ALMA discovers large “hot” cocoon around a small baby star

Scientists using the Atacama Large Millimeter/submillimeter Array (ALMA) discovered a large hot molecular cloud around a young star. This hot cloud is about 10 times larger than those found around typical solar-mass baby stars, which indicates that the star formation process has more diversity than ever thought.

Stars are formed in cold (–436° Fahrenheit [–260° Celsius]) gas and dust clouds. Infrared dark clouds (IRDCs) are dense regions of such clouds in which clusters of stars are formed. Since most stars are born as members of star clusters, investigating IRDCs has a crucial role in a comprehensive understanding of the star formation process.

A baby star is surrounded by the natal gas and dust cloud, and the cloud is warmed up from its center. Temperature of the central part of some, but not all, of such clouds reaches as high as –256° Fahrenheit (–160° Celsius). Astronomers call those clouds “hot cores” — it may not be hot on Earth but is hot enough for a cosmic cloud. Inside 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 abundant in hot cores.

An international research team led by Takeshi Sakai at the University of Electro-Communication in Japan used ALMA to observe an IRDC named G34.43+00.24 MM3 (MM3) in the constellation Aquila the Eagle. They discovered a young object from which the methanol molecular line is strongly emitted. A detailed investigation tells them that the temperature of the methanol gas is – 220° Fahrenheit (–140° Celsius). This shows that MM3 harbors a baby star surrounded by a hot core. The size of the hot core is as large as 800 by 300 astronomical units (1 AU equals the mean distance from the Sun to Earth, about 93 million miles [150 million kilometers]. Typical size of hot cores around low-mass young stars is several tens to a hundred AU; therefore, the hot core in MM3 is exceptionally large. “Thanks to the high sensitivity and spatial resolution [of ALMA], we need only a few hours to discover a previously unknown baby star,” said Sakai. “This is an important step to understand the star formation process in a cluster forming region.”

The team also observed radio emission from carbon sulfide (CS) and silicon monoxide (SiO) to reveal the detailed structure of the molecular outflow from the baby star. The speed of the emanated gas is 17 miles per second (28 km/s) and the extent is 4,400 AU. Based on these values, the team calculates the age of the outflow as only 740 years. Although molecular outflows are common features around protostars, an outflow as young as the one in MM3 is quite rare. In summary, ALMA finds that the protostar in MM3 is very young but has a giant hot core.

Why is the hot core in MM3 so large? In order to warm up such a large volume of gas, the baby star should emit much more energy than typical ones. Protostars produce emission by converting the gravitational energy of infalling material to 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 has not reached the reason with this observation yet. “ALMA’s spatial resolution improves much more in the near future,” Sakai said. “Then much detail of the infalling material toward the protostar can be revealed, and it helps us answer the mystery behind the diversity in star formation.”