Optical and near-infrared observations of the Fried Egg Nebula: Multiple shell ejections on a 100 yr timescale from a massive yellow hypergiant
Artikel i vetenskaplig tidskrift, 2020

Context. The fate of a massive star during the latest stages of its evolution is highly dependent on its mass-loss rate and geometry and therefore knowing the geometry of the circumstellar material close to the star and its surroundings is crucial. Aims. We aim to provide insight into the nature (i.e. geometry, rates) of mass-loss episodes, and in particular, the connection between the observed asymmetries due to the mass lost in a fast wind or during a previous, prodigious mass-losing phase. In this context, yellow hypergiants offer a good opportunity to study mass-loss events. Methods. We analysed a large set of optical and near-infrared data in spectroscopic and photometric, spectropolarimetric, and interferometric (GRAVITY/VLTI) modes, towards the yellow hypergiant IRAS 17163-3907. We used X-shooter optical observations to determine the spectral type of this yellow hypergiant and we present the first model-independent, reconstructed images of IRAS 17163-3907 at these wavelengths tracing milli-Arcsecond scales. Lastly, we applied a 2D radiative transfer model to fit the dereddened photometry and the radial profiles of published diffraction-limited VISIR images at 8.59 μm, 11.85 μm, and 12.81 μm simultaneously, adopting a revised distance determination using Gaia Data Release 2 measurements. Results. We constrain the spectral type of IRAS 17163-3907 to be slightly earlier than A6Ia (Teffâ ∼â 8500 K). The interferometric observables around the 2 μm window towards IRAS 17163-3907 show that the Brγ emission appears to be more extended and asymmetric than the Naâ » I and the continuum emission. Interestingly, the spectrum of IRAS 17163-3907 around 2 μm shows Mgâ » II emission that is not previously seen in other objects of its class. In addition, Brγ shows variability in a time interval of four months that is not seen towards Naâ » I. Lastly, in addition to the two known shells surrounding IRAS 17163-3907, we report on the existence of a third hot inner shell with a maximum dynamical age of only 30 yr. Conclusions. The 2 μm continuum originates directly from the star and not from hot dust surrounding the stellar object. The observed spectroscopic variability of Brγ could be a result of variability in the mass-loss rate. The interpretation of the presence of Naâ » I emission at closer distances to the star compared to Brγ has been a challenge in various studies. To address this, we examine several scenarios. We argue that the presence of a pseudo-photosphere, which was traditionally considered to be the prominent explanation, is not needed and that it is rather an optical depth effect. The three observed distinct mass-loss episodes are characterised by different mass-loss rates and can inform theories of mass-loss mechanisms, which is a topic still under debate both in theory and observations. We discuss these in the context of photospheric pulsations and wind bi-stability mechanisms.

Stars: individual: IRAS 17163-3907

Stars: AGB and post-AGB

Stars: evolution

Stars: mass-loss

Techniques: interferometric

Stars: imaging

Författare

E. Koumpia

University of Leeds

R. D. Oudmaijer

University of Leeds

V. Graham

University of Leeds

G. Banyard

KU Leuven

John H Black

Chalmers, Rymd-, geo- och miljövetenskap, Astronomi och plasmafysik, Galaktisk astrofysik

C. Wichittanakom

University of Leeds

Thammasat University

K. M. Ababakr

Erbil Polytechnic University

W. J. De Wit

European Southern Observatory Santiago

F. Millour

Observatoire de la Cote d'Azur

E. Lagadec

Observatoire de la Cote d'Azur

Sebastien Muller

Chalmers, Rymd-, geo- och miljövetenskap, Onsala rymdobservatorium, Onsala rymdobservatorium, Observationssupport

N. L. J. Cox

ACRI-ST

A. Zijlstra

University of Manchester

The University of Hong Kong

H. van Winckel

KU Leuven

M. Hillen

KU Leuven

R. Szczerba

Polish Academy of Sciences

J. S. Vink

Armagh Observatory

Sofia Wallström

KU Leuven

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 635 A183

Ämneskategorier

Astronomi, astrofysik och kosmologi

DOI

10.1051/0004-6361/201936177

Mer information

Senast uppdaterat

2020-04-27