Non-Radial Pulsations in Components of Symbiotic Stars

Photometric observations of symbiotic stars in the blue and in the red spectral regions make it possible to reveal non-radial oscillations both of the cool and of the hot components. Light variations of red giants in the symbiotic systems CI Cyg and AG Peg show several periods in the 10–80^d range, interpreted as p-mode pulsations. These modes are excited by a bright spot produced by radiation flux from the hot component. The spot moves on the red giant’s photosphere at a velocity close to the sound speed. During the active phase of the symbiotic star CH Cyg, at least 25 frequencies of oscillations in the 150–6000 s range of periods were found in the light of the white dwarf. Their features correspond to non-radial g-modes. In the frame of 2D gas dynamical non-adiabatic models, the interaction between gas flows and the accretion disk leads to formation of a system of shock waves propagating towards the compact object, which is one of possible mechanisms to excite non-radial pulsations of white dwarfs in symbiotic systems.     

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

We have re-analysed the long-term optical light curve (LC) of the symbiotic star Z Andromedae, covering 112 yr of mostly visual observations. Two strictly periodic cycles and one quasi-periodic cycle can be identified in this LC. A   P 1 = 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, that is, in time-intervals between outbursts, the LC is clearly modulated by a stable coherent period of   P 2 = 759.1  d. This is the well-known orbital period of the Z Andromedae binary system that has been measured also spectroscopically. A third coherent period of   P 3 = 658.4  d is modulating the intense fluctuations in the optical brightness of the system during outbursts. We attribute the trigger of the outburst 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 Pole and the South Pole 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 a retrograde direction. We briefly compare these findings in the LC of Z Andromedae to similar modulations that were identified in the LC of two other prototype symbiotics, BF Cyg and YY Her.     

Emission-line CCD imaging of three southern symbiotic stars

Symbiotic stars that are strong radio sources and have cool dust emitting in the infrared are expected to have extended emission nebulae around them. In order to search for such emission nebulae, we have carried out CCD imaging of three symbiotic stars (R Aqr, RR Tel and H1-36) with narrow-band filters centred at the emission lines of [O III] λ5007, Hα λ6563, [N II] λ6584, [S II] λ6717 + 6731. RR Tel and H1-36 images do not show any extended nebulosities around them. The CCD image of the R Aqr nebulosity in the high excitation [O m] line is different from its image in Hα and the low excitation lines of [N II] and [S II] indicating ionization-stratification in the nebula. In H1-36 the optical nebulosity (if it exists) is smaller than ∽2 arcsec while the radio image size is known to be large (∽5 arcsec). This behaviour is opposite to that seen in R Aqr in which the radio emission comes from the core region of a much larger optical nebulosity. Interstellar and/or circumstellar extinctions are suggested to be responsible for this difference     

Spectral and morphological studies of an “infrared” nebula in the region of Cyg OB7

Spectral and morphological studies of an infrared nebula in the neighborhood of Cyg OB7 discovered in 2007 are reported. It is shown that over the last several years the brightness and shape of the nebula have changed significantly. Spectral observations of the nebula made with the 2.6-m telescope at the Byurakan Observatory in 2004 and 2005 reveal the existence of a faint trace of continuum spectrum, indicating the existence of a star inside the dark cloud. Classification of these spectra shows that over one year the star’s spectral class changed from late G to early K. It is also shown that the absorption in the direction of the nebula is as high as 8^m–10^m. __________ Translated from Astrofizika, Vol. 50, No. 1, pp. 17–27 (February 2007).     

Variations in the brightness and polarization of W Cep

The hypothesis that W Cep has an eclipsing variability is discussed on the basis of our photometric and polarimetric observations of it from 1983–2003. Here we attempt to use our observations of W Cep to construct an average light curve, after separating out the slow variation, with the elements JD_min=2448105^d+2090^d. A strong variability is found from period to period at phases of 0.1–0.3 that may be caused by a variable flow of matter between the components. __________ Translated from Astrofizika, Vol. 49, No. 2, pp. 187–196 (May 2006).     

