arXiv subjects

Adam Burrows

Publications and source records attributed to Adam Burrows.

At least 253 records · Page 14Linked to original sources

arXiv2002

New CrH Opacities for the Study of L and Brown Dwarf Atmospheres

In this paper, we calculate new line lists and opacities for the 12 bands of the A$^6Σ^{+}$ -- X$^6Σ^{+}$ transitions of the CrH molecule. Identified in objects of the new L dwarf spectroscopic class (many of which are brown dwarfs), as well as in sunspots, the CrH molecule plays an important role in the diagnosis of low-temperature atmospheres. As a tentative first application of these opacities, we employ our new theoretical CrH data in an atmospheres code to obtain a CrH/H$_2$ number ratio for the skin of the L5 dwarf 2MASSI J1507038-151648 of $\sim 2-4\times 10^{-9}$, in rough agreement with chemical equilibrium expectations. Since in previous compilations the oscillator strength was off by more than an order of magnitude, this agreement represents a modest advance. However, in order to determine the CrH abundance in an L dwarf atmosphere, silicate clouds need to be incorporated into the model, and cloud modeling is still in a primitive stage of development. Nevertheless, one important step in L dwarf modeling is a reliable CrH opacity and this is what we have here attempted to provide.

astro-ph↗

arXiv2002

Theoretical Spectral Models of T Dwarfs at Short Wavelengths and Their Comparison with Data

We have generated new, self-consistent spectral and atmosphere models for the effective temperature range 600 K to 1300 K thought to encompass the known T dwarfs. For the first time, theoretical models are compared with a {\it family} of measured T dwarf spectra at wavelengths shortward of $\sim$1.0 micron. By defining spectral indices and standard colors in the optical and very near-infrared, we explore the theoretical systematics with \teff, gravity, and metallicity. We conclude that the short- wavelength range is rich in diagnostics that complement those in the near-infrared now used for spectral subtyping. We also conclude that the wings of the Na D and K I (7700Å) resonance lines and aggressive rainout of heavy metals (with the resulting enhancement of the sodium and potassium abundances at altitude) are required to fit the new data shortward of 1.0 \mic. Furthermore, we find that the water bands weaken with increasing gravity, that modest decreases in metallicity enhance the effect in the optical of the sodium and potassium lines, and that at low \teffs, in a reversal of the normal pattern, optical spectra become bluer with further decreases in \teff. Moreover, we conclude that T dwarf subtype is not a function of \teff alone, but that it is a non-trivial function of gravity and metallicity as well. As do Marley et al. (2001), we see evidence in early T dwarf atmospheres of a residual effect of clouds. With cloudless models, we obtain spectral fits to the two late T dwarfs with known parallaxes, but a residual effect of clouds on the emergent spectra of even late T dwarfs can not yet be discounted.

astro-ph↗

arXiv2001

The Spectra of T Dwarfs I: Near-Infrared Data and Spectral Classification

We present near-infrared spectra for a sample of T dwarfs, including eleven new discoveries made using the Two Micron All Sky Survey. These objects are distinguished from warmer (L-type) brown dwarfs by the presence of methane absorption bands in the 1--2.5 $\micron$ spectral region. A first attempt at a near-infrared classification scheme for T dwarfs is made, based on the strengths of CH$_4$ and H$_2$O bands and the shapes of the 1.25, 1.6, and 2.1 $\micron$ flux peaks. Subtypes T1 V through T8 V are defined, and spectral indices useful for classification are presented. The subclasses appear to follow a decreasing T$_{eff}$ scale, based on the evolution of CH$_4$ and H$_2$O bands and the properties of L and T dwarfs with known distances. However, we speculate that this scale is not linear with spectral type for cool dwarfs, due to the settling of dust layers below the photosphere and subsequent rapid evolution of spectral morphology around T$_{eff}$ $\sim$ 1300--1500 K. Similarities in near-infrared colors and continuity of spectral features suggest that the gap between the latest L dwarfs and earliest T dwarfs has been nearly bridged. This argument is strengthened by the possible role of CH$_4$ as a minor absorber shaping the K-band spectra of the latest L dwarfs. Finally, we discuss one peculiar T dwarf, 2MASS 0937+2931, which has very blue near-infrared colors (J-K$_s$ = $-0.89\pm$0.24) due to suppression of the 2.1 $\micron$ peak. The feature is likely caused by enhanced collision-induced H$_2$ absorption in a high pressure or low metallicity photosphere.

