Two decades of optical timing of the shortest-period binary star system HM Cancri

James Munday; T. R. Marsh; Mark Hollands; Ingrid Pelisoli; Danny Steeghs; Pasi Hakala; Elmé Breedt; Alex Brown; V. S. Dhillon; Martin J. Dyer; Matthew Green; Paul Kerry; S. P. Littlefair; Steven G. Parsons; Dave Sahman; Sorawit Somjit; Boonchoo Sukaum; James Wild

doi:10.1093/mnras/stac3385

paper

Two decades of optical timing of the shortest-period binary star system HM Cancri

arXiv:2211.09834 · doi:10.1093/mnras/stac3385

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 $f ¨_{0}$ is negative, where $f ¨_{0} = (- 5.38 \pm 2.10) \times 1 0^{- 27}$ 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 $2100 \pm 800$ yrs 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 ( $\approx 0.17$ M $_{⊙}$ ) coupled with a high accretor mass white dwarf ( $\approx 1.0$ M $_{⊙}$ ). Our measurements and models affirm that HM~Cancri is still one of the brightest verification binaries for the Laser Interferometer Space Antenna spacecraft.

12 pages (+5 pages appendix), 9 figures, 6 tables. Accepted for publication in MNRAS

Two decades of optical timing of the shortest-period binary star system HM Cancri

Abstract

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