OH/IR star

{{Short description|Type of star}}

{{more footnotes|date=June 2020}}

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File:VLT image of the surroundings of VY Canis Majoris seen with SPHERE.jpg image of the surroundings of the red hypergiant VY Canis Majoris]]

An OH/IR star is an asymptotic giant branch (AGB), a red supergiant (RSG), or a red hypergiant (RHG) star that shows strong OH maser emission and is unusually bright at near-infrared wavelengths.

In the very late stages of AGB evolution, a star develops a super-wind with extreme mass loss. The gas in the stellar wind condenses as it cools away from the star, forming molecules such as water (H₂O) and silicon monoxide (SiO). This can form grains of dust, mostly silicates, which obscure the star at shorter wavelengths, leading to a strong infrared source.{{cite journal|doi=10.1051/0004-6361:20020036 |title=Dust and the spectral energy distribution of the OH/IR star OH 127.8+0.0: Evidence for circumstellar metallic iron |journal=Astronomy and Astrophysics |volume=384 |issue=2 |pages=585 |year=2002 |last1=Kemper |first1=F. |last2=De Koter |first2=A. |last3=Waters |first3=L. B. F. M. |last4=Bouwman |first4=J. |last5=Tielens |first5=A. G. G. M. |bibcode=2002A&A...384..585K |arxiv = astro-ph/0201128 |s2cid=17649812 }} Hydroxyl (OH) radicals can be produced by photodissociation or collisional dissociation.{{cite journal|bibcode=1976ApJ...205..144G|title=OH-IR stars. I - Physical properties of circumstellar envelopes|journal=Astrophysical Journal|volume=205|pages=144|last1=Goldreich|first1=P.|last2=Scoville|first2=N.|year=1976|doi=10.1086/154257|url=https://authors.library.caltech.edu/37334/1/1976ApJ___205__144G.pdf}}

H₂O and OH can both be pumped to produce maser emission. OH masers in particular can give rise to a powerful maser action at 1612 MHz and this is regarded as a defining feature of the OH/IR stars. Many other AGB stars, such as Mira variables, show weaker OH masers at other wavelengths, such as 1667MHz or 22MHz.{{cite journal|doi=10.1086/341534|title=On Dead OH/IR Stars|journal=The Astrophysical Journal|volume=576|issue=1|pages=445–449|year=2002|last1=Lewis|first1=B. M.|bibcode=2002ApJ...576..445L|doi-access=free}}