What is the population of spectroscopic binaries?

Using associations of nearby young stars, the properties of those stars can be studied in detail. For example, it is well known that many stars are not “isolated objects” but evolve in space linked to a companion, i.e. they are binary systems, or even triple systems and more. Among these binaries there is also a very diverse zoology depending largely on the average physical separation between the pairs of stars. In particular, among the binaries with smaller distances between them are the spectroscopic binaries. In these systems, the two components are so close together that they cannot be resolved separately in images, even with the best telescopes we currently have.

Previous studies of populations of different ages pointed to 10% as a “universal” percentage of spectroscopic binaries. That is, on average, one out of every 10 stars observed in our galaxy is a spectroscopic binary. This result had been corroborated even by observing stars in SACY (Search for Associations Containing Young stars).

Recently, an international team of astrophysicists led by Sebastián Zúñiga-Fernández, PhD student at the Millennium Nucleus of Planetary Formation and the European Southern Observatory, ESO, published a paper in the prestigious scientific journal Astronomy & Astrophysics in which they study the fraction of spectroscopic binaries in the SACY sample, whose conclusions challenge the idea that the fraction of spectroscopic binaries is universal and does not exceed 10%.

Amelia Bayo, director of the Millennium Nucleus of Planetary Formation; Catalina Zamora, master’s student; Gabriel Corvalán, graduate in physics with mention in astronomy and collaborator of the NPF; and Johan Olofsson, associate researcher of the center, also participated in the research.

“Our results show a fraction of spectroscopic binaries between 20 and 30% for the three youngest associations. These percentages show signs of possible non-universal multiplicity among these groups. When analyzing the direction of the velocities of these three younger groups we found indications that they come from a different origin than the rest of the associations”, explains Sebastián Zúñiga-Fernández, who also belongs to the Institute of Physics and Astronomy of the University of Valparaíso.

These different origins, adds the scientist, could explain the difference in the proportion of spectroscopic binaries, so this fraction of 10% would not be a universal result for all young associations.

For this study, the team of researchers used archival observations from ESO to estimate the radial velocity and rotation speed of the stars. In addition, the authors collected measurements from the literature. In this way, by measuring how the velocities are changing as a function of time, new spectroscopic binaries were discovered in the SACY sample.

“This result opens up new challenges in understanding the formation and evolution of these binaries. We still need to better understand the relationship between the fraction of spectroscopic binaries and the original environment where the stars were born and spent their early years or how it affects whether they are accompanied by other objects. Studies of multiplicity in associations of young stars allow us to better understand the conditions in which these systems formed and the time scales of these processes, thus improving our understanding of stellar evolution,” the researcher points out.

Zúñiga-Fernández explains that it is necessary to verify these results with more spectroscopic observations and to follow up the binary candidates in order to confirm these systems and better characterize them.

“With more data for the binary candidates we can obtain the characteristics of these systems such as period, eccentricity and even their masses. This additional data can give us valuable information regarding the history of the formation of these systems, while at the same time we can understand where these differences come from,” he concludes.

Scientific publication

Image credit: NASA

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