I. Tuominen — Page 2

arXiv2000

Study of FK Comae Berenices. II. Spot evolution from 1994 to 1997

We present new surface (Doppler) images of the late-type single giant FK Com for June-July 1996, July-August 1996, April 1997 and June 1997. These images are compared with the previously published images from 1994 and 1995. The consecutive maps are cross-correlated to see the possible migration of the spots and the effects of differential rotation. The cross-correlation confirms an average longitudinal spot migration of 0.22+/-0.03 in phase within a year. This movement is probably an artifact caused by a difference between the accepted rotation period and the real photometric period for these years. If this is true, then the photometric rotation period for these years is 2.4037+/-0.0005 days. Measurements from these four years and six maps limit the surface differential rotation to alpha=0.0001+/-0.0002, where alpha is the difference between polar and equatorial angular velocities relative to the equatorial angular velocity.

astro-ph↗

arXiv2000

Chemical composition of evolved stars in the open cluster M 67

High-resolution spectra of six core helium-burning `clump' stars and three giants in the open cluster M 67 have been obtained with the SOFIN spectrograph on the Nordic Optical Telescope to investigate abundances of up to 25 chemical elements. Abundances of carbon were studied using the C2 Swan (0,1) band head at 5635.5 A. The wavelength interval 7980-8130 A with strong CN features was analysed in order to determine nitrogen abundances and 12C/13C isotope ratios. The oxygen abundances were determined from the [O I] line at 6300 A. The overall metallicity of the cluster stars was found to be close to solar ([Fe/H]=-0.03+-0.03). Compared with the Sun and other dwarf stars of the Galactic disk, as well as with dwarf stars of M 67 itself, abundances in the investigated stars suggest that carbon is depleted by about 0.2 dex, nitrogen is enhanced by about 0.2 dex and oxygen is unaltered. Among other mixing- sensitive chemical elements an overabundance of sodium may be suspected. The mean C/N and 12C/13C ratios are lowered to the values of 1.7+-0.2 and 24+-4 in the giants and to the values of 1.4+-0.2 and 16+-4 in the clump stars. These results suggest that extra mixing of CN-cycled material to the stellar surface takes place after the He-core flash. Abundances of heavy chemical elements in all nine stars were found to be almost identical and close to solar.

astro-ph↗