HR 5171

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HR 5171
HR 5171
Combined optical and infrared image
Credit: ESO/Digitized Sky Survey 2
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Centaurus
A
Right ascension 13h 47m 10.875s[1]
Declination −62° 35′ 23.06″[1]
Apparent magnitude (V) 6.1 - 7.5[2]
B
Right ascension 13h 47m 10.224s[1]
Declination −62° 35′ 17.40″[1]
Apparent magnitude (V) 9.83[1]
Characteristics
A
Spectral type K0 0-Ia[3]
B−V color index +2.499[1]
Variable type EB + SDOR?[4]
B
Spectral type B0 Ibp[5]
B−V color index +0.39[1]
Astrometry
Radial velocity (Rv) −38.20[6] km/s
Proper motion (μ) RA: −2.94[7] mas/yr
Dec.: −-2.54[7] mas/yr
Parallax (π) 1.35 ± 1.59 mas
Distance 11,700 ly
(3,600[2] pc)
Absolute magnitude (MV) −9.2[8] + −5.8[9]
Orbit[2]
Primary Aa
Companion Ab
Period (P) 1,304 ± 6 days
Semi-major axis (a) ~2,000 R
Eccentricity (e) 0
Inclination (i) >60°
Details
Aa
Mass 27 - 36[10] M
Radius 1,490 ± 540[10] R
Luminosity 630,000[10] L
Surface gravity (log g) −0.5 ± 0.6[10] cgs
Temperature 4,290 ± 760[10] 5,000[2] K
Ab
Mass 3 - 6[2] M
Radius 312 × 401[2] R
Temperature 4,800 - 5,200[2] K
Age 3.5[11] Myr
B
Luminosity 316,000[12] L
Surface gravity (log g) 3.0 - 3.5[5] cgs
Temperature 26,000[5] K
Age 4[11] Myr
Other designations
V766 Cen, HR 5171, HD 119796, HIP 67261, SAO 252448, CD−61°3988, WDS J13472-6235, AAVSO 1340-62
Database references
SIMBAD data

HR 5171, also known as V766 Centauri, is a triple star system in the constellation Centaurus, around 12,000 light years from Earth. It contains the largest known yellow hypergiant star which is also an eclipsing binary.

Observational history[edit]

HR 5171 was named by inclusion in the Harvard Revised Catalogue, later published as the Bright Star Catalogue. It was entry #5171, listed at magnitude 6.23 and spectral type K.[13] HR 5171 was catalogued as a double star in 1927.[14]

In 1956 HR 5171 was recorded at magnitude 6.4, spectral type G5p, and profoundly reddened.[15] In 1966 Corben recorded it as magnitude 6.51 and spectral type G5p, and noted it as being variable. A 1969 catalogue records a visual magnitude of 5.85 and a spectral type of A7V, presumably a case of mistaken identity.[16] In 1971, HR 5171 A was identified as a G8 hypergiant, reddened by over 3 magnitudes of interstellar extinction and also by half a magnitude of extinction from circumstellar material.[9] In 1979 it was confirmed as one of the brightest known stars with an absolute visual magnitude (MV) of −9.2.[8] The G8 spectral type was adjusted to K0 Ia+ in the revised MK system.[17]

In 1973 HR 5171 was formally recognised as variable star V766 in Centauri, based on Corben's 1966 catalogue.[18] At the time it was considered a "cool S Doradus variable", a class including stars such as Rho Cassiopeiae that are now known as the yellow hypergiants. These variables are usually classified as semiregular (SRd) due to variations which are sometimes well-defined, at other times nearly constant, and may show unpredictable fading. A detailed study showed variability in both brightness and spectral type with possible periods developing from 430 days to 494 days. The temperature was calculated to vary from nearly 5,000 K to below 4,000 K.[12]

