An article published in “Astrophysical Journal” describes the study of a brown dwarf that orbits a K-type star that was possible thanks to a gravitational microlensing event. An international team of astronomers used NASA’s Swift and Spitzer space telescopes to take advantage of that event, cataloged as OGLE-2015-BLG-1319, at a distance from its star that at which few of those objects were found, hence the name brown dwarf desert.
Brown dwarfs are a class of objects on the border between planet and star. Their mass is significantly higher than that of Jupiter but much smaller than that of the Sun or even of a red dwarf star. That’s not enough mass to ignite the nuclear reactions typical of stars and it’s for this reason that brown dwarfs are considered failed stars.
Over the years, thanks to the discovery of many new brown dwarfs, the study of these objects has become more sophisticated and targeted. An article published in “The Astrophysical Journal Supplement Series” showed the usefulness of this type of research in connection with those on the planets but brown dwarfs are revealing several interesting features.
In this specific research two space telescopes were used, a fact far from unprecedented but it’s the first time that the two are Spitzer and Swift. This was possible due to the microlensing event OGLE-2015-BLG-1319. Essentially, when a massive object such as the brown dwarf to be studied passes in front of a background star that star becomes brighter because the object in the foreground amplifies its light deflecting it with its gravity. The normal gravitational lenses are composed of galaxies or even galactic clusters, when it comes to a single star or a brown dwarf it’s called a microlens.
The Swift space telescope observed the system hosting the brown dwarf at the end of June 2015 but its location in low Earth orbit didnn’t allow it to take advantage of parallax, the apparent change in position of an object when viewed from two different points in space. These points are like multiple eyes to see an object’s distance. Using microlenses working models, the parallax allows to calculate the ratio between the mass of the object and its distance.
The Spitzer space telescope is much farther away from Earth so it was useful to get other observations of the same gravitational microlensing event. By combining the data collected by the two space telescopes and other ground-based telescopes, the researchers were able to estimate that the brown dwarf has a mass between 30 and 65 times that of the planet Jupiter. They have also been able to estimate that it orbits a K-class red dwarf star that has a mass about half that of the Sun.
The measure of the distance of the brown dwarf from its star is between two very different possibilities: a quarter of that of the Earth from the Sun or 45 times that of the Earth from the Sun. If the first estimate was correct it would be a very interesting discovery because so far only in 1% of systems with stars that have a mass comparable to that of the Sun there’s a brown dwarf orbiting it at a distance within three times that between Earth and the Sun. This is the area called the brown dwarf desert.
The hope is that in the future it will be possible to observe other gravitational microlensing events from multiple perspectives. Coordinating observations with different instruments and in particular of space telescopes in different orbits would provide more information on the observed objects. In the case of relatively small objects such as brown dwarfs that would improve the estimates on their characteristics and systems that host them.
