Footprints of baby planets in a gas disk
A new analysis of the ALMA data for a young star HL Tauri provides yet more firm evidence of baby planets around the star. Researchers uncovered two gaps in the gas disk around HL Tauri. 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.
|ALMA image of the dust disk around HL Tauri [Credit: ALMA (ESO/NAOJ/NRAO)]|
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.
|HCO+ gas (blue) and dust (red) distributions in the disk around HL Tauri. The ellipses show the locations of the gaps |
(radii of 30 and 70 au) [Credit: ALMA (ESO/NAOJ/NRAO), Yen et al.]
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, the lead author of the paper that appeared in the Astrophysical Journal Letters.
|Artist's concept of HL Tauri. The star is surrounded by the disk (shown in red) and thick envelope. The star ejects |
a bipolar collimated jet [Credit: Yin-Chih Tsai/ASIAA EPO]
The team also found that the gas density is high enough to harbor an infant planet around the inner gap. Comparing the structure of the inner gap to theoretical models, the team estimates the planet has a mass 0.8 times that of Jupiter.
On the other hand, the origin of the outer gap is still unclear. The team suggested the possible existence of a planet 2.1 times more massive than Jupiter, but the present research cannot eliminate the possibility that the gap is made by the drag between the dust particles and the gas. To solve this question, more data are needed.
"Our research clearly demonstrates that applying new data analysis techniques to existing data can uncover important facts, further increasing ALMA's already high scientific potential," commented Takakuwa. "Applying the same method to the datasets for other young stars, we expect to construct a systematic model of planet formation."
Source: National Institutes of Natural Sciences [May 25, 2016]