Variability of central stars of planetary nebulae with the zwicky transient facility. I. Methods, short-timescale variables, and the unusual nucleus of WeSb 1
Bhattacharjee S, Kulkarni SR, Kong AKH, Tam MS, Bond HE, El-Badry K, Caiazzo I, Chornay N, Graham MJ, Rodriguez AC, Zeimann GR, Fremling C, Drake AJ, Werner K, Rodriguez H, Prince TA, Laher RR, Chen TX, Riddle R. 2025. Variability of central stars of planetary nebulae with the zwicky transient facility. I. Methods, short-timescale variables, and the unusual nucleus of WeSb 1. Publications of the Astronomical Society of the Pacific. 137(2), 024201.
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Author
Bhattacharjee, Soumyadeep;
Kulkarni, S. R.;
Kong, Albert K.H.;
Tam, M. S.;
Bond, Howard E.;
El-Badry, Kareem;
Caiazzo, IlariaISTA
;
Chornay, Nicholas;
Graham, Matthew J.;
Rodriguez, Antonio C.;
Zeimann, Gregory R.;
Fremling, Christoffer
All

All
Department
Abstract
A complete understanding of the central stars of planetary nebulae (CSPNe) remains elusive. Over the past several decades, time-series photometry of CSPNe has yielded significant results including, but not limited to, discoveries of nearly 100 binary systems, insights into pulsations and winds in young white dwarfs, and studies of stars undergoing very late thermal pulses. We have undertaken a systematic study of optical photometric variability of cataloged CSPNe, using the light curves from the Zwicky Transient Facility (ZTF). By applying appropriate variability metrics, we arrive at a list of 94 highly variable CSPN candidates. Based on the timescales of the light-curve activity, we classify the variables broadly into short- and long-timescale variables. In this first paper in this series, we focus on the former, which is the majority class comprising 83 objects. We report periods for six sources for the first time, and recover several known periodic variables. Among the aperiodic sources, most exhibit a jitter around a median flux with a stable amplitude, and a few show outbursts. We draw attention to WeSb 1, which shows a different kind of variability: prominent deep and aperiodic dips, resembling transits from a dust/debris disk. We find strong evidence for a binary nature of WeSb 1 (possibly an F-type subgiant companion). The compactness of the emission lines and inferred high electron densities make WeSb 1 a candidate for either an EGB 6-type planetary nucleus, or a symbiotic system inside an evolved planetary nebula, both of which are rare objects. To demonstrate further promise with ZTF, we report three additional newly identified periodic sources that do not appear in the list of highly variable sources. Finally, we also introduce a two-dimensional metric space defined by the von Neumann statistics and Pearson Skew and demonstrate its effectiveness in identifying unique variables of astrophysical interest, like WeSb 1.
Publishing Year
Date Published
2025-02-01
Journal Title
Publications of the Astronomical Society of the Pacific
Publisher
IOP Publishing
Acknowledgement
This work is 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 grants No. AST-1440341 and AST-2034437 and a collaboration including current partners Caltech, IPAC, the Oskar Klein Center at Stockholm University, the University of Maryland, University of California, Berkeley, the University of Wisconsin at Milwaukee, University of Warwick, Ruhr University Bochum, Cornell University, Northwestern University, and Drexel University. Operations are conducted by COO, IPAC, and UW.
This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC; https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular, the institutions participating in the Gaia Multilateral Agreement.
We are grateful to the staffs of Palomar Observatory and the Hobby-Eberly Telescope for assistance with the observations and data management. The Liverpool Telescope is operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK Science and Technology Facilities Council.
The Low-Resolution Spectrograph 2 (LRS2) on HET was developed and funded by the University of Texas at Austin McDonald Observatory and Department of Astronomy, and by Pennsylvania State University. We thank the Leibniz-Institut für Astrophysik Potsdam (AIP) and the Institut für Astrophysik Göttingen (IAG) for their contributions to the construction of the integral field units. We acknowledge the Texas Advanced Computing Center (TACC) at The University of Texas at Austin for providing high performance computing, visualization, and storage resources that have contributed to the results reported within this paper.
