ALMA | ESO | NAOJ | NRAO | 2016 May 25
New analysis of ALMA data for HL Tauri provides yet more firm evidence of baby planets around the star. Researchers uncovered two gaps in the gas disk around the star. The locations of these gaps in the gas match the locations of gaps in the dust found in the ALMA high resolution image taken in 2014. This discovery supports the idea that planets form in much shorter timescales than previously thought and prompts a reconsideration of alternative planet formation scenarios.
In November 2014, ALMA released a startling image of HL Tauri and its dust disk. This image, the sharpest ever taken for this kind of object, clearly depicts several gaps in the dust disk around the star.
Astronomers have not yet reached a definitive answer for what makes the gaps in the dust disk. Because these disks are the sites of planet formation, some suggest that infant planets are the key; the dark gaps are carved by planets forming in the disk that attract or sweep away the dust along their orbits.
But others doubt the planet explanation because HL Tauri is very young, estimated to be only about a million years, and classical studies indicate that it takes more than tens of millions of years for planets to form from small dust. Those researchers propose other possible mechanisms to form the gaps: changes in the dust size through coalescence or destruction; or the formation of dust due to gas molecules freezing. ...
Even with ALMA's unprecedented sensitivity, it was not easy to reveal the distribution of gas in the disk. The team extracted the emissions from HCO+ gas molecules in the publicly available 2014 ALMA Long Baseline Campaign data and summed up the emissions in rings around the star to increase the effective sensitivity. This novel data analysis technique yielded the sharpest image ever of the gas distribution around a young star.
The image of HCO+ distribution reveals at least two gaps in the disk, at the radii of 28 and 69 astronomical units. "To our surprise, these gaps in the gas overlap with the dust gaps," said Yen ... "This supports the idea that the gaps are the footprints left by baby planets." The fact that the gaps in the dust and the gas match-up implies that the amount of material in the gaps likely decreases. This disfavors some of the theories that tried to explain the gaps solely by changes in the dust particles. A decrease in the amount of material in the gaps supports the planet formation theory, in spite of HL Tauri's young age. "Our results indicate that planets start to form much earlier than what we expected." Yen added. ...
Gas Gaps in the Protoplanetary Disk around the Young Protostar HL Tau - Hsi-Wei Yen et al
- Astrophysical Journal Letters 820(2):L25 (2016 Apr 01) DOI: 10.3847/2041-8205/820/2/L25
arXiv.org > astro-ph > arXiv:1603.01378 > 04 Mar 2016
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