arXiv subjects

Peter H. Hauschildt

Publications and source records attributed to Peter H. Hauschildt.

At least 73 records · Page 4Linked to original sources

arXiv2002

Determination of Primordial Metallicity and Mixing in the Type IIP Supernova 1993W

We present the results of a large grid of synthetic spectra and compare them to early spectroscopic observations of SN 1993W. This supernova was discovered close to its explosion date and at a recession velocity of 5400 km/s is located in the Hubble flow. We focus here on two early spectra that were obtained approximately 5 and 9 days after explosion. We parameterize the outer supernova envelope as a power-law density profile in homologous expansion. In order to extract information on the value of the parameters a large number of models was required. We show that very early spectra combined with detailed models can provide constraints on the value of the power law index, the ratio of hydrogen to helium in the surface of the progenitor, the progenitor metallicity and the amount of radioactive nickel mixed into the outer envelope of the supernova. The spectral fits reproduce the observed spectra exceedingly well. The spectral results combined with the early photometry predict that the explosion date was 4.7 \pm 0.7 days before the first spectrum was obtained. The ability to obtain the metallicity from early spectra make SN IIP attractive probes of chemical evolution in the universe and by showing that we have the ability to pin down the parameters of the progenitor and mixing during the supernova explosion, it is likely to make SN IIP useful cosmological distance indicators which are at the same time complementary to SNe Ia.

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arXiv2002

Methane and the Spectra of T Dwarfs

We have updated our PHOENIX model atmospheres and theoretical spectra for ultracool dwarfs with new opacity data for methane based on line strength predictions with the STDS software. By extending the line list to rotational levels of J=40 we can significantly improve the shape of the near-IR absorption features of CH_4, and in addition find an enhanced blanketing effect, resulting in up to 50% more flux emerging in the J band than seen in previous models, which may thus contribute to the brightening in J and blue IR colors observed in T dwarfs.

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arXiv2002

Highlights of Stellar Modeling with PHOENIX

We briefly describe the current version of the PHOENIX code. We then present some illustrative results from the modeling of Type Ia and Type II supernovae, hot stars, and irradiated giant planets. Good fits to observations can be obtained, when account is taken for spherically symmetric, line-blanketed, static or expanding atmospheres.

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arXiv2002

Model atmosphere analyses of post-common envelope stars

Understanding post-common-envelope evolution is important in the studies of close-binary systems. The majority of all interacting binaries with a compact object in their system (e.g. cataclysmic variables, X-ray binaries) are thought to have passed through a common-envelope (CE) phase. Current models of interacting binaries all assume, as a starting point, that there is no significant modification of the secondary star compared with a normal star of similar spectral type. The extent to which the CE significantly alters the composition of the secondary star has yet to be determined. We are studying the M-type secondary in the pre-CV system EG UMa, in order to determine its atmospheric parameters by comparison with synthetic spectra generated using PHOENIX. Absorption lines due to several elements have been used in investigating the stellar parameters of effective temperature, gravity and over elemental abundance. In addition, we are searching for anomalies due to s-process elements (such as Ba, Sr, Rb, Y), which may have been deposited in the atmosphere during the CE stage, and prove that CE evolution has occurred. EG UMa displays strong YO absorption bands, which are normally associated with giants and S stars. These bands may have been formed as a result of an Y-abundance enhancement introduced by the CE, which would be consistent with the observed possible Rb and Sr enrichment.

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arXiv2002

Detectibility of Hydrogen Mixing in Type Ia Supernova pre-Maximum Spectra

The presence of a small amount of hydrogen is expected in most single degenerate scenarios for producing a Type Ia supernova (SN Ia). While hydrogen may be detected in very early high resolution optical spectra, in early radio spectra, and in X-ray spectra, here we examine the possibility of detecting hydrogen in early low resolution spectra such as those that will be obtained by proposed large scale searches for nearby SNe Ia. We find that definitive detections will require both very early spectra (less than 5 days after explosion) and perhaps slightly higher amounts of hydrogen than are currently predicted to be mixed into the outer layers of SNe Ia. Thus, the non-detection of hydrogen so far does not in and of itself rule out any current progenitor models. Nevertheless, very early spectra of SNe Ia will provide significant clues to the amount of hydrogen present and hence to the nature of the SN Ia progenitor system. Spectral coverage in both the optical and IR will be required to definitively identify hydrogen in low resolution spectra.

