Stars young

Class II objects are T Tauri stars, young stars with a cool surface but high luminosity, which lie above the Main Sequence trend on a H-R diagram, and are identified as protostars on Fig. 2.4. These stars retain well-developed... 

Key words near infrared - stars - young stdlar objects - T Tbnri - binary... 

Figure 25. First light image of the Keck LGS AO system. The lens-like nebula at upper left is a disk of dust and gas surrounding the young star HK Tau B, The star is hidden from direct view, seen only in light reflected off the upper and lower surfaces of the disk.

Abstract. The astrophysical origins of the element fluorine remain uncertain due in part to the availability of just a small number of abundance results for this element, that has readily observable transitions only in the infrared via vibration-rotation lines of HF. In this paper, we discuss all the available Galactic fluorine abundances to date, and add results for field stars with metallicities between [Fe/H] = -0.5 and -1.0, plus two stars that are members of the Orion association. The fluorine abundances obtained for the young Orion members are found to be in agreement with the trend of [F/O] versus O observed for the disk and they are a good representation of the present day value in the Galactic disk. 

Fig. 1. The Galactic fluorine abundances obtained to date. Three samples are represented the disk of the Milky Way (crosses), including the two young Orion pre-main-sequence stars (open circles), and u> Centauri giants (filled circles).

Our analysis allows also to estimate the incidence of (3 Cephei stars in the LMC and SMC. They constitute of about 0.3% in the LMC, that is an order of magnitude more than in the SMC and about two orders less than in Galactic young clusters. 

Abstract. In this review I briefly discuss the theory of pre-main-sequence (PMS) Li depletion in low-mass (0.075 < M < 1.2 M ) stars and highlight those uncertain parameters which lead to substantial differences in model predictions. I then summarise observations of PMS stars in very young open clusters, clusters that have just reached the ZAMS and briefly highlight recent developments in the observation of Li in very low-mass PMS stars. 

Theory doesn t tell us what initial Li a star has, only what depletion it suffers. An accurate estimate of the initial Li abundance is therefore a pre-requisite before observations and models can be compared. The Sun is a unique exception, where we know the present abundance, A(Li) = 1.1 0.1 (where A(Li)= log[AT(Li)/AT(H)] + 12) and the initial abundance of A(Li)= 3.34 is obtained from meteorites. For recently born stars, the initial Li abundance is estimated from photospheric measurements in young T-Tauri stars, or from the hotter F stars of slightly older clusters, where theory suggests that no Li depletion can yet have taken place. Results vary from 3.0 < A(Li) < 3.4, somewhat dependent on assumed atmospheres, NLTE corrections and TeS scales [23,33]. It is of course quite possible that the initial Li, like Fe abundances in the solar neighbourhood, shows some cosmic scatter. Present observations certainly cannot rule this out, leading to about a 0.2 dex systematic uncertainty when comparing observations with Li depletion predictions. 

The originally proposed hypothesis that there might have been more than an episode of star formation within M 67 and that, accordingly, Li-rich/poor cluster stars might represent the young/old population ([10]) can be excluded, since we now know that Li at old ages is not necessarily low (see Fig. 1). The scatter in M 67 indeed reinforces the conclusion that at least one further parameter besides age and mass drives Li depletion, the possible additional parameters being the presence of planets, chemical composition ([10], [14]) and rotation and/or rotational history ([9]). 

If the scatter in Li is due to a dispersion in the initial rotational properties, one would expect that a dispersion is observed in all old clusters, unless they had a different initial distribution of rotation the latter hypothesis is rather unlikely, due to the fact that very similar distributions of rotational properties are currently derived for young clusters. As mentioned above (see also [23]), neither the three 2 Gyr clusters, nor NGC 188 show a significant star-to-star scatter. Although in all the four clusters fewer stars than in M 67 have been observed, both [18] and [22] provide convincing arguments that the lack of a scatter in these clusters is not due to low number statistics. 

How much the ages of young PMS object depend on the starting stellar evolution model Baraffe et al. (2002) show that the first million year(s) are very uncertain for low mass stars (the statement refers to masses O.IMq or smaller), as they... 

The basic energy source in young stars is the fusion of protons into the 4He nucleus in three steps. This process is occurring in our Sun and other main-sequence stars where it produces about 91 per cent of the total energy. The proton-proton cycle proceeds via three reaction steps ... 

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