The shortest period detached binary white dwarf system★

The Astrophysical Journal, 2012

We report on the discovery of the shortest period binary comprising a hot subdwarf star (CD−30 11223, GALEX J1411−3053) and a massive unseen companion. Photometric data from the All Sky Automated Survey show ellipsoidal variations of the hot subdwarf primary and spectroscopic series revealed an orbital period of 70.5 minutes. The large velocity amplitude suggests the presence of a massive white dwarf in the system (M 2 /M ⊙ 0.77) assuming a canonical mass for the hot subdwarf (0.48 M ⊙), although a white dwarf mass as low as 0.75 M ⊙ is allowable by postulating a subdwarf mass as low as 0.44 M ⊙. The amplitude of ellipsoidal variations and a high rotation velocity imposed a high-inclination to the system (i 68 •) and, possibly, observable secondary transits (i 74 •). At the lowest permissible inclination and assuming a subdwarf mass of ∼ 0.48 M ⊙ , the total mass of the system reaches the Chandrasekhar mass limit at 1.35 M ⊙ and would exceed it for a subdwarf mass above 0.48 M ⊙. The system should be considered, like its sibling KPD 1930+2752, a candidate progenitor for a type Ia supernova. The system should become semi-detached and initiate mass transfer within ≈ 30 Myr. Subject headings: binaries: close-white dwarfs * Based on observations made with ESO telescopes at the La Silla Paranal Observatory under programme ID 83.D

2010

The Laser Interferometer Space Antenna (LISA) will open the low-frequency (0.1-100 mHz) part of the gravitational wave spectrum to direct observation. Of order 3600 galactic close binary white dwarfs will be individually resolvable in its all-sky spectrum, of which a dozen systems are expected to be on the verge of merger, showing the effects of strong tidal heating and/or early onset of tidal mass transfer. Optical study of these systems would provide important insights into tidal dissipation mechanisms, and internal heating in merging white dwarfs that sets ignition conditions for potential type Ia supernovae. Theoretical modeling and instrumentation programs are needed now to enable a campaign for optical identifications to exploit this opportunity.

The Astrophysical Journal, 2021

We report the discovery of SDSS J133725.26+395237.7 (hereafter SDSS J1337+3952), a double-lined white dwarf (WD+WD) binary identified in early data from the fifth generation Sloan Digital Sky Survey (SDSS-V). The double-lined nature of the system enables us to fully determine its orbital and stellar parameters with follow-up Gemini spectroscopy and Swift UVOT ultraviolet fluxes. The system is nearby (d = 113 pc), and consists of a 0.51 M primary and a 0.32 M secondary. SDSS J1337+3952 is a powerful source of gravitational waves in the millihertz regime, and will be detectable by future space-based interferometers. Due to this gravitational wave emission, the binary orbit will shrink down to the point of interaction in ≈ 220 Myr. The inferred stellar masses indicate that SDSS J1337+3952 will likely not explode as a Type Ia supernova (SN Ia). Instead, the system will probably merge and evolve into a rapidly rotating helium star, and could produce an under-luminous thermonuclear supernova along the way. The continuing search for similar systems in SDSS-V will grow the statistical sample of double-degenerate binaries across parameter space, constraining models of binary evolution and SNe Ia.

The Astrophysical Journal, 2011

We describe new radial velocity and X-ray observations of extremely lowmass white dwarfs (ELM WDs, ∼ 0.2M ⊙) in the Sloan Digital Sky Survey Data Release 4 and the MMT Hypervelocity Star survey. We identify four new short period binaries, including two merger systems. These observations bring the total number of short period binary systems identified in our survey to 20. No main-sequence or neutron star companions are visible in the available optical photometry, radio, and X-ray data. Thus, the companions are most likely WDs. Twelve of these systems will merge within a Hubble time due to gravitational wave radiation. We have now tripled the number of known merging WD systems. We discuss the characteristics of this merger sample and potential links to underluminous supernovae, extreme helium stars, AM CVn systems, and other merger products. We provide new observational tests of the WD mass-period distribution and cooling models for ELM WDs. We also find evidence for a new formation channel for single low-mass WDs through binary mergers of two lower mass objects.

The Astrophysical Journal, 2010

We present radial velocity observations of four extremely low-mass (0.2 M ⊙) white dwarfs. All four stars show peak-to-peak radial velocity variations of 540-710 km s −1 with 1.0-5.9 hr periods. The optical photometry rules out mainsequence companions. In addition, no millisecond pulsar companions are detected in radio observations. Thus the invisible companions are most likely white dwarfs. Two of the systems are the shortest period binary white dwarfs yet discovered. Due to the loss of angular momentum through gravitational radiation, three of the systems will merge within 500 Myr. The remaining system will merge within a Hubble time. The mass functions for three of the systems imply companions more massive than 0.46 M ⊙ ; thus those are carbon/oxygen core white dwarfs. The unknown inclination angles prohibit a definitive conclusion about the future of these systems. However, the chance of a supernova Ia event is only 1% to 5%. These systems are likely to form single R Coronae Borealis stars, providing evidence for a white dwarf + white dwarf merger mechanism for these unusual objects. One of the systems, SDSS J105353.89+520031.0 has a 70% chance of having a low-mass white dwarf companion. This system will probably form a single helium-enriched subdwarf O star. All four white dwarf systems have unusal mass ratios of ≤ 0.2 − 0.8 that may also lead to the formation of AM CVn systems.

