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Posts: 131433
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WD2356-209
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Title: Alkali Line Profiles in Ultracool White Dwarfs
Authors: Derek Homeier, Nicole F. Allard, Christine M. S. Johnas, Peter H. Hauschildt, France Allard
(Version v2)

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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Ultra-cool dwarf
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Scientists from the University of Hertfordshire have discovered a rare binary system consisting of a white dwarf, a Sun-like star that has reached the end of its life, and an ultra-cool dwarf, which is the smallest kind of star. To make the discovery even more unusual, the co-orbiting pair has by far the widest separation ever detected in this type of binary system.

This is a record breaking discovery for a system of this kind. In the other few binary cases that are known, the objects are relatively close together. In this new system, the objects are 600 billion kilometres apart which is hundreds of times wider - Avril Day-Jones.

The group from Hertfordshire believes that the two objects formed at roughly the same time and were originally much closer together. During the death-throes of the white dwarfs progenitor star, forces induced when gas and dust from the star were thrown off into space caused the ultra-cool dwarf spiral out to its remote position.

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Posts: 131433
Date:
UltraCool White Dwarf
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COMBO-17 J114356.08-0144032

Christian Wolf of the University of Oxford has discovered a rare Ultra-cool white dwarf.

Only seven of these ultra cool white dwarfs, with surface temperatures below 4,000 kelvins, were known to exist before.

Regular white dwarfs are dead stars in which nuclear reactions have stopped.
Ultra-cool white dwarfs are much cooler and fainter than even the coldest white dwarf, making them nearly impossible to detect. Their extremely low temperatures suggest they somehow cooled faster than their normal counterparts.

The mechanism that could cause ultra-cool white dwarfs to cool so quickly is still unknown, but scientists believe it might be key to understanding how early stars in the galaxy formed and died.


Position(J2000): RA 11h43m56.s08 Dec -01.44'03.''21
Finding chart for COMBO-17 J114356.08-0144032 taken from a 5-hour R-band image obtained with WFI at the 2.2 m-telescope at La Silla in February 2000. The frame is 120'' on a side. North is up and East is on the left.


The dwarf, named COMBO-17 J114356.08-0144032, was found when Christian Wolf was comparing infrared magnitudes of stars from the COMBO-17 deep extragalactic survey data. COMBO-17 was a survey designed to pinpoint redshifts of distant galaxies and quasars. All of the magnitudes appeared as expected except for one, which further calculations could not explain.

"The unusual colours were real, and unlike everything I had seen before in COMBO-17" - Christian Wolf .


Colour-colour-diagram of white dwarfs: Large symbols show all known ultra-cool white dwarfs, where COMBO-17 J1143 is marked by a circle. Its original SDSS photometry is shown as the data point with error bars. Normal white dwarfs are shown as small symbols.
Objects which are redder than normal white dwarfs in r-i have M dwarf companions while bluer ones show collision-induced absorption (CIA) in optical bands.


The colours were similar to those of ultra-cool white dwarfs, and the star's spectrum was featureless - a sign that it was an ultra-cool white dwarf .
Due to the extreme faintest it may perhaps be the most distant.

due to this faintness, and lack of spectral features, little can be determined about the star's physical properties. The composition, the number of companions it has, and its mass cannot be calculated without spectral features.

"At this stage, we are still in the realm of complete speculation. We can only rule out certain scenarios"- Christian Wolf .

However, measuring the star's parallax may yield its luminosity.
The parallax for the first ultra-cool white dwarf, LHS 3250 , is known.
Ultra-cool white dwarfs SDSS J1337+00, LHS 3250, and LHS 1402 also share the same colours as this one.
The ultra-cool white dwarf, SDSS J0947, appears to be in a binary system with a slightly warmer (Teff~5000 K) white dwarf companion.
All have significant proper motions.
However, the information raised more questions than it answered.

"Unless we get more, we have no idea whether the result is typical for ultra-cool white dwarfs or not"- Christian Wolf .

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