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TOPIC: Brown Dwarfs


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Investigations by astronomers at the Harvard-Smithsonian Centre for Astrophysics (CfA) have led them to be able to decode the true nature of a mysterious object hiding inside a dark cosmic cloud 600 light-years away in the constellation Cygnus the Swan.
They found that the cloud, once thought to be featureless, contains a baby star, or possibly a failed star known as a "brown dwarf," that is still forming within its dusty cocoon.

The Spitzer space telescope discovered a very low mass object in the nearby dark cloud L1014. With the MMT and SMA astronomer confirmed that the source, L1014-IRS, is indeed protostellar and embedded within the cloud.
It drives a very small, low mass molecular outflow detected in CO 2-1 which has carved out a cavity seen in near-infrared scattered light. The luminosity of L1014-IRS is only 0.025-0.05 Lsun, and the inferred mass is of order 25 Jupiter masses, implying that it is either a very young protostar yet to accrete the bulk of its mass, or the first detected of an embedded, very young brown dwarf.

However, the embedded mystery object may still be growing so eventually it will become large enough to qualify as a small star.

A weak outflow of material was detected, using the SMA. Star formation theories predict such flows. The out flow is 10 times smaller in mass than anything seen before - confirmed both the low-mass nature of the object and its association with the surrounding dark cloud.

"The sensitivity and resolution of the Submillimetre Array with its multiple antennas were crucial in detecting the outflow" - Tyler Bourke.

"This object is the runt of the star formation family" - Tyler Bourke, CfA astronomer

Establishing the true nature of the object required the unique capabilities of the Submillimetre Array (SMA) in Hawaii.

"The SMA spotted what no single-dish telescope could see" - Tyler Bourke.

The puzzling object was discovered using a Smithsonian-developed infrared camera on board NASA's Spitzer Space Telescope. Spitzer studied the dusty cosmic cloud named L1014 as part of the Cores to Disks Legacy program. A core is the densest region of a cloud, massive enough to make a star like the sun.

L1014 was initially was classified as a "starless core" because it showed no evidence for star formation. Astronomers were surprised when Spitzer images revealed a faint infrared light source that appeared to be within the core.
Additional data were needed to confirm that the faint object was directly associated with the dark core, rather than being a chance superposition of a more distant, more mundane background object.

Expand (1.4Mb, 2200 x 1754)
At top left is the optical image of L1014, with contours of 1.2 mm dust emission, and the field-of-view of the Spitzer images indicated by the box. The position of young brown dwarf or protostar (dubbed L1014-IRS) is indicated. At top right is a 3-color image using the Spitzer data, colour-coded by wavelength. At bottom left is the 8-micron-only image, at bottom middle is a near-infrared image from the MMT revealing a scattered light nebula typically seen around young stellar objects, thought to be due to a cavity evacuated by an outflow. At bottom right is the confirmation that L1014-IRS drives a bipolar outflow, seen with the Submillimetre Array. The outflow velocities associate the infrared source with starless core L1014 at a distance of 200 parsecs, thus confirming its low luminosity and mass. Credit: Tyler Bourke & Tracy Huard (CfA)

Near-infrared observations by the MMT Observatory in Arizona revealed a scattered light nebula surrounding the faint central object in L1014.

"Light from the object is bouncing off surrounding dust and toward us. Reflection nebulosity like that is a fingerprint of an embedded object" - Tracy Huard, CfA astronomer, who took the MMT images.

The apparent size of the nebulosity indicated that the light source likely was located within L1014 and not in a more distant cloud. MMT data also gave investigators the orientation in space, or tilt, of the object within L1014. Astronomers then turned to the SMA for final confirmation.

"The Spitzer observations gave us hints to the nature of the object inside L1014. The MMT strengthened the association between the infrared source and the starless core. The Submillimetre Array clinched the case and revealed this object's true identity" - Tyler Bourke.

Astronomers hope to learn more about the early stages of star formation by studying young objects like the one still forming within L1014,

"The most elusive part of star formation is the moment of birth. In order to answer how it happens, you need examples of very young systems. This system is only about 10,000 to 100,000 years old - a baby as far as stars or brown dwarfs go" - Phil Myers, CfA astronomer.

"They're so young and faint that we can't tell how much mass they will accumulate. There's no prenatal test for these objects. We're not sure exactly what we'll get in the end!" - Phil Myers.

A paper by Tyler L. Bourke et al. covering the SMA observations will be published in an upcoming issue of The Astrophysical Journal Letters and is available online here
A second paper by Tracy L. Huard et al. covering the MMT observations will be published in The Astrophysical Journal and is available online here .
Adapted from source.



Posts: 131433
Brown Dwarfs

After decades of searching, astronomers discovered the first definite brown dwarf only ten years ago. Now, a new analysis of Hubble Space Telescope data implies our Galaxy has almost as many of these failed stars as it does normal stars like the Sun.
The Sun and most other stars power themselves by converting hydrogen into helium. In contrast, brown dwarfs have so little mass--less than 8 percent of the Sun's--that they never get hot enough to sustain this nuclear reaction. Instead, they convert gravitational energy into heat, glowing red, then fade as they cool.

Russell Ryan, Jr., Nimish Hathi, Seth Cohen, and Rogier Windhorst of Arizona State University used data from the Hubble Space Telescope to search for brown dwarfs in 15 different directions above and below the Milky Way's plane. By utilizing near-infrared data, the astronomers found 28 faint stars as red as those with the coolest spectral types, L and T.
The stars ranged in brightness from magnitude 21 to 25.

The brown dwarf candidates belong to the thin disk, the Galaxy's brightest component, which harbours the Sun and most other nearby stars. The thin disk is about 2,000 light-years thick; the brown dwarf candidates are in a disk that's 2,280 ± 330 light-years thick.
Extrapolating from the number of brown dwarfs they discovered, Ryan and his colleagues estimate the Galaxy has roughly 100 billion L- and T-type dwarfs. This number is comparable to the Milky Way's total of all other stars put together.
Thus, several brown dwarfs probably lurk unseen within just 12 light-years of the Sun. This volume of space contains more than two dozen main-sequence stars like the Sun--but only two known brown dwarfs.

Both these brown dwarfs orbit the orange dwarf star Epsilon Indi, which is 11.8 light-years from Earth. They are the closest known brown dwarfs to the Sun.
Despite their impressive number, brown dwarfs add little weight to the Galaxy and do not account for its dark matter.
Ryan's team estimates brown dwarfs contribute roughly a billion solar masses to the Milky Way--only 0.1 percent of the Galaxy's total. Altogether, the Galaxy has roughly a trillion solar masses, most of which is dark matter.
Ryan and his colleagues will publish their work in a future issue of Astrophysical Journal Letters.


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