arXiv subjects

Peter H. Hauschildt

Publications and source records attributed to Peter H. Hauschildt.

At least 37 records · Page 2Linked to original sources

arXiv2012

Modeling the near-UV band of GK stars, Paper II: NLTE models

We present a grid of atmospheric models and synthetic spectral energy distributions (SEDs) for late-type dwarfs and giants of solar and 1/3 solar metallicity with many opacity sources computed in self-consistent Non-Local Thermodynamic Equilibrium (NLTE), and compare them to the LTE grid of Short & Hauschildt (2010) (Paper I). We describe, for the first time, how the NLTE treatment affects the thermal equilibrium of the atmospheric structure (T(tau) relation) and the SED as a finely sampled function of Teff, log g, and [A/H] among solar metallicity and mildly metal poor red giants. We compare the computed SEDs to the library of observed spectrophotometry described in Paper I across the entire visible band, and in the blue and red regions of the spectrum separately. We find that for the giants of both metallicities, the NLTE models yield best fit Teff values that are ~30 to 90 K lower than those provided by LTE models, while providing greater consistency between \log g values, and, for Arcturus, Teff values, fitted separately to the blue and red spectral regions. There is marginal evidence that NLTE models give more consistent best fit Teff values between the red and blue bands for earlier spectral classes among the solar metallicity GK giants than they do for the later classes, but no model fits the blue band spectrum well for any class. For the two dwarf spectral classes that we are able to study, the effect of NLTE on derived parameters is less significant.

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arXiv2011

A 3D radiative transfer framework: XIII. OpenCL implementation

We discuss an implementation of our 3D radiative transfer (3DRT) framework with the OpenCL paradigm for general GPU computing. We implement the kernel for solving the 3DRT problem in Cartesian coordinates with periodic boundary conditions in the horizontal $(x,y)$ plane, including the construction of the nearest neighbor $\Lstar$ and the operator splitting step. We present the results of a small and a large test case and compare the timing of the 3DRT calculations for serial CPUs and various GPUs. The latest available GPUs can lead to significant speedups for both small and large grids compared to serial (single core) computations.

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arXiv2011

Detecting Planets around Very Cool Dwarfs at Near Infrared Wavelengths with the Radial Velocity Technique

Context. Radial velocity monitoring of very cool dwarfs such as late M- and hot L-dwarfs has become a promising tool to search for rocky planets as well as to follow-up planetary candidates around dwarfs found by transit surveys. These stars are faint at optical wavelengths, as their spectral flux distribution peaks at near-infrared (NIR) wavelengths. For this reason, it is desirable to measure the radial velocities in this wavelength regime. However, in the NIR there are only very few medium- and high-resolution spectrographs available which are mounted at large telescopes. In the near future, high-resolution spectrographs for the NIR will be built, which will allow us to search for rocky planets around cool M-dwarfs and L-dwarfs from radial velocities monitoring. Methods. Stellar atmosphere synthetic models for an M- and an L-dwarf with temperatures of 2200 K and 1800 K, respectively, and a theoretical spectrum of the Earth's transmission in the spectral range from 0.9 to 2.5 \mum are used. We simulate a series of Doppler-shifted spectra observed with different resolving powers and signal-to-noise ratios, and for different rotational broadenings of the dwarf. For different combinations of the input parameters we recover the radial velocity by means of cross-correlation with a high signal-to-noise ratio template and determine the associate uncertainties. Results. The highest precision in radial velocity measurements for the cool M-dwarf is found in the Y band around 1.0 \mum, while for the L-dwarf it is determined in the J band around 1.25 \mum. We note that synthetic models may lack of some faint absorption features or underestimate their abundances. Conversely, some instrumental/calibration aspects that are not taken into account in our estimations would rise the uncertainties.

