The tidal disruption event AT 2018hyz – I. Double-peaked emission lines and a flat Balmer decrement

P Short, M Nicholl, A Lawrence, S Gomez, I Arcavi, T Wevers, G Leloudas, S Schulze, J P Anderson, E Berger, P K Blanchard, J Burke, N Castro Segura, P Charalampopoulos, R Chornock, L Galbany, M Gromadzki, L J Herzog, D Hiramatsu, Keith HorneG Hosseinzadeh, D Andrew Howell, N Ihanec, C Inserra, E Kankare, K Maguire, C McCully, T E Müller Bravo, F Onori, J Sollerman, D R Young

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We present results from spectroscopic observations of AT 2018hyz, a transient discovered by the All-Sky Automated Survey for Supernova survey at an absolute magnitude of MV ∼ −20.2 mag, in the nucleus of a quiescent galaxy with strong Balmer absorption lines. AT 2018hyz shows a blue spectral continuum and broad emission lines, consistent with previous TDE candidates. High cadence follow-up spectra show broad Balmer lines and He i in early spectra, with He ii making an appearance after ∼70–100 d. The Balmer lines evolve from a smooth broad profile, through a boxy, asymmetric double-peaked phase consistent with accretion disc emission, and back to smooth at late times. The Balmer lines are unlike typical active galactic nucleus in that they show a flat Balmer decrement (Hα/Hβ ∼ 1.5), suggesting the lines are collisionally excited rather than being produced via photoionization. The flat Balmer decrement together with the complex profiles suggests that the emission lines originate in a disc chromosphere, analogous to those seen in cataclysmic variables. The low optical depth of material due to a possible partial disruption may be what allows us to observe these double-peaked, collisionally excited lines. The late appearance of He ii may be due to an expanding photosphere or outflow, or late-time shocks in debris collisions.
Original languageEnglish
Pages (from-to)4119-4133
Number of pages15
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Early online date02 Sept 2020
Publication statusPublished - Nov 2020


  • Space and Planetary Science
  • Astronomy and Astrophysics


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