An international team led by researchers at the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) has used a new infrared imaging technique to reveal dramatic moments in star and planet formation. These seem to occur when surrounding material falls toward very active baby stars, which then feed voraciously on it even as they remain hidden inside their birth clouds. The team used the HiCIAO (High Contrast Instrument for the Subaru Next-Generation Adaptive Optics) camera on the Subaru 8-meter Telescope in Hawaii to observe a set of newborn stars. The results of their work shed new light on our understanding of how stars and planets are born (Figure 1).
Figure 1: Circumstellar structures revealed by
Subaru-HiCIAO. The scale bars are shown in AUs (astronomical units). One
AU is the average distance from the sun to the earth. The gas and dust
surrounding baby stars (their food) are significantly more extended than
our solar system. Here we show the first observations of such complex
structures around active young stars. Figure without the labels is
linked here. (Credit: Science Advances, H. B. Liu.)
The Process of Star Birth
Stars are born when giant clouds of dust and gas collapse under the pull
of their own gravity. Planets are believed to be born at nearly the
same time as their stars in the same disk of material. However, there
are still a number of mysteries about the detailed physical processes
that occur as stars and planets form (Figure 2).
Figure 2: A schematic diagram of star and planet formation based on Green (2001)
Credit: ASIAA
The giant collections of dust and gas where stars form are called
"molecular clouds" because they are largely made up of molecules of
hydrogen and other gases. Over time, gravity in the densest regions of
these clouds gathers in the surrounding gas and dust, via a process
called "accretion". It is often assumed that this process is smooth and
continuous (Figure 2).
However, this steady infall explains only a small fraction of the final
mass of each star that is born in the cloud. Astronomers are still
working to understand when and how the remaining material is gathered in
during the process of star and planet birth.
A few stars are known to be associated with a sudden and violent
"feeding" frenzy from inside their stellar nursery. When they gluttonize
on the surrounding material, their visible light increases very
suddenly and dramatically, by a factor of about a hundred. These sudden
flareups in brightness are called "FU Orionis outbursts" because they
were first discovered toward the star FU Orionis.
Not many stars are found to be associated with such outbursts — only a
dozen out of thousands. However, astronomers speculate that all baby
stars may experience such outbursts as part of their growth. The reason
we only see FU Ori outbursts toward a few newborn stars is simply
because they are relatively quiet most of the time.
One key question about this mysterious facet of starbirth is "What
are the detailed physical mechanisms of these outbursts?" The answer
lies in the region surrounding the star. Astronomers know the optical
outbursts are associated with a disk of material close to the star,
called the "accretion disk". It becomes significantly brighter when the
disk gets heated up to temperatures similar to those of lava flows here
on Earth (around 700 to 1200 C or 1292 to 2182 F) like the one flowing
from Kilauea volcano area in the island of Hawaii. Several processes
have been proposed as triggers for such outbursts and astronomers have
been investigating them over the past few decades.
Finding a Mechanism for FU Ori Outbursts
An international team lead by Drs. Hauyu Baobab Liu and Hiro Takami,
two researchers at ASIAA, used a novel imaging technique available at
the Subaru Telescope to tackle this issue. The technique – imaging
polarimetry with coronagraphy – has tremendous advantages for imaging
the environments in the disks. In particular, its high angular
resolution and sensitivity allow astronomers to "see" the light from the
disk more easily. How does this work?
Circumstellar material is a mixture of gas and dust. The amount of
dust is significantly smaller than the amount of gas in the cloud, so it
has little effect on the motion of the material. However, dust
particles scatter (reflect) light from the central star, illuminating
all the surrounding material. The HiCIAO camera mounted on the Subaru
8.2-meter telescope, one of the largest optical and near-infrared (NIR)
telescopes in the world, is well-suited to observing this dim
circumstellar light. It successfully allowed the team to observe four
stars experiencing FU Ori outbursts.
Details of Four FU Ori Outbursts
The team's target stars are located 1,500-3,500 light-years from our
solar system. The images of these outbursting newborns were surprising
and fascinating, and nothing like anything previously observed around
young stars (Figure 1).
Three have unusual tails. One shows an "arm", a feature created by the
motion of material around the star. Another shows odd spiky features,
which may result from an optical outburst blowing away circumstellar gas
and dust. None of them match the picture of steady growth shown in Figure 2. Rather, they show a messy and chaotic environment, much like a human baby eating food.
