Photometric and spectrophotometric observations of the evolution of a strong outburst of the classical symbiotic star YY Herculis

Abstract

We present spectrophotometric (3400–7500 A) observations of the evolution of a strong outburst of the classical symbiotic star YY Her in 1993 and photoelectric UBV observations of the star’s eclipse in 1997. The duration of the phase of lowest brightness, when the U brightness had decreased by ∼1.3m, was ∼0.17 P orb (P orb is the orbital period). If this phase is due to the total eclipse of ∼70% of the radiation of the circumstellar envelope, this duration implies that the cool component of YY Her fills its Roche lobe, the bulk of the envelope’s volume emission measure is concentrated around the hot component in a region with rather sharp boundaries r Open image in new window R giant (R giant is the giant’s radius), and the line of sight is close to the binary orbital plane. We model fit the spectral energy distribution of YY Her to obtain estimates of the parameters of several structural components of the system. The red giant’s spectral type correlates with its visual brightness and does not correlate with the hot component’s brightness. At minimum brightness, the hot component’s luminosity fluctuates about that of its cool companion (L h, bol /L c, bol ≈0.9), and its temperature is T h ≈(9–11)×104 K. Activity of the hot component of YY Her is accompanied by increased brightness and reduced temperature (L h, bol ∝T h −5 ), though the relation between these parameters is not unique. At maximum brightness, L h, bol /L c, bol ≈10 and T h ≈6.0×104 K. If the red giant fills its Roche lobe, the hot component’s luminosity is ∼3.3 × 104L⊙. The active period of YY Her lasted about 5 years, and the activity decrease was not monotonic.