First Observational Proof of Existence of a Double-Degenerate, Super-Chandrasekhar System


Applying for time	Object visibility
Archive	Observing logs
Computing	Override programmes
Conferences and meetings	Planning observations
Detectors	Publications
Exposure time calculator	Schedules
Fault reports	Seminar programme
Feedback	Service programme
Filters	Site quality
[INGNEWS]	Studentship programme
Instruments	Time allocations
Isaac Newton Telescope	Weather and seeing
Library	William Herschel Telescope

An international group of astronomers has found the first pair of white dwarfs with a total combined mass unequivocally above the Chandrasekhar limit of 1.4 solar masses. The orbital period of the system is 4.2 hours, which means that the stars are close enough to spiral in due to the emission of gravitational waves and eventually merge in about 700 million years. This finding, which has been published in Nature, provides observational support to the double-degenerate path of formation of type Ia supernovae, so far a theoretical possibility under ongoing debate.

Close-up view of the bipolar planetary nebula Henize 2-428. This 2-h exposure in H-alpha 656.3 nm was obtained using the WFC on the INT. North is up and East is to the left. Credit: Santander-García et al (2015) [ JPG ].

Astronomers were looking for binaries in the heart of narrow-waisted, very collimated bipolar planetary nebulae in order to test the binary hypothesis, which predicts that angular momentum from a close companion could help shape the planetary nebula, giving it a bipolar morphology with the equator located along the orbital plane of the system. They used the Mercator telescope at La Palma to monitor the cores of the nebulae in search for periodic photometric variability. The shape of the light-curve of one of the objects of the sample, Henize 2-428, showed signs that both stars could be white dwarfs. The group then applied for observational time at the Isaac Newton Telescope (INT), where they obtained a new, more accurate light curve, and at the William Herschel Telescope (WHT), where they took a series of spectra in order to find radial-velocity Doppler-shifts from one or both stars. The spectra had too low signal-to-noise (S/N) ratio to measure the Doppler shifts but showed what seemed to be a double He II absorption line which would come from each of the stars.

A series of test spectra with the VLT in Chile confirmed that this was indeed the case, but the S/N was too low to allow for precise measurements. The next step was applying for time with Gran Telescopio de Canarias, with which the Doppler shifts could finally be measured and the orbit and stellar parameters solved.

The resulting system not only provided additional support for the binary hyphotesis (so far, the equator coincides with the orbital plane of the system in the eight cases in which there is enough data to solve the system), but hid a further surprise: the combined mass of the stars was above the Chandrasekhar limit even when considering uncertainties, and the stars were close enough to merge in a Hubble time.

Light curve of Henize 2-428 in two different photometric filters, and model. The shape of the periodic brightness variation is due to the stars being heavily distorted by each other’s tidal forces. The light curve data, together with radial velocity curves from the stars, allowed to reliably model the system’s orbital and stellar parameters. Credit: Santander-García et al (2015) [ PNG ].