AF Andromedae

AF Andromedae (AF And) is a luminous blue variable (LBV), a type of variable star. The star is one of the most luminous variables in M31, the Andromeda Galaxy.

AF Andromedae

A red band light curve for AF Andromedae, adapted from Joshi et al. (2019)[1]
Observation data Epoch J2000      Equinox J2000
Constellation	Andromeda
Right ascension	00h 43m 33.086s[2]
Declination	+41° 49′ 10.31″[2]
Characteristics
Spectral type	LBV
U−B color index	~ −0.9[3]
B−V color index	~ +0.1[3]
Variable type	LBV[4]
Astrometry
Radial velocity (Rv)	−152±9[5] km/s
Proper motion (μ)	RA: 0.088[2] mas/yr Dec.: −0.076[2] mas/yr
Parallax (π)	−0.0026±0.0674 mas
Distance	~2.5Mly ly (~780kpc pc)
Absolute magnitude (MV)	−8.2[6]
Details
Mass	50–120[7] M☉
Radius	63[4] R☉
Luminosity	1,500,000[6] L☉
Temperature	33,000±3,000 K (normal) 7,000 (outburst)[8] K
Other designations
AF Andromedae, AF And, HV 4013, 2MASS 00433308+4112103, Var 19
Database references
SIMBAD	data

Discovery

The star was discovered to be variable in 1927, with a photographic magnitude range of 15.3 to 16.5, at the Harvard College Observatory and designated HV 4013. It was considered to be the brightest variable star in M31.^[9][10] Two years later it was given the variable star designation AF Andromedae.^[11] Between 1917 and 1953, five or six major eruptions were detected and two or three minor ones. More eruptions were observed in 1970–74, 1987–92, 1998–2001, ^[8] and 2017.^[12]

AF And was often referred to as var 19, after its number in a Hubble list of variable stars in M31 and M33. It was identified as one of the five Hubble–Sandage variables: Var A, Var B, Var C, and Var 2 in M33, and Var 19 in M31.^[13][14][15] On the basis of color–color comparisons, it was assigned as spectral type B and described as related to the P Cygni variables. Observations from 1960 to 1970 showed irregular variations in the B (blue) magnitude between 15.5 and 17.6, with visual magnitudes somewhat brighter.^[13] The first detailed spectrum was published in 1975.^[5]

Spectrum

AF And in outbursts has a peculiar emission line spectrum described as very much like Eta Carinae, likely due to a dense stellar wind.^[5] When quiescent, the spectrum is similar to late Of or WN stars.^[16]

AF And has prominent allowed and forbidden Fe_II and hydrogen lines in its emission spectrum, as well as weaker He_I lines. The variability and lack of absorption lines defy a normal spectral classification, but it was suggested that it may be close to class A.^[5]

The 250.7 nm Fe_II line is unusually strong in emission. The same feature in Eta Carinae's spectrum has been attributed to a UV laser.^[16]

Properties

AF And was the brightest star in M31 when it was first noticed during an outburst, at an apparent magnitude around 15, over a million times more luminous than the Sun. Newer calculations give a luminosity slightly less than a million times that of the Sun.

The star's mass has not been calculated explicitly, but this type of star is massive, typically 50–120 M_☉.^[7]

See also

• Astronomy portal

• List of Andromeda's satellite galaxies
• M31-RV