In a 2014 paper, VLTI observations directly determined an unexpectedly large size for HR 5171 and revealed that it is a contact binary. A shell of material around the star has also been directly imaged.[2] In 2016, VLTI observations showed an even larger radius and an unexpectedly cool temperature for a K0 hypergiant.[10]

System[edit]

Artist's impression of HR 5171 and its companion star

The HR 5171 system contains at least three stars. The primary A is an eclipsing binary (components Aa and Ab, or A and C in the Catalog of Components of Double and Multiple Stars) with two yellow stars in contact and orbiting in 1,304 days. The companion has been detected directly by optical interferometry, and is approximately one third the size of the hypergiant primary. The two stars are in the common envelope phase where material surrounding both stars rotates synchronously with the stars themselves.

Component B, 9.4" away, is a B0 supergiant. It is a highly luminous massive star in itself but 3 magnitudes fainter than the yellow hypergiant. At 3.6 kpc, this is a projected separation of 35,000 AU although the actual separation could be larger.[2]

Variability[edit]

HR 5171 shows erratic changes in brightness and colour. HR 5171B is apparently stable, with the changes being due to physical changes in the hypergiant star, variations in the envelope, and eclipses between the two close companions.

The primary and secondary eclipses have depths of 0.21 and 0.14 magnitudes respectively at visual wavelengths. The light curve shows almost continuous variation due to the contact nature of the system, but there is a distinct flat bottom to the secondary eclipse where the secondary passes in front of the primary. The shape of the eclipse light curve suggests that the orbit is almost edge on to Earth, and that the secondary is slightly hotter than the primary.

The eclipses occur against a background of intrinsic variations. Statistically, the system has a mean magnitude of 6.54 and average variations of 0.23 magnitude over a period from the middle of the 20th century until 2013, but within this there are decades with relatively little variation and others which are much more active. Three deep minima have been observed, in 1975, 1993, and 2000, with the brightness dropping below 7th magnitude each time for around a year. Colour changes at these minima suggest a transfer of luminosity from the visual to the infrared, either as a result of cooling or recycling by the surrounding envelope. Following the deep minima, smaller brightness peaks are observed. Overall, the variability in brightness has been much stronger since 2000.

The variations in infrared brightness compared to visual brightness correspond quite well to the light curve, suggesting that brightness changes are related to colour or extinction changes, but there has been a secular trend in the B-V colour index. From 1942 until 1982, B-V steadily increased from around 1.8 to 2.6. Since then it has been approximately constant. This does not appear to be related to reddening as it is independent of the visual magnitude, so it suggests a change in the star itself. The most likely change is that the hypergiant has been cooling and increasing in size.

The variations are erratic, but a strong 657-day periodicity was noticed in Hipparcos photometry of HR 5171. More recent variations showed the strongest periodicity at around 3,300 days, but also showed other variation periods including one at 648 days. This persistent periodicity through all other variations is due to the eclipses twice every 1,304 days.[2]

It is classified in the General Catalogue of Variable Stars as a possible S Doradus variable, as well as an eclipsing variable.[4]

Properties[edit]

HR 5171Aa is the largest known yellow star. Its angular diameter has been measured twice using the AMBER instrument and VLTI. The results of the first set of observations are best represented as a 3.39 mas uniform disc with an extensive surrounding disc of decreasing intensity, plus a small companion located 1.45 mas from the centre of the primary disc. At a distance of 3.6 kpc, this corresponds to a size of 1,315 ± 260 R (568,000,000 ± 112,000,000 mi; 915,000,000 ± 181,000,000 km).[2] The second set of observations found a diameter to a Rosseland optical depth of 2/3 of 3.86 mas, corresponding to a radius of 1,490 R. Additionally, a 5.77 mas diameter ring of was resolved showing neutral sodium emission, with a larger fainter disc of dust.[10] If HR 5171Aa were to be put in the center of the Solar System, it would stretch to a distance of 1,036,593,000 kilometres (644,109,000 mi), between the orbits of Jupiter and Saturn. It has a volume approximately 3.308 billion times bigger than the Sun.[10]