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arXiv2010

Hydrogen Recombination with Multilevel atoms

Hydrogen recombination is one of the most important atomic processes in many astrophysical objects such as Type II supernova (SN~II) atmospheres, the high redshift universe during the cosmological recombination era, and H II regions in the interstellar medium. Accurate predictions of the ionization fraction can be quite different from those given by a simple solution if one takes into account many angular momentum sub-states, non-resonant processes, and calculates the rates of all atomic processes from the solution of the radiative transfer equation instead of using a Planck function under the assumption of thermal equilibrium. We use the general purpose model atmosphere code PHOENIX 1D to compare how the fundamental probabilities such as the photo-ionization probability, the escape probability, and the collisional de-excitation probability are affected by the presence of other metals in the environment, multiple angular momentum sub-states, and non-resonant processes. Our comparisons are based on a model of SN 1999em, a SNe Type II, 20 days after its explosion.

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arXiv2009

A 3D radiative transfer framework: VI. PHOENIX/3D example applications

We demonstrate the application of our 3D radiative transfer framework in the model atmosphere code PHOENIX/3D for a number of spectrum synthesis calculations for very different conditions. The 3DRT framework discussed in the previous papers of this series was added to our general-purpose model atmosphere code PHOENIX/1D and an extended 3D version PHOENIX/3D was created. The \phxT code is parallelized via the MPI library using a hierarchical domain decomposition and displays very good strong scaling. We present the results of several test cases for widely different atmosphere conditions and compare the 3D calculations with equivalent 1D models to assess the internal accuracy of the 3D modeling. In addition, we show the results for a number of parameterized 3D structures. With presently available computational resources it is possible to solve the full 3D radiative transfer (including scattering) problem with the same micro-physics as included in 1D modeling.

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arXiv2009

A 3D radiative transfer framework: V. Homologous Flows

Observations and theoretical calculations have shown the importance of non-spherically symmetric structures in supernovae. Thus, the interpretation of observed supernova spectra requires the ability to solve the transfer equation in 3-D moving atmospheres. We present an implementation of the solution of the radiative transfer equation in 3-D homologously expanding atmospheres in spherical coordinates. The implementation is exact to all orders in v/c. We use a new affine method that makes use of the fact that photons travel on straight lines. We compare our results in 3-D for spherically symmetric test problems with high velocity fields and find excellent agreement. Our well-tested 1-D results are based on methods where the momentum directions vary along the characteristic (co-moving momentum directions). Thus, we are able to verify both the analytic framework and its numerical implementation. Additionally, we have been able to test the parallelization over characteristics. Using 512^2 momentum angles we ran the code on 16,384 Opteron processors and achieved excellent scaling. It is now possible to calculate synthetic spectra from realistic 3D hydro simulations. This should open an era of progress in hydro modeling, similar to that that occurred in the 1980s when 1-D models were confronted with synthetic spectra.

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arXiv2009

A 3D radiative transfer framework: IV. spherical & cylindrical coordinate systems

We extend our framework for 3D radiative transfer calculations with a non-local operator splitting methods along (full) characteristics to spherical and cylindrical coordinate systems. These coordinate systems are better suited to a number of physical problems than Cartesian coordinates. The scattering problem for line transfer is solved via means of an operator splitting (OS) technique. The formal solution is based on a full characteristics method. The approximate $Λ$ operator is constructed considering nearest neighbors exactly. The code is parallelized over both wavelength and solid angle using the MPI library. We present the results of several test cases with different values of the thermalization parameter for the different coordinate systems. The results are directly compared to 1D plane parallel tests. The 3D results agree very well with the well-tested 1D calculations.

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arXiv2009

Optical and Near-Infrared Spectroscopy of the L Subdwarf SDSS J125637.13-022452.4

Red optical and near-infrared spectroscopy are presented for SDSS J125637.13-022452.4, one of only four L subdwarfs reported to date. These data confirm the low-temperature, metal-poor nature of this source, as indicated by prominent metal-hydride bands, alkali lines, and collision-induced H2 absorption. The optical and near-infrared spectra of SDSS J1256-0224 are similar to those of the sdL4 2MASS J16262034+3925190, and we derive a classification of sdL3.5 based on the preliminary scheme of Burgasser, Cruz, & Kirkpatrick. The kinematics of SDSS J1256-0224 are consistent with membership in the Galactic inner halo, with estimated $UVW$ space velocities indicating a slightly prograde, eccentric and inclined Galactic orbit (3.5 <~ R <~ 11 kpc; |Zmax| = 7.5 kpc). Comparison to synthetic spectra computed with the Phoenix code, including the recent implementation of kinetic condensate formation (Drift-Phoenix), indicate Teff ~ 2100-2500 K and [M/H] ~ -1.5 to -1.0 for logg ~ 5.0-5.5 (cgs), although there are clear discrepancies between model and observed spectra particularly in the red optical region. The stronger metal-oxide bands present in the Drift-Phoenix model spectra, a result of phase-non-equilibrium abundances of grain species, appears to contradict prior suggestions that grain formation is inhibited in metal-poor atmospheres; conclusive statements on the metallicity dependence of grain formation efficiency are as yet premature. In addition, an apparent shift in the temperature scale of L subdwarfs relative to L dwarfs may obviate the need for modified grain chemistry to explain some of the former's unique spectral characteristics.

