R Andromedae
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R Andromedae (R And) is a Mira-type variable star in the constellation Andromeda. Its spectral class is type S because it shows absorption bands of zirconium monoxide (ZrO) in its spectrum. It was among the stars found by Paul Merrill to show absorption lines of the unstable element technetium,[11] establishing that nucleosynthesis must be occurring in stars. The SH molecule was found for the first time outside earth in the atmosphere of this star.[12] The star is losing mass due to stellar winds at a rate of 1.09Template:E Template:Solar mass/yr.[5]
Variability
R Andromedae shows periodic variations in its brightness approximately every 409 days. The maximum brightness is not the same every cycle and can reach a peak magnitude of mv = 5.8, with the lowest known minima nearly 10 magnitudes fainter. The rise to maximum brightness is approximately twice as fast as the fall to minimum brightness. It is classified as a Mira variable. Those stars contract and expand regularly, changing size and temperature, and this causes the brightness variations.[2]
Properties
R Andromedae has a spectral type that varies as its brightness changes. At a typical maximum it is assigned a spectral type of S5/4.5e. This makes it an S-type star, a red giant similar to class M stars but with unusually strong molecular bands of ZrO in its spectrum compared to the titanium oxide (TiO) bands seen in other cool giants. S stars are intermediate between carbon stars and the more typical oxygen-rich giants. The S5 indicates its relative temperature, while the number after the slash is a measure of the relative C:O ratio, 4.5 meaning carbon is about 97% as abundant as oxygen. ZrO bands in R Andromedae are about twenty times stronger than those of TiO.[13] When it is fainter, the spectral type has been classified as late as S8,8e. On this older classification system for S stars, the number after the comma is an indication of the relative strength of ZrO and TiO bands which used to be considered to show the C:O ratio.[14]
R Andromedae, like all Mira variables, is an asymptotic giant branch star, one that has exhausted its core helium and is burning it in a shell outside the core and hydrogen in a shell closer to the surface. These stars undergo dredge-up events which cause convection of fusion products to the surface and anomalies such as enhanced carbon and zirconium. Asymptotic giant branch stars are very cool and luminous red giants; R Andromedae varies in temperature and luminosity but is typically about 2,500 K and Template:Solar luminosity. The angular diameter of R Andromedae has been measured at Script error: No such module "val". and Script error: No such module "val". on different dates, corresponding to radii of Template:Solar radius and Template:Solar radius respectively, assuming a distance of Script error: No such module "val"..[8] Other measurements based on the spectral energy distribution of the star give a luminosity of Template:Solar luminosity and a very cool temperature of Script error: No such module "val".,[7] which imply a very large radius of Template:Solar radius calculator.Template:Efn R Andromedae is one of the largest known stars.
Notes
References
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