The evolutionary picture of low mass protostars (T Tauri stars, M less than 2 M(sun)) is thought to be somewhat under control, i.e.,

• very slowly rotating cloud + trigger ===> collapse ===> star + disk formation ====> slow contraction and accretion + intense stellar winds (bipolar flows) ===> ignition of hydrogen burning and appearance of star on the Main Sequence.

  Now, what are some details of this process?

  • initially, the clouds which collapse are thought to be very slowly rotating (===>the spin may only be important in the later stages of collapse--initially it plays very little role in the formation process) and to not have uniform density (the clouds are initially denser in their centers than near their edges). The consequences of the former have been roughly discussed already while the latter condition leads to an inside-out collapse. Huh? What does this mean?
  • What happens is that the dense cores collapse faster than the less dense outer regions of the cloud. This follows from the free-fall time ~ 1/sqrt(Gxdensity). The initial collapse of the core is quite fast; time ~ 1/sqrt(6.7x10**(-8) x10**(-18) gm per cc) ~ 50,000 - 100,000 years or so. The lower density envelope takes longer to collapse accrete (collapse onto the protostar); time ~ millions of years or so. Roughly, speaking the Sun forms as shown here.
  • The inside-out collapse leads to the formation of the forming star in the center of the cloud which then slowly builds up its mass by accreting the outer layers of the cloud.
  • Another noteworthy aspect of this later stage of formation is that before the star actually gets hot enough to ignite nuclear fusion, an intense stellar wind is generated. Often times because the cloud was slowly rotating, a disk of material forms around the star. The disk collimates the intense stellar wind into 2 oppositely directed beams producing what is referred to as a bipolar flow.

    Outside of the interesting appearance of a bipolar flow, what are some consequences of the stellar wind? Well, for one thing ===> can cause the forming star to lose up to 0.4 M(sun)!! for another ===> can start to disrupt the cloud.

  • Is the disruption of the cloud important? Well, even though it takes several millions of years for the cloud to accrete onto the protostar, because the protostars are relatively low mass, it takes even longer to slowly contract and approach starhood. For the most part, the cloud has a chance to accrete onto the protostar before the violent stages of evolution begin.

    This is important in two senses:

    • the forming is star de-cloaked before it actually beomes a star. We can see T Tauri stars in the visible portion of the spectrum.
    • the cloud collapse process reaches completion before the cloud is disrupted and so, the newly formed star will be roughly the mass of the initially unstable cloud (minus the wind, ... )

Lower Mass Limit

As the forming stars slowly contract trying to reach densities and temperatures in their cores which are high enough to ignite fusion, an interesting thing happens to low mass clouds. Before their T's get high enough, their densities actually become large enough to make the electrons in the cores degnerate. Once the electrons become degenerate, it becomes very hard to compress them. Effectively what this means is that the cloud stops contracting (stops compressing).

The core of the star thus stops getting hotter and nuclear fusion is never ignited ===> the cloud never formally becomes a star. We refer to such objects as

• brown dwarfs
• planets (e.g., Jupiter)

An interesting comment is is that there is something strange about the star formation process. We might expect there to be a smooth distribution of objects (in mass) between stars and planet-sized things. That is, we might expect to see objects with mass spanning the range between Jupiter and the stars on the Main Sequence. Further, we know that the number of stars for a given mass is larger the smaller the mass of the star, e.g., M stars are much more common than O stars. If this trend continued, we would expect to see many more planets and brown dwarfs than M stars.

Surprisingly, there appears to be a gap between the lowest mass stars and brown dwarfs and planets in terms of mass. There is not a smooth distribution in terms of their mass. The process of star apparently is different for star-sized things than it is for planet-sized things.

Information provided by: http://zebu.uoregon.edu