3 July, 2014
Dynamical interaction of star-forming gas was found in a star-forming region by observations with the Atacama Large Millimeter/submillimeter Array (ALMA). This is a remarkable observation result that disproves a conventional assumption that stars are formed by slow contraction of gas clouds.
Computer simulation of multiple star formation in turbulence. Newborn stars are in complex motion and their influence ripples across the surrounding environment in a wave-like formation and makes an extended gas structure which looks like a bow. Credit: Tomoaki Matsumoto (Hosei University)
A research team led by Kazuki Tokuda and Toshikazu Onishi at Osaka Prefecture University conducted ALMA observations of a high-density gas cloud called MC27/L1521F in the constellationTaurus. From past observation results, it was confirmed that MC27 has a new-born star. And by this observation the research team found a new starless high-density core, which is considered to be very close to the initial stage of star formation, right next to the new-born star. Also, the research team detected an extended gas cloud around MC27 which is assumed to be formed by dynamic gravitational interaction of two or more gas cores. Such dynamical kinematics of star formation which was newly found by this observation will be a key factor in understanding star formation process starting from gas clouds.
Stars are formed in molecular cloud cores composed of dust and gas which have several solar masses within 0.1 light years. It has already been known that a protostar (baby star) is formed at the center of the molecular cloud cores with increased density of gas and dust, but the distribution of gas and dust around a protostar, or their distribution and kinematics at the birth of a protostar have not yet been well understood.
To detect a clue to these mysteries, it is necessary to conduct detailed investigation of the environment surrounding a newborn protostar soon after birth or to observe high-density molecular cloud cores at the earliest phase of protostar formation. The research team led by Kazuki Tokuda, a graduate student at Osaka Prefecture University, and Toshikazu Onishi, a professor at the same university, conducted ALMA observations of a molecular cloud core MC27 (about 450 light years away from the Earth in the direction of the constellation Taurus, the Bull) which contains a newborn protostar. By past observations with the Nobeyama Radio Observatory (NRO) 45-m Telescope of the National Astronomical Observatory of Japan (NAOJ), it was found that MC27 is a very high-density molecular cloud core. Also NASA’s Spitzer Space Telescope discovered a very low-luminosity protostar deep inside the high-density molecular cloud core. The research team carried out detailed observations of dust continuum emission at the center of MC27 and molecular lines of HCO+ from high-density gas to study the properties of gas and dust shortly after the birth of a protostar using ALMA that has high sensitivity and high resolution.
The obtained results were far beyond the researcher’s expectations. They found that that the center of MC27 contains not only gas surrounding the protostar but also two high-density gas condensations (Figure 1). One of the gas condensations MMS-2 at a distance of 200 au from the protostar  is the highest-density molecular cloud core (with tens of millions of gas molecules within 1 cubic centimeter) ever found in low-mass star forming regions. The research team presumes that MMS-2 is at a stage very close to the birth of a protostar.
Fig. 1: A false-color composite image: ALMA observations of dust continuum emission (green) and high-density gas emission (red), and Spitzer observation of infrared emission (blue). While the infrared image shows only the central protostar, ALMA observation results show high-density molecular cloud cores and an extended gas cloud. Credit: Kazuki Tokuda (Osaka Prefecture University) / ALMA(ESO/NAOJ/NRAO) / NASA / JPL-Caltech
A newborn protostar is surrounded by a large amount of gas and dust, which are pulled toward the growing protostar by gravity. Since the interstellar matter around the protostars is absorbed into the protostar or blown away within hundreds of thousands of years after the birth of a protostar , it is not easy to probe the conditions of gas and dust at the birth of a protostar. Kazuki Tokuda says, “It was very exciting when we found a star-forming gas condensation right next to the protostar. We might say we are witnessing star-forming gas at the very moment of birth of a star. We will study further and gain better understanding of star formation mechanism.”
