A new class of pulsating white dwarf of extremely low mass: the fourth and fifth members

作者：Hermes J J^*; Montgomery M H; Gianninas A; Winget D E; Brown Warren R; Harrold Samuel T; Bell Keaton J; Kenyon Scott J; Kilic Mukremin; Castanheira Barbara G

来源：Monthly Notices of the Royal Astronomical Society, 2013, 436(4): 3573-3580.

DOI：10.1093/mnras/stt1835

• Stars: individual: SDSS J161431
• 28+191219
• 4
• Stars: individual: SDSS J222859
• 93+362359
• 6
• Stars: oscillations
• white dwarfs
• Galaxy: stellar content

We report the discovery of two new pulsating extremely low-mass (ELM) white dwarfs (WDs), SDSS J161431.28+191219.4 (hereafter J1614) and SDSS J222859.93+362359.6 (hereafter J2228). Both WDs have masses %26lt; 0.25 M-circle dot and thus likely harbour helium cores. Spectral fits indicate these are the two coolest pulsating WDs ever found. J1614 has T-eff = 8880 +/- 170 K and log g = 6.66 +/- 0.14, which corresponds to a similar to 0.19 M-circle dot WD. J2228 is considerably cooler, with a T-eff = 7870 +/- 120 K and log g = 6.03 +/- 0.08, which corresponds to an similar to 0.16 M-circle dot WD, making it the coolest and lowest mass pulsating WD known. There are multiple ELM WDs with effective temperatures between the warmest and coolest known ELM pulsators that do not pulsate to observable amplitudes, which questions the purity of the instability strip for low-mass WDs. In contrast to the CO-core ZZ Ceti stars, which are believed to represent a stage in the evolution of all such WDs, ELM WDs may not all evolve as a simple cooling sequence through an instability strip. Both stars exhibit long-period variability (1184-6235 s) consistent with non-radial g-mode pulsations. Although ELM WDs are preferentially found in close binary systems, both J1614 and J2228 do not exhibit significant radial-velocity variability, and are perhaps in low-inclination systems or have low-mass companions. These are the fourth and fifth pulsating ELM WDs known, all of which have hydrogen-dominated atmospheres, establishing these objects as a new class of pulsating WD.

• 出版日期2013-12