Alpha Circini

Alpha Circini (α Cir, α Circini) is a variable star in the faint, southern, circumpolar constellation of Circinus. At an apparent visual magnitude of 3.19,^[4] it is the brightest star in the constellation and can be readily seen with the naked eye from the southern hemisphere to as far north as 25° north latitude. Parallax measurements of this star yield an estimated distance of 54.0 light-years (16.6 parsecs) from the Earth.^[1]

A light curve for Alpha Circini, adapted from Bruntt et al. (2009)^[10]

α Circini

Location of α Circini (circled)
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Circinus
Right ascension	14h 42m 30.41958s[1]
Declination	−64° 58′ 30.4934″[1]
Apparent magnitude (V)	3.18 - 3.21[2]
Characteristics
Spectral type	A7 Vp SrCrEu[3]
U−B color index	+0.12[4]
B−V color index	+0.24[4]
Variable type	roAp[2]
Astrometry
Radial velocity (Rv)	+7.2[5] km/s
Proper motion (μ)	RA: −192.53[1] mas/yr Dec.: −233.51[1] mas/yr
Parallax (π)	60.35 ± 0.14 mas[1]
Distance	54.0 ± 0.1 ly (16.57 ± 0.04 pc)
Absolute magnitude (MV)	+2.18[6]
Details
Mass	1.5–1.7[7] M☉
Radius	1.967 ± 0.066[8] R☉
Luminosity	10.51 ± 0.60[7] L☉
Surface gravity (log g)	4.1[7] cgs
Temperature	7,500[7] K
Metallicity [Fe/H]	0.13[9] dex
Rotation	4.4790 ± 0.0001 days[10]
Rotational velocity (v sin i)	13.0 ± 1.5[10] km/s
Age	~12 million[11] years
Other designations
17. G Circini,[12] CD−64° 86, HD 128898, HIP 71908, HR 5463, SAO 252853.[13]
Database references
SIMBAD	data

This star belongs to a class of variables known as rapidly oscillating Ap stars. It oscillates with multiple, non-radial pulsation cycles and a dominant cycle of 6.8 minutes.^[10] The spectrum shows peculiar features caused by chemical stratification of the outer atmosphere. It displays a moderate deficiency of carbon, nitrogen and oxygen, while there is an overabundance of chromium (Cr).^[7] The stellar classification of A7 Vp SrCrEu^[3] indicates that this is a main sequence star with enhanced levels of strontium (Sr), chromium, and europium (Eu) in its atmosphere (compared to a typical star like the Sun).^[14]

The mass of Alpha Circini is about 150% to 170% the mass of the Sun^[7] and it has double the Sun's radius,^[8] while the luminosity is more than 10 times that of the Sun. The effective temperature of the outer envelope is about 7,500 K, giving it the white hue typical of A-type stars.^[7][15] It is rotating with a period of 4.5 days and the pole is inclined by about 37 ± 4° to the line of sight from the Earth.^[10]

Based upon its location and motion through space, Alpha Circini is a candidate member of a stellar kinematic group known as the Beta Pictoris moving group. This group shares a common origin and has an estimated age of about 12 million years. At the birth of this group, Alpha Circini was estimated to be located at a distance of about 91 ly (28 pc) from the center of the assemblage.^[11]

References

1. van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357, S2CID 18759600
2. Samus, 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. Bibcode:2009yCat....102025S.
3. Gray, R. O.; et al. (July 2006), "Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 parsecs: The Northern Sample I", The Astronomical Journal, 132 (1): 161–170, arXiv:astro-ph/0603770, Bibcode:2006AJ....132..161G, doi:10.1086/504637, S2CID 119476992
4. 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
5. Evans, D. S. (June 20–24, 1966), Batten, Alan Henry; Heard, John Frederick (eds.), "The Revision of the General Catalogue of Radial Velocities", Determination of Radial Velocities and Their Applications, Proceedings from IAU Symposium No. 30, 30, University of Toronto: International Astronomical Union: 57, Bibcode:1967IAUS...30...57E
6. Ammler-von Eiff, Matthias; Reiners, Ansgar (June 2012), "New measurements of rotation and differential rotation in A-F stars: are there two populations of differentially rotating stars?", Astronomy & Astrophysics, 542: A116, arXiv:1204.2459, Bibcode:2012A&A...542A.116A, doi:10.1051/0004-6361/201118724, S2CID 53666672.
7. Kochukhov, O.; Shulyak, D.; Ryabchikova, T. (June 2009), "A self-consistent empirical model atmosphere, abundance and stratification analysis of the benchmark roAp star α Circini", Astronomy and Astrophysics, 499 (3): 851–863, arXiv:0903.3512, Bibcode:2009A&A...499..851K, doi:10.1051/0004-6361/200911653, S2CID 17511410
8. Bruntt, H.; et al. (June 2008), "The fundamental parameters of the roAp star α Circini", Monthly Notices of the Royal Astronomical Society, 386 (4): 2039–2046, arXiv:0803.1518, Bibcode:2008MNRAS.386.2039B, doi:10.1111/j.1365-2966.2008.13167.x, S2CID 13859127
9. North, P.; Berthet, S.; Lanz, T. (January 1994), "The nature of the F STR lambda 4077 stars. 3: Spectroscopy of the barium dwarfs and other CP stars", Astronomy and Astrophysics, 281 (3): 775–796, Bibcode:1994A&A...281..775N
10. Bruntt, H.; et al. (June 2009), "Asteroseismic analysis of the roAp star α Circini: 84d of high-precision photometry from the WIRE satellite", Monthly Notices of the Royal Astronomical Society, 396 (2): 1189–1201, arXiv:0903.3967, Bibcode:2009MNRAS.396.1189B, doi:10.1111/j.1365-2966.2009.14804.x, S2CID 119292022
11. Nakajima, Tadashi; Morino, Jun-Ichi; Fukagawa, Misato (September 2010), "Potential Members of Stellar Kinematical Groups within 20 pc of the Sun", The Astronomical Journal, 140 (3): 713–722, Bibcode:2010AJ....140..713N, doi:10.1088/0004-6256/140/3/713
12. Gould, Benjamin Apthorp (1879), "Uranometria Argentina", Resultados del Observatorio Nacional Argentino en Cordoba, 1, Buenos Aires: 385, Bibcode:1879RNAO....1.....G. See VizieR catalogue V/135A.
13. "* alf Cir". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2012-01-04.
14. Kaler, James B., "ALPHA CIR (Alpha Circini)", Stars, University of Illinois, retrieved 2012-01-04
15. "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on February 22, 2012, retrieved 2012-01-16