* Astronomy

Blobrana · L

RE: NLTT 11748

Blobrana · L

Unique Eclipsing Binary Star System Discovered by UCSB Astrophysicists

Astrophysicists at UC Santa Barbara are the first scientists to identify two white dwarf stars in an eclipsing binary system, allowing for the first direct radius measurement of a rare white dwarf composed of pure helium. The results will be published in the Astrophysical Journal Letters. These observations are the first to confirm a theory about a certain type of white dwarf star.
The story began with observations by Justin Steinfadt, a UCSB physics graduate student who has been monitoring white dwarf stars as part of his Ph.D. thesis with Lars Bildsten, a professor and permanent member of UCSB's Kavli Institute for Theoretical Physics, and Steve Howell, an astronomer at the National Optical Astronomy Observatory (NOAO) in Tucson, Ariz.
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Blobrana · L

Scientists have identified two white dwarf stars in an eclipsing binary system, allowing for the first direct radius measurement of a rare white dwarf composed of pure helium.
Brief eclipses were discovered during observations of the star NLTT 11748 with the Faulkes Telescope North of the Las Cumbres Observatory Global Telescope (LCOGT), a UCSB-affiliated institution. NLTT 11748 is one of the few very low-mass, helium-core white dwarfs that are under careful study for their brightness variations.
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Title: Discovery of the Eclipsing Detached Double White Dwarf Binary NLTT 11748
Authors: Justin D. R. Steinfadt, David L. Kaplan, Avi Shporer, Lars Bildsten, Steve B. Howell

We report the discovery of the first eclipsing detached double white dwarf (WD) binary. In a pulsation search, the low-mass helium core WD NLTT 11748 was targeted for fast (approx 1-min) differential photometry with the Las Cumbres Observatory's Faulkes Telescope North. Rather than pulsations, we discovered approx 180-s 3-6% dips in the photometry. Subsequent radial velocity measurements of the primary from the Keck telescope found variations with a semi-amplitude K_1 = 271 ±3 km/s, and confirmed the dips as eclipses caused by an orbiting WD with a mass M_2 = 0.648-0.771 M_sun for M_1 = 0.1-0.2 M_sun. We detect both the primary and secondary eclipse during the P_orb = 5.64 hr orbit and measure the secondary's brightness to be 3.5 ±0.3% of the primary at SDSS-g'. Assuming that the secondary follows the mass-radius relation of a cold C/O WD and including the effects of microlensing in the binary, the primary eclipse yields a primary radius of R_1 = 0.043-0.039 R_sun for M_1 = 0.1-0.2 M_sun; consistent with the theoretically expected values for a helium core WD with a thick, stably burning hydrogen envelope. Though nearby (at approx 150 pc), the gravitational wave strain from NLTT 11748 is likely not adequate for direct detection by the the Laser Interferometer Space Antenna. Future observational efforts will determine M_1, yielding accurate WD mass-radius measurement of both components, as well as a clearer indication of the binary's fate once contact is reached.

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