Cienciaes.com: The dramatic life of the stars “Water sources”. We spoke with Carmen Sánchez Contreras

2021-12-27 19:17:50

Our planet revolves around a star that, compared to others, wanders alone in this area of ​​the Milky Way. This “solitude” is not usual, in fact, most of the stars that exist in the galaxy form double, triple systems, even in larger sets. The relationship between stars and their companions is very interesting because depending on their size, age, or the distance that separates them, their evolution can follow very different paths. Some binary stars are even so close to each other that they exchange matter and can even merge, generating in a few moments an enormous emission of energy that illuminates the entire galaxy. Observing one of these events is difficult, because, as Carmen Sánchez Contreras, a researcher at the CSIC at the Center for Astrobiology, they last a very short time. However, the remnants that remain after such a dramatic event are observable.

Carmen Sánchez has participated in an investigation that has made it possible to study fifteen unusual stars that have recently gone through an episode of this type and are known as “water sources”. The result of the study has been published in the scientific journal Nature Astronomy and is signed by an international group of researchers including scientists from the Chalmers University of Technology in Onsala, Sweden, the Institute of Astrophysics of Andalusia and the Center for Astrobiology (CAB, CSIC).

The study helps to understand one of the stellar evolutionary paths in which binary systems are involved in which both stars are so close that, at one point in their evolution, they orbit within a common envelope of gas and dust. This envelope is created when the largest and most massive star, following evolutionary paths similar to the one that the Sun will follow in the distant future, upon finishing the hydrogen that is fused inside its core, expands its atmosphere becoming a red giant. When that happens in a very close double system, the envelope of gas and dust thrown off by the more massive star eventually engulfs the companion star. The collision with the gases slows down the speed of the smaller star and it begins to fall little by little in a spiral towards the more massive one. The end of this process can lead to two situations: The most dramatic ends with the merger of both stars in a highly energetic process that could give rise to a type of supernova. However, if the envelope of the ejected red giant star surpasses the companion before the two actually merge, a compact binary system will be created.

The team of researchers has studied the 15 objects with the Atacama Large Millimeter / submillimeter Array (ALMA). ALMA is a complex of 66 antennas located on the Llano de Chajnantor, located 5,000 meters above sea level, in the Chilean Atacama desert. These antennas, fifty of which have diameters of 12 meters, are capable of observing the sky in unison, generating a radio image that is equivalent to that of a single radio telescope 16 kilometers in diameter. As the electromagnetic emissions have their origin in the atoms and molecules of the stars, the collected signal is studied in its different frequencies and with this it is possible to identify the chemical elements and molecules that generated them.

As a star expands in the red giant phase, the physical conditions of pressure and temperature of the atoms and molecules it contains change in the cloud. These variations allow the existence of certain molecules that could not exist under other conditions. Thus, at certain distances from the main star, molecules such as SiO (silicon monoxide), H2O (water), OH, CO, etc. are detected. The radio emissions created travel to us, are captured with ALMA And its study allows us to detect not only the molecules that originated them, but also the speeds at which the gas they are part of expands. Normally, the signals that come from water molecules tend to come from regions that have a certain expansion rate and are common in many astronomical objects. However, in the 15 astronomical objects that have been part of this study, the researchers have been able to verify that these emissions were much faster than expected, in other words, the water emanated faster, which is why these objects were identified as “water sources”.

The common enveloping event that originates from water sources has a very short duration, barely tens or hundreds of years, but over time it can give rise to planetary nebulae that, due to the interaction of the two stars They take on irregular shapes. The study in which Carmen Sánchez participates also reveals that the sources of water were thought to originate from more massive stars, on the order of between 4 and 8 times the mass of the Sun, but this study shows that this is not the case, that start from less massive stars, similar to the Sun in size or less than 4 solar masses.

I invite you to listen to Carmen Sánchez Contreras, a researcher at the Higher Council for Scientific Research (CSIC) in the astrophysics department of the Center for Astrobiology.

Reference:

Khouri, T., Vlemmings, W.H.T., Tafoya, D. et al. Observational identification of a sample of likely recent common-envelope events. Nat Astron (2021). https://doi.org/10.1038/s41550-021-01528-4