This image shows the G205.46-14.56 clump located in the Orion Molecular Cloud Complex. The yellow contours show the dense cores discovered by JCMT, and the zoomed-in pictures show the 1.3mm continuum emission of ALMA observation. These observations give insight into the formation of various stellar systems in dense cores. Image Credit: Qiuyi Luo et al. 2022.

How to get several star systems

Stars are formed inside massive clouds of gas and dust called molecular clouds. The nebulous hypothesis explains how it works. According to that hypothesis, dense nuclei inside these clouds of hydrogen collapse due to instability and form stars. The nebulous hypothesis is much more detailed than the short version, but that is the basic idea.

The problem is that it only explains how individual stars are formed. But about half of the Milky Way’s stars are binary pairs or more stars. The nebula hypothesis does not clearly explain how these stars are formed.

Most stars with about the same mass as our sun or larger are not single stars. Most are members of several star systems, especially binary stars. While nebula theory explains how single stars are formed, there are competing theories about how multiple stars are formed.

First of all, after a molecular cloud collapses into a star, it forms a rotating disk of gas and dust around the young protostar, which is called a circumferential disk. A theory that explains how multiple stars are formed says that a pair or more young protostars are fragments of a mother disk that was once much larger. Another theory states that the young protostars form independently, then capture each other in an orbital arrangement.

This is an ALMA image of a young protostar, called a T Tauri star.  They are less than 10 million years old and are representative of the type of young stars found in stellar nurseries such as the Orion Cloud Complex.  It shows the disk that surrounds the young star, from which planets will eventually be formed.  The researchers behind this new study examined the dense nuclei that form young stars like this one to find differences between nuclei that formed multiple stars and those that formed single stars like our sun.  Image credit: ALMA (ESO / NAOJ / NRAO)
T Tauri stars are less than 10 million years old and represent the type of young stars found in stellar nurseries such as the Orion Cloud Complex. It shows the disk that surrounds the young star, from which planets will eventually be formed. The researchers behind this new study examined the dense nuclei that form young stars like this one to find differences between nuclei that formed multiple stars and those that formed single stars like our sun. Image credit: ALMA (ESO / NAOJ / NRAO)

But when stars form inside a molecular cloud, it begins with a dense nucleus inside the cloud. This nucleus initiates the gravitational collapse that collects enough gas in one place to form a star. The question is what is different with some of these nuclei that cause more stars to form compared to single stars?

That’s what astronomers at Hawaiis do The James Clerk Maxwell Telescope (JCMT) wanted to understand.

JCMT is a 15-meter radio telescope at the Mauna Kea Observatory in Hawaii. The telescope’s submillimeter observations allow it to observe the molecular clouds where the stars are born. The researchers used it to observe Orion Molecular Cloud Complex (OMCC), the closest active star chamber to Earth, which is still about 1,500 light-years away. The OMCC contains two giant molecular clouds (GMCs), Orion A and Orion B. They also used observations from ALMA and Japan The Nobeyama telescope.

The team saw several star systems form in the Orion complex and made important discoveries about the process. They presented their results in an article published in The Astrophysical Journal. The magazine is “ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP): How do dense core properties affect the diversity of protostars?The first author is Qiuyi Luo, a Ph.D. student at Shanghai Astronomical Observatory.

“During the transition phase from a prestellar to a protostellar cloud core, one or more protostars may form within a single gas core,” the paper writes. “However, the detailed physical processes for this transition are still unclear.”

For this study, the research team collected observations of 43 protostellar nuclei in Orion’s molecular cloud complex with JCMT. Then they used the powerful ALMA telescope to study the internal structure of the nuclei.

This image shows the G205.46-14.56 lump located in the Orion Molecular Cloud Complex.  The yellow contours show the dense nuclei detected by JCMT, and the zoomed-in images show 1.3 mm continuum emission from ALMA observation.  These observations provide insight into the formation of various star systems in dense nuclei.  Image credit: Qiuyi Luo et al.  2022
This image shows the G205.46-14.56 lump located in the Orion Molecular Cloud Complex. The yellow contours show the dense nuclei detected by JCMT, and the zoomed-in images show 1.3 mm continuum emission from ALMA observation. These observations provide insight into the formation of various star systems in dense nuclei. Image credit: Qiuyi Luo et al. 2022

Research shows that about 30% of the 43 dense nuclei form binary or multiple stars, and the rest form only single stars. Astronomers measured and estimated the sizes and masses of the nuclei. They found that binary / multiple cores have higher densities and masses, although the sizes of all cores are not very different.

