12 Canis Majoris

12 Canis Majoris is a variable star located about 707 light years away from the Sun in the southern constellation of Canis Major.^[12] It has the variable star designation HK Canis Majoris; 12 Canis Majoris is the Flamsteed designation. This body is just barely visible to the naked eye as a dim, blue-white hued star with a baseline apparent visual magnitude of +6.07.^[3] It is moving away from the Earth with a heliocentric radial velocity of +16 km/s.^[3] This is the brightest star in the vicinity of the open cluster NGC 2287,^[13] although it is probably not a member based on its proper motion.^[14]

12 Canis Majoris

A visual band light curve for HK Canis Majoris (2020)[1]
Observation data Epoch J2000      Equinox J2000
Constellation	Canis Major
Right ascension	06h 47m 01.48353s[2]
Declination	−21° 00′ 55.4517″[2]
Apparent magnitude (V)	+6.07[3]
Characteristics
Spectral type	B7 II/III[4] or B5 V[5]
B−V color index	−0.159±0.004[3]
Variable type	SX Ari[6]
Astrometry
Radial velocity (Rv)	+16.4±2.5[3] km/s
Proper motion (μ)	RA: −14.568[2] mas/yr Dec.: +5.778[2] mas/yr
Parallax (π)	4.6155±0.0543 mas
Distance	707 ± 8 ly (217 ± 3 pc)
Absolute magnitude (MV)	−0.81[3]
Details
Mass	4.8[7] M☉
Radius	2.73[8] R☉
Luminosity	537[9] L☉
Surface gravity (log g)	4.23[5] cgs
Temperature	15,830±60[9] K
Metallicity [Fe/H]	−0.37[10] dex
Rotation	2.18045 d[7]
Rotational velocity (v sin i)	65[7] km/s
Age	134[11] Myr
Other designations
12 CMa, HK CMa, BD−20°1576, FK5 817, GC 8884, HD 49333, HIP 32504, HR 2509, SAO 172318[12]
Database references
SIMBAD	data

The Bayer designation ρ Canis Majoris/Rho Canis Majoris has been applied to this star in the past, but this is now rarely seen.^[15] Perhaps the best-known use was by Charles Messier when describing the cluster Messier 41: "cluster of stars below Sirius, near Rho Canis Majoris".^[16]

This star has a stellar classification of B7 II/III,^[4] matching the spectrum of a B-type star intermediate between a giant and bright giant.^[4] (Cidale et al. (2007) show a class of B5 V,^[5] which would indicate it is instead a B-type main-sequence star.) It is a magnetic Bp star^[17] of the helium–weak variety (CP4), with the spectrum displaying evidence for vertical stratification of helium in the atmosphere.^[18]

Holger Pedersen and Bjarne Thomsen discovered that 12 Canis Majoris is a variable star, in 1977.^[19] It was given its variable star designation in 1981.^[20] Samus et al. (2017) classify it as an SX Arietis variable that varies in brightness by about 0.05 magnitudes over a period of 2.18045 days.^[6] It has 4.8 times the mass of the Sun and 2.73^[8] times the Sun's radius. The star is radiating 537 times the Sun's luminosity from its photosphere at an effective temperature of 15,830 K.