Astronomy:R99

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R99 (HD 269445) is a star in the Large Magellanic Cloud in the constellation Dorado. It is classified as a possible luminous blue variable and is one of the most luminous stars known.

R99 has a peculiar spectrum that has been described as OBf:pe,[1] "unclassifiable", peculiar WN10, "similar to the unusual LBV HD 5980",[2] "unique", and Ofpe/WN9.[3] The Ofpe/WN9 type remains even though other stars of this type have been reclassified to types between WN9 and WN11.[4] R99 has significant differences from those other stars which preclude it being given a simple WN spectral type: the ultraviolet spectrum is strongly blanketed over a different range of wavelengths; highly ionised iron lines are seen in absorption instead of emission; the Hi lines are unusually narrow and have no P Cygni profile; a lack of any significant absorption features near Hδ; a number of metal lines are unusually strong or weak compared to other stars of the type; there is a small unexplained infrared excess.[5]

The wind structure of R99 may be significantly different from most WR stars and LBVs. The normal temperature-stratified WR wind is accelerated to terminal velocity, causing lines of different ionisation levels of Helium to be created at different distances from the star. This does not seem to apply to R99.[5] Significant polarisation of the spectrum continuum is also seen, suggesting an asymmetric wind. This has not been observed in other WR stars.[6]

R99 shows brightness variations of about 0.3 magnitude over a period of decades, and smaller amplitudes with the strongest periods at two and ten days. The colour also varies, with the star being bluer at minimum light.[7] It has been classified as a Luminous Blue Variable on account of the variability and spectrum, although it has never been observed in outburst.[8] Others still list it only as a candidate.[9][10]

References

  1. Walborn, N. R. (1977). "Spectral classification of O and B0 supergiants in the Magellanic Clouds". Astrophysical Journal 215: 53. doi:10.1086/155334. Bibcode1977ApJ...215...53W. 
  2. Crowther, P. A.; Smith, L. J. (1997). "Fundamental parameters of Wolf-Rayet stars. VI. Large Magellanic Cloud WNL stars". Astronomy and Astrophysics 320: 500. Bibcode1997A&A...320..500C. 
  3. Bohannan, Bruce; Walborn, Nolan R. (1989). "The Ofpe/WN9 class in the Large Magellanic Cloud". Publications of the Astronomical Society of the Pacific 101: 520. doi:10.1086/132463. Bibcode1989PASP..101..520B. 
  4. 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. doi:10.1111/j.1745-3933.2008.00517.x. Bibcode2008MNRAS.389L..38S. 
  5. 5.0 5.1 Pasquali, Anna; Langer, Norbert; Schmutz, Werner; Leitherer, Claus; Nota, Antonella; Hubeny, Ivan; Moffat, Anthony F. J. (1997). "O Stars in Transition. II. Fundamental Properties and Evolutionary Status of Ofpe/WN9 Stars from HST Ultraviolet Observations". The Astrophysical Journal 478 (1): 340–357. doi:10.1086/303767. Bibcode1997ApJ...478..340P. 
  6. Vink, Jorick S. (2007). "Constraining GRB progenitor models by probing Wolf-Rayet wind geometries in the Large Magellanic Cloud". Astronomy and Astrophysics 469 (2): 707–711. doi:10.1051/0004-6361:20077302. Bibcode2007A&A...469..707V. 
  7. Van Genderen, A. M.; Sterken, C.; De Groot, M.; Reijns, R. A. (1998). "Light variations of massive stars (alpha Cyg variables). XV. The LMC supergiants R99 (LBV), R103, R123 (LBV) and R128". Astronomy and Astrophysics 332: 857. Bibcode1998A&A...332..857V. 
  8. Van Genderen, A. M. (2001). "S Doradus variables in the Galaxy and the Magellanic Clouds". Astronomy and Astrophysics 366 (2): 508–531. doi:10.1051/0004-6361:20000022. Bibcode2001A&A...366..508V. 
  9. Smith, Nathan; Tombleson, Ryan (2015). "Luminous blue variables are antisocial: Their isolation implies that they are kicked mass gainers in binary evolution". Monthly Notices of the Royal Astronomical Society 447 (1): 598–617. doi:10.1093/mnras/stu2430. Bibcode2015MNRAS.447..598S. 
  10. Richardson, Noel D.; Mehner, Andrea (2018). "The 2018 Census of Luminous Blue Variables in the Local Group". Research Notes of the American Astronomical Society 2 (3): 121. doi:10.3847/2515-5172/aad1f3. Bibcode2018RNAAS...2c.121R. 

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