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How Many Brown Dwarfs in the Milky Way?

Interesting news keeps coming out of the National Astronomy Meeting in the UK. Today it involves brown dwarfs and their distribution throughout the galaxy, a lively question given how recently we’ve begun to study these ‘failed stars.’ Maybe we need a better name than ‘brown dwarfs,’ for that matter, since these objects are low enough in mass that they cannot sustain stable hydrogen fusion in their core. In a murky intermediary zone between planet and star, they can produce planets of their own but straddle all our contemporary definitions.

At the NAS meeting, an international team led by Koraljka Muzic (University of Lisbon) has reported on its work on brown dwarfs in clusters. It seems a sensible approach — go to the places where young stars are forming and try to figure out how many brown dwarfs emerge alongside them. That could help to give us an overview, because the brown dwarfs we’ve already found (beginning with the first, in 1995) are generally within 1500 light years of the Sun. Cluster studies should help us gain some sense of their broader distribution.

Clusters, after all, give birth to all kinds of stars from tens of solar masses down to the brown dwarfs we’re looking for, and the so-called initial mass function (IMF) showing the distribution of the different masses in young clusters has received plenty of attention. A key figure in its earlier study was Edwin Salpeter, an emigrant from Austria to Australia who wound up teaching at Cornell University. His work on the initial mass function led to values still accepted today down to stars of about half a solar mass, but things below that get murkier.

Image: False-colour 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 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. Credit: Koraljka Muzic, University of Lisbon, Portugal / Aleks Scholz, University of St Andrews, UK / Rainer Schoedel, University of Granada, Spain / Vincent Geers, UKATC Edinburgh, UK / 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.

Thus the need to update the IMF in the low mass range, a key part of what Muzic and team call Substellar Objects in Nearby Young Clusters, their survey to examine substellar populations in nearby star-forming regions. Recent studies have indicated that a single underlying initial mass function can explain the ratios of stars to brown dwarfs in the clusters studied. But most of the clusters studied to date have been loose groups of low mass stars.

Having already looked at five nearby star-forming regions, Muzic’s group chose to study brown dwarf formation in more massive, dense embedded clusters and homed in on the more distant cluster RCW 38, some 5500 light years out in the constellation Vela. This is a young cluster (less than a million years) that is twice as dense as the Orion Nebula Cluster and orders of magnitude denser than other nearby star forming regions. Packed with massive stars, it seems the ideal place to determine whether different starting conditions produce a different ratio of brown dwarfs to other types of star.

From the paper:

According to various BD formation theories, stellar density is expected to affect the production of very-low mass objects (high densities favor higher production rate of BDs), as well as the presence of massive OB stars capable of stripping the material around nearby pre-stellar cores until leaving an object not massive enough to form another star. In addition to the theoretical expectations, we discussed several observational hints for environmental differences in the nearby star forming regions from the literature, which, however, require further investigation. To that end, we choose to study a cluster that is several orders of magnitude denser than any of the nearby star forming regions (except for the ONC [Orion Nebula Cluster]), and also, unlike them, rich in massive stars.

Working with adaptive optics observations via the NAOS-CONICA imager at the European Space Agency’s Very Large Telescope, the researchers found values for brown dwarf formation that agreed with other young star-forming regions, even those much less dense. The results were clearcut, as the paper notes:

…leaving no evidence for environmental differences in the efficiency of the production of BDs and very-low mass stars possibly caused by high stellar densities or a presence of numerous massive stars.

And this:

In all regions studied so far, the star/BD ratio is between 2 and ∼ 5, i.e. for each 10 low-mass stars between 2 and 5 BDs are expected to be formed. This is also consistent with estimates of the star/BD ratio in the field (∼ 5; Bihain & Scholz 2016). The sum of these results clearly shows that brown dwarf formation is a universal process and accompanies star formation in diverse star forming environments across the Galaxy.

