Astronomers enable search for small soon-to-be stars

May 22, 2021

The Cat's Paw Nebula contains large clouds of gas and dust that are actively giving rise to new stars. A recent study on one of the earliest stages of star formation analyzed NGC 6334S, a molecular cloud that appears black in the center-right of this false-color image. (NASA/JPL-Caltech)

Using a newly applied observational technique, astronomers found the first collection of several low-mass clumps of dust and gas that will likely birth stars in an emerging star cluster, which may provide new information to astronomers about how stars form.

Peering into a dark molecular cloud in the Cat's Paw Nebula, an international collaboration of scientists identified nine previously unseen clumps of dust and gas that appear primed to someday collapse into new stars.

The researchers found them by looking for a type of ammonia gas that they contain, a fresh approach that could lead to many more discoveries of these low-mass pre-stellar cores, the team said in a paper published April 29 in The Astrophysical Journal Letters.

Stars are born when dust and gas collapse so densely under the force of gravity that atoms fuse and give off immense energy, igniting nuclear fusion. Predating fusion is a pre-stellar core, also known as a starless core, which is a dense collection of gravitationally bound material with conditions ripe to someday form one or multiple stars.

Many pre-stellar cores several times the mass of the sun have been previously discovered, but data on low-mass stars is lacking, according to the paper's authors. Shanghuo Li, the lead author, said it is known that high-mass and low-mass stars form alongside each other, but the latter are rarely found using heat signals known as dust continuum emissions because of these stars' very low temperature.

"Some people tried to find the low-mass objects in high-mass-forming [regions], but they fail because they just use a continuum emission," said Li, a postdoctoral astrophysics researcher at the Korea Astronomy and Space Science Institute. "You have to find a good tracer to look at those high-mass objects."

Rather than using continuum emissions, the researchers looked for light emitted by molecules of ammonia that carry one atom of deuterium, a rare isotope of hydrogen with a nucleus that contains a neutron. Ammonia effectively indicates the presence of cold, dense gas, Li said, and deuterium ammonia does an even better job.

The study focused on a region known as NGC 6334S in the Cat's Paw Nebula, located in the constellation Scorpius, that contains a massive molecular cloud with newly formed and actively forming stars. The cloud emits practically no light in the infrared spectrum but is "visible" in radio-wave frequencies, which includes emission lines from deuterium ammonia.

This region contains matter 1,300 times more massive than the sun and is in an early stage of becoming a star cluster, which may be more common in the Milky Way galaxy than previously believed. It is an "ideal laboratory" to search for and study early star formation, the researchers wrote in the paper.

"We want to look at the earliest stage in this very complex structure," Li said, "and try to understand how those stars, including low-mass and high-mass, they form in this earliest stage."

The team conducted radio-wave measurements of NGC 6334S using the Atacama Large Millimeter/submillimeter Array in Chile and the Karl G. Jansky Very Large Array in New Mexico. Li and other scientists previously used the same data to find that the motions of the molecular cloud's material are of low turbulence, which implies that the region would form many low-mass stars and therefore several low-mass pre-stellar cores.

The radio-wave emission spectrum of deuterium ammonia highlighted 17 low-mass cores with no sign of a young star in them. Of those, nine are gravitationally bound and were therefore identified as "excellent" pre-stellar core candidates, the researchers said.

This group of pre-stellar cores is the first to be discovered within a star cluster-forming cloud, and the cores ranged in size from 0.13 to 0.87 times the sun's mass.

Put together with the larger pre-stellar cores, the low-mass cores fill in a gap in understanding how the dust and gas in these cores behave, which is described by the core mass function. This information is "critical" to understanding the formation of stars and star clusters in the Milky Way galaxy and other galaxies, Li said.

And while deuterium ammonia has been used before to find high-mass cores, this study is the first to use it to find low-mass stars, according to Li. He said that it could be used to effectively track down other pre-stellar cores of a similar size, although more effective tracer molecules may also exist.

Li said he and his colleagues have already applied for more telescope time to look for low-mass pre-stellar cores elsewhere in the night sky, where high-mass cores have been identified.

The study, "A low-mass cold and quiescent core population in a massive star protocluster," published April 29 in The Astrophysical Journal Letters, was authored by Shanghuo Li, Korea Astronomy and Space Science Institute, Shanghai Astronomical Observatory, Center for Astrophysics | Harvard & Smithsonian and University of Chinese Academy of Sciences; Xing Lu, National Astronomical Observatory of Japan; Qizhou Zhang, Howard Smith and Shaye Strom, Center for Astrophysics | Harvard & Smithsonian; Chang Won Lee, Kee-Tae Kim and Shinyoung Kim, Korea Astronomy and Space Science Institute and University of Science and Technology; Patricio Sanhueza, National Astronomical Observatory of Japan and SOKENDAI; Henrik Beuther, Max Planck Institute for Astronomy; Izaskun Jimenez-Serra, Spanish Astrobiology Center; Keping Qiu and Fei Li, Nanjing University; Aina Palau, National Autonomous University of Mexico; Siyi Feng, National Astronomical Observatories, Academia Sinica Institute of Astronomy and Astrophysics and National Astronomical Observatory of Japan; Thushara Pillai, Boston University; Hong-Li Liu, Yunnan University; Josep Miquel Girart, Institute of Space Sciences; Tie Liu, Junzhi Wang and Juan Li, Shanghai Astronomical Observatory; Ke Wang and Nannan Yue, Peking University; Hauyu Baobab Liu, Academia Sinica Institute of Astronomy and Astrophysics; Di Li, National Astronomical Observatories and University of KwaZulu-Natal; and Jeong-Eun Lee, Kyung Hee University.

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