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We are trying to reduce the largest uncertainties in using white dwarf stars as Galactic chronometers by understanding the details of carbon crystalliazation that currently result in a 1–2 Gyr uncertainty in the ages of the oldest white dwarf stars. We expect the coolest white dwarf stars to have crystallized interiors, but theory also predicts hotter white dwarf stars, if they are massive enough, will also have some core crystallization. BPM 37093 is the first discovered of only a handful of known massive white dwarf stars that are also pulsating DAV, or ZZ Ceti, variables. Our approach is to use the pulsations to constrain the core composition and amount of crystallization. Here we report our analysis of 4 hours of continuous time series spectroscopy of BPM 37093 with Gemini South combined with simultaneous time-series photometry from Mt. John (New Zealand), SAAO, PROMPT, and Complejo Astronomico El Leoncito (CASLEO, Argentina).

Accreting white dwarfs have recently been shown to exhibit non-radial pulsations similar to their non-interacting counterparts. This allows us to probe the interior of the accreting white dwarf using seismology, and may be the only way to determine masses for non-eclipsing cataclysmic variables. Improving our understanding of accreting white dwarfs will have implications for models of supernovae Type Ia. Pulsating white dwarfs in cataclysmic variables are also useful in establishing the effects of accretion on pulsations. A search for nonradial pulsations among suitable candidates has led to the discovery of twelve such systems known to date. With the goal of establishing an instability strip (or strips) for these pulsating accretors, we acquired HST ultra-violet time-series spectroscopy of six pulsating white dwarfs in cataclysmic variables in 2007 and 2008. This approach enables us to measure the effective temperature of the white dwarf using the co-added spectrum, and to simultaneously characterize the pulsations. We also intended to constrain the pulsation mode identification by comparing the ultra-violet amplitudes to those from near-simultaneous ground-based photometry. Our preliminary results indicate a broad instability strip in the temperature range of 10500–15400 K.

Two recently discovered white dwarfs, WDJ041246.84\\(+\\)754942.26 and WDJ165335.21\\(-\\)100116.33, exhibit H\\(\\alpha\\) and H\\(\\beta\\) 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 WDJ041246.84\\(+\\)754942.26 the strength of the H\\(\\alpha\\) and H\\(\\beta\\) emission cores varies in anti-phase 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.

Temporal variations in metal-line strengths in H-atmosphere white dwarfs hold the potential to test the timescales of graviational settling theory. These short timescales, in turn, require that DAZs are currently accreting. Such temporal variations would also indicate that accretion from a circumstellar dust disk can be episodic. We are compiling increasing evidence for time-variable Ca and Mg line-strength variations in the best studied DAZ, G29-38. Our evidence to date supports the gravitational settling timescales of Koester & Wilken (2006) and episodic accretion from G29-38's debris disk. Furthermore, we have detected evidence for time-variable accretion with a timescale 24 hours, and typical variability of ∼4% during the 100 days of our autumn 2007 monitoring campaign.

Infrared excesses around white dwarf stars indicate the presence of various astrophysical objects of interest, including companions and debris disks. In this second paper of a series, we present follow-up observations of infrared excess candidates from Gaia and unWISE discussed in the first paper, Paper I. We report space-based infrared photometry at 3.6 and 4.5 micron for 174 white dwarfs from the Spitzer Space Telescope and ground-based near-infrared J, H, and K photometry of 235 white dwarfs from Gemini Observatory with significant overlap between Spitzer and Gemini observations. This data is used to confirm or rule-out the observed unWISE infrared excess. From the unWISE-selected candidate sample, the most promising infrared excess sample comes from both colour and flux excess, which has a Spitzer confirmation rate of 95%. We also discuss a method to distinguish infrared excess caused by stellar or sub-stellar companions from potential dust disks. In total, we confirm the infrared excess around 62 white dwarfs, 10 of which are likely to be stellar companions. The remaining 52 bright white dwarf with infrared excess beyond two microns has the potential to double the known sample of white dwarfs with dusty exoplanetary debris disks. Follow-up high-resolution spectroscopic studies of a fraction of confirmed excess white dwarfs in this sample have discovered emission from gaseous dust disks. Additional investigations will be able to expand the parameter space from which dust disks around white dwarfs are found.

