Eta Trianguli Australis

Eta Trianguli Australis (η Trianguli Australis) is a solitary,^[9] blue-white hued star in the southern constellation Triangulum Australe. It is sometimes given a superscript: η¹ Trianguli Australis, though it is the only star that is commonly referred to by this Bayer designation. Lacaille named a close by star as Eta as well, which was inconsistently followed by Francis Baily, who used the name for the brighter or both stars in two different publications. Despite their faintness, Gould upheld their Bayer designation as they were closer than 25 degrees to the south celestial pole. The second Eta is now designated as HD 150550.^[10]

Eta Trianguli Australis

Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Triangulum Australe
Right ascension	16h 41m 23.10722s[1]
Declination	−68° 17′ 46.0412″[1]
Apparent magnitude (V)	+5.91[2]
Characteristics
Spectral type	B7 V[3] or B7 IVe[4]
U−B color index	−0.42[2]
B−V color index	−0.08[2]
Astrometry
Radial velocity (Rv)	−10.0±7.6[5] km/s
Proper motion (μ)	RA: −14.067[1] mas/yr Dec.: −11.535[1] mas/yr
Parallax (π)	4.2679 ± 0.0842 mas[1]
Distance	760 ± 20 ly (234 ± 5 pc)
Absolute magnitude (MV)	−0.36[6]
Details[7]
Mass	3.76±0.12 M☉
Luminosity	318 L☉
Temperature	12,162 K
Rotational velocity (v sin i)	234 km/s
Age	199[6] Myr
Other designations
η TrA, CPD−68° 2789, GC 22379, HD 149671, HIP 81710, HR 6172, SAO 253649[8]
Database references
SIMBAD	data

This star is faintly visible to the naked eye with an apparent visual magnitude of +5.91.^[2] Based upon an annual parallax shift of just 4.93 mas as seen from the Earth, it is located roughly 760 light years from the Sun. At that distance, the visual magnitude of the star is diminished by an extinction of 0.28 due to interstellar dust.^[6] The system appears to be moving closer to the Sun with a radial velocity of around −10 km/s.^[5]

Eta Trianguli Australis is a B-type main-sequence star with a stellar classification of B7 V.^[3] Although it has been classified as a B7 IVe Be subgiant star in the past,^[4] Jaschek and Jaschek (1992) found no trace of emission in the spectrum.^[11] Grady et al. (1989) reported only "weak or narrow double-peaked Hα emission, but no emission at Hβ".^[12] The star shows a relatively high peculiar velocity of 40±20 km/s.^[4] It is spinning rapidly with a projected rotational velocity of 234 km/s.^[7]

References

1. Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
2. Johnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory, 4 (99): 99, Bibcode:1966CoLPL...4...99J.
3. Houk, Nancy (1979), Michigan catalogue of two-dimensional spectral types for the HD stars, vol. 1, Ann Arbor, Michigan: Dept. of Astronomy, University of Michigan, Bibcode:1978mcts.book.....H.
4. Berger, D. H.; Gies, D. R. (July 2001), "A Search for High-Velocity Be Stars", The Astrophysical Journal, 555 (1): 364–367, Bibcode:2001ApJ...555..364B, doi:10.1086/321461.
5. de Bruijne, J. H. J.; Eilers, A.-C. (October 2012), "Radial velocities for the HIPPARCOS-Gaia Hundred-Thousand-Proper-Motion project", Astronomy & Astrophysics, 546: 14, arXiv:1208.3048, Bibcode:2012A&A...546A..61D, doi:10.1051/0004-6361/201219219, S2CID 59451347, A61.
6. Gontcharov, G. A. (November 2012), "Spatial distribution and kinematics of OB stars", Astronomy Letters, 38 (11): 694–706, arXiv:1606.09028, Bibcode:2012AstL...38..694G, doi:10.1134/S1063773712110035, S2CID 119108982.
7. Zorec, J.; Royer, F. (January 2012), "Rotational velocities of A-type stars. IV. Evolution of rotational velocities", Astronomy & Astrophysics, 537: A120, arXiv:1201.2052, Bibcode:2012A&A...537A.120Z, doi:10.1051/0004-6361/201117691, S2CID 55586789.
8. "eta01 TrA". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2017-09-14.{{cite web}}: CS1 maint: postscript (link)
9. Eggleton, P. P.; Tokovinin, A. A. (September 2008), "A catalogue of multiplicity among bright stellar systems", Monthly Notices of the Royal Astronomical Society, 389 (2): 869–879, arXiv:0806.2878, Bibcode:2008MNRAS.389..869E, doi:10.1111/j.1365-2966.2008.13596.x, S2CID 14878976.
10. Wagman, Morton (2003), Lost Stars: Lost, Missing and Troublesome Stars from the Catalogues of Johannes Bayer, Nicholas Louis de Lacaille, John Flamsteed, and Sundry Others, Blacksburg, VA: The McDonald & Woodward Publishing Company, pp. 303–04, Bibcode:2003lslm.book.....W, ISBN 978-0-939923-78-6
11. Jaschek, C.; Jaschek, M. (November 1992), "A southern Be star survey - Spectra and envelope radii", Astronomy and Astrophysics Supplement Series, 95 (3): 535–540, Bibcode:1992A&AS...95..535J.
12. Grady, C. A.; et al. (April 1, 1989), "Highly ionized stellar winds in Be stars. II - Winds in B6-B9.5e stars", Astrophysical Journal, Part 1, 339: 403–419, Bibcode:1989ApJ...339..403G, doi:10.1086/167306.