R Octantis

Not to be confused with Rho Octantis.

R Octantis, also known as HD 40857, is a solitary, red hued variable star located in the southern circumpolar constellation Octans. It has an apparent magnitude that varies in-between 6.4 and 13.2 within 405 days.^[3] At is maximum, it is barely visible to the naked eye. The object is located relatively far at a distance of about 1,900 light years based on parallax measurements from Gaia DR3,^[2] but is receding with a heliocentric radial velocity of 46 km/s.^[6]

R Octantis

The visual band light curve of R Octantis, from AAVSO data[1]
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Octans
Right ascension	05h 26m 06.19562s[2]
Declination	−86° 23′ 17.7741″[2]
Apparent magnitude (V)	6.4 – 13.2[3]
Characteristics
Spectral type	M5.5e[4] (M5.3e – M8.4e[3])
Variable type	Mira[5]
Astrometry
Radial velocity (Rv)	46±5[6] km/s
Proper motion (μ)	RA: 0.540 mas/yr[2] Dec.: 31.079 mas/yr[2]
Parallax (π)	1.7152±0.0862 mas[2]
Distance	1,900 ± 100 ly (580 ± 30 pc)
Details
Mass	6.6[7] M☉
Radius	466[7] R☉
Luminosity	9,304[8] L☉
Temperature	2,571[8] K
Other designations
R Octantis, CD−86°52, CPD−86°72, HD 40857, HIP 25412[9]
Database references
SIMBAD	data

R Octantis has been known to have a peculiar spectra since 1892.^[10] It was revealed to have emission lines in its spectrum in a 1954 paper^[11] and was found to be a long period variable a year later.^[12] In 1966, R Octantis was officially classified as a Mira variable.^[4] Later observations reveal it to have an overabundance of oxygen on its outer layers.^[13]

The object has an average stellar classification of M5.5e, indicating that it is a M-type star with emission lines in its spectrum.^[4] However, this can range from M5.3 to M8.4e.^[3] It is currently on the asymptotic giant branch, generating energy by fusing hydrogen and helium shells around an inert carbon core. As a result, it has expanded to 466 times the radius of the Sun.^[7] R Octantis radiates over 9,000 times the luminosity of the Sun from its photosphere at an effective temperature of 2,571 K,^[8] which is cooler than most Mira variables.