R Andromedae

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 1.09×10⁻⁶ M_☉/yr.^[5]

R Andromedae

Location of R Andromedae (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Andromeda
Right ascension	00h 24m 01.946s[1]
Declination	+38° 34′ 37.37″[1]
Apparent magnitude (V)	5.8 - 15.2[2]
Characteristics
Spectral type	S3,5e-S8,8e(M7e)[2]
B−V color index	1.97[3]
Variable type	Mira[2]
Astrometry
Radial velocity (Rv)	−6.40 [4] km/s
Proper motion (μ)	RA: −16.021±0.148[1] mas/yr Dec.: −32.794±0.141[1] mas/yr
Parallax (π)	2.6444±0.1600 mas[1]
Distance	1,230 ± 70 ly (380 ± 20 pc)
Absolute magnitude (MV)	−5.19[5]
Details
Mass	3.7[6] M☉
Radius	730[7][a] or 476±120 – 493±129[8] R☉
Luminosity	6,300[7] L☉
Surface gravity (log g)	−1.02[9] cgs
Temperature	1,900 – 2,500[9] K
Metallicity [Fe/H]	−1.01[10] dex
Other designations
HR 90, BD +37°58, HD 1967, SAO 53860, HIP 1901.
Database references
SIMBAD	data

Variability

R Andromedae light curve

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 m_v = 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 6,300 L_☉. The angular diameter of R Andromedae has been measured at 8.63±1.42 mas and 8.32±1.27 mas on different dates, corresponding to radii of 493±129 R_☉ and 476±120 R_☉ respectively, assuming a distance of 532 pc.^[8] Other measurements based on the spectral energy distribution of the star give a luminosity of 6,000 L_☉ and a very cool temperature of 1,900 K,^[7] which imply a very large radius of 730 R_☉.^[a] R Andromedae is one of the largest known stars.

Notes

1. Applying the Stefan–Boltzmann law with a nominal solar effective temperature of 5,772 K:

   ${\displaystyle {\sqrt {{\biggl (}{\frac {5,772}{1,900}}{\biggr )}^{4}\cdot 6,000}}=730\ R_{\odot }}$.