Pre-main sequence

The current status of HF abundances from infrared spectroscopy in samples of red-giants from different Galactic stellar populations are summarized in Figure 1. The abundance results displayed in this figure are from Cunha et al. (2003), plus new results for stars at the lowest metallicities, as well as two Orion pre-main-sequence stars. The run of fluorine with metallicity is now probed between oxygen abundances from roughly 7.7 to 8.7. 

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).

Abstract. We use intermediate resolution (II, 19 300) spectroscopic observations in the spectral region including the Li 6708 A line to study 341 stars in the star forming region (SFR) NGC 6530. Based on the optical color-magnitude diagrams (CMD), they are G, K and early M type pre-main sequence (PMS) cluster candidates. 72% of them are probable cluster members since are X-ray sources detected in a Chandra-ACIS observation ([2]). We use our spectroscopic measurements to confirm cluster membership by means of radial velocities and to investigate the Li abundance of cluster members. 

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. 

R.J. Jeffries Pre-main-sequence lithium depletion . In This volume. 

R.D. Jeffries Pre main sequence mixing lithium in young open clusters . This Vol. 

Clarifying Problems in the Description of Pre Main Sequence Evolution... 

What observers would like to understand when they are in front of the numerous recent sets of pre main sequence tracks is -or should be- the following ... 

The uncertainty in the age of pre main sequence stars is therefore of the order of the thermal timescale at the luminosity of D-burning smaller than a few times 105 yr for normal T Tauri, and larger than 106 yr for very low mass stars and brown dwarfs (BD). In fact, comparing observations spanning a wide range of masses we could even constrain the models, for example we can ascertain whether the Stahler et al. (1986) picture of collapse is valid also in the BD regime, or... 

Generally convection is calibrated by requiring that its free parameter(s) are chosen to reproduce the solar radius at the solar age. However, it is also possible to use models which do not fit the solar location, and in fact these seem to reproduce much better two observational constraints of the pre main sequence, namely the PMS Lithium depletion and the HR diagram location of some binary stars for which masses are known. 

Low (<1 solar mass) Middle (5-10 solar masses) High (>20 solar masses) Protostar — pre-main sequence — main sequence — red giant — planetary nebula — white dwarf — black dwarf Protostar - main sequence — red giant — planetary nebula or supernova —> white dwarf or neutron star Protostar — main sequence —> supergiant — supernova — neutron star... 

The birth of a protostar and its life as a pre-main-sequence star, its descent to the main sequence and death, starting with a red giant leading to planetary nebula and ending in white and black dwarfs. This sequence varies with mass... 

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. 

As stars become older, lithium at their surface becomes gradually depleted by mixing with deeper layers at temperatures above 2.5 x 106 K where it is destroyed by the (p, a) reaction, Eq. (4.49). This destruction takes place more rapidly in cooler stars with deeper outer convection zones, so that there is a trend for lithium abundance to decrease with both stellar age and diminishing surface temperature in cooler stars some depletion takes place already in the pre-main-sequence stage. Thus, in the young Pleiades cluster ( 108 yr), lithium has its standard abundance down to Teff = 5500 K (type G5), whereas in the older Hyades cluster ( 6 x 108yr) it is noticeably depleted below Tc t = 6300 K (F7) and also in... 

Fig. 5.2. Pre-main-sequence contraction of stars with different masses. Adapted from Iben (1965).

Tracks followed by 1,3, and 5 M pre-main sequence stars as they evolve toward the main sequence are shown on an H-R diagram. Pre-main sequence stars shine primarily by conversion of gravitational potential energy to heat, although energy released by burning of deuterium and other elements also plays a role. 

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. 

Itis ofinterestthatLi is easily destroyed by (p, alpha) reactions atT = 2.5 MK or greater, so that when newly born stars arrive on the Main Sequence, they have already destroyed Li throughout most of their interiors by these nuclear interactions in the deep convection of the pre-Main-Sequence evolution. The observed Li has survived only in the outermost few percent by mass of the interior of the stars. 

Class II. Sources with spectral index —1.5 < cyir < 0. These are pre-main-sequence stars with observable accretion discs (classical T Tauri stars). 

FU Orionis star (FU Ori-type star or Fuor) a pre-main-sequence star that recently underwent an extreme brightening at visual wavelengths, typically of five magnitudes or more. They are named after their prototype FU Orionis. These stars rapidly brighten, then remain almost steady or slowly decline by a magnitude or two over a period of decades. It is proposed that the brightening is driven by a runaway disk accretion, a result of an instability. 

The fact that the emission from H20 and OH comes from many different but very intense spots (1 — 100 AU in linear dimensions) separated by distances of about 10,000 AU lends support to the suggestion that this radiation is emitted from massive protostars in their pre-Main Sequence adiabatic contraction state (Mezgerand Robinson, 1968 Mezger, 1971). 

Fig. 4. The pre-main sequence evolution of 0.5 to 1.5 solar mass stars in the Hertzsprung-Russell diagram. The lines represent theoretical evolutionary tracks, while the dots represent observational data for T Tauri stars (Cohen and Kuhi, 1979). The corresponding ages and ratios of UV flux to present solar UV flux are indicated for a solar-mass star.

For a protostar evolving to the main sequence, theoretical models can be used to predict a few observational quantities such as surface temperature and luminosity. However, the models are not sufficient to predict accurately the ultraviolet flux emitted by a pre-main sequence star. Thus we employ ultraviolet observations of young solar-like stars as the best estimate of the UV flux emitted by the young sun. 

Eeigelson E. D., Garmire G. P., and Pravdo S. H. (2002b) Magnetic flaring in the pre main-sequence sun and implications for the early solar system. Astrophys. J. 572, 335-349. 

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