Pulsations, eruptions, and evolution of four yellow hypergiants

Aims. We aim to explore the variable photometric and stellar properties of four yellow hypergiants (YHGs), HR 8752, HR 5171A, ρ Cas, and HD 179821, and their pulsations of hundreds of days, and long-term variations (LTVs) of years. We also aim to explore light and colour curves for characteristics b...

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Bibliographic Details
Published in:Astronomy and astrophysics (Berlin) 2019-11, Vol.631, p.A48
Main Authors: van Genderen, A. M., Lobel, A., Nieuwenhuijzen, H., Henry, G. W., de Jager, C., Blown, E., Di Scala, G., van Ballegoij, E. J.
Format: Article
Language:English
Subjects:
Absorption
Astronomical models
Color
Mathematical analysis
Opacity
Photometry
Polycyclic aromatic hydrocarbons
Properties (attributes)
stars: massive
stars: oscillations
Stellar evolution
Stellar oscillations
supergiants
techniques: photometric
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Summary:Aims. We aim to explore the variable photometric and stellar properties of four yellow hypergiants (YHGs), HR 8752, HR 5171A, ρ Cas, and HD 179821, and their pulsations of hundreds of days, and long-term variations (LTVs) of years. We also aim to explore light and colour curves for characteristics betraying evolutionary loops and eruptive episodes and to investigate trends of quasi-periods and the possible need for distance revisions. Methods. We tackled multi-colour and visual photometric data sets, looked for photometric indications betraying eruptions or enhanced mass-loss episodes, calculated stellar properties mainly using a previously published temperature calibration, and investigated the nature of LTVs and their influence on quasi-periods and stellar properties. Results. Based on driven one-zone stellar oscillation models, the pulsations can be characterised as “weakly chaotic”. The BV photometry revealed a high-opacity layer in the atmospheres. When the temperature rises the mass loss increases as well, consequently, as the density of the high-opacity layer. As a result, the absorption in B and V grow. The absorption in B, presumably of the order of one to a few 0.m 1 0 . m 1 $ 0{{\overset{\text{ m}}{.}}}1 $ , is always higher than in V. This difference renders redder and variable (B − V) colour indexes, but the absorption law is unknown. This property of YHGs is unpredictable and explains why spectroscopic temperatures (reddening independent) are always higher than photometric ones, but the difference decreases with the temperature. A new (weak) eruption of ρ Cas has been identified. We propose shorter distances for ρ Cas and HR 5171A than the accepted ones. Therefore, a correction to decrease the blue luminescence of HR 5171A by polycyclic aromatic hydrocarbon (PAH) molecules is necessary, and HR 5171A would no longer be a member of the cluster Gum48d. HR 5171A is only subject to one source of light variation, not by two as the literature suggests. Eruptive episodes (lasting one to two years), of YHGs prefer relatively cool circumstances when a red evolutionary loop (RL) has shifted the star to the red on the HR diagram. After the eruption, a blue loop evolution (BL) is triggered lasting one to a few decades. We claim that in addition to HR 8752, also the other three YHGs have shown similar cycles over the last 70 years. This supports the suspicion that HD 179821 might be a YHG (with a possible eruptive episode between 1925 and 1960). The range
ISSN:0004-6361
1432-0746
DOI:10.1051/0004-6361/201834358