Formulario de Uso de Recursos –FUR–

The quantity o f th e o r e tic a l work published on the stru ctu re and evolution!' o f raain-sequence sta rs is con sid erab le. For a wider d iscu ssio n o f

th e o r e tic a l and ob servation al r e su lts the reader is referred to review papers by Iben (1967, 1974) and referen ces th erein .

E xtensive c a lib ra tio n s o f evolutionary tracks for hydrogen-burning phases have been made most recen tly by Mengel, Sw eigart, Demarque and Gross

(1979) and Hej'.lesen (1980), These ca lib ra tio n s are u sefu l both as to o ls for the study o f ob servation al data for s ta r -c lu s te r s (Iben, 1967) and

(in the current study) as comparison models for the in v e stig a tio n o f new input p h y sics, P ersian , Refsdal and S ta b e ll(1974) and S ta b ell (1975) in v e stig a te

the in flu en ce of the op acity values and m ixing-length on main-sequence s ta r s , f fin d in g that 1) increased o p a c itie s anywhere in a main-sequence sta r lead to a decreased lum inosity and e ff e c tiv e temperature, e s p e c ia lly when the op acity in crease occurs in the h o tter (lo g T >6.5) part o f the sta r ,

2) the slope o f the main-sequence is unaffected by changes to the op acity data, but is in flu en ced by the ra tio o f the mixing length to pressure sc a le h eigh t t/Hp , 3) an in crease in the op acity throughout the sta r leads to

an in crease in the evolutionary tim escale (for population II s t a r s ) , but the change in the ZAMS p o sitio n cancels out th is e f f e c t when isochrones are co n stru cted ,, 4) the value o f does in flu en ce the p o sitio n o f the isochrones leading to an uncertainty in the estim ated age o f MIS, based on the p o sitio n o f the main-sequence tu rn -o ff p o in t, o f approximately 10% due to the uncertainty in the value o f the m ixing-length alon e.

In the fo llow in g sectio n s we address three asp ects o f s t e lla r stru ctu re and evolu tion in main-sequence sta r s ,

1) Stothers (1974a) compares the stru ctu re of ZAMS models based on the Cox-Stevart (1965) and Carson o p a c itie s . He uses com positions

(Xe,2!e ) - (0, 739, 0,021) and ( Xcj ^ ) * (0,602, 0.044) for models w ith

" ^ ' '

1

masses ^ 120. D ifferen ces between models based on the two s e ts of o p a c itie s are found to be sim ila r to those produced by choosing a d iffe r e n t in terp o la tio n procedure in the op acity ta b le s. Assuming that the ‘ true' | op acity li e s between the Cox-Stewart and Carson o p a c itie s , th is r e s u lt

im plies that any s ig n ific a n t uncertainty in the stru ctu re of the ZAMS is not due to errors in the o p acity. We re-examine th is claim , and extend the - study to lower m e t a llic it ie s and lower m asses, comparing the op acity

uncertainty w ith the uncertainty in the treatm ent o f convective envelopes. I 2) In view o f the s e n s it iv it y o f hydrogen-burning s h e lls to the op acity tj found for h orizon tal branch sta rs in chapter 4 we re-examine the ev olu tio n o f main-sequence sta rs using the Carson o p a c itie s . : |

3) . Jimenez and G arcia-Pelayo (1982) re-examine observations o f the

ap sid al motion constant k, for a large number o f e c lip s in g binary system s. ;h

They fin d a co rr ela tio n between changes in and th e.su rfa ce gravity ^ during m-s evo lu tion which in d ic a te s a marginal preference for th e o r e tic a l values o f for models constructed w ith the Carson o p a c itie s . We ca lcu la te

during evolutionary sequences for a larger range in mass and com position | than h ith erto attem pted in order to assess these r e s u lts .

In the course o f these stu d ies a number o f featu res w ith important consequences for the theory o f hydrogen-burning sta rs have been e sta b lish e d but for which time is not a v a ila b le for a f u l l in v e s tig a tio n to be undertaken^ These featu res are noted at the appropriate p oin ts in the follow in g d iscu ssiop

144

5 .2 ZERO-AGE MAIN-SEQUENCE MODELS 1

The zero-age main-sequence models have been ca lcu la ted w ith the Henyey % code described in chapter 2, and w ith the input physics described in chapter 3i;

The zero-age sta r is assumed to have a homogeneous com position and the CNO | abundances in hydrogen-burning zones are assumed to be in equilibrium . The | com positions and m ixing-lengths used are described w ith the models in tab le 5.2

In an attem pt to determine the e f f e c t of using the Carson o p a c itie s in the zero-age m odels, the S tellin g w erf (1975) op acity formula was used to