Pismis 24-1

Pismis 24-1, also known as HD 319718, is the brightest star of the open cluster Pismis 24 within the nebula NGC 6357 about 6,500 light-years away. It was once thought to be the most massive star known, but is composed of at least three individual objects, each still among the most luminous and most massive stars known.

Pismis 24-1

Pismis 24-1 is the brightest point of light at the centre of this optical image of the NGC 6357 region.
Credit: ESO/IDA/Danish 1.5 m/ R. Gendler, U.G. Jørgensen, J. Skottfelt, K. Harpsøe
Observation data
Epoch J2000      Equinox J2000
Constellation Scorpius
NE
Right ascension 17h 24m 43.497s[1]
Declination –34° 11′ 56.86″[1]
Apparent magnitude (V) 11.00[2]
SW
Right ascension 17h 24m 43.481s[1]
Declination –34° 11′ 57.21″[1]
Apparent magnitude (V) 11.10[2]
Characteristics
NE
Spectral type O3.5If*[3]
Variable type eclipsing[4]
SW
Spectral type O4III(f+)[3]
Astrometry
Radial velocity (Rv)−2.0[5] km/s
Proper motion (μ) RA: −0.1[6] mas/yr
Dec.: −1.3[6] mas/yr
Distance6,500[7] ly
(2,000[7] pc)
Absolute magnitude (MV)−7.50[3]
NE
Absolute magnitude (MV)−6.41[4]
SW
Absolute magnitude (MV)−6.28[4]
Details
NE
Mass74[1] M☉
Radius18 R☉
Luminosity776,000[1] L☉
Temperature42,500/41,500[4] K
SW
Mass66[1] M☉
Radius17 R☉
Luminosity646,000[1] L☉
Temperature~40,000[1] K
Other designations
CD−34°11671, 2MASS J17244349-3411570, CCDM J17247-3412A
NE: Pismis 24-1NE, HD 319718A
SW: Pismis 24-1SW, HD 319718B
Database references
SIMBADdata

Discovery

Pismis 24-1 was first catalogued as HD 319718, later resolved into both Pismis 24-1 and the fainter Pismis 24-16. The surrounding H II region NGC 6357 is prominent, but the compact 10th-magnitude open cluster Pismis 24 was not identified until 1959.[8]

In 1973, Pismis 24 was resolved into 15 components of which 12 were considered member stars. The brightest was numbered first as Pismis 24-1 and tentatively considered a supergiant.[9] It was later resolved into an O3.5 supergiant spectroscopic binary and an O4 giant star separated by approximately 500 AU.[3]

System
A red band light curve for Pismis 24-1, adapted from Barr Domínguez et al. (2013)[4]

Pismis 24-1 has been resolved visually into two components, usually labelled as NE and SW from their orientation with each other. Pismis 24-1NE is slightly more luminous and hotter than 24-1SW, but is known to be a spectroscopic binary.[4] This is surprising given the spectral luminosity classes, because it would make the individual supergiant stars less luminous than a single cooler giant star. It could be that the interaction between the components of 24-1NE is confusing its classification, or the O4 giant may also be a close binary.[3]

Pismis 24-1 is actually a shallow eclipsing binary with a period of 2.4 days. It is presumed to be the NE component which produces the eclipses, but the separate light curves for the components have not been resolved. The light curve is symmetrical, indicating a near circular orbit, and the two eclipsing stars have very similar masses and temperatures.[4]

The Catalog of Components of Double and Multiple Stars lists Pismis 24-1 as having two fainter companions 5.5 and 16.4 arcseconds distant.[10] This is not surprising since it is a member of a rich open cluster only 1.5 arcminutes across.[7]

Properties
Detail of NGC 6537 with Pismis 24 (infrared and optical image)

The two components of Pismis 24-1NE cannot be detected separately, but analysis of their eclipses shows that they are almost identical, with temperatures around 42,000 K. The pair combined is nearly 800,000 times as luminous as the sun, making each individual star likely to be under 400,000 L☉. The spectral type of the combined object is O3.5 If* indicating an expanded star with strong emission lines of highly ionised nitrogen. Separate spectral signatures cannot be detected so it is assumed that both stars have similar spectra. The mass as a single object has been calculated to be 74 M☉ but each star would have a smaller mass. Hard X-ray radiation from the vicinity of Pismis 24-1 is assumed to be caused by the colliding winds of these two supergiants.[4]

