Site Map
Contacts
Follow us on Facebook Follow us on Twitter YouTube channel
Centro de Astrofísica da Universidade do Porto

On the width of the theoretical lower main sequence.
Consequences for the determination of the ΔY/ΔZ ratio in the solar neighbourhood.

J. M. Fernandes, Y. Lebreton, A. Baglin

Abstract
We discuss the relation between the main sequence thickness and chemical composition variations. With the help of theoretical zero-age main sequence models between 0.7 and 1.0Msun, computed with an advanced physical description of the stellar interiors, in a range of metallicities typical from Population I stars (0.008<=Z<=0.034), we establish a theoretical relation between the relative helium to metal enrichment, ΔY/ΔZ, and the main-sequence broadening, represented in this work by ΔMbol. The locus in the Hertzsprung-Russell diagram of 69 low mass stars closer than 25pc is analyzed, taking into account the present available metallicities for 25 of them. Models show that ΔY/ΔZ should be higher than 2.0 in the solar neighbourhood to account for the observational main sequence width. As a consequence an helium abundance lower limit of Y=0.246 is found in the solar neighbourhood. This value is presently loosely constrained by the observational uncertainties. A new step towards the ΔY/ΔZ determination is expected in the very near future, when accurate values of the stellar luminosities as determined by Hipparcos will permit more definite conclusions.

Astronomy and Astrophysics
Volume 311, Page 127
July 1996

>> ADS

Institute of Astrophysics and Space Sciences

Institute of Astrophysics and Space Sciences (IA) is a new but long anticipated research infrastructure with a national dimension. It embodies a bold but feasible vision for the development of Astronomy, Astrophysics and Space Sciences in Portugal, taking full advantage and fully realizing the potential created by the national membership of the European Space Agency (ESA) and the European Southern Observatory (ESO). IA resulted from the merging the two most prominent research units in the field in Portugal: the Centre for Astrophysics of the University of Porto (CAUP) and the Center for Astronomy and Astrophysics of the University of Lisbon (CAAUL). It currently hosts more than two-thirds of all active researchers working in Space Sciences in Portugal, and is responsible for an even greater fraction of the national productivity in international ISI journals in the area of Space Sciences. This is the scientific area with the highest relative impact factor (1.65 times above the international average) and the field with the highest average number of citations per article for Portugal.

Proceed on CAUP's website|Go to IA website