Angular Momentum Problem
First discovered as a shortcoming to the Condensation Theory[1], the angular momentum problem in star formation is a fundamental problem in astrophysics and astronomy that tries to understand the conservation of angular momentum during the processes of stellar formation.
The Condensation Theory, or the Nebular Theory, is the leading hypothesis that explains the characteristics of the solar system, the organization of planets into Terrestrial and Jovian and their differences, and the existence of asteroids and comets. The theory states star formation begins within vast molecular clouds that are cold, dense regions of interstellar space, composed of dust, gas, hydrogen, helium, and even heavier elements[2]The gas within a molecular cloud experiences a gravitational collapse, causing the cloud to spin even faster and flatten itself into a disk. When this collapse occurs to form a star, angular momentum should be conserved, similar to when an ice skater pulls in their arms to spin faster. Based on this, we would expect to always see the central region of the disk rotating faster than the outer regions. But this is not what we consistently observe. When we observe young stars, in the early protoplanetary stages just after collapse, we notice that the rotational speed of the protoplanetary disk varies. This angular momentum conservation mystery presents several challenges during the star formation process, collectively referred to as the "angular momentum problem."
The key questions and challenges related to the angular momentum problem in star formation are as follows:
Conservation of Angular Momentum[3]:
If we are observing outer regions spinning just as fast or even faster than the central region of the disk, then how is angular momentum still conserved and redistributed during the collapse of the molecular cloud? This contradiction between conservation of angular momentum and the observed properties of young stars and their disks is the biggest weakness Astronomers and Astrophysicists are working to resolve.
Formation of Protoplanetary Disks:[4]
Protoplanetary disks are crucial for the formation of planets; understanding the observed momentum and how these disks form is crucial to understanding planetary formation and dynamics. It is currently unclear how angular momentum is transported from the collapse of the molecular cloud to the new protoplanetary disk[5], and the exact mechanisms leading to its formation are still not fully understood either.
Magnetic Fields and Turbulence:
It is believed magnetic fields play a crucial role in star formation as they provide a counter pressure against gravitational collapse, they influence the gas and dust particles, and, most importantly, in this case, they can redistribute angular momentum[6]. Turbulence within the molecular cloud is also thought to redistribute angular momentum, but is still not quite understood. This leads us to the next challenge: understanding the relationship between magnetic fields, turbulence, and their effect on angular momentum.
The Accretion Process
The star’s rotational rate and disk evolution can be greatly impacted by the accretion of material from the disk onto the central star[7]. If Astronomers and Astrophysicists understand this accretion, it can give more insight into how it impacts angular momentum.
Key Indicators of Angular Momentum Problem
Kepler's Laws of Planetary Motion[8]
Kepler's second Law states that a planet orbiting a star sweeps out an equal area of space in equal amounts of time. This means that the planet does not have a constant velocity along its orbit; instead, the velocity varies depending on its distance from the star along its path, to keep angular momentum conserved.
Protostellar Disk Observations[9]
If angular momentum were perfectly conserved during star formation, we would expect to see the central region of the disk rotating faster than the outer regions. However, we see that the protoplanetary disks have a considerable amount of angular momentum, which makes their existence difficult to explain.
Spectroscopic Observations
When we use spectroscopy to study the spectra of young stars and their disks, we have further evidence that shows us that these disks have significant rotational velocities[10]. This offers insight into the angular momentum distribution within the disks by giving us information on the velocity and motion of gas in different regions of the disk, the disk structure, gas dynamic and accretion, and even any disk anomalies.
References
- ↑ "Chapter 15, Section 2". lifeng.lamost.org.
- ↑ Inutsuka, Shu-ichiro; Inoue, Tsuyoshi; Iwasaki, Kazunari; Hosokawa, Takashi (August 2015). "The formation and destruction of molecular clouds and galactic star formation: An origin for the cloud mass function and star formation efficiency". Astronomy & Astrophysics. 580: A49. arXiv:1505.04696. Bibcode:2015A&A...580A..49I. doi:10.1051/0004-6361/201425584. Unknown parameter
|s2cid=ignored (help) - ↑ College, Department of Physics and Astronomy at Douglas. "9.6 Angular Momentum and Its Conservation".
- ↑ "Planet Formation | Center for Astrophysics". www.cfa.harvard.edu.
- ↑ "Protoplanetary Disks and Their Evolution". astrobites.
- ↑ "The Role of Magnetic Fields in Star Formation | Center for Astrophysics". pweb.cfa.harvard.edu.
- ↑ Hartmann, Lee (20 November 2008). "Accretion Processes in Star Formation". doi:10.1017/CBO9780511552090.009.
- ↑ "Orbits and Kepler's Laws - NASA Science". NASA.
- ↑ Armitage, Philip J. (22 September 2011). "Dynamics of Protoplanetary Disks". Annual Review of Astronomy and Astrophysics. 49 (1): 195–236. arXiv:1011.1496. Bibcode:2011ARA&A..49..195A. doi:10.1146/annurev-astro-081710-102521. Unknown parameter
|s2cid=ignored (help) - ↑ Usui, Tadashi; Saitō, Mamoru; Tomita, Akihiko (May 2001). "Spectroscopic Observations of the Star Formation Activities in the Central Regions of Early-Type Spiral Galaxies". The Astronomical Journal. 121 (5): 2483–2498. Bibcode:2001AJ....121.2483U. doi:10.1086/320389. Unknown parameter
|s2cid=ignored (help)
Burkert, A. 2009, Galactic Disk Formation and the Angular Momentum Problem, arxiv:0908.1409 Zavala, J., Okamoto, T., & Frenk, C. S. 2008, Bulges versus discs: the evolution of angular momentum in cosmological simulations of galaxy formation, MNRAS 387: 364-370
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