The Astrophysical Journal, 2012

We identify two new tidally distorted white dwarfs (WDs), SDSS J174140.49+652638.7 and J211921.96−001825.8 (hereafter J1741 and J2119). Both stars are extremely low-mass (ELM, ≤ 0.2M ⊙) WDs in short-period, detached binary systems. High-speed photometric observations obtained at the McDonald Observatory reveal ellipsoidal variations and Doppler beaming in both systems; J1741, with a minimum companion mass of 1.1 M ⊙ , has one of the strongest Doppler beaming signals ever observed in a binary system (0.59±0.06% amplitude). We use the observed ellipsoidal variations to constrain the radius of each WD. For J1741, the star's radius must exceed 0.074 R ⊙. For J2119, the radius exceeds 0.10 R ⊙. These indirect radius measurements are comparable to the radius measurements for the bloated WD companions to A-stars found by the Kepler spacecraft, and they constitute some of the largest radii inferred for any WD. Surprisingly, J1741 also appears to show a 0.23±0.06% reflection effect, and we discuss possible sources for this excess heating. Both J1741 and J2119 are strong gravitational wave sources, and the time-of-minimum of the ellipsoidal variations can be used to detect the orbital period decay. This may be possible on a timescale of a decade or less.

The Astrophysical Journal, 2009

We report on the detection of the two shortest period non-interacting white dwarf binary systems.

Monthly Notices of the Royal Astronomical Society, 2014

We present ULTRACAM photometry and X-Shooter spectroscopy of the eclipsing double white dwarf binary CSS 41177, the only such system that is also a double-lined spectroscopic binary. Combined modelling of the light curves and radial velocities yield masses and radii for both white dwarfs without the need to assume mass-radius relations. We find that the primary white dwarf has a mass of M 1 = 0.38 ± 0.02 M ⊙ and a radius of R 1 = 0.0222 ± 0.0004 R ⊙ . The secondary white dwarf's mass and radius are M 2 = 0.32 ± 0.01 M ⊙ and R 2 = 0.0207 ± 0.0004 R ⊙ , and its temperature and surface gravity (T 2 = 11678 ± 313 K, log(g 2 ) = 7.32 ± 0.02) put it close to the white dwarf instability strip. However, we find no evidence for pulsations to roughly 0.5% relative amplitude. Both masses and radii are consistent with helium white dwarf models with thin hydrogen envelopes of 10 −4 M * . The two stars will merge in 1.14 ± 0.07 Gyr due to angular momentum loss via gravitational wave emission.

arXiv (Cornell University), 2023

We report the discovery of SDSS J022932.28+713002.7, a nascent extremely low-mass (ELM) white dwarf (WD) orbiting a massive (> 1 M ⊙ at 2σ confidence) companion with a period of 36 hours. We use a combination of spectroscopy, including data from the ongoing SDSS-V survey, and photometry to measure the stellar parameters for the primary pre-ELM white dwarf. The lightcurve of the primary WD exhibits ellipsoidal variation, which we combine with radial velocity data and PHOEBE binary simulations to estimate the mass of the invisible companion. We find that the primary WD has mass M 1 = 0.18 +0.02 -0.02 M ⊙ and the unseen secondary has mass M 2 = 1.19 +0.21 -0.14 M ⊙ . The mass of the companion suggests that it is most likely a near-Chandrasekhar mass white dwarf or a neutron star. It is likely that the system recently went through a Roche lobe overflow from the visible primary onto the invisible secondary. The dynamical configuration of the binary is consistent with the theoretical evolutionary tracks for such objects, and the primary is currently in its contraction phase. The measured orbital period puts this system on a stable evolutionary path which, within a few Gyrs, will lead to a contracted ELM white dwarf orbiting a massive compact companion.

2010

An XMM-Newton observation performed in May 2008 has confirmed that the 13 seconds pulsations in the X-ray binary HD 49798/RX J0648.0-4418 are due to a rapidly rotating white dwarf. From the pulse time delays induced by the 1.55 days orbital motion, and the system's inclination, constrained by the duration of the X-ray eclipse discovered in this observation, we could derive a mass of 1.28±0.05 M ⊙ for the white dwarf. The future evolution of this post common envelope binary system will likely involve a new phase of mass accretion through Rochelobe overflow that could drive the already massive white dwarf above the Chandrasekhar limit and produce a Type Ia supernova.