|ALMA Discovers Large “Hot” Cocoon around a Small Baby Star||
Monday, 07 October 2013
A large hot molecular cloud around a very young star was discovered by ALMA. This hot cloud is about ten times larger than those found around typical solar-mass baby stars, which indicates that the star formation process has more diversity than ever thought. This result was published in the Astrophysical Journal on September 20th, 2013.
Stars are formed inside cold (-260 degrees Celsius) clouds made of gas and dust. Infrared Dark Clouds (IRDC) are dense regions of such clouds, and it is though that they are the places in which clusters of stars are formed. Since most of stars are born as members of star clusters, investigating IRDCs is crucial to understanding the star formation process at large.
A baby star is thus surrounded by the gas and dust clouds in which it forms, and as it evolves it will heat up the clouds from the center. The temperature of the central part of some, but not all, of such clouds reaches as high as -160 degrees Celsius. Astronomers call such cloud centers "hot cores" – it may not be hot on the Earth, but it is hot enough for a cosmic cloud. Inside these hot cores, various molecules, originally trapped in the ice mantle around dust particles, are sublimated. Organic molecules such as methanol (CH3OH), ethyl cyanide (CH3CH2CN), and methyl formate (HCOOCH3) are therefore abundant in hot cores.
An international research team, led by Takeshi Sakai at the University of Electro-Communication, Japan, used ALMA to observe an IRDC named G34.43+00.24 MM3 (which we will refer to as MM3) in the constellation of Aquila (the Eagle). They discovered a young object with very strong methanol line emission. A detailed investigation revealed that the temperature of the methanol gas is -140 degrees Celsius, confirming that MM3 harbors a baby star surrounded by a hot core. The size of the hot core however is as large as 800 astronomical units (au, 1 au equals the distance between the Sun and the Earth; 150 million km). The typical size of hot cores around low-mass young stars is several tens to hundred of au, therefore the hot core in MM3 is exceptionally large. "Thanks to the extraordinarily high sensitivity of ALMA we needed only a few hours to discover a previously unknown baby star. This is an unprecedented step towards understanding how the star formation process in a cluster forming region works", Sakai says.
The team also observed radio emission from carbon sulfide (CS) and silicon monoxide (SiO) to reveal the detailed structure of a molecular outflow originating from the baby star. The speed of the emanated gas is 28 km/s and the extent is 4,400 au. Based on these values, the team calculated that the age of the outflow is only 740 years. Although molecular outflows are common features around protostars an e outflow as young as the one in MM3 is quite rare. It is the first time that such a young protostar is observed in a giant hot core.
So why is the hot core in MM3 is so large? In order to heat up the large volume of gas, the baby star should emit much more energy than a typical one. Protostars produce emission by converting the gravitational energy of infalling material into thermal energy. The large size of the hot core in MM3 is possibly due to a higher mass infalling rate than ever thought. The other possibility is that two or more protostars are embedded in the hot core. The research team cannot rule out either scenario yet this observation. "ALMA’s spatial resolution will improve dramatically in the near future", Sakai says, "Then we will be able to resolve the details of the infalling material toward the protostar, and this will help us unveil the mystery behind the diversity in star formation."
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.
The research findings are presented in the article "ALMA Observations of the IRDC Clump G34.43+00.24 MM3: Hot Core and Molecular Outflows", published in the Astrophysical Journal, Vol. 775, of September 20, 2013.
The research team members are: T. Sakai (University of Electro-Communication, Japan), N. Sakai (The University of Tokyo), J. B. Foster (Yale University), P. Sanhueza、J. M. Jackson (Boston University), M. Kassis (Keck Observatory), K. Furuya, Y. Aikawa (Kobe University), T. Hirota (National Astronomical Observatory of Japan), S. Yamamoto (The University of Tokyo).
This research was supported by KAKENHI (21224002, 23740146, 25400225 and 25108005).
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