astro-ph↗

arXiv2001

The Physics of Protoneutron Star Winds: Implications for r-Process Nucleosynthesis

We solve the general-relativistic steady-state eigenvalue problem of neutrino-driven protoneutron star winds, which immediately follow core-collapse supernova explosions. We provide velocity, density, temperature, and composition profiles and explore the systematics and structures generic to such a wind for a variety of protoneutron star characteristics. Furthermore, we derive the entropy, dynamical timescale, and neutron-to-seed ratio in the general relativistic framework essential in assessing this site as a candidate for $r$-process nucleosynthesis. Generally, we find that for a given mass outflow rate ($\dot{M}$), the dynamical timescale of the wind is significantly shorter than previously thought. We argue against the existence or viability of a high entropy ($\gtrsim300$ per k$_{B}$ per baryon), long dynamical timescale $r$-process epoch. In support of this conclusion, we model the protoneutron star cooling phase, calculate nucleosynthetic yields in our steady-state profiles, and estimate the integrated mass loss. We find that transonic winds enter a high entropy phase only with very low $\dot{M}$ ($\lesssim1\times10^{-9}$ M$_\odot$ s$^{-1}$) and extremely long dynamical timescale ($τ_ρ\gtrsim0.5$ seconds). Our results support the possible existence of an early $r$-process epoch at modest entropy ($\sim150$) and very short dynamical timescale, consistent in our calculations with a very massive or very compact protoneutron star that contracts rapidly after the preceding supernova. We explore possible modifications to our models, which might yield significant $r$-process nucleosynthesis generically. Finally, we speculate on the effect of fallback and shocks on both the wind physics and nucleosynthesis.

astro-ph↗

arXiv2001

The Theory of Brown Dwarfs and Extrasolar Giant Planets

Straddling the traditional realms of the planets and the stars, objects below the edge of the main sequence have such unique properties, and are being discovered in such quantities, that one can rightly claim that a new field at the interface of planetary science and and astronomy is being born. In this review, we explore the essential elements of the theory of brown dwarfs and giant planets, as well as of the new spectroscopic classes L and T. To this end, we describe their evolution, spectra, atmospheric compositions, chemistry, physics, and nuclear phases and explain the basic systematics of substellar-mass objects across three orders of magnitude in both mass and age and a factor of 30 in effective temperature. Moreover, we discuss the distinctive features of those extrasolar giant planets that are irradiated by a central primary, in particular their reflection spectra, albedos, and transits. Aspects of the latest theory of Jupiter and Saturn are also presented. Throughout, we highlight the effects of condensates, clouds, molecular abundances, and molecular/atomic opacities in brown dwarf and giant planet atmospheres and summarize the resulting spectral diagnostics. Where possible, the theory is put in its current observational context.

astro-ph↗

arXiv2001

HST Time-Series Photometry of the Transiting Planet of HD 209458

We have observed 4 transits of the planet of HD 209458 using the STIS spectrograph on HST. Summing the recorded counts over wavelength between 582 nm and 638 nm yields a photometric time series with 80 s time sampling and relative precision of about 1.1E-4 per sample. The folded light curve can be fit within observational errors using a model consisting of an opaque circular planet transiting a limb-darkened stellar disk. In this way we estimate the planetary radius R_p = 1.347 +/- 0.060 R_Jup, the orbital inclination i = 86.68 +/- 0.14 degrees, the stellar radius R_* = 1.146 +/- 0.050 R_solar, and one parameter describing the stellar limb darkening. Our estimated radius is smaller than those from earlier studies, but is consistent within measurement errors, and is also consistent with theoretical estimates of the radii of irradiated Jupiter-like planets. Satellites or rings orbiting the planet would, if large enough, be apparent from distortions of the light curve or from irregularities in the transit timings. We find no evidence for either satellites or rings, with upper limits on satellite radius and mass of 1.2 Earth radii and 3 Earth masses, respectively. Opaque rings, if present, must be smaller than 1.8 planetary radii in radial extent. The high level of photometric precision attained in this experiment confirms the feasibility of photometric detection of Earth-sized planets circling Sun-like stars.

