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Register a new userOrbital period analysis of eclipsing Z Cam-type Dwarf Nova EM Cygni: Evidence of magnetic braking and a third body
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Combining with our newest CCD times of light minimum of EM Cygni, all 45 available times of light minimum including 7 data with large scatters are compiled and the updated O-C analysis is made. The bestfit for the O-C diagram of EM Cygni is a quadratic-plus-sinusoidal fit. The secular orbital period decrease rate -2.5(pm 0.3)x10^{-11} s s^{-1} means that magnetic braking effect with a rate of mass loss via stellar wind, 2.3x10^{-10}Msunyr^{-1}, is needed for explaining the observed orbital period decrease. Moreover, for explaining the significant cyclical period change with a period of sim 17.74(pm 0.01)yr shown in the O-C diagram, magnetic activity cycles and light travel-time effect are discussed in detail. The O-C diagram of EM Cygni cannot totally rule the possibility of multi-periodic modulation out due to the gaps presented after 25000 cycles. Based on the hypothesis of a K-type third star in literature, light trave-time effect may be a more plausible explanation. However, the low orbital inclination of the third body (sim 7.4 degree) suggests that the hypothetic K-type third star may be captured by EM Cygni. But assuming the spectral contamination from a block of circumbinary material instead of a K-type third star, the third star may be a brown dwarf in case of the coplanar orbit with parent binary.
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Photometry of Leo5 = 1H 1025+220 show that it is a dwarf nova of the Z Cam subtype. Two long standstills have been observed in the last five years.
Long term optical monitoring of the dwarf nova OQ Car has been conducted to study the previously unknown behaviour of this star system. The observations have shown OQ Car to have frequent dwarf nova outbursts and revealed the first recorded standstill of this star system. Based on this, we conclude that OQ Car is a new member of the Z Cam type dwarf novae.
The defining characteristic of Z Cam stars are standstills in their light curves. Some Z Cams exhibit atypical behaviour by going into outburst from a standstill. It has previously been suggested that UY Pup had been a Z Cam star, but it was ruled out due to its long-term light curve. However, in December 2015 UY Pup went into outburst and unexpectedly entered into a short standstill instead of returning to quiescence. Furthermore, UY Pup exhibited additional unusual behaviour with two outbursts detected during its standstill. After this standstill UY Pup made a brief excursion to a quiescence state and slowly rose to a longer and well-defined standstill, where it again went into another outburst. Through comparative analysis, research, and observational data of UY Pup it is evident and thus concluded that it is indeed a Z Cam star, in which renders it to be one of only four known anomalous Z Cam stars.
New eclipse timings of the Z Cam-type dwarf nova AY Psc were measured and the orbital ephemeris was revised. Based on the long-term AAVSO data, moreover, the outburst behaviors were also explored. Our analysis suggests that the normal outbursts are quasi-periodic, with an amplitude of $sim2.5(pm0.1)$ mag and a period of $sim18.3(pm0.7)$ days. The amplitude vs. recurrence-time relation of AY Psc is discussed, and we concluded that this relation may represents general properties of dwarf nova (DN) outbursts. The observed standstill ends with an outburst, which is inconsistent with the general picture of Z Cam-type stars. This unusual behavior was considered to be related to the mass-transfer outbursts. Moreover, the average luminosity is brighter during standstills than during outburst cycles. The changes in brightness marks the variations in $dot{M}_{2}$ due to the disc of AY Psc is nearly steady state. $dot{M}_{2}$ value was limited to the range from $6.35times10^{-9}$ to $1.18times10^{-8}M_{odot}yr^{-1}$. More detailed examination shows that there are a few small outbursts presence during standstills. These events with amplitudes of $sim0.5-0.9$ mag are very similar to the stunted outbursts reported in some NLs. We discussed several possible mechanisms and suggested that the most reasonable mechanism for these stunted outbursts is a changing mass-transfer rate.
In this paper we analyzed the behavior of the unusual dwarf nova EM Cyg using the data obtained in April-October, 2007 in Vyhorlat observatory (Slovak Republic) and in September, 2006 in Crimean Astrophysical Observatory (Ukraine). During our observations EM Cyg has shown outbursts in every 15-40 days. Because on the light curves of EM Cyg the partial eclipse of an accretion disc is observed we applied the eclipse mapping technique to reconstruct the temperature distribution in eclipsed parts of the disc. Calculations of the accretion rate in the system were made for the quiescent and the outburst states of activity for different distances.
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