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arXiv2002

Detailed Spectroscopic Analysis of SN 1987A: The Distance to the LMC using the SEAM method

Supernova 1987A remains the most well-studied supernova to date. Observations produced excellent broad-band photometric and spectroscopic coverage over a wide wavelength range at all epochs. We model the observed spectra from Day 1 to Day 81 using a hydrodynamical model. We show that good agreement can be obtained at times up to about 60 days, if we allow for extended nickel mixing. Later than about 60 days the observed Balmer lines become stronger than our models can reproduce. We show that this is likely due to a more complicated distribution of gamma-rays than we allow for in our spherically symmetric calculations. We present synthetic light curves in UBVRIJHK and a synthetic bolometric light curve. Using this broad baseline of detailed spectroscopic models we find a distance modulus mu = 18.5 +/- 0.2 using the SEAM method of determining distances to supernovae. We find that the explosion time agrees with that of the neutrino burst and is constrained at 68% confidence to within +/- 0.9 days. We argue that the weak Balmer lines of our detailed model calculations casts doubt on the accuracy of the purely photometric EPM method. We also suggest that Type IIP supernovae will be most useful as distance indicators at early times due to a variety of effects.

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arXiv2002

Non-LTE Effects of Na I in the Atmosphere of HD209458b

The recent announcement that sodium absorption has been observed in the atmosphere of HD209458b, the only EGP observed to transit its parent star, is the first direct detection of an EGP atmosphere. We explore the possibility that neutral sodium is {\em not} in local thermodynamic equilibrium (LTE) in the outer atmosphere of irradiated EGPs and that the sodium concentration may be underestimated by models that make the LTE assumption. Our results indicate that it may not be necessary to invoke excessive photoionization, low metallicity, or even high altitude clouds to explain the observations.

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arXiv2001

Atmospheric Analysis of the M/L- and M/T-Dwarf Binary Systems LHS 102 and Gliese 229

We present 0.9-2.5um spectroscopy with R~800 and 1.12-1.22um spectroscopy with R~5800 for the M dwarfs Gl 229A and LHS 102A, and for the L dwarf LHS 102B. We also report IZJHKL' photometry for both components of the LHS 102 system, and L' photometry for Gl 229A. The data are combined with previously published spectroscopy and photometry to produce flux distributions for each component of the kinematically old disk M/L-dwarf binary system LHS 102 and the kinematically young disk M/T-dwarf binary system Gliese 229. The data are analyzed using synthetic spectra generated by the latest "AMES-dusty" and "AMES-cond" models by Allard & Hauschildt. Although the models are not able to reproduce the overall slope of the infrared flux distribution of the L dwarf, most likely due to the treatment of dust in the photosphere, the data for the M dwarfs and the T dwarf are well matched. We find that the Gl 229 system is metal-poor despite having kinematics of the young disk, and that the LHS 102 system has solar metallicity. The observed luminosities and derived temperatures and gravities are consistent with evolutionary model predictions if the Gl 229 system is very young (age ~30 Myr) with masses (A,B) of (0.38,>0.007)M(sun), and the LHS 102 system is older, aged 1-10 Gyr with masses (A,B) of (0.19,0.07)M(sun).

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arXiv2001

Effective Temperatures of late L dwarfs and the onset of Methane signatures

We present a spectral analysis of a sample of late L dwarfs. We use our latest model atmospheres and synthetic spectra and optical and K band spectra to determine effective temperatures. We derive effective temperatures of 1400 K to 1700 K for L8 to L6 dwarfs. The analysis demonstrates that our recent models that rain out the formed dust completely are applicable to optical spectra of late L dwarfs and that more consistent models are needed for intermediate L dwarfs and for infrared spectra. We compare the results for the effective temperatures with the temperatures of the onset of Methane formation. Our models predict Methane absorption at 3.3micron to occur at about 400 K higher temperatures than Methane absorption at 2.2micron. This is consistent with our data and previous observations which show Methane absorption at 3.3micron but not at 2.2micron in late L dwarfs.

