ALMA-resolved salt emission traces the chemical footprint and inner wind morphology of VY Canis Majoris

Abstract

Context. At the end of their lives, most stars lose a significant amount of mass through a stellar wind. The specific physical and chemical circumstances that lead to the onset of the stellar wind for cool luminous stars are not yet understood. Complex geometrical morphologies in the circumstellar envelopes prove that various dynamical and chemical processes are interlocked and their relative contributions are not easy to disentangle. Aims. We aim to study the inner-wind structure (R < 250 R ) of the well-known red supergiant VY CMa, the archetype for the class of luminous red supergiant stars suffering high mass loss. Specifically, the objective is to unravel the density structure in the inner envelope and to examine the chemical interaction between some gas and dust species. Methods. We analyse high spatial resolution (∼ 0 . 24 × 0 . 13) ALMA Science Verification (SV) data in band 7 in which four thermal emission lines of gaseous sodium chloride (NaCl) are present at high signal-to-noise ratio. Results. For the first time, the NaCl emission in the inner wind region of VY CMa is spatially resolved. The ALMA observations reveal the contribution of up to four different spatial regions. The NaCl emission pattern is different compared to the dust continuum and TiO2 emission already analysed from the ALMA SV data. The emission can be reconciled with an axisymmetric geometry, where the lower density polar/rotation axis has a position angle of ∼50• measured from north to east. However, this picture can not capture the full morphological diversity, and discrete mass ejection events need to be invoked to explain localized higher-density regions. The velocity traced by the gaseous NaCl line profiles is significantly lower than the average wind terminal velocity, and much slower than some of the fastest mass ejections, signalling a wide range of characteristic speeds for the mass loss. Gaseous NaCl is detected far beyond the main dust condensation region. Realising the refractory nature of this metal halide, this hints at a chemical process preventing all NaCl from condensing onto dust grains. We show that in the case of the ratio of the surface binding temperature to the grain temperature being ∼50, only some 10% of NaCl remains in gaseous form, while for lower values of this ratio thermal desorption efficiently evaporates NaCl. Photodesorption by stellar photons seems not to be a viable explanation for the detection of gaseous NaCl at 220 R from the central star, and instead, we propose shock-induced sputtering driven by localized mass ejection events as alternative. Conclusions. The analysis of the NaCl lines demonstrate the capabilities of ALMA to decode the geometric morphologies and chemical pathways prevailing in the winds of evolved stars. These early ALMA results prove that the envelopes surrounding evolved stars are far from homogeneous, and that a variety of dynamical and chemical processes dictate the wind structure.

Orion SrcI’s Disk Is Salty

“…To our knowledge, SrcI is only the fifth astronomical source in which KCl and NaCl have been detected. The only previous detections of vibrationally excited NaCl were in the v=1 level in IRC+10216 (Quintana-Lacaci et al 2016) and VY CMa (Decin et al 2016), while SrcI exhibits clear emission up to v=6 in both NaCl and KCl.…”

Massive dust clumps in the envelope of the red supergiant VY Canis Majoris

“…tribution within the nebula than the continuum. None of the spatial structures, except for SW Clump (Kamiński et al 2013;Decin et al 2016;De Beck et al 2015), can be uniquely identified as a counterpart of any of the major continuum clumps. In particular, as noted before by O'Gorman et al (2015), molecular emission seems to "avoid" regions of strong continuum emission.…”

“…The extended SW Clump has been relatively well characterized in molecular and atomic gas (Kamiński et al 2013;Humphreys et al 2005;Decin et al 2016). Its CO spectrum displays at least four distinct velocity components centered at about -16, 18, 38, and 53 km s −1 .…”

“…Our understanding of the mass-loss mechanism in red supergiants -and in VY CMa in particular -is possibly poorest among all cool evolved stars. Evolutionary modelers often assume crudely calibrated massloss formula (e.g de Jager et al 1988) and consequently mass loss is considered a major source of uncertainty in theoretical astrophysics (e.g. Georgy & Ekström 2015).…”

“…Observed at sufficient angular resolutions, the envelope has revealed a wide diversity of structures, e.g. knots, arcs, clumps, rings, at scales ranging from mas to arcsec, and in all accessible wavelength regimes, from visual to radio (Smith et al 2001;Humphreys et al 2005Humphreys et al , 2007Kamiński et al 2013;Decin et al 2016;Richards et al 2014;O'Gorman et al 2015;Gordon et al 2019). Their structure and distibution is very erratic and Submillimeter Array Fellow the envelope does not appear to have any preferred axis of symmetry.…”