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Early-time spectral nature of AT2023vto (Hβ misidentification versus spectral evolution)

Ascertain whether the early-time optical spectrum of AT2023vto contained a genuine Hβ emission feature that was misidentified, or whether the spectrum evolved from being dominated by Hβ to He II λ4686 within approximately 30 rest-frame days.

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Background

AT2023vto was initially classified as a SLSN-II based on a reported Hβ feature, but the authors’ later MMT spectrum shows a dominant broad He II λ4686 line with no Hβ, leading them to reclassify the transient as a TDE-He.

Because the earlier spectrum (at ∼26 days observed phase) is not publicly available, the authors cannot determine whether the initial Hβ identification was erroneous or whether the spectrum underwent rapid evolution from Hβ to He II, an evolution claimed in another TDE (AT2017eqx) but typically over longer timescales.

References

Since the earlier spectrum used for the initial classification (observed phase of 26 days; Poidevin et al. 2023) is not publicly available, we cannot determine if the broad feature we observe was misidentified as Hβ, or whether the spectrum has evolved from being dominated by Hβ to He IIλ4686; such a transition was claimed for the TDE AT2017eqx (Nicholl et al. 2019), but over a much longer timescale of ≈ 100 d compared to only ≈ 30 d in this case.

— AT2023vto: An Exceptionally Luminous Helium Tidal Disruption Event from a Massive Star (2408.01482 - Kumar et al., 2 Aug 2024) in Section 2.4, Optical Spectroscopy