astro-ph↗

arXiv2000

Alkali Metals and the Color of Brown Dwarfs

I summarize some of the consequences for the optical and very-near-infrared spectra of T dwarfs (in particular) and brown dwarfs (in general) of their possible dominance by the neutral alkali metal lines. As a byproduct of this study, I estimate the true optical color of ``brown'' dwarfs.

astro-ph↗

arXiv2000

Neutrino Processes in Supernovae and the Physics of Protoneutron Star Winds

In preparation for a set of hydrodynamical simulations of core-collapse supernovae and protoneutron star winds, we investigate the rates of production and thermalization of $ν_μ$ and $ν_τ$ neutrinos in dense nuclear matter. Included are contributions from electron scattering, electron-positron annihilation, nucleon-nucleon bremsstrahlung, and nucleon scattering. We find that nucleon scattering dominates electron scattering as a thermalization process at neutrino energies greater than $\sim$15 MeV. In addition, nucleon-nucleon bremsstrahlung dominates electron-positron annihilation as a production mechanism at low neutrino energies, near and below the $ν_μ$ and $ν_τ$ neutrinospheres. Furthermore, we have begun a study of steady-state general relativistic protoneutron star winds employing simple neutrino heating and cooling terms. From this analysis we obtain acceleration profiles as well as asymptotic lepton fractions and baryon entropies essential in assessing the wind as a potential site for $r$-process nucleosynthesis.

astro-ph↗

arXiv2000

An Improved Red Spectrum of the Methane or T-dwarf SDSS 1624+0029: Role of the Alkali Metals

A Keck~II low resolution spectrum shortward of ome-micron is presented for SDSS 1624+0029, the first field methane or T dwarf discovered in the Sloan Digital Sky Survey. Significant flux is detected down to the spectrum's short wavelength limit of 6200Å. The spectrum exhibits a broad absorption feature centered at 7700Å, which we interpret as the K~I 7665/7699 resonance doublet. The observed flux declines shortward of 7000Å, due most likely to the red wing of the Na~I doublet. Both Cs~I doublet lines are detected more strongly than in an earlier red spectrum. Neither Li~I absorption nor H$α$ emission are detected. An exploratory model fit to the spectrum suggests that the shape of the red spectrum can be primarily accounted for by the broad wings of the K~I and Na~I doublets. This behavior is consistent with the argument proffered by Burrows, Marley and Sharp that strong alkali absorption is principally responsible for depressing T dwarf spectra shortward of 1$μ$m. In particular, there seems no compelling reason at this time to introduce dust or an additional opacity source in the atmosphere of the SDSS object. The width of the K~I and strengths of the Cs~I lines also indicate that the Sloan object is warmer than Gl~229B.

astro-ph↗

arXiv2000

Albedo and Reflection Spectra of Extrasolar Giant Planets

We generate theoretical albedo and reflection spectra for a full range of extrasolar giant planet (EGP) models, from Jovian to 51-Pegasi class objects. Our albedo modeling utilizes the latest atomic and molecular cross sections, a Mie theory treatment of extinction by condensates, a variety of particle size distributions, and an extension of the Feautrier radiative transfer method which allows for a general treatment of the scattering phase function. We find that due to qualitative similarities in the compositions and spectra of objects within each of five broad effective temperature ranges, it is natural to establish five representative EGP albedo classes: a ``Jovian'' class (T$_{\rm eff} \lesssim 150$ K; Class I) with tropospheric ammonia clouds, a ``water cloud'' class (T$_{\rm eff} \sim 250$ K; Class II) primarily affected by condensed H$_2$O, a ``clear'' class (T$_{\rm eff} \gtrsim 350$ K; Class III) which lacks clouds, and two high-temperature classes: Class IV (900 K $\lesssim$ T$_{\rm{eff}}$ $\lesssim$ 1500 K) for which alkali metal absorption predominates, and Class V (T$_{\rm{eff}}$ $\gtrsim$ 1500 K and/or low surface gravity ($\lesssim$ 10$^3$ cm s$^{-2}$)) for which a high silicate layer shields a significant fraction of the incident radiation from alkali metal and molecular absorption. The resonance lines of sodium and potassium are expected to be salient features in the reflection spectra of Class III, IV, and V objects. We derive Bond albedos and effective temperatures for the full set of known EGPs and explore the possible effects of non-equilibrium condensed products of photolysis above or within principal cloud decks. As in Jupiter, such species can lower the UV/blue albedo substantially, even if present in relatively small mixing ratios.

