Main sequence stars

In fact, the sun is not a first-generation main-sequence star since spectroscopic evidence shows the presence of many heavier elements thought to be formed in other types of stars and subsequently distributed throughout the galaxy for eventual accretion into later generations of main-sequence stars. In the presence of heavier elements, particularly carbon and nitrogen, a catalytic sequence of nuclear reactions aids the fusion of protons to helium (H. A. Bethe... 

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).

We are currently working on detached systems in the old open clusters NGC 2243, NGC 188 and NGC 6791. Presently we have the most complete and best data available for NGC 188 (see Fig. 1 and caption for more details) and NGC 2243. In both clusters the detached system is located close to the cluster turnoff and consists of two quite similar stars. In NGC 6791 we were able to determine the period for the system called V20 [1] and have subsequently obtained photometry for this system covering both eclipses. Due to the faintness (V = 17.34) we have not yet obtained radial velocities for the system which is comprised of a star very close to the cluster turnoff and a main-sequence star approximately 2.2 magnitudes (V) fainter. 

From observations of 11 main-sequence stars belonging to the Galactic halo, Spite Spite [27] concluded that the lithium abundance was essentially independent of metallicity for halo stars hotter than 5600 K, and inferred that the Li abundance was hardly altered from the Big Bang. Two decades of work has followed, increasing the number of stars observed and the range of metallicity that they span, in an effort to establish the primordial Li abundance more securely. 

Massive stars play an important role in numerous astrophysical contexts that range from the understanding of starburst environments to the chemical evolution in the early Universe. It is therefore crucial that their evolution be fully and consistently understood. A variety of observations of hot stars reveal discrepancies with the standard evolutionary models (see [1] for review) He and N excesses have been observed in O and B main sequence stars and large depletions of B accompanied by N enhancements are seen in B stars and A-F supergiants [2,3,4,5], All of these suggest the presence of excess-mixing, and have led to the development of a new generation of evolutionary models which incorporate rotation (full reviews in [1], [6], [7]). 

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... 

The B-N object may be considered to be a zero-age main sequence star that evolves with increasing surface temperature and luminosity at optical wavelengths. The descent from the right-hand upper quarter of the HR diagram, along with what has been called the Hayashi track or birth lines, precedes the entrance onto the... 

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 ... 

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... 

Main sequence star The majority of stars, 92 per cent, that fall on the leading diagonal of the Herzprung-Russell diagram. 

Fig. 4.12. Stellar lithium abundances (log of the number per 1012 H atoms) among main-sequence stars as a function of metallicity. The full-drawn curve shows the prediction of a numerical Galactic chemical evolution model, while the broken-line curve gives the sum of a primordial component and an additional component proportional to iron and normalized to meteoritic abundance. Adapted from Matteucci, D Antona and Timmes (1995).

At sufficiently high densities (e.g. cores of upper main-sequence stars), the > sign virtually becomes an equality (adiabatic stratification), but at lower densities (e.g. envelopes of the Sun and cooler stars) an exact calculation is very difficult and in most models a crude approximation based on mixing-length theory is used. In a situation where the chemical composition changes with depth, Eq. (5.24) (known as the Schwarzschild criterion) needs to be replaced by more complicated considerations. 

Fig. 5.1. Opacity of stellar material with X = 0.7, Z = 0.02 (roughly solar composition) as a function of temperature and the parameter log R, where R = p/T is approximately constant throughout a main-sequence star, corresponding to a polytrope with n = 3 (see Appendix 4) e.g. at 1 M0 log/ varies from —1.5 at the centre to 0.0 in the envelope, while at 10 M0 the corresponding range is from -3.5 to -4.0. (Density in units of gmcm-3, T(, in units of 106K, opacity in units of cm2 gm-1.) OP and OPAL refer to two independent opacity calculation projects. After Badnell, Bautista, Butler et al. (2005).

Table 5.1. Homology relations for main-sequence stars...

The synthesis of helium follows a somewhat indirectpath. The longest part is the first, because it involves the transformation of a proton into a neutron, and such transmutations proceed via the weak interaction (slow at these temperatures). The leisurely pace of these reactions confers long life upon main-sequence stars. 

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. 

What are the main nuclear reactions that power main sequence stars What happens when this energy source is exhausted ... 

Basu, S. and Rana, N. C. (1992) Multiplicity-corrected mass function of main-sequence stars in the solar neighborhood. Astrophysical Journal, 393, 373-384. 

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. 

Abstract Although once it was thought that main-sequence stars are remarkably homogeneous with respect to their chemical composition, the upper main-sequence stars (30000 > Te > 7000) show a variaety of chemically peculiar stars besides the so-called normal stars. Those include the Am, Ap, A Bootis, He-deficient, and He-rich stars. This review summarizes the current data, which are necessary to construct and test the theoretical models of these stars. In the second half of the review we concentrate on Li. In the lower main-sequecnce stars abundances of Li have been determined in hundreds of stars. Some of the remarkable results are ... 

ABSTRACT Constraints that abundance anomalies observed on main sequence stars put on turbulence, meridional circulation and mass loss are reviewed. The emphasis is on recent observations of Li abundances. 

Fig. 2 Meridional circulation stream lines (full lines) as a function of the angle from the polar axis. The position of convection zones is indicated by dotted lines. That identified by A is for a 12000 R main sequence star while that indicated by B is for a 6400 K star.

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