Photometric variability in a warm, strongly magnetic DQ white dwarf, SDSS J103655.39+652252.2

• Published in: arXiv.org 2013-04
• Main Authors: Williams, Kurtis A, Winget, D E, Montgomery, M H, Dufour, Patrick, Kepler, S O, Hermes, J J, Falcon, Ross E, Winget, K I, Bolte, Michael, Rubin, K H R
• Format: Article
• Language: English
• Subjects: Amplitudes; Atmospheres; Atmospheric models; Carbon; Diagnostic systems; Field strength; Helium; Magnetic fields; Modulation; Photometry; Pulse shape; White dwarf stars
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1304.3165

We present the discovery of photometric variability in the DQ white dwarf SDSS J103655.39+652252.2 (SDSS J1036+6522). Time-series photometry reveals a coherent monoperiodic modulation at a period of 1115.64751(67) s with an amplitude of 0.442% +/- 0.024%; no other periodic modulations are observed with amplitudes >~0.13%. The period, amplitude, and phase of this modulation are constant within errors over 16 months. The spectrum of SDSS J1036+6522 shows magnetic splitting of carbon lines, and we use Paschen-Back formalism to develop a grid of model atmospheres for mixed carbon and helium atmospheres. Our models, while reliant on several simplistic assumptions, nevertheless match the major spectral and photometric properties of the star with a self-consistent set of parameters: Teff~15,500 K, log g ~9, log(C/He)=-1.0, and a mean magnetic field strength of 3.0 +/- 0.2 MG. The temperature and abundances strongly suggest that SDSS J1036+6522 is a transition object between the hot, carbon-dominated DQs and the cool, He-dominated DQs. The variability of SDSS J1036+6522 has characteristics similar to those of the variable hot carbon-atmosphere white dwarfs (DQVs), however, its temperature is significantly cooler. The pulse profile of SDSS J1036+6522 is nearly sinusoidal, in contrast with the significantly asymmetric pulse shapes of the known magnetic DQVs. If the variability in SDSS J1036+6522 is due to the same mechanism as other DQVs, then the pulse shape is not a definitive diagnostic on the absence of a strong magnetic field in DQVs. It remains unclear whether the root cause of the variability in SDSS J1036+6522 and the other hot DQVs is the same.