The luminosity has been calculated from Spectral energy distribution (SED) fitting to be 630,000 L.[10] The effective temperature derived from matching infrared spectra is 5,000 K,[2] while the temperature calculated from a radius of 1,490 R and luminosity of 630,000 L is 4,290 ± 760 K. At 4,290 K, it has been described as a red supergiant rather than a yellow hypergiant.[10]

The star has been found to be getting larger in size over the time of its examination and is one of the very few stars that have been found to be changing their temperature with difference in size: HR 5171 Aa is getting cooler as it grows.[2]

The secondary HR 5171 Ab is a luminous yellow star with a radius about a third that of the primary star and an almost identical temperature. It is much less massive, estimated at only a tenth of the mass of the primary. Its exact properties can only be estimated from models since it is barely resolved from its larger companion and its spectrum cannot be distinguished.[2]

The hot companion HR 5171 B is a B0 supergiant, 316,000 times as luminous as the sun. Although it is about half the bolometric luminosity of HR 5171A, it is three magnitudes fainter as much of its radiation is in the ultraviolet.

Evolution[edit]

The evolutionary history of HR 5171A is complicated by its uncertain and unusual physical properties and binary companion. As a single star with a temperature of 4,290 K, its properties correspond to a non-rotating star with an initial mass of 32 - 40 M, or possibly a rotating star of initial mass 25 M, which is several million years old and near its coolest temperature and largest size. Such stars are too massive to produce type II-P supernovae at the red supergiant stage and will evolve to higher temperatures, likely producing a different type of supernova explosion.[10] With a temperature of 5,000 K, it would be a slightly more evolved star, having left the red supergiant phase. The primary star is probably undergoing wind roche lobe overflow (WRLOF) with a portion of the material being transferred to the secondary. This is a possible evolutionary path to a stripped-envelope Wolf-Rayet binary system. The interaction between the pair should spin up the primary to synchronous rotation, which is a possible path to fast-spinning Luminous Blue Variables or B[e] stars.[2]

See also[edit]

References[edit]

  1. ^ a b c d e f g Høg, 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. Bibcode:2000A&A...355L..27H. ISBN 0333750888. doi:10.1888/0333750888/2862. 
  2. ^ a b c d e f g h i j k l m n o Chesneau, O.; Meilland, A.; Chapellier, E.; Millour, F.; Van Genderen, A. M.; Nazé, Y.; Smith, N.; Spang, A.; Smoker, J. V.; Dessart, L.; Kanaan, S.; Bendjoya, Ph.; Feast, M. W.; Groh, J. H.; Lobel, A.; Nardetto, N.; Otero, S.; Oudmaijer, R. D.; Tekola, A. G.; Whitelock, P. A.; Arcos, C.; Curé, M.; Vanzi, L. (2014). "The yellow hypergiant HR 5171 A: Resolving a massive interacting binary in the common envelope phase". Astronomy & Astrophysics. 563: A71. Bibcode:2014A&A...563A..71C. arXiv:1401.2628v2Freely accessible. doi:10.1051/0004-6361/201322421. 
  3. ^ Keenan, P. C.; McNeil, R. C. (1989). "The Perkins catalog of revised MK types for the cooler stars". The Astrophysical Journal Supplement Series. 71: 245. Bibcode:1989ApJS...71..245K. doi:10.1086/191373. 
  4. ^ a b 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. 
  5. ^ a b c Van Genderen, A. M.; Nieuwenhuijzen, H.; Lobel, A. (2015). "An early detection of blue luminescence by neutral PAHs in the direction of the yellow hypergiant HR 5171A?". Astronomy & Astrophysics. 583: A98. Bibcode:2015A&A...583A..98V. arXiv:1509.07421Freely accessible. doi:10.1051/0004-6361/201526392. 
  6. ^ Gontcharov, G. A. (2006). "Pulkovo Compilation of Radial Velocities for 35 495 Hipparcos stars in a common system". Astronomy Letters. 32 (11): 759. Bibcode:2006AstL...32..759G. doi:10.1134/S1063773706110065. 
  7. ^ a b Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653. Bibcode:2007A&A...474..653V. arXiv:0708.1752Freely accessible. doi:10.1051/0004-6361:20078357. 
  8. ^ a b Humphreys, R. M. (1978). "Studies of luminous stars in nearby galaxies. I. Supergiants and O stars in the Milky Way". Astrophysical Journal. 38: 309. Bibcode:1978ApJS...38..309H. doi:10.1086/190559. 
  9. ^ a b Humphreys, R. M.; Strecker, D. W.; Ney, E. P. (1971). "High-luminosity G supergiants". Astrophysical Journal. 167: L35. Bibcode:1971ApJ...167L..35H. doi:10.1086/180755. 
  10. ^ a b c d e f g h i j k Wittkowski, M.; Arroyo-Torres, B.; Marcaide, J. M.; Abellan, F. J.; Chiavassa, A.; Guirado, J. C. (2017). "VLTI/AMBER spectro-interferometry of the late-type supergiants V766 Cen (=HR 5171 A), σ Oph, BM Sco, and HD 206859". Astronomy & Astrophysics. 597: A9. Bibcode:2017A&A...597A...9W. arXiv:1610.01927Freely accessible. doi:10.1051/0004-6361/201629349. 
  11. ^ a b Karr, J. L.; Manoj, P.; Ohashi, N. (2009). "Gum 48d: An Evolved H II Region with Ongoing Star Formation". The Astrophysical Journal. 697: 133. Bibcode:2009ApJ...697..133K. arXiv:0903.0934Freely accessible. doi:10.1088/0004-637X/697/1/133. 
  12. ^ a b Van Genderen, A. M. (1992). "Light variations of massive stars (Alpha Cygni variables). XII - the photometric history of the G8Ia(+) hypergiant V766 CEN ({{{1}}}) during the years 1953-1991 and its interpretation". Astronomy and Astrophysics. 257: 177. Bibcode:1992A&A...257..177V. 
  13. ^ Pickering, Edward Charles (1908). "Revised Harvard photometry : A catalogue of the positions, photometric magnitudes and spectra of 9110 stars, mainly of the magnitude 6.50, and brighter observed with the 2 and 4 inch meridian photometers". Annals of the Astronomical Observatory of Harvard College. 50. Bibcode:1908AnHar..50....1P. 
  14. ^ Innes, R. T. A.; Dawson, B. H.; Van Den Bos, W. H. (1927). "Southern double star catalogue -19 deg. To -90 deg". Johannesburg. Bibcode:1927sdsc.book.....I. 
  15. ^ Stoy, R. H. (1956). "Photoelectric Magnitudes and Colours for 270 Southern Stars". Monthly Notes of the Astronomical Society of South Africa. 15: 96. Bibcode:1956MNSSA..15...96S. 
  16. ^ Cowley, A.; Cowley, C.; Jaschek, M.; Jaschek, C. (1969). "A study of the bright a stars. I. A catalogue of spectral classifications". Astronomical Journal. 74: 375. Bibcode:1969AJ.....74..375C. doi:10.1086/110819. 
  17. ^ Keenan, P. C.; Pitts, R. E. (1980). "Revised MK spectral types for G, K, and M stars". Astrophysical Journal Supplement Series. 42: 541. Bibcode:1980ApJS...42..541K. doi:10.1086/190662. 
  18. ^ Kukarkin, B. V.; Kholopov, P. N.; Kukarkina, N. P.; Perova, N. B. (1973). "59th Name-List of Variable Stars". Information Bulletin on Variable Stars. 834: 1. Bibcode:1973IBVS..834....1K. 
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