A mystery solved: the mass ratio of the dwarf nova EM Cygni |
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| Authors: | R C North T R Marsh C K J Moran U Kolb R C Smith R Stehle |
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| Institution: | University of Southampton, Department of Physics, Highfield, Southampton SO17 1BJ; The Open University, Department of Physics, Walton Hall, Milton Keynes MK7 6AA; University of Sussex, Astronomy Centre, Brighton BN1 9QJ; University of Leicester, Astronomy Group, University Road, Leicester LE1 7RH |
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| Abstract: | We have discovered that the spectrum of the well-known dwarf nova EM Cyg is contaminated by light from a K25V star (in addition to the K-type mass donor star). The K25V star contributes approximately 16 per cent of the light from the system and if not taken into account has a considerable effect upon radial velocity measurements of the mass donor star. We obtain a new radial velocity amplitude for the mass donor star of K 2=202±3 km s1, compared with the value of K 2=135±3 km s1 obtained in Stover, Robinson & Nather's classic study of EM Cyg. The revised value of the amplitude, combined with a measurement of rotational broadening of the mass donor, v sin i =140±6 km s1, leads to a new mass ratio of q M 2 M 1=0.88±0.05. This solves a long-standing problem with EM Cyg, because Stover et al.'s measurements indicated a mass ratio q >1, a value that should have led to dynamically unstable mass transfer for the secondary mass deduced by Stover et al. The revised value of the mass ratio, combined with the orbital inclination i =67±2°, leads to masses of 0.99±0.12 M and 1.12±0.08 M for the mass donor and white dwarf respectively. The mass donor is evolved, because it has a later spectral type (K3) than its mass would imply.
We discuss whether the K star could be physically associated with EM Cyg or not, and present the results of the spectroscopic study. |
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| Keywords: | accretion accretion discs techniques: radial velocities binaries: spectroscopic stars: individual: EM Cyg novae cataclysmic variables |
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