution and pulsation models, utilising estimates of host cluster properties to constrain stellar physical parameters. We ...present a catalogue of 86 luminous (F814W<19) variable stars in M31 clusters identified by mining the archival Panchromatic Hubble Andromeda Treasury (PHAT) survey using a combination of statistical analysis of sparse PHAT light curves and difference imaging. We determine the evolutionary phases and initial masses of these variable stars by matching them with theoretical isochrones generated using host cluster properties from the literature. We calculate the probability of PHAT photometry being blended due to the highly crowded nature of cluster environments for each cluster-variable star, using these probabilities to inform our level of confidence in the derived properties of each star. Our 86 cluster-variable stars have initial masses between 0.8--67 \(M_{\odot}\). Their evolutionary phases span the main sequence, more evolved hydrogen- and helium-burning phases, and the post-asymptotic giant branch. We identify numerous candidate variable star types: RV Tauri variables, red supergiants and slowly pulsating B-type supergiants, along with Wolf Rayet stars, \(\alpha\) Cygni and Mira variables, a classical Cepheid and a possible super-asymptotic giant. We characterise 12 cluster-variable stars at higher confidence based on their difference image quality and lower blending probability. Ours is the first systematic study of variable stars in extragalactic stellar clusters leveraging the superior resolution of the Hubble Space Telescope and demonstrating the unique power of stellar clusters in constraining the fundamental properties of variable stars.
We present a new grid of cloudy atmosphere and evolution models for substellar objects. These models include the effect of refractory cloud species, including silicate clouds, on the spectra and ...evolution. We include effective temperatures from 900 to 2400 K and surface gravities from log g=3.5-5.5, appropriate for a broad range of objects with masses between 1 and 84 Jupiter masses. Model pressure-temperature structures are calculated assuming radiative-convective and chemical equilibrium. We consider the effect of both clouds and metallicity on the atmospheric structure, resulting spectra, and thermal evolution of substellar worlds. We parameterize clouds using the Ackerman & Marley (2001) cloud model, including cloud parameter fsed values from 1-8; we include three metallicities (-0.5, 0.0, and +0.5). Refractory clouds and metallicity both alter the evolution of substellar objects, changing the inferred temperature at a given age by up to 100-200 K. We compare to the observed photometry of brown dwarfs, finding broad agreement with the measured photometry. We publish the spectra, evolution, and other data products online with open access.
The detection of disk-integrated polarization from Luhman 16A and B in H-band, and subsequent modeling, has been interpreted in the framework of zonal cloud bands on these bodies. Recently, Tan and ...Showman (2021) investigated three-dimensional atmospheric circulation and cloud structures of brown dwarfs with general circulation models (GCMs), and their simulations yield complex cloud distributions showing some aspects of zonal jets, but also complex vortices that cannot be captured by a simple model. Here we use these 3D GCMs specific to Luhman 16A and B, along with the three-dimensional Monte Carlo radiative transfer code ARTES, to calculate their polarization signals. We adopt the 3D temperature-pressure and cloud profiles from the GCMs as our input atmospheric structures. Our polarization calculations at 1.6 \(\mu\)m agree well with the measured degree of linear polarization from both Luhman 16 A and B. Our calculations reproduce the measured polarization for both the objects with cloud particle sizes between 0.5-1 \,\(\mu\)m for Luhman 16 A and 5 \,\(\mu\)m for Luhman 16 B. We find that the degree of linear polarization can vary on hour-long timescales over the course of a rotation period. We also show that models with azimuthally symmetric band-like cloud geometries, typically used for interpreting polarimetry observations of brown dwarfs, over-predict the polarization signal if the cloud patterns do not include complex vortices within these bands. This exploratory work shows that GCMs are promising for modeling and interpreting polarization signals of brown dwarfs.
WASP-69 b is a hot, inflated, Saturn-mass planet 0.26 Mjup with a zero-albedo equilibrium temperature of 963 K. Here, we report the JWST 2 to 12 um emission spectrum of the planet consisting of two ...eclipses observed with NIRCam grism time series and one eclipse observed with MIRI LRS. The emission spectrum shows absorption features of water vapor, carbon dioxide and carbon monoxide, but no strong evidence for methane. WASP-69 b's emission spectrum is poorly fit by cloud-free homogeneous models. We find three possible model scenarios for the planet: 1) a Scattering Model that raises the brightness at short wavelengths with a free Geometric Albedo parameter 2) a Cloud Layer model that includes high altitude silicate aerosols to moderate long wavelength emission and 3) a Two-Region model that includes significant dayside inhomogeneity and cloud opacity with two different temperature-pressure profiles. In all cases, aerosols are needed to fit the spectrum of the planet. The Scattering model requires an unexpectedly high Geometric Albedo of 0.64. Our atmospheric retrievals indicate inefficient redistribution of heat and an inhomogeneous dayside distribution, which is tentatively supported by MIRI LRS broadband eclipse maps that show a central concentration of brightness. Our more plausible models (2 and 3) retrieve chemical abundances enriched in heavy elements relative to solar composition by 6x to 14x solar and a C/O ratio of 0.65 to 0.94, whereas the less plausible highly reflective scenario (1) retrieves a slightly lower metallicity and lower C/O ratio.
We report observations of the atmospheric transmission spectrum of the sub-Neptune exoplanet GJ 3470 b taken using the Near-Infrared Camera (NIRCam) on JWST. Combined with two archival HST/WFC3 ...transit observations and fifteen archival Spitzer transit observations, we detect water, methane, sulfur dioxide, and carbon dioxide in the atmosphere of GJ 3470 b, each with a significance of >3-sigma. GJ 3470 b is the lowest mass -- and coldest -- exoplanet known to show a substantial sulfur dioxide feature in its spectrum, at \(M_{p}\)=11.2\({\,{\rm M}_{\oplus}}\) and \(T_{eq}\)=600\(\,\)K. This indicates disequilibrium photochemistry drives sulfur dioxide production in exoplanet atmospheres over a wider range of masses and temperatures than has been reported or expected. The water, carbon dioxide, and sulfur dioxide abundances we measure indicate an atmospheric metallicity of approximately \(100\times\) Solar. We see further evidence for disequilibrium chemistry in our inferred methane abundance, which is significantly lower than expected from equilibrium models consistent with our measured water and carbon dioxide abundances.
