Free Astronomy Magazine July-August 2019

49 SPACE CHRONICLES Very Large Telescope (VLT), in Chile, the researchers were able to accu- rately measure the system for the first time. The article is the first to be pub- lished using HiPERCAM data. Ac- cording to Vikram Dhillon, an astrophysicist at the University of Sheffield, an IAC affiliate researcher and one of those responsible for the HiPERCAM project: “Our meas- urement of the mass and radius of this ancient star confirms a key pre- diction of the theory of stellar structure and demonstrates the unique capabilities of our new cam- era HiPERCAM on the 10.4m Gran Telescopio Canarias” . This camera can take one picture every millisecond, unlike other cam- eras that normally only take one picture every few minutes. Thanks to its high capture speed, HiPER- CAM makes it possible to study in unprecedented detail objects with rapid variations in brightness due to phenomena such as eclipses and explosions. “The observations with HiPERCAM in GTC have allowed to obtain the curves of light of this object in up to 5 filters simultaneously with an unprecedented precision, thanks to the telescope light collecting capac- ity. The HiPERCAM/GTC combina- tion opens an extraordinary door to the study of objects with temporal variability, providing a very high temporal resolution and covering a range of magnitudes unattainable for other telescopes” , explains An- tonio Cabrera, head of scientific op- erations at the GTC. With these values, together with the temperature and the luminosity of the cool subdwarf star also ob- tained from the observations, the authors were able to validate, for the first time, the theoretical rela- tions between the mass, radius, lu- minosity, and temperature for the oldest stars in our Galaxy. A rtist’s impression of an eclipsing binary system composed by a cool subdwarf (yellow) and a white dwarf (white). [Mark Garlick] Alberto Rebassa Mansergas, who led the study. Because old stars are faint and they are relatively rare in the vicinity of the Sun, few cool subdwarfs are known in our solar neighbourhood. Currently, the ra- dius of only 88 and the masses of six cool subdwarfs have been esti- mated. However, no mass and ra- dius values for the same cool subdwarf had been accurately measured, leaving the theo- retical studies for such stars untested until now. In their work, the researchers have found the first cool subdwarf in an eclips- ing binary system, a system where two stars orbit one an- other, in this case, a cool subdwarf and a white dwarf. “Eclipsing binaries offer the opportu- nity to measure di- rectly the masses and radii of the two components with unprecedented pre- cision” , argues Re- bassa Mansergas. Until now scientists did not have a camera powerful enough to get accurate measure- ments of stellar compo- nents such as mass and radius. Thanks to the use of the HiPERCAM instrument in- stalled in the GTC, in the Roque de los Muchachos Observatory (Garafía, La Palma), combined with data from the X-Shooter instrument of the Unit 2 telescope of ESO’s ! JULY-AUGUST 2019