Vista / VVV
False-color image shows the central star (inside the gray circle) that illuminates part of the nebula (pink), with the other region in darkness (blue)Vista / VVVTwelve years of observations in an obscure region of the Scorpion constellation, neighbor of the Milky Way’s center, have led to the discovery of a celestial object shrouded in a singular setting. An international team of astronomers identified a young, variable star, which changes its brightness over time while immersed in a nebula—a cloud of cosmic gas and dust—which also changes its luminosity periodically.
“Every four years or so, the star blinks and its brightness diminishes for a certain period. One area of the nebula blinks in synchrony with it, while the other part behaves in the opposite way,” says astrophysicist Roberto Saito of the Federal University of Santa Catarina (UFSC), lead author of an article published in Astrophysical Journal Letters in November describing this exotic celestial body. “If the star lights up, the other part goes out and vice versa.”
This standard of luminosity variation was observed over three complete cycles of four years. The blinking and disappearing of the star and the nebula are attributed to a wavelike phenomenon known as light echo, similar to what happens when sound causes reverberations. The star emits a light which, when it reaches the nebula, is reflected back and illuminates the gas and dust cloud.
Due to the finite velocity of the light and considerable size of the nebula, its different regions are, to the outside observer, illuminated by the central star at different moments. The star emits light in all directions, and the light that comes directly to Earth illuminates the region of the cloud closest to us, while light emitted in the opposite direction takes longer to get here because it has to go to the most distant part of the nebula before being reflected back toward Earth. “When this happens, the star has gone dark again,” explains Saito.
In space, light echoes are commonly observed in novas and supernovas. A nova is a bright explosion produced when an enormous mass of gas is transferred from one large and relatively cold star to another which is smaller, but warmer, in a binary system. When giant stars get to the end of their life cycle and undergo a violent nuclear explosion, this burst of light and energy is called a supernova. Light echoes, as described in the article, have never before been registered in a variable star.
Unlike any type of star listed in the catalogs of astronomical objects, this star from the Scorpion constellation received the name WIT-12. The letters stand for the question “what is this?”, used to name celestial bodies not falling into any known class of objects, and therefore grouped into a separate category. The numeral indicates that the star is the 12th celestial body to be considered a WIT, nomenclature adopted by the project Vista Variables in the Via Lactea (VVV). Saito has also participated in the discovery of other WITs from VVV data (see Pesquisa FAPESP issue nº 274).
Since 2010 this initiative has mapped, in close infrared frequencies, around a billion stars in the Milky Way plane with the Vista telescope of the European Southern Observatory (ESO), located in Cerro Paranal, Chile. The Vista observations used different color filters over time, enabling initial identification of a nebula whose brightness altered periodically. It was then possible to associate the changing brightness of the gas and dust cloud to a light source—also variable—situated in its center: probably a star.
Amanda Smith / University of Cambridge
Artistic impression of another WIT—number 8—formed of a system with two starsAmanda Smith / University of CambridgeYoung stellar object
To determine the characteristics of this object, the authors of the paper had to draw upon the services of another telescope situated in Chile. They used the Soar, one of whose partners is the Brazilian Astrophysics Laboratory (LNA), of the Ministry of Science, Technology, and Innovation (MCTI), to obtain the star’s spectrum.
This type of representation splits the light emitted by the star into its constituent colors (different wavelengths) from which certain parameters can be inferred, such as chemical composition, temperature, and intrinsic luminosity. “Close infrared analysis of the spectrum enabled classification of the light source inside the nebula as a young stellar object (YSO),” says Brazilian astrophysicist Felipe Navarete, another author of the study. The researcher operates in La Serena, Chile, home to the Soar, for NOIRLab, an agency that operates telescopes associated to the US National Science Foundation (NSF).
The available data suggest that this is a young red star; this type of celestial body, which is in the earliest beginnings of its existence, is usually relatively cold, with a mass not much bigger than that of the Sun, and typically formed only a few million years ago. It is also commonplace for a YSO to still be surrounded by a gas and dust cloud.
For astrophysicist Augusto Damineli, of the Institute of Astronomy, Geophysics, and Atmospheric Sciences at the University of São Paulo (IAG-USP), who is not involved in studies into the star or gas/dust cloud, the results demonstrated in the article are the beginning, and not the end, of work with WIT-12. “We know it’s a variable star that emits light echoes to the nebula surrounding it,” considers Damineli. “Significant observation investment was required to make this affirmation. Even so, there is not enough information to answer the question ‘what is it?’”
He expects that the use of new analysis methods, possibly with the help of AI, and more powerful observation instruments coming online, can throw some light on the nature of nonstandard stars, not least because mysterious objects such as WIT are set to be registered with more constancy as new telescopes, such as the Vera Rubin Observatory in Chile, commence operation, says the USP astrophysicist.
Scientific article
SAITO, R. K. 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. Vol. 958, no. 1. Nov. 14, 2023.
