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More than 36 yr have passed since the discovery of the infrared excess from circumstellar dust orbiting the white dwarf G29-38, which at 17.5 pc it is the nearest and brightest of its class. The precise morphology of the orbiting dust remains only marginally constrained by existing data, subject to model-dependent inferences, and thus fundamental questions of its dynamical origin and evolution persist. This study presents a means to constrain the geometric distribution of the emitting dust using stellar pulsations measured at optical wavelengths as a variable illumination source of the dust, which reradiates primarily in the infrared. By combining optical photometry from the Whole Earth Telescope with 0.7–2.5μm spectroscopy obtained with SpeX at NASA’s Infrared Telescope Facility, we detect luminosity variations at all observed wavelengths, with variations at most wavelengths corresponding to the behavior of the pulsating stellar photosphere, but toward the longest wavelengths the light curves probe the corresponding time variability of the circumstellar dust. In addition to developing methodology, we find the pulsation amplitudes decrease with increasing wavelength for principal pulsation modes, yet increase beyond ≈2μm for nonlinear combination frequencies. We interpret these results as combination modes derived from the principal modes of identicalℓvalues and discuss the implications for the morphology of the warm dust. We also draw attention to some discrepancies between our findings and theoretical expectations for the results of the nonlinearity imposed by the surface convection zone on mode–mode interactions and on the behavior of the first harmonic of the highest-amplitude pulsation mode.

 

Two recently discovered white dwarfs, WD J041246.84 + 754942.26 and WD J165335.21 − 100116.33, exhibit Hα and Hβ Balmer line emission similar to stars in the emerging DAHe class, yet intriguingly have not been found to have detectable magnetic fields. These white dwarfs are assigned the spectral type DAe. We present detailed follow-up of the two known DAe stars using new time-domain spectroscopic observations and analysis of the latest photometric time-series data from TESS and ZTF. We measure the upper magnetic field strength limit of both stars as B < 0.05 MG. The DAe white dwarfs exhibit photometric and spectroscopic variability, where in the case of WD J041246.84 + 754942.26 the strength of the Hα and Hβ emission cores varies in antiphase with its photometric variability over the spin period, which is the same phase relationship seen in DAHe stars. The DAe white dwarfs closely cluster in one region of the Gaia Hertzsprung–Russell diagram together with the DAHe stars. We discuss current theories on non-magnetic and magnetic mechanisms which could explain the characteristics observed in DAe white dwarfs, but additional data are required to unambiguously determine the origin of these stars.

 

This paper reports the ULTRACAM discovery of dipolar surface spots in two cool magnetic white dwarfs with Balmer emission lines, while a third system exhibits a single spot, similar to the prototype GD 356. The light curves are modelled with simple, circular, isothermal dark spots, yielding relatively large regions with minimum angular radii of 20°. For those stars with two light-curve minima, the dual spots are likely observed at high inclination (or colatitude); however, identical and antipodal spots cannot simultaneously reproduce both the distinct minima depths and the phases of the light-curve maxima. The amplitudes of the multiband photometric variability reported here are all several times larger than that observed in the prototype GD 356; nevertheless, all DAHe stars with available data appear to have light-curve amplitudes that increase towards the blue in correlated ratios. This behaviour is consistent with cool spots that produce higher contrasts at shorter wavelengths, with remarkably similar spectral properties given the diversity of magnetic field strengths and rotation rates. These findings support the interpretation that some magnetic white dwarfs generate intrinsic chromospheres as they cool, and that no external source is responsible for the observed temperature inversion. Spectroscopic time-series data for DAHe stars is paramount for further characterization, where it is important to obtain well-sampled data, and consider wavelength shifts, equivalent widths, and spectropolarimetry.

 

We report the discovery of two apparently isolated stellar remnants that exhibit rotationally modulated magnetic Balmer emission, adding to the emerging DAHe class of white dwarf stars. While the previously discovered members of this class show Zeeman-split triplet emission features corresponding to single magnetic field strengths, these two new objects exhibit significant fluctuations in their apparent magnetic field strengths with variability phase. The Zeeman-split hydrogen emission lines in LP 705−64 broaden from 9.4 to 22.2 MG over an apparent spin period of 72.629 min. Similarly, WD J143019.29−562358.33 varies from 5.8  to 8.9 MG over its apparent 86.394 min rotation period. This brings the DAHe class of white dwarfs to at least five objects, all with effective temperatures within 500 K of 8000 K and masses ranging from $0.65\,\,{\text{to}}\,\,0.83\, {\rm M}_{\odot }$.

