Wednesday, March 24, 2021

Star Formation - Part II

On the first part we saw some basic historic concepts about star formation (i.e. Jeans mass). We then went through the first steps of star formation. This means we saw what are the stellar nurseries of the universe, which are better known as interstellar clouds. We closed the section, with the initial collapse of an interstellar cloud, which eventually leads to the formation of stars. Since we still have a long way to go to the actual formation of stars, let’s see what happens next.

Prior continuing with the process of star formation it would be useful to introduce some terms that are important in describing the process.

What is a protostar?

So let’s start with the definition of the protostar. A protostar is a very young star that still gathers mass from the parent molecular cloud. This is the earliest phase in the evolution of stars. This starts when a fragment from the cloud collapses due to gravity and within it, the protostar core forms. As in-falling keeps on accumulating mass on the protostar, a pre-main-sequence star is formed. This keeps on contracting, allowing for hydrogen fusion to start, thus becoming a main-sequence star. For a Sun-like star, this phase lasts around 500,000 years.

What is the Hertzsprung-Russell diagram?

The Hertzsprung–Russell diagram (H-R diagram), is a scatter plot of stars that displays the relationship between the star’s absolute magnitude or luminosity, versus its effective temperature, or stellar classification. The diagram, unlike what its name suggests, it was created independently around 1910 by Ejnar Hertzsprung and Henry Norris Russell.

What is the Hayashi limit?

The Hayashi limit is a constraint upon the maximum radius of a star given its mass. When the inward force of gravity is balanced with the outward pressure from the core, then the star cannot exceed the radius defined by the Hayashi limit. Its name comes after Chushiro Hayashi, a Japanese astrophysicist.

What is the Henyey track?

The Henyey track is an evolutionary path of pre-main-sequence stars with masses > 0.5 M☉ in the H-R diagram, after the end of the Hayashi track. This describes the evolutionary path of more massive pre-main-sequence stars, where a star can remain in radiative equilibrium during a certain time period on its contraction to the main sequence. Thus, the collapse is very slow, and the star’s luminosity remains constant. Its name comes after the American astronomer Louis George Henyey.

Formation of the protostar

We saw in the previous section the process of the molecular cloud collapse, and what parameters may trigger it. So let’s see what happens next, by overviewing the characteristics of a protostar and its formation process. The collapse of the new protostellar structure will continue, on the condition that the gravitational binding energy gets eliminated. However, the protostellar cloud becomes opaque, thus it cannot further emit. This means that the energy is removed through another medium. Here the presence of dust becomes crucial. At this point, the dust within the cloud is heated to a temperature of 60-100 K. (-213°C—173.15°C). Thus, the excess energy can be emitted in the form of far-infrared radiation to which the cloud is transparent. This allows the further collapse of the cloud.

The density profile of the cloud increases towards the center of the cloud, thus its core that will become opaque first. This happens at a density of 10-13 g/cm3. This forms a core (known as the first hydrostatic core) that can further collapse. Its temperature increases, since gas falling towards it, creates shock waves.

When the core temperature reaches around 2,000K, the thermal energy will segregate the H2 molecules. This is followed by the ionization of the hydrogen and helium atoms. This absorbs the energy of the contraction, which allows it to continue on a timescale comparable with the free-fall. When the density of the in-falling material reaches a density of about 10-8 g/cm3, then the material is sufficiently transparent to allow energy radiated from the protostar to escape. A combination of convection within the protostar and radiation from the exterior allows the star to contract further. This is halted when the gas is hot enough, so its internal pressure can support the protostar from further collapse (this state is known as hydrostatic equilibrium). When this phase is completed, congratulations, you now have a protostar.

By this point, a circumstellar disk has been formed thus, accretion to the protostar continues. When temperature and density are high enough, deuterium (this is an isotope of hydrogen, with one proton and one neutron in its core) begins, and the outward pressure slows down the collapse. Material from the cloud continues to fall into the protostar. At this stage, a bipolar jet is produced, which has a bow shape. Such object is known as Herbig-Haro (HH) objects, and they are amongst the most spectacular objects someone can observe. Through this mechanism, excessive angular momentum is expelled, allowing the star to form.

The Orion Nebula is the example of star formation. From observations we have found around 700 hundred stars at various stages of the star forming process. Image Credit: Credit: NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA), the Hubble Space Telescope Orion Treasury Project Team and L. Ricci (ESO).

When the surrounding gas and dust envelope disperses and accretion stops, the star is now at its pre-main-sequence stage (PMS). The distinctive difference with a main-sequence star (like our Sun) is the energy source (i.e., gravitational contraction vs. hydrogen fusion).

The PMS star follows what is known as the Hayashi track on the Hertzsprung-Russell diagram. The contraction will continue, and it stops till the Hayashi limit is reached. After this point, the contraction continues on a thermal time scale. This the amount of time needed for the star to radiate its total kinetic energy at its current luminosity rate. For our Sun, this scale is 30 million years. Stars with mass less than > 0.5 M☉ will be main sequence stars. More massive PMS at the end of the Hayashi track, collapse slowly near hydrostatic equilibrium, following the Henyey track.

Finally, hydrogen fusion begins in the core of the star, and any remaining surrounding material is moved away. After this point, the protostellar phase ends, and the main sequence phase starts.

The process is well understood for star around 1 M☉ (or less). However, for high-mass stars, the length of the star formation process is comparable to the other timescales of their evolution (i.e., short) but the process is not that clear and well defined.

Observations of stellar nurseries

At the beginning of the post, we saw that the initial view of star formation was rather simplistic since it ignored plenty of elements that we went through in the previous section. The reason for this was not due to lack of knowledge, but lack of observations.

All the main features of star formation are not observable at optical wavelengths. The formation and evolution of a protostar are hidden by a dense cloud of dust and gas inherited by the parent molecular cloud. Often though these cocoons like structures can be in silhouette against bright emission from surrounding gas.High resolution snapshot of a newborn star surrounded by dust. The source is known as HH-212 (or the definition of protostar example) and it is located in the Orion nebula. Such protostar images were impossible to obtain in the past, today thank to sub-millimeter telescopes it is possible. Image Credit:ALMA (ESO/NAOJ/NRAO)/Lee et al.

The early stages of a star’s life can be seen in infrared light, which can penetrate dust much more efficiently than optical light. Thus observations with Spitzer Space Telescopes, Wide-field Infrared Survey Explorer (WISE), or even Hubble Space Telescope have provided observations of high importance in our understanding of star formation.

Equally important are observations in X-ray, since emission from such objects is 100-100,000 times stronger than main-sequence stars. For low-mass stars, X-rays are emitted by the heating in their stellar coronas, while for more massive stars, it comes from shock waves in their stellar winds.

Note that the formation of individual stars can only be directly observed in our own Galaxy. In distant galaxies, this is done through the detection of unique spectral features.

Star formation process – Low-mass & high-mass star formation

The formation mechanism of stars varies with their mass. For low-mass star formation, we have observational evidence that suggests that low-mass stars form by the gravitational collapse of local rotating density enhancements within the cloud. The collapse of a rotating cloud of gas and dust leads to the formation of an accretion disk, which feeds matter to the protostar. For stars with a mass around 8 M☉and higher, the mechanism of star formation is not well understood.

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