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Discovery of an old nova remnant in the Galactic globular cluster M 22⋆

P. Weilbacher, M. M. Roth, S. Dreizler, B. Giesers, T.-O. Husser, S. Kamann, J. Brinchmann, W. Kollatschny, A. Monreal-Ibero, K. B. Schmidt, M. Wendt, L. Wisotzki, R. Bacon

Abstract
A nova is a cataclysmic event on the surface of a white dwarf in a binary system that increases the overall brightness by several orders of magnitude. Although binary systems with a white dwarf are expected to be overabundant in globular clusters compared with in the Galaxy, only two novae from Galactic globular clusters have been observed. We present the discovery of an emission nebula in the Galactic globular cluster M 22 (NGC 6656) in observations made with the integral-field spectrograph MUSE. We extracted the spectrum of the nebula and used the radial velocity determined from the emission lines to confirm that the nebula is part of NGC 6656. Emission-line ratios were used to determine the electron temperature and density. It is estimated to have a mass of 1–17 × 10−5M. This mass and the emission-line ratios indicate that the nebula is a nova remnant. Its position coincides with the reported location of a “guest star”, an ancient Chinese term for transients, observed in May 48 BCE. With this discovery, this nova may be one of the oldest confirmed extra-solar events recorded in human history.

Keywords
globular clusters: individual: NGC 6656; novae; cataclysmic variables; techniques: imaging spectroscopy; Astrophysics - Solar and Stellar Astrophysics; Astrophysics - Astrophysics of Galaxies

Astronomy and Astrophysics
Volume 626, Article Number A69, Number of pages 6
2019 June

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia