Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994aas...185.9801p&link_type=abstract
American Astronomical Society, 185th AAS Meeting, #98.01; Bulletin of the American Astronomical Society, Vol. 26, p.1478
Astronomy and Astrophysics
Astronomy
Scientific paper
Solar-type stars at the zero-age main sequence (ZAMS) present a set of boundary conditions for theoretical models of stellar angular momentum and dynamo evolution. For pre-main sequence (PMS) stars, the ZAMS represents the end-point of evolution dominated by gravitational contraction and interactions with circumstellar accretion disks. For main sequence stars, the ZAMS represents a starting point where the subsequent evolution of the star is dominated by magnetic braking. Knowledge of ZAMS stellar parameters, such as rotation rates and activity levels, is therefore crucial for accurate theoretical model predictions. To this end, we have undertaken a program of ground- and space-based observations to measure photometric rotation periods and X-ray luminosities for low-mass stars in two southern clusters, IC 2391 and IC 2602. With cluster ages of 30-50 Myr, IC 2391 and IC 2602 are ideal laboratories in which to observe conditions at the ZAMS since the solar-type stars in these clusters have not been on the main sequence long enough to undergo significant magnetic braking. For the G and K dwarfs in IC 2391 we find a factor of ~ 10--20 spread in the distribution of X-ray luminosities. Among the IC 2391 and IC 2602 K dwarfs, we observe a factor of ~ 10 spread in the distribution of rotation periods. These results show conclusively that stars arrive on the ZAMS with a wide range of rotation rates and coronal activity levels. When compared to data from clusters like the Pleiades and the Hyades, we find there are stars in IC 2391 and IC 2602 with rotation rates and activity levels similar to those observed in the older clusters. These data show that no one empirical formula, such as the classic Skumanich t(-1/2) scaling law, can describe the time decay of dynamo activity or rotation rate for each and every star.
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