Atmosphere Code PHOENIX Research Articles

Molecular Line Opacity of LiCl in the Mid‐Infrared Spectra of Brown Dwarfs

We present a complete line list for the X 1Sigma+ electronic ground state of LiCl computed using fully quantum-mechanical techniques. This list includes transition energies and oscillator strengths in the spectral region 0.3-39,640.7 cm-1 for all allowed rovibrational transitions in absorption within the electronic ground state. The calculations were performed using an accurate hybrid potential constructed from a spectral inversion fit of experimental data and from recent multi-reference single- and double-excitation configuration interaction calculations. The line list was incorporated into the stellar atmosphere code PHOENIX to compute spectra for a range of young to old T dwarf models. The possibility of observing a signature of LiCl in absorption near 15.8 microns is addressed and the proposal to use this feature to estimate the total lithium elemental abundance for these cool objects is discussed.

Influence of chemi-ionization and chemi-recombination processes on the population of hydrogen Rydberg states in atmospheres of late type dwarfs

We study the influence of a group of chemi-ionization and chemi-recombination processes on the populations of higher states of hydrogen in the layers of a stellar atmosphere. The group of processes includes ionization: H*(n) + H(1s) ⇒ H + 2 +e, H*(n)+H(1s) ⇒ H(1s)+H + +e, and inverse recombination: H + 2 +e ⇒ H*(n)+H(1s), H(1s)+H + +e ⇒ H*(n)+H(1s), where H*(n) is the hydrogen atom in a state with the principal quantum number n » 1, and H + 2 is the hydrogen molecular ion in a weakly bound rho-vibrational state of the ground state. These processes have been treated within the framework of the semi-classical approximation, developed in several previous papers, and have been included in the general stellar atmosphere code PHOENIX. We present results for an M dwarf atmosphere with T e f f = 3800 K and find that the inclusion of chemi-ionization and chemi-recombination processes is significant in the low temperature parts of the atmosphere.

Radiative accelerations in stellar atmospheres

We present calculations of radiative accelerations obtained with the general purpose model atmosphere code PHOENIX. The accelerations are computed simultaneously for all the elements He–Ga; Kr–Nb; Ba; La, using the opacity sampling method. The calculations are mainly performed with the LTE approximation. Tests are made to evaluate the influence of non-LTE effects and the validity of the radiative flux obtained through the diffusion approximation. It is shown that the abundances supported in the atmospheres of HgMn and He-weak stars are generally consistent with those observed.

SN 1984A and Delayed‐Detonation Models of Type Ia Supernovae

SN 1984A shows unusually large expansion velocities in lines from freshly synthesized material, relative to typical Type Ia supernovae (SNe Ia). SN 1984A is an example of a group of SNe Ia which have very large blueshifts of the P Cygni features but otherwise normal spectra. We have modeled several early spectra of SN 1984A with the multipurpose non-LTE (NLTE) model atmosphere and spectrum synthesis code PHOENIX. We have used as input two delayed-detonation models: CS15DD3 (Iwamoto et al.) and DD21c (Hoflich, Wheeler, & Thielemann). These models show line-expansion velocities which are larger than that for a typical deflagration model like W7, which we have previously shown to fit the spectra of normal SNe Ia quite well. We find these delayed-detonation models to be reasonable approximations to large absorption feature blueshift SNe Ia, like SN 1984A. Higher densities of newly synthesized intermediate-mass elements at higher velocities, v > 15,000 km s-1, are found in delayed-detonation models than in deflagration models. We find that this increase in density at high velocities is responsible for the larger blueshifts in the synthetic spectra. We show that the variations in line width in observed SNe Ia are likely due to density variations in the outer, high-velocity layers of their atmospheres.

Analysis of Type IIn SN 1998S: Effects of Circumstellar Interaction on Observed Spectra

We present spectral analysis of early observations of the Type IIn supernova 1998S using the general non-local thermodynamic equilibrium atmosphere code PHOENIX. We model both the underlying supernova spectrum and the overlying circumstellar interaction region and produce spectra in good agreement with observations. The early spectra are well fitted by lines produced primarily in the circumstellar region itself, and later spectra are due primarily to the supernova ejecta. Intermediate spectra are affected by both regions. A mass-loss rate of order ~ 0.0001-0.001 M☉ yr-1 is inferred for a wind speed of 100-1000 km s-1. We discuss how future self-consistent models will better clarify the underlying progenitor structure.

Multiwavelength analyses of the extraordinary nova LMC 1991

LMC 91 was a very fast, classical nova and the brightest nova ever observed in the Large Magellanic Cloud. It was extensively observed during both its early optically thick and its nebular evolution in the optical and UV wavelength regions. We successfully fit all the optically thick spectra using a grid of spherically symmetric, non-LTE, line-blanketed, expanding synthetic spectra created with the model atmosphere code PHOENIX. The emission lines of the nebular spectra have been fitted using an optimization technique for the emission-line luminosities predicted by the photoionization code CLOUDY. Our analyses show the following: the bolometric luminosity was super-Eddington before visual maximum and reached the ejected mass was and nuclear burning on the white dwarf ceased after ∼100 days. The elemental abundances (by number) with respect to solar of the ejecta are and all other elements ∼0.1. These abundances were determined from both the optically thick and nebular analyses. The extreme luminosity, high ejected mass, rapid turn-off time, and low metallicity (except for enhanced CNO) represent the extreme values of observational nova parameters and thus present interesting challenges for understanding the nature of the outburst of LMC 91.