We are searching for new pulsating DB white dwarf stars (DBVs) based on the newly found white dwarf stars from the spectra obtained by the Sloan Digital Sky Survey. DBVs pulsate at hotter temperature ranges than their better known cousins, DAVs or ZZ Ceti stars. Since the evolution of white dwarf stars is characterized by cooling, asteroseismological studies of DBVs give us opportunities to study white dwarf structure at a different evolutionary stage than the DAVs. The hottest DBVs are thought to have neutrino luminosities exceeding their photon luminosities (Winget et al. 2004), a quantity measurable through asteroseismology. Therefore, they can also be used to study neutrino physics in the stellar interior. At the time of the meeting, we reported on the nine new DBVs, doubling the number of previously known DBVs. Here we report the new nine pulsators' lightcurves and power spectra.

The success of asteroseismology lies in the correct identification of the normal modes of oscillation. The Whole Earth Telescope (WET) identified the normal modes of a helium white dwarf pulsator, GD358, by analyzing the period distribution of the pulsation modes. Another way to identify modes is by comparing pulsation amplitudes in the UV to the optical. To cross-calibrate the two mode identification methods, we observed CD358 in August, 1996 with the Hubble Space Telescope (HST) to obtain the UV data while observing nearly simultaneously from the ground. During our observations, GD358 went through a very drastic amplitude modulation in a time scale of hours. These short time scale amplitude changes made the direct UV to optical amplitude determinations difficult. We successfully eliminate the possibility that the 423s mode, the dominant mode at the time of these observations, is an ℓ = 3 or 4 g-mode pulsation, but we cannot unambiguously decide if it is an ℓ = 1 or 2. Theoretical calculations indicate that the massive pulsating DA white dwarf BPM 37093 has a crystallized interior (Winget et al. 1997; Kanaan 1996; Montgomery 1998). Crystallization was predicted theoretically 40 years ago (Kirshnitz 1960; Abrisokov 1961; Salpeter 1961) although uncertainties in the nature and extent of crystallization, as well as its associated effects, are the largest sources of uncertainty in calculating the ages of the coolest white dwarf stars—important chronometers of the galactic disk. The WET observed BPM 37093 in April 1998 and again in April 1999, simultaneously with the HST, in hopes of using both the period distribution and the amplitude comparison method to identify the ℓ value of the modes and measure the first crystallized mass-fraction of a stellar interior. Here we also rule out the possibility of the observed modes being ℓ = 3 and higher and demonstrate that not all the observed modes are ℓ = 1. If all the observed modes are ℓ = 2, then we conclude that the crystallization mass fraction is between 0–80%, depending mainly on the surface H layer. In the end, we evaluate the amplitude comparison method and address advantages and problems using this method compared to other mode identification methods.

We present a dedicated search for new pulsating helium-atmosphere (DBV) white dwarfs from the Sloan Digital Sky Survey using the McDonald 2.1m Otto Struve Telescope. In total we observed 55 DB and DBA white dwarfs with spectroscopic temperatures between 19,000 and 35,000K. We find 19 new DBVs and place upper limits on variability for the remaining 36 objects. In combination with previously known DBVs, we use these objects to provide an update to the empirical extent of the DB instability strip. With our sample of new DBVs, the red edge is better constrained, as we nearly double the number of DBVs known between 20,000 and 24,000K. We do not find any new DBVs hotter than PG 0112+104, the current hottest DBV at \\(T_{\\mathrm{eff}}\\,{\\approx}\\) 31,000K, but do find pulsations in four DBVs with temperatures between 27,000 and 30,000K, improving empirical constraints on the poorly defined blue edge. We investigate the ensemble pulsation properties of all currently known DBVs, finding that the weighted mean period and total pulsation power exhibit trends with effective temperature that are qualitatively similar to the pulsating hydrogen-atmosphere white dwarfs.

