Scientists have directly imaged eight dim objects accompanying very bright stars in the Gaia data catalog, including so-called “failed stars,” otherwise known as brown dwarfs.
The stars and their companions were initially identified from the millions of stars in the Gaia catalog. They were considered ideal for follow-up investigations with the ground-based instrument GRAVITY, 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 already achieved the first direct observation of an extrasolar planet, or “exoplanet.”
Following up on the Gaia observations, GRAVITY directly recorded light signals from companions around eight bright stars, seven of which were theoretically undiscovered objects.
Three of the companion objects are small and faint stars, and the other five are brown dwarfs. These form like stars and have more mass than gas giant planets, but they do not have enough mass to trigger the fusion of hydrogen into helium in their cores, as main sequence stars do. Hence their nickname “failed stars”.
Related: Gaia probe reveals ‘gold mine’ of more than 500,000 undiscovered stars
One of the brown dwarfs spotted by GRAVITY orbits its parent star at a distance roughly equal to the distance between Earth and the Sun. This is the first time one of these failed stars has been seen directly so close to its host star. “We have shown that it is possible to capture an image of a faint companion even when it is orbiting very close to its bright host,” team leader and European Southern Observatory (ESO) scientist Thomas Winterhalder said in a statement. “This achievement underscores the remarkable synergy between Gaia and GRAVITY. Only Gaia can identify such close host star and ‘hidden’ companion systems, and then GRAVITY can take over and image the smaller, fainter object with unprecedented precision.”
Firefly on the lighthouse
Direct observation of dim objects such as small, faint stars or brown dwarfs around bright stars is not difficult. In fact, observing their light signals is similar to seeing the light of a firefly sitting on a glowing lighthouse. Understandably, any attempt to show the light of a firefly is washed out by the brighter light of a beacon, and the same is true of bright stars and their faint companions.
While Gaia cannot directly detect 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 “wobbles” away from Earth (and Gaia), the wavelength of the light lengthens, 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 Doppler shift, a phenomenon that affects sound waves on Earth. For example, when an ambulance is rushing towards you with its siren blaring, the sound waves are compressed and the siren is louder, similar to a blueshift. As the ambulance passes you, the sound wavelengths are stretched and the siren is low-slung, as is the red shift of the light from the star as it recedes.
This red and blue shift effect is slight, but Gaia is sensitive enough to notice it. The small companions of these stars in the Gaia sample lie at small angular separations of a few tens of milliseconds from their bright parent stars, about a quarter of that when viewed from a distance of about 100 kilometers.
“In our observations, the Gaia data acts as a kind of pointer,” 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 unprecedentedly accurate measurements of star motions and positions are essential to pointing our instrument in the right direction in the sky.” “
The cooperation of Gaia and GRAVITY helped the team overcome the mere revelation of these companions. The two datasets also allowed the team to separate the masses of the stars and the masses of the companions. In addition, measuring the differences in the wavelengths of light from the stars and their companions, as well as combining this information with the above mass estimates, allowed the team to infer the ages of the companions.
This revealed that the brown dwarfs were less luminous than expected at the observed ages and masses, meaning that these bodies themselves could be orbiting another smaller and even fainter companion, perhaps even an elusive exomoon.
The strength of the Gaia-GRAVITY tag team means that scientists will soon be able to use the two instruments to image smaller companions around bright stars, specifically exoplanets.
“The ability to capture minute motions of close pairs in the sky is unique to the Gaia mission. The next catalog to be made available as part of the fourth data release (DR4) will contain an even richer collection of stars with potentially smaller companions,” said the European Space Agency (ESA) scientist Gaia Johannes Sahlmann. “This result represents a new breakthrough 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.
Originally published on Space.com.