Discovery of a 310 Day Period from the Enshrouded Massive System NaSt1 (WR 122)

Creators: Lau, Ryan M.; Tinyanont, Samaporn; Hankins, Matthew J.; Ashley, Michael C. B.; De, Kishalay; Filippenko, Alexei V.; Hillenbrand, Lynne A.; Kasliwal, Mansi M.; Mauerhan, Jon C.; Moffat, Anthony F. J.; Moore, Anna M.; Smith, Nathan; Soon, Jamie; Soria, Roberto; Travouillon, Tony; van der Hucht, Karel A.; Williams, Peredur M.; Zheng, WeiKang

Abstract

We present optical and infrared (IR) light curves of NaSt1, also known as Wolf–Rayet 122, with observations from Palomar Gattini-IR (PGIR), the Zwicky Transient Facility (ZTF), the Katzman Automatic Imaging Telescope, the Asteroid Terrestrial-impact Last Alert System, and the All-Sky Automated Survey for Supernovae (ASAS-SN). We identify a P = 309.7 ± 0.7 day photometric period from the optical and IR light curves that reveal periodic, sinusoidal variability between 2014 July and 2021 July. We also present historical IR light curves taken between 1983 July and 1989 May, which show variability consistent with the period of the present-day light curves. In the past, NaSt1 was brighter in the J band with larger variability amplitudes than the present-day PGIR values, suggesting that NaSt1 exhibits variability on longer (≳decade) timescales. Sinusoidal fits to the recent optical and IR light curves show that the amplitude of NaSt1's variability differs at various wavelengths and also reveal significant phase offsets of 17.0 ± 2.5 day between the ZTF r and PGIR J light curves. We interpret the 310 day photometric period from NaSt1 as the orbital period of an enshrouded massive binary. We suggest that the photometric variability of NaSt1 may arise from variations in the line-of-sight optical depth toward circumstellar optical/IR-emitting regions throughout its orbit due to colliding-wind dust formation. We speculate that past mass transfer in NaSt1 may have been triggered by Roche-lobe overflow (RLOF) during an eruptive phase of an Ofpe/WN9 star. Lastly, we argue that NaSt1 is no longer undergoing RLOF mass transfer.

Additional Information

© 2021. The American Astronomical Society. Received 2021 March 15; revised 2021 August 27; accepted 2021 August 27; published 2021 November 15. We thank T. Jayasinghe for discussion of the technical details of the filter properties in the ASAS-SN survey. We also thank M. Munoz for an enlightening discussion of Of?p stars. R.M.L. acknowledges the Japan Aerospace Exploration Agency's International Top Young Fellowship (ITYF). A.F.J. M. is grateful for financial assistance from NSERC (Canada). M.M.K. acknowledges the Heising-Simons foundation for support via a Scialog fellowship of the Research Corporation. M.M.K. and A.M.M. acknowledge the Mt. Cuba Astronomical Foundation. A.V.F. is grateful for financial assistance from the bTABASGO Foundation, the Christopher R. Redlich Fund, the U.C. Berkeley Miller Institute for Basic Research in Science (in which he is a Miller Senior Fellow), and many individual donors. M.M.K. acknowledges generous support from the David and Lucille Packard Foundation. J.S. is supported by an Australian Government Research Training Program (RTP) Scholarship. Palomar Gattini-IR (PGIR) is generously funded by Caltech, Australian National University, the Mt. Cuba Astronomical Foundation, the Heising-Simons Foundation, and the Binational Science Foundation. PGIR is a collaborative project among Caltech, Australian National University, University of New South Wales, Columbia University, and the Weizmann Institute of Science. Based in part on observations obtained with the Samuel Oschin 48 inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation (NSF) under grant AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories. Operations are conducted by Caltech Optical Observatories, IPAC, and the University of Washington. We thank the Las Cumbres Observatory and its staff for its continuing support of the ASAS-SN project. LCOGT observations were performed as part of DDT award 2019B003 to E.G. ASAS-SN is supported by the Gordon and Betty Moore Foundation through grant GBMF5490 to the Ohio State University, and NSF grants AST-1515927 and AST-1908570. Development of ASAS-SN has been supported by NSF grant AST-0908816, the Mt. Cuba Astronomical Foundation, the Center for Cosmology and AstroParticle Physics at the Ohio State University, the Chinese Academy of Sciences South America Center for Astronomy (CAS- SACA), the Villum Foundation, and George Skestos. UKIRT is owned by the University of Hawaii (UH) and operated by the UH Institute for Astronomy; operations are enabled through the cooperation of the East Asian Observatory. When the data reported here were acquired, UKIRT was operated by the Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the UK. Research at Lick Observatory is partially supported by a generous gift from Google. KAIT and its ongoing operation were made possible by donations from Sun Microsystems, Inc., the Hewlett-Packard Company, AutoScope Corporation, Lick Observatory, the NSF, the University of California, the Sylvia and Jim Katzman Foundation, and the TABASGO Foundation. This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. The Asteroid Terrestrial-impact Last Alert System (ATLAS) project is primarily funded to search for near-Earth objects (NEOs) through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from the survey area. This work was partially funded by Kepler/K2 grant J1944/80NSSC19K0112 and HST GO-15889, and STFC grants ST/T000198/1 and ST/S006109/1. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queenʼs University Belfast, the Space Telescope Science Institute, the South African Astronomical Observatory, and The Millennium Institute of Astrophysics (MAS), Chile. This research made use of Astropy,22 a community-developed core Python package for Astronomy (Astropy Collaboration et al. 2013, 2018). Facilities: PGIR, ATLAS, KAIT, ZTF, ASAS-SN, AEOS (BASS), UKIRT, ESO 1-m.

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