Academia Sinica Institute of Astronomy and Astrophysics

Academia Sinica: Astronomers Spot Gluttonous Baby Stars

posted Feb 23, 2016, 10:51 PM by Lauren Huang   [ updated Feb 23, 2016, 11:06 PM ]

Release time: February 25, 2016, 08:30 am (CST)

An international team led by Dr. Hauyu Baobab Liu and Dr. Michihiro Takami from the Academia Sinica, Institute of Astronomy and Astrophysics have found that stars may not accumulate their mass steadily, as was previously thought, but in a series of violent events that manifest as sharp stellar brightening. The researchers used the High-Contrast Coronographic Imager for Adaptive Optics (HiCIAO) camera at the Subaru 8.2-m telescope to observe and capture images of the complex structures around the newborn stars. These complex structures are the key to trigger active accretion events of the young stars, and may be related to the formation of planets. The research results were published in the American Association for the Advancement of Science (AAAS) online magazine Science Advances on February 5, 2016.

Fig. 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. Credit: Science Advances, H. B. Liu.

A few stars are known to be associated with a sudden and violent “feeding”, characterized by a fast and dramatic increase of visible light by a factor of at least a hundred. These phenomena are called “FU Orionis outbursts”, as they were first discovered toward the star FU Orionis. Not so many stars have been 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 observed steady and continuous accretion can actually only explains a small faction (1-10%) of the final mass of a newborn star. Therefore, astronomers have been intrigued by how the other 90-99% of the mass of the star is formed, and what the growing process of a star looks like. Over a decade, there have been several theories put forward. Out of these theories, Drs. Liu and Takami considered one related to gas clouds, gravity and interaction to be a more reasonable explanation.

They used HiCIAO, camera at the Subaru 8.2-m telescope located at the summit of Mauna Kea, Hawaii to observe four stars with FU Ori outbursts, located 1500-3500 light years from our solar system. The high-resolution near-infrared images of these stars were surprising and fascinating, and were nothing like anything previously observed around young stars (Figure 1). Three of the stars were observed to have unusual tails. One shows an arm, a feature that is the result of 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 images matched the picture of steady shown in Figure 2. Rather, they show a messy and chaotic environment.

Fig. 2 A diagram of smooth, continuous star formation. Image Credit: ASIAA (adapted from Green, 2001)

To better understand the observed structures, theorists in the team extensively studied one of the mechanisms proposed for the FU Ori outbursts. According to this theory, gravity in circumstellar gas (+dust) results in complicated structures that look like cream stirred into coffee (Figure 3, left). These structures fall onto the star at irregular intervals. The team conducted further computer simulations for scattered light. Although more simulations are required to match the simulations to the observed images, the images obtained so far suggest that this is a promising explanation for the nature of FU Ori outbursts.

Fig. 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.

Studying these structures may also shed light on how some planetary systems are born. Astronomers know some exoplanets (planets around other stars) are found extremely far away from the central star, sometimes at more than a thousand times the distance between the Sun and the Earth. This is significantly larger than the orbit of Neptune (about 30 times the distance between the Sun and Earth) and orbits explained by standard theories of planet formation. Simulations of complicated circumstellar structures like the ones seen here also predict that some dense clumps of material may become gas giant planets. This would naturally explain the presence of exoplanets with the large orbits described above.

The research was supported by the Ministry of Science and Technology (MOST) of Taiwan, the Russian Ministry of Education and Science, a Hubble Fellowship, and the Submillimeter Array.

The complete list of authors and their affiliations is: Hauyu Baobab Liu, Academia Sinica Institute of Astronomy and Astrophysics; Michihiro Takami, Academia Sinica Institute of Astronomy and Astrophysics; Tomoyuki Kudo, National Astronomical Observatory of Japan; Jun Hashimoto, National Astronomical Observatory of Japan; Ruobing Dong, Academia Sinica Institute of Astronomy and Astrophysics, 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, National Astronomical Observatory of Japan; Misato Fukagawa, National Astronomical Observatory of Japan; Motohide Tamura, National Astronomical Observatory of Japan, University of Tokyo; Thomas Henning, Max-Planck-Institute for Astronomy, 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; Toru Tsuribe, College of Science, Ibaraki University, Japan.

