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1. The Emerging Class of Double-faced White Dwarfs

   https://doi.org/10.3847/1538-4357/adbd3a  

   Moss, Adam; Kilic, Mukremin; Bergeron, Pierre; Jewett, Gracyn; Brown, Warren R (April 2025, The Astrophysical Journal)

   Abstract We report our findings on a spectroscopic survey of seven unresolved DA+DB binary white dwarf candidates. We have discovered extreme spectroscopic variations in one of these candidates, SDSS J084716.21+484220.40. Previous analysis failed to reproduce the optical spectrum using a single object with a homogeneous atmosphere. Our time-resolved spectroscopy reveals a double-faced white dwarf that switches between a DBA and DA spectral type over 6.5 or 8.9 hr due to varying surface abundances. We also provide time-series spectroscopy of the magnetic DBA, SDSS J085618.94+161103.6 (LB 8915), and confirm an inhomogeneous atmosphere. We employ an atmosphere model with hydrogen caps and a helium belt that yields excellent fits to our time-resolved spectra. We use the oblique rotator model to derive the system geometry for both targets. With the addition of these two objects, the emerging class of double-faced white dwarfs now consists of seven members. We summarize the properties of this new class of objects, and discuss how magnetism impacts the convective processes and leads to the formation of double-faced white dwarfs. We identify cooler versions of white dwarfs with inhomogeneous atmospheres among the cool magnetic DA white dwarf sample, where the Hαline is shallower than expected based on pure hydrogen atmosphere models. 

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2. A Magnetic White Dwarf with Five Hα Components

   https://doi.org/10.1093/mnras/stz2394  

   Kilic, Mukremin; Rolland, B; Bergeron, P; Vanderbosch, Z; Benni, P; Garlitz, J (August 2019, Monthly Notices of the Royal Astronomical Society)

   G183−35 is an unusual white dwarf that shows an Hα line split into five components, instead of the usual three components seen in strongly magnetic white dwarfs. Potential explanations for the unusual set of lines includes a double degenerate system containing two magnetic white dwarfs and/or rotational modulation of a complex magnetic field structure. Here we present time-resolved spectroscopy of G183−35 obtained at the Gemini Observatory. These data reveal two sets of absorption lines that appear and disappear over a period of about 4 hours. We also detect low-level (0.2%) variability in optical photometry at the same period. We demonstrate that the spectroscopic and photometric variability can be explained by the presence of spots on the surface of the white dwarf and a change in the average field strength from about 4.6 MG to 6.2 MG. The observed variability is clearly due to G183−35’s relatively short spin period. However, rotational modulation of a complex magnetic field by itself cannot explain the changes seen in the central Hα component. An additional source of variability in the line profiles, most likely due to a chemically inhomogeneous surface composition, is also needed. We propose further observations of similar objects to test this scenario. 

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3. An Isolated White Dwarf with a 70 s Spin Period

   https://doi.org/10.3847/2041-8213/ac3b60  

   Kilic, Mukremin; Kosakowski, Alekzander; Moss, Adam G.; Bergeron, P.; Conly, Annamarie A. (December 2021, The Astrophysical Journal Letters)

   Abstract We report the discovery of an isolated white dwarf with a spin period of 70 s. We obtained high-speed photometry of three ultramassive white dwarfs within 100 pc and discovered significant variability in one. SDSS J221141.80+113604.4 is a 1.27M_⊙(assuming a CO core) magnetic white dwarf that shows 2.9% brightness variations in the BG40 filter with a 70.32 ± 0.04 s period, becoming the fastest spinning isolated white dwarf currently known. A detailed model atmosphere analysis shows that it has a mixed hydrogen and helium atmosphere with a dipole field strength ofB_d= 15 MG. Given its large mass, fast rotation, strong magnetic field, unusual atmospheric composition, and relatively large tangential velocity for its cooling age, J2211+1136 displays all of the signatures of a double white dwarf merger remnant. Long-term monitoring of the spin evolution of J2211+1136 and other fast-spinning isolated white dwarfs opens a new discovery space for substellar and planetary mass companions around white dwarfs. In addition, the discovery of such fast rotators outside of the ZZ Ceti instability strip suggests that some should also exist within the strip. Hence, some of the monoperiodic variables found within the instability strip may be fast-spinning white dwarfs impersonating ZZ Ceti pulsators. 

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4. A cool, magnetic white dwarf accreting planetary debris

   https://doi.org/10.1093/mnras/stad3370  

   Vennes, Stéphane; Kawka, Adela; Klein, Beth L.; Zuckerman, B.; Weinberger, Alycia J.; Melis, Carl (October 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present an analysis of spectroscopic data of the cool, highly magnetic, and polluted white dwarf 2MASS J0916−4215. The atmosphere of the white dwarf is dominated by hydrogen, but numerous spectral lines of magnesium, calcium, titanium, chromium, iron, and strontium, along with Li i, Na i, Al i, and K i lines, are found in the incomplete Paschen–Back regime, most visibly, in the case of Ca ii lines. Extensive new calculations of the Paschen–Back effect in several spectral lines are presented and results of the calculations are tabulated for the Ca ii H&K doublet. The abundance pattern shows a large lithium and strontium excess, which may be viewed as a signature of planetary debris akin to Earth’s continental crust accreted on to the star, although the scarcity of silicon indicates possible dilution in bulk Earth material. Accurate abundance measurements proved sensitive to the value of the broadening parameter due to collisions with neutral hydrogen ($$\Gamma$$H i), particularly in saturated lines such as the resonance lines of Ca i and Ca ii. We found that $$\Gamma$$H i if formulated with values from the literature could be overestimated by a factor of 10 in most resonance lines. 

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5. The 100 pc White Dwarf Sample in the SDSS Footprint

   Kilic, Mukremin; Bergeron, P.; Kosakowski, Alekzander; Brown, Warren R.; Agueros, Marcel A.; Blouin, Simon (July 2020, The Astrophysical journal)

   We present follow-up spectroscopy of 711 white dwarfs within 100 pc, and present a detailed model atmosphere analysis of the 100 pc white dwarf sample in the SDSS footprint. Our spectroscopic follow-up is complete for 83% of the white dwarfs hotter than 6000 K, where the atmospheric composition can be constrained reliably. We identify 1508 DA white dwarfs with pure hydrogen atmospheres. The DA mass distribution has an extremely narrow peak at $$0.59~M_{\odot}$$, and reveals a shoulder from relatively massive white dwarfs with $M=0.7$$-$$0.9~M_{\odot}$$. Comparing this distribution with binary population synthesis models, we find that the contribution from single stars that form through mergers cannot explain the over-abundance of massive white dwarfs. In addition, the mass distribution of cool DAs shows a near absence of $$M>1~M_{\odot}$$ white dwarfs. The pile-up of 0.7-$$0.9~M_{\odot}$$ and the disappearance of $$M>1~M_{\odot}$$ white dwarfs is consistent with the effects of core crystallization. Even though the evolutionary models predict the location of the pile-up correctly, the delay from the latent heat of crystallization by itself is insufficient to create a significant pile-up, and additional cooling delays from related effects like phase separation are necessary. We also discuss the population of infrared-faint (ultracool) white dwarfs, and demonstrate for the first time the existence of a well defined sequence in color and magnitude. Curiously, this sequence is connected to a region in the color-magnitude diagrams where the number of helium-dominated atmosphere white dwarfs is low. This suggests that the infrared-faint white dwarfs likely have mixed H/He atmospheres. 

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