The Secret Origin Story of Brown Dwarfs

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Sometimes called failed stars, brown dwarfs straddle the line between star and planet. Too massive to be “just” a planet, but lacking enough material to start fusion and become a full-fledged star, brown dwarfs are sort of the middle child of cosmic objects. Only first detected in the 1990s, their origins have been a mystery for astronomers. But a researchers from Canada and Austria now think they have an answer for the question: where do brown dwarfs come from?

If there’s enough mass in a cloud of cosmic material to start falling in upon itself, gradually spinning and collapsing under its own gravity to compress and form a star, why are there brown dwarfs? They’re not merely oversized planets — they aren’t in orbit around a star. They’re not stars that “cooled off” — those are white dwarfs (and are something else entirely.) The material that makes up a brown dwarf probably shouldn’t have even had enough mass and angular momentum to start the whole process off to begin with, yet they’re out there… and, as astronomers are finding out now that they know how to look for them, there’s quite a lot.

So how did they form?

According to research by Shantanu Basu of the University of Western Ontario and  Eduard I. Vorobyov from the University of Vienna in Austria and Russia’s Southern Federal University, brown dwarfs may have been flung out of other protostellar disks as they were forming, taking clumps of material with them to complete their development.

Basu and Vorobyov modeled the dynamics of protostellar disks, the clouds of gas and dust that form “real” stars. (Our own solar system formed from one such disk nearly five billion years ago.) What they found was that given enough angular momentum — that is, spin — the disk could easily eject larger clumps of material while still having enough left over to eventually form a star.

Model of how a clump of low-mass material gets ejected from a disk (S. Basu/E. Vorobyev)

The ejected clumps would then continue condensing into a massive object, but never quite enough to begin hydrogen fusion. Rather than stars, they become brown dwarfs — still radiating heat but nothing like a true star. (And they’re not really brown, by the way… they’re probably more of a dull red.)

In fact a single protostellar disk could eject more than one clump during its development, Basu and Vorobyov found, leading to the creation of multiple brown dwarfs.

If this scenario is indeed the way brown dwarfs form, it stands to reason that the Universe may be full of them. Since they are not very luminous and difficult to detect at long distances, the researchers suggest that brown dwarfs may be part of the answer to the dark matter mystery.

“There could be significant mass in the universe that is locked up in brown dwarfs and contribute at least part of the budget for the universe’s missing dark matter,” Basu said. “And the common idea that the first stars in the early universe were only of very high mass may also need revision.”

Based on this hypothesis, with the potential number of brown dwarfs that could be in our galaxy alone we may find that these “failed stars” are actually quite successful after all.

The team’s research paper was accepted on March 1 into The Astrophysical Journal.

Read more on the University of Western Ontario’s news release here.

The Zooniverse is Expanding: The Milky Way Project Begins Today

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From the folks that brought you the addictive citizen science projects Galaxy Zoo and Moon Zoo (among others), comes yet another way to explore our Universe and help out scientists at the same time. The Milky Way Project invites members of the public to look at images from infrared surveys of our Milky Way and flag features such as gas bubbles, knots of gas and dust and star clusters.

As with the other Zooniverse projects, the participation of the public is a core feature. Accompanying the Milky Way Project is a way for Zooniverse members – lovingly called “zooites” – to discuss the images they’ve cataloged. Called Milky Way Talk, users can submit images they find curious or just plain beautiful to the talk forum for discussion.

The Milky Way Project uses data from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) and the Multiband Imaging Photometer for Spitzer Galactic Plane Survey (MIPSGAL). These two surveys have imaged the Milky Way in infrared light at different frequencies. GLIMPSE at 3.6, 4.5, 5.8, and 8 microns, and MIPSGAL at 24 and 70 microns. In the infrared, things that don’t emit much visible light – such as large gas clouds excited by stellar radiation – are apparent in images.

The new project aims at cataloging bubbles, star clusters, knots of gas and dark nebulae. All of these objects are interesting in their own ways.

Bubbles – large structures of gas in the galactic plane – belie areas where young stars are altering the interstellar medium that surrounds them. They heat up the dust and/or ionize the gas that surrounds them, and the flow of particles from the star pushes the diffuse material surrounding out into bubble shapes.

The green knots are where the gas and dust are more dense, and might be regions that contain stellar nurseries. Similarly, dark nebulae – nebulae that appear darker than the surrounding gas – are of interest to astronomers because they may also point to stellar formation of high-mass stars.

