Optical and near-infrared observations of the Fried Egg Nebula

Abstract

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 (Teff ∼ 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 Na I and the continuum emission. Interestingly, the spectrum of IRAS 17163−3907 around 2 μm shows Mg 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 Na 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 Na 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.

The first interferometric survey of massive YSOs in the K-band

“…Notably, at high accretion rates (M acc >=10 −3 M ⊙ /yr), the accreting MYSO is expected to be bloated (Hosokawa et al 2010), and therefore cool, which can arguably prevent the direct ionisation of the disc. However, Simon et al (1983) showed that winds of MYSOs can be so dense that hydrogen is collisionally excited to its n=2 state, which makes its ionisation from cooler stars possible (see also, Koumpia et al 2020;Drew et al 1998;Drew 1998). Indeed, a bloated star could also explain the narrow single line profiles of Brγ observed in MYSOs in various studies (e.g., Pomohaci et al 2017), which is in contrast to the relatively broader and double-peaked lines predicted by disc models around hot main sequence stars (Sim et al 2005).…”

“…Wanting to limit the algorithm dependency on the reconstructed images, we proceed to a thorough comparison of WISARD with the Multi-aperture ImageReconstruction Algorithm (MIRA; Thiébaut 2008) and BSMEM (Baron & Young 2008). A detailed description of the initialisation of the image reconstruction algorithms is presented in Koumpia et al (2020).…”

Wind-envelope interaction as the origin of the slow cyclic brightness variations of luminous blue variables

“…Moreover, multidimensional calculations including a local treatment of the radiative acceleration in the stellar envelope, and hydrodynamic wind models able to investigate the complex radiative acceleration in the supersonic outflows (Sundqvist et al 2019;Sander et al 2020;Björklund et al 2020), can give insights into the ability of our scenario to reproduce the observed phenomenology of these massive stars. Furthermore, our models establish the framework required for quantitative predictions on the formation of circumstellar nebulae surrounding LBVs (Nota et al 1995;Weis 2003;Agliozzo et al 2014), due to wind-wind interaction and variable mass-loss rates (Vink & de Koter 2002;Koumpia et al 2020), as well as their imprint on supernovae (Chevalier & Fransson 1994;Kotak & Vink 2006;Boian & Groh 2018).…”

“…giants as an empirical limit for the triggering of the S Dor instability (Smith et al 2004;Davies et al 2018), as well as the formation of shells and nebulae due to the changing wind properties (e.g., Koumpia et al 2020). For post-main-sequence stars, we expect a significantly lower initial-mass limit because the L/M is higher (higher by more than an order of magnitude than the main-sequence) reached in the late phases of stellar evolution (Brott et al 2011;Langer 2012).…”

Superadiabaticity and the metallicity independence of the Humphreys-Davidson limit

“…Here we discuss the effects of reduced mass loss on our results, by replacing the standard RSG metallicity-independent mass-loss rates of de Jager with the Fe-dependent recipe of Vink et al (2001) in the yellow supergiant regime (5500 K < 𝑇 eff < 10 kK). The mass loss in the YSG regime are highly uncertain (see Lobel et al 2003;Gordon & Humphreys 2019;Andrews et al 2019;Koumpia et al 2020;Grassitelli et al 2021) and understanding the variation of mass loss as a function of effective temperature is crucial. The RSG rates by de Jager are still used for cooler temperatures.…”