Avi Shaforer, a postdoctoral fellow from UCSB and LCOGT, brought his expertise from the field of planets outside the solar system to the new discovery in which it was determined that one of the white dwarfs is of an extremely rare type - it is composed of helium, and not oxygen and carbon like most white dwarfs, including its companion to the system
Astrophysicists at the University of California, Santa Barbara (UCSB) are the first to identify two white dwarfs in a double star system, making it possible for the first time to directly measure the radius of a rare white dwarf composed of pure helium. The results will be published in the Astrophysical Journal Letters. These observations are the first to confirm a theory about this type of white dwarfs.
The story began when Justin Steinfadt, a PhD student in physics at UCSB, observed white dwarfs as part of his doctoral thesis with Lars Bildsten, a professor and faculty member at UCSB's Kavli Institute for Theoretical Physics (KITP) and Steve Howell of the Astronomical Optical Observatory. National (NOAO) in Tucson Arizona.
The short eclipses were discovered during an observation of the star NLTT 11748 with the Fulks North telescope, which is located in Hawaii and belongs to LCOGT, an institute affiliated with UCSB. The star, NLTT 11748, is one of a small number of low-mass, helium-core white dwarfs that are under careful study of their brightness variations. A quick photo of the star - as one exposure every minute or so, found that in several successive photos of the star it looks a little pale. Steinft quickly realized the importance of the unexpected discovery. "We watch many stars, but I still think we were lucky," he said.
Avi Shaforer, a postdoctoral fellow from UCSB and LCOGT, is an Israeli astronomer living in California. Shaporer made the first observations with the Fulks North telescope, and brought his expertise in the field of extrasolar planets to the new discovery. "We knew something was out of the ordinary, especially when we confirmed these low sections the next night as well." Shaforer said. The scientists observed the three-minute eclipses of the double star, which occur twice during 5.6 hours - the amount of time it takes the stars to complete one revolution around each other.
The excitement surrounding the discovery and the need to confirm it quickly led to the use of a 10-meter diameter telescope at the Keck Observatory located on Mauna Kea in Hawaii, just five weeks after the first observation. The team will be strengthened by David Kaplan, Amit Hubble and a postdoctoral student at KITP. Bildsten and Kaplan arranged the use of Keck by exchanging time they had reserved for another project with Jeff Marcy of Berkeley.
During the night, the researchers were able to measure the changes in the Doppler shift of the star NLTT 11748, which result from the changes in the speed of the brightest star in the system as it orbits its companion white dwarf, which is much paler but more massive than it. "It's amazing to witness how quickly the halo stars change their speed in minutes." said Kaplan, who was present at Keck during the sightings.
One of the stars in the binary system that was discovered is a rare white dwarf whose shell is made of helium, and whose mass is only 10-20% of that of the Sun. The existence of these stars was known for over twenty years. Theoretical works have predicted that these stars are hotter and larger than ordinary white dwarfs, but so far their size has not been directly measured. The observations of NLTT 11748 by the group of researchers led to the first measurement of the radius of a star of this type and to the confirmation of the theory.
The other star in the binary system is also a white dwarf, although of the more common type, and it is composed mainly of carbon and oxygen and has a melt of about 70% of the mass of the Sun. This star is more massive, but also smaller than the other white dwarf. The light it emits is 30 times paler than that of its double system counterpart.
Shaporer and Kaplan even observed that when the massive white dwarf whips its partner it causes a gravitational refraction of the light coming from the partner. This rare phenomenon, which is actually the bending of the light rays under the influence of gravity, results in a small amplification of the light during the eclipse, which makes the eclipse appear slightly shallower.
Bildsten noted that the discovery was made possible by the effective collaboration between researchers from different institutes, KITP, UCSB and LCOGT. "An interesting and fascinating possibility is what will happen in 6-10 billion years" Bildsten said. "The double system emits gravitational waves at a rate that would require the two white dwarfs to come into contact. What will happen then - no one can guess."
An animated illustration of the small white dwarf passing the larger of the two
For a press release from the University of California at Santa Barbara
Dr. Avi Shaforer is a post-doctoral fellow at the University of California, Santa Barbara (UCSB) and Mitzpe Las-Combes (LCOGT). Shaforer did his doctoral thesis at Tel Aviv University under the guidance of Professor Zvi Maza, which he completed in 2009. His main area of research is exoplanets and he also studies double-loud stars containing a compact object (black hole, neutron star or white dwarf). As part of his work, he uses many telescopes around the world and in space, which also include the telescopes at the Wise Observatory in the Negev. Sporer was involved in a number of interesting astronomical discoveries, such as the discovery of the angel planets HAT-P-2b, CoRoT-7b and CoRoT-9b, the discovery of the defects of the planet GJ436b and the measurement of the mass of the stellar black hole M33 X-7. So far he has been involved in 41 articles published in the professional literature and his work has won him several awards, including the Zeev Frankel Award of the Israel Physics Society.
More of the topic in Hayadan:
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to Max Power
As far as I understand, the helium white dwarf was created in a process of nuclear combustion that stops in the helium stage due to its small mass, as indicated in the article, only about 10-20% of the mass of the (our) Sun.
The larger white dwarf burns to the oxygen-carbon phase due to its greater mass, about 70% solar mass. In general, the greater the star's mass, it manages to burn into heavier elements, up to iron!
And the nuclear combustion time is getting shorter as the mass of the star increases.
As indicated in the article, the dipole slowly collapses due to the emission of gravitational energy.
When the two dwarfs merge, they will, together, have a mass less than the mass of the Sun,
In my humble opinion as:
0.8 solar masses = (6% mass that becomes gravitational energy) – (0.7 + 0.15) the sum of the masses of the two dwarfs
From the little I know on the subject, a mass of 0.8 solar masses is the lower limit of mass required for a supernova explosion that leaves behind an iron core at the end of nuclear combustion.
That is, if the total mass of the two dwarfs is not heavy enough to create a supernova, nuclear combustion will start again, which will stop in the carbon-oxygen phase, similar to the large dwarf.
Of course, the prediction may change, if in more accurate measurements later, different values for the dipole masses will be measured.
The paper does not explain how a helium white dwarf was formed, apparently there was not enough mass to reach the temperatures that allow helium fusion.