G\\,29\\(-\\)38 (TIC~422526868) is one of the brightest (\\(V=13.1\\)) and closest (\\(d = 17.51\\)\\,pc) pulsating white dwarfs with a hydrogen-rich atmosphere (DAV/ZZ Ceti class). It was observed by the {\\sl TESS} spacecraft in sectors 42 and 56. The atmosphere of G~29\\(-\\)38 is polluted by heavy elements that are expected to sink out of visible layers on short timescales. The photometric {\\sl TESS} data set spans \\(\\sim 51\\) days in total, and from this, we identified 56 significant pulsation frequencies, that include rotational frequency multiplets. In addition, we identified 30 combination frequencies in each sector. The oscillation frequencies that we found are associated with \\(g\\)-mode pulsations, with periods spanning from \\(\\sim\\) 260 s to \\(\\sim\\) 1400 s. We identified %three distinct rotational frequency triplets with a mean separation \\(\\delta \\nu_{\\ell=1}\\) of 4.67 \\(\\mu\\)Hz and a quintuplet with a mean separation \\(\\delta \\nu_{\\ell=2}\\) of 6.67 \\(\\mu\\)Hz, from which we estimated a rotation period of about \\(1.35 \\pm 0.1\\) days. We determined a constant period spacing of 41.20~s for \\(\\ell= 1\\) modes and 22.58\\,s for \\(\\ell= 2\\) modes. We performed period-to-period fit analyses and found an asteroseismological model with \\(M_{\\star}/M_{\\odot}=0.632 \\pm 0.03\\), \\(T_{\\rm eff}=11\\, 635\\pm 178\\) K, and \\(\\log{g}=8.048\\pm0.005\\) (with a hydrogen envelope mass of \\(M_{\\rm H}\\sim 5.6\\times 10^{-5}M_{\\star}\\)), in good agreement with the values derived from spectroscopy. We obtained an asteroseismic distance of 17.54 pc, which is in excellent agreement with that provided by {\\sl Gaia} (17.51 pc).

The collection of high-quality photometric data by space telescopes is revolutionizing the area of white-dwarf asteroseismology. Among the different kinds of pulsating white dwarfs, there are those that have He-rich atmospheres, and they are called DBVs or V777 Her variable stars. The archetype of these pulsating white dwarfs, GD~358, is the focus of the present paper. We report a thorough asteroseismological analysis of the DBV star GD~358 (TIC~219074038) based on new high-precision photometric data gathered by the {\\it TESS} space mission combined with data taken from the Earth. In total, we detected 26 periodicities from the {\\it TESS} light curve of this DBV star using a standard pre-whitening. The oscillation frequencies are associated with nonradial \\(g\\)(gravity)-mode pulsations with periods from \\(\\sim 422\\) s to \\(\\sim 1087\\) s. Moreover, we detected 8 combination frequencies between \\(\\sim 543\\) s and \\(\\sim 295\\) s. We combined these data with a huge amount of observations from the ground. We found a constant period spacing of \\(39.25\\pm0.17\\) s, which helped us to infer its mass (\\(M_{\\star}= 0.588\\pm0.024 M_{\\sun}\\)) and constrain the harmonic degree \\(\\ell\\) of the modes. We carried out a period-fit analysis on GD~358, and we were successful in finding an asteroseismological model with a stellar mass (\\(M_{\\star}= 0.584^{+0.025}_{-0.019} M_{\\sun}\\)), in line with the spectroscopic mass (\\(M_{\\star}= 0.560\\pm0.028 M_{\\sun}\\)). We found that the frequency splittings vary according to the radial order of the modes, suggesting differential rotation. Obtaining a seismological made it possible to estimate the seismological distance (\\(d_{\\rm seis}= 42.85\\pm 0.73\\) pc) of GD~358, which is in very good accordance with the precise astrometric distance measured by {\\it GAIA} EDR3 (\\(\\pi= 23.244\\pm 0.024, d_{\\rm GAIA}= 43.02\\pm 0.04\\)~pc).