The complete article is available at the Science Advances online journal website
http://advances.sciencemag.org/content/2/2/e1500875

Media contacts:
Dr. Michihiro Takami, Institute of Astrophysics and Astronomy, Academia Sinica, hiro@asiaa.sinica.edu.tw (Tel) +886-2-2366-5311
Ms. Pearl Huang, Department of Secretariat, Central Office of Administration, Academia Sinica pearlhuang@gate.sinica.edu.tw
(Tel) +886-2-2789-8820 (M) + 886-912-831-188
Ms. Mei-Hui Lin, Department of Secretariat, Central Office of Administration, Academia Sinica mhlin313@gate.sinica.edu.tw
(Tel) +886-2-2789-8821 (M)+ 886- 963-712-720

Astronomers "Weigh" a Galaxy's Black Hole by Studying the Einstein Ring Phenomenon

posted Sep 28, 2015, 12:55 AM by Lauren Huang   [ updated Sep 29, 2015, 8:49 PM ]

(Release time: Taiwan time September 30, 2015, 08:30 am)

Astronomers at the Institute of Astronomy and Astrophysics (ASIAA), Postdoctoral Fellow Dr. Kenneth Wong, Assistant Research Fellow Dr. Sherry Suyu and Associate Research Fellow Dr. Satoki Matsushita have recently analyzed the highest-ever resolution images of SDP.81, a gravitational lens, taken by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. From observations of this ring-shaped image known as an Einstein Ring (a result of the gravitational lensing effect), the team calculated that the supermassive black hole located near the center of the lensing galaxy may contain over 300 million times the mass of the sun. Measuring the masses of more distant black holes is the key to understanding the formation and evolution of black holes and their host galaxies. This study marks a new era of research using ALMA and its unparalleled capability for cutting-edge scientific endeavors. The research was published in The Astrophysical Journal on September 28, 2015

 















ASIAA astronomers have determined that the foreground galaxy in the SDP. 81 system, whose mass is lensing the background source into the Einstein Ring, contains a supermassive black hole that has more than 300 million times the mass of the Sun. Credit: ALMA (NRAO/ESO/NAOJ)/ Kenneth Wong (ASIAA).

The data on the gravitational lensing system SDP.81, which were taken in November 2014 to test the high­-resolution capabilities of the full ALMA array, were released worldwide in February of 2015. This rare yet beautiful “ring” image displays a spectacular phenomenon, caused by gravitational effects, that occurs through a chance alignment of three objects: the Earth and two other galaxies, all located in a straight line separated by a great distance of 12 billion light years.

 












ALMA image of the gravitational lens system SDP.81. The bright orange sections of the ring (ALMA's highest resolution observation ever) reveals the glowing dust in this distant galaxy. The fainter lower-resolution portions of the ring trace the millimeter wavelength light emitted by carbon monoxide. Forming an Einstein ring is a rare phenomenon. Credit: ALMA (NRAO/ESO/NAOJ); B. Saxton NRAO/AUI/NSF

The team further explained that in this Einstein Ring system, there are in fact, two galaxies: the foreground galaxy, which is 4 billion light years away, and the background galaxy, whose light has taken 12 billion years to reach us. The gravity of the massive foreground galaxy deflects the light from the background galaxy and creates the ring structure. The background galaxy contains a large amount of dust that has been heated by vigorous star formation, causing it to shine brightly in submillimeter light. ALMA is a sub-millimeter telescope which is best for observing dusty objects.

