Fastest Known Binary Star

My high-school students are probably tiring of my telling them how much information about a star can be divined simply from measuring its light, especially when the observations involve multiple spectral regions. Well, they're going to hear it again, when I tell them the amazing story of HM Cancri.

With a visual magnitude of 21, this star wasn't even known to astronomers until 1999, when the orbiting Rosat observatory discovered an X-ray source that brightens and dims every 5.4 minutes. To observers, such isolated X-ray emitters usually scream "interacting binary" — including a degenerate has-been of a star, a white dwarf or neutron star, that's pulling matter off a hapless companion. The exiting stream slams onto the dwarf or neutron star (or onto a disk around it) violently enough to generate X rays.

But could HM Cancri's twosome actually be whirling around each other in less time than I usually need to take out the trash? No other known binary system has a period this short (the closest contender, V407 Vulpeculae, clocks in at 9½ minutes). In the past decade theorists have offered two alternative interpretations, which require a binary system with a dense object either rotating or orbiting with a 5.4-minute period.

New observations seem to clinch the idea that HM Cancri's two stars really are locked in a 5.4-minute whirlwind dance. An international team led by Gijs Roelofs (Harvard-Smithsonian Center for Astrophysics) captured the system's spectrum last year using the Keck I telescope on Mauna Kea. The researchers had been doggedly persistent — they'd wanted to make these observations in 2005, 2006, and 2007, but bad weather scuttled their plans each time.

The Keck spectra reveal blue-light emission from hot neutral helium (presumed to come from the donor star) and from ionized helium (from the hotter, denser star accreting the matter). What makes the case ironclad is that these emissions are Doppler-shifted in opposite directions. When one source is moving away from us, the other is moving toward us, and vice versa, clinching the case for a fast binary. A single neutron star, no matter how fast it's spinning, can't match the Keck observations. The artist's concept at upper right shown the presumed situation.

Roelofs and his colleagues calculate that HM Cancri's stars are only about 3 Earth diameters apart — so close that the donor star (also a white dwarf, with about a quarter of the Sun's mass) must be swollen and distended by the pull of its heavier companion (with about half a Sun's mass). A double peak in the ionized-helium emission also implies that a fast-spinning ring of matter encircles the accreting star, either as a belt on its surface or as a disk above it.

One remaining loose end is finding an explanation for why the paired stars are gradually twirling faster and faster. Since the total mass is conserved, the mass transfer can't explain it. "The binary HM Cancri is a real challenge for our understanding of stellar and binary evolution," notes team member Gijs Nelemans (Radboud University, The Netherlands) in a press release. The team also hopes to pin down the system's distance, which is only roughly estimated to be 16,000 light-years.

No matter how these stars became so closely entwined (they're much too close to have been born that way), HM Cancri's tight whirlabout must make it one of the strongest sources of gravitational waves in the galaxy. As high-energy astrophysicist Tod Strohmayer (NASA-Goddard Space Flight Center) points out, "This object is likely radiating more energy in gravitational waves than in electromagnetic energy." That in itself could account for the pair's orbital-energy loss and period speedup. Not surprisingly, it's high on the list of targets for the European Space Agency's proposed (but as yet unfunded) Laser Interferometer Space Antenna, designed to detect sources of low-frequency gravitational waves using a triad of widely separated spacecraft.