Pismis 24-1SW is apparently a single star with a spectral type of O4 III(f+), indicating a temperature around 40,000 K and emission lines of ionised nitrogen, silicon and helium. The luminosity is around 650,000 L☉, the radius 17 R☉, and the mass 66 M☉. It is classified as a giant star on the basis of its spectrum, but the hottest O stars develop these spectral features while still burning hydrogen in their cores, as a result of vigorous convection and powerful stellar winds.[4]

When first modelled, Pismis 24-1 was thought to be a single star with a mass of 300 M☉ or more, higher than expected to be theoretically possible. This estimate has decreased as the star was discovered to be a binary, then a triple, and as newer models of stellar atmospheres have been developed. The latest mass estimates are well within theoretical expectations for star formation without exotic explanations.[3]

References
  1. Fang, M.; Van Boekel, R.; King, R. R.; Henning, T.; Bouwman, J.; Doi, Y.; Okamoto, Y. K.; Roccatagliata, V.; Sicilia-Aguilar, A. (2012). "Star formation and disk properties in Pismis 24". Astronomy & Astrophysics. 539: A119. arXiv:1201.0833. Bibcode:2012A&A...539A.119F. doi:10.1051/0004-6361/201015914. S2CID 73612793.
  2. Skiff, B. A. (2014). "VizieR Online Data Catalog: Catalogue of Stellar Spectral Classifications (Skiff, 2009-2014)". VizieR On-line Data Catalog: B/Mk. Originally Published in: Lowell Observatory (October 2014). 1. Bibcode:2014yCat....1.2023S.
  3. Maíz Apellániz, J.; Walborn, Nolan R.; Morrell, N. I.; Niemela, V. S.; Nelan, E. P. (2007). "Pismis 24-1: The Stellar Upper Mass Limit Preserved". The Astrophysical Journal. 660 (2): 1480–1485. arXiv:astro-ph/0612012. Bibcode:2007ApJ...660.1480M. doi:10.1086/513098. S2CID 15936535.
  4. Barr Domínguez, A.; Chini, R.; Pozo Nuñez, F.; Haas, M.; Hackstein, M.; Drass, H.; Lemke, R.; Murphy, M. (2013). "Eclipsing high-mass binaries. I. Light curves and system parameters for CPD - 51° 8946, PISMIS 24-1, and HD 319702". Astronomy & Astrophysics. 557: A13. arXiv:1306.5482. Bibcode:2013A&A...557A..13B. doi:10.1051/0004-6361/201321642. S2CID 118623790.
  5. Crampton, D. (1972). "Radial velocities of southern B stars determined at the Radcliffe Observatory - VI. Stars in H II regions". Monthly Notices of the Royal Astronomical Society. 158: 85–98. Bibcode:1972MNRAS.158...85C. doi:10.1093/mnras/158.1.85.
  6. 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. doi:10.1888/0333750888/2862.
  7. Lima, E. F.; Bica, E.; Bonatto, C.; Saito, R. K. (2014). "Probing embedded star clusters in the HII complex NGC 6357 with VVV". Astronomy & Astrophysics. 568: A16. arXiv:1406.2413. Bibcode:2014A&A...568A..16L. doi:10.1051/0004-6361/201323050. S2CID 56229887.
  8. Pišmiš, P. (1959). "New Southern Star Clusters (Spanish Title: Nuevos Cumulos Estelares en regiones del sur)". Boletín de los Observatorios de Tonantzintla y Tacubaya. 2 (18): 37. Bibcode:1959BOTT....2r..37P.
  9. Moffat, A. F. J.; Vogt, N. (1973). "Southern open stars clusters. III. UBV-Hbeta photometry of 28 clusters between galactic longitudes 297d and 353d". Astronomy and Astrophysics Supplement Series. 10: 135. Bibcode:1973A&AS...10..135M.
  10. Dommanget, J.; Nys, O. (1994). "Catalogue des composantes d'etoiles doubles et multiples (CCDM) premiere edition - Catalogue of the components of double and multiple stars (CCDM) first edition". Com. De l'Observ. Royal de Belgique. 115: 1. Bibcode:1994CoORB.115....1D.
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