This single-lined spectroscopic binary has a physical separation of at least 1.25 ± 0.01 AU, an orbital period of 2.32 years, and a high eccentricity (ovalness) of 0.45.[3] The visible component is an Ap star with a stellar classification of Ap(SiCr),[4] although the effective temperature of 11,300 K is a closer match to a spectral type of B8.5p.[1]P. Renson and associates (1991) gave a spectral type of A0p EuCrSi,[9] indicating the spectrum shows peculiarities in the europium, chromium, and silicon bands. It is classified as an Alpha2 Canum Venaticorum variable with a brightness that varies from 6.10 down to 6.12 in magnitude.[6]
KQ Vel is a frequently-studied object that is often used as a magnetic standard star. The magnetic field of this star has a dipole strength of 7.5 kG, while displaying additional quadropole and octopolemoments. It is inclined to the rotation axis by 16°. The mean longitudinal field is almost constant, showing a strength of −2 kG.[8] The star is less than halfway through its main sequence lifespan, but is rotating very slowly with a period of around 2,800 days. It has three times the mass and 2.5 times the radius of the Sun, while radiating 105 times the Sun's luminosity.[8]
The mass function of the system indicates that the secondary must have more than two times the mass of the Sun, yet there is no sign of it in otherwise high-quality spectra.[1] A main sequence stellar companion of this mass would have a spectral type earlier than A5V.[1] This led to the suggestion that this companion must be a compact object, either a black hole, a neutron star, or a pair of white dwarfs.[8] In 2018 the companion was detected in the near infrared using the Pioneer instrument at the VLIT observatory, showing an H-band magnitude difference of 1.8±0.03 at an angular separation of 18.72±0.02 mas. This corresponds to a projected linear separation of a little over 2 AU.[1]
X-ray observations with the Chandra X-ray Observatory in 2016 strongly suggested that the companion is a neutron star, which would make KQ Velorum the first known pair of strongly magnetic Ap star and neutron star that have been discovered.[15] Radio emission has also been detected from the secondary object, raising the possibility that the secondary is actually itself a binary containing a magnetically active star.[16]
If KQ Velorum B is a neutron star, the system would be the product of a supernova explosion, possibly of the electron capture type that would not significantly disrupt the orbit. The strongly magnetic Ap star would likely be the result of a merger, perhaps from an W UMa close binary. The current neutron star may have been the tertiary member of the system, and it gained mass during a Roche lobe overflow of the binary components.[15]
^ abHouk, N. (1978), Michigan catalogue of two-dimensional spectral types for the HD stars, vol. 2, Dept. of Astronomy, University of Michigan, Bibcode:1978mcts.book.....H.
^ abRenson, P.; et al. (September 1991), "General catalogue of AP and AM stars", Astronomy and Astrophysics Supplement Series, 89: 429, Bibcode:1991A&AS...89..429R.
^Hensberge, H. (January 1993), Dworetsky, M. M.; Castelli, F.; Faraggiana, R. (eds.), "Longterm Variability in Cp-Stars", Peculiar versus Normal Phenomena in A-type and Related Stars. International Astronomical Union, Colloquium No. 138, held in Trieste, Italy, July 1992, vol. 44, no. 138, San Francisco, CA: Astronomical Society of the Pacific, p. 547, Bibcode:1993ASPC...44..547H, ISBN0937707635.
^Borra, E. F.; Landstreet, J. D. (December 1975), "Discovery of magnetic fields in four southern Ap stars", Publications of the Astronomical Society of the Pacific, 87: 961–963, Bibcode:1975PASP...87..961B, doi:10.1086/129880, S2CID121297510.
^Mathys, G.; et al. (June 1997), "The mean magnetic field modulus of AP stars", Astronomy and Astrophysics Supplement Series, 123 (2): 353–402, Bibcode:1997A&AS..123..353M, doi:10.1051/aas:1997103.