Star clusters and galaxies outside of the Milky Way may also be visible in some of the images. Though the cataloging of these objects isn’t the main focus of the project, zooites can flag them in the images for later discussion. Just like in the other Zooniverse projects, which use data from robotic surveys, there is always the chance that you will be the first person ever to look at something in one of the images. You could even be like Galaxy Zoo member Hanny and discover something that astronomers will spend telescope time looking at!

This image is full of objects that are interesting to astronomers for study. You can help them pick out which things to study. Image Credit: Spitzer/The Milky Way Project

The GLIMPSE-MIPSGAL surveys were performed by the Spitzer Space Telescope. Over 440,000 images – all taken in the infrared – are in the catalog and need to be sifted through. This is a serious undertaking, one that cannot be accomplished by graduate students in astronomy alone.

In cataloging these bubbles for subsequent analysis, Milky Way Project members can help astronomers understand both the interstellar medium and the stars themselves imaged by the survey. It will also help them to make a map of the Milky Way’s stellar formation regions.

As with the other Zooniverse projects, this newest addition relies on the human brain’s ability to pick out patterns. Diffuse or oddly-shaped bubbles – such as those that appear “popped” or are elliptical – are difficult for a computer to analyze. So, it’s up to willing members of the public to help out the astronomy community. The Zooniverse community boasts over 350,000 members participating in their various projects.

A little cataloging and research of these gas bubbles has already been done by researchers. The Milky Way Project site references work by Churchwell, et. al, who cataloged over 600 of the bubbles and discovered that 75% of the bubbles they looked at were created by type B4-B9 stars, while 0-B3 stars make up the remainder (for more on what these stellar types mean, click here).

A zoomable map that uses images from the surveys – and has labeled a lot of the bubbles that have been already cataloged by the researchers- is available at Alien Earths.

For an extensive treatment of just how important these bubbles are to understanding stars and their formation, the paper “IR Dust Bubbles: Probing the Detailed Structure and Young Massive Stellar Populations of Galactic HII Regions” by Watson, et. al is available here.

If you want to get cracking on drawing bubbles and cataloging interesting features of our Milky Way, take the tutorial and sign up today.

Sources: The Milky Way Project, Arxiv, GLIMPSE

A Young Star Grows Up

Remember when you were young and how Mom always told you to eat everything on your plate so you would get big? Well, there’s a young star heeding that advice about 2,600 light years from Earth in the constellation Monoceros. Known as MWC 147, this young stellar object is devouring everything on its “plate,” the disk of gas and dust that surrounds it. Astronomers are witnessing how this star is gaining mass, and is on its way to becoming an adult.

Using the Very Large Telescope Interferometer, ESO (European Organization for Astronomical Research in the Southern Hemisphere) astronomers have peered into the disc of material surrounding MWC 147, witnessing how the star gains its mass as it matures. This star is increasing in mass at a rate of seven millionths of a solar mass per year. Ah, these young stars. It seems like they grow up so fast these days.

MWC 147 is less than half a million years old. If our 4.6 billion year old Sun is considered to be middle-aged, MWC 147 would be a 1-day-old baby. This star is in the family of Herbig Ae/Be objects. These are stars that have a few times the mass of our Sun and are still forming, increasing in mass by swallowing material present in a surrounding disc.

Being 6.6 times more massive than the Sun, however, MWC 147 will only live for about 35 million years, or to draw again the comparison with a person, about 100 days, instead of the 80 year equivalent of our Sun.

We’re still learning about the morphology of the inner environment of these young stars, and everything we can discover helps us to better understand how stars and their surrounding planets form.

The observations by the ESO astronomers show that the temperature changes in this area are much steeper than predicted by current models, indicating that most of the near-infrared emission emerges from hot material located very close to the star, within one or two times the Earth-Sun distance (1-2 AU). This also implies that dust cannot exist so close to the star, since the strong energy radiated by the star heats up and ultimately destroys the dust grains.

“We have performed detailed numerical simulations to understand these observations and reached the conclusion that we observe not only the outer dust disc, but also measure strong emission from a hot inner gaseous disc. This suggests that the disc is not a passive one, simply reprocessing the light from the star,” explained astronomer Stefan Kraus. “Instead, the disc is active, and we see the material, which is just transported from the outer disc parts towards the forming star.”

Also of note is the beautiful image of the region surrounding MWC 147, which I’ll post below. The number of stars in this image is incredible, and is reminiscent of the “grains of sandâ€? comment by Carl Sagan. This is a wide field image taken by Stephane Guisard of ESO with a 200 mm lens.

The Region Surrounding MWC 147.  Image Credit:  Stéphane Guisard (ESO)

Original News Source: ESO Press Release