astro-ph↗

arXiv2000

The Near-Infrared and Optical Spectra of Methane Dwarfs and Brown Dwarfs

We identify the pressure--broadened red wings of the saturated potassium resonance lines at 7700 Åas the source of anomalous absorption seen in the near-infrared spectra of Gliese 229B and, by extension, of methane dwarfs in general. This conclusion is supported by the recent work of Tsuji {\it et al.} 1999, though unlike them we find that dust need not be invoked to explain the spectra of methane dwarfs shortward of 1 micron. We find that a combination of enhanced alkali abundances due to rainout and a more realistic non-Lorentzian theory of resonant line shapes may be all that is needed to properly account for these spectra from 0.5 to 1.0 microns. The WFPC2 $I$ measurement of Gliese 229B is also consistent with this theory. Furthermore, a combination of the blue wings of this K I resonance doublet, the red wings of the Na D lines at 5890 Å, and, perhaps, the Li I line at 6708 Åcan explain in a natural way the observed WFPC2 $R$ band flux of Gliese 229B. Hence, we conclude that the neutral alkali metals play a central role in the near-infrared and optical spectra of methane dwarfs and that their lines have the potential to provide crucial diagnostics of brown dwarfs. We speculate on the systematics of the near-infrared and optical spectra of methane dwarfs, for a given mass and composition, that stems from the progressive burial with decreasing \teff of the alkali metal atoms to larger pressures and depths.

astro-ph↗

arXiv2000

Mu and Tau Neutrino Thermalization and Production in Supernovae: Processes and Timescales

We investigate the rates of production and thermalization of $ν_μ$ and $ν_τ$ neutrinos at temperatures and densities relevant to core-collapse supernovae and protoneutron stars. Included are contributions from electron scattering, electron-positron annihilation, nucleon-nucleon bremsstrahlung, and nucleon scattering. For the scattering processes, in order to incorporate the full scattering kinematics at arbitrary degeneracy, the structure function formalism developed by Reddy et al. (1998) and Burrows and Sawyer (1998) is employed. Furthermore, we derive formulae for the total and differential rates of nucleon-nucleon bremsstrahlung for arbitrary nucleon degeneracy in asymmetric matter. We find that electron scattering dominates nucleon scattering as a thermalization process at low neutrino energies ($ε_ν\lesssim 10$ MeV), but that nucleon scattering is always faster than or comparable to electron scattering above $ε_ν\simeq10$ MeV. In addition, for $ρ\gtrsim 10^{13}$ g cm$^{-3}$, $T\lesssim14$ MeV, and neutrino energies $\lesssim60$ MeV, nucleon-nucleon bremsstrahlung always dominates electron-positron annihilation as a production mechanism for $ν_μ$ and $ν_τ$ neutrinos.

astro-ph↗

arXiv1999

Type Ia Supernova Explosions in Binary Systems: The Impact on the Secondary Star and its Consequences

One method of discriminating between the many Type Ia progenitor scenarios is by searching for contaminating hydrogen and helium stripped from the companion star. We present several high-resolution 2-D numerical simulations of the impact of a Type Ia supernova explosion with hydrogen-rich main sequence, subgiant, and red giant companions. The binary parameters were chosen to represent several classes of single-degenerate Type Ia progenitor models that have been suggested in the literature. We use realistic stellar models and supernova debris profiles to represent each binary system. For each scenario, we explore the hydrodynamics of the supernova-secondary interaction, calculate the amount of stellar material stripped from the secondary and the kick delivered by the impact, and construct the velocity and solid angle distributions of the stripped material. After the impact, the main sequence star is puffed up, much like a pre-main sequence star. The subgiant companion behaves similarly. The secondary will not be contaminated by much supernova debris from the initial impact, but it may accrete low-velocity iron-group elements (or oxygen and silicon if the ejecta is radially mixed) at late times. A He pre-white dwarf will be left behind after a supernova explosion with a red giant companion.

