T Tauri winds

Figure 2. Hj and CO formation/destruction network in the T-Tauri wind. Only the principal formation and destruction routes for Hj and CO are shown. Hj is formed only by the H" " route shown and is primarily destroyed by the temperature sensitive collisional dissociation reaction. There are two CO formation routes high temperature fast reactions involving H2 and temperature-independent slow reactions involving H. Once formed, the CO is chemically highly stable.

It has been shown that the primary T Tauri wind is basically atomic with little molecular formation. The only route of H2 formation is via n which is a mechanism that has been proposed in models of the chemistry of the early universe (e.g. Lepp and Shull 1984). 

Fig. 2.2 The state of the incipient solar system during the T Tauri phase of the young sun. The central region around the sun was blown free from the primeval dust cloud. Behind the shock front is the disc with the remaining solar nebula, which contained the matter formed by the influence of the solar wind on the primeval solar nebula. From Gaffey (1997)... 

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. 

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. 

Depletion of rare gases in Earth s atmosphere in comparison with cosmic abim dances suggests tliat any primary atmosphere captured at the planet s early ic cretion could have been lost by an impact with one or more large bodies dm mg the later stages of Ore accretion [66,74], and by T-tauri solar winds of high energy particles which could readily blow volatile elements out of the iimci Solar System [75],... 

Class III objects are weak line T Tauri stars (T Tauri stars in which the characteristic emission lines are only weakly observed in their optical spectra) and have little or no evidence of a disk. At this stage of solar nebula evolution, which may last between 3 and 30 Ma, the sun has formed, and the material of the nebula is being dissipated by solar winds in the inner part. In the outer part of the nebula material is dissipated by photo-evaporation caused by UV radiation from the solar wind. A positive pressure gradient near the inner edge of the nebula facilitates planetesimal formation. 

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. 

The chemistry in a dynamical model of a dark cloud has been explored in calculations by Charnley et al. (1988a). In this model, the wind of a T Tauri star blows a cavity in the molecular cloud, and impinges on clumps of dense molecular gas within the cavity. Close to the star, these impacts produce strong shocks in the wind and ionize it. The wind erodes the clumps and is mass loaded with atomic and molecular material. This mass loaded, partially ionized material is decelerated by a weak reverse shock at the cavity boundary, and accumulates there, becoming incorporated ultimately in a new clump from which a new star will form. In this model, the main chemical evolution takes place in this material at the cavity boundary the chemical development is truncated when heavy atoms and molecules strike and stick to grain surfaces. This accretion time is on the order of 10 /n yr, or 10 yr in dark clouds. Thus, the chemistry has available to it, post shock, about one million years. 

The study of the chemical and physical processes occurring in T Tauri outflows is of great importance as these stars have been proposed as the energy source of Herbig-Haro objects (Schwartz 1978 Canto 1978) and turbulence in molecular clouds (Norman and Silk 1980, Franco 1983). In this work, the physical model of the outflow is taken from Hartmann, Edwards and Avrett (1982) in which the primary stellar wind (i.e. wind that has not interacted with its environment) is ionized and heated by Alfven waves in the star s convection zone to reach a terminal velocity (of about 230 kms ) and a maximum temperature (of 20,000 K, cf. Hartmann et al. model no. 2) at z = 3 to 5, where z is the radial ordinate in units of stellar radii (r t 2 x 10 cm). Thereafter the wind expands and cools radlatively and adiabatically. Other parameters for the model are the initial wind density at z = 1 (oq lO - cm ), the density at z = 5 (n < 5 X 10 to 10 cm ) and the stellar photospheric temperature 4000 E). The cooling rate of the wind is obviously dependent on the physical conditions within the ejecta and in any case is by no means certain. Hartmann et al. suggest a... 

The ionization structure of the wind is also very important in determining the chemistry. Since the photospheric temperature is so low, the initial ionization is simply taken to be that of material in LTG at z = 3 to 5 where the ejecta temperature is at a maximum, so that the ionization is simply given by the balance of radiative recombination with the collisional ionization rates. Once the ejecta cools below about 15000 K, recombination occurs. For T Tauri ejecta temperatures and densities (at z = 10 where T 15000 K) this recombination will occur over a characteristic recombination length of Az 6. Thus the conditions within the ejecta are changing from being hot, dense and ionized to being relatively cool, diffuse and neutral. 

The lunar soil is an ideal repository for implanted solar wind elements, as are certain gas-rich meteorites. Deuterium is depleted relative to the terrestrial standard in these materials, the D H ratio of <3 X 10 being consistent with the hypothesis that D is converted into He in the proto-Sun. Ion probe mass spectrometry has been used to study Mg, P, Ti, Cr and Fe which are present to enhanced levels in lunar minerals, indicating an exposure age of approximately 6 X 1Q4 y. The isotopic data indicate that the light isotopes of a number of elements have been preferentially lost from lunar material because of volatilization by micrometeorites or solar wind bombardment. There is some indication, from a study of Ne in gas-rich meteorites, of a large solar flare irradiation during the early history of the Solar System, perhaps related to the T-Tauri phase of the Sun. 

Carr, Tokunaga, Najita, Shu (UCB), and Glassgold (NYU) have observed the CO V = 2-0 bandhead of sever YSOs with CSHELL. They have detected a very broad bandhead in the object WL16 and have fit the profile with a model of a rotating disk around the central star (Carr et aL 1993). Hamann (OSU), Simon (SUNY), and Carr have observed infrared [Fe lines to constrain the densities of YSO environments, trace extended YSO winds, and probe the red outfiow lobes. They have detected [Fe H] in one T Tauri object so far which appears to arise in a weU-coUimated, high velocity jet. 

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