Wednesday, December 18, 2024

 SPACE / COSMOS

First ever binary star found near our galaxy’s supermassive black hole




ESO

Location of binary star D9 in the Milky Way 

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This image indicates the location of the newly discovered binary star D9, which is orbiting Sagittarius A*, the supermassive black hole at the centre of our galaxy. It is the first star pair ever found near a supermassive black hole. The cut-out shows  the binary system as detected by the SINFONI spectrograph on ESO’s Very Large Telescope. While the two stars cannot be discerned separately in this image, the binary nature of D9 was revealed by the spectra captured by SINFONI over several years. These spectra showed that the light emitted by hydrogen gas around D9 oscillates periodically towards red and blue wavelengths as the two stars orbit each other.

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Credit: ESO/F. Peißker et al., S. Guisard




An international team of researchers has detected a binary star orbiting close to Sagittarius A*, the supermassive black hole at the centre of our galaxy. It is the first time a stellar pair has been found in the vicinity of a supermassive black hole. The discovery, based on data collected by the European Southern Observatory’s Very Large Telescope (ESO’s VLT), helps us understand how stars survive in environments with extreme gravity, and could pave the way for the detection of planets close to Sagittarius A*.

Black holes are not as destructive as we thought,” says Florian Peißker, a researcher at the University of Cologne, Germany, and lead author of the study published today in Nature Communications. Binary stars, pairs of stars orbiting each other, are very common in the Universe, but they had never before been found near a supermassive black hole, where the intense gravity can make stellar systems unstable.

This new discovery shows that some binaries can briefly thrive, even under destructive conditions. D9, as the newly discovered binary star is called, was detected just in time: it is estimated to be only 2.7 million years old, and the strong gravitational force of the nearby black hole will probably cause it to merge into a single star within just one million years, a very narrow timespan for such a young system.

This provides only a brief window on cosmic timescales to observe such a binary system — and we succeeded!” explains co-author Emma Bordier, a researcher also at the University of Cologne and a former student at ESO.

For many years, scientists also thought that the extreme environment near a supermassive black hole prevented new stars from forming there. Several young stars found in close proximity to Sagittarius A* have disproved this assumption. The discovery of the young binary star now shows that even stellar pairs have the potential to form in these harsh conditions. “The D9 system shows clear signs of the presence of gas and dust around the stars, which suggests that it could be a very young stellar system that must have formed in the vicinity of the supermassive black hole,” explains co-author Michal Zajaček, a researcher at Masaryk University, Czechia, and the University of Cologne.

The newly discovered binary was found in a dense cluster of stars and other objects orbiting Sagittarius A*, called the S cluster. Most enigmatic in this cluster are the G objects, which behave like stars but look like clouds of gas and dust. 

It was during their observations of these mysterious objects that the team found a surprising pattern in D9. The data obtained with the VLT’s ERIS instrument, combined with archival data from the SINFONI instrument, revealed recurring variations in the velocity of the star, indicating D9 was actually two stars orbiting each other. “I thought that my analysis was wrong,” Peißker says, “but the spectroscopic pattern covered about 15 years, and it was clear this detection is indeed the first binary observed in the S cluster.”

The results shed new light on what the mysterious G objects could be. The team proposes that they might actually be a combination of binary stars that have not yet merged and the leftover material from already merged stars.

The precise nature of many of the objects orbiting Sagittarius A*, as well as how they could have formed so close to the supermassive black hole, remain a mystery. But soon, the GRAVITY+ upgrade to the VLT Interferometer and the METIS instrument on ESO’s Extremely Large Telescope (ELT), under construction in Chile, could change this. Both facilities will allow the team to carry out even more detailed observations of the Galactic centre, revealing the nature of known objects and undoubtedly uncovering more binary stars and young systems. “Our discovery lets us speculate about the presence of planets, since these are often formed around young stars. It seems plausible that the detection of planets in the Galactic centre is just a matter of time,” concludes Peißker.

More information

This research was presented in the paper “A binary system in the S cluster close to the supermassive black hole Sagittarius A*” published today in Nature Communications (doi: 10.1038/s41467-024-54748-3).

The team is composed of F. Peißker (Institute of Physics I, University of Cologne, Germany [University of Cologne]), M. Zajaček (Department of Theoretical Physics and Astrophysics, Masaryk University, Brno, Czechia; University of Cologne), L. Labadie (University of Cologne), E. Bordier (University of Cologne), A. Eckart (University of Cologne; Max Planck Institute for Radio Astronomy, Bonn, Germany), M. Melamed (University of Cologne), and V. Karas (Astronomical Institute, Czech Academy of Sciences, Prague, Czechia).

The European Southern Observatory (ESO) enables scientists worldwide to discover the secrets of the Universe for the benefit of all. We design, build and operate world-class observatories on the ground — which astronomers use to tackle exciting questions and spread the fascination of astronomy — and promote international collaboration for astronomy. Established as an intergovernmental organisation in 1962, today ESO is supported by 16 Member States (Austria, Belgium, Czechia, Denmark, France, Finland, Germany, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom), along with the host state of Chile and with Australia as a Strategic Partner. ESO’s headquarters and its visitor centre and planetarium, the ESO Supernova, are located close to Munich in Germany, while the Chilean Atacama Desert, a marvellous place with unique conditions to observe the sky, hosts our telescopes. ESO operates three observing sites: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its Very Large Telescope Interferometer, as well as survey telescopes such as VISTA. Also at Paranal ESO will host and operate the Cherenkov Telescope Array South, the world’s largest and most sensitive gamma-ray observatory. Together with international partners, ESO operates ALMA on Chajnantor, a facility that observes the skies in the millimetre and submillimetre range. At Cerro Armazones, near Paranal, we are building “the world’s biggest eye on the sky” — ESO’s Extremely Large Telescope. From our offices in Santiago, Chile we support our operations in the country and engage with Chilean partners and society. 

