When Do Stalled Stars Resume Spinning Down? Advancing Gyrochronology with Ruprecht 147

Creators: Curtis, Jason Lee; Agüeros, Marcel A.; Matt, Sean P.; Covey, Kevin R.; Douglas, Stephanie T.; Angus, Ruth; Saar, Steven H.; Cody, Ann Marie; Vanderburg, Andrew; Law, Nicholas M.; Kraus, Adam L.; Latham, David W.; Baranec, Christoph; Riddle, Reed; Ziegler, Carl; Lund, Mikkel N.; Torres, Guillermo; Meibom, Søren; Silva Aguirre, Victor; Wright, Jason T.

Abstract

Recent measurements of rotation periods 〈P_(rot)〉 in the benchmark open clusters Praesepe (670 Myr), NGC 6811 (1 Gyr), and NGC 752 (1.4 Gyr) demonstrate that, after converging onto a tight sequence of slowly rotating stars in mass–period space, stars temporarily stop spinning down. These data also show that the duration of this epoch of stalled spin-down increases toward lower masses. To determine when stalled stars resume spinning down, we use data from the K2 mission and the Palomar Transient Factory to measure P_(rot) for 58 dwarf members of the 2.7 Gyr old cluster Ruprecht 147, 39 of which satisfy our criteria designed to remove short-period or near-equal-mass binaries. Combined with the Kepler P_(rot) data for the approximately coeval cluster NGC 6819 (30 stars with M_★ > 0.85 M_⊙, our new measurements more than double the number of ≈2.5 Gyr benchmark rotators and extend this sample down to ≈0.55 M_⊙. The slowly rotating sequence for this joint sample appears relatively flat (22 ± 2 days) compared to sequences for younger clusters. This sequence also intersects the Kepler intermediate-period gap, demonstrating that this gap was not created by a lull in star formation. We calculate the time at which stars resume spinning down and find that 0.55 M_⊙ stars remain stalled for at least 1.3 Gyr. To accurately age-date low-mass stars in the field, gyrochronology formulae must be modified to account for this stalling timescale. Empirically tuning a core–envelope coupling model with open cluster data can account for most of the apparent stalling effect. However, alternative explanations, e.g., a temporary reduction in the magnetic braking torque, cannot yet be ruled out.

Additional Information

© 2020. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2020 April 10; revised 2020 September 27; accepted 2020 September 28; published 2020 November 27. J.L.C. is supported by the National Science Foundation Astronomy and Astrophysics Postdoctoral Fellowship under award AST-1602662 and the National Aeronautics and Space Administration under grant NNX16AE64G issued through the K2 Guest Observer Program (GO 7035). M.A.A. acknowledges support provided by the NSF through grant AST-1255419. S.T.D. acknowledges support provided by the NSF through grant AST-1701468. S.P.M. is supported by the European Research Council under the European Union's Horizon 2020 research and innovation program (agreement No. 682393, AWESoMeStars). S.H.S. acknowledges support by NASA Heliophysics LWS grant NNX16AB79G. C.Z. is supported by a Dunlap Fellowship at the Dunlap Institute for Astronomy & Astrophysics, funded through an endowment established by the Dunlap family and the University of Toronto. Funding for the Stellar Astrophysics Centre is provided by The Danish National Research Foundation (grant agreement No. DNRF106). The Center for Exoplanets and Habitable Worlds and the Penn State Extraterrestrial Intelligence Center are supported by the Pennsylvania State University and the Eberly College of Science. We thank Sydney Barnes, Mark Giampapa, Jennifer van Saders, Travis Metcalfe, Eric Mamajek, Garrett Somers, Federico Spada, David Gruner, Rayna Rampalli, and the attendees of the Thinkshop 16 meeting hosted at the Leibniz Institute for Astrophysics in Potsdam, Germany (AIP) in 2019,42 for enlightening conversations on rotation, activity, and our Ruprecht 147 results. We acknowledge preliminary work on the PTF data by Leo J. Liu (not used in this study). We are grateful to the K2 Guest Observer office and Ball Aerospace for repositioning the Campaign 7 field to accommodate Ruprecht 147; the staff at the various observatories cited in this study; the Harvard–Smithsonian Center for Astrophysics telescope allocation committee for granting access to Magellan and TRES; Iván Ramírez for assistance acquiring and analyzing MIKE spectra; John O'Meara and John Bochanski for assistance with MagE; Edward Villanueva and Dan Kelson for their assistance with MIKE data; Jeff Valenti, Debra Fischer, and John M. Brewer for assistance with SME; Katja Poppenhaeger for supporting our petition to repoint K2 Campaign 7, Fabienne Bastien for supporting our GO 7035 proposal, and Jennifer van Saders and Florian Gallet for sharing rotational isochrones generated from their angular momentum evolution models (van Saders & Pinsonneault 2013; Gallet & Bouvier 2015). This paper includes data collected by the Kepler and K2 missions, which are funded by the NASA Science Mission directorate. We obtained these data from the Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NNX09AF08G and by other grants and contracts. This work has made use of data from the European Space Agency (ESA) mission Gaia,43 processed by the Gaia Data Processing and Analysis Consortium (DPAC).44 Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This work made use of the https://gaia-kepler.fun cross-match database, created by Megan Bedell. This paper is based on observations obtained with the Samuel Oschin Telescope as part of the Palomar Transient Factory project, a scientific collaboration between the California Institute of Technology, Columbia University, Las Cumbres Observatory, the Lawrence Berkeley National Laboratory, the National Energy Research Scientific Computing Center, the University of Oxford, and the Weizmann Institute of Science. The Robo-AO system was developed by collaborating partner institutions, the California Institute of Technology and the Inter-University Centre for Astronomy and Astrophysics, and with the support of the National Science Foundation under grant Nos. AST-0906060, AST-0960343, and AST-1207891, the Mt. Cuba Astronomical Foundation, and by a gift from Samuel Oschin. This work utilized SOLIS data obtained by the NSO Integrated Synoptic Program (NISP), managed by the National Solar Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under a cooperative agreement with the National Science Foundation. This research has made use of NASA's Astrophysics Data System and the VizieR (Ochsenbein et al. 2000) and SIMBAD (Wenger et al. 2000) databases, operated at CDS, Strasbourg, France. Facilities: Gaia - , K2 - , PTF - , PO:1.5 m (Robo-AO). - Software: The IDL Astronomy User's Library (Landsman 1993), K2fov (Mullally et al. 2016).

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