From ALMA(CL) : “Stellar Eggs near Galactic Center Hatching into Baby Stars”

From ALMA(CL)

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ALMA pseudo-color composite image of the gas outflows from baby stars in the Galactic Center region. Gas moving toward us is shown in blue and gas moving away from us is shown in red. Credit: From ALMA(CL), Lu et al.

Astronomers found a number of baby stars hiding around the center of the Milky Way using the Atacama Large Millimeter/submillimeter Array (ALMA). Previous studies had suggested that the environment there is too harsh to form stars because of the strong tidal forces, strong magnetic fields, high energy particles, and frequent supernova explosions. These findings indicate that star formation is more resilient than researchers thought. These observations suggest there is ubiquitous star formation activity hidden deep in dense molecular gas, which may allow for the possibility of a future burst of star formation around the Galactic Center.

“It is like hearing babies’ cries in a place we expected to be barren,” says Xing Lu, an astronomer at the NAOJ. “It is very difficult for babies to be born and grow up healthily in an environment that is too noisy and unstable. However, our observations prove that even in the strongly disturbed areas around the Galactic Center, baby stars still form.”

Stars are formed in cosmic clouds gathered by gravity. If something interferes with the gravity driven processes, star formation will be suppressed. There are many potential sources of interference in the Central Molecular Zone (CMZ) of the Milky Way, located within a radius of 1000 light-years from the Galactic Center. Examples include strong turbulence which stirs up the clouds and prevents them from contracting, or strong magnetic fields can support the gas against self-gravitational collapse. In fact, previous observations indicated that star formation here is much less efficient; with the exception of one active star forming region called Sagittarius B2 (Sgr B2).

Lu and his colleagues used ALMA to tackle the mystery of suppressed star formation in most of the CMZ. The target regions contain an ample amount of gas, but no star formation has been expected. Contrary to the traditional picture, the team discovered more than 800 dense cores of gas and dust particles in the CMZ. “The discovery leads to the question of whether they are actually ‘stellar eggs’ or not,” says Lu. To look for telltale signs of star formation indicative of stellar eggs, the team again used ALMA to search for energetic gas outflows, which are like the birth cries of baby stars. Thanks to ALMA’s high sensitivity and high spatial resolution, for the first time, they detected 43 small and faint outflows in the clouds. This is unambiguous evidence of ongoing star formation. It turned out that many baby stars were hiding in the regions that were thought to be unsuitable for stellar growth.

The small number of detected outflows is another mystery. Considering the fact that more than 800 “stellar eggs” have been found, the small number of “stellar babies” might indicate that the star formation activity in the CMZ is in the very early phase. “Although a large number of outflows might be still hidden in the regions, our results may suggest we are seeing the beginning of the next wave of active star formation,” says Lu.

“Although previous observations have suggested that overall star formation rates are suppressed to about 10% in the giant molecular clouds in the Galactic Center, this observation shows that the star formation processes hidden in dense molecular gas clouds are not very different from those of the Solar neighborhood,” explains Shu-ichiro Inutsuka, a professor at Nagoya University [名古屋大学; Nagoya daigaku](JP) and a co-author of the research paper. “The ratio of the number of star-forming cores to star-less cores seems to be only a few times smaller than that in the Solar neighborhood. This can be regarded as the ratio of their respective lifetimes. We think that the average duration of the star-less core stage in the Galactic Center might be somewhat longer than in the Solar neighborhood. More research is needed to explain why it is so.”

The research team is now analyzing ALMA’s higher resolution observation data for the CMZ and aims to study the properties of the accretion disks around the baby stars which drive the gas outflows. By comparing with other star forming regions, they hope to better understand star formation in the CMZ, from clouds to protostars, and from chemistry to magnetic fields.

Additional Information

These observation results were presented in Xing Lu et al. “ALMA Observations of Massive Clouds in the Central Molecular Zone: Ubiquitous Protostellar Outflows” in The Astrophysical Journal on March 16, 2021.

This research was supported by the Japan Society of Promotion of Science (JSPS) KAKENHI (No. 18K13589 & 20K14528), the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through an Emmy Noether Research Group (grant number KR4801/1-1), the DFG Sachbeihilfe (grant number KR4801/2-1), the SFB 881 “The Milky Way System” (subproject B2), the European Union’s Horizon 2020 research and innovation programme via the ERC Starting Grant MUSTANG (grant agreement number 714907), and the National Science Foundation under Award No. 1816715.

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The Atacama Large Millimeter/submillimeter Array (ALMA)(CL) , 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 Organization for Astronomical Research in the Southern Hemisphere (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 European Southern Observatory(EU), on behalf of North America by the National Radio Astronomy Observatory (NRAO), which is managed by Associated Universities, Inc. (US) 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.

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ALMA is a time machine!

