Rotational Velocities and Ages of Solar-Type Stars.
Abstract
The sun and other main-sequence stars of similar mass are known to rotate much more slowly than more massive main-sequence stars. This transition from rapid to slow rotation is coincident with the onset of convective energy transport in the cooler, less massive stars. Also, the cooler stars show evidence of chromospheric activity that is generally lacking in the hot stars. Few good measurements of rotational velocities (v sin i's) exist for solar-type stars because of the difficulty of the observations. This study was undertaken to provide the basic data needed to discuss the rotational history of solar-type stars and the relation of angular momentum loss to other physical phenomena. High resolution spectral line profiles were obtained for 82 stars and have been modeled to yield v sin i's. The available Li data hae been reanalyzed to derive an age calibration for field stars. The Li--age relation indicates that 1 M(,(CIRCLE)) stars have undergone pre-main-sequence Li depletion by a factor of 2 to 5. The Li data for G2V stars are consistent with a uniform initial Li abundance. There are stars hotter than 6200(DEGREES)K that have depleted Li by a mechanism different from that in 1 M(,(CIRCLE)) stars. The field star and cluster data indicate <v sin i> (PROPORTIONAL) t('- 1/2) (t = age), a relation that has been predicted from models of angular momentum loss through a stellar wind forced to corotate by a magnetic field. Observations of very young stars indicate that the sun could not have been rotating as rapidly as an extrapolation of this relation suggests, thus very young 1 M(,(CIRCLE)) stars lose angular momentum less efficiently than main -sequence stars. Chromospheric Ca II K emission is well correlated with rotation. Stars cooler than 6000(DEGREES)K show much stronger K emission than somewhat hotter stars with the same rotational velocity. There is no significant difference in the rotational properties of stars in the interval F8V to G2V, despite the very different rates at which these stars deplete Li. 1 M(,(CIRCLE)) stars in the Hyades cluster rotate faster than field stars of their age, as do higher-mass Hyades stars. The distribution of v sin i's about the mean for all ages considered together is consistent with a Maxwell-Boltzmann distribution of true rotational velocities. The rms deviation about the mean is 0.30 of the mean. The sun's rotation is 0.77 <v> for its age, and thus is within 1 s.d. of the mean, so that the sun does not have significantly less angular momentum than a typical 1 M(,(CIRCLE)) star of its age. Rapid rotation does not enhance Li depletion, nor is it likely that it enhances the mass loss rate or the Alfven radius. It is possible, but not certain, that slow rotation inhibits Li depletion.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1980
- Bibcode:
- 1980PhDT........17S
- Keywords:
-
- Physics: Astronomy and Astrophysics;
- Angular Momentum;
- Stellar Evolution;
- Stellar Rotation;
- Velocity Measurement;
- Line Spectra;
- Lithium;
- Solar Rotation;
- Stellar Mass;
- Astrophysics