Interesting indeed! The brown dwarf population of the galaxy, based on this survey, is at least 25 billion and may range as high as 100 billion brown dwarfs, which would mean a brown dwarf for every hydrogen-burning star. The range results from the difficulty in observing such small, faint objects, raising the prospect that even the high number here may be an underestimate. Another cause for the variation: Star formation rates in the Milky Way seem to have been higher in the past. If this is the case, the 100 billion number seems more likely.

And note this: The calculations on the brown dwarf population were derived only for brown dwarfs more massive than 0.03 solar masses. Compare this to Jupiter’s mass, which is 0.00095 that of the Sun. Between the two is a wide range of possibilities.

“It seems that brown dwarfs form in abundance in a variety of star clusters,” says Ray Jayawardhana (York University), a member of the research team. “They are ubiquitous denizens of our Milky Way galaxy.”

The paper is Muzic, et al., “The Low-Mass Content of the Massive Young Star Cluster RCW 38,” submitted to Monthly Notices of the Royal Astronomical Society (preprint).

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{ 36 comments… add one }
  • Spaceman July 7, 2017, 10:17

    Interesting study! I have always been intrigued by brown dwarfs. You mentioned that the details of the IMF below half a solar mass are murky, but I had been under the impression that the IMF is already well-characterized down to the hydrogen burning limit but not below it? Though not addressed in this particular study, is there any emerging evidence as to how common planets might be around brown dwarfs? Third question: are there any ideas as to what a brown dwarf might look like from up close, say, to an orbiting interstellar spaceship? Last question: could brown dwarfs provide energy for any existing civilizations in the distant future– long after the stars have burned out?

  • Mike July 7, 2017, 10:28

    If the brown dwarf population is that numerous it should be seen eventually in the gravitational lensing surveys. The WISE survey didn’t seem to indicate this large a population. A more sensitive infrared survey would be helpful here.

    • Ron S. July 7, 2017, 20:40

      As I understand it (and Paul pointed out) the galactic distribution may be uneven and since lensing surveys only work for nearby BD we must be careful to not blindly extrapolate. Since birth rate is a distance independent survey this study can be helpful. Conversely if the distribution is fairly even and birth rate in clusters is high that would imply other BD sources are less fecund than currently thought. All very interesting.

      As for a better name: stellar tween?

      • Paul Gilster July 8, 2017, 10:15

        “As for a better name: stellar tween?”

        Ha! Implies a certain adolescent quality to brown dwarfs that’s sort of appealing!

  • RAS July 7, 2017, 11:54

    Wouldn’t WFIRST fulfill that role?

  • ljk July 7, 2017, 12:05

    Paul Gilster said in the main article:

    “Maybe we need a better name than ‘brown dwarfs,’ for that matter, since these objects are low enough in mass that they cannot sustain stable hydrogen fusion in their core. In a murky intermediary zone between planet and star, they can produce planets of their own but straddle all our contemporary definitions.”

    I always thought that black holes should be called collapsars to go along with pulsars and quasars, but BH seems here to stay.

  • FrankH July 7, 2017, 16:21

    I think “Failed stars” is almost as bad as “Super Earth”. If a BD – or even Jupiter – can be a failed star, then so can the Earth.

    Substellar dwarf would limit the range to something not a star, but somewhat distinct from a planet by the way they form; slightly less murky since Jupiter and Saturn may fall in this group.

    Brown dwarf is the shortest, most descriptive name we have. Something that cannot sustain hydrogen (or hydrogen isotope) fusion for long and is more massive than the largest planet, whatever that may be. They are the astronomical equivalent of “I know it when I see it”.

  • Alex Tolley July 7, 2017, 16:22

    That frequency of BD production would seem to imply that we have missed more local BDs. New observations might be of help.

    Given their frequency, perhaps it is time to think up more uses for BDs in interstellar flight. If they are mostly hydrogen, then they might be good places to refuel as they are cool and approachable, unlike stars. What other uses might they have?

    • Brett Bellmore July 7, 2017, 20:39

      Refueling between stars is basically pointless; Newton’s laws dominate.

