Tidal Circularization in the Open Cluster NGC 6819 Sylvana Yelda Advisor: Robert Mathieu

Tidal Circularization Orbits constantly change –Earth-Moon –Hot Jupiters

NGC 6819 WIYN Open Cluster Study (WOCS) Very concentrated, rich cluster –~ solar metallicity Intermediate age: ~ 2.4 Gyr ~ 300 pc above Galactic plane –Cygnus

Hole et al. 2005

Searching for Binaries Radial velocity –Member: rv NGC6819 =2.17 km s -1 –Candidate binary: rms > 2.0 km s -1 Spectroscopic binaries Cross-correlation function

Searching for Binaries SB1 vs. SB2

Apastron Periastron Apastron Orbital Solutions

P= e=0.445 ± 0.036

P= e=0.723 ± 0.055

P= e=0.021 ± *Longest period circular orbit

P=1.847 e=0.022 ± *Shortest period circular orbit a~0.03 AU!

P=4.850 e=0.016 ± 0.008

Angular Momentum Conserved Rotational → Orbital –Star rotates slower Eccentricity decreases –L orbital ↑ corresponds to e ↓ (circular)

Tidal Forces Tidal Force: 1/d 3 Misalignment of tidal bulges w.r.t. line joining centers produces torque –Rotational angular momentum transferred to orbital ( τ= dL/dt) Transition from eccentric to circular –Closest binaries → Circular orbits –Distant binaries → Nonzero eccentricities

Tidal Circularization Period Period at which typical binary (e ~ 0.35) has circularized in the age of population Meibom & Mathieu 2005

n=39

Summary Short-period → Circular Longer-period → Non-zero eccentricities CP of NGC6819 consistent with observed trend With increasing age, longer period binaries become circularized

Thank you! Robert Mathieu Soeren Meibom Aaron Geller Tabetha Hole Ella Braden Ed Mierkiewicz REU Students UW Faculty & Staff