NGC 3603-A1

NGC 3603-A1 (HD 97950A1) is a double-eclipsing binary star system located at the centre of the HD 97950 cluster in the NGC 3603 star-forming region, about 25,000 light years from Earth. Both stars are of spectral type WN6h and among the most luminous and most massive known.

NGC 3603-A1

A1 is the brightest (and top right) of the three barely-resolved stars at the centre in this HST image of the central region of HD 97950.
Credit: NASA, ESA and Wolfgang Brandner (MPIA), Boyke Rochau (MPIA) and Andrea Stolte (University of Cologne)
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Carina
Right ascension 11h 15m 07.305s[1]
Declination −61° 15′ 38.43″[1]
Apparent magnitude (V) 11.18[1]
Characteristics
Spectral type WN6h+WN6h[2]
B−V color index 1.03[1]
Variable type EA[3]
Astrometry
Proper motion (μ) RA: 2.4[4] mas/yr
Dec.: 2.8[4] mas/yr
Distance7,600[2] pc
Absolute magnitude (MV)−8.13[2]
Orbit[3]
PrimaryA1a
CompanionA2b
Period (P)3.7724 days
Eccentricity (e)0
Inclination (i)71°
Semi-amplitude (K1)
(primary)		330 ± 20 km/s
Semi-amplitude (K2)
(secondary)		433 ± 53 km/s
Details
A1a
Mass120[2] M☉
Radius29[2] R☉
Luminosity2,500,000[2] L☉
Temperature42,000[2] K
Age1.5[2] Myr
A1b
Mass92[2] M☉
Radius26[2] R☉
Luminosity1,500,000[2] L☉
Temperature40,000[2] K
Age1.5[2] Myr
Other designations
NGC 3603-A1, CD-60°3452A1, CPD-60°2732A1, HD 97950A1, HIP 54948A1, WR 43a, UCAC2 4794917, AAVSO 1110-60, NGC 3603 MDS 30
Database references
SIMBADdata

HD 97950 was catalogued as a star, but was known to be a dense cluster or close multiple star. In 1926, the six brightest members were given letters from A to F,[5] although several of them have since been resolved into more than one star.[6] Star A was first resolved into three components using speckle interferometry, although they can now be directly imaged using space-based or adaptive optics.[1] Component A1 was finally determined to be a spectroscopic binary.[7]

The two component stars of NGC 3603-A1 circle each other every 3.77 days, and show brightness variations of about 0.3 magnitudes due to eclipses. The stars orbit very close to each other, separated by barely their own diameters and at or near to filling their roche lobes.[7]

The masses of A1a and A1b determined from the orbital parameters are 116 ± 31 M☉ and 89 ± 16 M☉respectively.[3] This makes them the two most massive stars directly measured, i.e. with their masses determined (using Keplerian orbits), and not estimated from models. The masses estimated from analysis of the physical properties are slightly higher at 120 M☉ and 92 M☉.

Each component is a Wolf-Rayet (WR) star, with spectra dominated by strong broadened emission lines. Type WN6 indicates that ionised nitrogen lines are strong in comparison to ionised carbon lines, and the suffix h indicates that hydrogen is also seen in the spectrum. This type of WR star is not the classical stripped helium-burning aged star, but a young highly luminous object with CNO cycle fusion products showing at the surface due to strong conventional and rotational mixing, and high mass loss rates from the atmosphere. The emission lines are generated in the stellar wind and the photosphere is completely hidden. The surface fraction of hydrogen is still estimated to be 60-70%.[2]

Although the stars are very young, around 1.5 million years old, they have already lost a considerable fraction of their initial masses. The initial masses are estimated to have been 148 M☉ and 106 M☉, meaning they have lost 28 M☉ and 14 M☉ respectively.[2]

References
  1. Melena, Nicholas W.; Massey, Philip; Morrell, Nidia I.; Zangari, Amanda M. (2008). "The Massive Star Content of NGC 3603". The Astronomical Journal. 135 (3): 878–891. arXiv:0712.2621. Bibcode:2008AJ....135..878M. doi:10.1088/0004-6256/135/3/878. ISSN 0004-6256.
  2. Crowther, P. A.; Schnurr, O.; Hirschi, R.; Yusof, N.; Parker, R. J.; Goodwin, S. P.; Kassim, H. A. (2010). "The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 M⊙ stellar mass limit". Monthly Notices of the Royal Astronomical Society. 408 (2): 731–751. arXiv:1007.3284. Bibcode:2010MNRAS.408..731C. doi:10.1111/j.1365-2966.2010.17167.x.
  3. Schnurr, O.; Casoli, J.; Chené, A. -N.; Moffat, A. F. J.; St-Louis, N. (2008). "The very massive binary NGC 3603-A1". Monthly Notices of the Royal Astronomical Society: Letters. 389 (1): L38–L42. arXiv:0806.2815. Bibcode:2008MNRAS.389L..38S. doi:10.1111/j.1745-3933.2008.00517.x.
  4. Zacharias, N.; et al. (2004). "The Second U.S. Naval Observatory CCD Astrograph Catalog (UCAC2)". CDS/ADC Collection of Electronic Catalogues. 1289 (5): 3043–3059. arXiv:astro-ph/0403060. Bibcode:2003yCat.1289....0Z. doi:10.1086/386353.
  5. Van Den Bos, W. H. (1928). "Another nebulous multiple star". Bulletin of the Astronomical Institutes of the Netherlands. 4: 261. Bibcode:1928BAN.....4..261V.
  6. Moffat, Anthony F. J.; Drissen, Laurent; Shara, Michael M. (1994). "NGC 3603 and its Wolf-Rayet stars: Galactic clone of R136 at the core of 30 Doradus, but without the massive surrounding cluster halo". Astrophysical Journal. 436: 183. Bibcode:1994ApJ...436..183M. doi:10.1086/174891.
  7. Moffat, A. F. J.; Poitras, V.; Marchenko, S. V.; Shara, M. M.; Zurek, D. R.; Bergeron, E.; Antokhina, E. A. (2004). "Hubble Space TelescopeNICMOS Variability Study of Massive Stars in the Young Dense Galactic Starburst NGC 3603". The Astronomical Journal. 128 (6): 2854–2861. Bibcode:2004AJ....128.2854M. doi:10.1086/425878. ISSN 0004-6256.
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