This figure from the study shows the exemplary core G196.92-10.37.  (a) is a JCMT image with a Spitzer image on top of it.  The yellow circle is the zoomed area in (b.) (B) shows continuum contour levels.  (c) shows ALMA data and also shows that the nucleus forms three stars: A, B and C. Image credit: Qiuyi Luo et al.  2022
This figure from the study shows the exemplary core G196.92-10.37. (a) is a JCMT image with a Spitzer image on top of it. The yellow circle is the zoomed area in (b.) (B) shows continuum contour levels. (c) shows ALMA data and also indicates that the nucleus forms three stars: A, B and C. Image credit: Qiuyi Luo et al. 2022

“This is understandable,” said lead author Qiuyi Luo. “Denser nuclei are much easier to fragment due to disturbances caused by self-gravity inside molecular nuclei.”

From there, the team turned to Japan’s 45-meter Nobayama radio telescope. They observed what is known as the N2H + J = 1-0 molecular line in all 43 dense nuclei. N2H + is diazenylium, one of the first ions ever found in interstellar clouds. This molecular line is easily observed through the Earth’s atmosphere with fine precision. Astronomers use it to map the density and velocity of gas in molecular clouds.

These observations showed that dense nuclei forming multiple stars are more turbulent than nuclei forming single stars.

This figure from the study shows the Mach number for gas in the dense nuclei measured with the N2H + line.  Higher Mach numbers mean more turbulence, and this figure shows that binary and multiple star nuclei are more turbulent than single star nuclei.  Image credit: Qiuyi Luo et al.  2022
This figure from the study shows the Mach number for gas in the dense cores measured by N2hrs+ line. Higher Mach numbers mean more turbulence, and this figure shows that binary and multiple star nuclei are more turbulent than single star nuclei. Image credit: Qiuyi Luo et al. 2022

“These Nobeyama observations provide a good measure of turbulence levels in dense nuclei. Our findings suggest that binary / multiple stars tend to form in more turbulent nuclei,” said Professor Ken’ichi Tatematsu, who led the Nobeyama observations.

Lead author Qiuyi Luo summarized the results of the study in a press release. “In a word, we found that binary / multiple stars tend to form in denser and more turbulent molecular nuclei in this study.”

This figure from the study shows the gas velocity in two of the dense cores.  Blue indicates lower speed and red indicates higher speed.  The arrows show the directions of the local increasing velocity gradients, with the lengths indicating their size.  The upper core, marked in orange, is a binary core, and the lower core marked in black is a single core.  Image credit: Qiuyi Luo et al.  2022
This figure from the study shows the gas velocity in two of the dense cores. Blue indicates lower speed and red indicates higher speed. The arrows show the directions of the local increasing velocity gradients, with the lengths indicating their size. The upper core, marked with orange, is a binary core, and the lower core, marked with black, is a single core. Image credit: Qiuyi Luo et al. 2022

Co-author Sheng-Yuan Liu added, “JCMT has proven to be an excellent tool for uncovering these amazing nurseries for ALMA follow-ups. a more thorough understanding of star formation. “

The researchers also found that the stars in each binary or multiple arrangement are usually in very different evolutionary stages. The more developed protostars are generally further from the center of the dense nuclei than their younger counterparts. This indicates that as the stars evolve, they migrate out of their birth cores.

This study shows some differences between nuclei that form single stars and nuclei that form binary and multiple stars. But that’s just the beginning: there is much more to learn and many more questions.

One of the questions is what role does magnetic field play in star formation? Star-forming clouds can be highly magnetized. Magnetic fields from the interstellar medium penetrate star-forming clouds, and astronomers know that magnetic fields can affect the rate of star formation. Do they play a role in determining whether a single star is formed versus several stars?

This figure is from a separate study that simulated the effect of magnetic fields on star-forming regions.  The left is a simulated star-forming region without a magnetic field, the right is with a magnetic field.  Each white circle is a protostar, and red indicates gas moving at high speeds.  Without magnetism, the mass collapses into a central area with less effluent gas.  With magnetism, the protostars are more scattered and more gas escapes.  This seems to indicate that magnetic fields inhibit the formation of dense structures.  Image credit: Krumholz and Federrath 2019.
This figure is from a separate study that simulated the effect of magnetic fields on star-forming regions. The left is a simulated star-forming region without a magnetic field, the right is with a magnetic field. Each white circle is a protostar, and red indicates gas moving at high speeds. Without magnetism, the mass collapses into a central area with less effluent gas. With magnetism, the protostars are more scattered and more gas escapes. This seems to indicate that magnetic fields inhibit the formation of dense structures. Image credit: Krumholz and Federrath 2019.

“We have not yet looked at the effect of magnetic fields in our analysis,” says corresponding author Tie Liu, who was also the leader of the ALMA observations. “Magnetic fields can suppress fragmentation in dense nuclei, so we are pleased to focus the next step of our research in this area with JCMT.”

The authors point out that the low sample size inhibits their results. Forty-three dense nuclei may not have enough data to draw conclusions from, especially since they are all from the same molecular cloud. The study was also limited by the resolution of the various observatories and telescopes used in the study.

“Our results could be further tested using future observations with higher spatial and spectral resolution against a more complete dense core sample in different molecular clouds found in widely differing environments,” they conclude.

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