To understand the structures observed around these newborn stars, theorists on the team extensively studied one of several mechanisms proposed to explain FU Ori outbursts. It suggests that gravity in circumstellar gas and dust clouds creates complicated structures that look like cream stirred into coffee (Figure 3, left). These oddly shaped collections of material fall onto the star at irregular intervals. The team also conducted further computer simulations for scattered light from the outburst. Although more simulations are required to match the simulations to the observed images, these images show that this is a promising explanation for the nature of FU Ori outbursts (Figure 3).
Figure 3: Images made from computer simulations based
on one theory for violent growth of a star. (Left) Simulations of the
motion of circumstellar materials falling onto a baby star. (Middle and
right). Models of how we would observe the structure in scattered light,
seen from two different angles.
(Credit: Science Advances, H. B. Liu.)
Studying these structures may also reveal how some planetary systems
are born. Astronomers know some exoplanets (planets around other stars)
are found extremely far away from their central stars. Sometimes they
orbit more than a thousand times the distance between the Sun and Earth,
and significantly larger than the orbit of Neptune (which is about 30
times the distance between the Sun and Earth). These distances are also
much larger than orbits explained by standard theories of planet
formation. Simulations of complicated circumstellar structures like the
ones seen in the HiCIAO views also predict that some dense clumps in the
material may become gas giant planets. This would naturally explain the
presence of exoplanets with such large orbits.
In spite of these exciting new results, there is a still great deal
more work to do to understand the mechanisms of star and planet birth.
More detailed comparisons between observation and theory are needed.
Further observations, particularly with the Atacama Large
Millimeter/Submillimeter Array, will take our gaze more deeply into
circumstellar gas and dust clouds. The array allows observations of the
surrounding dust and gas with unprecedented angular resolution and
sensitivity. Astronomers are also planning to construct telescopes
significantly larger than Subaru in the coming decades – including the
Thirty Meter Telescope (TMT) and the European Extremely Large Telescope.
These should allow detailed studies of regions very close to newborn
stars.
Note:
- Astronomical Unit (AU) is a unit of distance. 1 AU corresponds to the average distance between the Earth and the Sun.
Paper and Research Team:
This research was supported by the Ministry of Science and Technology
(MoST) of Taiwan (Grant Nos. 103-2112-M-001-029 and
104-2119-M-001-018). E.I.V. acknowledges the support from the Russian
Ministry of Education and Science Grant 3.961.2014/K and RFBR grant
14-02-00719. R.D. was supported by Hubble Fellowship. M.M.D.
acknowledges support from the Submillimeter Array through an SMA
Postdoctoral Fellowship.
These observational results were published by Liu et al. as "Circumstellar Disks of the Most Vigorously Accreting Young Stars" in the Science Advances on February 5, 2016.
These observational results were published by Liu et al. as "Circumstellar Disks of the Most Vigorously Accreting Young Stars" in the Science Advances on February 5, 2016.
This research was conducted by:
- Hauyu Baobab Liu (Academia Sinica Institute of Astronomy and Astrophysics, Taiwan / European Southern Observatory, EU)
- Michihiro Takami (Academia Sinica Institute of Astronomy and Astrophysics, Taiwan)
- Tomoyuki Kudo (Subaru Telescope, National Astronomical Observatory of Japan, USA)
- Jun Hashimoto (National Astronomical Observatory of Japan, Japan)
- Ruobing Dong (Academia Sinica Institute of Astronomy and Astrophysics, Taiwan / Hubble Fellow / Department of Astronomy, UC Berkeley, USA)
- Eduard I. Vorobyov (Department of Astrophysics, University of Vienna, Austria / Research Institute of Physics, Southern Federal University, Russia)
- Tae-Soo Pyo (Subaru Telescope, National Astronomical Observatory of Japan, USA)
- Misato Fukagawa (National Astronomical Observatory of Japan, Japan)
- Motohide Tamura (National Astronomical Observatory of Japan, Japan / Department of Astronomy, Graduate School of Science, The University of Tokyo, Japan)
- Thomas Henning (Max-Planck-Institut für Astronomie, Germany)
- Michael M. Dunham (Harvard-Smithsonian Center for Astrophysics, USA)
- Jennifer Karr (Academia Sinica Institute of Astronomy and Astrophysics, Taiwan)
- Nobuhiko Kusakabe (National Astronomical Observatory of Japan, Japan)
- Toru Tsuribe (College of Science, Ibaraki University, Japan)
