Search

In a big town where the night sky is virtually invisible and if often only a virtual experience a telescope is the best channel for the real sky. Even a small telescope. Moon, planets, double stars and some deep sky objects become real objects viewed through an eyepiece or by means of modern CCD In the last few years, in the nice location of our Astrogarden, we did a lot of public outreach and educational activities using small telescopes. Our results show that these activities are very efficient in the broadening of understanding of astronomy and science with the public, in stimulating scientific interests with the young people and in developing useful skills in university students.

In this paper we study in detail the variability of the symbiotic nova AG Peg in the IUE range during the period 1978-1995. We find that the luminosity of the hot component decreased steadily from about 1850 L. in 1978 to 430 L. in 1995 while its effective temperature remained nearly constant, 86500 +/- 1300 degrees K. At the same time, the mass loss rate decreased by a factor of 4-5 although the ejection velocity remained constant. The observed fainting of both the narrow and the broad emission lines is ascribed to the decrease of the luminosity and ionizing flux from the hot source. The narrow emission lines and the long wavelength UV continuum show periodic variations arising from a dense and asymmetric nebular region associated with the cool star. Radial velocity variations of the narrow lines indicate that this region is receding from the center of mass of the system, and support the presence of the ablation tail suggested by Penston and Alien (1985). No periodic flux variations are seen in the short wavelength ultraviolet continuum and in the broad emission lines, which are both formed in the fast wind from the hot component.

We present the results of an extensive campaign of coordinated X-ray (ROSAT) and UV (IUE) observations of the symbiotic star AG Dra during a long period of quiescence (1990-1993) followed by two optical outbursts in 1994 and 1995. The hot component (i.e. X-ray emitting compact object) turns out to be very luminous during the quiescent phase: (9.5+/-1.5)*10^36 (D/2.5 kpc)^2 erg/s. This suggests that the compact object is burning hydrogen-rich matter on its surface even in the quiescent (as defined optically) state at a rate of (3.2+/-0.5)*10^-8 (D/2.5 kpc)^2 M_sun/yr. Assuming a steady state, i.e. burning at precisely the accretion supply rate, this high rate suggests a Roche lobe filling cool companion though Bondi-Hoyle accretion from the companion wind cannot be excluded. We have discovered a remarkable decrease of the X-ray flux during both optical maxima. In the UV these events were characterized by a large increase of the emission line and continuum fluxes. The anticorrelation of X-ray/UV flux and optical brightness evolution is very likely due to a temperature decrease of the hot component which could be the result of an increased mass transfer to the burning compact object, causing it to slowly expand to about twice its original size during each optical outburst., accepted for publ. in A&A, 17 pages incl. 5 Figs

Following a previous study by Altamore et al. (1982, Paper I) of the ultraviolet spectrum of KQ Pup, and the spectral Atlas of this star by Altamore et al. (1991, Paper II), we analyze coude and CAT CES optical spectra of the object observed during 1969-1984, and compare with IUE data obtained in 1979. Besides the M-star photospheric absorptions, the optical spectrum shows sharp emissions of [S II], [Cu II], [Fe II], Fe II, and [Ni II]. They probably arise in an intermediate region of the M-star wind ionized by the hot star radiation, where also the P Cygni UV lines of ionized metals are formed. The region is more extended and closer to the M-star near periastron passage. The earlier Balmer lines present a double peaked emission with a sharp central absorption which in 1979 and 1983 appears double suggesting the presence of streams or clouds of matter in the system environment. The redshifted emission component disappears in the higher series members, giving rise to an inverse P Cygni profile. Some Balmer lines display very broad absorption components which are identified as the density broadened hot star photospheric lines. He I is probably present with a rotationally broadened absorption at 3819 angstrom largely hidden by the rich M-star spectrum. In the UV the resonance lines and the excited lines of the doubly ionized metals are broad and redshifted by about +37 km s-1 with respect to the M-star radial velocity, implying line formation in opaque and turbulent matter flowing near and towards the surface of the hot star. From the blue-IR colour indices and the 2200 angstrom interstellar band we derive for KQ Pup a colour excess of E(B-V) = 0.14 +/- 0.03. The ultraviolet to infrared energy distribution appears bimodal with the hot component dominating below about 3600 angstrom and the M star above 4000 angstrom. We have fitted the dereddened energy distribution with a combination of a 30 000 K black body and of a M-supergiant spectrum (alpha-Ori), and derived for KQ Pup a distance at 1400 +/- 200 pc. At this distance the hot component results a slightly underluminous BOV star with a radius of 4.35R.. The mass ratio of the components of the binary system seems to be close to unity. We propose for KQ Pup a binary model with a highly mass losing cool supergiant primary and an unevolved early-type secondary. The optical and UV emission lines are formed in the regions of the cool star wind which are ionized by the hot star radiation. A shock is probably produced near the hot star by the interaction of the cool star wind with the hot star wind. Instabilities probably occur near the shock which produce cool clouds from the M-star wind, which may penetrate the hot star wind and be accreted.

