Uncovering Stellar Mysteries – Astrophysicists Observe Blue Supergiants in Detail

Researchers have conducted a comprehensive study on 750 blue supergiant stars within 6,500 light-years of Earth using the IACOB project, providing valuable insights into their evolutionary phase termed “stellar adolescence.” The study’s findings pave the way for understanding the nature, evolution, and critical role of these stars in the universe, with future works aiming to elucidate their physical and chemical properties.

Stars serve as the fundamental building blocks of galaxies and, by extension, the entire observable universe. Among the diverse spectrum of stars, those with a mass exceeding 8 times that of the Sun are labeled as massive stars. Their potent radiation and formidable stellar winds exert a significant influence on the surrounding interstellar medium.

Inside these stars, elements beyond hydrogen and helium are formed, playing a pivotal role in the chemical transformation of galaxies and laying the foundation for life’s emergence. Furthermore, when these stars meet their end as supernovae, they give rise to neutron stars and black holes of stellar mass. All of this implies that their nature and evolution are critical for astrophysics.

In this context, blue supergiants is the term used to define those massive stars which are in an intermediate stage of their lives, a critical time, which could be described as “stellar adolescence” which will determine the rest of their lives and their final fates. Given the complexity of this evolutionary phase, previous research based on samples of a few tens of these stars has not managed to obtain sufficient information to understand them in detail.

Image of the double cluster h and xi Persei, in the constellation of Perseus, with the blue supergiants in the study shown with crosses and including a typical spectrum from the sample. Credit: Abel de Burgos Sierra (IAC)

In the published study observations have been taken of some 750 blue supergiant stars in a volume within 6,500 light-years of Earth, which makes this one of the most complete and highest quality samples obtained until now. For this study, the project IACOB of the IAC has spent over 15 years obtaining spectra (which are the fingerprints of stars) of high quality and high resolution of massive stars, which includes an exhaustive search for blue supergiants in the Milky Way in order to examine the large majority of them. These observations were mainly carried out with the NOT and Mercatortelescopes at the Roque de los Muchachos observatory on the island of La Palma.

“The analysis of this sample,” explains Abel de Burgos Sierra, a researcher at the IAC and the ULL, and first author of the article “will allow us to tackle some of the questions about the evolutionary nature and the physical properties of these objects which have gone for decades without solutions, because they were less well known that other types of stars with less mass, even though they are important in many fields of modern astrophysics.”

Image of the Milky Way with the blue supergiant stars from the sample superposed. Credit: DSS/Abel de Burgos Sierra (IAC)

To choose the sample, a new method of flagging has been used based on an easily identifiable tracer in the spectra of these stars (the shape of the profile of the H-neta line). Using a simple measurement this new method permits the rapid and effective identification of stars within a specific range of temperature and surface gravity. Using this the researchers did not need to derive these quantities using the normal methods of spectral analysis using complex model stellar atmospheres.

“This will be very important for identifying this type of star when the next major spectroscopic surveys of massive stars such as WEAVE-SCIP from the Roque de los Muchachos or 4MIDABLE-LRfrom La Silla, in Chile, start to observe thousands of spectra of the stars in our galaxy every night during the coming five years,” comments Sergio Simón-Díaz, and IAC researcher, a co-author of the article and Principal Investigator (IP) of the IACOB project, an international collaboration led by the IAC, whose aim is to produce the largest database ever of the spectra of massive stars in the Milky Way.

The next step, in which De Burgos is already working as part of his doctoral thesis, is to obtain accurate data about the physical parameters (mass, temperature, luminosity) and the chemical abundances (He, C, N, O, Si) for the sample of 750 blue supergiants. “This will help to answer some of the most interesting still unanswered questions which will give us a better understanding of this “adolescent phase” of massive stars,” concludes Miguel A. Urbaneja, a researcher at the University of Innsbruck and a co-author of the article.

Reference: “The IACOB project. IX. Building a modern empirical database of Galactic O9 – B9 supergiants: sample selection, description, and completeness” by A. de Burgos, S. Simón-Díaz, M. A. Urbaneja and I. Negueruela, 26 June 2023, Astronomy and Astrophysics.
DOI: 10.1051/0004-6361/202346179