HR 5171
Observation data J2000.0
| ||
---|---|---|
Constellation | Centaurus
| |
A | ||
Right ascension | 13h 47m 10.864s[1] | |
Declination | −62° 35′ 22.95″[1] | |
Apparent magnitude (V) | 6.1 - 7.5[2] | |
B | ||
Right ascension | 13h 47m 09.953s[3] | |
Declination | −62° 35′ 15.99″[4] | |
Apparent magnitude (V) | 9.83[4] | |
Characteristics | ||
A | ||
Spectral type | K0 0-Ia[5] | |
B−V color index | +2.499[4] | |
Variable type | ||
B | ||
Spectral type | B0 Ibp[7] | |
B−V color index | +0.39[4] | |
A | ||
Distance | 10,000 ± 400 ly (3,100 ± 100 pc) | |
Absolute magnitude (MV) | −5.8[12] | |
Inclination (i) | >60° | |
Details | ||
Aa | ||
Myr | ||
Ab | ||
Mass | 5+15 −3[14] M☉ | |
Radius | 312 - 401,[2] 650±150[14] R☉ | |
Temperature | 4,800 - 5,200[2] K | |
B | ||
Myr | ||
AAVSO 1340-62 | ||
Database references | ||
SIMBAD | data |
HR 5171, also known as V766 Centauri, is a
A recent publication (2019) rules out this hypothesis, and the distance of HR 5171 has been revised to 5,200 ± 1,600 light years and its radius to 3-5 AU (i.e. 650 to 1080 times that of the Sun).[9]
An optical companion, HR 5171B, may or may not be at the same distance as the yellow supergiant.[9]
System
![](http://upload.wikimedia.org/wikipedia/commons/thumb/e/eb/V766_Centauri.tiff/lossy-page1-310px-V766_Centauri.tiff.jpg)
The HR 5171 system contains at least three stars. The primary A is an
Component B, located 9.4
Observational history
HR 5171 was named by inclusion in the
In 1956, HR 5171 was recorded at magnitude 6.4, spectral type G5p, and profoundly reddened.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/d/d0/V766CenLightCurve.png/220px-V766CenLightCurve.png)
In 1973 HR 5171 was formally recognised as variable star V766 Centauri, based on Corben's 1966 catalogue.[22] 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 semi-regular (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. Surface temperature was calculated to vary from nearly 5,000K to below 4,000K.[15]
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.[13] Further interferometry imaged the secondary star transiting the primary.[14]
Distance
![](http://upload.wikimedia.org/wikipedia/commons/thumb/c/ca/The_field_around_yellow_hypergiant_star_HR_5171.jpg/240px-The_field_around_yellow_hypergiant_star_HR_5171.jpg)
(ESO/Digitized Sky Survey 2)
HR 5171 appears near the centre of the
Early calculations based on the assumed luminosity of HR 5171B gave a distance of 3.2 kpc and 3.2 magnitudes of interstellar extinction. Comparison of HR 5171A with similar stars in the Magellanic Clouds imply a distance of 3.7 kpc. An average distance based on all these calculations is 3.6 kpc,[12] which is still the widely accepted distance although there are reasons to think it could be closer.[7]
Gum 48d is also catalogued as RCW 80, although the designation RCW 80 is sometimes used for the more distant supernova remnant G309.2-00.6 which overlaps it.[7] The open cluster NGC 5281 lies 19' from HR 5171, projected against the supernova remnant but only about 1,200 parsecs from Earth.[23]
Spectrum
The spectrum of HR 5171 is easily separated into a luminous yellow star and a hot blue supergiant. The third component, HR 5171Ab, is not resolved and its spectral type is uncertain. Both stars show 3-4 magnitudes of reddening due to dust extinction.
The yellow star has been defined as the spectral standard for K0 0-Ia stars.
The blue companion has been classified as B0 Ibp, a hot supergiant of normal luminosity, with some uncertainty. The spectral peculiarity code indicates that its
Variability
This section needs additional citations for verification. (October 2019) |
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 minima 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 minimum 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 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.[6]
Properties
The angular diameter of HR 5171A has been published three times using measurements from the Very Large Telescope, twice with the AMBER interferometer and once with the PIONIER interferometer. In all cases, an unexpectedly large diameter was found, between about 3.3 and 4.1 mas, well over 1,000 R☉ at the accepted distance of 3.6 kpc.
The earliest AMBER interferometry was at a range of infrared wavelengths in March 2012. The best-fit model was a sharply-defined uniform disk with a small bright spot towards its edge, all surrounded by a fainter extended envelope. The uniform disk, taken to be the photosphere of the larger star, was 3.39 mas across, corresponding to a radius of 1,315 ± 260 solar radii (915,000,000 ± 181,000,000 km; 6.12 ± 1.21 au). The size of the smaller disk, assumed to be the secondary star, was not well-defined.[2] The second set of AMBER observations were made in the K-band in April 2014. The best fits for a uniform disk and the Rosseland radius of a model atmosphere were almost identical at 3.87 mas and 3.86 mas respectively, corresponding to a radius of 1,492 ± 540 R☉ (6.94 ± 2.51 au).[13] The PIONIER observations were made across six different infrared wavelengths during 2016 and 2017. Aperture synthesis was used to produce an image of HR 5171 at three different phases of the orbit. In two of the images, the secondary star is visible in front of the primary, and in the third it is expected to be behind the primary and was not visible. Modelled as a Rosseland stellar atmosphere surrounded by an extended uniform disk, the photosphere was found to be between 3.3 mas and 4.8 mas. Overall, the radius of the primary was calculated to be 1,575 ± 400 R☉ (7.32 ± 1.86 au) and 650 ± 150 R☉ (450,000,000 ± 100,000,000 km) for the secondary.[14] The radii are statistically consistent with each other, but more representative of an extreme red supergiant rather than a yellow hypergiant. It is unclear whether this is due to binary interaction or misinterpretation of the unusual and highly reddened spectrum.[2]
The luminosity has been calculated from spectral energy distribution (SED) fitting to be 630,000 L☉, assuming a distance of 3.7 kpc and 3.2 magnitudes of interstellar extinction.[11] This is considerably more luminous than expected for any red supergiant and extreme even for a yellow hypergiant.[7] 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.[13]
The close 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. From the shape of the eclipse light curve, it is 12% as luminous as the primary and slightly hotter. It is much less massive, estimated at only a tenth of the mass of the primary. Its exact properties can only be predicted 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 according to a 1992 paper. Although it is about half the
Evolution
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
References
- ^ S2CID 244398875. Gaia DR3 record for this source at VizieR.
- ^ S2CID 52108686.
- ^ S2CID 244398875. Gaia DR3 record for this source at VizieR.
- ^ ISBN 0333750888.
- doi:10.1086/191373.
- ^ Bibcode:2009yCat....102025S.
- ^ S2CID 56270146.
- S2CID 119231169.
- ^ S2CID 203836020.
- ^ S2CID 17962808.
- ^ doi:10.1086/190559.
- ^ doi:10.1086/180755.
- ^ S2CID 55679854.
- ^ S2CID 54740936.
- ^ Bibcode:1992A&A...257..177V.
- ^ Jim Kaler. "V766 Centauri". Retrieved 2015-11-21.
- Bibcode:1908AnHar..50....1P.
- Bibcode:1927sdsc.book.....I.
- Bibcode:1956MNSSA..15...96S.
- doi:10.1086/110819.
- ^ doi:10.1086/190662.
- Bibcode:1973IBVS..834....1K.
- S2CID 15997425.