Rotational Evolution and Radii of Young Sun-like Stars in NGC 2264 Laurin M. Gray & Katherine L. Rhode Indiana University, Bloomington, IN (USA) Background & Motivation • Understanding the origin and evolution of the Solar System and other Sun-like stars and planetary systems is a primary goal of modern astrophysics. • Studying T Tauri stars provides insight into the early interactions between Sun-like stars and their protoplanetary disks, and the time scales governing disk dissipation and planet formation. • NGC 2264 is a well-studied, nearby (~760 pc) open cluster with an age of ~3 Myr and a large T Tauri star population. The wealth of literature data make it an ideal target for developing our methods. Observations & Methods • The WIYN 3.5-m telescope and Hydra multi-object spectrograph were used to obtain high-dispersion (R ~ 21,500) spectra of stars in NGC 2264 and measure the stars’ projected rotation velocity (v sin(i)) values. • Our final NGC 2264 sample includes ~250 T Tauri stars. Roughly 30 are likely spectroscopic binaries; we excluded these stars from some analysis because they can yield overestimated v sin(i) values. • We use cross-correlation to measure v sin(i) and can measure values down to ~11 km/s. Our cross-correlation template stars have a range of temperatures matching those of our sample stars. We also observed v sin(i) standard stars to check our measurement accuracy. Future Work Open Cluster IC 5070 NGC 2264 IC 348 Upper Sco h Persei Age (Myr) 1-3 2-3 2-6 8-10 ~14 Distance (pc) 795 760 315 140 2100 References Baraffe et al. 2015, A&A, 577, A42 Jackson et al. 2018, MNRAS, 476, 3245 Somers et al. 2020, ApJ, 891:29 Acknowledgements We thank Catrina Hamilton-Drager and Luisa Rebull for sharing data and helping us design the project. We also thank the IU Astronomy Department and IU College of Arts & Sciences for providing funding. We have selected four additional clusters (see table), ranging from 1-14 Myr in age, for similar analysis. This will allow us to investigate the rotational evolution of low-mass stars during the timescale when protoplanetary disks dissipate. HR diagram of NGC 2264 stars used in the model comparison, with isochrones (yellow dashed lines) and mass tracks (black dotted lines) from the starspot-free Baraffe et al. 2015 model (left) and Somers et al. 2020 model with 51% starspot coverage (right). The main sequence is approximated at 100 Myr by the solid orange line. The measured v sin(i) values of the stars are indicated by the color bar, and open symbols mark stars with v sin(i) below our 11 km/s resolution limit. Vertical gray lines mark temperature-based subgroups used in the model comparison. At the age of NGC 2264 (~3 Myr), the model with starspots fits the data better than the starspot-free model. Spectra of a K2 cross-correlation template star (top) and three NGC 2264 stars (bottom three), with our v sin(i) values marked. The absorption lines broaden as v sin(i) increases. Stars with Accreting Disks May Rotate Slower Stars in Binary Systems May Rotate Faster Starspots Are Needed to Match Measured Radii Questions? Contact me at: grayla@iu.edu • A 2-sample K-S statistical test finds that the v sin(i) distributions do not have a statistically significant difference (p=0.126), but the large fraction of stars that lie below our resolution limit could mask a genuine difference in the two samples. • The fact that a higher proportion of CTTSs have low v sin(i) values compared to WTTSs seems to imply that stars with accreting disks may rotate more slowly than stars without active accretion. • 59% (49/83) of disked, CTTS stars have v sin(i) <11 km/s, while only 36% (47/130) of WTTS stars have v sin(i) below this limit. • Binarity can cause overly wide CCF peaks, resulting in overestimated v sin(i) values; this analysis only includes binaries with clearly resolved CCF peaks or single-lined binaries identified by differing radial velocities at different epochs. Stars identified as “single sy