W Orionis

W Orionis is a carbon star in the constellation Orion, approximately 400 parsecs (1,300 ly) away. It varies regularly in brightness between extremes of magnitude 4.4 and 6.9 roughly every 7 months. When it is near its maximum brightness, it is faintly visible to the naked eye of an observer with good observing conditions.

W Orionis

The visual band light curve for W Orionis, plotted from ASAS data[1]
Observation data Epoch J2000      Equinox J2000
Constellation	Orion
Right ascension	05h 05m 23.72142s[2]
Declination	+01° 10′ 39.4512″[2]
Apparent magnitude (V)	4.4 - 6.9[3]
Characteristics
Evolutionary stage	AGB
Spectral type	C-N5 C25.5[4] (C5,4(N5)[5])
U−B color index	+6.84[6]
B−V color index	+3.42[6]
Variable type	SRb[5]
Astrometry
Radial velocity (Rv)	+16.50[7] km/s
Proper motion (μ)	RA: +7.5[2] mas/yr Dec.: -1.4[2] mas/yr
Parallax (π)	2.65±0.95 mas[2]
Distance	approx. 1,200 ly (approx. 400 pc)
Absolute magnitude (MV)	−5.76[8]
Details
Mass	1-2[9] M☉
Radius	406[9] R☉
Luminosity	6,761[9] L☉
Surface gravity (log g)	−0.60[9] cgs
Temperature	2,600[9] K
Metallicity [Fe/H]	0.0[9] dex
Other designations
W Ori, BD+00°939, HD 32736, HIP 23680
Database references
SIMBAD	data

Variability

Evelyn Leland discovered that the star is a variable star based on observations done in the last decades of the 19th century, when it was known as BD +00°939. The discovery was announced in 1895.^[10] It was listed with its variable star designation, W Orionis, in Annie Jump Cannon's 1907 work Second Catalog of Variable Stars.^[11] W Orionis is a semiregular variable with an approximately 212‑day cycle.^[5] A long secondary period of 2,450 days has also been reported.^[12]

Properties

The angular diameter of W Orionis has been measured using interferometry and a value of 9.7 mas is found. Although it is known to be a pulsating variable star, no changes in the diameter were seen.^[9]

Technetium has not been detected in W Orionis, an unexpected result since this s-process element should be dredged up in all thermally-pulsating AGB stars and especially in carbon stars.^[9]