A Be star that turned bright and blue during a major outburst

¹ Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Beijing, 100101, PR China
² European Organisation for Astronomical Research in the Southern Hemisphere (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
³ School of Astronomy and Space Science, University of Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, PR China
⁴ Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, PR China
⁵ New Cornerstone Science Laboratory, National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Beijing 100101, PR China
⁶ European Organisation for Astronomical Research in the Southern Hemisphere (ESO), Casilla 19001, Santiago 19, Chile
⁷ Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Av. Gran Bretaña 1111, Valparaíso, Chile
⁸ Astronomical Observatory Belgrade, Volgina 7, 11060 Belgrade, Serbia
⁹ Nanjing Institute of Astronomical Optics & Technology, National Astronomical Observatories, Chinese Academy of Sciences, Nanjing 210042, PR China
¹⁰ Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão 1226, Cidade Universitária, 05508-900 São Paulo, Sp, Brazil
¹¹ Department of Astronomy, University of Wisconsin-Madison, 2535 Sterling Hall 475 N Charter Street, Madison, WI 53706, USA
¹² Kavli Institute for Theoretical Physics, University of Califonia, Santa Barbara, CA 93106, USA
¹³ Astroserver.org, Főtér 1, 8533 Malomsok, Hungary

^★ Corresponding authors; wangw@nao.cas.cn; dbaade@eso.org

Received: 29 October 2024
Accepted: 29 December 2024

Abstract

Context. Classical Be stars are rapidly rotating B-type stars exhibiting Balmer emission lines originating from circumstellar Keplerian gaseous disks, which likely form through episodic mass-ejection events. The currently favored model for Be star disks is the viscous decretion disk (VDD) model. However, the mechanism behind the mass ejection process during the formation of a VDD is a mystery.

Aims. We present peculiar behavior of the Be star KIC 9715425, which exhibited several minor outbursts (MIBs) and one major outburst (MAB) observed as brightenings in broadband photometry, as well as transient Hα line emission. The variability may provide valuable keys to understanding the nature of Be outbursts.

Methods. Based on Kepler, the All-Sky Automated Survey for Supernovae (ASAS-SN) and the Transiting Exoplanet Survey Satellite time-series photometry, the Large Sky Area Multi-Object Fiber Spectroscopic Telescope spectroscopy, multi-wavelength observations in the near-ultraviolet (NUV) from the Galaxy Evolution Explorer (GALEX) and optical from Gaia, Xinglong 2.16 m telescope and Isaac Newton Telescope of KIC 9715425 covering its outbursts, we determined fundamental parameters of the central star and the circumstellar disk. A frequency analysis and spectral modeling were carried out to characterize the events.

Results. During the major outburst, we find a 36% flux increase in the GALEX NUV band compared to 25% in the Kepler band, suggesting that whatever produced the flux increase should be hotter than the central B-type star. This is contradictory to the conventional scenario that the VDD should be cooler. In addition, such a high flux increase can only be accounted for by a luminous disk, which should produce a much stronger Hα emission than observed. The origin of this anomalously hot component remains unexplained.

Conclusions. Except for the bluing, the available observations of KIC 9715425 are perfectly compatible with being a normal Be star. If the bluing was physically related to the MAB, the most economical effort to identify the nature of the underlying process could be adaptive NUV and far-ultraviolet monitoring of Be stars with cyclically repeating MABs.