nucleosynthesis processes occurring in the inner regions, which in turn depends on the mass of the progenitor star and on its initial metallicity. In our galaxy, we expect stars to be born with an almost solar C/O ratio (C/O $\sim0.5$, @AllendePrieto_02), that may change during the evolution of the star depending on its initial mass. Theoretical models [see, e.g., @Marigo_03; @Karakas_09] predict that the less massive progenitors (M$_{*}$ $\lesssim1.5$–2 M$_\odot$) will remain O-rich during their whole evolution. Stars with masses in the range $\sim$ 2–4 M$_\odot$ may evolve from O-rich to C-rich due to the third dredge-up process that enriches the surface of the AGB star with carbon. In the most massive progenitors (M$_{*}$ $\gtrsim$ 4–5 M$_\odot$), the hot bottom burning process occurs efficiently and, since it converts carbon into nitrogen via the CN cycle, it can prevent the formation of a C-rich star.

The value of C/O in a dust-forming stellar envelope defines whether the dust particles will be C-rich or O-rich. If carbon is more abundant than oxygen, all the oxygen atoms will be trapped in CO molecules and carbon atoms will be available to form C-rich compounds, such as graphite, amorphous carbon, or silicon carbide (SiC). If 