Surface Oscillations in Overdense Plasmas Irradiated by Ultrashort Laser Pulses

The generation of electron surface oscillations in overdense plasmas irradiated at normal incidence by an intense laser pulse is investigated. Two-dimensional (2D) particle-in-cell simulations show a transition from a planar, electrostatic oscillation at $2\omega$, with $\omega$ the laser frequency, to a 2D electromagnetic oscillation at frequency $\omega$ and wavevector $k>\omega/c$. A new electron parametric instability, involving the decay of a 1D electrostatic oscillation into two surface waves, is introduced to explain the basic features of the 2D oscillations. This effect leads to the rippling of the plasma surface within a few laser cycles, and is likely to have a strong impact on laser interaction with solid targets.Comment: 9 pages (LaTeX, Revtex4), 4 GIF color figures, accepted for publication in Phys. Rev. Let

New rotation periods in the Pleiades: Interpreting activity indicators

We present results of photometric monitoring campaigns of G, K and M dwarfs in the Pleiades carried out in 1994, 1995 and 1996. We have determined rotation periods for 18 stars in this cluster. In this paper, we examine the validity of using observables such as X-ray activity and amplitude of photometric variations as indicators of angular momentum loss. We report the discovery of cool, slow rotators with high amplitudes of variation. This contradicts previous conclusions about the use of amplitudes as an alternate diagnostic of the saturation of angular momentum loss. We show that the X-ray data can be used as observational indicators of mass-dependent saturation in the angular momentum loss proposed on theoretical grounds.Comment: 24 pages, LaTex (AASTeX); includes 8 postscript figures and 4 Latex tables. To appear in ApJ, Feb. 1, 1998. Postscript version of preprint can be obtained from http://casa.colorado.edu/~anitak/pubs.htm

Deep MMT Transit Survey of the Open Cluster M37 III: Stellar Rotation at 550 Myr

In the course of conducting a deep (14.5 ~< r ~< 23), 20 night survey for transiting planets in the rich ~550 Myr old open cluster M37 we have measured the rotation periods of 575 stars which lie near the cluster main sequence, with masses 0.2 Msun ~< M ~< 1.3 Msun. This is the largest sample of rotation periods for a cluster older than 500 Myr. Using this rich sample we investigate a number of relations between rotation period, color and the amplitude of photometric variability. Stars with M >~ 0.8 Msun show a tight correlation between period and mass with heavier stars rotating more rapidly. There is a group of 4 stars with P > 15 days that fall well above this relation, which, if real, would present a significant challenge to theories of stellar angular momentum evolution. Below 0.8 Msun the stars continue to follow the period-mass correlation but with a broad tail of rapid rotators that expands to shorter periods with decreasing mass. We combine these results with observations of other open clusters to test the standard theory of lower-main sequence stellar angular momentum evolution. We find that the model reproduces the observations for solar mass stars, but discrepancies are apparent for stars with 0.6 ~< M ~< 1.0 Msun. We also find that for late-K through early-M dwarf stars in this cluster rapid rotators tend to be bluer than slow rotators in B-V but redder than slow rotators in V-I_{C}. This result supports the hypothesis that the significant discrepancy between the observed and predicted temperatures and radii of low-mass main sequence stars is due to stellar activity.Comment: Replaced with version accepted to ApJ. 104 pages, 7 tables, 26 figure