Scientists have directly imaged eight dark objects accompanying very bright stars in the Gaia data catalogue, including so-called “failed stars”, otherwise known as brown dwarfs.
Stars and their companions are initially identified from the millions of stars in the Gaia catalog. They were deemed ideal for follow-up studies with the ground-based GRAVITY instrument, an advanced near-infrared interferometer located on the Very Large Telescope (VLT) atop Cerro Paranal in Chile. By combining infrared light from multiple telescopes, a process called interferometry, GRAVITY has now achieved the first direct observation of an extrasolar planet, or “exoplanet”.
Following the Gaia observations, GRAVITY directly observed light signals from satellites around the eight bright stars, seven of which were theorized to be previously undiscovered objects.
Three of the companion objects are small and faint stars, and the remaining five are brown dwarfs. The latter form as stars and have a greater mass than the gas giant planets, but do not have enough mass to cause hydrogen to fuse with helium in their cores, as main-sequence stars do. Hence their nickname “failed stars”.
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One of the brown dwarfs observed by GRAVITY orbits its parent star at a distance roughly equal to the distance between Earth and the sun. This marks the first time one of these failed stars has been seen directly so close to its host star.
“We have demonstrated that it is possible to image a faint companion even when it orbits very close to its bright host,” team leader and European Southern Observatory (ESO) scientist Thomas Winterhalder said in a statement. “This achievement highlights the remarkable synergy between Gaia and GRAVITY. Only Gaia can identify such narrow systems containing a star and a ‘hidden’ satellite, and then GRAVITY can take over to image the smaller, fainter object with unprecedented precision.”
Firefly on a lighthouse
Direct observation of faint objects such as small, faint stars or brown dwarfs around bright stars is no mean feat. In fact, noticing their light signals is akin to seeing the light of a firefly sitting on a shining lighthouse. Understandably, any attempt to image the firefly’s light is washed out by the brighter light of the lighthouse, and the same is true of bright stars and their faint companions.
While Gaia cannot directly observe the dim companions of these stars, the space telescope has been able to infer their presence. This is because when a brown dwarf, or small star in general, orbits a larger, brighter star, its gravity pulls on the parent star and this causes a “wobble” in the motion of the larger, brighter star.
As this star “moves away” from Earth (and Gaia), the wavelength of the light is stretched, shifting it toward the red end of the electromagnetic spectrum. Conversely, as it wobbles toward Earth, the light’s wavelengths shorten, shifting the light toward the blue end of the electromagnetic spectrum.
This red and blue shift effect is analogous to the Doppler shift, the phenomenon that affects sound waves on Earth. For example, as an ambulance rushes towards you with its siren blaring, the sound waves are compressed and the siren is louder, similar to blueshift. As the ambulance passes you, the wavelengths of the sound are stretched and the siren is quiet, just like the red shift of the light from the star as it moves away.
This red and blue shift effect is small, but Gaia is sensitive enough to notice it. The small companions of these stars in the Gaia sample lie at small separation angles from their bright parent stars of a few tens of milliarcseconds, which is about the size of a quarter as seen from a distance of about 62 miles (100 kilometers).
“In our observations, the Gaia data acts as a kind of signpost,” explained Thomas. “The part of the sky that we can see with GRAVITY is very small, so we need to know where to look. Gaia’s unparalleled precision measurements of the motions and positions of the stars are essential to point our instrument in the right direction in the sky. “
The collaboration between Gaia and GRAVITY helped the team go beyond simply discovering these satellites. The two data sets also allowed the team to separate the masses of the stars and the masses of the satellites. Furthermore, measuring the differences in the wavelengths of light from the stars and their satellites, and combining this information with the aforementioned mass estimates, allowed the team to conclude that ages to comrades.
This revealed that the brown dwarfs were less luminous than expected at the observed ages and masses, suggesting that these bodies themselves may be orbited by another smaller and even fainter companion, perhaps even elusive exomoons.
The strength of the Gaia-GRAVITY marker team means that scientists may soon be able to use these two instruments to image smaller moons around bright stars, namely exoplanets.
“The ability to reveal the small motions of close pairs in the sky is unique to the Gaia mission. The next catalogue, which will be made available as part of the fourth data release (DR4), will contain an even richer collection of stars with potentially smaller satellites,” said European Space Agency (ESA) Gaia scientist Johannes Salman. ” This result opens new ground in the search for planets in our galaxy and promises us glimpses of new distant worlds.”
The team’s research was published June 10 in the journal Astronomy and Astrophysics.