R Aquarii spectra revisited by SUMA

We analyse the optical spectra and the UV spectral evolution of the jets and of the HII region inside the R Aquarii binary system by the code SUMA which consistently accounts for shock and photoionization. The temperature of the hot star results 80,000 K as for a white dwarf. We find that the shock velocity in the NE jet increased between 1983 and 1989. The spectral evolution between 1989 and 1991 of the SW jet indicates that a larger contribution from low density-velocity matter affects the 1991 spectra. The evolution of the UV spectra from 8/11/1980 to 26/5/1991 in the HII region indicates that the reverse shock is actually a standing shock. The results obtained by modelling the line spectra are cross-checked by the fit of the continuum SED. It is found that a black-body temperature of 2800 K reproduces the radiation from the red giant. A black-body emission component corresponding to 1000 K is emitted by dust in the surrounding of the red giant. Model calculations confirm that the radio emission is of thermal origin. We found that the NE jet bulk emission is at a distance of about 2 (15) cm from the internal system, while the distance of the SW jet bulk is about 6 (14) cm. The distance of the reverse shock from the hot source in the internal region is < 9 (13) cm.Comment: 9 pages, MN LaTeX style (including 6 Tables) + 5 PostScript figures. To appear in The Monthly Notices of the Royal Astronomical Societ

The giant star of the symbiotic system YY Her: Rotation, Tidal wave, Solar-type cycle and Spots

We analyze the historical light curve of the symbiotic star YY Her, from 1890 up to December 2005. A secular declining trend is detected, at a rate of ~.01 magn in 1000 d, suggesting that the system could belong to the sub-class of symbiotic novae. Several outburst events are superposed on this slow decline. Three independent periodicities are identified in the light curve. A quasi-periodicity of 4650.7 d is detected for the outburst occurrence. We suggest that it is a signature of a solar-type magnetic dynamo cycle in the giant component. A period of 593.2 d modulates the quiescent light curve and it is identified as the binary period of the system. During outburst events the system shows a stable periodic oscillation of 551.4 d. We suggest that it is the rotation period of the giant.The secondary minima detected at some epochs of quiescence are probably due to dark spots on the surface of the rotating giant. The difference between the frequencies of these two last periods is the frequency of a tidal wave in the outer layers of the giant. A period which is a beat between the magnetic cycle and the tidal wave period is also apparent in the light curve. YY Her is a third symbiotic system exhibiting these cycles in their light curve, suggesting that a magnetic dynamo process is prevalent in the giant components of symbiotic stars, playing an important role in the outburst mechanism of some of these systems.Comment: 8 pages, 8 figures. Accepted by MNRA

Discovery of the 1.80 hr Spin Period of the White Dwarf of the Symbiotic System BF Cyg

We report on the discovery of a coherent periodicity in the B light curve of the symbiotic star BF Cyg. The signal was detected in some sections of the light curve of the star recorded in the year 2003 as double hump periodic variations with an amplitude of ~7 mmag. In the year 2004 the signal was also present in only a subsection of the light curve. In that year, the system was about twice as bright and the amplitude of the oscillations was about half of what it was in 2003. In 2004 the cycle structure was of a single hump, the phase of which coincided with the phase of one of the humps in the 2003 cycle. No periodic signal was detected in a third, short series of observations performed in the year 2007, when the star was three magnitudes brighter than in 2003. We interpret the periodicity as the spin period of the white dwarf component of this interacting binary system. We suggest that the signal in 2003 originated in two hot spots on or near the surface of the white dwarf, most likely around the two antipodes of an oblique dipole magnetic field of this star. Magnetic field lines funneled accreted matter from the wind of the cool component to the pole areas, where the falling material created the hot spots. This process is apparently intermittent in its nature. In 2004, the activity near only one pole was enhanced enough to raise the signal above the threshold of our detection ability.Comment: 7 pages, 5 figures, accepted for publication in MNRA

Activity cycle of the giant star of Z Andromedae and its spin period

We have reanalyzed the long-term optical light curve (LC) of the symbiotic star Z Andromedae, covering 112--yr of mostly visual observations. Two strictly periodic and one quasi-periodic cycles can be identified in this LC. A P1=7550 d quasi periodicity characterizes the repetition time of the outburst episodes of this symbiotic star. Six such events have been recorded so far. During quiescence states of the system, i.e. in time intervals between outbursts, the LC is clearly modulated by a stable coherent period of P2=759.1 d. This is the well known orbital period of the Z And binary system that have been measured also spectroscopically. A third coherent period of P3=658.4 d is modulating the intense fluctuations in the optical brightness of the system during outbursts. We attribute the trigger of the outbursts phenomenon and the clock that drives it, to a solar type magnetic dynamo cycle that operates in the convection and the outer layers of the giant star of the system. We suggest that the intense surface activity of the giant star during maximum phases of its magnetic cycle is especially enhanced in one or two antipode regions, fixed in the atmosphere of the star and rotating with it. Such spots could be active regions around the North and the South poles of a general magnetic dipole field of the star. The P3 periodicity is half the beat of the binary orbital period of the system and the spin period of the giant. The latter is then either 482 or 1790 d. If only one pole is active on the surface of the giant, P3 is the beat period itself, and the spin period is 352 d. It could also be 5000 d if the giant is rotating in retrograde direction. We briefly compare these findings in the LC of Z And to similar modulations that were identified in the LC of two other prototype symbiotics, BF Cyg and YY Her.Comment: 9 pages, 4 figures, Accepted for publication in MNRA