 

The distribution of white dwarf rotation periods provides a means for constraining angular momentum evolution during the late stages of stellar evolution, as well as insight into the physics and remnants of double degenerate mergers. Although the rotational distribution of low-mass white dwarfs is relatively well constrained via asteroseismology, that of high-mass white dwarfs, which can arise from either intermediate-mass stellar evolution or white dwarf mergers, is not. Photometric variability in white dwarfs due to rotation of a spotted star is rapidly increasing the sample size of high-mass white dwarfs with measured rotation periods. We present the discovery of 22.4 minute photometric variability in the light curve of EGGR 156, a strongly magnetic, ultramassive white dwarf. We interpret this variability as rapid rotation, and our data suggest that EGGR 156 is the remnant of a double degenerate merger. Finally, we calculate the rate of period change in rapidly-rotating, massive, magnetic WDs due to magnetic dipole radiation. In many cases, including EGGR 156, the period change is not currently detectable over reasonable timescales, indicating that these WDs could be very precise clocks. For the most highly-magnetic, rapidly-rotating massive WDs, such as ZTF J1901+1450 and RE J0317−853, the period change should be detectable and may help constrain the structure and evolution of these exotic white dwarfs.

 

This work combines spectroscopic and photometric data of the polluted white dwarf WD 0141−675, which has a now retracted astrometric super-Jupiter candidate, and investigates the most promising ways to confirm Gaia astrometric planetary candidates and obtain follow-up data. Obtaining precise radial velocity measurements for white dwarfs is challenging due to their intrinsic faint magnitudes, lack of spectral absorption lines, and broad spectral features. However, dedicated radial velocity campaigns are capable of confirming close-in giant exoplanets (a few MJup) around polluted white dwarfs, where additional metal lines aid radial velocity measurements. Infrared emission from these giant exoplanets is shown to be detectable with JWST Mid-Infrared Instrument (MIRI) and will provide constraints on the formation of the planet. Using the initial Gaia astrometric solution for WD 0141−675 as a case study, if there were a planet with a 33.65 d period or less with a nearly edge-on orbit, (1) ground-based radial velocity monitoring limits the mass to <15.4 MJup, and (2) space-based infrared photometry shows a lack of infrared excess and in a cloud-free planetary cooling scenario, a substellar companion would have to be <16 MJup and be older than 3.7 Gyr. These results demonstrate how radial velocities and infrared photometry can probe the mass of the objects producing some of the astrometric signals, and rule out parts of the brown dwarf and planet mass parameter space. Therefore, combining astrometric data with spectroscopic and photometric data is crucial to both confirm and characterize astrometric planet candidates around white dwarfs.

 

AM CVn-type systems are ultracompact, helium-accreting binary systems that are evolutionarily linked to the progenitors of thermonuclear supernovae and are expected to be strong Galactic sources of gravitational waves detectable to upcoming space-based interferometers. AM CVn binaries with orbital periods ≲20–23 min exist in a constant high state with a permanently ionized accretion disc. We present the discovery of TIC 378898110, a bright (G = 14.3 mag), nearby (309.3 ± 1.8 pc), high-state AM CVn binary discovered in TESS two-minute-cadence photometry. At optical wavelengths, this is the third-brightest AM CVn binary known. The photometry of the system shows a 23.07172(6) min periodicity, which is likely to be the ‘superhump’ period and implies an orbital period in the range 22–23 min. There is no detectable spectroscopic variability. The system underwent an unusual, year-long brightening event during which the dominant photometric period changed to a shorter period (constrained to 20.5 ± 2.0 min), which we suggest may be evidence for the onset of disc-edge eclipses. The estimated mass transfer rate, $\log (\dot{M} / \mathrm{M_\odot } \, \mathrm{yr}^{-1}) = -6.8 \pm 1.0$, is unusually high and may suggest a high-mass or thermally inflated donor. The binary is detected as an X-ray source, with a flux of $9.2 ^{+4.2}_{-1.8} \times 10^{-13}$ erg cm−2 s−1 in the 0.3–10 keV range. TIC 378898110 is the shortest-period binary system discovered with TESS, and its large predicted gravitational-wave amplitude makes it a compelling verification binary for future space-based gravitational wave detectors.

