purpose: rederive gyrochronology coefficients using new photometric time series data on stars in M34
Abstract
- photometric time series data used to determine stellar rotation periods
- radial velocity survey used to determine membership
- gyrochronology typically carries ~15% error
- comes up with an age for M34 of 240 Myr
Introduction
- as stars age, ang. momentum is lost through the evolution of magnetic fields (marked by chromospheric activity)
- open clusters are good for studying gyrochronology because they cover many masses with well known ages
- Skumanich found vsini ~ t^(-0.5)
- vsini measurements not enough
- periodicity is difficult to measure
- requires a star with some sort of periodic variation
- LOTS of telescope time - enough to see at least two cycles
- Meibom 2009 described M35
- This paper focuses on M34
Observations, Data Reduction, and Rotation Period Determination
- Meibom 2009 has more information about this process
- Observations for M34 were done at the same time and in the same way as those for M35
- Used WIYN 0.9m telescope on Kitt Peak
- observations taken every hour for 5-6 hrs per night for 16 nights
- Used Scargle 1982 algorithm to detect periods in unevenly sampled data
- handled the false alarm probability using a Monte Carlo calculation
Results
- Resulting color-period plot is extremely clean because of the long baseline observations
- a clear rotational isochrone shows up in the diagram
- Results are very similar to other works (Irwin et al 2006, James et al 2010
Analysis and Discussion
- comparison with other clusters helps determine the timing of the transition between I and C rotational stages
- All clusters show two sequences (I and C)
- Turnoff for G stars ~ 150 Myr
- Turnoff for K stars ~ 150-300 Myr
- Turnoff for M stars ~ 600 Myr
- stars spend little time in the gap between C and I sequences
- rough dP/dt can be estimated for the Pleiades
- ~.07 days/Myr
- Skumanich spin down rate (~t^(1/2)) describes the slowing that stars experience once they have evolved to the I sequence, and begin to lose ang. momentum due to stellar winds, etc.
- Meibom tested this by Skumanich-ly spinning up the Hyades by 405 Myrs to match the age of M34
- G stars seem to follow the Skumanich path, but K dwarfs spin down slower
- (this seems like an easy enough test, and should be done for other spectral types...)
- Assuming different ages for M35 (from the first paper), Meibom tested the variability of the gyro coefficients
- found that only the a term was significantly affected by changes in age
- Used a nonlinear least squares fit of P(t,B-V) with three different sets of coefficients to determine the age of M34
- Found a range of ages, all which coincide (including errors) with the stellar evolution age for the cluster
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