Main sequence

The primary structure of a peptide is given by its amino acid sequence plus any disulfide bonds between two cysteine residues. The primary structure is detemnined by a systematic approach in which the protein is cleaved to smaller fragments, even individual amino acids. The smaller fragments are sequenced and the main sequence deduced by finding regions of overlap among the smaller peptides. 

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... 

More massive stars in the upper part of the main-.sequence diagram (i.e. star.s with masses in the range 1.4-3.5 M ) have a somewhat different hi.story to that considered in the preceding sections. We have seen (p. 6) that such stars consume their hydrogen much more rapidly than do smaller stars and hence spend less... 

This relationship has been predicted by the stellar evolution theory, as resulting essentially from the so called first and second dredge-up episodes. In Fig. la we see the corresponding predictions by the theory (Marigo, 2001). The sensitivity of the predicted abundances both to the metallicity at the epoch of formation of the progenitors and to their stellar mass in the Main Sequence is evident, making such diagrams powerful tools to trace back the evolutionary history of individual PNe. Uncertainties prevent however us to pursue now this... 

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. 

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. 

Fig. 10 from our study of a large sample of stars in M5 (Cohen, Briley Stetson 2002) (not reproduced here due to limits of length) is typical of the GCs studied thus far in such detail. It shows a strong anti-correlation between C and N abundances, i.e. conversion of C into N, with strong-to-star variations in derived C and N abundances seen at all luminosities probed. (This sample contains mostly stars at the base of the RGB and just below the main sequence turnoff,... 

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. 

The difference in the Li abundances in the G-stars of the Pleiades and the Sun, combined with the probable similarities in their overall chemical composition tell us that PMS Li depletion cannot be the whole story. Another mechanism, additional to convective mixing, must be responsible for Li depletion whilst solar-type stars are on the main-sequence. Recent PMS models that have their convective treatments tuned to match the structure of the Sun reproduce the mass dependence of Li depletion, but deplete too much Li compared with the Pleiades, and can even explain the solar A (Li) in the case of full spectrum turbulence models [9]. The over-depletion with respect to the Pleiades gets worse at lower masses. Better fits to the Pleiades data are achieved with PMS models that feature relatively inefficient convection with smaller mixing lengths. 

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. 

Abstract. We report on preliminary results of VLT/FLAMES observations of the old open clusters NGC 2506, Mel 66 and Cr 261, obtained as part of our Guaranteed Time on this instrument. We focus in particular on the very old cluster Cr 261, one of the oldest open clusters in the Galaxy. We compare the derived Li abundances with those of other old clusters, and we discuss briefly Li depletion on the main-sequence from the age of the Hyades to 8 Gyr. 

S. Randich Mixing on the main sequence lithium and beryllium in old open clusters . This volume. 

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]). 

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