We thank the anonymous referee for the detailed comments, which improved the clarity of the manuscript significantly. We also thank Gunter Cibis for pointing out typographical errors in the names of a few PNe in the first draft. S.B. expresses gratitude to Kishalay De for providing the Gattini-IR and WISE data. S.B. thanks Frank J. Masci and Zachary P. Vanderbosch for useful discussions and suggestions regarding solving the issues with ZTF forced photometry on extended sources. S.B. also thanks Jim Fuller, Charles C. Steidel, Lynne Hillenbrand, and Adolfo Carvalho for useful discussions on methods and science. S.B. also thanks David O. Cook for providing access to his CLU image cutout service to generate the WeSb 1 image. S.B. acknowledges the financial support from the Wallace L. W. Sargent Graduate Fellowship during the first year of his graduate studies at Caltech. N.C. was supported through the Cancer Research UK grant A24042. S.B. thanks Martina Veresvarka for drawing our attention to the TESS light curves of WeSb 1.
We have used Python packages Numpy (Harris et al. 2020), SciPy (Virtanen et al. 2020), Matplotlib (Hunter 2007), Pandas (pandas development team 2020), Astropy (Astropy Collaboration et al. 2013, 2018), and Astroquery (Ginsburg et al. 2019) at various stages of this research.
Volume
137
Issue
2
Article Number
024201
ISSN
IST-REx-ID
Cite this
Bhattacharjee S, Kulkarni SR, Kong AKH, et al. Variability of central stars of planetary nebulae with the zwicky transient facility. I. Methods, short-timescale variables, and the unusual nucleus of WeSb 1. Publications of the Astronomical Society of the Pacific. 2025;137(2). doi:10.1088/1538-3873/ada702
Bhattacharjee, S., Kulkarni, S. R., Kong, A. K. H., Tam, M. S., Bond, H. E., El-Badry, K., … Riddle, R. (2025). Variability of central stars of planetary nebulae with the zwicky transient facility. I. Methods, short-timescale variables, and the unusual nucleus of WeSb 1. Publications of the Astronomical Society of the Pacific. IOP Publishing. https://doi.org/10.1088/1538-3873/ada702
Bhattacharjee, Soumyadeep, S. R. Kulkarni, Albert K.H. Kong, M. S. Tam, Howard E. Bond, Kareem El-Badry, Ilaria Caiazzo, et al. “Variability of Central Stars of Planetary Nebulae with the Zwicky Transient Facility. I. Methods, Short-Timescale Variables, and the Unusual Nucleus of WeSb 1.” Publications of the Astronomical Society of the Pacific. IOP Publishing, 2025. https://doi.org/10.1088/1538-3873/ada702.
S. Bhattacharjee et al., “Variability of central stars of planetary nebulae with the zwicky transient facility. I. Methods, short-timescale variables, and the unusual nucleus of WeSb 1,” Publications of the Astronomical Society of the Pacific, vol. 137, no. 2. IOP Publishing, 2025.
Bhattacharjee S, Kulkarni SR, Kong AKH, Tam MS, Bond HE, El-Badry K, Caiazzo I, Chornay N, Graham MJ, Rodriguez AC, Zeimann GR, Fremling C, Drake AJ, Werner K, Rodriguez H, Prince TA, Laher RR, Chen TX, Riddle R. 2025. Variability of central stars of planetary nebulae with the zwicky transient facility. I. Methods, short-timescale variables, and the unusual nucleus of WeSb 1. Publications of the Astronomical Society of the Pacific. 137(2), 024201.
Bhattacharjee, Soumyadeep, et al. “Variability of Central Stars of Planetary Nebulae with the Zwicky Transient Facility. I. Methods, Short-Timescale Variables, and the Unusual Nucleus of WeSb 1.” Publications of the Astronomical Society of the Pacific, vol. 137, no. 2, 024201, IOP Publishing, 2025, doi:10.1088/1538-3873/ada702.
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