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Register a new userAG Pegasi - now a classical symbiotic star in outburst?
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Optical spectroscopy study of the recent AG Peg outburst observed during the second half of 2015 is presented. Considerable variations of the intensity and the shape of the spectral features as well as the changes of the hot component parameters, caused by the outburst, are discussed and certain similarities between the outburst of AG Peg and the outburst of a classical symbiotic stars are shown. It seems that after the end of the symbiotic nova phase, AG Peg became a member of the classical symbiotic stars group.
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Symbiotic stars often contain white dwarfs with quasi-steady shell burning on their surfaces. However, in most symbiotics, the origin of this burning is unclear. In symbiotic slow novae, however, it is linked to a past thermonuclear runaway. In June 2015, the symbiotic slow nova AG Peg was seen in only its second optical outburst since 1850. This recent outburst was of much shorter duration and lower amplitude than the earlier eruption, and it contained multiple peaks -- like outbursts in classical symbiotic stars such as Z And. We report Swift X-ray and UV observations of AG Peg made between June 2015 and January 2016. The X-ray flux was markedly variable on a time scale of days, particularly during four days near optical maximum, when the X-rays became bright and soft. This strong X-ray variability continued for another month, after which the X-rays hardened as the optical flux declined. The UV flux was high throughout the outburst, consistent with quasi-steady shell burning on the white dwarf. Given that accretion disks around white dwarfs with shell burning do not generally produce detectable X-rays (due to Compton-cooling of the boundary layer), the X-rays probably originated via shocks in the ejecta. As the X-ray photo-electric absorption did not vary significantly, the X-ray variability may directly link to the properties of the shocked material. AG Pegs transition from a slow symbiotic nova (which drove the 1850 outburst) to a classical symbiotic star suggests that shell burning in at least some symbiotic stars is residual burning from prior novae.
AG Peg is known as the slowest symbiotic nova, which experienced its nova-like outburst around 1850. After 165 years, during June of 2015, it erupted again showing characteristics of the Z And-type outburst. The primary objective is to determine basic characteristics, the nature and type of the 2015 outburst of AG Peg. We achieved this aim by modelling the spectral energy distribution using low-resolution spectroscopy (330-750 nm), medium-resolution spectroscopy (420-720 nm; R=11000), and $UBVR_{rm C}I_{rm C}$ photometry covering the 2015 outburst with a high cadence. Optical observations were complemented with the archival HST and FUSE spectra from the preceding quiescence. During the outburst, the luminosity of the hot component was in the range of 2-11$times 10^{37}(d/0.8{rm kpc})^2$ erg/s. To generate the maximum luminosity the white dwarf (WD) had to accrete at $sim 3times 10^{-7}$ M$_{odot}yr^{-1}$, which exceeds the stable-burning limit and thus led to blowing optically thick wind from the WD. We determined its mass-loss rate to a few $times 10^{-6}$ M$_{odot}yr^{-1}$. At the high temperature of the ionising source, $1.5-2.3times 10^5$ K, the wind converted a fraction of the WDs photospheric radiation into the nebular emission that dominated the optical. A one order of magnitude increase of the emission measure, from a few $times 10^{59}(d/0.8 {rm kpc})^2$ cm$^{-3}$ during quiescence, to a few $times 10^{60}(d/0.8,{rm kpc})^2$ cm$^{-3}$ during the outburst, caused a 2 mag brightening in the LC, which is classified as the Z And-type of the outburst. The very high nebular emission and the presence of a disk-like HI region encompassing the WD, as indicated by a significant broadening and high flux of the Raman-scattered OVI 6825 AA line during the outburst, is consistent with the ionisation structure of hot components in symbiotic stars during active phases.
We present an analysis of the XMM-Newton observation of the symbiotic star AG Peg, obtained after the end of its 2015 outburst. The X-ray emission of AG Peg is soft and of thermal origin. AG Peg is an X-ray source of class beta of the X-ray sources amongst the symbiotic stars, whose X-ray spectrum is well matched by a two-temperature optically-thin plasma emission (kT_1 ~ 0.14 keV and kT_2 ~ 0.66 keV). The X-ray emission of the class beta sources is believed to originate from colliding stellar winds (CSW) in binary system. If we adopt the CSW picture, the theoretical CSW spectra match well the observed properties of the XMM-Newton spectra of AG Peg. However, we need a solid evidence that a massive-enough hot-star wind is present in the post-outburst state of AG Peg to proof the validity of the CSW picture for this symbiotic binary. No short-term X-ray variability is detected while the UV emission of AG Peg shows stochastic variability (flickering) on time-scales of minutes and hours.
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.
We present pre- and post-outburst observations of the new FU Orionis-like young stellar object PTF 10qpf (also known as LkHa 188-G4 and HBC 722). Prior to this outburst, LkHa 188-G4 was classified as a classical T Tauri star on the basis of its optical emission-line spectrum superposed on a K8-type photosphere, and its photometric variability. The mid-infrared spectral index of LkHa 188-G4 indicates a Class II-type object. LkHa 188-G4 exhibited a steady rise by ~1 mag over ~11 months starting in Aug. 2009, before a subsequent more abrupt rise of > 3 mag on a time scale of ~2 months. Observations taken during the eruption exhibit the defining characteristics of FU Orionis variables: (i) an increase in brightness by > 4 mag, (ii) a bright optical/near-infrared reflection nebula appeared, (iii) optical spectra are consistent with a G supergiant and dominated by absorption lines, the only exception being Halpha which is characterized by a P Cygni profile, (iv) near-infrared spectra resemble those of late K--M giants/supergiants with enhanced absorption seen in the molecular bands of CO and H_2O, and (v) outflow signatures in H and He are seen in the form of blueshifted absorption profiles. LkHa 188-G4 is the first member of the FU Orionis-like class with a well-sampled optical to mid-infrared spectral energy distribution in the pre-outburst phase. The association of the PTF 10qpf outburst with the previously identified classical T Tauri star LkHa 188-G4 (HBC 722) provides strong evidence that FU Orionis-like eruptions represent periods of enhanced disk accretion and outflow, likely triggered by instabilities in the disk. The early identification of PTF 10qpf as an FU Orionis-like variable will enable detailed photometric and spectroscopic observations during its post-outburst evolution for comparison with other known outbursting objects.
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