Bipolar outflows

Class I obj ects also have bipolar outflows, but they are less powerful and less well collimated than those of Class 0 objects. This stage lasts 100 000 to 200 000 years. Class //objects, also known as classical T Tauri stars, are pre-main-sequence stars with optically thick proto-planetary disks. They are no longer embedded in their parent cloud, and they are observed in optical and infrared wavelengths. They still exhibit bipolar outflows and strong stellar winds. This stage lasts from 1-10 million years. Class ///objects are the so-called weak line or naked T-Tauri stars. They have optically thin disks, perhaps debris disks in some cases, and there are no outflows or other evidence of accretion. They are observed in the visible and near infrared and have strong X-ray emission. These stars may have planets around them, although they cannot be observed. 

Some further discussions on outflows are given in Chapters 3 and 8. A detailed overview on the present state of theory and observation of jets and bipolar outflows is found in Konigl Ruden (1993), Konigl Pudritz (2000), and Pudritz el al. (2007). 

Protostellar phases class —I, 0, 1, II, III objects. 04.2.2.2 Ubiquity of bipolar outflows from earliest phases ASTROPHYSICAL ANALOGUES FOR THE SOLAR NEBULA... 

Ubiquity of bipolar outflows from earliest phases... 

Figure 3 The prototypical young star with a bipolar outflow, L1551IRS5 (Snell et al, 1980), now known to harbor a binary protostar, with a separation of 45 AU, and where each protostar is orbited by a disk with a radius of —10 AU and a mass of — 0.05 solar masses (Snell et al., 1980) (reproduced by permission of University of Chicago Press and American Astronomical Society from Astrophys. 1980, 239, L17-L22).

If an X-wind is responsible for driving bipolar outflows, then there are possibly important implications for the thermal processing of solids and the production of short-lived radioisotopes (Shu et al., 1996, 2001). The basic idea is that some of the solids that spiral inward and approach the boundary layer between the solar nebula and the protosun will be lifted upward by the same magnetically driven wind that powers bipolar outflows. While close to the protosun, these solids will be subject to heating by the solar radiation... 

Two types of jet or outflow models have been proposed. Liffman and Brown (1996) suggested that chondrules formed in a bipolar outflow by ablation of planetesimals and were then injected into the asteroid belt. The observed correlation between rim thickness and chondrule size may have arisen when chondrules reentered the dusty nebula at hypersonic speeds (Liffman and Toscano, 2000). 

Jets and outflows arising from low mass protostars can produce some of the most spectacular images of the star formation process. The HH 46/47 system is a striking example of a low mass protostar, surrounded by a circumstellar disk, ejecting a jet and creating a bipolar outflow perpendicular to the disk (Figure 14). The central protostar lies inside a dark cloud (known as a Bok... 

A second consequence is that the outflow which occurs due to a star hitting the 12-limit, is predicted to be highly bipolar. This is illustrated by a hydrodynamical simulation of the giant eruption of r] Car by Langer el al. (1999). The fact that virtually all LBV nebulae are highly bipolar supports the idea that the D-limit is actually involved in the LBV instability. 

T Tauri star An unstable young variable star in its pre-main sequence phase (see Hertzsprung-Russell diagram). The instability, brought about by the beginning of nuclear fusion in the core of the star, causes pulsations and stellar winds, possibly with bipolar outflows. Groups of such stars, often associated with Herbig-Haro objects, are called T Tauri associations. 

Bipolar outflow source L1228 (see text). The exciting star is indicated. (Data from L. Haikala). 

In an attempt to contribute theoretically towards an understanding of the physics of bipolar-flows, we have developed a model aimed at explaining the major observational features of outflow sources. The initial scenario required by the model is that of a central stellar object embedded in a parental cloud of gas and dust forming a halo whose shape deviates slightly from spherical symmetry, an assumption which should be met in practically all realistic cases. 

The radial inflow is a transient phenomenon lasting only about C2-3) x 10 years if we assume a typical radial scale of lO AU for the non-rotating halo which supplies the radial disk flow. When the non-rotating halo is exhausted, the radial inflow and the well-collimated bipolar outflow stop, leaving behind a rotating faint disk around the stellar object. 

Thus, the four major observational features associated with bipolar outflows,... 

Andr4 P., Ward-Thompson, D., and Baisony, M. (2000). Prom Pre-Stellar Cores to Protostars The Initial Conditions of Star Eormation. In Protostars and Planets IV (V. Mannings, A. P. Boss, and S. S. Russell, eds.), in press. University of Arizona Press, Tucson, AZ. Bachiller, R. (1996). Bipolar molecular outflows from young stars and protostars, Ann. Rev. Astron. Astrophys. 34,111-154. 

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