STELLAR ROTATION IN M35: MASS-PERIOD RELATIONS, SPIN-DOWN RATES, AND GYROCHRONOLOGY

Abstract

We present the results of a 5 month photometric time-series survey for\nstellar rotation over a 40'x40' field on the 150 Myr open cluster M35. We\nreport rotation periods for 441 stars and determine their cluster membership\nand binarity based on a decade-long radial-velocity survey, proper-motion\nmeasurements, and multi-band photometric observations. We find that 310 of the\nstars with measured rotation periods are late-type members of M35. Their\ndistribution of rotation periods span more than two orders of magnitude from\n~0.1-15 days, not constrained by the sampling frequency and the time-span of\nthe survey. With an age between the zero-age main-sequence and the Hyades, and\nwith ~6 times more rotation periods than measured in the Pleiades, M35 permit\ndetailed studies of early rotational evolution of late-type stars. Nearly 80%\nof the 310 rotators lie on two distinct sequences in the color-period plane,\ndefining clear relations between stellar rotation period and color (mass). The\nM35 color-period diagram enables us to determine timescales for the transition\nbetween the two rotational states for G and K dwarfs, respectively. These\ntimescales are inversely related to the mass of the convective envelope, and\noffer constraints on the rates of internal and external angular momentum\ntransport and of the evolution of stellar dynamos. A comparison to the Hyades,\nconfirm the Skumanich (1972) spindown-dependence for G dwarfs on one rotational\nstate, but suggest that K dwarfs spin down more slowly. The locations of the\nrotational sequences in the M35 color-period diagram support the use of\nrotational isochrones to determine ages for coeval stellar populations. We use\nsuch gyrochronology to determine "gyro-ages" for M35. We use the M35 data to\nevaluate new color dependencies for the rotational isochrones.\n