Scientists have directly imaged eight dark objects that lie next to very bright stars in the Gaia data catalog, including so-called “failed stars”, also known as brown dwarfs.
The stars and their companions were originally identified from millions of stars in the Gaia catalogue. They were considered ideal for further study using the ground-based GRAVITY instrument, a sophisticated near-infrared interferometer on the Very Large Telescope (VLT) at the top of Cerro Paranal in Chile. By combining infrared light from several telescopes, a process called interferometry, GRAVITY has already made the first direct observation of an extrasolar planet, or “exoplanet.”
Following the Gaia observations, GRAVITY directly detected light signals from companions of the eight bright stars, seven of which were previously undiscovered theoretical objects.
Three of the companions are small and faint stars, the other five are brown dwarfs. The latter form like stars and have more mass than gas giants, but not enough mass to trigger the fusion of hydrogen into helium in their core, as is the case with main sequence stars. Hence their nickname “failed stars”.
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One of the brown dwarfs discovered by GRAVITY orbits its parent star at a distance roughly equal to the distance between the Earth and the Sun. This is the first time one of these failed stars has been directly observed so close to its parent star. “We have shown that it is possible to capture an image of a faint companion even when it orbits very close to its bright host star,” said team leader and European Southern Observatory (ESO) scientist Thomas Winterhalder in a statement. “This achievement highlights the remarkable synergy between Gaia and GRAVITY. Only Gaia can identify such close systems hosting a star and a ‘hidden’ companion, and then GRAVITY can take over to image the smaller and fainter object with unprecedented accuracy.”
A firefly on a lighthouse
Directly observing faint objects such as small, faint stars or brown dwarfs around bright stars is no easy task. In fact, detecting their light signals is akin to seeing the light of a firefly sitting on a glowing lighthouse. Understandably, any attempt to image the firefly’s light will be outshone by the brighter light of the lighthouse, and the same is true for bright stars and their faint companions.
While Gaia cannot directly detect the faint companions of these stars, the space telescope was able to infer their presence. This is because when a brown dwarf, or a small star in general, orbits a larger, brighter star, its gravity tugs on the parent star and this causes a “wobble” in the motion of the larger, brighter star.
As this star tumbles away from Earth (and Gaia), the wavelength of light expands, shifting toward the red end of the electromagnetic spectrum. Conversely, as it tumbles toward Earth, the wavelength of light shortens, 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, when an ambulance speeds toward you with its siren blaring, the sound waves are compressed and the siren is higher pitched, similar to blue shift. As the ambulance passes you, the wavelengths of the sound expand and the siren is lower pitched, just like the red shift of light from a star as it moves away.
This red- and blueshift effect is tiny, but Gaia is sensitive enough to detect it. The small companions to these stars in the Gaia sample lie at tiny angles of a few dozen milliarcseconds from their bright parent stars, which is about the size of a quarter viewed from a distance of about 100 kilometers.
“Gaia data acts as a kind of guidepost in our observations,” explained Thomas. “The part of the sky we can see with GRAVITY is very small, so we need to know where to look. Gaia’s unprecedentedly precise measurements of the motions and positions of stars are essential to pointing our instrument in the right direction in the sky.”
The collaboration between Gaia and GRAVITY helped the team go beyond simply discovering these companions. The two data sets also allowed the team to separate the masses of the stars and the masses of the companions. In addition, by measuring the differences in the wavelengths of light from the stars and their companions, and combining this information with the mass estimates above, the team was able to deduce the ages of the companions.
The brown dwarfs were found to be less luminous than expected given their observed age and mass, suggesting that these bodies may themselves be orbited by another, smaller and even dimmer companion, perhaps even an elusive exomoon.
The power of the Gaia-GRAVITY tag team means that scientists could soon use these two instruments to image smaller companions around bright stars, called exoplanets.
“The ability to decipher the tiny motions of closely spaced pairs in the sky is unique to the Gaia mission. The next catalogue, made available as part of the fourth data release (DR4), will contain an even more extensive collection of stars with possibly smaller companions,” said Johannes Sahlmann, Gaia scientist at the European Space Agency (ESA). “This result breaks new ground in the search for planets in our galaxy and promises to give us insights into new distant worlds.”
The team’s research was published June 10 in the journal Astronomy and Astrophysics.
Originally) released on Space.com.