Milky Way could be home to 100 billion 'failed stars'

July 5, 2017, Royal Astronomical Society
A false-color near-infrared image of the core of the young massive cluster RCW 38 taken with the adaptive-optics camera NACO at the ESO's Very Large Telescope. RCW 38 lies at a distance of about 5500 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. These candidate brown dwarfs might weigh only a few tens of Jupiter masses, or about 100 times less than the most massive stars seen towards the centre of the image. Credit: Koraljka Muzic, University of Lisbon, Portugal / Aleks Scholz, University of St Andrews, UK / Rainer Schoedel, University of Granada, Spain / Vincent Geers, UKATC / Ray Jayawardhana, York University, Canada / Joana Ascenso, University of Lisbon, University of Porto, Portugal / Lucas Cieza, University Diego Portales, Santiago, Chile. The study is based on observations conducted with the VLT at the European Southern Observatory.

Our galaxy could have 100 billion brown dwarfs or more, according to work by an international team of astronomers, led by Koraljka Muzic from the University of Lisbon and Aleks Scholz from the University of St Andrews. On Thursday 6 July Scholz will present their survey of dense star clusters, where brown dwarfs are abundant, at the National Astronomy Meeting at the University of Hull.

Brown dwarfs are objects intermediate in mass between stars and planets, with masses too low to sustain stable hydrogen fusion in their core, the hallmark of stars like the Sun. After the initial discovery of in 1995, scientists quickly realised 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 1500 light years, simply because these objects are faint and therefore difficult to observe. Most of those detected are located in nearby star forming regions, which are all fairly small and have a low density of stars.

In 2006 the team began a new search for brown dwarfs, observing five nearby star forming regions. The Substellar Objects in Nearby Young Clusters (SONYC) survey included the star NGC 1333, 1000 light years away in the constellation of Perseus. That had about half as many brown dwarfs as stars, a higher proportion than seen before.

A false-color near-infrared image of the core of the young massive cluster RCW 38 taken with the adaptive-optics camera NACO at the ESO's Very Large Telescope. RCW 38 lies at a distance of about 5500 light years from the Sun. The field of view of the central image is approximately 1 arc minute, or 1.5 light years across. Credit: Koraljka Muzic, University of Lisbon, Portugal / Aleks Scholz, University of St Andrews, UK / Rainer Schoedel, University of Granada, Spain / Vincent Geers, UKATC / Ray Jayawardhana, York University, Canada / Joana Ascenso, University of Lisbon, University of Porto, Portugal / Lucas Cieza, University Diego Portales, Santiago, Chile. The study is based on observations conducted with the VLT at the European Southern Observatory.

To establish whether NGC 1333 was unusual, in 2016 the team turned to another more distant star cluster, RCW 38, in the constellation of Vela. This has a high density of more massive stars, and very different conditions to other clusters.

RCW 38 is 5500 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, Scholz, Muzic and their collaborators used the NACO adaptive optics camera on the European Southern Observatory's Very Large Telescope, observing the cluster for a total of 3 hours, and combining this with earlier work.

The researchers found just as many brown dwarfs in RCW 38 - about half as many as there are stars - and realised 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.

Artist's impression of a T-type brown dwarf. Credit: NASA / JPL-Caltech

Scholz says: "We've found a lot of brown dwarfs in these clusters. And whatever the cluster type, the brown dwarfs are really common. Brown dwarfs form alongside in clusters, so our work suggests there are a huge number of brown dwarfs out there."

From the SONYC survey, Scholz and team leader Koraljka Muzic, estimate that our galaxy, 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.

Explore further: The missing brown dwarfs

More information: The new work appears in: "The low-mass content of the massive young star cluster RCW 38", K. Muzic, R. Schoedel, A. Scholz, V. C. Geers, R. Jayawardhana, J. Ascenso, and L. A. Cieza, Monthly Notices of the Royal Astronomical Society, submitted.

The results will be presented as part of a talk at NAM2017 by co-author and SONYC lead Aleks Scholz on Thursday 6 July.

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13 comments

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ursiny33
not rated yet Jul 06, 2017
They are future food for double and triple star solar systems
Da Schneib
not rated yet Jul 06, 2017
Huh. Interesting but not really surprising.

