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The envelopes of Red Supergiants (RSGs) have a unique chemical environment not seen in other types of stars. They foster an oxygen-rich synthesis but are tempered by sporadic and chaotic mass loss, which distorts the envelope and creates complex outflow sub-structures consisting of knots, clumps, and arcs. Near the stellar photosphere, molecules and grains form under approximate LTE conditions, as predicted by chemical models. However, the complicated outflows appear to have distinct chemistries generated by shocks and dust destruction. Various RSG envelopes have been probed for their molecular content, mostly by radio and millimeter observations; however, VY Canis Majoris (VY CMa) and NML Cygni (NML Cyg) display the highest chemical complexity, and also the most complicated envelope structure. Thus far, over 29 different molecules have been identified in the envelopes of RSGs. Some molecules are common for circumstellar gas, including CO, SiO, HCN and H2O, which have abundances of ∼10−6–10−4, relative to H2. More exotic oxides have additionally been discovered, such as AlO, AlOH, PO, TiO2, and VO, with abundances of ∼10−9–10−7. RSG shells support intricate maser emission in OH, H2O and SiO, as well. Studies of isotope ratios in molecules suggest dredge-up at least into the H-burning shell, but further exploration is needed.
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The Arizona Radio Observatory 1 mm Spectral Survey of the Hypergiant Star NML Cygni (215–285 GHz)
Abstract A sensitive (1σrms ≤ 3 mK; 2 MHz resolution) 1 mm spectral survey (214.5–285.5 GHz) of the envelope of the oxygen-rich supergiant star NML Cygni (NML Cyg) has been conducted using the 10 m Submillimeter Telescope of the Arizona Radio Observatory. These data represent the first spectral line survey of NML Cyg and are complementary to a previous 1 mm survey of the envelope of a similar hypergiant, VY Canis Majoris (VY CMa). The complete NML Cyg data set is presented here. In the survey, 104 emission lines were observed, arising from 17 different molecules and 4 unidentified features. Many of the observed features have complex line profiles, arising from asymmetric outflows characteristic of hypergiant stars. While most of the lines in the survey arise from SiO, SO, SO2, and SiS, CO had the strongest emission. Five other C-bearing species are identified in the survey (HCN, CN, HCO+, CS, and HNC), demonstrating an active carbon chemistry despite the O-rich environment. Moreover, NS was observed, but not NO, although favorable transitions of both molecules lie in the surveyed region. Sulfur chemistry appears to be prominent in NML Cyg and plays an important role in the collimated outflows. The refractory species observed, NaCl and AlO, have narrow emission lines, indicating that these molecules do not reach the terminal expansion velocity. NaCl and AlO likely condense into dust grains at r < 50R*. From NaCl, the chlorine isotope ratio was determined to be35Cl/37Cl = 3.85 ± 0.30.
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- Award ID(s):
- 1907910
- PAR ID:
- 10378363
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astronomical Journal
- Volume:
- 164
- Issue:
- 6
- ISSN:
- 0004-6256
- Format(s):
- Medium: X Size: Article No. 230
- Size(s):
- Article No. 230
- Sponsoring Org:
- National Science Foundation
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