 
The left panel shows the foreground lensing galaxy (observed with Hubble), and the gravitational lens system SDP.81, which forms an almost perfect Einstein Ring but is hardly visible. The middle image shows the sharp ALMA image of the Einstein Ring. The foreground lensing galaxy is invisible to ALMA, as it does not emit strong submillimeter-wavelength light. The resulting reconstructed image of the distant galaxy (right) using sophisticated models of the magnifying gravitational lens reveals fine structures within the ring that have never been seen before: several giant clouds of dust and cold molecular gas, which are the birthplaces of stars and planets. Credit: ALMA (NRAO/ESO/NAOJ)/Y. Tamura (The University of Tokyo)/Mark Swinbank (Durham University).

By analyzing the high-­resolution data and modeling the gravitational lensing effect, the team determined that the massive lensing galaxy contains over 350 billion times the mass of the sun within the ring. Wong, together with Suyu and Matsushita, analyzed the central regions of SDP.81 and found the predicted central image of the background galaxy to be extremely faint. Lensing theory predicts that the central image of a lensing system is very sensitive to the mass of a supermassive black hole in the lens galaxy:­­ the more massive the black hole, the fainter the central image. From this, they calculated that the supermassive black hole, located very close to the center of the SDP.81, may contain over 300 million times the mass of the sun.

The first author of the article, Dr. Kenneth Wong, explained the information behind this image. He says almost all massive galaxies seem to have supermassive black holes at their centers, “they can be millions, or even billions of times more massive than the sun. However, we can only directly calculate the mass for very nearby galaxies. With ALMA, we now have the sensitivity to look for the central image of the lens, which can allow us to determine the mass of much more distant black holes.” Measuring the masses of more distant black holes is the key to understanding their relationship with their host galaxies and how they grow over time, he added.

“This is the highest-­resolution image of a gravitational lens that has ever been taken,” said Wong, “The amount of detail in this image is much greater than even space telescope observations,”

“ALMA opens a new frontier to weigh supermassive black holes at centers of lensing galaxies, which cannot be done previously with optical telescopes,” said co­author Sherry Suyu of ASIAA.

“With the high­-resolution capabilities of ALMA, which we helped to realize in the last 5 years, it is now possible to perform new science that cannot be done with any other instrument,” said co­author Satoki Matsushita of ASIAA.

Drs. Wong, Suyu, and Matsushita, along with collaborators in Japan, have been awarded time on ALMA within the next year to observe a more distant gravitational lens similar to SDP.81, one which they anticipate will have a brighter central image that could directly constrain the mass of the supermassive black hole. With the capabilities of ALMA, they hope to continue investigating the formation and evolution of these massive galaxies and their supermassive black holes.

The Atacama Large Millimeter/submillimeter Array (ALMA), whose construction was participated by teams across the globe, is the biggest radio telescope array in the world. Construction began in 1998 and the telescope was inaugurated in March 2013. ALMA is composed of 54 12-meter antennae and 12 7-meter antennae that all work together at millimeter and submillimeter wavelengths to achieve an angular resolution of 0.01 arcseconds. ALMA provides advanced accurate data for studies of the relic radiation from the early Universe, molecular gas, and interstellar dust.

The full article, entitled “The Innermost Mass Distribution of the Gravitational Lens SDP.81 from ALMA Observations”, is available from The Astrophysical Journal website at: http://stacks.iop.org/0004-637X/811/115.

The complete list of authors is: Kenneth C. Wong, Sherry H. Suyu, and Satoki Matsushita.

Media contacts:
Dr. Sherry H. Suyu, Assistant Research Fellow, Institute of Astrophysics and Astronomy, Academia Sinica suyu@asiaa.sinica.edu.tw (Tel) 886-2-2366-5477
Ms. Mei-Hui Lin, Department of Secretariat, Central Office of Administration, Academia Sinica mhlin313@gate.sinica.edu.tw (Tel) +886-2-2789-8821 (M)+ 886-963-712-720
Ms. Pearl Huang, Department of Secretariat, Central Office of Administration,
Academia Sinica pearlhuang@gate.sinica.edu.tw (Tel) +886-2-2789-8820 (M) + 886-912-831-188

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