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arXiv2008

Lithium Depletion of Nearby Young Stellar Associations

We estimate cluster ages from lithium depletion in five pre-main-sequence groups found within 100 pc of the Sun: TW Hydrae Association, Eta Chamaeleontis Cluster, Beta Pictoris Moving Group, Tucanae-Horologium Association and AB Doradus Moving Group. We determine surface gravities, effective temperatures and lithium abundances for over 900 spectra through least squares fitting to model-atmosphere spectra. For each group, we compare the dependence of lithium abundance on temperature with isochrones from pre-main-sequence evolutionary tracks to obtain model dependent ages. We find that the Eta Chamaelontis Cluster and the TW Hydrae Association are the youngest, with ages of 12+/-6 Myr and 12+/-8 Myr, respectively, followed by the Beta Pictoris Moving Group at 21+/-9 Myr, the Tucanae-Horologium Association at 27+/-11 Myr, and the AB Doradus Moving Group at an age of at least 45 Myr (where we can only set a lower limit since the models -- unlike real stars -- do not show much lithium depletion beyond this age). Here, the ordering is robust, but the precise ages depend on our choice of both atmospheric and evolutionary models. As a result, while our ages are consistent with estimates based on Hertzsprung-Russell isochrone fitting and dynamical expansion, they are not yet more precise. Our observations do show that with improved models, much stronger constraints should be feasible: the intrinsic uncertainties, as measured from the scatter between measurements from different spectra of the same star, are very low: around 10 K in effective temperature, 0.05 dex in surface gravity, and 0.03 dex in lithium abundance.

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arXiv2008

A 3D radiative transfer framework: III. periodic boundary conditions

We present a general method to solve radiative transfer problems including scattering in the continuum as well as in lines in 3D configurations with periodic boundary conditions. he scattering problem for line transfer is solved via means of an operator splitting (OS) technique. The formal solution is based on a full characteristics method. The approximate $Λ$ operator is constructed considering nearest neighbors exactly. The code is parallelized over both wavelength and solid angle using the MPI library. We present the results of several test cases with different values of the thermalization parameter and two choices for the temperature structure. The results are directly compared to 1D plane parallel tests. The 3D results agree very well with the well-tested 1D calculations.

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arXiv2008

Consistent simulations of substellar atmospheres and non-equilibrium dust-cloud formation

We aim to understand cloud formation in substellar objects. We combined the non-equilibrium, stationary cloud model of Helling, Woitke & Thi (2008; seed formation, growth, evaporation, gravitational settling, element conservation) with the general-purpose model atmosphere code PHOENIX (radiative transfer, hydrostatic equilibrium, mixing length theory, chemical equilibrium) in order to consistently calculate cloud formation and radiative transfer with their feedback on convection and gas phase depletion. We calculate the complete 1D model atmosphere structure and the chemical details of the cloud layers. The DRIFT-PHOENIX models enable the first stellar atmosphere simulation that is based on the actual cloud formation process. The resulting (T,p) profiles differ considerably from the previous limiting PHOENIX cases DUSTY and COND. A tentative comparison with observations demonstrates that the determination of effective temperatures based on simple cloud models has to be applied with care. Based on our new models, we suggest a mean Teff=1800K for the L-dwarf twin-binary system DENIS J0205-1159 which is up to 500K hotter than suggested in the literature. We show transition spectra for gas-giant planets which form dust clouds in their atmospheres and evaluate photometric fluxes for a WASP-1 type system.