Preliminary Spectral Analysis of the Type II Supernova 1999em

We have calculated fast direct spectral model fits to two early-time spectra of the Type-II plateau SN 1999em, using the SYNOW synthetic spectrum code. The first is an extremely early blue optical spectrum and the second a combined HST and optical spectrum obtained one week later. Spectroscopically this supernova appears to be a normal Type II and these fits are in excellent agreement with the observed spectra. Our direct analysis suggests the presence of enhanced nitrogen. We have further studied these spectra with the full NLTE general model atmosphere code PHOENIX. While we do not find confirmation for enhanced nitrogen (nor do we rule it out), we do require enhanced helium. An even more intriguing possible line identification is complicated Balmer and He I lines, which we show falls naturally out of the detailed calculations with a shallow density gradient. We also show that very early spectra such as those presented here combined with sophisticated spectral modeling allows an independent estimate of the total reddening to the supernova, since when the spectrum is very blue, dereddening leads to changes in the blue flux that cannot be reproduced by altering the ``temperature'' of the emitted radiation. These results are extremely encouraging since they imply that detailed modeling of early spectra can shed light on both the abundances and total extinction of SNe II, the latter improving their utility and reliability as distance indicators.

A Grid of Non‐LTE Model Atmospheres for White Dwarfs in Cataclysmic Variables

We have calculated a grid of non-LTE (NLTE) line-blanketed model atmospheres for white dwarfs in cataclysmic variable (CV) systems using our stellar atmosphere code PHOENIX. The grid covers the temperature range 20,000-500,000 K for solar abundances and 10-2 and 10-4 solar metal abundance. The effective temperature resolution is 10,000 K in the range 20,000-200,000 K and is 20,000 K in the range 200,000-500,000 K. The models include a large number of NLTE levels for light metals such as CNO and heavy metals such as Ni and Fe. The necessity of using self-consistent NLTE line-blanketed model atmospheres for the analysis of white dwarfs is well known and reaffirmed here. We discuss the importance of metal opacity, especially metals heavier than Ca, in the atmospheres of CV primaries. We also compare our grid to previously published models and discuss the general improvements offered by our models. In addition, a specific model from our grid is compared to a Hubble Space Telescope observation of U Gem and predictions for Far-Ultraviolet Spectroscopic Explorer (FUSE) observations are made.

Metallicity Effects in Non‐LTE Model Atmospheres of Type Ia Supernovae

We have calculated a grid of photospheric phase atmospheres of Type Ia supernovae (SNe Ia) with metallicities from 10 times to 1/30 the solar metallicity in the C+O layer of the deflagration model, W7. We have modeled the spectra using the multipurpose non-LTE model atmosphere and spectrum synthesis code PHOENIX. We show models for the epochs 7, 10, 15, 20, and 35 days after explosion. When compared to observed spectra obtained at the approximately corresponding epochs, these synthetic spectra fit reasonably well. The spectra show variation in the overall level of the UV continuum with lower fluxes for models with higher metallicity in the unburned C+O layer. This is consistent with the classical surface cooling and line-blocking effect due to metals in the outer layers of C+O. The UV features also move consistently to the blue with higher metallicity, demonstrating that they are forming at shallower and faster layers in the atmosphere. The potentially most useful effect is the blueward movement of the Si II feature at 6150 Å with increasing C+O layer metallicity. We also demonstrate the more complex effects of metallicity variations by modifying the 54Fe content of the incomplete burning zone in W7 at maximum light. We briefly address some shortcomings of the W7 model when compared to observations. Finally, we identify that the split in the Ca H+K feature produced in W7 and observed in some SNe Ia is due to a blending effect of Ca II and Si II and does not necessarily represent a complex abundance or ionization effect in Ca II.

Model atmospheres for M (sub)dwarf stars. 1: The base model grid

We have calculated a grid of more than 700 model atmospheres valid for a wide range of parameters encompassing the coolest known M dwarfs, M subdwarfs, and brown dwarf candidates: 1500 less than or equal to T(sub eff) less than or equal to 4000 K, 3.5 less than or equal to log g less than or equal to 5.5, and -4.0 less than or equal to (M/H) less than or equal to +0.5. Our equation of state includes 105 molecules and up to 27 ionization stages of 39 elements. In the calculations of the base grid of model atmospheres presented here, we include over 300 molecular bands of four molecules (TiO, VO, CaH, FeH) in the JOLA approximation, the water opacity of Ludwig (1971), collision-induced opacities, b-f and f-f atomic processes, as well as about 2 million spectral lines selected from a list with more than 42 million atomic and 24 million molecular (H2, CH, NH, OH, MgH, SiH, C2, CN, CO, SiO) lines. High-resolution synthetic spectra are obtained using an opacity sampling method. The model atmospheres and spectra are calculated with the generalized stellar atmosphere code PHOENIX, assuming LTE, plane-parallel geometry, energy (radiative plus convective) conservation, and hydrostatic equilibrium. The model spectra give close agreement with observations of M dwarfs across a wide spectral range from the blue to the near-IR, with one notable exception: the fit to the water bands. We discuss several practical applications of our model grid, e.g., broadband colors derived from the synthetic spectra. In light of current efforts to identify genuine brown dwarfs, we also show how low-resolution spectra of cool dwarfs vary with surface gravity, and how the high-regulation line profile of the Li I resonance doublet depends on the Li abundance.