ALMA-In Search of our Cosmic Origins

The Atacama Large Millimeter/submillimeter Array (ALMA) is an astronomical interferometer of 66 radio telescopes in the Atacama Desert of northern Chile, which observe electromagnetic radiation at millimeter and submillimeter wavelengths. The array has been constructed on the 5,000 m (16,000 ft) elevation Chajnantor plateau – near the Llano de Chajnantor Observatory and the ESO Atacama Pathfinder Experiment (CL). This location was chosen for its high elevation and low humidity, factors which are crucial to reduce noise and decrease signal attenuation due to Earth’s atmosphere. ALMA provides insight on star birth during the early Stelliferous era and detailed imaging of local star and planet formation.

ALMA is an international partnership among Europe, the United States, Canada, Japan, South Korea, Taiwan, and Chile. Costing about US$1.4 billion, it is the most expensive ground-based telescope in operation. ALMA began scientific observations in the second half of 2011 and the first images were released to the press on 3 October 2011. The array has been fully operational since March 2013.

Overview

The initial ALMA array is composed of 66 high-precision antennas, and operates at wavelengths of 3.6 to 0.32 millimeters (31 to 1000 GHz). The array has much higher sensitivity and higher resolution than earlier submillimeter telescopes such as the single-dish James Clerk Maxwell Telescope or existing interferometer networks such as the Submillimeter Array or the Institut de Radio Astronomie Millimétrique Plateau de Bure interferometer(FR) Plateau de Bure facility.

IRAM-Institut de Radio Astronomie Millimétrique Plateau de Bure interferometer (FR) at an elevation of 2550 meters, the telescope currently consists of ten antennas, each 15 meters in diameter.interferometer, Located in the French Alpes on the wide and isolated Plateau de Bure at an elevation of 2550 meters.

The antennas can be moved across the desert plateau over distances from 150 m to 16 km, which will give ALMA a powerful variable “zoom”, similar in its concept to that employed at the centimetre-wavelength Very Large Array (VLA) site in New Mexico, United States.

The high sensitivity is mainly achieved through the large numbers of antenna dishes that will make up the array.

The telescopes were provided by the European, North American and East Asian partners of ALMA. The American and European partners each provided twenty-five 12-meter diameter antennas, that compose the main array. The participating East Asian countries are contributing 16 antennas (four 12-meter diameter and twelve 7-meter diameter antennas) in the form of the Atacama Compact Array (ACA), which is part of the enhanced ALMA.

By using smaller antennas than the main ALMA array, larger fields of view can be imaged at a given frequency using ACA. Placing the antennas closer together enables the imaging of sources of larger angular extent. The ACA works together with the main array in order to enhance the latter’s wide-field imaging capability.

ALMA has its conceptual roots in three astronomical projects — the Millimeter Array (MMA) of the United States, the Large Southern Array (LSA) of Europe, and the Large Millimeter Array (LMA) of Japan.

The first step toward the creation of what would become ALMA came in 1997, when the National Radio Astronomy Observatory (NRAO) and the European Southern Observatory (ESO) agreed to pursue a common project that merged the MMA and LSA. The merged array combined the sensitivity of the LSA with the frequency coverage and superior site of the MMA. ESO and NRAO worked together in technical, science, and management groups to define and organize a joint project between the two observatories with participation by Canada and Spain (the latter became a member of ESO later).

A series of resolutions and agreements led to the choice of “Atacama Large Millimeter Array”, or ALMA, as the name of the new array in March 1999 and the signing of the ALMA Agreement on 25 February 2003, between the North American and European parties. (“Alma” means “soul” in Spanish and “learned” or “knowledgeable” in Arabic.) Following mutual discussions over several years, the ALMA Project received a proposal from the National Astronomical Observatory of Japan (NAOJ) whereby Japan would provide the ACA (Atacama Compact Array) and three additional receiver bands for the large array, to form Enhanced ALMA. Further discussions between ALMA and NAOJ led to the signing of a high-level agreement on 14 September 2004 that makes Japan an official participant in Enhanced ALMA, to be known as the Atacama Large Millimeter/submillimeter Array. A groundbreaking ceremony was held on November 6, 2003 and the ALMA logo was unveiled.

During an early stage of the planning of ALMA, it was decided to employ ALMA antennas designed and constructed by known companies in North America, Europe, and Japan, rather than using one single design. This was mainly for political reasons. Although very different approaches have been chosen by the providers, each of the antenna designs appears to be able to meet ALMA’s stringent requirements. The components designed and manufactured across Europe were transported by specialist aerospace and astrospace logistics company Route To Space Alliance, 26 in total which were delivered to Antwerp for onward shipment to Chile.

Partners

European Southern Observatory (EU) and the European Regional Support Centre
National Science Foundation (US) via the National Radio Astronomy Observatory (US) and the North American ALMA Science Center (US)
National Research Council Canada [Conseil national de recherches Canada] (CA)
National Astronomical Observatory of Japan (JP) under the National Institute of Natural Sciences (自然科学研究機構, Shizenkagaku kenkyuukikou) (JP)
ALMA-Taiwan at the Academia Sinica Institute of Astronomy & Astrophysics [中央研究院天文及天文物理研究所](TW)
Republic of Chile