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.
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.
Figure 1. ALMA image of the dust disk around HL Tauri
Credit: ALMA (ESO/NAOJ/NRAO)
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.
More data was needed to determine which theory is correct. We know that
the disks around young stars contain gas in addition to the dust. In
fact, in general the amount of gas is 100 times larger than that of
dust. The research team led by Dr. Hsi-Wei Yen at Academia Sinica
Institute of Astronomy and Astrophysics in Taiwan and Professor
Shigehisa Takakuwa at Kagoshima University, Japan, focused on the
distribution of gas in the disk to better understand the true nature of
the disk. If the dust gaps are caused by the variance of the dust
properties, that wouldn't directly affect the gas, so no gaps would be
seen in the gas distribution. If on the other hand, the gaps in the dust
are caused by the gravity of forming planets, the gravity would be
expected to created gaps in the gas as well.
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. "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.
Figure 2. 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.
Credit: ALMA (ESO/NAOJ/NRAO), Yen et al.
Figure 3. 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."
These observation results were published as Yen et al. "Gas Gaps in the
Protoplanetary Disk around the Young Protostar HL Tau" in the
Astrophysical Journal Letters, issued in April 2016.
The research team members are:
Hsi-Wei Yen (Academia Sinica Institute of Astronomy and Astrophysics, Taiwan), Hauyu Baobab Liu (European Southern Observatory, Germany), Pin-Gao Gu, Naomi Hirano, Chin-Fei Lee (Academia Sinica Institute of Astronomy and Astrophysics, Taiwan), Evaria Puspitaningrum (Institut Teknologi Bandung, Indonesia), and Shigehisa Takakuwa (Kagoshima University, Japan)
This research is supported by the Ministry of Science and Technology, Taiwan.
Credit: Clem & Adri Bacri-Normier (wingsforscience.com)/ESO
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The Atacama Large Millimeter/submillimeter Array (ALMA), an
international astronomy facility, is a partnership of the European
Organisation for Astronomical Research in the Southern Hemisphere (ESO),
the U.S. National Science Foundation (NSF) and the National Institutes
of Natural Sciences (NINS) of Japan in cooperation with the Republic of
Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in
cooperation with the National Research Council of Canada (NRC) and the
National Science Council of Taiwan (NSC) and by NINS in cooperation with
the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space
Science Institute (KASI). ALMA construction and operations are led by
ESO on behalf of its Member States; by the National Radio Astronomy
Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on
behalf of North America; and by the National Astronomical Observatory of
Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO)
provides the unified leadership and management of the construction,
commissioning and operation of ALMA.