astro-ph↗

arXiv1998

Nucleosynthesis in Type II supernovae and the abundances in metal-poor stars

We explore the effects on nucleosynthesis in Type II supernovae of various parameters (mass cut, neutron excess, explosion energy, progenitor mass) in order to explain the observed trends of the iron-peak element abundance ratios ([Cr/Fe], [Mn/Fe], [Co/Fe] and [Ni/Fe]) in halo stars as a function of metallicity for the range $ -4 \le$ [Fe/H] $\le -2.5$. [Cr/Fe] and [Mn/Fe] decrease with decreasing [Fe/H], while [Co/Fe] behaves the opposite way and increases. We show that such a behavior can be explained by a variation of mass cuts in Type II supernovae as a function of progenitor mass, which provides a changing mix of nucleosynthesis from an alpha-rich freeze-out of Si-burning and incomplete Si-burning. This explanation is consistent with the amount of ejected $^{56}$Ni determined from modeling the early light curves of individual supernovae. We also suggest that the ratio [H/Fe] of halo stars is mainly determined by the mass of interstellar hydrogen mixed with the ejecta of a single supernova which is larger for larger explosion energy and the larger Strömgren radius of the progenitor.

astro-ph↗

arXiv1998

New Ideas in the Theory of Extrasolar Giant Planets and Brown Dwarfs

We summarize and extend recent work on the theory of extrasolar giant planets (EGPs) and brown dwarfs, paying particular attention to Gliese 229 B, the albedos of EGPs, the compositions of substellar atmospheres, the connections with the giant planets in the solar system, cloud physics, and non-gray spectral synthesis. The role of condensates in altering the optical spectrum of Gliese 229 B is explored, as are the systematics of the reflection spectra from extrasolar giant planets near their primaries. In addition, we discuss the role of convection and disequilibrium chemistry in explaining the anomalous detection of CO in Gliese 229 B. Throughout, we highlight the distinctive chemistry that defines this new class of objects and set goals for future study.

astro-ph↗

arXiv1998

On the Systematics of Core-Collapse Explosions

Recent observations of supernovae, supernova remnants, and radio pulsars suggest that there are correlations between pulsar kicks and spins, infrared and gamma-ray line profiles, supernova polarizations, and ejecta debris fields. A framework is emerging in which explosion asymmetries play a central role. The new perspective meshes recent multi-dimensional theoretical investigations of the explosion mechanism with trends in $^{56}$Ni yields and explosion kinetic energies. These trends imply that the mass above which black holes form after collapse is $\sim$30 M$_{\odot}$ and that supernova explosion energies may vary by as much as a factor of four. In addition, new neutrino-matter opacity calculations reveal that the inner cores of protoneutron stars are more transparent than hitherto suspected. This may have consequences for the delayed neutrino-driven mechanism of explosion itself.

astro-ph↗

arXiv1998

Many-Body Corrections to Charged-Current Neutrino Absorption Rates in Nuclear Matter

Including nucleon--nucleon correlations due to both Fermi statistics and nuclear forces, we have developed a general formalism for calculating the charged--current neutrino--nucleon absorption rates in nuclear matter. We find that at one half nuclear density many--body effects alone suppress the rates by a factor of two and that the suppression factors increase to $\sim$5 at $4\times10^{14}$ g cm$^{-3}$. The associated increase in the neutrino--matter mean--free--paths parallels that found for neutral--current interactions and opens up interesting possibilities in the context of the delayed supernova mechanism and protoneutron star cooling.

astro-ph↗

arXiv1998

The Effects of Correlations on Neutrino Opacities in Nuclear Matter

Including nucleon-nucleon correlations due to both Fermi statistics and nuclear forces, we have developed a general formalism for calculating the neutral-current neutrino-nucleon opacities in nuclear matter. We derive corrections to the dynamic structure factors due to both density and spin correlations and find that neutrino-nucleon cross sections are suppressed by large factors around and above nuclear density. In addition, we find that the spectrum of energy transfers in neutrino scattering is considerably broadened by the interactions in the medium. An identifiable component of this broadening comes from the absorption and emission of quanta of collective modes akin to the Gamow-Teller and Giant Dipole resonances in nuclei (zero-sound; spin waves), with Čerenkov kinematics. Under the assumption that both the charged-current and the neutral-current cross sections are decreased by many-body effects, we calculate a set of ad hoc protoneutron star cooling models to gauge the potential importance of the new opacities to the supernova itself. We find that after many hundreds of milliseconds to seconds the driving neutrino luminosities might be increased by from 10% to 100%. However, the actual consequences, if any, of these new neutrino opacities remain to be determined.

astro-ph↗