No abstract

Helium core white dwarfs (WDs) with mass M 0.20 M undergo several Gyr of stable hydrogen burning as they evolve. We show that in a certain range of WD and hydrogen envelope masses, these WDs may exhibit g-mode pulsations similar to their passively cooling, more massive carbon/oxygen core counterparts, the ZZ Cetis. Our models with stably burning hydrogen envelopes on helium cores yield g-mode periods and period spacings longer than the canonical ZZ Cetis by nearly a factor of 2. We show that core composition and structure can be probed using seismology since the g-mode eigenfunctions predominantly reside in the helium core. Though we have not carried out a fully nonadiabatic stability analysis, the scaling of the thermal time in the convective zone with surface gravity highlights several low-mass helium WDs that should be observed in search of pulsations: NLTT 11748, SDSS J0822+2753, and the companion to PSR J1012+5307. Seismological studies of these He core WDs may prove especially fruitful, as their luminosity is related (via stable hydrogen burning) to the hydrogen envelope mass, which eliminates one model parameter.

We report on the first ground-based measurement of the relativistic beaming effect (aka Doppler boosting). We observed the beaming effect in the detached, non-interacting eclipsing double white dwarf (WD) binary NLTT 11748. Our observations were motivated by the system's high mass ratio and low luminosity ratio, leading to a large beaming-induced variability amplitude at the orbital period of 5.6 hr. We observed the system during 3 nights at the 2.0m Faulkes Telescope North with the SDSS-g ′ filter, and fitted the data simultaneously for the beaming, ellipsoidal and reflection effects. Our fitted relative beaming amplitude is (3.0 ± 0.4) × 10 −3 , consistent with the expected amplitude from a blackbody spectrum given the photometric primary radial velocity amplitude and effective temperature. This result is a first step in testing the relation between the photometric beaming amplitude and the spectroscopic radial velocity amplitude in NLTT 11748 and similar systems. We did not identify any variability due to the ellipsoidal or reflection effects, consistent with their expected undetectable amplitude for this system. Low-mass, helium-core WDs are expected to reside -2in binary systems where in some of those systems the binary companion is a faint C/O WD and the two stars are detached and non-interacting, as in the case of NLTT 11748. The beaming effect can be used to search for the faint binary companion in those systems using wide-band photometry.

The newfound prevalence of extremely low mass (ELM, M He < 0.2 M ) helium white dwarfs (WDs) in tight binaries with more massive WDs has raised our interest in understanding the nature of their mass transfer. Possessing small (M env ∼ 10 −3 M ) but thick hydrogen envelopes, these objects have larger radii than cold WDs and so initiate mass transfer of H-rich material at orbital periods of 6-10 minutes. Building on the original work of D'Antona et al., we confirm the 10 6 yr period of continued inspiral with mass transfer of H-rich matter and highlight the fact that the inspiraling direct-impact double WD binary HM Cancri likely has an ELM WD donor. The ELM WDs have less of a radius expansion under mass loss, thus enabling a larger range of donor masses that can stably transfer matter and become a He mass transferring AM CVn binary. Even once in the long-lived AM CVn mass transferring stage, these He WDs have larger radii due to their higher entropy from the prolonged H-burning stage.

We have discovered a partially eclipsing white dwarf, low-mass M dwarf binary (3.015114 hour orbital period), SDSS J143547.87+373338.5, from May 2007 observations at the WIYN telescope. Here we present blue band photometry of three eclipses. Eclipse fitting gives main sequence solutions to the M dwarf companion of M S = 0.15 − 0.35M ⊙ and R S = 0.17 − 0.32R ⊙ . Analysis of the SDSS spectrum constrains the M dwarf further to be of type M4-M6 with M S = 0.11 − 0.20M ⊙ . Once full radial velocity curves are measured, high precision determinations of the masses and radii of both components will be easily obtained without any knowledge of stellar structure or evolution. ZZ Ceti pulsations from the white dwarf were not found at our 4 mmag detection limit.