RY Sagittarii

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RY Sagittarii
Observation data
Epoch J2000.0 ICRS      Equinox J2000.0 ICRS
Constellation Sagittarius
Right ascension 19h 16m 32.77s[1]
Declination −33° 31′ 20.34″ [1]
Apparent magnitude (V) 5.8-14.0[2]
Spectral type G0Iaep[2]
Variable type R CrB variable
Radial velocity (Rv) -23[3] km/s
Proper motion (μ) RA: 10.33[3] mas/yr
Dec.: -0.41[3] mas/yr
Parallax (π) 1.29[1] mas
Distance 9049[3] ly
Absolute magnitude (MV) -5.64[3]
Radius 60 R
Other designations
HIP 94730, HD 180093, HR 7296, SAO 211117
Database references

RY Sagittarii is an orange supergiant and an R Coronae Borealis type variable star in the constellation Sagittarius.[1] Although it ostensibly has the spectrum of a G-type star, it differs markedly from most in that it has almost no hydrogen and a lot of carbon. The star is so remote that its parallax, distance and hence luminosity were impossible to calculate with any accuracy.[4] The Hipparcos satellite calculated its parallax at 1.29 milliarcseconds,[5] yielding a distance of 1,716.6 light-years (526.32 parsecs) from Earth. Its effective temperature has been calculated at 7250 K.[6]

It is one of the three brightest R Coronae Borealis stars visible to Earth observers, along with R Coronae Borealis and V854 Centauri.[7] It is also a pulsating variable, with a semiregular period of 38 days.[4] Its light curve has been studied for over a hundred years and is typical for the class, characterised by a sudden drop in brightness of several magnitudes over a few weeks before gradually brightening over the following several months. The timing between these dimmings is irregular.[4] The cause of the drop in magnitude is the presence of dust clouds of carbon obscuring (and most likely ejected from) the star, though the mechanism how this might occur is not known.[4][8] Extensive clouds have been detected with ESO's Very Large Telescope Interferometer.[9]

Colonel Ernest Elliott Markwick first came across what became known as RY Sagittarii during searches for variable stars while posted in Gibraltar.[10] He recorded it dimming from magnitude 7 in July 1893 to fainter than 11 by 23 October that year, and brightening to magnitude 6.4 by November 1894.[11] Edward Charles Pickering wrote that it was a "remarkable object",[11] and "nearly got away".[10] The spectrum was first noted to be peculiar at the time,[11] and by 1953 it was classified as a R Coronae Borealis variable, along with a handful of other stars.[12]

Mystified by its origins, Danziger postulated possible explanations as forming from a helium cloud, an aged star that had exhausted its hydrogen, or a star that had somehow thrown off its hydrogen envelope, though noted there was no evidence of such an envelope. He conceded that knowledge of star evolution was not advanced enough to come up with an explanation.[13]


  1. ^ a b c d "V*RY Sgr". SIMBAD Astronomical Database(Centre de donées astronomiques de Strasbourg). Retrieved 8 August 2013. 
  2. ^ a b "V RY Sgr". The International Variable Star Index. Retrieved 8 August 2013. 
  3. ^ a b c d e "HIP 94730", Wolfram Alpha, retrieved 2013-08-08 
  4. ^ a b c d Clayton, G. C. (1996). "The R Coronae Borealis Stars". Publications of the Astronomical Society of the Pacific. 108: 225. Bibcode:1996PASP..108..225C. doi:10.1086/133715. 
  5. ^ van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752Freely accessible, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357. 
  6. ^ Clayton, Geoffrey C.; Geballe, T. R.; Zhang, Wanshu (2013). "Variable Winds and Dust Formation in R Coronae Borealis Stars". The Astronomical Journal. 146 (2): 9. arXiv:1305.5047Freely accessible. Bibcode:2013AJ....146...23C. doi:10.1088/0004-6256/146/2/23. 23. 
  7. ^ Skuljan, L.; Cottrell, P. L. (2002). "Recent declines of RS Telescopii, UW Centauri, and V Coronae Australis". The Observatory. 122: 322–29. Bibcode:2002Obs...122..322S. 
  8. ^ Davis, Kate (January 2000). "R Coronae Borealis". Variable Star of the Month. Retrieved 6 July 2014. 
  9. ^ de Laverny, Patrick (3 August 2007). "Star Caught Smoking: VLTI Snapshots Dusty Puff Around Variable Star". European Southern Observatory. Retrieved 13 March 2015. 
  10. ^ a b Shears, Jeremy (2012). "Ernest Elliott Markwick: Variable stars and military campaigns" (PDF). The Journal of the British Astronomical Association. 122 (6): 335–48. 
  11. ^ a b c Pickering, E. C. (1896). "Harvard College Observatory, Circular no. 7. Ten New Variable Stars". Astrophysical Journal. 4: 138–42. Bibcode:1896ApJ.....4..138P. doi:10.1086/140256. 
  12. ^ Bidelman, William P. (1953). "The Spectra of Certain Stars whose Atmospheres may BE Deficient in Hydrogen". Astrophysical Journal. 117: 25. Bibcode:1953ApJ...117...25B. doi:10.1086/145665. 
  13. ^ I. J., Danziger (August 2013), "A high-dispersion spectral study of RY Sagittarii", Monthly Notices of the Royal Astronomical Society, 130: 199, Bibcode:1965MNRAS.130..199D, doi:10.1093/mnras/130.3.199.