Young stellar objects

At the centre of the cloud is the young stellar object destined to become the Sun. It accounts for approximately 99.9 per cent of the mass of the nebula and there are various examples of this in the heavens, including the classic pre-main sequence T-Tauri star. The star continues to evolve, blowing off bipolar jets (see Figure 4.5) and beginning a solar wind of particles. Of course, the star does not reach its full luminous intensity and the best theories suggest that the Sun was some 30 per cent less luminous when the Earth began to form. 

Giant molecular cloud (GMC) A region of space with a larger molecular density of 10s cm-3 and a rich chemical composition. The GMC may also contain young stellar objects. 

Herbig-Haro object A bright object associated with young stellar object, probably due to a region of ionisation associated with high-speed polar jets. 

Young stellar object (YSO) Young stellar object - a protostar that is beginning to shine but at low temperature so it has a spectrum with a maximum in the infrared. 

Young stellar objects are frequently classified into four classes that characterize four important evolutionary stages ... 

All young stellar objects show indications for stellar winds and outflows. These phenomena are always observed to occur in systems that undergo mass accretion that interacts with magnetic fields and rotation. They are not limited to star formation but are also observed in other cases, e.g. during accretion onto central black holes in galaxies. 

Table 2.3 Observed properties of some young stellar objects and their accretion disks spectral type, effective temperature Teg, luminosity L, estimated stellar mass M, stellar radius Rt, accretion rate M, disk radius T isk as observed by dust emission, inclination of disk with respect to sight line, and disk mass Mdisk estimated from submillimeter dust emission...

Figure 2.11 The HH-30 object, a young stellar object showing two thin jets flowing out from the central region of an accretion disk. The outflow velocity in the jets is 90-270 km s-1. The two bowl-shaped regions are starlight scattered by the dust in the uppermost layers of the disk. The dark lane in between is the accretion disk seen side-on. The radial optical depth in the disk is too high for starlight to penetrate in this direction. The radial extension of the disk is 425 AU. (Photo credit Hubble Space Telescope, NASA/ESA and STScI.)...

In heavily obscured regions with ongoing star formation one observes the so-called Herbig-Haro (HH) objects thin collimated jets of matter rapidly flowing (up to several hundred kilometers per second) out from young stellar objects. An example is shown in Fig. 2.11. These jets are mainly associated with Class 0 and I objects but sometimes are also observed for T Tauri stars. The outflows interact with... 

Beckwith S. V. W., Sargent A. 1., Chini R. S., and Gusten R. (1990) A survey for circumstellar disks around young stellar objects. Astron, J. 99, 924-945. 

Feigelson E. D. and Montmerle T. (1999) High energy processes in young stellar objects. Ann. Rev. Astron. Astrophys. 37, 363-408. 

Strom S. E., Strom K. M., and Edwards S. (1988) Energetic winds and circumstellar disks associated with low mass young stellar objects. In Galactic and Extragalactic Star Formation (ed. R. Pudritz). NATO Advanced Study Institute, Reidel, Dordrecht, The Netherlands, pp. 53-68. 

Astronomical observations of molecular clouds and young stellar objects provide the basis for our understanding of the early solar system (Cameron, 1995 Alexander et al., 2001). The first stage in this process is when a fragment of an interstellar molecular cloud collapses to form a disk-like nebula, or proto-planetary disk. This process normally takes... 

The final stage in the condensation of a solar nebula is that of planetary formation. Planets are thought to form in a protoplane-tary disk of the type observed around young stellar objects such as Beta Pictoris (Section 2.3.1.1). The lifetime of such dust-laden protoplanetary disks is of the order of 107 years. Until recently we knew very little about processes operating in such protoplanetary disks, and much of what we did know was drawn from the study of the solar system. However, the recent discovery of planetary systems around other stars suggest that our solar system may not be characteristic of planetary systems in general and many... 

The study of such species is important cosmochemically, but is quite difficult at microwave frequencies where the rotational spectra are weak, and nearly impossible at IR or optical wavelengths due to the extinction present in dense molecular clouds and young stellar objects. 

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