A new estimate suggests that the Milky Way’s population of brown dwarfs — objects too large to be a planet but too small to be a star — exceeds 25 billion.
False-color near-infrared image of the core of the young massive cluster RCW 38 taken with the adaptive-optics camera NACO on ESO’s Very Large Telescope. RCW 38 lies at a distance of about 5,500 light years from the Sun. The field of view of the central image is approximately 1 arc minute, or 1.5 light-years across. The insets, each spanning about 0.07 light-years on a side, show a subset of the faintest and least massive cluster candidate brown dwarfs (indicated by arrows) of RCW 38 discovered in this new image. Image credit: Muzic et al.
Brown dwarfs are cool, dim objects that are difficult to detect. Sometimes called failed stars, they are too small to sustain hydrogen fusion reactions at their cores, yet they have star-like attributes.
Typically, brown dwarfs have masses between 13 and 80 Jupiters — too massive to be planets, yet they possess some planet-like characteristics. Their temperatures can range from nearly as hot as a star to as cool as a planet.
After the initial discovery of brown dwarfs in 1995, astronomers quickly realized that they are a natural by-product of processes that primarily lead to the formation of stars and planets.
All of the thousands of brown dwarfs found so far are relatively close to the Sun, the overwhelming majority within 1,500 light years.
In 2006, Dr. Koraljka Muzic, an astronomer at the Universities of Lisbon and Diego Portales, and co-authors began a new search for brown dwarfs, observing five nearby star forming regions.
As part of the Substellar Objects in Nearby Young Clusters (SONYC) survey, the researchers observed NGC 1333, a star cluster some 1,000 light-years away in the constellation Perseus. That cluster had about half as many brown dwarfs as stars, a higher proportion than seen before.
To establish whether NGC 1333 was unusual, in 2016 they turned to another more distant star cluster, RCW 38, in the constellation Vela. This has a high density of more massive stars, and very different conditions to other clusters.
RCW 38 is approximately 5,500 light years away, meaning that the brown dwarfs are both faint, and hard to pick out next to the brighter stars.
To get a clear image of RCW 38, the astronomers used ESO’s Very Large Telescope, observing the cluster for a total of 3 hours, and combining this with earlier work.
They found just as many brown dwarfs in RCW 38 — about half as many as there are stars — and realized that the environment where the stars form, whether stars are more or less massive, tightly packed or less crowded, has only a small effect on how brown dwarfs form.
“We’ve found a lot of brown dwarfs in these clusters. And whatever the cluster type, the brown dwarfs are really common,” said team member Dr. Aleks Scholz, an observational astronomer at the University of St Andrews.
“Brown dwarfs form alongside stars in clusters, so our work suggests there are a huge number of brown dwarfs out there.”
From the SONYC survey, the authors estimate that the Milky Way has a minimum of between 25 and 100 billion brown dwarfs.
There are many smaller, fainter brown dwarfs too, so this could be a significant underestimate, and the survey confirms these dim objects are ubiquitous.
The results will be published in the Monthly Notices of the Royal Astronomical Society, but have been published on arXiv.org ahead of time.
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Koraljka Muzic et al. 2017. The low-mass content of the massive young star cluster RCW 38. MNRAS, submitted for publication; arXiv: 1707.00277