      You expend fuel to get up to speed, and then cruise along at that speed without further expenditure of fuel, until you reach your destination and expend the rest to come to a halt.

      If you stop at an intermediate point to refuel, you have to spend the fuel you would have used at your destination to stop there, and then spend the fuel you just picked up to resume your flight. Absolutely nothing gained, and you lose time due to having to slow down and accelerate again.

      Not at all like a planetary surface, where friction means you have to continuously expend fuel to keep moving, and can run out before you reach your destination.

      You wouldn’t stop at a brown dwarf unless you had some other purpose in doing so.

      • Alex Tolley July 9, 2017, 16:48

        Refueling between stars is basically pointless; Newton’s laws dominate.

        For a fast starship I agree. However, this may not be the case for slow world ships where fusion is used to create power for the ship rather than velocity. I am envisaging O’Neills and larger, slowly traveling outward from Sol and taking on board fuel for energy production. One needn’t even slow down such a ship, it might be going slow enough to scoop up the fuel on a close flyby, or possibly deploy a scoop ship to slow down, scoop up the fuel and then rendezvous with the mothership. Such slow ships might be traveling at very small fractions of c as they are nomadic, the inhabitants having no desire to colonize a new planet rather than living in the comfort of their manufactured world. Such ships may have started in the Oort cloud using the water there for their hydrogen fuel source, then later, when their technology had improved, ventured out among the stars.

        Such ships could be vast, as large or larger than the SW’s “Death Star”.

    • Robert G July 8, 2017, 9:15

      Communications posts, taking advantage of gravitational lensing, and with presumably less grief from chromosphere interactions?

    • Michael July 8, 2017, 16:30

      Perhaps Gravo lensing beacons for astronavigation. They could also be ‘lit’ up by neutron bombardment to produce deuterium for fusion over time provided they are heavy enough. They would also make powerful observation platforms with their mass and great density.

      • Brett Bellmore July 9, 2017, 7:57

        Where you going to get the neutrons? Manufacturing neutrons when all you’ve got is Hydrogen is energetically expensive, you might even use up all the power you could get from fusing the Deuterium.

        We are eventually going to have to learn to fuse pure hydrogen, but it’s going to be a tough one.

        • Michael July 10, 2017, 9:54

          We could use the neutrons from normal fusion allowing them to flow to the BD over time.

          • Brett Bellmore July 10, 2017, 17:58

            I’m unaware of any fusion reaction that produces neutrons without requiring neutron rich inputs, but perhaps I’ve overlooked one.

            • Michael July 13, 2017, 6:45

              Producing neutrons would be a problem, perhaps the breeding of tritium, from lithium, and deuterium to be fed to the ‘planet’ would be better.

  • Frank Smith July 7, 2017, 17:07

    It would be nice if a BD was found about 1 light year out from the earth. It would be a great objective for testing various propulsion schemes. We could see results without waiting 400 years.

    Also the possibility of a gravitational assist the way various solar system missions have taken advantage of a free boost in speed.

    • Antonio July 8, 2017, 6:45

      You can’t have a gravitational assist from an almost static body, like a BD orbiting the Sun in the Oort Cloud. You would have only a gravitational drag.

      • Brett Bellmore July 8, 2017, 12:12

        Not true, the classic slingshot maneuver would work. But the gain wouldn’t be very great, if you already had enough speed to get to the BD in a reasonable amount of time.

        • Frank Smith July 8, 2017, 13:00

          I was thinking along the lines of a non-gravitationally bound BD, but Brett is probably right. If we had a 1 tenth light speed probe adding 20 kps or so is not much of a gain.

          • Michael July 8, 2017, 16:32

            Due to their powerful magnetic fields energy could be extracted to bend their spacecraft’s trajectories by a greater amount.

        • Antonio July 8, 2017, 18:17

          Brett Bellmore: No, you will not gain speed. There will be no loss either, I was wrong with that part. https://en.wikipedia.org/wiki/Gravity_assist

          • Michael July 9, 2017, 9:39

            You can gain speed by using the galactic momentum component of the BD, but compared to the spacecraft velocity component it is very little.

