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Register a new userDiscovery of Molecular Hydrogen in White Dwarf Atmospheres
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S. Xu
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With the Cosmic Origins Spectrograph onboard the Hubble Space Telescope, we have detected molecular hydrogen in the atmospheres of three white dwarfs with effective temperatures below 14,000 K, G29-38, GD 133 and GD 31. This discovery provides new independent constraints on the stellar temperature and surface gravity of white dwarfs.
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Dense, He-rich atmospheres of cool white dwarfs represent a challenge to the modeling. This is because these atmospheres are constituted of a dense fluid in which strong multi-atomic interactions determine their physics and chemistry. Therefore, the ideal-gas-based description of absorption is no longer adequate, which makes the opacities of these atmospheres difficult to model. This is illustrated with severe problems in fitting the spectra of cool, He-rich stars. Good description of the infrared (IR) opacity is essential for proper assignment of the atmospheric parameters of these stars. Using methods of computational quantum chemistry we simulate the IR absorption of dense He/H media. We found a significant IR absorption from He atoms (He-He-He CIA opacity) and a strong pressure distortion of the H$_2$-He collision-induced absorption (CIA). We discuss the implication of these results for interpretation of the spectra of cool stars.
We present the discovery of the first T dwarf + white dwarf binary system LSPM 1459+0857AB, confirmed through common proper motion and spectroscopy. The white dwarf is a high proper motion object from the LSPM catalogue that we confirm spectroscopically to be a relatively cool (Teff=5535+-45K) and magnetic (B~2MG) hydrogen-rich white dwarf, with an age of at least 4.8Gyrs. The T dwarf is a recent discovery from the UKIRT Infrared Deep Sky Survey (ULAS 1459+0857), and has a spectral type of T4.5+-0.5 and a distance in the range 43-69pc. With an age constraint (inferred from the white dwarf) of >4.8Gyrs we estimate Teff=1200-1500K and logg=5.4-5.5 for ULAS 1459+0857, making it a benchmark T dwarf with well constrained surface gravity. We also compare the T dwarf spectra with the latest LYON group atmospheric model predictions, which despite some shortcomings are in general agreement with the observed properties of ULAS 1459+0857. The separation of the binary components (16,500-26,500AU, or 365 arcseconds on the sky) is consistent with an evolved version of the more common brown dwarf + main-sequence binary systems now known, and although the system has a wide separation, it is shown to be statistically robust as a non spurious association. The observed colours of the T dwarf show that it is relatively bright in the z band compared to other T dwarfs of similar type, and further investigation is warranted to explore the possibility that this could be a more generic indicator of older T dwarfs. Future observations of this binary system will provide even stronger constraints on the T dwarf properties, and additional systems will combine to give a more comprehensively robust test of the model atmospheres in this temperature regime.
We report the discovery of J1953-1019, the first resolved triple white dwarf system. The triplet consists of an inner white dwarf binary and a wider companion. Using Gaia DR2 photometry and astrometry combined with our follow-up spectroscopy, we derive effective temperatures, surface gravities, masses and cooling ages of the three components. All three white dwarfs have pure-hydrogen (DA) atmospheres, masses of 0.60-0.63 Msun and cooling ages of 40-290 Myr. We adopt eight initial-to-final mass relations to estimate the main sequence progenitor masses (which we find to be similar for the three components, 1.6-2.6 Msun) and lifetimes. The differences between the derived cooling times and main sequence lifetimes agree for most of the adopted initial-to-final mass relations, hence the three white dwarfs in J1953-1019 are consistent with coeval evolution. Furthermore, we calculate the projected orbital separations of the inner white dwarf binary (303.25 +- 0.01 au) and of the centre of mass of the inner binary and the outer companion (6398.97 +- 0.09 au). From these values, and taking into account a wide range of possible configurations for the triplet to be currently dynamically stable, we analyse the future evolution of the system. We find that a collision between the two inner white dwarfs due to Lidov-Kozai oscillations is unlikely, though if it occurs it could result in a sub-Chandrasekhar Type Ia supernova explosion.
Both CO and SiO have been observed at early and late phases in SN 1987A. H_2 was predicted to form at roughly the same time as these molecules, but was not detected at early epochs. Here we report the detection of NIR lines from H_2 at 2.12 mu and 2.40 mu in VLT/SINFONI spectra obtained between days 6489 and 10,120. The emission is concentrated to the core of the supernova in contrast to H-alpha and approximately coincides with the [Si I]/[Fe II] emission detected previously in the ejecta. Different excitation mechanisms and power sources of the emission are discussed. From the nearly constant H_2 luminosities we favour excitation resulting from the 44Ti decay.
I was one of the six people most closely involved in the discovery of the first magnetic white dwarf in 1970, now 50 years ago. Thinking back on this event, I have realised that the discovery occurred when and how it did because of a series of lucky coincidences along a strange and winding path. In this paper I recount the events as I recall them, and reflect on how those unlikely coincidences helped us to succeed.
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