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Register a new userThe hot and the cool otbursts in the symbiotic system AG Draconis
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In this papper we present the analyses of the six (1998, 1997, 2001, 2002, 2003 and 2005) last outbursts of AG Draconis on the basis of low resolution visual spectroscopy. A new method to determine the Zanstras temperature of the hot ionizing source from the optical Hb and HeII emission lines has been used. As a results we obtained the evolution of the individual outburst on the H-R diagram.
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The modeling of UV and optical spectra emitted from the symbiotic system AG Draconis, adopting collision of the winds, predicts soft X-ray bremsstrahlung from nebulae downstream of the reverse shock with velocities > 150 km/s and intensities comparable to those of the white dwarf black body flux. At outbursts, the envelop of debris, which corresponds to the nebula downstream of the high velocity shocks (700-1000 km/s) accompanying the blast wave, absorbs the black body soft X-ray flux from the white dwarf, explains the broad component of the H and He lines, and leads to low optical-UV-X-ray continuum fluxes. The high optical-UV flux observed at the outbursts is explained by bremsstrahlung downstream of the reverse shock between the stars. The depletion of C, N, O, and Mg relative to H indicates that they are trapped into dust grains and/or into diatomic molecules, suggesting that the collision of the wind from the white dwarf with the dusty shells, ejected from the red giant with about 1 year periodicity, leads to the U-band fluctuations during the major bursts.
AG Dra is a symbiotic variable consisting of a metal poor, yellow giant mass donor under-filling its Roche lobe, and a hot accreting white dwarf, possibly surrounded by an optically thick, bright accretion disk which could be present from wind accretion. We constructed NLTE synthetic spectral models for white dwarf spectra and optically thick accretion disk spectra to model a FUSE spectrum of AG Dra, obtained when the hot component is viewed in front of the yellow giant. The spectrum has been de-reddened (E(B-V) = 0.05) and the model fitting carried out, with the distance regarded as a free parameter, but required to be larger than the Hipparcos lower limit of 1 kpc. We find that the best-fitting model is a bare accreting white dwarf with Mwd = 0.4 Msun, Teff = 80,000K and a model-derived distance of 1543 pc. Higher temperatures are ruled out due to excess flux at the shortest wavelengths while a lower temperature decreases the distance below 1 kpc. Any accretion disk which might be present is a only a minor contributor to the FUV flux. This raises the possibility that the soft X-rays originate from a very hot boundary layer between a putative accretion disk and the accreting star.
We present high resolution spectroscopy of the yellow symbiotic star AG Draconis with ESPaDOnS at the {it Canada-France-Hawaii Telescope}. Our analysis is focused on the profiles of Raman scattered ion{O}{VI} features centered at 6825 AA and 7082 AA, which are formed through Raman scattering of ion{O}{VI}$lambdalambda$1032 and 1038 with atomic hydrogen. These features are found to exhibit double component profiles with conspicuously enhanced red parts. Assuming that the ion{O}{vi} emission region constitutes a part of the accretion flow around the white dwarf, Monte Carlo simulations for ion{O}{VI} line radiative transfer are performed to find that the overall profiles are well fit with the accretion flow azimuthally asymmetric with more matter on the entering side than on the opposite side. As the mass loss rate of the giant component is increased, we find that the flux ratio $F(6825)/F(7082)$ of Raman 6825 and 7082 features decreases and that our observational data are consistent with a mass loss rate $dot Msim 2 times 10^{-7} {rm M_{odot} yr^{-1}}$. We also find that additional bipolar components moving away with a speed $sim 70{rm km s^{-1}}$ provide considerably improved fit to the red wing parts of Raman features. The possibility that the two Raman profiles differ is briefly discussed in relation to the local variation of the ion{O}{VI} doublet flux ratio.
Symbiotic stars often exhibit broad wings around Balmer emission lines, whose origin is still controversial. We present the high resolution spectra of the S type symbiotic stars Z Andromedae and AG Draconis obtained with the ESPaDOnS and the 3.6 m Canada France Hawaii Telescope to investigate the broad wings around H$alpha$ and H$beta$. When H$alpha$ and H$beta$ lines are overplotted in the Doppler space, it is noted that H$alpha$ profiles are overall broader than H$beta$ in these two objects. Adopting a Monte Carlo approach, we consider the formation of broad wings of H$alpha$ and H$beta$ through Raman scattering of far UV radiation around Ly$beta$ and Ly$gamma$ and Thomson scattering by free electrons. Raman scattering wings are simulated by choosing an H I region with a neutral hydrogen column density $N_{HI}$ and a covering factor $CF$. For Thomson wings, the ionized scattering region is assumed to cover fully the Balmer emission nebula and is characterized by the electron temperature $T_e$ and the electron column density $N_e$. Thomson wings of H$alpha$ and H$beta$ have the same width that is proportional to $T_e^{1/2}$. However, Raman wings of H$alpha$ are overall three times wider than H$beta$ counterparts, which is attributed to different cross section for Ly$beta$ and Ly$gamma$. Normalized to have the same peak values and presented in the Doppler factor space. H$alpha$ wings of Z And and AG Dra are observed to be significantly wider than H$beta$ counterpart, favoring the Raman scattering origin of broad Balmer wings.
Symbiotic binary AG~Draconis (AG~Dra) has an well-established outburst behavior based on an extensive observational history. Usually, the system undergoes a 9--15~yr period of quiescence with a constant average energy emitted, during which the systems orbital period of $sim$550~d can be seen at shorter wavelengths (particularly in the U-band) as well as a shorter period of $sim$355~d thought to be due to pulsations of the cool component. After a quiescent period, the marker of an active period is usually a major (cool) outburst of up to $textrm{V}=8.4$~mag, followed by a series of minor (hot) outbursts repeating at a period of approximately 1~yr. However, in 2016 April after a 9-year period of quiescence AG~Dra exhibited unusual behavior: it began an active phase with a minor outburst followed by two more minor outbursts repeating at an interval of $sim$1~yr. We present R-band observations of AG~Dras 2018 April minor outburst and an analysis of the outburst mechanism and reports on the systems activity levels following the time of its next expected outburst. By considering the brightening and cooling times, the scale of the outburst, and its temperature evolution we have determined that this outburst was of disk instability nature.
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