 

Context. The TESS space mission has recently demonstrated its great potential to discover new pulsating white dwarf and pre-white dwarf stars, and to detect periodicities with high precision in already known white-dwarf pulsators. Aims. We report the discovery of two new pulsating He-rich atmosphere white dwarfs (DBVs) and present a detailed asteroseismological analysis of three already known DBV stars employing observations collected by the TESS mission along with ground-based data. Methods. We processed and analyzed TESS observations of the three already known DBV stars PG 1351+489 (TIC 471015205), EC 20058−5234 (TIC 101622737), and EC 04207−4748 (TIC 153708460), and the two new DBV pulsators WDJ152738.4−50207.4 (TIC 150808542) and WD 1708−871 (TIC 451533898), whose variability is reported for the first time in this paper. We also carried out a detailed asteroseismological analysis using fully evolutionary DB white-dwarf models built considering the complete evolution of the progenitor stars. We constrained the stellar mass of three of these target stars by means of the observed period spacing, and derived a representative asteroseismological model using the individual periods, when possible. Results. We extracted frequencies from the TESS light curves of these DBV stars using a standard pre-whitening procedure to derive the potential pulsation frequencies. All the oscillation frequencies that we found are associated with g -mode pulsations with periods spanning from ∼190 s to ∼936 s. We find hints of rotation from frequency triplets in some of the targets, including the two new DBVs. For three targets, we find constant period spacings, which allowed us to infer their stellar masses and constrain the harmonic degree ℓ of the modes. We also performed period-to-period fit analyses and found an asteroseismological model for three targets, with stellar masses generally compatible with the spectroscopic masses. Obtaining seismological models allowed us to estimate the seismological distances and compare them with the precise astrometric distances measured with Gaia . We find a good agreement between the seismic and the astrometric distances for three stars (PG 1351+489, EC 20058-5234, and WD 1708-871); although, for the other two stars (EC 04207-4748 and WD J152738.4-50207), the discrepancies are substantial. Conclusions. The high-quality data from the TESS mission continue to provide important clues which can be used to help determine the internal structure of pulsating pre-white dwarf and white dwarf stars through the tools of asteroseismology. 

We present the discovery of the eclipsing double white dwarf (WD) binary WDJ 022558.21−692025.38 that has an orbital period of 47.19 min. Following identification with the Transiting Exoplanet Survey Satellite, we obtained time series ground based spectroscopy and high-speed multiband ULTRACAM photometry which indicate a primary DA WD of mass $0.40\pm 0.04\, \text{M}_\odot$ and a $0.28\pm 0.02\, \text{M}_\odot$ mass secondary WD, which is likely of type DA as well. The system becomes the third-closest eclipsing double WD binary discovered with a distance of approximately 400 pc and will be a detectable source for upcoming gravitational wave detectors in the mHz frequency range. Its orbital decay will be measurable photometrically within 10 yr to a precision of better than 1 per cent. The fate of the binary is to merge in approximately 41 Myr, likely forming a single, more massive WD.

 

We present the photometric data from TESS for two known ZZ Ceti stars, PG 1541 + 651 and BPM 31594. Before TESS, both objects only had observations from short runs from ground-based facilities, with three and one period detected, respectively. The TESS data allowed the detection of multiple periodicities, 12 for PG 1541 + 651, and six for BPM 31594, which enables us to perform a detailed asteroseismological study. For both objects, we found a representative asteroseismic model with canonical stellar mass ∼0.61M⊙ and thick hydrogen envelopes, thicker than 10−5.3M*. The detection of triplets in the Fourier transform also allowed us to estimate mean rotation periods, being ∼22 h for PG 1541 + 651 and 11.6 h for BPM 31594, which is consistent with a range of values reported for other ZZ Ceti stars.