Abstract
ABSTRACT Despite their large impact on stellar and galactic evolution, the properties of outflows from red supergiants are not well characterized. We used the Onsala 20m telescope to perform a spectral survey at 3 and 4 mm (68–116 GHz) of the red supergiant NML Cyg, alongside the yellow hypergiant IRC + 10420. Our observations of NML Cyg were combined with complementary archival data to enable a search for signatures of morphological complexity in the circumstellar environment, using emission lines from 15 molecular species. The recovered parameters imply the presence of three distinct, coherent, and persistent components, comprised of blue-shifted and red-shifted components, in addition to an underlying outflow centred at the stellar systemic velocity. Furthermore, to reproduce 12CO emission with 3D radiative transfer models required a spherical outflow with three superposed conical outflows, one towards and one away from the observer, and one in the plane of the sky. These components are higher in density than the spherical outflow by up to an order of magnitude. We hence propose that NML Cyg’s circumstellar environment consists of a small number of high-density large-scale coherent outflows embedded in a spherical wind. This would make the mass-loss history similar to that of VY CMa, and distinct from μ Cep, where the outflow contains many randomly distributed smaller clumps. A possible correlation between stellar properties, outflow structures, and content is critical in understanding the evolution of massive stars and their environmental impact.
Highlights
Massive stars, with initial masses > 8 M, are important astrophysical objects that act as powerful sources of chemical and mechanical enrichment through feedback processes from their stellar winds
This paper has presented the results of a spectral survey carried out over 68–116 GHz towards the red supergiants (RSGs) NML Cyg with the 3 and 4 mm receivers on the OSO 20 m radio telescope
We detected emission from 15 lines pertaining to 10 molecular species around NML Cyg
Summary
With initial masses > 8 M , are important astrophysical objects that act as powerful sources of chemical and mechanical enrichment through feedback processes from their stellar winds. Key stages for feedback from massive stars are the short-lived post-main sequence phases of red supergiants (RSGs) and yellow hypergiants (YHGs), which experience strong mass loss, at rates reaching up to 10−3 M yr−1 via episodic outflows (de Jager 1998; Levesque 2010). These stellar stages are characterized by high luminosities (105 L ) and relatively cool temperatures (Teff ∼ 3000–8000 K), allowing for the formation of molecules and dust close to the star. All of these previous studies show evidence of a rich chemistry present in the environments surrounding massive evolved stars, for which a full understanding has not been reached
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