An international team of scientists, led by the Polytechnic University of Catalonia BarcelonaTech (UPC) and the Institute of Space Studies of Catalonia (IEEC), has measured for the first time the stellar parameters of a class of very old stars, known as cold sub-subanga stars, in our galaxy, the Milky Way.
These stars are apparently very similar to our sun, but their mass and radius are smaller, since they formed during the early phases of the Milky Way. And here is precisely why they make them so interesting for astronomers: because they contain important information about their structure and chemical evolution.
The work, whose results are published in the journal Nature Astronomy, has been carried out in collaboration with researchers from the University of Sheffield, the Institute of Astrophysics of the Canary Islands (IAC), the Great Canary Telescope (GTC) and the National Astronomical Observatories of the Chinese Academy of Sciences.
As reported by the IAC in a statement, when the Milky Way was formed, the first stars were composed mainly of hydrogen. In Astronomy, heavier elements than hydrogen and helium are considered metals, and their presence in a star determines their metallicity. As time went by and the stars were dying, the content of these metals in the Galaxy and in the new stars that were born was increasing. Therefore, old stars have a lower metallicity than younger ones.
"Since old stars can reveal important information about the structure and chemical evolution of the Milky Way, It is essential that astronomers determine their most basic stellar parameters, such as mass and radius. " explains the researcher from UPC and IEEC Alberto Rebassa Mansergas, who has directed the study.
The problem is that the old stars are weak and relatively rare around the Sun, so there are few cold subenanas known in our solar neighborhood. Currently, the radios of 88 cold subenanas and the masses of only six.
"Up until now, the values of the mass and the radius of the same cold sub-field had not been accurately measured, which left the theoretical studies on these stars unproven," recalls the IAC.
Researchers have found the first cold subenana in a system in which two stars orbit each other; in this case, it is a cold subenana and a white dwarf.
The finding has been possible thanks to the use for the first time of a camera powerful enough to obtain accurate measurements of stellar parameters such as mass and radius, HiPERCAM, installed at the Roque de los Muchachos Observatory (Garafía, La Palma) , combined with data from the X-Shooter instrument of the Unit 2 telescope of the Very Large Telescope (VLT) of the European Southern Observatory (ESO), in Chile.
This camera can take a picture every millisecond, unlike other cameras that normally only take a picture every few minutes. Thanks to its high speed of capture, HiPERCAM allows to study with unprecedented detail objects with fast variations of brightness, due to phenomena such as eclipses and explosions.
With these values, together with the temperature and luminosity of the cold sub-star, also obtained from the observations, the authors were able to validate, for the first time, the theoretical relationship between mass, radius, luminosity and temperature of the oldest stars of our galaxy.