Protostar

A protostar is a large mass that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. The protostellar phase is an early stage in the process of star formation. For a one solar-mass star it lasts about 10,000,000 years.[1] It starts with a core of increased density in a molecular cloud and ends with the formation of a pre-main-sequence star (either a T Tauri star if below two solar masses or a Herbig Ae/Be star if between two and eight solar masses[n 1]), which then develops into a main sequence star. This is heralded by the T Tauri wind, a type of super solar wind that marks the change from the star accreting mass into radiating energy.

History

The existence of 'protostars' was first proposed and postulated by Soviet-Armenian scientist, Viktor Ambartsumian.[2]

Ambartsumian's research was in the so-called 'continuous emission', observed in the spectra of young stars of the T Tauri type and their associated neighbor stars. As opposed to the classical hypotheses suggesting that stars formed singly as a result of condensation of small masses of diffuse matter, the new hypothesis postulated the existence of massive star-forming bodies, "proto-stars". The process of disintegration of proto-stars is responsible for the formation of multiple members in star associations.

Role in stellar evolution

Main article: Star formation

Star formation begins in giant molecular clouds. These clouds are initially balanced between gravitational forces, which work to collapse the cloud, and pressure forces (primarily from the gas) which work to keep the cloud from collapsing. When these forces fall out of balance, such as due to a supernova shock wave, the cloud begins to collapse and fragment into smaller and smaller fragments. The smallest of these fragments begin contracting and become protostars.

As the cloud continues to contract, it begins to increase in temperature. The temperature increase is not caused by nuclear reactions but rather by the conversion of gravitational energy to thermal kinetic energy. As a particle (atom or molecule) falls towards the centre of the contracting fragment, its gravitational energy decreases. As the total energy of the particle must remain constant (due to conservation of energy), the reduction in gravitational potential energy results in an increase in the particle's kinetic energy. The kinetic energy of a group of particles is the thermal kinetic energy, or temperature, of the cloud. The more the cloud contracts the more the temperature increases.

Collisions between molecules often leave them in excited states which can emit radiation as those states decay. At the temperatures of a protostar (10 to 20 kelvins) most of the radiation is in the microwave or infrared range of the spectrum. At this early stage of star formation, most of this radiation escapes, preventing a rapid rise in temperature of the cloud. This stage of protostar evolution is known as the isothermal phase.

As the cloud contracts the number density of the molecules increases, making it more difficult for the emitted radiation to escape. In effect, the gas becomes opaque to the radiation and the temperature within the cloud will begin to rise more rapidly. The gas cloud still has much more gas at this stage, called a Class 0 protostar.

As the system evolves, more and more emission starts to come from the protostar rather than the surrounding dust and gas. In the Class I stage, the protostar is now about the same mass as the surrounding envelope.

The next stage of protostar evolution for star less than two solar masses is the classic T Tauri star (a.k.a. Class II protostar). In this phase, the temperature increases substantially and this disk becomes substantially smaller than the protostar. In the final stage of protostar evolution, the temperature rises and the surrounding material becomes an order of magnitude smaller, becoming a Class III protostar ('weak' T Tauri star).[3] For protostars between two and eight solar masses, the next stage, instead of a T Tauri star, is a Herbig Ae/Be star. Pre-main-sequence stars over eight solar masses are not observed because they have already moved on to the main sequence before they can blow away their surrounding dark nebula.

Infrared measurements taken by the 2MASS and WISE astronomical surveys have been particularly effective at unveiling numerous protostars and their host star clusters.[4][5]

Classes of protostars

For details of observational classification, see Young stellar object.
Class peak emission duration (Years) description
0 submillimeter 104 early accretion
I far-infrared 105 main accretion phase
II near-infrared 106 classic T Tauri star
III visible 107[3] 'weak line' T Tauri star

[6]

Gallery

Video about the protostar V1647 Orionis and its X-ray emission (2004).
Protostar outburst - HOPS 383 (2015).
A protostar inside a Bok globule (Artist's image).
Stellar cluster RCW 38, around the young star IRS2, a system of two massive stars and protostars.

See also

Wikimedia Commons has media related to Protostars.

Notes

  1. ↑ Stars more massive than eight solar masses are not observed in the pre-main-sequence star phase, as by the time they blow away their surrounding dark nebula, they are already on the main sequence

References

  1. ↑ "How the Sun Came to Be: Stellar Evolution". Retrieved April 10, 2015.
  2. ↑ "Abstract". SpringerLink. Retrieved 2011-01-12.
  3. ↑ 3.0 3.1 Lecture notes from an astronomy course
  4. ↑ Froebrich, D.; Scholz, A.; Raftery, C. L. (2007). A systematic survey for infrared star clusters with |b| <20° using 2MASS, MNRAS, 347, 2
  5. ↑ Majaess, D. (2013). Discovering protostars and their host clusters via WISE, ApSS, 344, 1
  6. ↑ Larson, R.B. (2003), The physics of star formation'', Reports on Progress in Physics, vol. 66, issue 10, pp. 1651–1697

External links