Two decades of optical timing of the shortest-period binary star system HM Cancri. Issue 4 (23rd November 2022)

• Record Type: Journal Article
• Authors: Munday, James; Marsh, T R; Hollands, Mark; Pelisoli, Ingrid; Steeghs, Danny; Hakala, Pasi; Breedt, Elmé; Brown, Alex; Dhillon, V S; Dyer, Martin J; Green, Matthew; Kerry, Paul; Littlefair, S P; Parsons, Steven G; Sahman, Dave; Somjit, Sorawit; Sukaum, Boonchoo; Wild, James
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 518:Issue 4(2023)
• Page Start: 5123
• Page End: 5139
• Publication Date: 2022-11-23
• Subjects: gravitational waves -- binaries: close -- individual: RX J0806.3+1527, HM Cancri -- stars: white dwarfs; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stac3385
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: The shortest-period binary star system known to date, RX J0806.3+1527 (HM Cancri), has now been observed in the optical for more than two decades. Although it is thought to be a double degenerate binary undergoing mass transfer, an early surprise was that its orbital frequency, f 0, is currently increasing as the result of gravitational wave radiation. This is unusual since it was expected that the mass donor was degenerate and would expand on mass loss, leading to a decreasing f 0 . We exploit two decades of high-speed photometry to precisely quantify the trajectory of HM Cancri, allowing us to find that $\ddot{f}_0$ is negative, where $\ddot{f}_0~=~(-5.38\pm 2.10)\times 10^{-27}$ Hz s −2 . Coupled with our positive frequency derivative, we show that mass transfer is counteracting gravitational-wave dominated orbital decay and that HM Cancri will turn around within 2100 ± 800 yr from now. We present Hubble Space Telescope ultra-violet spectra which display Lyman-α absorption, indicative of the presence of hydrogen accreted from the donor star. We use these pieces of information to explore a grid of permitted donor and accretor masses with the Modules for Experiments in Stellar Astrophysics suite, finding models in good accordance with many of the observed properties for a cool and initially hydrogen-rich extremely low mass white dwarf (≈0.17 M⊙ ) coupled with a high-accretor mass white dwarf (≈1.0 M⊙ ). Our measurements and models affirm that HM Cancri is still one of the brightest verification binaries for the Laser Interferometer Space Antenna spacecraft.