Fig. 2: An artist’s conception of the center of a molecular cloud core MC 27 based on the ALMA observa tion results. Credit: NAOJ
Fig. 3: An artist’s conception of the center of a molecular cloud core MC27 based on the ALMA observation results with descriptions of each component. Credit: NAOJ
Another finding of this observation is an outflow of gas from the protostar. This outflow is smaller than the outflows ever observed around other protostars. From its dimensions and velocity, the outflow from the protostar is supposed to have occurred from several decades to 200 years ago, which indicates that the protostar is very young. ALMA captured the site where a newborn star just after birth and an “egg of a star” are growing together.
A further surprising result is the existence of an extended gas cloud which looks like a tail of MMS-2. The length of the cloud reaches 200 au and its velocity is somewhat different from that of the molecular cloud core. Shu-ichiro Inutsuka, a professor at Nagoya University and one of the research collaborators, says, “This long-shaped structure is supposed to be formed by strong gravitational interaction between fast-moving molecular cloud cores.” In physics, “turbulence” indicates a state of gas flow where gas moves around chaotically. When a swirling turbulence occurs, gas clouds are broken apart into smaller clouds. These reshaped small gas clouds are constantly on the move while pulling each other by gravity and their influence ripples across the surrounding environment in a wave-like formation and makes an extended gas structure which looks like a bow.
The research team assumes the extended gas structure found in MC27 would be associated with such dynamic gas movement as described. Tomoaki Matsumoto, a professor at Hosei University and a member of the research team, did computer simulations of gas clouds where strong turbulence is taking place. As a result of the study, it was found that small gas clouds could form stars inside them while revolving around each other and evolve into a multiple star system with multiple orbits around each other. The obtained results can be interpreted as an initial phase of multiple star formation process.
Toshikazu Onishi, a member of the research team says, “We first obtained observational data of MMS-2, an extended gas cloud, and a gas outflow at a very early stage by using ALMA. These results will greatly contribute to the establishment of multiple star formation theory. We expect that further detailed observations of MC27 and future ALMA observations of other molecular cloud cores will bring rapid progress in understanding of the star formation mechanism and its process.”
 Au (astronomical unit) is a unit of distance based on the mean distance from the Earth to the Sun. 1 au is approximately 150 million km. The radius of Neptune’s orbit, which is the outermost orbit of our solar system, is about 30 au.
 The lifetime of a solar-mass star is about 10 billion years. Its period as a protostar is of hundreds of thousands of years and it is just a fraction of the total life-span, which is equivalent to a day of 100 years of human life. This means a protostar is compared to a newborn baby just after birth.
The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA is funded in Europe by the European Southern Observatory (ESO), in North America by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and in East Asia by the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Academia Sinica (AS) in Taiwan. ALMA construction and operations are led on behalf of Europe by ESO, on behalf of North America by the National Radio Astronomy Observatory (NRAO), which is managed by Associated Universities, Inc. (AUI) and on behalf of East Asia by the National Astronomical Observatory of Japan (NAOJ). The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.
These observation results were published in The Astrophysical Journal Letters (June 11, 2014) as Tokuda et al. “ALMA Observations of a High-density Core in Taurus: Dynamical Gas Interaction at the Possible Site of a Multiple Star Formation”.
The research team members are: Kazuki Tokuda (Graduate student at Osaka Prefecture University) Toshikazu Onishi (Professor at Osaka Prefecture University) Kazuya Saigo (Project assistant professor at the National Astronomical Observatory of Japan) Akiko Kawamura (Project associate professor at the National Astronomical Observatory of Japan) Yasuo Fukui (Professor at Nagoya University) Tomoaki Matsumoto (Professor at Hosei University) Shu-ichiro Inutsuka (Professot at Nagoya University) Masahiro Machida (Associate professor at Kyushu University) Kengo Tomida (Research Fellow of the Japan Society for the Promotion of Science, Princeton University/the University of Tokyo) Kengo Tachihara (Associate professor at Nagoya University)
This research was supported by the Japan Society of the Promotion of Science (JSPS) grants (22244014, 23403001, 23540270).
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