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arXiv2007

Detailed Spectral Modeling of a 3-D Pulsating Reverse Detonation Model: Too Much Nickel

We calculate detailed NLTE synthetic spectra of a Pulsating Reverse Detonation (PRD) model, a novel explosion mechanism for Type Ia supernovae. While the hydro models are calculated in 3-D, the spectra use an angle averaged hydro model and thus some of the 3-D details are lost, but the overall average should be a good representation of the average observed spectra. We study the model at 3 epochs: maximum light, seven days prior to maximum light, and 5 days after maximum light. At maximum the defining Si II feature is prominent, but there is also a prominent C II feature, not usually observed in normal SNe Ia near maximum. We compare to the early spectrum of SN 2006D which did show a prominent C II feature, but the fit to the observations is not compelling. Finally we compare to the post-maximum UV+optical spectrum of SN 1992A. With the broad spectral coverage it is clear that the iron-peak elements on the outside of the model push too much flux to the red and thus the particular PRD realizations studied would be intrinsically far redder than observed SNe Ia. We briefly discuss variations that could improve future PRD models.

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arXiv2007

Alkali Line Profiles in Ultracool White Dwarfs

We present PHOENIX atmosphere models for metal-rich cool white dwarfs using improved line shapes for the Na I and K I resonance doublets. Profiles for collisional broadening due to H2 and He based on the adiabatic representation show strong deviations from Van der Waals interaction at short distances. Comparison with observed spectra that show extremely broadened Na I lines indicates that a He-rich atmospheric composition is required to explain the line strengths and spectral energy distributions. Our current synthetic spectra, using an expansion in powers of density to the third order optimised for brown dwarf atmosphere conditions, significantly underestimate the observed absorption in the far wings, even predicting smaller total line strength than a Lorentzian profile. This is due to the handling of multiple perturber interactions becoming inadequate for the extreme densities of the coolest white dwarfs. The density expansion would have to be extended at least to the 7th order for an accurate treatment of such conditions and might break down altogether in the densest objects. The results of a direct calculation of the unified profile should therefore be used for model atmospheres of cool metal-rich white dwarfs. Qualitative comparison of the full adiabatic profile to the spectrum of WD2356-209 indicates good agreement with the observed line shape. Observations of the coolest white dwarfs may therefore serve as a laboratory for testing the physics of the deeper atmospheres and interiors of brown dwarfs and giant planets.

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arXiv2007

A 3D radiative transfer framework: II. line transfer problems

Higher resolution telescopes as well as 3D numerical simulations will require the development of detailed 3D radiative transfer calculations. Building upon our previous work we extend our method to include both continuum and line transfer. We present a general method to calculate radiative transfer including scattering in the continuum as well as in lines in 3D static atmospheres. The scattering problem for line transfer is solved via means of an operator splitting (OS) technique. The formal solution is based on a long-characteristics method. The approximate $Λ$ operator is constructed considering nearest neighbors {\em exactly}. The code is parallelized over both wavelength and solid angle using the MPI library. We present the results of several test cases with different values of the thermalization parameter and two choices for the temperature structure. The results are directly compared to 1D spherical tests. With our current grid setup the interior resolution is much lower in 3D than in 1D, nevertheless the 3D results agree very well with the well-tested 1D calculations. We show that with relatively simple parallelization that the code scales to very large number of processors which is mandatory for practical applications. Advances in modern computers will make realistic 3D radiative transfer calculations possible in the near future. Our current code scales to very large numbers of processors, but requires larger memory per processor at high spatial resolution.

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arXiv2007

Reddening, Abundances, and Line Formation in SNe II

We present detailed NLTE spectral synthesis models of the Type II supernova 2005cs, which occurred in M51 and for which the explosion time is well determined. We show that previous estimates for the reddening were significantly too high and briefly discuss how this will effect the inferred progenitor mass. We also show that standard CNO-burning enhanced abundances require far too large an oxygen depletion, although there is evidence for a single optical N II line and the sodium abundance shows clear evidence for enhancement over solar both as expected from CNO processing. Finally we calculate a distance using the SEAM method. Given the broad range of distances to M51 in the literature, the determination of a distance using Cepheid variables would be quite valuable.