            • Antonio July 9, 2017, 18:12

              Nope. In this scenario, the BD is orbiting the Sun, so the Sun, the BD and the ship are all sharing the same galactic momentum. Put another way, the BD will not have any galactic momentum in the reference frame of the Sun.

              • Michael July 10, 2017, 12:55

                It was not stated that it was in orbit around our sun.

                • Antonio July 10, 2017, 18:34

                  Ok. Then it could be used, if you knew the trajectory and velocity in advance, but the gain would be minimal.

          • Mauldred July 10, 2017, 9:01

            An Oberth’s manoeuver would still work.

            Approach the BD as close as possible, then fire the engines prograde while you’re just a few thousands kilometers above its cloud tops, you’ll convert some potential energy into kinetic energy.

            • Brett Bellmore July 10, 2017, 18:00

              That’s what I meant by “a classic slingshot maneuver”.

  • Christopher Phoenix July 7, 2017, 21:55

    Interesting study! It seems that brown dwarfs are commonplace inhabitants of our galaxy. Maybe it will turn out that there are even more of these dim, low-mass objects than hydrogen-burning stars, once we count those below 0.03 solar masses.

    Since brown dwarfs seem to be so common, it is probably a good time to ponder their astrobiological potential. Brown dwarfs radiate infrared radiation as they slowly contract, converting gravitational energy into heat. Studies suggest that massive brown dwarfs might have a narrow habitable zone due to this. This zone will shrink as the primary cools, but could there be time for life to get started all the same?

    This isn’t the only possibility. The under-ice ocean on Europa shows us that there are more possible habitats for life than Earth-like planets warmed by a hydrogen-burning stars. If tidal heating can keep Europa warm around Jupiter, I expect this can be just as important for a satellite of a brown dwarf. Then consider that as-yet-uncounted range of objects between Jupiter’s 0.00095 solar masses and the 0.03 solar masses that is the lower limit for this current study. Could they all be potential habitats for life?

    This also makes me wonder how brown dwarfs might be important to intelligent life from this planet, i.e., us! Mallove and Matloff note that if there is a brown dwarf closer to us than the nearest hydrogen burning star (Proxima Centauri at 4.2 lys), it could be a valuable stepping stone to interstellar travel, much like the Moon was for solar system exploration. If they are as common as the new study suggests, maybe there’s some hope of finding one nearby?

    It is interesting to ponder how brown dwarfs might be of use to interstellar travelers. I suspect they aren’t too helpful for gravitational assists, and smaller objects tend to be easier to harvest hydrogen from (then again, maybe this won’t be too big a problem by the time we can reach one). But if we have transitioned to a life in space, maybe we will choose to colonize brown dwarf systems. They probably have strong magnetic fields, like Jupiter, offering plentiful opportunities for electrodynamic tether power in addition to fusion fuel. This alternative to the traditional idea of migrating to other Sun-like stars with habitable planets deserves some investigation, I think.

  • Michael July 8, 2017, 16:52

    ‘Studies suggest that massive brown dwarfs might have a narrow habitable zone due to this. This zone will shrink as the primary cools, but could there be time for life to get started all the same? ‘

    This article may go some way to explain the HB zone around BD over time.

    https://planetplanet.net/2014/10/09/real-life-sci-fi-world-4-earth-around-a-brown-dwarf/

  • Geoffrey Hillend July 9, 2017, 17:52

    I doubt anyone one would want to colonize a brown dwarf system. There is no light so the atmospheres of the planets would be frozen. Any planet close enough to have internal heat caused by tidal forces from the gravity of a BD like Io around Jupiter would also orbit through a powerful, rotating magnetic field which would strip particles from the atmosphere of the planet or any volcanic plumes and accelerate them away from the planet. Or charged particles trapped in a high voltage electromagnetic field flux tube would collide with the planet in a dangerous radiation zone.

    A brown dwarf as close a Jupiter to a star would have a much higher radiation zone due to the solar wind particles which become trapped in the BD magnetic field.

