Nonlocal thermodynamic equilibrium (NLTE) calculations of hot white dwarf (WD) model atmospheres are the cornerstone of modern flux calibrations for the Hubble Space Telescope and for the CALSPEC ...database. These theoretical spectral energy distributions (SEDs) provide the relative flux versus wavelength, and only the absolute flux level remains to be set by reconciling the measured absolute flux of Vega in the visible with the Midcourse Space Experiment values for Sirius in the mid-IR. The most recent SEDs calculated by the tlusty and tmap NLTE model atmosphere codes for the primary WDs G191-B2B, GD 153, and GD 71 show improved agreement to 1% from 1500 to 30 m, in comparison to the previous 1% consistency only from 2000 to 5 m. These new NLTE models of hot WDs now provide consistent flux standards from the far UV to the mid-IR.
Atmospheric compositions can provide powerful diagnostics of formation and migration histories of planetary systems. We investigate constraints on atmospheric abundances of H2O, Na, and K, in a ...sample of transiting exoplanets using the latest transmission spectra and new H2 broadened opacities of Na and K. Our sample of 19 exoplanets spans from cool mini-Neptunes to hot Jupiters, with equilibrium temperatures between ∼300 and 2700 K. Using homogeneous Bayesian retrievals we report atmospheric abundances of Na, K, and H2O, and their detection significances, confirming 6 planets with strong Na detections, 6 with K, and 14 with H2O. We find a mass-metallicity trend of increasing H2O abundances with decreasing mass, spanning generally substellar values for gas giants and stellar/superstellar for Neptunes and mini-Neptunes. However, the overall trend in H2O abundances, from mini-Neptunes to hot Jupiters, is significantly lower than the mass-metallicity relation for carbon in the solar system giant planets and similar predictions for exoplanets. On the other hand, the Na and K abundances for the gas giants are stellar or superstellar, consistent with each other, and generally consistent with the solar system metallicity trend. The H2O abundances in hot gas giants are likely due to low oxygen abundances relative to other elements rather than low overall metallicities, and provide new constraints on their formation mechanisms. The differing trends in the abundances of species argue against the use of chemical equilibrium models with metallicity as one free parameter in atmospheric retrievals, as different elements can be differently enhanced.
We present a systematic study of the spectral consequences of departures from chemical equilibrium in the atmospheres of L and T dwarfs, and for even cooler dwarfs. The temperature/pressure profiles ...of the nonequllibrium models are fully consistent with the nonequilibrium chemistry. Our grid of nonequilibrlum models includes spectra for effective temperatures from 200 to 1800 K, three surface gravities, four possible values of the coefficient of eddy diffusion in the radiative zone, and three different CO/CH sub(4) chemical reaction prescriptions. We also provide clear and cloudy model variants. We find, in keeping with previous studies, that there are essentially only two spectral regions where the effects of departures from chemical equilibrium can Influence the predicted spectrum. These are in the M ( similar to 4-5 urn) and N (8-14 mu n) bands due to CO and NH 3, respectively. The overabundance of CO translates into flux suppressions of at most similar to 40% between effective temperatures of 600 and 1800 K. The effect is largest around T sub(elf) approximately 1100 K. The underabundance of ammonia translates into flux enhancements of no more than similar to 20% for the T sub(elf) range from 300 to 1800 K, with the largest effects at the lowest values of T sub(elf). The magnitude of the departure from chemical equilibrium increases with decreasing gravity, with increasing eddy diffusion coefficient, and with decreasing speed of the CO/CH4 reaction. Although these effects are modest, they lead to better fits with the measured T dwarf spectra. Furthermore, the suppression in the M band due to nonequilibrium enhancements in the CO abundance disappears below similar to 500 K, and is only partial above similar to 500 K, preserving the M-band flux as a useful diagnostic of cool atmospheres and maintaining its importance for searches for the cooler brown dwarfs beyond the T dwarfs.
Abstract
We present an outline of basic assumptions and governing structural equations describing atmospheres of sub-stellar mass objects, in particular the extrasolar giant planets and brown dwarfs. ...Although most of the presentation of the physical and numerical background is generic, details of the implementation pertain mostly to the code cooltlusty.
We also present a review of numerical approaches and computer codes devised to solve the structural equations, and make a critical evaluation of their efficiency and accuracy.
