Eta Geminorum is the star's
Bayer designation. The traditional names Tejat Prior, Propus (from the
Greek, meaning forward foot) and Praepes and Pish Pai (from the
Persian Pīshpāy, پیشپای, meaning foreleg). In 2016, the
International Astronomical Union organized a
Working Group on Star Names (WGSN)[15] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[16] included a table of the first two batches of names approved by the WGSN; which included Propus for this star.
This star, along with
γ Gem (Alhena),
μ Gem (Tejat Posterior),
ν Gem and
ξ Gem (Alzirr) were Al Han'ah, "the brand" (on the neck of the camel). They also were associated in Al Nuḥātai, the dual form of Al Nuḥāt, "a Camel's Hump".[13]
η Geminorum lies at the foot of the
Castor side of Gemini, about two degrees west of
μ Geminorum and two degrees southeast of the bright
open clusterM35. Between the two stars are several faint areas of nebulosity. η Gem just to the west of the supernova remnant shell
IC 443. Further east around μ Gem is the
emission nebulaS249. In between is the small faint emission nebula
IC 444 around the 7th magnitude
12 Geminorum.
η Geminorum is 0.9 degree south of the
ecliptic, so it can be
occulted by the
Moon[18] and, rarely, by
planets. The last occultation by a planet took place on July 27, 1910, by
Venus,[19] and the next to last on July 11, 1837, by
Mercury.[citation needed]
Variability
In 1865,
Julius Schmidt first reported that η Geminorum was a variable star. The light variations were described by Schmidt and other observers as having long maxima of constant brightness, minima of greatly varying size and shape, and a period around 231 days.[21] The star was classified as both a
semiregular variable and an
eclipsing variable. The eclipse period has been set at about eight years, corresponding to the orbit of an unseen companion. The eclipses were initially questioned, but special projects led to the eclipses of February 1980, April 1988 and October 2012 being well observed. Eclipses have depths of about half a magnitude and durations of several weeks.[22][20]
The semi-regular variations have been classified as type SRa, indicating relatively predictable periodicity with some variations in amplitude and light curve shape. These types of variable are considered to be very similar to
Mira variables, but with smaller amplitudes.[2] Many long-period variables show long secondary periods, typically ten times longer than the main period, but these changes have not been detected for η Geminorum. The main period has been refined to an average of 234 days.[23]
System
η Geminorum is a triple system, with the luminous class M star having a close companion known only from radial velocity variations, and a more distant companion resolved visually.
In 1881,
Burnham observed that η Geminorum had a close companion (η Gem B). At that time the separation was measured to be 1.08".[24] This has now increased to 1.65" and an orbit has been calculated to be 474 years long and rather eccentric.[21] Little is known about the companion, although it is 6th magnitude. It is given a G0 spectral type and is assumed to be a giant on the basis of its brightness.[4]
In 1902,
William Wallace Campbell reported that η Geminorum A showed radial velocity variations. The assumption was that the star was a spectroscopic binary, although no period or other orbital parameters were determined.[25] An orbit calculated in 1944 is essentially unchanged today, with a period of 2,983 days and an eccentricity of 0.53. Observations were made looking for sign of eclipses corresponding to the derived orbit, but the evidence was regarded as inconclusive,[26] and the eclipses were not confirmed until much later. Due to the appearance of the spectrum, the spectroscopic companion is suspected to be a fainter M-class star.[4] Since the secondary star alone would be too small to cause the observed eclipses, it is probably surrounded by a
circumstellar disk.[9]
Evolution
The luminous main component of η Geminorum is an
asymptotic giant branch star, a highly evolved cool luminous star that was originally 2-8 M☉ on the main sequence.[27]
^
abcdeHøg, E.; Fabricius, C.; Makarov, V. V.; Urban, S.; Corbin, T.; Wycoff, G.; Bastian, U.; Schwekendiek, P.; Wicenec, A. (2000). "The Tycho-2 catalogue of the 2.5 million brightest stars". Astronomy and Astrophysics. 355: L27.
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2000A&A...355L..27H.
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^
abcSamus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1.
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^
abcdeHunsch, Matthias; Schmitt, Jurgen H. M. M.; Schroder, Klaus-Peter; Zickgraf, Franz-Josef (1998). "On the X-ray emission from M-type giants". Astronomy and Astrophysics. 330: 225.
Bibcode:
1998A&A...330..225H.
^
abTorres, Guillermo; Sakano, Kristy (2022). "η Geminorum: An eclipsing semiregular variable star orbited by a companion surrounded by an extended disc". Monthly Notices of the Royal Astronomical Society. 516 (2): 2514–2521.
arXiv:2208.07375.
Bibcode:
2022MNRAS.516.2514T.
doi:
10.1093/mnras/stac2322.
^Huang, Y.; Liu, X.-W.; Yuan, H.-B.; Xiang, M.-S.; Chen, B.-Q.; Zhang, H.-W. (2015). "Empirical metallicity-dependent calibrations of effective temperature against colours for dwarfs and giants based on interferometric data". Monthly Notices of the Royal Astronomical Society. 454 (3): 2863.
arXiv:1508.06080.
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2015MNRAS.454.2863H.
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10.1093/mnras/stv1991.
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^Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub.
ISBN978-1-931559-44-7.
^White, Nathaniel M.; Feierman, Barry H. (September 1987). "A Catalog of Stellar Angular Diameters Measured by Lunar Occultation". Astronomical Journal. 94: 751.
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1987AJ.....94..751W.
doi:
10.1086/114513.
^Können, G. P.; Van Maanen, J. (April 1981). "Planetary occultations of bright stars". Journal of the British Astronomical Association. 91: 148–157.
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1981JBAA...91..148K.
^Percy, J. R.; Nasui, C. O.; Henry, G. W. (2008). "Long-Term Photometric Variability of 13 Bright Pulsating Red Giants". The Journal of the American Association of Variable Star Observers. 36 (1): 139.
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2008JAVSO..36..139P.
^Clerke, A. M. (1902). "The system of eta Geminorum". The Observatory. 25: 389.
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