  • Daniel Duffy July 9, 2017, 20:00

    http://nextbigfuture.com/2009/11/could-nasa-wide-infrared-survey.html

    “The other headline would be the discovery of a brown dwarf that is even closer to Earth than the nearest star, the Alpha Centauri system at 4.3 light-years. Brown dwarfs are objects that form along with stars but do not have enough mass to trigger or sustain nuclear fusion. They are so cool and dim very little is known about their distribution in the galaxy”

    and:

    http://www.scifi.com/sfw/issue183/labnotes.html

    “What if space is littered with these failed stars, scattered between the bright ones like a stellar Polynesia, making interstellar travel a series of short hops, rather than a single gigantic one? What if a simple fusion reactor carried just enough fuel to push a spacecraft to our solar system’s Planet X in reasonable time? What if it could refuel there, harvesting just enough hydrogen or deuterium or helium to limp along to another dark neighbor, and another, and another? Granted, it would take a long, long time to get to Alpha Centauri that way, and probably a much, much longer time to find a planet somewhere that looked even remotely like our rain- and sun-drenched Earth. But given the likelihood of tidally warmed moons, and the obvious possibilities for life there, we may just find that the cold, dark spaces are where most of the action is anyway.”

    There may be dozens or hundreds of mini-solar systems between Sol and Alpha Centauri. With the discovery of BDs, free floating planets between the stars, and extra-solar planetoids like Sedna, future space explorers may find plenty to keep them occupied in our own solar neighborhood for centuries to come. While not the galaxy spanning empires and federations of science fiction, it would be enough for our species to explore far into the future.

    And since these mini-solar systems and planets are a stone’s throw away, they can be reached without exotic warp drives or hyperspace jumps. Simple solar sails, laser sails or nuclear rockets will do just fine. Exploration missions can visit and return in a matter of years, instead of centuries or millennium. Interstellar “empires” and “federations” can be created using slower than light space travel. Maybe Capt. Kirk and Obi Wan Kenobi wouldn’t be impressed, but we’ll be half way to Alpha Centauri.

    What if BDs turn out to be scattered by the dozens or hundreds in the space between the stars? And what if most of them have mini-solar systems (like Jupiter and Saturn) capable of supporting life because there is enough heat is generated by the BD to allow liquid water and photosynthesis based on infrared frequencies? It’s easy to imagine life based on infrared photosynthesis on moons orbiting brown dwarfs which give off heat but not light. Not just imagine it, we already know of such life here on Earth, green sulfur bacteria. And if BDs floating between the stars greatly outnumber suns, then visible light spectrum based life may be the exception instead of the rule.

    In addition to infrared based life, Cornell researchers have modeled methane based life forms that don’t use water and could live in the liquid methane seas of Titan. Methane based life forms by themselves are a fascinating concept. But ironically the potential “Goldilocks” zone for such life is far greater (extending across the range of Jovian worlds out to the Kuiper belt) than our narrow zone for water based life forms.

    So “life as we know it” based on water and the visible light spectrum photosynthesis may be the rare exception in a universe dominated by methane based life and life that utilizes infrared photosynthesis.

  • Antonio July 10, 2017, 11:44

    “Granted, it would take a long, long time to get to Alpha Centauri that way, and probably a much, much longer time to find a planet somewhere that looked even remotely like our rain- and sun-drenched Earth.”

    Why on hell would you search for that!? You can obtain energy and matter from the most abundant bodies out there, and travel freely between them. The Universe is yours!! Who needs a second Earth!?

  • Rob Flores July 11, 2017, 11:44

    I am just glad that we dont have one Incoming to our solar system.
    Jupiter x 10, mass would certainly re-arrange things even if it only came to the proximity of the KBO band.

  • kzb July 12, 2017, 7:14

    But how do we reconcile this result with WISE?
    This study finds the star/BD ratio to be 2.1 +/- 0.6.
    What did WISE find? From memory I think it was about 6.
    So why the discrepancy?

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