Tau Canis Majoris

Tau Canis Majoris is a multiple star system in the constellation Canis Major. Its name is a Bayer designation that is Latinized from τ Canis Majoris, and abbreviated Tau CMa or τ CMa. This system is approximately 5,000 light years distant from Earth and is the brightest member of the open cluster NGC 2362.

Tau Canis Majoris

Location of τ CMa (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Canis Major
Right ascension	07h 18m 42.48642s[1]
Declination	−24° 57′ 15.7413″[1]
Apparent magnitude (V)	4.40[2] (4.89 + 5.33 + 9.70[3])
Characteristics
Spectral type	O9II[4] (O9II + B0.5V + B0.5V[5])
U−B color index	−0.99[2]
B−V color index	−0.15[2]
Variable type	β Lyr[6]
Astrometry
Radial velocity (Rv)	+33.80[7] km/s
Proper motion (μ)	RA: −2.31 mas/yr[1] Dec.: 5.02 mas/yr[1]
Parallax (π)	1.09±0.59 mas[1]
Distance	1,570[8] pc
Absolute magnitude (MV)	−5.9 + −3.6 + −3.6[5]
Details
τ CMa Aa
Mass	50[9] M☉
Radius	19.8[10] R☉
Luminosity	280,000[11] L☉
Surface gravity (log g)	3.5[10] cgs
Temperature	32,000[11] K
τ CMa Ab1
Mass	17.8[9] M☉
τ CMa Ab2
Mass	17.8[9] M☉
Other designations
τ Canis Majoris, 30 Canis Majoris, CD−24°5176, GC 9736, HD 57061, HIP 35415, HR 2782, SAO 173446, ADS 5977, CCDM 07187-2457[12]
Database references
SIMBAD	data

This star is sometimes known as the 'Mexican Jumping Star' by amateur astronomers, because it can appear to 'jump around' with respect to the other stars in the cluster because of its marked contrast in brightness.^[13]

System

τ CMa at the center of NGC 2362

τ Canis Majoris lies at the center of the very young open cluster NGC 2362, which contains several hundred stars. It is by far the brightest member of the cluster and the only one to have evolved away from the main sequence.

A number of stars were catalogued by John Herschel as companions of the primary, τ CMa A: component B is a 10th magnitude star 8.6 arc-seconds distant; component C is a 14th magnitude star 14.2 arc-seconds away; and component D is an 8th magnitude star at 85 arc-seconds.^[3][14] These stars are all thought to be main sequence members of NGC 2362. A period of 94,000 years has been suggested for the AB pair assuming they are gravitationally bound.^[15]

In 1951, component A was resolved into a double separated by only 0.15", with an estimated orbital period of 250 years.^[16] The Washington Double Star Catalog lists the pair as magnitude 4.89 Aa and magnitude 5.33 Ab,^[3] but the CCDM designates the components as A and P.^[17] A 10th magnitude component E was discovered in 2010. It is less than 1" away from the 4.89 magnitude main component.^[18]

A light curve for Tau Canis Majoris, plotted from TESS data^[19]

The primary component A is itself a spectroscopic binary with a period of 154.918 days. Its variable radial velocity was discovered in 1906 and the first orbital elements published in 1928.^[20] More recently, Hipparcos satellite data revealed the existence of a 1.282 day eclipsing binary within the system, with two equal minima where the brightness dips by half a magnitude.^[5] The short period eclipsing pair is the fainter component of the longer period spectroscopic binary, forming an unusual triple star with the most massive component orbiting the pair of smaller stars.^[9] These three stars are referred to as Aa, Ab1, and Ab2; not to be confused with the fainter visible star 0.15 arc-seconds distant.^[17]

The relative brightnesses of the three spectroscopic components have been calculated from the eclipses and orbital movement. The spectrum, mass, and luminosity is dominated by component Aa which is now considered to be an O9 bright giant. It is thought to have a mass around 50 M_☉, a temperature of 32,000 K, and a luminosity of 280,000 L_☉. The two eclipsing stars are almost identical class B-type main-sequence stars with masses around 18 M_☉.

The properties of the minor components of the τ CMa grouping are poorly known. Component D has been described as having spectral type B2V, but this is uncertain.^[15] UW Canis Majoris is another 4th magnitude star less than half a degree away, and is itself an eclipsing binary system associated with NGC 2362. It has been catalogued as τ² CMa, but that name is now rarely used.^[21]