In this work we have analyzed the Li and rotation observations for 120 subgiant stars from Lèbre et al. (1999). We used the HIPPARCOS trigonometric parallax measurements to locate precisely our objects in the HR diagram and to determine the individual mass and evolutionary status for all stars in the sample.
We have compared observed Li abundances with predictions of Li dilution caused by the deepening of the convective envelope on the subgiant branch. Our models show that the beginning of the theoretical lithium dilution is a function of stellar mass and coincides with the observational features. Stars with masses show a large range in abundance before the turnoff, indicating lithium depletion in the previous phases. Stars with masses between 1.2 and (i.e., in the dip region) show ALi values in agreement with what is found in the open clusters. We note that many stars with masses higher than show lithium depletion up to two orders of magnitude before the start of the dilution at Log . The process that depletes Li in these objects while on the main sequence should however preserve the boron abundance, which is in agreement with the standard dilution predictions in the underabundant Li giants of the Hyades, as shown recently by Duncan et al. (1998). Because these specie burn at different depths than lithium, future observations of boron for our sample will provide powerful additional constraints and will confirm the similarity between field and cluster evolved stars.
Our analysis confirms that low mass stars leave the main sequence with a low rotational rate, while more massive stars are slowed only when reaching the subgiant branch. A very slight increase of the depth of the convective envelope seems to be sufficient for the magnetic braking to take place at this phase. Even if the decrease in the behavior of Lithium is nearly the rotational discontinuity, our interpretation of the observations shows that lithium and rotation discontinuities are independent.
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000