Star changes its brightness every four years and lights up part of the nebula that surrounds it – 01/21/2024 – Science

Twelve years of observations in an obscure region of the Scorpius constellation, neighboring the center of the Milky Way, led to the discovery of a celestial object surrounded by a unique context. An international team of astronomers has identified a young variable star, which changes brightness over time, immersed in a nebula, a cloud of gas and cosmic dust, which also periodically changes its luminosity.

“Every four years, approximately, the star blinks and decreases its brightness for a certain time. A region of the nebula blinks in synchrony with it, while another part behaves in the opposite way”, says astrophysicist Roberto Saito, from the Federal University de Santa Catarina (UFSC), main author of an article published in November in the Astrophysical Journal Letters that describes the exotic celestial object. “If the star lights up, that part goes out and vice versa.”

This pattern of luminosity variation was observed during three complete cycles of four years. The blinking and blinking between the star and the nebula is attributed to a wave phenomenon called luminous echo, similar to what occurs with the reverberation produced by sound. The star emits light that, when it encounters the nebula, is reflected back and illuminates the cloud of gas and dust.

Due to the finite speed of light and the large size of the nebula, its different regions are, from the point of view of an external observer, illuminated by the central star at different times. The star emits light in all directions. The part of the light that comes directly to Earth illuminates the cloud region closest to us. Light emitted in the opposite direction takes longer to get here because it has to travel to the most distant portion of the nebula before being reflected back towards Earth. “When this happens, the star has already darkened again,” explains Saito.

In space, light echoes are commonly observed in novae and supernovae. A nova is the bright explosion produced when a huge mass of gas is transferred from a large, relatively cool star to a small but very hot star in a binary system. When giant stars reach the end of their life cycle and suffer a violent nuclear explosion, this burst of light and energy is called a supernova. In a variable star, like the one described in the article, light echoes had never been recorded.

Because it does not resemble any type of star present in catalogs of astronomical objects, the star in the constellation Scorpio was named WIT-12. The lyrics refer to the question, in English, “what is this?” (what is this?), used to indicate celestial bodies that do not fit into any known class of objects and are grouped in a separate category. The numeral indicates that the star is the twelfth celestial body to be considered a WIT, a nomenclature adopted by the Vista Variables in the Via Lactea (VVV) project. Saito also participated in the discovery of other WITs based on VVV data (see Pesquisa Fapesp nº 274).

Since 2010, this initiative has mapped, at near-infrared frequencies, around 1 billion stars in the plane of the Milky Way with the Vista telescope of the European Southern Observatory (ESO), located in Cerro Paranal, Chile. Vista’s observations used different color filters over time and made it possible to identify, initially, the existence of a nebula whose brightness changed periodically. It was then possible to associate the change in luminosity of the gas and dust cloud with a source of brightness, also variable, located in its center — probably a star.

Young stellar object

To determine the characteristics of this celestial object, the authors of the work had to resort to the services of another telescope located in Chile. They used Soar —which has the National Astrophysics Laboratory (LNA), of the Ministry of Science, Technology and Innovation (MCTI) as one of its partners— to obtain the star’s spectrum.

This type of recording decomposes the light emitted by a star into its constituent colors (different wavelengths) from which some parameters can be inferred, such as its chemical composition, temperature and intrinsic luminosity. “Analysis of the near-infrared spectrum allowed us to classify the light source inside the nebula as a young stellar object [YSO, young stellar object]”, comments Brazilian astrophysicist Felipe Navarete, another author of the study. The researcher works in La Serena, Chile, where Soar is located, for NOIRLab, an entity that operates telescopes associated with the National Science Foundation (NSF) in the United States.

Available data suggest that it is a young red star. This type of celestial body, which is in the early days of its existence, tends to be relatively cold, with a mass not much greater than that of the Sun, and having been formed just a few million years ago. It is also common for a YSO to still be surrounded by a cloud of gas and dust.

For astrophysicist Augusto Damineli, from the Institute of Astronomy, Geophysics and Atmospheric Sciences at the University of São Paulo (IAG-USP), who is not involved in studies with the star and the gas and dust cloud, the results presented in the article are the beginning, not the end, of work with WIT-12. “We know that it is a variable star that emits luminous echoes to the nebula that surrounds it”, ponders Damineli. “It took a significant observational investment to be able to make this statement. Still, it is completely insufficient to answer the question ‘what is it?’.”

He hopes that the use of new analysis methods, possibly with the help of artificial intelligence, and the entry into operation of more powerful observation instruments can shed light on the nature of non-standard stars. Especially because mysterious objects such as WIT must be recorded more consistently as new telescopes, such as the Vera Rubin Observatory, in Chile, begin to operate, says the USP astrophysicist.

Scientific article
SAITO, RK et al. VVV-WIT-12 and Its Fashionable Nebula: A 4 yr long-period young stellar object with a light echo? The Astrophysical Journal Letters. v. 958, no. 1. 14 Nov. 2023.