Panchromatic evolution of three luminous red novae: Forbidden hugs in pandemic times — IV. (arXiv:2208.02782v1 [astro-ph.SR])

Panchromatic evolution of three luminous red novae: Forbidden hugs in pandemic times — IV. (arXiv:2208.02782v1 [astro-ph.SR])
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We present photometric and spectroscopic data on three extragalactic luminous
red novae (LRNe): AT2018bwo, AT2021afy, and A2021blu. AT2018bwo was discovered
in NGC45 (at about 6.8 Mpc) a few weeks after the outburst onset. During the
monitoring period, the transient reached a peak luminosity of 10^40 erg/s.
AT2021afy, hosted by UGC10043 (~49.2 Mpc), showed a double-peaked light curve,
with an early maximum reaching a luminosity of 2.1×10^41 erg/s. Finally, for
AT2021blu in UGC5829 (~8.6 Mpc), the pre-outburst phase was well-monitored by
several photometric surveys, and the object showed a slow luminosity rise
before the outburst. The light curve of AT2021blu was sampled with an
unprecedented cadence until the object disappeared behind the Sun, and was then
recovered at late phases. The light curve of LRN AT2021blu shows a double peak,
with a prominent early maximum reaching a luminosity of 6.5×10^40 erg/s, half
that of AT2021afy. The spectra of AT2021afy and AT 2021blu display the expected
evolution for LRNe: a blue continuum dominated by prominent Balmer lines in
emission during the first peak, and a redder continuum consistent with that of
a K-type star with narrow absorption metal lines during the second, broad
maximum. The spectra of AT2018bwo are markedly different, with a very red
continuum dominated by broad molecular features in absorption. As these spectra
closely resemble those of LRNe after the second peak, probably AT 2018bwo was
discovered at the very late evolutionary stages. This would explain its fast
evolution and the spectral properties compatible with that of an M-type star.
From the analysis of deep frames of the LRN sites years before the outburst,
and considerations of the light curves, the quiescent progenitor systems of the
three LRNe were likely massive, with primaries ranging from about 13Mo for
AT2018bwo, to 13-18Mo for AT 2021blu, and over 40Mo for AT2021afy.

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