T Tauri stage

We may thus put constraints from chondrules on the presence of dust and gas 2 Myr after the first solid formation. Again, it is not possible at present to correlate directly the chondrule formation period with the evolutionary stage of the protoplanetary disk, but high-temperature events capable of melting chondrule precursors could have occurred during the active stage of the protoplanetary disk (classical T Tauri stage). 

As the star approaches the Main Sequence, it enters one of the most dramatic periods of its lifetime, the T-Tauri stage. The name T-Tauri comes from the first star observed that exhibited the events associated with this stage of stellar evolution. T-Tauri stars eject large proportions of their mass in violent bursts. 

Artistic rendering of four observed stages of star formation, (a) Class 0 object a deeply embedded hydrostatic core surrounded by a dense accretion disk. Strong bipolar jets remove angular momentum, (b) Class I object protostar in the later part of the main accretion phase, (c) Class II object or T Tauri star pre-main-sequence star with optically thick protoplanetary disk, (d) Class III object or naked T Tauri star star has an optically thin disk and thus can be directly observed. Some may have planets. 

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. 

The initial evolutionary stages (Classes 0 and I) are optically hidden by the dust of the collapsing envelope. These phases can be only observed in the far-infrared. Optically visible are the Class II and III objects, that form the long-known class of T Tauri stars. They have reached almost their final mass ( 90% see Beckwith et al. 1990) at the transition from Class I to Class II, which occurs for solar-mass stars approximately 2 x 105 yr after the onset of collapse (see Table 2.2). 

T Tauri stars very young, low-mass stars, less than 10 million years old and under 3 solar-masses, that are still undergoing gravitational contraction. T Tauri stars represent an intermediate stage between a protostar and a low-mass main-sequence star like the Sun. The prototype for this class of stars is T Tau. 

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. 

Once the core temperature is high enough (hydrogen and helium must be fully ionized) the opacity ( bf+Kes) fall sufficiently for a radiative core to form. At this point the star contracts at constant L towards the main-sequence, and the convective envelope shrinks. By this stage, the stars have mostly emerged from their dusty cocoon and become visible, low-mass stars as T Tauri stars and intermediate mass stars as Herbig Ae/Be stars. 

If all solar-type stars are indeed monbers of multiple star systems by the time they readi the T Tauri star stage of evolution, rou y 2 — 3 X 10 years (Simon et al. 1993), then multiple star systems are the main product of star formation. Such systems must take form early in the star s lifetime as opposed later via a mechanism such as capture. This leads our attention towards multiple star formation mechanisms such as core fragmentation (e.g.. Boss 1993 BonneU and Bastien 1993) or disk fragmentation (e.g., Adams, Ruden, and Shu 1989). To distinguish between these models it is necessary to determine the masses and es of the components in multiple star systons. 

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