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Young exoplanet’s atmosphere unexpectedly differs from its birthplace



New study shows planet formation might be more complicated than previously thought



Northwestern University

PDS 70 

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The natal disk of PDS 70 with new planet PDS 70b (bright spot on the right). By studying this system, researchers uncovered a mismatched composition of gases in the planet’s atmosphere compared to gases within the disk.

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Credit: ESO/A. Müller et al.




Just as some children physically resemble their parents, many scientists have long thought that developing planets should resemble the swirling disk of gas and dust that births them.

But, in a new study, a Northwestern University-led team of astrophysicists discovered the resemblance might be looser than previously thought. By studying a still-forming exoplanet and its surrounding natal disk, the researchers uncovered a mismatched composition of gases in the planet’s atmosphere compared to gases within the disk.

The surprising finding potentially confirms long-held skepticism that scientists’ current model of planet formation is too simplified.

The study will be published on Wednesday (Dec. 18) in the Astrophysical Journal Letters. It marks the first time physicists have compared information from an exoplanet, its natal disk and host star.

“For observational astrophysicists, one widely accepted picture of planet formation was likely too simplified,” said Northwestern’s Chih-Chun “Dino” Hsu, who led the study. “According to that simplified picture, the ratio of carbon and oxygen gases in a planet’s atmosphere should match the ratio of carbon and oxygen gases in its natal disk — assuming the planet accretes materials through gases in its disk. Instead, we found a planet with a carbon and oxygen ratio that is much lower compared to its disk. Now, we can confirm suspicions that the picture of planet formation was too simplified.”

Hsu is a postdoctoral associate at the Center for Interdisciplinary Exploration and Research in Astrophysics(CIERA). He is advised by Jason Wang, an assistant professor of physics and astronomy at Northwestern’s Weinberg College of Arts and Sciences and member of CIERA.

Searching for visible birth material

All planets are born from a natal disk, a rotating disk of gas and dust that surrounds a new star. Over millions of years, gravity pulls gas and dust together to form clumps, which eventually grow into planets. Until recently, it was impossible to obtain a direct view of a natal disk in order to track a planet’s birth. Most observable exoplanets are too old, so their natal disks have already disappeared.

The exception, however, is PDS 70, a natal disk that envelopes two fledgling gas-giant exoplanets — similar to Jupiter — called PDS 70b and PDS 70c. Located just 366 million lightyears from Earth within the constellation Centaurus, the planets are, at most, a youthful 5 million years old.

“This is a system where we see both planets still forming as well as the materials from which they formed,” Wang said. “Previous studies have analyzed this disk of gas to understand its composition. For the first time, we were able to measure the composition of the still-forming planet itself and see how similar the materials are in the planet compared to the materials in the disk.”

Examining planetary fingerprints

To measure the materials, Hsu, Wang and their team examined the light emitted from PDS 70b. This light, or spectra, is like a fingerprint, revealing an object’s composition, motion, temperature and other characteristics. Each molecule or element produces its own spectrum. So, by studying these spectra, researchers can pinpoint the specific molecules or elements within an object.

In previous work, Wang co-developed new photonics technologies that enable astronomers to capture the spectrum of targeted faint objects near much brighter stars. The researchers used this technique to zero in on the faint features of the young planetary system.

“These new tools make it possible to take a really detailed spectra of faint objects next to really bright objects,” Wang said. “Because the challenge here is there’s a really faint planet next to a really bright star. It’s hard to isolate the light of the planet in order to analyze its atmosphere.”

With the spectra, the researchers obtained information about carbon monoxide and water from PDS 70b. From that, they calculated the inferred ratio of carbon and oxygen within the planet’s atmosphere. Then, they compared that ratio to previously reported measurements of gases in the disk.

“We initially expected the carbon-to-oxygen ratio in the planet might be similar to the disk,” Hsu said. “But, instead, we found the carbon, relative to oxygen, in the planet was much lower than the ratio in the disk. That was a bit surprising, and it shows that our widely accepted picture of planet formation was too simplified.”

Solid components might make the difference

To explain this mismatch, Hsu and Wang think two different scenarios might be at play. One explanation is the planet might have formed before its disk became enriched in carbon. Another explanation is the planet might have grown mostly by absorbing large amounts of solid materials in addition to gases. While the spectra show only gases, some of the carbon and oxygen initially could be accreted from solid — trapped in ice and dust.

“If the planet preferentially absorbed ice and dust, then that ice and dust would have evaporated before going into the planet,” Wang said. “So, it might be telling us that we can’t just compare gas versus gas. The solid components might be making a big difference in the carbon to oxygen ratio.”

For this study, the team only studied PDS 70b. Next, they plan to observe the spectra from the other planet in the PDS 70 system.

“By studying these two planets together, we can understand the system’s formation history even better,” Hsu said. “But, also, this is just one system. Ideally, we need to identify more of them to better understand how planets form.”

The study, “PDS 70b shows stellar-like carbon-to-oxygen ratio,” was supported by the Heising-Simons Foundation, the Simons Foundation and the National Science Foundation.

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