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Register a new userDiscovery of photospheric CaX emission lines in the far-UV spectrum of the hottest known white dwarf (KPD0005+5106)
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For the first time, we have identified photospheric emission lines in the far-UV spectrum of a white dwarf. They were discovered in the Far Ultraviolet Spectroscopic Explorer spectrum of the hot (Teff~200,000 K) DO white dwarf KPD0005+5106 and they stem from extremely highly ionized calcium (CaX 1137, 1159 Ang). Their photospheric origin is confirmed by non-LTE line-formation calculations. This is the highest ionisation stage of any element ever observed in a stellar photosphere. Calcium has never been detected before in any hot white dwarf or central star of planetary nebula. The calcium abundance determination for KPD0005+5106 (1-10 times solar) is difficult, because the line strengths are rather sensitive to current uncertainties in the knowledge of effective temperature and surface gravity. We discuss the possibility that the calcium abundance is much lower than expected from diffusion/levitation equilibrium theory. The same emission lines are exhibited by the [WCE]-type central star NGC2371. Another CaX line pair (1461, 1504 Ang) is probably present in a Hubble Space Telescope spectrum of the PG1159-type central star NGC246.
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KPD0005+5106 is the hottest known helium-rich white dwarf. We have identified NeVIII lines in UV and optical spectra and conclude that it is significantly hotter than previously thought, namely Teff=200,000 K instead of 120,000 K. This is a possible explanation for the observed hard X-ray emission as being of photospheric origin. Concerning its evolutionary state, we suggest that KPD0005+5106 is not a descendant of a PG1159 star but more probably related to the O(He) stars and RCrB stars.
The compact object in the interacting binary AR Sco has widely been presumed to be a rapidly rotating, magnetized white dwarf (WD), but it has never been detected directly. Isolating its spectrum has proven difficult because the spin-down of the WD generates pulsed synchrotron radiation that far outshines the WDs photosphere. As a result, a previous study of AR Sco was unable to detect the WD in the averaged far-ultraviolet spectrum from a Hubble Space Telescope (HST) observation. In an effort to unveil the WDs spectrum, we reanalyze these HST observations by calculating the average spectrum in the troughs between synchrotron pulses. We identify weak spectral features from the previously unseen WD and estimate its surface temperature to be 11500$pm$500K. Additionally, during the synchrotron pulses, we detect broad Lyman-$alpha$ absorption consistent with hot WD spectral models. We infer the presence of a pair of hotspots, with temperatures between 23000K and 28000K, near the magnetic poles of the WD. As the WD is not expected to be accreting from its companion, we describe two possible mechanisms for heating the magnetic poles. The Lyman-$alpha$ absorption of the hotspots appears relatively undistorted by Zeeman splitting, constraining the WDs field strength to be 100 MG, but the data are insufficient to search for the subtle Zeeman splits expected at lower field strengths.
We spectroscopically measure multiple hydrogen Balmer line profiles from laboratory plasmas to investigate the theoretical line profiles used in white dwarf atmosphere models. X-ray radiation produced at the Z Pulsed Power Facility at Sandia National Laboratories initiates plasma formation in a hydrogen-filled gas cell, replicating white dwarf photospheric conditions. Here we present time-resolved measurements of H$beta$ and fit this line using different theoretical line profiles to diagnose electron density, $n_{rm e}$, and $n=2$ level population, $n_2$. Aided by synthetic tests, we characterize the validity of our diagnostic method for this experimental platform. During a single experiment, we infer a continuous range of electron densities increasing from $n_{rm e}sim4$ to $sim30times10^{16},$cm$^{-3}$ throughout a 120-ns evolution of our plasma. Also, we observe $n_2$ to be initially elevated with respect to local thermodynamic equilibrium (LTE); it then equilibrates within $sim55,$ns to become consistent with LTE. This supports our electron-temperature determination of $T_{rm e}sim1.3,$eV ($sim15,000,$K) after this time. At $n_{rm e}gtrsim10^{17},$cm$^{-3}$, we find that computer-simulation-based line-profile calculations provide better fits (lower reduced $chi^2$) than the line profiles currently used in the white dwarf astronomy community. The inferred conditions, however, are in good quantitative agreement. This work establishes an experimental foundation for the future investigation of relative shapes and strengths between different hydrogen Balmer lines.
We report ground-based observations at 0.91 microns of the occultation of the hot Jupiter WASP-33b by its A5 host star. We measure the planet to be 0.109 +/- 0.030 per cent as bright as its host star at 0.91 microns. This corresponds to a brightness temperature, T_B = 3620 +200 -250 K, significantly higher than the zero-albedo equilibrium temperature for both isotropic re-radiation (2750 +/- 37 K) and uniform day-side only re-radiation (3271 +/- 44 K), but consistent with the zero-redistribution temperature (3515 +/- 47 K). This indicates that the heat redistribution from the day-side of WASP-33b to the night side is inefficient, and further suggest that there is immediate re-radiation, and therefore little or no redistribution, of heat within the day-side. We also detected the stellar pulsations of WASP-33, which we model as the sum of four sinusoids, with periods of between 42 and 77 minutes and amplitudes of 0.5 to 1.5 mmag.
We have detected an ionized nebula around the hot DO white dwarf KPD 0005+5106, and used the [OIII]/H-alpha ratios and nebular velocities to separate this nebula from the background HII region of AO Cas. The angular size of the [OIII] nebula of KPD 0005+5106 is ~3 deg. The velocity of this nebula is similar to those of the local interstellar HI gas and the interstellar/circumstellar absorption lines in UV spectra of KPD 0005+5106, but has a large offset from those of the stellar photospheric lines. The mass of the ionized nebula, ~70 Msun, indicates that it consists of interstellar material and that the velocity offset between the star and the ambient medium should not be interpreted as a wind outflow. We have also analyzed the ROSAT PSPC observation of KPD 0005+5106 to determine its hard X-ray luminosity. Using the L_X/L_bol relation for late-type stars and the lack of obvious near-IR excess of KPD 0005+5106, we exclude the possible existence of a binary companion with coronal activity. Without a wind outflow, the presence of stellar OVIII emission requires that X-rays at energies greater than 0.871 keV are present in the vicinity of KPD 0005+5106. This hard X-ray emission is most puzzling as neither photospheric emission at such high energies nor a high-temperature corona is expected from current stellar atmospheric models of KPD 0005+5106. X-ray observations with high angular resolution and sensitivity are needed to confirm the positional coincidence and to enable X-ray spectral analyses for constraining the physical origin of the hard X-ray emission from KPD 0005+5106.
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