Modeling RR Tel through the Evolution of the Spectra

We investigate the evolution of RR Tel after the outburst by fitting the emission spectra in two epochs. The first one (1978) is characterized by large fluctuations in the light curve and the second one (1993) by the slow fading trend. In the frame of a colliding wind model two shocks are present: the reverse shock propagates in the direction of the white dwarf and the other one expands towards or beyond the giant. The results of our modeling show that in 1993 the expanding shock has overcome the system and is propagating in the nearby ISM. The large fluctuations observed in the 1978 light curve result from line intensity rather than from continuum variation. These variations are explained by fragmentation of matter at the time of head-on collision of the winds from the two stars. A high velocity (500 km/s) wind component is revealed from the fit of the SED of the continuum in the X-ray range in 1978, but is quite unobservable in the line profiles. The geometrical thickness of the emitting clumps is the critical parameter which can explain the short time scale variabilities of the spectrum and the trend of slow line intensity decrease.Comment: 26 pages, LaTeX (including 5 Tables) + 6 PostScript figures. To appear in "The Astrophysical Journal

The historical light curve of the symbiotic star AG Draconis: intense, magnetically induced cyclic activity

We analyze an optical light curve of the symbiotic system AG Draconis covering the last 120 years of its history. During the first 32 years the system was in a quiescence state. Around the year 1922 the star's quiescence luminosity brightened by 0.29 mag. The last 82 years of the light curve (LC) are characterized by a series of outbursts of 1-2 magnitude in brightness and about 100 days in duration. The outbursts are distributed along the time axis in 6 clusters with a quasi-periodic cycle of some 5300 days. The time intervals among the outbursts themselves are integral numbers of the period 373.5 days. During quiescence states the LC oscillates with the binary period of the system of 550 d. The LC contains also a weak periodic signal with a period of 350 d, attributed to pulsations of the giant star. Another period of 1160 d is also present in the light curve, being the sidereal rotation period of the giant star. We suggest that the outbursts are events of intense mass transfer from the giant onto the hot component. These are modulated by an interplay between a solar-like magnetic dynamo cycle operating in the outer layers of the giant, and a tidal deformation of these layers that circulates the surface of the giant with the synodic diurnal period of 373.5 Earth days. AG Dra is the 5th symbiotic system with a light curve that reflects such an intense magnetic and magnetically modulated activity. (Abridged)Comment: 10 pages, 4 figures. Accepted for publication in MNRA

Hidden Subluminous sd/wd among the FAUST UV sources toward OPHIUCHUS

A UV image in the direction of Ophiuchus, obtained with the FAUST instrument is analysed. Suitable candidates as unrecognized subluminous stars are selected comparing the observed UV flux to the predicted one. The uv-excess objects were observed at the 1.0 m Wise telescope. This method yields to the detection of eight broad Balmer lines objects. Six are classified as sds and two wds, comparing the Hbeta line profile with that of stellar model atmospheres.Comment: 2 pages, including 2 figures. To appear in the Proceedings of the 13th European Workshop on White Dwarfs. NATO Science Series II, Kluwer Academic Publishe

Principal component analysis of IUE galaxy spectra

We analyse the UV spectral energy distribution of a sample of normal galaxies listed in the IUE-INES Guide No. 2-Normal Galaxies (Formiggini & Brosch, 2000) using a Principal Component Analysis. The sample consists of the IUE-SW spectra of the central regions of 118 galaxies, where the IUE aperture included more than 1 per cent of the galaxy size. The principal components are associated with the main components observed in the UV spectra of galaxies. The first component, accounting for the largest source of diversity, can be associated with the UV continuum emission. The second component represents the UV contribution of an underlying evolved stellar population. The third component is sensitive to the amount of activity in the central regions of galaxies and measures the strength of star formation events. In all the samples analysed here the principal component representative of star-forming activity accounts for a significant percentage of the variance. The fractional contribution to the SED by the evolved stars and by the young population are similar. Projecting the SEDs onto their eigenspectra, we find that none of the coefficients of the principal components can outline an internal correlation or can correlate with the optical morphological types. In a sub-sample of 43 galaxies, consisting of almost only compact and BCD galaxies, the third principal component defines a sequence related to the degree of starburst activity of the galaxy.Comment: 13 pages, incl. 14 figures. Accepted by MNRA