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The center of the Milky Way, shrouded in clouds and gas, has long been a source of intrigue for astronomers. The earlier identification of its supermassive black hole (SMBH) decades ago revealed numerous mysteries. Among them was the strange absence of binary stars in this galactic region, where massive stars usually exist in pairs in the spiral arms. At the core, however, almost all stars seemed to be solitary.
Known as S stars, these celestial bodies orbit the Milky Way’s center at remarkable speeds, with their only apparent companions being G objects — dust and gas clouds that move along similar paths. A recent study published in Nature Communications has unveiled that these G objects are more complex than previously thought, as one of them has provided solid evidence of a binary star system, identified as D9, orbiting at the galactic center. This system consists of two stars that complete an orbit around one another approximately once every year, suggesting that there are likely other similar systems concealed within the core region.
First author Florian Peissker from the University of Cologne expressed that “The D9 system is actually a missing link.” He elaborated that it clarifies both the nature of G objects and the lack of binary S stars since the latter were originally classified as G objects.
A Glimpse into Birth
Peissker utilized the ERIS and SINFONI spectrometers on the Very Large Telescope in Chile, monitoring D9’s behavior over a span of 15 years, analyzing nightly data for recurring velocity changes. By observing the radial velocity — similar to detecting an exoplanet through its effect on its star — astronomers determined the existence of two stars in mutual orbit.
The research team estimates that the stars in the D9 system are relatively youthful at about 2.7 million years old, with an orbital period around the SMBH of several hundred years. It is anticipated that within a million years, the two stars will merge into a single entity, potentially accounting for the observed scarcity of binary stars at the galactic core. Peissker articulated that when the “dust” clears around G objects, they are often resultant from merging S stars.
This revelation may also provide insight into another enigma regarding S stars, which, if they were originally captured from the outer galaxy, would need to be around a thousand times older to have completed their inward journey. In contrast, if they were formed through the merging of binary systems concealed within the dust, their “rebirth” would explain their younger appearance.
The G objects have been perplexing for some time, though past observations offered hints of their nature. In 2014, a G object dubbed G2 passed close to the SMBH, traveling at a substantial fraction of the speed of light. Contrary to expectations, G2 emerged intact after a close encounter with the extreme gravitational forces, suggesting it harbored a dense body, possibly a protostar, at its core. The discovery of D9 supports this theory.
Precision and Perseverance
Studying objects situated 26,000 light-years away, obscured by dust and gas, presents significant challenges. Accurate measurements are essential, with a strong “signal to noise” ratio being crucial. One common method employed is stacking months of observations while slightly altering fields of view to produce a mosaic that mitigates instrumental anomalies.
An innovative approach, conceived by Peissker during a bike ride home, involved analyzing data from every night over an extended period, systematically filtering observations to enhance the likelihood of identifying binaries like D9, which must navigate the robust gravitational forces close to the SMBH.
Peissker noted, “I wrote down all the values and Doppler-shifted radial velocities of these objects for every night for a year.” Upon detecting a periodic pattern in D9’s movement, he extended this analysis over the entire 15 years.
Particularly, he utilized spectral readings indicative of ionized hydrogen emissions, referred to as Brackett-gamma lines, to track the Doppler effect, a phenomenon where wavelengths appear stretched or compressed based on the objects’ motion. The cyclical radial velocities observed signaled the interaction between the twin stars.
Peissker contends that G objects cannot merely be “coreless clouds,” as dust and gas subjected to intense stellar winds would not survive without a hidden stellar core anchoring them. The team’s serendipitous findings were, according to Peissker, a matter of fortunate timing.
“We were super lucky because D9 is on the descending part of its orbit,” he remarked, referencing the 200-year orbit of the binary. If D9 were ascending, its speed would complicate observations. Fortunately for astronomers, its slower descent allowed for clearer spectroscopic attributes, ultimately leading to this significant discovery regarding the binary star system.
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www.astronomy.com