HD 108 is a massive, peculiarstar in the northerncircumpolar constellation ofCassiopeia. At anapparent visual magnitude of 7.40,[3] it is too faint to be visible to thenaked eye. HD 108 is located at an estimated distance of 6,230 ± 360light-years (1.91 ± 0.11 kpc) from the Sun,[6] but is drifting closer with a line of sight velocity of −63 km/s.[5] Based on itsproper motion, it is a likely member of the Cas OB5 association of co-moving stars.[4]
This star was discovered to have strongH-alphaemission lines in 1925, suggesting it is aBe star with astellar classification of O6e.[9] In 1940, the spectrum of this star was found to have changed significantly. It was noted as belonging to a set of peculiar stars of class 'Of', based on certain He II and N III emission lines atwavelengths of 4634, 4640, and4686 Å.[10] Thespectrum displayed evidence of mass loss from a high velocitystellar wind.[11] In addition to the Of emission lines, HD 108 shows unusual C III emission lines around4650 Å with a comparable strength to the nearby N III lines. In 1971,N. Walborn introduced the Of?p category to classify the spectrum of this star andHD 148937.[12]
In 1974,radial velocity measurements of singly-ionized helium lines from this star showed a periodicity of 4.612 days, suggesting the presence of an unseen closely orbiting companion. This object appeared to be orbiting with an eccentricity of 0.44.[11] The isolated location of this star in the sky indicated this is a high velocityrunaway star system.[11] The ejection of this system from a youngopen cluster of stars is difficult to explain through many-body interactions, and may instead be the result of aType II supernova explosion. This indicates the hypothetical companion could be a collapsedsupernova remnant; either aneutron star or a stellar massblack hole.[13]
Measurement of theinfrared excess and wind velocity in 1981 indicated a mass loss rate of5×10−5M☉·yr−1. This is one of the highest rates found forO-type stars and is more in line with aWolf-Rayet star.Radiation pressure alone is insufficient to explain this rate.[14] Ultraviolet measurement of the spectrum demonstrated variations in wind velocity of40 km/s and the changes did not support a binary system.[15]
Apolarimetric study of the star in 1994 showed random variability but with systematic nightly changes. The data proved similar to that observed for singleOB supergiants andWolf-Rayet stars. It may be explained by random ejection of plasma blobs in the stellar wind, or by an equatorial disk combined with jets at the poles.[16] This star maintained a steady brightness from 1988 until 1994, then it began a monotonic decline. It showed a decrease of 0.06 in magnitude by 1999.[17][18] Emission lines reached a minimum strength during 2007–2008, then began to increase.[6]
In 2001, a study of 30 years of spectroscopic observations of HD 108 failed to detect any periodicity that would support the presence of a companion.[19]X-ray observations with theXMM-Newton space observatory showed that theemission is very stable over long periods. This is in contrast to the optical spectrum which shows significant line variations on timescales of decades.[20] The X-ray luminosity for this star is too low for it to be considered anX-ray binary.[12]
The detection of a dipolemagnetic field around HD 108 was announced in 2010. The longitudinal magnetic field has a measured strength of−168±35 Gauss. The detection ofZeeman features in certain spectral lines suggested the presence of material around the star.[21] The stable nature of the magnetic field indicated a very low rotation rate of 55 years. This would require a spin-down period of 8.5 Myr, which is much longer than the star's estimated age of 3.3 Myr. To explain this, the average magnetic field needs to be twice as strong as measured.[7]
^abcdDucati, J. R. (2002), "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system",CDS/ADC Collection of Electronic Catalogues, vol. 2237,Bibcode:2002yCat.2237....0D.
^abcShultz, M.; Wade, G. A. (July 2017), "Confirming the oblique rotator model for the extremely slowly rotating O8f?p star HD 108",Monthly Notices of the Royal Astronomical Society,468 (4):3985–3992,arXiv:1703.08996,Bibcode:2017MNRAS.468.3985S,doi:10.1093/mnras/stx759.
^abNazé, Y.; et al. (January 2006), Wilson, A. (ed.), "The Peculiar Of?p Stars HD 108 and HD 191612: The X-ray View",Proceedings of The X-ray Universe 2005 (ESA SP-604). 26-30 September 2005, El Escorial, Madrid, Spain, vol. 604, p. 99,Bibcode:2006ESASP.604...99N.
^Ferrari-Toniolo, M.; et al. (October 1981), "Infrared excess and mass-loss rate of the extreme of star HD 108",Publications of the Astronomical Society of the Pacific,93 (555):633–635,Bibcode:1981PASP...93..633F,doi:10.1086/130900,JSTOR40677870.
^Hutchings, J. B.; van Heteren, J. (October 1981), "IUE spectroscopy of hot binary stars HD 108,HD 149404 and HD 163181",Publications of the Astronomical Society of the Pacific,93:626–629,Bibcode:1981PASP...93..626H,doi:10.1086/130898.
^Barannikov, A. A. (March 1999), "Long-term spectroscopic and photoelectric variability of the peculiar runaway O7I fp star HD 108",Astronomy Letters,25 (3):169–178,Bibcode:1999AstL...25..169B.
^Barannikov, A. A. (February 2007), "Long-Term Optical Light Variations of the Peculiar Massive Runaway Star HD 108",Information Bulletin on Variable Stars,5756: 1,Bibcode:2007IBVS.5756....1B.
^Hubrig, S.; et al. (August 2010), "Measurements of mean longitudinal magnetic fields in the Of?p stars HD 108 and HD 191612",Astronomische Nachrichten,331 (8): 781,Bibcode:2010AN....331..781H,doi:10.1002/asna.201011412.
^Nazé, Y.; et al. (November 2005), Rauw, G.; et al. (eds.), "The peculiar Of?p stars HD 108 and HD 191612",Proceedings of "Massive Stars and High-Energy Emission in OB Associations", a workshop of the JENAM 2005, "Distant Worlds", held in Liège (Belgium), July 4 - 7, 2005, pp. 31–34,Bibcode:2005mshe.work...31N.
Andrillat, Y.; et al. (1973), "Recent observations of the spectrum of the Of-star HD 108",Astronomy and Astrophysics (in French),29: 171,Bibcode:1973A&A....29..171A.
