A Constraint on Zsolar from Fits of Isochrones to the Color-Magnitude Diagram of M67

• The mass at which a transition is made between stars that have radiative or convective cores throughout the core H burning phase is a fairly sensitive function of Z (particularly, the CNO abundances). As a consequence, the ~4 Gyr, open cluster M67 provides a constraint on Zsolar (and the solar heavy-element mixture) because (1) high-resolution spectroscopy indicates that this system has virtually the same metal abundances as the Sun, and (2) its turnoff stars have masses just above the lower limit for sustained core convection on the main sequence. In this study, evolutionary tracks and isochrones using the latest MARCS model atmospheres as boundary conditions have been computed for 0.6-1.4 Msolar on the assumption of a metals mix (implying Zsolar~0.0125) based on the solar abundances derived by M. Asplund and collaborators using 3D model atmospheres. These calculations do not predict a turnoff gap where one is observed in M67. No such difficulty is found if the analysis uses isochrones for Zsolar=0.0165, assuming the Grevesse and Sauval mix of heavy elements. Our findings, like the inferences from helioseismology, indicate a problem with the abundances of Asplund and collaborators. However, it is possible that low-Z models with diffusive processes taken into account will be less problematic.

Subject headings

NATURVETENSKAP  -- Fysik -- Astronomi, astrofysik och kosmologi (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Astronomy, Astrophysics and Cosmology (hsv//eng)

Keyword

Stars: Hertzsprung-Russell Diagram

Galaxy: Open Clusters and Associations: Individual: Messier Number: M67

Stars: Atmospheres

Stars: Evolution

Sun: Abundances

Astronomy and astrophysics

Astronomi och astrofysik

Publication and Content Type

ref (subject category)

art (subject category)