On the behavior stellar rotation in the solar neighbourhood

de Freitas Daniel Brito, danielbrito@dfte.ufrn.br, DFTE/UFRN
De Medeiros Jose Renan, DFTE/UFRN

Abstract
Different studies point for a rotation-age connection following a {\it v}$_{rot}$ $\propto$ t$^{\alpha}$ relationship. The value of the $\alpha$--parameter has a strong role in the evolutionary behaviour of rotation because it indicates how strong the spindown is once stars evolve. The present study aims at improving this picture on the basis of an enlarged analysis taking into account the role of mass, metallicity or galatic latitude on the rotation--age relationship, based on an unprecedented sample of about 8200 field F and G--type dwarf stars in the solar neighbourhood. We found that, irrespective of any physical parameters, the decay of rotation with age for solar--type field dwarf stars in the solar neighbourhood qualitatively parallels the results obtained in previous studies for solar--type stars in stellar clusters, following a power-law decay {\it v}$_{rot}$ $\propto$ t$^{\alpha}$. Nevertheless, the $\alpha$--exponent clearly depends on mass, indicating that the rotation-age relationship is not continuous. We also observed that the power-law decay laws proposed so far are not able to match the bulk of the data of rotation versus age in the present sample of field F and G solar--type dwarf stars. Futhermore, we found that field G--type dwarf stars in the solar neighbourhood rotate faster than the Sun, with an average rotational velocity about two times greater than the solar rotational velocity. Finally, comparing the average of the rotational velocity $<${\it v} sin{\it i}$>$ of F and G--type dwarf stars in the present sample and in young stellar open clusters, we found that the stars in the field rotate faster than those in open clusters, with such a discrepancy increasing for the younger clusters.