A new and unusual LBV-like outburst from a Wolf–Rayet star in the outskirts of M33

Creators: Smith, Nathan; Andrews, Jennifer E.; Moe, Maxwell; Milne, Peter; Bilinski, Christopher; Kilpatrick, Charles D.; Fong, Wen-Fai; Badenes, Carles; Filippenko, Alexei V.; Kasliwal, Mansi; Silverman, Jeffrey M.

Abstract

MCA-1B (also called UIT003) is a luminous hot star in the western outskirts of M33, classified over 20 yr ago with a spectral type of Ofpe/WN9 and identified then as a candidate luminous blue variable (LBV). Palomar Transient Factory data reveal that this star brightened in 2010, with a light curve resembling that of the classic LBV star AF And in M31. Other Ofpe/WN9 stars have erupted as LBVs, but MCA-1B was unusual because it remained hot. It showed a WN-type spectrum throughout its eruption, whereas LBVs usually get much cooler. MCA-1B showed an almost four-fold increase in bolometric luminosity and a doubling of its radius, but its temperature stayed ≳29 kK. As it faded, it shifted to even hotter temperatures, exhibiting a WN7/WN8-type spectrum, and doubling its wind speed. MCA-1B is reminiscent of some supernova impostors, and its location resembles the isolated environment of SN 2009ip. It is most similar to HD 5980 (in the Small Magellanic Cloud) and GR 290 (also in M33). Whereas these two LBVs exhibited B-type spectra in eruption, MCA-1B is the first clear case where a Wolf–Rayet (WR) spectrum persisted at all times. Together, MCA-1B, HD 5980, and GR 290 constitute a class of WN-type LBVs, distinct from S Doradus LBVs. They are most interesting in the context of LBVs at low metallicity, a possible post-LBV/WR transition in binaries, and as likely Type Ibn supernova progenitors.

Additional Information

© 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model). Received: 25 October 2019; Revision received: 04 January 2020; Accepted: 06 January 2020; Published: 14 January 2020. We thank Dovi Poznanski and Eran Ofek for assistance with the PTF photometry, and Yi Cao, Brad Cenko, Ryan Foley, and Shri Kulkarni for assistance with both photometric and spectroscopic observations obtained through PTF. We are grateful to Yogesh Joshi for sending a table of photometry for AF And prior to publication, and we thank Paul Crowther for providing spectra of the Ofpe/WN9 comparison stars in Fig. 5. Support for NS was provided by NSF award AST-1515559, and by the National Aeronautics and Space Administration (NASA) through HST grant AR-14316 from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555. AVF's research has been generously supported by the TABASGO Foundation, the Christopher R. Redlich Fund, and the Miller Institute for Basic Research in Science (U.C. Berkeley). This paper is based in part on observations obtained with the Samuel Oschin Telescope and the 60 inch Telescope at the Palomar Observatory 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. Observations using Steward Observatory facilities were obtained as part of the observing program AZTEC: Arizona Transient Exploration and Characterization, which receives support from NSF grant AST-1515559. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA; the observatory was made possible by the generous financial support of the W. M. Keck Foundation. Research at Lick Observatory is partially supported by a generous gift from Google. Some observations reported here were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution. This paper uses data taken with the MODS spectrographs built with funding from NSF grant AST-9987045 and the NSF Telescope System Instrumentation Program (TSIP), with additional funds from the Ohio Board of Regents and the Ohio State University Office of Research. We are grateful for the assistance of the staffs at the various observatories where data were obtained. Facilities: HST (WFPC2), Keck:I (LRIS), Keck II (DEIMOS), LBT (MODS), MMT (Bluechannel), Lick: 3m (Kast), PO:1.2m, 1.5m, SO: Bok (B&C, SPOL), SO:Kuiper (Mont4K) SO:Super-LOTIS.

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