New IUE observations of the innermost region of the galactic globular cluster M5 (NGC 5904) are presented and discussed. Addition of IUE spectra and identification of spectral features were obtained from the present and previous IUE observations of M5. Synthetic models of the cluster were constructed by taking into account optical and UV data. A comparison between theory and observation allowed for some estimates of the stellar content of M5. However, attention is called to the limits of validity of synthetic models, at least when they are compared with IUE observations.

The Herbig Be star HD 200775, associated with the extended reflection nebulae NGC 7023, has been observed in the ultraviolet with the IUE satellite, in the optical with the 90-cm Schmidt telescope of Campo Imperatore Observatory, and in the infrared with the 2.3 m telescope of Wyoming University in a coordinated campaign made during August 1978. The ultraviolet low-dispersion spectrum shows several strong absorption resonance lines (C II, C IV, Si II, Si IV, Al III . . .) probably formed in an expanding envelope. From the IR observations up to 12.6 microns, a value of the mass loss rate expansion velocity ratio of 2.3 x 10 to the -9th solar masses/year per km/sec is derived. The infrared energy distribution shows also the presence of a dust shell at a mean distance of about 3 x 10 to the 14th cm from the central star. A value of the color excess of 0.25 + or - 0.05 is derived from the 2200-A interstellar band and from the gradient of the UV continuum.

The observation of the X-ray spectrum of peculiar emission line stars, whose atmospheric envelopes are characterized by extreme physical conditions, represents an important tool to investigate the processes of formation of high temperature plasmas. The authors discuss the case of the superluminous object η Car showing both high and low temperature regions and an asymmetric wind. They present new EXOSAT observations of symbiotic stars. AG Draconis observed at a minimum phase shows a low temperature X-ray spectrum. A weak flux from the Mira variable R Aqr observed near maximum has been also detected.

In the framework of a phenomenological study of the ultraviolet properties of classical novae in outburst, we have selected 12 objects among the best monitored at low resolution with the IUE satellite, and studied the temporal evolution of the ultraviolet continuum and of the O I 1300 Angstrom line flux during the early post-outburst phase. We confirm that the UV flux maximum takes place systematically after the visual maximum and that its time delay is a linear function of t(3). A linear dependence on t3 is also found for the duration of the UV outburst and for the time the O I line flux reaches a maximum. This latter time marks the start of the transition phase to nebular conditions. Within the uncertainties imposed by the sample of objects used and by the observational errors, these results suggest a quite homogeneous behaviour of classical novae in the ultraviolet range.

There are some fundamental aspects of the "symbiotic phenomenon" which are poorly understood. Modern technology now allows to observe faint stars with efficient very high resolution spectrographs, such as the SARG mounted at TNG, which will be crucial for our understanding of the nature and evolutionary stage of symbiotic stars and related sytems. Of great importance will be the determination of luminosity, rotation, mass loss, chemical composition of the cool stellar components which is basic for settling the mass transfer in the system and tracing back its past history. High resolution observations are also needed to derive the velocity, temperature and abundance gradients in the nebula, and to unveil peculiar excitation processes.

The recent 1994-1995 active phase of AG Draconis has given us for the first time the opportunity to follow the full X-ray behaviour of a symbiotic star during two successive outbursts and to compare with its quiescence X-ray emission. With \ros observations we have discovered a remarkable decrease of the X-ray flux during both optical maxima, followed by a gradual recovering to the pre-outburst flux. In the UV the events were characterized by a large increase of the emission line and continuum fluxes, comparable to the behaviour of AG Dra during the 1980-81 active phase. The anticorrelation of X-ray/UV flux and optical brightness evolution is shown to very likely be due to a temperature decrease of the hot component. Such a temperature decrease could be produced by an increased mass transfer to the burning compact object, causing it to slowly expand to about twice its original size., Comment: 12 pages postscript incl. figures, Proc. of Workshop on Supersoft X-Ray Sources, to appear in Lecture Notes in Physics vol. 472 (1996)

An atlas of the high-resolution UV spectrum of Eta Car from 1200 to 1974 A and from 2200 to 3230 A is presented, based on IUE observations made between 1978 and 1980. The fluxes and equivalent widths of the emission and absorption features, and the stellar continuum in line-free regions are presented. The profiles displayed by the most intense emission suggest line formation in an asymmetric envelope. Many of the observed features may be explained if Eta Car is an intermediate, possible binary, F-type hypergiant in a short living stage, which holds a massive wind heated by dissipation of mechanical energy. 61 refs.