The next question is, do they form in nascent solar systems and escape, or form from patches of nebula that aren't dense enough to form stars?
dnatwork
5 / 5 (1) Jul 06, 2017
I think the next questions are: what's the mass of all these things that we couldn't see before, and how much less dark matter is needed now?
Da Schneib
5 / 5 (4) Jul 06, 2017
Two problems with that, @dnat. First, we don't see enough microlensing events for brown dwarfs to be ubiquitous enough to account for dark matter; second, the amount needed would be on the close order of ten times as many brown dwarfs as stars, more than an order of magnitude difference from what we see.

We've known that for over a decade now.
katesisco
1 / 5 (3) Jul 06, 2017
Thinking brown dwarfs are the product of sun's blowing gas bubbles from the corona. These gas bubbles do not condense to form a visible matter object until their lattice structure changes. And perhaps they also become less dense/magnetic and lose spherical shape and become gas clouds. Like the one NASA says we are in now.
jonnyuzi
not rated yet Jul 06, 2017
That's simply because all the White Dwarfs were racist and moved into a more hospitable galaxy.
Steelwolf
not rated yet Jul 06, 2017
I would imagine that most brown dwarfs actually form normally in large dense clouds but when a slightly older star forms and then blows away the gas cloud it tends to shut down the formation of the brown dwarfs as well.

Perhaps they also act as seeds for the next round of gascooling, densification and infall, forming seeds for the next full on active stars.
StudentofSpiritualTeaching
1 / 5 (1) Jul 08, 2017
Huh. Interesting but not really surprising.

The next question is, do they form in nascent solar systems and escape, or form from patches of nebula that aren't dense enough to form stars?


The next question is: "Where is our solar system's brown dwarf right now?" Answer: causing a bit of havoc in the Oort Cloud.
Mayday
not rated yet Jul 08, 2017
Are you sure that they're not just leaky Dyson spheres?
Shootist
5 / 5 (1) Jul 08, 2017
Huh. Interesting but not really surprising.

The next question is, do they form in nascent solar systems and escape, or form from patches of nebula that aren't dense enough to form stars?


The next question is: "Where is our solar system's brown dwarf right now?" Answer: causing a bit of havoc in the Oort Cloud.


Hardly, a mass 20-30 times Jupiter would have been noted long ago. We have yet to pin down their locations but it is realized that there is a Neptune-mass object disturbing Kuiper Belt objects and potentially an Earth mass object as well.
AstroRudy2
1 / 5 (1) Jul 10, 2017
To Da Schneib. This population was seen in microlensing in quasar lens system Q0957+561A,B by R. Schild in 1996 and then confirmed in Q2237+0305 by Vakulik et al (several papers in era 2003 - 2008 by V.Vakulik and colleagues in Kharkov published in MNRAS. They are numerous enough to account for missing baryon dark matter, and formed in early Universe as understood by Carl Gibson. Because MACHO microlensing searches assume uniform distribution they cannot find the clumped distribution. The objects do not have a mean mass but a M^-3 power law mass distribution (Theo Nieuwenhuizen et al 2011.
Da Schneib
5 / 5 (2) Jul 10, 2017
So now we're supposed to have a hundred times more brown dwarfs than we see, *and* they're all clumped together so MACHO microlensing searches can't find them?

Sounds pretty unlikely.
AstroRudy2
2 / 5 (2) Jul 10, 2017
The objects found are probably not escapees, but rather the Carl H. Gibson primordial population that he called "Primordial Fog Particles (PFP).
The predecesser to this stqr formation cloud was found by the European Herschel space telescope, which compiled the HERSCHEL DARK CLOUD CATALOGUE of 1300 such objects, all at measured temperature ~12K. These are clouds of these brown dwarf objects thermostated to13.8 K by the TRIPLE POINT TEMPERATURE OF HYDROGEN at low density, 13.8K. At this temperature hydrogen gas freezes directly to hydrogen ice but it has to dump lots of fusion energy so the planetary mass objects are forming snow and radiating furiously at 13.8 K. The Herschel Dark Clouds have measured sizes around 5 parsecs = 17 light years. The clouds have bizarre properties and have never been explained.

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