Oxygen-rich Mass Loss with a Pinch of Salt: NaCl in the Circumstellar Gas of IK Tauri and VY Canis Majoris

The NaCl molecule has been observed in the circumstellar envelopes of VY Canis Majoris (VY CMa) and IK Tauri (IK Tau)—the first identifications of a metal refractory in oxygen-rich shells of evolved stars. Five rotational transitions of NaCl at 1 and 2 mm were detected toward VY CMa and three 1 mm lines were observed toward IK Tau, using the telescopes of the Arizona Radio Observatory. In both objects, the line widths of the NaCl profiles were extremely narrow relative to those of other molecules, indicating that sodium chloride has not reached the terminal outflow velocity in either star, likely a result of early condensation onto grains. Modeling the observed spectra suggests abundances, relative to H2, of f ~ 5 × 10-9 in VY CMa and f ~ 4 × 10-9 in IK Tau, with source sizes of 0.5'' and 0.3'', respectively. The extent of these sources is consistent with the size of the dust acceleration zones in both stars. NaCl therefore appears to be at least as abundant in O-rich shells as compared to C-rich envelopes, where f ~ (0.2-2) × 10-9, although it appears to condense out earlier in the O-rich case. Chemical equilibrium calculations indicate that NaCl is the major carrier of sodium at T ~ 1100 K for oxygen-rich stars, with predicted fractional abundances in good agreement with the observations. These measurements suggest that crystalline salt may be an important condensate for sodium in both C- and O-rich circumstellar shells.