A new outburst of the yellow hypergiant star Rho Cas. (arXiv:1812.03065v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Kraus_M/0/1/0/all/0/1">Michaela Kraus</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kolka_I/0/1/0/all/0/1">Indrek Kolka</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Aret_A/0/1/0/all/0/1">Anna Aret</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nickeler_D/0/1/0/all/0/1">Dieter H. Nickeler</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Maravelias_G/0/1/0/all/0/1">Grigoris Maravelias</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Eenmae_T/0/1/0/all/0/1">Tõnis Eenmäe</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lobel_A/0/1/0/all/0/1">Alex Lobel</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Klochkova_V/0/1/0/all/0/1">Valentina G. Klochkova</a>
Yellow hypergiants are evolved massive stars that were suggested to be in
post-red supergiant stage. Post-red supergiants that evolve back to the blue,
hot side of the Hertzsprung-Russell diagram can intersect a temperature domain
in which their atmospheres become unstable against pulsations (the Yellow Void
or Yellow Wall), and the stars can experience outbursts with short, but violent
mass eruptions. The yellow hypergiant Rho Cas is famous for its historical and
recent outbursts, during which the star develops a cool, optically thick wind
with a very brief but high mass-loss rate, causing a sudden drop in the light
curve. Here we report on a new outburst of Rho Cas which occurred in 2013,
accompanied by a temperature decrease of ~3000 K and a brightness drop of 0.6
mag. During the outburst TiO bands appear, together with many low excitation
metallic atmospheric lines characteristic for a later spectral type. With this
new outburst, it appears that the time interval between individual events
decreases, which might indicate that Rho Cas is preparing for a major eruption
that could help the star to pass through the Yellow Void. We also analysed the
emission features that appear during phases of maximum brightness and find that
they vary synchronous with the emission in the prominent [CaII] lines. We
conclude that the occasionally detected emission in the spectra of Rho Cas, as
well as certain asymmetries seen in the absorption lines of low to
medium-excitation potential, are circumstellar in nature, and we discuss the
possible origin of this material.
Yellow hypergiants are evolved massive stars that were suggested to be in
post-red supergiant stage. Post-red supergiants that evolve back to the blue,
hot side of the Hertzsprung-Russell diagram can intersect a temperature domain
in which their atmospheres become unstable against pulsations (the Yellow Void
or Yellow Wall), and the stars can experience outbursts with short, but violent
mass eruptions. The yellow hypergiant Rho Cas is famous for its historical and
recent outbursts, during which the star develops a cool, optically thick wind
with a very brief but high mass-loss rate, causing a sudden drop in the light
curve. Here we report on a new outburst of Rho Cas which occurred in 2013,
accompanied by a temperature decrease of ~3000 K and a brightness drop of 0.6
mag. During the outburst TiO bands appear, together with many low excitation
metallic atmospheric lines characteristic for a later spectral type. With this
new outburst, it appears that the time interval between individual events
decreases, which might indicate that Rho Cas is preparing for a major eruption
that could help the star to pass through the Yellow Void. We also analysed the
emission features that appear during phases of maximum brightness and find that
they vary synchronous with the emission in the prominent [CaII] lines. We
conclude that the occasionally detected emission in the spectra of Rho Cas, as
well as certain asymmetries seen in the absorption lines of low to
medium-excitation potential, are circumstellar in nature, and we discuss the
possible origin of this material.
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