Abstract
Abstract We report the discovery of the fifth self-lensing binary in which a low-mass white dwarf (WD) gravitationally magnifies its 15th magnitude G-star companion, KIC 8145411, during eclipses. The system was identified from a pair of such self-lensing events in the Kepler photometry, and was followed up with the Tillinghast Reflector Echelle Spectrograph (TRES) on the 1.5 m telescope at the Fred Lawrence Whipple Observatory and the High-Dispersion Spectrograph (HDS) on the Subaru 8.2 m telescope. A joint analysis of the TRES radial velocities, the HDS spectrum, and the Kepler photometry of the primary star determines the WD mass 0.20 ± 0.01 M ⊙, orbital semimajor axis 1.28 ± 0.03 au, and orbital eccentricity 0.14 ± 0.02. Because such extremely low-mass WDs cannot be formed in isolation within the age of the Galaxy, their formation is believed to involve binary interactions that truncated evolution of the WD progenitor. However, the observed orbit of the KIC 8145411 system is at least 10 times wider than required for this scenario to work. The presence of this system in the Kepler sample, along with its similarities to field blue straggler binaries presumably containing WDs, may suggest that some 10% of post-asymptotic giant branch binaries with Sun-like primaries contain such anomalous WDs.
Highlights
The lowest-mass known white dwarfs (WDs) in the Galaxy have masses below 0.2 M (e.g., Kilic et al 2007a)
Because the Galaxy is not old enough to produce them through single-star evolution, these so-called extremely low-mass (ELM) WDs with 0.25 M are considered to be remnants of mass transfer during the red-giant branch (RGB) phase, in which hydrogen envelope of the WD progenitor was stripped to leave a degenerate helium core with 0.45 M, before the star ignites helium and enters the asymptotic giant branch (AGB) phase (e.g., Marsh et al 1995)
For low-mass WDs identified as companions to millisecond pulsars, the mass transfer model successfully explained the correlation between their orbital periods and WD masses (Rappaport et al 1995; Tauris & Savonije 1999), as well as the rejuvenated nature of the pulsars
Summary
The lowest-mass known white dwarfs (WDs) in the Galaxy have masses below 0.2 M (e.g., Kilic et al 2007a). The WD was identified from periodic brightening of KIC 8145411 due to in-eclipse microlensing in the archival photometry of the Kepler spacecraft (Borucki et al 2010). This “self-lensing” has been used to detect four WDs in binaries with orbital periods 88–728 days (Kruse & Agol 2014; Kawahara et al 2018), and here we report the fifth such case. We characterize the system combining the Kepler photometry and groundbased spectroscopy (Section 2), and make a strong case that the companion is an ELM WD even though its light is not observed (Section 3). We briefly comment on possible formation paths (Section 6)
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