Since 2016, the Southern Photometric Local Universe Survey (S-PLUS) has been dedicated to mapping half of the sky in the Southern Hemisphere. The task has been assigned to the T80S, a small Brazilian robotic telescope installed in the Cerro Tololo Inter-American Observatory, in the Chilean Andes. The diameter of its primary mirror is just 83 centimeters. On nights with good viewing conditions, the T80S, which has a wide field of view, is dedicated to scanning the southern sky. Around 80% of the area to be observed within the scope of the S-PLUS project has already been mapped. Records of approximately 111 million celestial bodies, such as galaxies, stars, and asteroids, have been obtained. Amid such a vast number of objects, surprising discoveries emerge from time to time.
This is the case of an article published in February in the scientific journal Astrophysical Journal Letters (APJL), led by a group of Brazilian researchers. The study presents 10 new binary systems, each composed of two stars classified as cataclysmic variables. These previously unknown pairs of stars were identified using an alternative technique, designed by the researchers and based on the features of the T80S.
“By finding systems previously unidentified in earlier mapping surveys, we are enhancing the discovery of so-called exotic cataclysmic variables,” says astrophysicist Raimundo Lopes de Oliveira, of the Federal University of Sergipe (UFS), lead author of the study. “Only a few examples of these atypical systems are known. Yet, according to theoretical models, they should represent the majority of the systems.”
To confirm that they were cataclysmic variables and validate the new methodology, Lopes de Oliveira and his colleagues from the article conducted a pilot project. After identifying 10 new candidate cataclysmic variables from the S-Plus data, they confirmed the status of these systems through observations using the Gemini South telescope, also in Chile, and the US Space Agency’s (NASA) Swift X-ray space telescope.
Cataclysmic variables are systems made up of two stars in very close proximity. The distance between them varies over time but tends to be similar to that between the Earth and the Moon, around 380,000 kilometers. One is always a white dwarf, a classification given to an extremely dense and compact dying star in its final evolutionary stage. The other, the companion star, is typically a common star, similar to the Sun. Their close proximity causes the companion star to lose matter, which is then drawn to the white dwarf. The exchange of matter causes intense radiation emission and sometimes leads to the formation of an accretion disk around the white dwarf. These systems present pronounced variations in brightness over time, occasionally experiencing intense eruptions.
It is not possible to discover these objects from just a single image of the sky. The two stars are so close together that they cannot be distinguished. The brightness of one overlaps with that of the other, making them appear as a single object against the dark sky, similar to other stars. Even the expected oscillation in the brightness of cataclysmic variables is not an easy clue to obtain. After all, it is unknown where in the sky these binary systems are located, or when, with what intensity, or for how long their brightness will vary.
The most traditional approach used to find this type of binary system is based on detecting fluctuations in brightness, usually in the optical spectrum (visible light), and detecting X-ray emissions. The spectroscopy technique, which breaks down the captured light, is important for these studies, but it only allows one or a few objects to be observed at a time, and only the brightest ones. As a result, it is time-consuming and, consequently, expensive.
However, thanks to a unique feature of the camera installed in the T80S, Lopes de Oliveira and colleagues of the S-Plus survey were able to develop a method to search for cataclysmic variables based on the observations made by the Brazilian telescope. Although it primarily mapped the southern sky in visible light, with a small part in infrared and ultraviolet, the T80S camera is equipped with 12 filters, or bands, which can be understood as 12 different perspectives for each observed region of the sky.
Each filter represents a different color. So, instead of generating a single optical record resulting from the combined radiation emitted at all frequencies, the telescope produces 12 images, one for each color. Each image reveals the amount of light emitted within a specific wavelength range. “S-Plus, with its observations across different regions of the electromagnetic spectrum, offers us an alternative to spectroscopy,” says Lopes de Oliveira.
“It is the only survey of the Southern Hemisphere that utilizes a camera with this band system,” says astrophysicist Cláudia Mendes de Oliveira, of the Institute of Astronomy, Geophysics, and Atmospheric Sciences at the University of São Paulo (IAG-USP), one of the coauthors of the article in ApJL. In addition to coordinating the S-Plus survey, she was responsible for the design and installation of the T80S in Chile, a project funded by FAPESP.
The multicolored images from S-Plus enable the creation of a graph that indicates the light intensity of each spectral region observed by the T80S. Based on previous studies of cataclysmic variables, the researchers found that, in the data from the survey, this type of binary system tends to produce a V-shaped graph, with an excess of emissions in the bluer colors, a dip at longer wavelengths, and another excess in the redder regions.
“We were able to see the manifestation of bluish light, which comes from the white dwarf, and the spectral lines resulting from the matter that it is capturing, as well as the redder emission from the cooler companion star,” says astrophysicist Amanda de Araújo, who is doing a PhD at UFS, under the guidance of Lopes de Oliveira, and is the author of the new study.
For Brazilian astrophysicist Ingrid Pelisoli, from Warwick University in the UK, the technique proposed for identifying cataclysmic variables using S-Plus survey data is extremely interesting. “The proposed method is very clever,” comments Pelisoli, who also studies these binary systems but did not participate in the work with S-Plus data. “It is an intermediate approach between spectroscopy and photometry, which is normally used for the initial characterization of stars.” In classic photometric mapping, an object’s light is captured by one or a few filters—but not with 12 bands like S-Plus.
Based on the typical spectral signatures of cataclysmic variables, the S-Plus researchers hope to develop an automated process, possibly with the help of artificial intelligence, that can independently search for this type of binary system among the millions of celestial objects covered by the survey. If everything goes as expected, the Brazilian survey should continue discovering new explosive pairs of stars.
The story above was published with the title “Stellar banquet” in issue in issue 349 of march/2025.
Project
Rydberg atoms interacting with microwave fields (nº 21/06371-7); Grant Mechanism Regular Research Grant; Principal Investigator Luis Marcassa (USP); Investment R$145,756.68.
Scientific article
KONDO, J. D. M. et al. Multiphoton-dressed Rydberg excitations in a microwave cavity with ultracold Rb atoms. Physical Review A. Dec. 2, 2024.
Republish
