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1. The 40 pc sample of white dwarfs from Gaia

   https://doi.org/10.1093/mnras/stad3773  

   O’Brien, Mairi W.; Tremblay, P-E; Klein, B. L.; Koester, D.; Melis, C.; Bédard, A.; Cukanovaite, E.; Cunningham, T.; Doyle, A. E.; Gänsicke, B. T.; et al (December 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present a comprehensive overview of a volume-complete sample of white dwarfs located within 40 pc of the Sun, a significant proportion of which were detected in Gaia Data Release 3 (DR3). Our DR3 sample contains 1076 spectroscopically confirmed white dwarfs, with just five candidates within the volume remaining unconfirmed (> 99 per cent spectroscopic completeness). Additionally, 28 white dwarfs were not in our initial selection from Gaia DR3, most of which are in unresolved binaries. We use Gaia DR3 photometry and astrometry to determine a uniform set of white dwarf parameters, including mass, effective temperature, and cooling age. We assess the demographics of the 40 pc sample, specifically magnetic fields, binarity, space density, and mass distributions. 

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2. 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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3. A 500 pc volume-limited sample of hot subluminous stars: I. Space density, scale height, and population properties

   https://doi.org/10.1051/0004-6361/202348319  

   Dawson, H; Geier, S; Heber, U; Pelisoli, I; Dorsch, M; Schaffenroth, V; Reindl, N; Culpan, R; Pritzkuleit, M; Vos, J; et al (June 2024, Astronomy & Astrophysics)

   We present the first volume-limited sample of spectroscopically confirmed hot subluminous stars out to 500 pc, defined using the accurate parallax measurements from theGaiaspace mission data release 3 (DR3). The sample comprises a total of 397 members, with 305 (~77%) identified as hot subdwarf stars, including 83 newly discovered systems. Of these, we observe that 178 (~58%) are hydrogen-rich sdBs, 65 are sdOBs (~21%), 32 are sdOs (~11%), and 30 are He-sdO/Bs (~10%). Among them, 48 (~16%) exhibit an infrared excess in their spectral energy distribution fits, suggesting a composite binary system. The hot subdwarf population is estimated to be 90% complete, assuming that most missing systems are these composite binaries located within the main sequence (MS) in theGaiacolour-magnitude diagram. The remaining sources in the sample include cataclysmic variables, blue horizontal branch stars, hot white dwarfs, and MS stars. We derived the mid-plane densityρ₀and scale height h_zfor the non-composite hot subdwarf star population using a hyperbolic sechant profile (sech²). The best-fit values areρ₀= 5.17 ± 0.33 × 10⁻⁷stars pc⁻³and h_z= 281 ± 62 pc. When accounting for the composite-colour hot subdwarfs and their estimated completeness, the mid-plane density increases toρ₀= 6.15_−0.53^+1.16× 10⁻⁷stars pc⁻³. This corrected space density is an order of magnitude lower than predicted by population synthesis studies, supporting previous observational estimates. 

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4. The ubiquity of carbon dredge-up in hydrogen-deficient white dwarfs as revealed by GALEX

   https://doi.org/10.1093/mnrasl/slad105  

   Blouin, Simon; Kilic, Mukremin; Bédard, Antoine; Tremblay, Pier-Emmanuel (July 2023, Monthly Notices of the Royal Astronomical Society: Letters)

   ABSTRACT The convective dredge-up of carbon from the interiors of hydrogen-deficient white dwarfs has long been invoked to explain the presence of carbon absorption features in the spectra of cool DQ stars ($$T_{\rm eff} \lt 10\,000\,$$K). It has been hypothesized that this transport process is not limited to DQ white dwarfs and also operates, albeit less efficiently, in non-DQ hydrogen-deficient white dwarfs within the same temperature range. This non-DQ population is predominantly composed of DC white dwarfs, which exhibit featureless optical spectra. However, no direct observational evidence of ubiquitous carbon pollution in DC stars has thus far been uncovered. In this Letter, we analyse data from the Galaxy Evolution Explorer to reveal the photometric signature of ultraviolet carbon lines in most DC white dwarfs in the $$8500\, {\rm K} \le T_{\rm eff} \le 10\,500\,$$K temperature range. Our results show that the vast majority of hydrogen-deficient white dwarfs experience carbon dredge-up at some point in their evolution. 

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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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