Pi1 Gruis

π¹ Gruis (Pi¹ Gruis) is a semiregular variable star in the constellation Grus around 530 light-years from Earth. It forms a close double star with π² Gru four arc-minutes away.

For other stars with this Bayer designation, see π Gruis.

π¹ Gruis

Surface of the red giant star π1 Gruis.[1]
Observation data Epoch J2000      Equinox J2000
Constellation	Grus
Right ascension	22h 22m 44.20571s[2]
Declination	−45° 56′ 52.6115″[2]
Apparent magnitude (V)	5.31 - 7.01[3] + 10.4[4]
Characteristics
Evolutionary stage	AGB[5]
Spectral type	S5,7:[6] + G0V[4]
Variable type	SRb[7]
Astrometry
Parallax (π)	6.13±0.76 mas[2]
Distance	approx. 530 ly (approx. 160 pc)
Absolute magnitude (MV)	G0: +4.3[4]
Orbit[4]
Primary	A
Companion	B
Period (P)	6,200 years
Semi-major axis (a)	2.8" (460 AU)
Orbit[8]
Primary	A
Companion	C
Period (P)	11.0+1.7 −1.5 years
Semi-major axis (a)	7.05+0.54 −0.57 AU
Eccentricity (e)	0.35+0.18 −0.17
Inclination (i)	31.75–38.04°
Longitude of the node (Ω)	321.7+5.91 −5.67°
Periastron epoch (T)	2015.06+1.09 −1.14
Argument of periastron (ω) (primary)	359.00+7.87 −11.26°
Details[8]
A
Mass	1.54–2.19 M☉
Radius	319±23 R☉
Luminosity	7,440 L☉
Temperature	2,300[9] – 3,100[4] K
B
Mass	1.06 M☉
Radius	1.10 R☉
Luminosity	1.35 L☉
Temperature	5,930 K
C
Mass	0.86+0.22 −0.20 M☉
Radius	If main sequence: 0.80+0.34 −0.17 R☉ If white dwarf: 0.009±0.002 R☉
Luminosity	If main sequence: 0.41+1.10 −0.31 L☉
Temperature	If main sequence: 5,170+820 −1,070 K
Other designations
CD−46°14292, HR 8521, HD 212087, SAO 231105, HIP 110478
Database references
SIMBAD	data

Characteristics

A visual band light curve for π¹ Gruis, adapted from Mayer et al. (2014)^[4]

π¹ Gruis is an asymptotic giant branch (AGB) star of spectral type S5.^[5] It is one of the brightest members of a class of stars known as S stars.^[10] It is also a semi-regular variable star ranging from apparent magnitude 5.3 to 7.0 over a period of 198.8 days.^[11]

An ageing star, π¹ Gruis is thought to be well on its way transitioning from a red giant to a planetary nebula. A shell of material has been detected at a distance of 0.91 light-years (0.28 parsecs), which is estimated to have been ejected 21,000 years ago.^[12] Closer to the star, there appears to be a cavity within 200 AU, suggesting a drop off in the ejection of material in the past 90 years. The presence of companions makes the shape of the shell irregular rather than spherical.^[10][8]

The primary star has a measured diameter of 18.37 milliarcseconds,^[13] corresponding to a size 350 times that of the Sun.^[1]

Companions

π¹ Gruis has two companion stars forming a gravitationally bound stellar system.

The inner, named π¹ Gruis C, is 7.05+0.54
−0.57 AU away from the primary and has an orbital period of 11 years. Preliminary data suggest an orbital eccentricity of 0.35+0.18
−0.17, albeit the orbit may be circular. Its mass is measured at 0.86+0.20
−0.22 M_☉, suggesting it is either a K-type main-sequence star or white dwarf. It is accreting matter from the red giant primary, and as of such the system may be undergoing novae every 900 years.^[8]

The outer companion, named π¹ Gruis B, is of apparent magnitude 10.9 and is sunlike in properties—a yellow main sequence star of spectral type G0V. Separated by 2.71″ (projected separation of 453±33 AU), the pair is thought to be gravitationally bound.^[13]

History of observations

The star was catalogued by French explorer and astronomer Nicolas Louis de Lacaille in 1756 but not given a name. Instead, he gave the Bayer designation of "π Gruis" to π². It was Thomas Brisbane who designated this star as π¹.^[14] Annie Jump Cannon was the first to report its unusual spectrum, sending a plate of its spectrograph made in 1895 to Paul W. Merrill and noting its similarity to R Andromedae. Merrill selected these two stars along with R Cygni to be the three prototypes of the S star class.^[15] π¹ Gruis was one of the first 17 stars defined as S-stars by Merrill in 1922; the only star not observed from Mount Wilson due to its southerly location in the sky.^[16] Analysis of its spectrum showed bands indicating the presence of technetium,^[17] as well as oxides of zirconium, lanthanum, cerium and yttrium but not titanium nor barium which have been recorded in other S stars.^[18]