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arXiv2001

The Limiting Effects of Dust in Brown Dwarf Model Atmospheres

We present opacity sampling model atmospheres, synthetic spectra and colors for brown dwarfs and very low mass stars in two limiting case of dust grain formation: 1) inefficient gravitational settling i.e. the dust is distributed according to the chemical equilibrium predictions, 2) efficient gravitational settling i.e. the dust forms and depletes refractory elements from the gas, but their opacity does not affect the thermal structure. The models include the formation of over 600 gas phase species, and 1000 liquids and crystals, and the opacities of 30 different types of grains including corundum (Al$_2$O$_3$), the magnesium aluminum spinel MgAl$_2$O$_4$, iron, enstatite (MgSiO$_3$), forsterite (Mg$_2$SiO$_4$), amorphous carbon, SiC, and a number of calcium silicates. The models extend from the beginning of the grain formation regime well into the condensation regime of water ice ($\teff= 3000 - 100$ K) and encompasses the range of $\log g= 2.5 - 6.0$ at solar metallicity. We find that silicate dust grains can form abundantly in the outer atmospheric layers of red and brown dwarfs with spectral type later than M8. The greenhouse effects of dust opacities provide a natural explanation for the peculiarly red spectroscopic distribution of the latest M dwarfs and young brown dwarfs. The grainless (Cond) models on the other hand, correspond closely to methane brown dwarfs such as Gliese 229B. We also recover that the $λ$5891,5897ÅNa I D and $λ$7687,7701ÅK I resonance doublets plays a critical role in T dwarfs where their red wing define the pseudo-continuum from the $I$ to the $Z$ bandpass.

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arXiv2001

Infrared Spectroscopy of Substellar Objects in Orion

We present broad band spectra of a sample of 21 low luminosity sources in the Trapezium Cluster, with masses in the range 0.008 - 0.10 Msun (assuming an age of 1 Myr). These were selected for low extinction in most cases and are located west of the brighter nebulosity. The spectra are in the H bandpass (1.4-1.95 um) and K bandpass (1.9-2.5 um) also for most of the brighter sources, with a resolution of 50 nm. They were taken with UKIRT using the CGS4 spectrometer. Absorption by water vapour bands is detected in all the substellar candidates except one, which is a highly reddened object with strong H2 emission and an anomalously blue (I-J) colour, implying that it is a very young cluster member with circumstellar matter. The observation of prominent water vapour bands confirms the low Effective Temperatures implied by our (I-J) colour measurements in an earlier paper and would imply late M or L spectral types if these were older field dwarfs. However, the profiles of the H bandpass spectra are very different from those of field dwarfs with similar water absorption strength, demonstrating that they are not foreground or background objects. In addition, the CO absorption bands at 2.3 um and the NaI absorption feature at 2.21 um are very weak for such cool sources. All these features are quite well reproduced by the AMES-Dusty-1999 model atmospheres of Allard et al.(2000,2001), and arise from the low gravities predicted for the Trapezium sources. This represents a new proof of the substellar status of our sources, independent of the statistical arguments for low contamination, which are reexamined here. The very late spectral types of the planetary mass objects and very low mass brown dwarfs demonstrate that they are cluster members, since they are too luminous to be field dwarfs in the background.

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arXiv2001

Parallel Implementation of the PHOENIX Generalized Stellar Atmosphere Program. III: A parallel algorithm for direct opacity sampling

We describe two parallel algorithms for line opacity calculations based on a local file and on a global file approach. The performance and scalability of both approaches is discussed for different test cases and very different parallel computing systems. The results show that a global file approach is more efficient on high-performance parallel supercomputers with dedicated parallel I/O subsystem whereas the local file approach is very useful on farms of workstations, e.g., cheap PC clusters.