We have established a network of 19 faint (16.5 mag < V < 19 mag) northern and equatorial DA white dwarfs (WDs) as spectrophotometric standards for present and future wide-field observatories. Our ...analysis infers spectral energy distribution (SED) models for the stars that are tied to the three CALSPEC primary standards. Our SED models are consistent with panchromatic Hubble Space Telescope photometry to better than 1%. The excellent agreement between observations and models validates the use of non-LTE DA WD atmospheres extinguished by interstellar dust as accurate spectrophotometric references. Our standards are accessible from both hemispheres and suitable for ground- and space-based observatories covering the ultraviolet to the near-infrared. The high precision of these faint sources makes our network of standards ideally suited for any experiment that has very stringent requirements on flux calibration, such as studies of dark energy using the Large Synoptic Survey Telescope and the Wide-field Infrared Survey Telescope.
We report on non-LTE Ne abundances for a sample of B-type stellar members of the Orion association. The abundances were derived by means of non-LTE folly metal-blanketed model atmospheres and ...extensive model atoms with updated atomic data. We find that these young stars have a very homogeneous abundance of A(Ne) = 8.11 c 0.04. This abundance is higher by 60.3 dex than the currently adopted solar value, A(Ne) = 7.84, which is derived from lines produced in the corona and active regions. The general agreement between the abundances of C, N, and O derived for B stars with the solar abundances of these elements derived from three-dimensional hydrodynamical models atmospheres strongly suggests that the abundance patterns of the light elements in the Sun and B stars are broadly similar. If this hypothesis is true, then the Ne abundance derived here will help to reconcile solar models with helioseismological observations.
Self-consistent vertical structure models together with non-LTE radiative transfer should produce spectra from accretion disks around black holes, which differ from multitemperature blackbodies at ...levels that may be observed. High-resolution, high signal-to-noise observations warrant spectral modeling that both accounts for relativistic effects and treats the physics of radiative transfer in detail. In Davis et al. we presented spectral models that accounted for non-LTE effects, Compton scattering, and the opacities due to ions of abundant metals. Using a modification of this method, we have tabulated spectra for black hole masses typical of Galactic binaries. We make them publicly available for spectral fitting as an XSPEC model. These models represent the most complete realization of standard accretion disk theory to date. Thus, they are well suited both for testing the theory's applicability to observed systems and for constraining properties of the black holes, including their spins.
Abstract Hot DA white dwarfs (DAWDs) have fully radiative pure hydrogen atmospheres that are the least complicated to model. Pulsationally stable, they are fully characterized by their effective ...temperature T eff and surface gravity log g , which can be deduced from their optical spectra and used in model atmospheres to predict their spectral energy distributions (SEDs). Based on this, three bright DAWDs have defined the spectrophotometric flux scale of the CALSPEC system of the Hubble Space Telescope (HST). In this paper we add 32 new fainter (16.5 < V < 19.5) DAWDs spread over the whole sky and within the dynamic range of large telescopes. Using ground-based spectra and panchromatic photometry with HST/WFC3, a new hierarchical analysis process demonstrates consistency between model and observed fluxes above the terrestrial atmosphere to <0.004 mag rms from 2700 to 7750 Å and to 0.008 mag rms at 1.6 μ m for the total set of 35 DAWDs. These DAWDs are thus established as spectrophotometric standards with unprecedented accuracy from the near-ultraviolet to the near-infrared, suitable for both ground- and space-based observatories. They are embedded in existing surveys like the Sloan Digital Sky Survey, Pan-STARRS, and Gaia, and will be naturally included in the Large Synoptic Survey Telescope survey by the Rubin Observatory. With additional data and analysis to extend the validity of their SEDs further into the infrared, these spectrophotometric standard stars could be used for JWST, as well as for the Roman and Euclid observatories.
We present calculations of non-LTE, relativistic accretion disk models applicable to the high/soft state of black hole X-ray binaries. We include the effects of thermal Comptonization and bound-free ...and free-free opacities of all abundant ion species. Taking into account the relativistic propagation of photons from the local disk surface to an observer at infinity, we present spectra calculated for a variety of accretion rates, black hole spin parameters, disk inclinations, and stress prescriptions. We also consider nonzero inner torques on the disk and explore different vertical dissipation profiles, including some that are motivated by recent radiation magnetohydrodynamic (MHD) simulations of magnetorotational turbulence. Bound-free metal opacity generally produces significantly less spectral hardening than previous models that only considered Compton scattering and free-free opacity. We find that the resulting effective photosphere usually lies at a small fraction of the total column depth, producing spectra that are remarkably independent of the stress prescription and vertical structure assumptions. We provide detailed comparisons between our models and the widely used multicolor disk model. Frequency-dependent discrepancies exist that may affect the parameters of other spectral components when this simpler disk model is used to fit modern X-ray data. For a given source, our models predict that the luminosity in the high/soft state should approximately scale with the fourth power of the empirically inferred maximum temperature, but with a slight hardening at high luminosities. This is in good agreement with observations.
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