HD 191612 is abinary star system in the northernconstellation ofCygnus, the swan. Based onparallax measurements, it is located at a distance of 6,100 light years from theSun. It has anapparent visual magnitude of 7.81,[1] which is too faint to be readily visible with the naked eye, requiring a small telescope to view. HD 191612 is drifting closer with a heliocentricradial velocity of −28 km/s.[6] It lies within theCygnus OB 3 association of recently-formed stars.[8]
The Of?pstellar classification was first proposed by American astronomerN. R. Walborn in 1972. This rare class consists of hot, massiveO-type stars with a peculiar spectrum displayingP Cygni profilehydrogen lines plus specificemission lines for doubly-ionized carbon and nitrogen atoms that are comparable in strength. In 1973, the star HD 191612 was included in this group by Walborn with a class of O6.5f?pe, suggesting it is host to a spherically expandingshell.[9]
In 1989, this star was tentatively identified as anX-ray source using theEinstein Observatory. The X-ray luminosity of this star is relatively high, comparable to someX-ray binaries withaccreting companions. When combined with the unique spectral features, this suggested an orbitingcollapsed star was colliding with thestellar wind from the primary and creating X-ray emittingshocks.[10]
A study of massiveOB stars in 1992 found a class (O6.5 III f) for HD 191612 that was different from the one reported by Walborn in 1972.[11] This led to a 2003 study that showed HD 191612 alternated between two spectral states. At least four of these transformations had been observed since 1950. These changes did not occur over short time periods and noradial velocity variations were observed, which mostly ruled out aninteracting binary system as the cause.[12]
A light curve for HD 191612, plotted from Hipparcos data,[13] folded with the 538 day period from theGCVS[3]
A periodic, symmetrical, low amplitudelight curve was reported by theHipparcos satellite. This showed a cycle time of~540 d, which seemed too long to be explained by normal rotation orpulsation. The reported period successfully predicted the observed spectral changes.[14] In 2006, amagnetic field was discovered based on aZeeman signature in the spectrum of HD 191612. This was only the second O-type star found to have a strong magnetic field, afterTheta1 Orionis C. The polar field strength was estimated at−1.5 kG. This detection suggested that the rotation of the star may have been significantly slowed through confinement of the stellar wind by the magnetic field.[15]
By 2007, long term spectroscopic observations had shown systematic variations in the radial velocity that did not match the light curve period. Instead, these suggested a binary star system with anorbital period of1,542±14 d and aneccentricity of0.438±0.038. The observations were consistent with aB-type main sequence companion having about half the mass of the primary.[4]
In 2011, the magnetic field of the primary was confirmed and was found to vary with a period of537.6 d, matching the spectral variation. This finding supported a dipoleoblique rotator model, with the magnetic field oriented at an angle to the stellar poles. With an adoptedaxial tilt of 30°, the best fit model has a polar magnetic field strength of2,450±400 G and an obliquity of67°±5°. The magnetic braking time of 0.33 Myr is much lower than the star's estimated age of 3–4 Myr, demonstrating that this effect is sufficient to produce the inferred slow rotation period of537.6 d.[5]
The strong stellar wind of this star madestar spots less likely as a source of itsphotometric variability. Instead, it may due to light absorption by circumstellar clouds, in combination with wind blanketing under the influence of the magnetic field.[16]
^Rao, Anjali; Gandhi, Poshak; Knigge, Christian; Paice, John A.; Leigh, Nathan W. C.; Boubert, Douglas (June 2020), "Kinematic study of the association Cyg OB3 with Gaia DR2",Monthly Notices of the Royal Astronomical Society,495 (1):1491–1500,arXiv:1908.00810,Bibcode:2020MNRAS.495.1491R,doi:10.1093/mnras/staa1217.
^Walborn, N. R. (December 1973), "The space distribution of the O stars in the solar neighborhood",Astronomical Journal,78:1067–1083,Bibcode:1973AJ.....78.1067W,doi:10.1086/111509.
^Herrero, A.; et al. (July 1992), "Intrinsic parameters of galactic luminous OB stars",Astronomy and Astrophysics,261:209–234,Bibcode:1992A&A...261..209H.
Marcolino, W. L. F.; et al. (May 2013), "Phase-resolved ultraviolet spectroscopy of the magnetic Of?p star HD 191612",Monthly Notices of the Royal Astronomical Society,431 (3):2253–2260,arXiv:1302.4708,Bibcode:2013MNRAS.431.2253M,doi:10.1093/mnras/stt323.
Sundqvist, Jon O.; et al. (June 2012), "A dynamical magnetosphere model for periodic Hα emission from the slowly rotating magnetic O star HD 191612",Monthly Notices of the Royal Astronomical Society: Letters,423 (1):L21 –L25,arXiv:1203.1050,Bibcode:2012MNRAS.423L..21S,doi:10.1111/j.1745-3933.2012.01248.x.
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.
