4. The Vsini main features
In Fig. 6 we present the behaviour of rotation in the HR diagram for our sample stars, including the dwarfs and giants. We define several Vsini intervals as in Lèbre et al. (1999): Slow rotators correspond to Vsini 10 km:s -1 , moderate rotators to 10 km:s -1 Vsini 40 km:s -1 , and high rotators to Vsini 40 km:s -1 . Fig. 6 shows clearly the now well established rotational discontinuity along the subgiant branch near , here indicated by two arrows (Gray & Nagar 1985; De Medeiros & Mayor 1989, 1990). An interesting feature to discuss is the influence of stellar mass on the rotational discontinuity. First of all, one observes that all the subgiant stars with mass lower than about present Vsini values lower than 10.0 km:s -1 . Observations in young galactic clusters (see Gaigé 1993 and references therein) show that these low-mass stars actually acquire a slow rotation early on the main sequence, as explained by the magnetic braking scenario for main sequence stars (Kraft 1967; Schrijver & Pols 1993).
Before the rotational discontinuity, the subgiants with mass larger than present a broad range of Vsini values. Because their very thin surface convective envelopes are not an efficient site for magnetic field generation via a dynamo process, these stars are not expected to experience significant angular momentum loss during their main sequence evolution. This result is again in agreement with the data in open clusters.
As can be seen in Table 2 (see also Fig. 4), the effective temperature at which the convective envelope starts to deepen depends slightly on the stellar mass. We also give the depth of the convective envelope at the effective temperature of the rotational discontinuity, . For the masses lower than , the observed rotation discontinuity occurs just when the convective envelope starts deepening. We thus see that if the magnetic braking plays a relevant role in the rotational discontinuity, it requires only a very small change in the mass of the convective envelope. Above , our sample is very sparse (due in particular to the very rapid evolution of such stars in the Hertzsprung gap) and we have no data for single stars on the left of the rotation discontinuity exhibited by lower stellar masses. We cannot thus discuss further the impact of the deepening of the convective envelope on the braking in these more massive stars.
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000