SW Virginis

SW Virginis is a variable star in the equatorial constellation of Virgo, abbreviated SW Vir. It is a variable star that ranges in brightness from an apparent visual magnitude of 6.65 down to 7.95,^[4] with a pulsation period of 153.8 days.^[13] The star is located at a distance of approximately 590 light years from the Sun based on parallax measurements,^[3] but is drifting closer with a radial velocity of −16 km/s.^[9] SW Vir is located near the ecliptic and is subject to lunar occultation, which has allowed its angular diameter to be directly measured.^[14][15][11]

SW Virginis

A visual band light curve for SW Virginis, adapted from Percy et al. (2001)[1]
Observation data Epoch J2000.0      Equinox J2000.0
Constellation	Virgo[2]
Right ascension	13h 14m 04.383s[3]
Declination	−02° 48′ 25.13″[3]
Apparent magnitude (V)	6.65 to 7.95[4]
Characteristics
Evolutionary stage	AGB[5]
Spectral type	M7III:[6]
U−B color index	0.72[7]
B−V color index	1.67[7]
Variable type	SRb[8]
Astrometry
Radial velocity (Rv)	−15.78±3.51[9] km/s
Proper motion (μ)	RA: −35.131 mas/yr[3] Dec.: −2.769 mas/yr[3]
Parallax (π)	5.5526±0.4437 mas[3]
Distance	590 ± 50 ly (180 ± 10 pc)
Details
Mass	1–1.25[10] M☉
Radius	244±42[11] R☉
Luminosity	4,500±1,100[10] L☉
Surface gravity (log g)	−0.3±0.1[10] cgs
Temperature	3,060±130[11] K
Metallicity [Fe/H]	~0.0[10] dex
Other designations
SW Vir, BD−02°3653, HD 114961, HIP 64569, SAO 139236, PPM 179000[12]
Database references
SIMBAD	data

The variability of this star was discovered by W. P. Fleming and announced in 1901, when it was known as BD−02°3653.^[16] It was given its variable star designation, SW Virginis, in 1912.^[17] The star was initially catalogued as an irregular variable but later was classed as a semi-regular variable of type SRb.^[4] The stellar classification of M7III: indicates this is an evolved red giant, with the trailing colon indicating some uncertainty about the classification.^[6] In 2003, the isotope ⁹⁹Tc was detected in the atmosphere of SW Vir. The short lifespan of this element is an indicator that the star is undergoing the third dredge-up during the thermally pulsating phase of the asymptotic giant branch (AGB).^[5]

This is an oxygen-rich non-Mira AGB star^[18][19] that is shedding mass at a rate of 4×10⁻⁷ M_☉·yr⁻¹.^[20] An infrared excess indicates the star has a circumstellar dust envelope consisting of grains of matter that have condensed out of expelled gas.^[21] This dust shell displays an asymmetric appearance.^[18][19] Polarization of light from these dust grains indicate a typical size of less than a μm.^[19] The shell's thermal (non-maser) mm-wave spectral line emission from carbon monoxide was detected in 1986, and showed that the circumstellar envelope is expanding at 8.6 km/sec.^[22] The spectrum of molecular water has been detected in the star's extended outer atmosphere – out to twice the stellar radius – with a column density of 10¹⁹–10²⁰ cm^–2.^[23]