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1. CSS1603+19: a low-mass polar near the cataclysmic variable period minimum

   https://doi.org/10.1093/mnras/stad1156  

   Liu, Yiqi ; Hwang, Hsiang-Chih ; Zakamska, Nadia L. ; Thorstensen, John R. ( April 2023 , Monthly Notices of the Royal Astronomical Society)

   CSS1603+19 is a cataclysmic variable (CV) with an orbital period of 81.96 min, near the minimal period of CVs. It is unusual in having a strong mid-infrared excess inconsistent with thermal emission from a brown dwarf companion. Here, we present time-resolved multiwavelength observations of this system. WISE photometry indicates that the mid-infrared excess displays a one-magnitude eclipsing-like variability during the orbit. We obtained near-infrared and optical spectroscopy using Gemini, MDM, and APO telescopes. Near-infrared spectra show possible cyclotron features indicating that the white dwarf has a magnetic field of about 5 MG. Optical and near-infrared spectra display double-peaked emission lines, with both components showing strong radial velocity variations during the orbital period and with the broad component leading the narrow component stably by about 0.2 of the orbital phase. We construct a physical model informed by existing observations of the system and determine that one component likely originates from the accretion column on to the magnetized white dwarf in synchronous rotation with the orbital motion and the other from the Roche overflow point. This allows us to constrain the masses of the binary components to be M1 > 0.24 M⊙ for the white dwarf accretor and M2 = 0.0644 ± 0.0074 M⊙ for themore »donor. We classify the system as an AM Herculis star, or a polar. It has likely completed its stint on the period gap, but has not yet gone through the period bounce.

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2. 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 1.27 M ⊙ (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 of B 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-spinningmore »white dwarfs impersonating ZZ Ceti pulsators.« less
3. MOBSTER – III. HD 62658: a magnetic Bp star in an eclipsing binary with a non-magnetic ‘identical twin’

   https://doi.org/10.1093/mnras/stz2846  

   Shultz, M. E. ; Johnston, C. ; Labadie-Bartz, J. ; Petit, V. ; David-Uraz, A. ; Kochukhov, Ø. ; Wade, G. A. ; Pepper, J. ; Stassun, K. G. ; Rodriguez, J. E. ; et al ( October 2019 , Monthly Notices of the Royal Astronomical Society)

   HD 62658 (B9p V) is a little-studied chemically peculiar star. Light curves obtained by the Kilodegree Extremely Little Telescope (KELT) and Transiting Exoplanet Survey Satellite (TESS) show clear eclipses with a period of about 4.75 d, as well as out-of-eclipse brightness modulation with the same 4.75 d period, consistent with synchronized rotational modulation of surface chemical spots. High-resolution ESPaDOnS circular spectropolarimetry shows a clear Zeeman signature in the line profile of the primary; there is no indication of a magnetic field in the secondary. PHOEBE modelling of the light curve and radial velocities indicates that the two components have almost identical masses of about 3 M⊙. The primary’s longitudinal magnetic field 〈Bz〉 varies between about +100 and −250 G, suggesting a surface magnetic dipole strength Bd = 850 G. Bayesian analysis of the Stokes V profiles indicates Bd = 650 G for the primary and Bd < 110 G for the secondary. The primary’s line profiles are highly variable, consistent with the hypothesis that the out-of-eclipse brightness modulation is a consequence of rotational modulation of that star’s chemical spots. We also detect a residual signal in the light curve after removal of the orbital and rotational modulations, which might be pulsational in origin; this could be consistent withmore »the weak line profile variability of the secondary. This system represents an excellent opportunity to examine the consequences of magnetic fields for stellar structure via comparison of two stars that are essentially identical with the exception that one is magnetic. The existence of such a system furthermore suggests that purely environmental explanations for the origin of fossil magnetic fields are incomplete.

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4. Two new white dwarfs with variable magnetic Balmer emission lines

   https://doi.org/10.1093/mnras/stad760  

   Reding, Joshua S. ; Hermes, J. J. ; Clemens, J. C. ; Hegedus, R. J. ; Kaiser, B. C. ( March 2023 , Monthly Notices of the Royal Astronomical Society)

   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 $0.65\,\,{\text{to}}\,\,0.83\, {\rm M}_{\odot }$.
5. DAHe white dwarfs from the DESI Survey

   https://doi.org/10.1093/mnras/stad727  

   Manser, Christopher J. ; Gänsicke, Boris T. ; Inight, Keith ; Robert, Akshay ; Ahlen, S. ; Allende Prieto, C. ; Brooks, D. ; Cooper, A. P. ; de la Macorra, A. ; Font-Ribera, A. ; et al ( March 2023 , Monthly Notices of the Royal Astronomical Society)

   A new class of white dwarfs, dubbed DAHe, that present Zeeman-split Balmer lines in emission has recently emerged. However, the physical origin of these emission lines remains unclear. We present here a sample of 21 newly identified DAHe systems and determine magnetic field strengths and (for a subset) periods that span the ranges of ≃6.5–147 MG and ≃0.4–36 h, respectively. All but four of these systems were identified from the Dark Energy Spectroscopic Instrument survey sample of more than 47 000 white dwarf candidates observed during its first year of observations. We present detailed analysis of the new DAHe WD J161634.36+541011.51 with a spin period of 95.3 min, which exhibits an anticorrelation between broad-band flux and Balmer line strength that is typically observed for this class of systems. All DAHe systems cluster closely on the Gaia Hertzsprung–Russell diagram where they represent ≃1 per cent of white dwarfs within that region. This grouping further solidifies their unexplained emergence at relatively late cooling times and we discuss this in context of current formation theories. Nine of the new DAHe systems are identifiable from Sloan Digital Sky Survey spectra of white dwarfs that had been previously classified as featureless DC-type systems. We suggest high-S/N (signal-to-noise ratios), unbiased observations ofmore »DCs as a possible route for discovering additional DAHe systems.

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