Molecules and Outflows in NML Cygni: New Insights from a 1 mm Spectral Line Survey

Abstract

A sensitive (1σ rms ≤ 3 mK) 1 mm spectral survey (214.5-285.5 GHz) of the oxygen-rich circumstellar envelope of the red hypergiant NML Cygni (NML Cyg) has been conducted using the Sub-millimeter Telescope (SMT) of the Arizona Radio Observatory (ARO). Over 100 spectral lines were detected, arising from 17 different molecules, including the carbon-bearing species CO, HCN, HCO+, CN, and HNC; sulfur- and silicon-containing compounds H2S, SO, SO2, SiO, and SiS; and more exotic NaCl and AlO. The 1 mm spectrum of NML Cyg closely resembles that of VY Canis Majoris (VY CMa) suggesting that the chemistries of hypergiant stars are similar. The line profiles in NML Cyg consist of multiple velocity features, particularly evident in SO2 and SO. In addition to a spherical wind at the star's systemic velocity, the spectra suggest an asymmetric, blueshifted component near V LSR = -21 3 km s-1 and a collimated, redshifted component near 15 3 km s-1, positioned ∼34 and ∼12 , respectively, from the line of sight. The red- and blueshifted flows appear to be randomly oriented, and likely trace sporadic mass loss events. Their LSR velocities align closely with those of 22 GHz water masers, suggesting an NE-SE orientation. The winds may also be associated with the asymmetric nebula in F555W HST images but extending to 5″ (∼600R ∗). NML Cyg appears to be another example of rare, massive stars with collimated, episodic ejections, analogous to Betelgeuse and VY CMa, lending support for an important new mass loss mechanism - surface activity. © 2021. The American Astronomical Society. All rights reserved.