A New Class of Large-amplitude Radial-mode Hot Subdwarf Pulsators

Creators: Kupfer, Thomas; Bauer, Evan B.; Burdge, Kevin B.; Bellm, Eric C.; Bildsten, Lars; Fuller, Jim; Hermes, J. J.; Kulkarni, Shrinivas R.; Prince, Thomas A.; van Roestel, Jan; Dekany, Richard; Duev, Dmitry A.; Feeney, Michael; Giomi, Matteo; Graham, Matthew J.; Kaye, Stephen; Laher, Russ R.; Masci, Frank J.; Porter, Michael; Riddle, Reed; Shupe, David L.; Smith, Roger M.; Soumagnac, Maayane T.; Szkody, Paula; Ward, Charlotte

Abstract

Using high-cadence observations from the Zwicky Transient Facility at low Galactic latitudes, we have discovered a new class of pulsating, hot compact stars. We have found four candidates, exhibiting blue colors (g − r ≤ −0.1 mag), pulsation amplitudes of >5%, and pulsation periods of 200–475 s. Fourier transforms of the light curves show only one dominant frequency. Phase-resolved spectroscopy for three objects reveals significant radial velocity, T_(eff), and log(g) variations over the pulsation cycle, which are consistent with large-amplitude radial oscillations. The mean T_(eff) and log(g) for these stars are consistent with hot subdwarf B (sdB) effective temperatures and surface gravities. We calculate evolutionary tracks using MESA and adiabatic pulsations using GYRE for low-mass, helium-core pre-white dwarfs (pre-WDs) and low-mass helium-burning stars. Comparison of low-order radial oscillation mode periods with the observed pulsation periods show better agreement with the pre-WD models. Therefore, we suggest that these new pulsators and blue large-amplitude pulsators (BLAPs) could be members of the same class of pulsators, composed of young ≈0.25–0.35 M_⊙ helium-core pre-WDs.

Additional Information

© 2019 The American Astronomical Society. Received 2019 May 13; revised 2019 May 29; accepted 2019 June 1; published 2019 June 18. Based on observations obtained with the Samuel Oschin Telescope 48 inch and the 60 inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under grant No. 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 COO, IPAC, and UW. This work makes use of observations from the LCOGT network. 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 the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This research was supported in part by the National Science Foundation through grants AST-1514737, ACI-1663688, and at the KITP by grant PHY-1748958. This research benefited from interactions that were funded by the Gordon and Betty Moore Foundation through Grant GBMF5076. Facilities: PO:1.2 m (ZTF) - , LCOGT (Spectral) - , Hale (DBSP - , Chimera) - , Keck:I (LRIS). - Software: Lpipe (Perley 2019), PyRAF (Bellm & Sesar 2016), Gatspy (VanderPlas & Ivezić 2015), Period04 (Lenz & Breger 2005), FITSB2 (Napiwotzki et al. 2004), MESA (Paxton et al. 2011, 2013, 2015, 2018), GYRE (Townsend & Teitler 2013), Matplotlib (Hunter 2007), Astropy (Astropy Collaboration et al. 2013, 2018), Numpy (Oliphant 2015).

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