Thursday, January 6, 2022

Stars Form Much Faster Than Previously Assumed

There may be two factors at work in explaining the "impossible early galaxy observations" discussed in my previous post at this blog.

One of the factors, discussed in that post, is that gravitational based explanations of dark matter phenomena suggest that stars form galaxy structures much more quickly than they do in a cold dark matter paradigm.

Yesterday's new paper suggests that there is also a second factor at work which has nothing to do with the dark matter and dark energy phenomena debates. 

Stars form much more quickly from clouds of hydrogen gas than historically assumed, because magnetic fields in those clouds of hydrogen gas are much weaker than they were previously believed to be. There are plausible reasons for this to be the case, although it isn't yet clear which one of these reasons is actually the cause of this reality. 

But, this second factor is still a much less important one than the gravity v. dark matter distinction in my previous post, because it takes much longer for galaxies to coalesce from individual stars (on the order of hundreds of millions or billions of years) than it does for stars to form from hydrogen gas. The "classical view" is that it takes about ten million years for a typical star to form, while this observations suggests that one million years is closer to the mark. This difference is a mere rounding error relative to the time required to form a galaxy from individual stars.

Astronomers have long thought it takes millions of years for the seeds of stars like the Sun to come together. Clouds of mostly hydrogen gas coalesce under gravity into prestellar cores dense enough to collapse and spark nuclear fusion, while magnetic forces hold matter in place and slow down the process. But observations using the world’s largest radio telescope are casting doubt on this long gestational period. Researchers have zoomed in on a prestellar core in a giant gas cloud—a nursery for hundreds of baby stars—and found the tiny embryo may be forming 10 times faster than thought, thanks to weak magnetic fields.

“If this is proven to be the case in other gas clouds, it will be revolutionary for the star formation community,” says Paola Caselli from the Max Planck Institute for Extraterrestrial Physics, who was not involved with the research. 
Studying star birth and the tug of war between gravity and magnetic forces has been a challenge because the magnetic fields can be 100,000 times weaker than Earth’s. The only direct way to detect them comes from a phenomenon called the Zeeman effect, in which the magnetic fields cause so-called spectral lines to split in a way that depends on the strength of the field. These spectral lines are bright or dark patterns where atoms or molecules emit or absorb specific wavelengths of light. For gas clouds, the Zeeman splitting occurs in radio wavelengths, so radio telescopes are needed. And the dishes must be big in order to zoom in on a small region of space and reveal such a subtle effect. . . .
In a study published today [January 5, 2022] in Nature, researchers report a magnetic field strength of 4 microgauss—no stronger than in the outer layer. “If the standard theory worked, the magnetic field needs to be much stronger to resist a 100-fold increase in cloud density. That didn’t happen,” says Di Li, the chief scientist of FAST who led the study.

“The paper basically says that gravity wins in the cloud: That’s where stars start to form, not in the dense core,” Caselli adds. “That’s a very big statement.”

The finding implies that a gas cloud could evolve into a stellar embryo 10 times quicker than previously thought, says lead author Tao-Chung Ching of the Chinese Academy of Sciences’s National Astronomical Observatories.

 From here. The paper and its abstract are as follows:

Magnetic fields have an important role in the evolution of interstellar medium and star formation. 
As the only direct probe of interstellar field strength, credible Zeeman measurements remain sparse owing to the lack of suitable Zeeman probes, particularly for cold, molecular gas. 
Here we report the detection of a magnetic field of +3.8 ± 0.3 microgauss through the HI narrow self-absorption (HINSA) towards L1544 —a well-studied prototypical prestellar core in an early transition between starless and protostellar phases characterized by a high central number density and a low central temperature. 
A combined analysis of the Zeeman measurements of quasar HI absorption, HI emission, OH emission and HINSA reveals a coherent magnetic field from the atomic cold neutral medium (CNM) to the molecular envelope. The molecular envelope traced by the HINSA is found to be magnetically supercritical, with a field strength comparable to that of the surrounding diffuse, magnetically subcritical CNM despite a large increase in density. 
The reduction of the magnetic flux relative to the mass, which is necessary for star formation, thus seems to have already happened during the transition from the diffuse CNM to the molecular gas traced by the HINSA. This is earlier than envisioned in the classical picture where magnetically supercritical cores capable of collapsing into stars form out of magnetically subcritical envelopes.

The body text explains that:
[T]he molecular envelope of the L1544 core traced by HINSA is at least 13 times less magnetized relative to its mass compared with its ambient CNM. This is different from the ‘classic’ theory of low-mass star formation, which envisions the transition from magnetic subcriticality to supercriticality occurring as the supercritical core forms out of the magnetically supported (subcritical) envelope. Our results suggest that the transition from magnetic subcriticality to supercriticality occurs earlier, during the formation of the molecular envelope, favouring the more rapidly evolving scenario of core formation and evolution for L1544 over the slower, magnetically retarded scenario. In other words, by the time that the molecular envelope is formed, the problem of excessive magnetic flux as a fundamental obstacle to gravitational collapse and star formation is already resolved.
As comment from a peer reviewer disclosed with the article is also notable (and honestly, the paper would have been better if it had been taken more strongly to heart by the authors than they do in minimal deference to it, the substance of the paper is solid but the exposition in the paper is lacking):
They state that their result shows that the reduction in magnetic flux relative to mass occurs earlier than envisioned in the “classical” theory of star formation, but the discussion of this is rather limited (lines 141-149). Yet this is THE astrophysical result of the paper. They should briefly describe the “classical” prediction, namely that mass/flux is reduced by gravity driving neutrals through the ions and magnetic field by a process called ambipolar diffusion. They should explicitly state exactly why their result is contrary to this prediction; that is, what is the argument that the regime sampled by the H I self-absorption is not gravitationally contracting with ambipolar diffusion producing a subcritical region as in the “classical” theory. Further, they should briefly suggest how their result might be explained theoretically if ambipolar diffusion does not. 
Two major possibilities are (1) formation of molecular clouds by flows along flux tubes (i.e., Vazquez Semadeni et al., MNRAS 414, 2511, 2011) and (2) magnetic reconnection (i.e., Lazarian et al., ApJ 757, 154, 2012). 
In (1), for a relatively small distance along a flux tube, as sampled by a small telescope beam, initially there will be little mass and λ will be measured to be highly subcritical. As mass flows into the region of the cloud (not due to gravity but due to colliding interstellar flows), λ (again as sampled in a small telescope beam) increases. Thus clouds start out as atomic and subcritical and accumulate mass over large distances to become molecular and supercritical as they evolve, becoming self-gravitating at about the same time. 
In (2), as the amplitude of turbulence as well as the scale of turbulent motions decrease from the envelope to the core of a cloud, the diffusion of the magnetic field is faster in the envelope. As a result, the magnetic flux trapped during the collapse in the envelope is being released faster than the flux trapped in the core, resulting in much weaker fields in envelopes than in cores. 
Both of these “non-classical” theories would seem capable of explaining the observational result of this paper.

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