The Discovery of Oxygen‐rich Chromospheres in Two Massive DQ White Dwarfs

Provençal, J. L.; Shipman, H. L.; MacDonald, J.

doi:10.1086/430121

Cited by 11 publications

(9 citation statements)

References 30 publications

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“…If this emission is real, the most plausible identification is O i. We note that a few cool DQ WDs show oxygen emission lines (Provencal et al 2005), the origin of which is still uncertain. A higher S/N spectrum will be required to determine whether this feature in SDSS is genuine.…”

Section: Field Measurement For An Offset Dipolementioning

confidence: 83%

The incidence of magnetic fields in cool DZ white dwarfs

Hollands¹,

Gänsicke²,

Koester³

2015

Monthly Notices of the Royal Astronomical Society

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Little is known about the incidence of magnetic fields among the coolest white dwarfs. Their spectra usually do not exhibit any absorption lines as the bound-bound opacities of hydrogen and helium are vanishingly small. Probing these stars for the presence of magnetic fields is therefore extremely challenging. However, external pollution of a cool white dwarf by, e.g., planetary debris, leads to the appearance of metal lines in its spectral energy distribution. These lines provide a unique tool to identify and measure magnetism in the coolest and oldest white dwarfs in the Galaxy.We report the identification of 7 strongly metal polluted, cool (T eff < 8000 K) white dwarfs with magnetic field strengths ranging from 1.9 to 9.6 MG. An analysis of our larger magnitude-limited sample of cool DZ yields a lower limit on the magnetic incidence of 13 ± 4 percent, noticeably much higher than among hot DA white dwarfs.1 WDs showing only hydrogen/helium lines in their spectra are classified DA/DB, with only metal lines as DZ, and without any spectral lines as DC. Magnetic DA WDs where magnetism is dec 2015 RAS

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Section: Field Measurement For An Offset Dipolementioning

confidence: 83%

The incidence of magnetic fields in cool DZ white dwarfs

Hollands¹,

Gänsicke²,

Koester³

2015

Monthly Notices of the Royal Astronomical Society

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“…This is the first detection of photospheric emission lines in this spectral range of any hot (pre-) white dwarf. Provencal et al (2005) discovered low-ionisation emission lines in HST/STIS UV spectra of two relatively cool (T eff ≈ 12 500 K) He-rich white dwarfs (spectral type DQ). It was shown, however, that they are chromospheric in origin.…”

Section: Discussionmentioning

confidence: 97%

Discovery of photospheric Ca X emission lines in the far-UV spectrum of the hottest known white dwarf (KPD 0005+5106)

Werner¹,

Rauch²,

Kruk

2008

A&A

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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 (T eff ≈ 200 000 K) DO white dwarf KPD 0005+5106 and they stem from extremely highly ionized calcium (Ca x λλ 1137, 1159 Å). 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 KPD 0005+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 equi-

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“…It is tempting to speculate a bit further based on this potentially new insight into the white dwarf evolution, and include the only other two white dwarfs suspected to have chromospheric emission; the massive DQe stars G227-5 and G35-26. Both stars have emission lines seen only in the ultraviolet, where only the heavy elements C and O are seen towards both stars, with further lines of N (plus Mg and Si possibly) observed from G35-36 (Provencal, Shipman & MacDonald 2005). If the DAHe stars are emitting from intrinsic chromospheres and all have hydrogen atmospheres, it is tempting to co-identify these two as non-DA counterparts.…”

Section: Intrinsic Chromospheric Activity In White Dwarfsmentioning

confidence: 99%

A test of the planet–star unipolar inductor for magnetic white dwarfs

Walters

Farihi

Marsh

et al. 2021

Monthly Notices of the Royal Astronomical Society

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Despite thousands of spectroscopic detections, only four isolated white dwarfs exhibit Balmer emission lines. The temperature inversion mechanism is a puzzle over 30 years old that has defied conventional explanations. One hypothesis is a unipolar inductor that achieves surface heating via ohmic dissipation of a current loop between a conducting planet and a magnetic white dwarf. To investigate this model, new time-resolved spectroscopy, spectropolarimetry, and photometry of the prototype GD 356 are studied. The emission features vary in strength on the rotational period, but in anti-phase with the light curve, consistent with a cool surface spot beneath an optically thin chromosphere. Possible changes in the line profiles are observed at the same photometric phase, potentially suggesting modest evolution of the emission region, while the magnetic field varies by 10 per cent over a full rotation. These comprehensive data reveal neither changes to the photometric period, nor additional signals such as might be expected from an orbiting body. A closer examination of the unipolar inductor model finds points of potential failure: the observed rapid stellar rotation will inhibit current carriers due to the centrifugal force, there may be no supply of magnetospheric ions, and no anti-phase flux changes are expected from ohmic surface heating. Together with the highly similar properties of the four cool, emission-line white dwarfs, these facts indicate that the chromospheric emission is intrinsic. A tantalizing possibility is that intrinsic chromospheres may manifest in (magnetic) white dwarfs, and in distinct parts of the HR diagram based on structure and composition.

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The Discovery of Oxygen‐rich Chromospheres in Two Massive DQ White Dwarfs

Cited by 11 publications

References 30 publications

The incidence of magnetic fields in cool DZ white dwarfs

The incidence of magnetic fields in cool DZ white dwarfs

Discovery of photospheric Ca X emission lines in the far-UV spectrum of the hottest known white dwarf (KPD 0005+5106)

A test of the planet–star unipolar inductor for magnetic white dwarfs

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