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Irregular emission cycles in the Oe star HD 60848 | | Gregor Rauw
; Yael Naze
; Fran Campos
; Joan Guarro Flo
; Umberto Sollecchia
; | | Date: |
17 Aug 2020 | | Abstract: | We analyse the spectroscopic and photometric variability of the Oe star HD
60848 over the last twenty five years. The spectra reveal recurrent, but
irregular cycles of increased circumstellar emission lines. These cycles are
highly asymmetric displaying a slow increase over about 6 years, followed by a
fast decay within about 6 months. Our analysis focuses on the most recent cycle
(2013 - 2020). The equivalent width and velocity separation of the emission
peaks indicate variations of the outer disk radius by a factor ~ 2.2, although
the variability appears more complex than expected from first principle
relations for optically thin Keplerian disks. We observe a time delay between
the variations of the strengths of He I 5876 on the one hand and H-alpha and
H-beta on the other hand. We interpret this behaviour in a two-step disk growth
scenario, where the disk first expands radially before its density increases. A
difference in behaviour is also seen between H-alpha and the H I Paschen lines,
with the latter displaying a more symmetric cycle, similar to the photometric
variability. The rather fast decays of the H-alpha emission observed in 2001,
2009 and 2018 - 2019 suggest that the strong radiation field and early spectral
type of the star lead to a faster dissipation of the disk than in later-type Be
stars, as theoretically expected. We discuss X-ray observations of the star
both during a high and a low-emission state. The X-ray spectrum is soft at both
epochs, and the X-ray fluxes are only marginally different and remain
consistent with the canonical Lx/Lbol relation of O-type stars. These results
indicate that the circumstellar decretion disk of HD 60848 has essentially no
impact on the star’s X-ray emission, and that the latter most likely arises
inside the stellar wind. | | Source: | arXiv, 2008.07228 | | Services: | Forum | Review | PDF | Favorites | |
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