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Baby Star's 'Placenta' Precisely Measured for the First Time

The disk that feeds a baby star has been precisely measured for the first time, revealing details that have, until now, remained a mystery.

It is well known that as a massive cloud of gas collapses under its own gravity, baby stars may form. The intense gravitational collapse kicks off fusion processes that begin the coalescence of more matter that feeds into a newborn star. Though the general process is fairly well understood, the details are not.

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For example, a stellar embryo growing inside a gas cloud isn't "fed" directly from that cloud; matter from the cloud spirals toward the baby star, creating a rapidly-swirling, hot disk. The star is therefore fed by the disk, which is itself fed by gas from the surrounding cloud. This disk acts almost like a mother's placenta; it's the placenta that provides nutrients for the developing embryo, not the mother herself.

But astronomers have not been able to precisely observe where the disk around a newborn star ends (the "placenta") and where the inner boundary of the gas cloud (the "mother") begins. Now, astronomers using the stunningly powerful Atacama Large Millimeter/submillimeter Array (ALMA) have seen this boundary, a direct observation that will undoubtedly improve star (and planetary) formation models.

"The disks around young stars are the places where planets will be formed," said Yusuke Aso, of the University of Tokyo and lead author of a paper published in the Astrophysical Journal. "To understand the formation mechanism of a disk, we need to differentiate the disk from the outer envelope precisely and pinpoint the location of its boundary."

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Zooming in on a protostar named TMC-1A, which is located around 450 light-years away in the constellation Taurus, Aso's team were able to see its spinning inner disk (the protoplanetary disk) and differentiate it from the cloud feeding it. ALMA's extreme precision at measuring velocity distributions was key to this endeavor.

In the case of TMC-1A, the transition boundary from spinning disk to surrounding gas cloud envelope was measured to extend 90 AU (astronomical units; where 1 AU is the average distance the Earth orbits the sun) from the central baby star, a distance 3-times bigger than the orbit of Neptune. What's more, the ALMA observations revealed the protostar's disk obeys Keplarian motion; the material closest to the star orbits faster, whereas the material further out orbits slower.

This is important: using the rotation speed of the disk's gas, the researchers could calculate the mass of the baby star. This stellar infant "weighs in" at a healthy 0.68 times (68 percent, or roughly two-thirds) the mass of our sun. They were also able to deduce the rate matter was falling from the disk onto the star - one-millionth of the mass of our sun is falling into TMC-1A every year at a speed of 1 kilometer per second.

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Interestingly, this mass in-fall speed is much less than what would be expected if the gas was falling at freefall speed (i.e. if nothing was impeding its flow). This slower than expected in-fall velocity could be down to the protostar's young magnetic field buffering the in-falling material, throttling the quantity of matter that feeds it.

"We expect that as the baby star grows, the boundary between the disk and the infall region moves outward," said Aso. "We are sure that future ALMA observations will reveal such evolution."

In summary, astronomers have taken an interstellar ultrasound of a star that is in the process of growing inside its stellar nursery, revealing unparalleled detail in how protostars form. And that's just awesome.

Source: NAOJ via Physorg.com

This is an artist's impression of the baby star TMC-1A. The star is located in the center and surrounded by a rotating gas disk. Gas is in-falling to the disk from the envelope further out.

The Atacama Large Millimeter/submillimeter Array (ALMA) is finally complete, after the project's final 12-meter antenna was handed over on Sept. 30, 2013. The 66th dish, shown here, is the last of 25 European-built instruments. The Joint ALMA Observatory (JAO) is a collaboration between the European Southern Observatory (ESO), the National Radio Astronomy Observatory (NRAO) and the National Astronomical Observatory of Japan (NAOJ).

All 66 millimetre/submillimetre-wave radio antennas are expected to be operational by the end of 2013, working together as one large telescope. ALMA will operate as an interferometer, spread over 16 kilometers of the Chajnantor Plateau in the Atacama Desert, Chile.

ALMA is sensitive to millimetre and submillimetre wavelengths, between infrared light and radio waves in the electromagnetic spectrum, a range that will help astronomers peel back the veil on distant objects in the Cosmos.

The giant antenna transporter, called Otto, delivers the final antenna to the array on Sept. 30, 2013.

The final dish was built by the European AEM Consortium, the largest of the project's contracts. North America delivered 25 12-meter antennas and East Asia delivered 16 (four 12-meter and twelve 7-meter).

"This is an important milestone for the ALMA Observatory since it enables astronomers in Europe and elsewhere to use the complete ALMA telescope, with its full sensitivity and collecting area," said Wolfgang Wild, the European ALMA Project Manager.

An artist's impression of the complete ALMA array in the Atacama Desert.

Possibly breaking the record for altitude record for a radio controlled hexacopter, this aerial photograph of ALMA in the extreme environment of the Atacama Desert in Chile was taken earlier this year.