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X Per and V725 Tau: infrared variations on timescales longer than fifteen years

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Abstract

The results of infrared observations of the two Be stars X Per and V725 Tau, which are the optical components of X-ray binary systems, obtained in 1994–2016 are presented. The observations cover Be-star phases as well as shell phases. The data analysis shows that the radiation observed from the binaries at 1.25, 3.5, and 5 μm can be explained as the combined radiation from the optical components and variable sources (shells/disks) that emit as blackbodies (BBs). Emission from a source with the color temperature T c ~1000−1500 K was detected for X Per at λ ≥ 3.5 μm. The highest IR-brightness variation amplitudes for X Per were 0.9−1.2m (JHK magnitudes) and ~1.45m (LM magnitudes); for V725 Tau, they were 1.1−1.4m and ~1.7m (L magnitudes). The parameters of the optical components and interstellar extinction during the Be phases were estimated: the color excesswasE(B−V) = 0.65±0.08m and 0.77 ± 0.03m for X Per and V725 Tau, respectively. Light from the variable sources (disks/shells) was distinguished and their color temperatures, radii, and luminosities estimated for different observation epochs in a BB model. The variations of the binaries’ IR brightness and colors are shown to be due to changing parameters of the variable sources. The mean color temperature of the cool source (disk/shell) and the mean radius and mean luminosity of X Per are 9500± 2630 K, (35 ± 10) R⊙, and (9100± 540) L⊙. For V725 Tau, these parameters are 6200 ± 940 K, (27 ± 6) R⊙, and (980 ± 420) L⊙. The 1.25–5 μm radiation from X Per at different epochs can be represented as a sum of contributions from at least three sources: the optical component and two objects emitting as BBs. To reproduce the 1.25–3.5 μm radiation from V725 Tau, two components are sufficient: the optical component and a single variable BB object. For both binary systems, orbital variations of the IR brightness can be noted near the Be-star phase. The amplitudes of the J-band variations of X Per and V725 Tau are about 0.3m and 0.1m, respectively.

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Authors and Affiliations

  1. Sternberg Astronomical Institute, Lomonosov Moscow State University, Universitetskii pr. 13, Moscow, 119991, Russia

    O. G. Taranova & V. I. Shenavrin

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  1. O. G. Taranova
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  2. V. I. Shenavrin
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Correspondence to O. G. Taranova.

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Original Russian Text © O.G. Taranova, V.I. Shenavrin, 2017, published in Astronomicheskii Zhurnal, 2017, Vol. 94, No. 11, pp. 981–993.

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Taranova, O.G., Shenavrin, V.I. X Per and V725 Tau: infrared variations on timescales longer than fifteen years. Astron. Rep. 61, 983–995 (2017). https://doi.org/10.1134/S1063772917100080

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  • DOI: https://doi.org/10.1134/S1063772917100080

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