University Astronomers Discover a New Mechanism of Pulsating Star Formation
2012-04-13
The international research team led by Prof. Grzegorz Pietrzyński from the Astronomical Observatory of the University of Warsaw (AOUW) has published a fascinating paper in the prestigious journal Nature, reporting surprising findings about a unique binary star system near the centre of the Milky Way.
The system, bearing the catalogue name OGLE-BLG-RRLR-02792, comprises a red giant and a star pulsating in a way characteristic of a large group of variable stars, known as RR Lyrae stars. Stars of this class are extremely important in contemporary astrophysics, since they are often used as standard candles to measure galactic distances. Although they have been known for over a century, they still conceal many secrets and remain the subject of intensive study.
Binary stars that eclipse (passing in front of each other as they orbit a shared centre of mass) offer astronomers a chance to estimate star mass and size with great precision. Prof. Igor Soszyńskis discovery of OGLE-BLG-RRLR-02792 therefore provided a unique opportunity to study a RR Lyrae star in a new way: as well as changes in brightness resulting from RR Lyrae pulsations, observations of the system have also registered eclipses.
It should be noted that this represents another discovery of an extremely rare object made on the basis of unique data gathered as part of the Polish OGLE project, directed by Prof. Andrzej Udalski. Even though there are tens of thousands of RR Lyrae stars (the majority of which have been discovered by the OGLE team), never before had a binary eclipsing system been identified including a star of this type.
We thought that the OGLE-BLG-RRLR-02792 system would be the first instance where we would be able to measure the mass of an RR Lyrae star, allowing us to test our understanding of these distance indicators, says Prof. Soszyński, discoverer of the OGLE-BLG-RRLR-02792 system.
Using the worlds largest and most modern telescopes (VLT, Magellan), additional observations of the system were conducted as part of the Araucaria project. In combination with the brightness measurements taken by the OGLE team, these observations enabled the physical parameters of this unusual system to be pinned down.
The results came as quite a shock: the mass of the pulsating star turned out to be just a quarter of that of our own Sun! says Prof. Pietrzyński, leader of the team and first author of the paper. That finding contradicts our current understanding of RR Lyrae stars. It has been commonly thought that their mass should be around double of what we found. This discovery changes our thinking about the physical processes governing the evolution of stars, he added.
The mystery of the OGLE-BLG-RRLR-02792 star's low mass was solved using evolutionary models calculated by Prof. Kazimierz Stępień, also from the AOUW. The models predict that during their evolution, the stars comprising the binary system were so close together that they may have exchanged mass, adds Prof. Stępień.
As a result, the initially larger star lost its external layers, and its heated core is currently pulsating, closely imitating a classical RR Lyrae star. A century after the discovery of the first RR Lyrae stars, we have found a completely new, previously unknown mechanism forming pulsating stars, which behave identically to classical RR Lyrae stars even though they have completely different origins and physical properties, concludes Prof. Pietrzyński.
The newly discovered mechanism underlying the formation of these very specific stars should help us gain a better understanding of some of the most important problems faced by contemporary astrophysics, involving the occurrence of anomalous stars, the numbers of white dwarfs, and what is known as the UV excess observed in old elliptical galaxies.
The potential existence of a sizeable number of such pulsating stars similar to the one now found in the OGLE-BLG-RRLR-02792 system could be a source of errors in distance calculations made using RR Lyrae stars, since they are observationally indistinguishable. As stars of this new type emit less radiation, using them to determine distances may introduce a large margin of error. However, estimates of the numbers of such stars indicate that they are relatively rare; therefore they should not significantly interfere with estimates of distance based on larger samples of classical RR Lyrae stars.
The OGLE-BLG-RRLR-02792 system has been registered during a very short phase in its stellar evolution. Relatively soon in a few tens of thousands of years the pulsations will fade. Generally speaking, the probability of detecting large numbers of such objects is low, and only by observing hundreds of millions of stars through the OGLE project will we be able to capture such unique instances, adds Prof. Udalski, leader of the OGLE project.
The team analysing the OGLE-BLG-RRLR-02792 system includes four other Polish astronomers: Dr. Dariusz Graczyk, currently working as part of the Araucaria project team at the University of Concepcion in Chile, and Dr. Bogumił Pilecki, Piotr Konorski and Marek Górski from the AOUW.
The Araucaria project, conducted jointly with astronomers from Chile, the US and Italy, aims to determine a precise scale of distances in the universe. In recent years, it has gained recognition for its precise measurements of distances to nearby galaxies, including to the Large Magellanic Cloud, which is used as a standard of distances in the universe. Leader of the team, Prof. Grzegorz Pietrzyński, has recently been awarded a prestigious research grant as part of the Foundation for Polish Science TEAM programme, and the IDEAS Plus programme financed by the Polish Ministry of Science and Higher Education for the development of his research and team.
The OGLE project, conducted at the Las Campanas Observatory in Chile, is one of the worlds largest sky scanning endeavours. For over 20 years the project has been conducted by Polish astronomers from the AOUW, and it has made numerous world-class discoveries in some of the most important fields of contemporary astrophysics. The OGLE project is co-financed by the Polish Ministry of Science and Higher Education and the European Research Council (an IDEAS programme Advanced Grant for Prof. Udalski).
Read the full article in Nature here.