摘要：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 that 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
experiencing high mass loss. Specifically, the objective is to unravel the density
structure in the inner envelope and to examine the chemical interaction between 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 cannot
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 that prevents 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 does not seem to be a
viable explanation for the detection of gaseous NaCl at 220 R⋆ from the central star,
so instead, we propose shock-induced sputtering driven by localized mass ejection events
as an alternative.
Conclusions. The analysis of the NaCl lines demonstrates 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.