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arXiv2006

Near-IR integral-field spectroscopy of the companion to GQ Lup

The first substellar companion of possibly planetary mass around a normal star, GQ Lup, has been directly imaged (Neuhaeuser et al., 2005). Besides the unknown formation history, the mass of such an object is a criterion to decide about its true nature. We aim to determine the physical properties of the GQ Lup companion - effective temperate and surface gravity, and thus its mass independently from evolutionary models. We use the adaptive optics integral-field spectrograph SINFONI at the VLT for near-infrared spectroscopy from 1.1 to 2.5 um with a resolution of R = 2500--4000. We compare these spectra with synthetic atmospheric models (GAIA v2.0 cond). From the complete set of spectra we find a consistent effective temperature of Teff = 2600 +/- 100 K and surface gravity of log{g} = 3.7 +/- 0.5. Combined with a slightly revised luminosity of log(L/L_(\sun)) = -2.25 +/- 0.24 for the companion, we determine a radius of R = 3.50 (+1.50/-1.03) Jupiterradii and thus a mass of ~ 25 Jupitermasses. Due to the large uncertainty of the surface gravity, the mass could range from 4 to 155 Jupitermasses. By comparing the paramaters of the companion of GQ Lup to the ones of 2MASS J05352184-0546085, published by Stassun et al. (2006), we conclude that the companion to GQ Lup A has a mass lower than 36 Jupitermasses.

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arXiv2006

Accurate M Dwarf Metallicities from Spectral Synthesis: A Critical Test of Model Atmospheres

We describe a method for accurately determining M dwarf metallicities with spectral synthesis based on abundance analyses of visual binary stars. We obtained high resolution, high signal-to-noise spectra of each component of five visual binary pairs at McDonald Observatory. The spectral types of the components range from F7 to K3 V for the primaries and M0.5 to M3.5 V for the secondaries. We have determined the metallicities of the primaries differentially with respect to the Sun by fitting synthetic spectra to Fe \textsc{i} line profiles in the observed spectra. In the course of our analysis of the M dwarf secondaries we have made significant improvements to the PHOENIX cool-star model atmospheres and the spectrum analysis code MOOG. Our analysis yields a RMS deviation of 0.11 dex in metallicity values between the binary pairs. We estimate the uncertainties in the derived stellar parameters for the M dwarfs to be 48 K, 0.10 dex, 0.12 dex, 0.15 km s$^{-1}$, and 0.20 km s$^{-1}$ for {T}$_{eff}$, log \textsl{g}, [M/H], $ξ$, and $η$ respectively. Accurate stellar evolutionary models are needed to progress further in the analysis of cool-star spectra; the new model atmospheres warrant recalculation of the evolutionary models.

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arXiv2006

Resolving Stellar Atmospheres I: The H alpha line and comparisons to microlensing observations

We present work on H alpha spectral line characteristics in PHOENIX stellar model atmospheres and their comparison to microlensing observations. We examine in detail the H alpha equivalent width (EW) and the line shape characteristics for effective temperatures of 4500K< Teff < 5600K where H alpha is a strong spectral feature. We find that H alpha EW in models calculated under the assumption of local thermodynamic equilibrium (LTE) is up to 15% smaller than in models without this assumption, non-LTE models (NLTE) and that line shapes vary significantly for the two model types. A comparison with available high quality microlensing data, capable of tracing H alpha absorption across the face of one G5III giant, shows that the LTE model that fits the EW best is about 100K hotter than and the best-fitting NLTE model has a similar Teff as predicted by the spectral type analysis of the observed star but agree within the uncertainties of the observationally derived temperature. Neither LTE nor NLTE models fit the line shape well. We suspect unmodelled chromospheric emission. Line shape diagnostics suggest lower gravities than derived for the star and are unacceptable low in the case of the LTE models. We show that EW alone is insufficient for comparison to stellar model atmospheres, but combined with a new shape parameter we define is promising. In stellar parameter ranges where the H alpha line is strong, a NLTE approach of modeling stellar atmospheres is not only beneficial but mandatory.

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