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arXiv2001

Irradiated Planets

We have modeled irradiated planets located near a dM5 and a G2 primary star. The impinging radiation field was explicitly included in the solution of the radiative transfer equation and in the computation of the atmospheric structure. We find that large errors in both the thermal and reflected flux will result from models which do not include the impinging radiation in a self-consistent manner. A cool (teff = 500K) and a hot (teff = 1000K) planet were modeled at various orbital separations from both the dM5 and the G2 primary. In all scenarios, we compared the effects of the irradiation in two extreme cases: one where dust clouds form and remain suspended in the atmosphere, and another where dust clouds form but completely settle out of the atmosphere. The atmospheric structure and emergent spectrum strongly depend on the presence or absence of dust clouds. We find that, in the absence of dust opacity, the impinging radiation significantly alters the innermost layers of an EGP atmosphere and that they are actually brighter in the optical than dusty planets. Our models also indicate that the planet-to-star brightness ratio in the optical will be less that 1 x 10^-5 for objects like tau bootis which is consistent with recently reported upper limit values.

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arXiv2001

NLTE Synthetic Spectral Fits to the Type Ia Supernova 1994D in NGC 4526

We have fit the normal, well observed, Type Ia Supernova (SN Ia) SN 1994D with non-LTE spectra of the deflagration model W7. We find that well before maximum luminosity W7 fits the optical spectra of SN 1994D. After maximum brightness the quality of the fits weakens as the spectrum forms in a core rich in iron peak elements. We show the basic structure of W7 is likely to be representative of the typical SN Ia. We have shown that like W7, the typical SN Ia has a layer of unburned C+O composition at v > 15000 \kmps, followed by layers of C-burned and O-burned material with a density structure similar to W7. We present UVOIR (UBVRIJKH) synthetic photometry and colors and compare with observation. We have computed the distance to the host galaxy, NGC 4526, obtaining a distance modulus of μ= 30.8 \pm 0.3. We discuss further application of this direct measurement of SNe Ia distances. We also discuss some simple modifications to W7 that could improve the quality of the fits to the observations.

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arXiv2001

Nickel Mixing in the Outer Layers of SN 1987A

Supernova 1987A remains the most well-observed and well-studied supernova to date. Observations produced excellent broad-band photometric and spectroscopic coverage over a wide wavelength range at all epochs. Here, we focus on the very early spectroscopic observations. Only recently have numerical models been of sufficient detail to accurately explain the observed spectra. In SN 1987A, good agreement has been found between observed and synthetic spectra for day one, but by day four, the predicted Balmer lines become much weaker than the observed lines. We present the results of work based on a radiation-hydrodynamic model by Blinnikov and collaborators. Synthetic non-LTE spectra generated from this model by the general radiation transfer code PHOENIX strongly support the theory that significant mixing of nickel into the outer envelope is required to maintain strong Balmer lines. Preliminary results suggest a lower limit to the average nickel mass of 1.0 \times 10^{-5} solar masses is required above 5000 \kmps by day four. PHOENIX models thus have the potential to be a sensitive probe for nickel mixing in the outer layers of a supernova.

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arXiv2001

Analysis of Keck HIRES spectra of early L-type dwarfs

We present analyses of high resolution and medium resolution spectra of early L dwarfs. We used our latest set of model atmospheres to reproduce and analyze the observed features. We can model the optical flux and the atomic line profiles with the best accuracy to date. The models used to reproduce the observations include dust condensation and dust opacities. Compared to previous studies using older models we find that our dust treatment is much improved. The derived parameters for the objects are well in the expected range for old very low mass objects. This is also supported by the absence of Li in most of the objects. For the objects showing Li we can be almost certain that those are brown dwarfs. However, a spectral analysis in general, and this one in particular can only very roughly determine mass and age.

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arXiv2000

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.

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arXiv2000

Analysis of the Type IIn Supernova 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 \tt 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 fit 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 $\dot M \sim 0.0001-0.001$\msol yr$^{-1}$ is inferred for a wind speed of 100-1000 \kmps. We discuss how future self-consistent models will better clarify the underlying progenitor structure.

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