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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.
| 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 | |
| Database references | |
| SIMBAD | data |
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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]
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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 FeII and hydrogen lines in its emission spectrum, as well as weaker HeI 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 FeII line is unusually strong in emission. The same feature in Eta Carinae's spectrum has been attributed to a UV laser.[16]
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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]
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