New Plateau de Bure observations of M 1–92; unveiling the core

M 1–92 is a very well studied bipolar pPN that can be considered an archetype of this type of sources; it shows a clear axial symmetry, along with the kinematics characteristic of this class of envelopes around post-AGB stars. We performed sub-arcsecond resolution observations of the J=2–1 rotational line of ¹³CO in M 1–92 with the new extended configurations of the IRAM Plateau de Bure interferometer, for studying the morphology and velocity field of the molecular gas better in the nebula, particularly in its central parts. We found that the equatorial structure dividing the two lobes is a thin flat disk, which expands radially with a velocity proportional to the distance to the central stellar system. The kinetic age of this equatorial flow is very similar to that measured in the two lobes, suggesting that the whole structure was formed as a result of a single event some 1200 yr ago, after which the nebula reached an expansion velocity field with axial symmetry. The small widths and velocity dispersion in the gas forming the lobe walls confirm that the acceleration responsible for the nebular shape could not last more than 100–120 yr. In view of the similarity to η Car, we speculate on the possibility that the whole nebula was formed as a result of a magneto-rotational explosion in a common-envelope system. The study of the possible importance of this mechanism in the context of global PNe and pPNe reshaping should be one on the fields in which future ALMA observations will make a crucial contribution. Based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).     

Notices to investigation of symbiotic binaries V. Physical parameters derived from UBV magnitudes

In the optical, the spectrum of symbiotic binaries consists of contributions from the cool giant, symbiotic nebula and the hot star. Strong emission lines are superposed on the continuum. In this paper, we introduce a simple method to extract individual components of radiation from photometric UBV magnitudes. We applied the method to classical symbiotic stars AX Per, AG Dra, AG Peg and Z And, the symbiotic novae RR Tel and V1016 Cyg and the classical nova V1974 Cyg during its nebular phase. We estimated the electron temperature and emission measure of the nebula in these systems and the V magnitude of the giant in the symbiotic objects. Our results are in a good agreement with those obtained independently by a precious modelling the UV-IR SED.     

Study of nonstationary processes in the close binary system RY Scuti

A new analysis is made of old electrophotometric data on RY Scuti, a close binary of an early spectral class, taken at the Abastumani Astrophysical Observatory, Georgia, during 1972–1985 and at the Maidanak Observatory, Uzbekistan, during 1979–1991. Nonstationary processes occurring from period to period, from month to month, and from year to year are identified in RY Scuti. These variations range from hundredths to tenths of a stellar magnitude. In addition, periodic changes in the light curve of this system with a period of several years have been identified near the first maximum. This result is of considerable interest in connection with similar brightness changes observed in bright blue variable LBV stars, and also is related to the ejection of a nebula surrounding the RY Scuti system. Translated from Astrofizika, Vol. 52, No. 2, pp. 275–289 (May 2009).     

The putative nebula of the Wolf-Rayet WR 60 star: a case of mistaken identity and reclassification as a new supernova remnant G310.5+0.8

We present narrow band AAO/UKST Hα images and medium and low resolution optical spectra of a nebula shell putatively associated with the Wolf-Rayet star WR 60. We also present the first identification of this shell in the radio regime at 843 MHz and at 4850 MHz from the Sydney University Molonglo Sky Survey (SUMSS), and from the Parkes-MIT-NRAO (PMN) survey respectively. This radio emission closely follows the optical emission. The optical spectra from the shell exhibits the typical shock excitation signatures sometimes seen in Wolf-Rayet stellar ejecta but also common to supernova remnants. A key finding however, is that the WR 60 star, is not, in fact, anywhere near the geometrical centre of the putative arcuate nebula ejecta as had been previously stated. This was due to an erroneous positional identification for the star in the literature which we now correct. This new identification calls into serious question any association of the nebula with WR 60 as such nebula are usually quite well centred on the WR stars themselves. We now propose that this fact combined with our new optical spectra, deeper Hα imaging and newly identified radio structures actually imply that the WR 60 nebula should be reclassified as an unassociated new supernova remnant which we designate G310.5+0.8.     

Notices to investigation of symbiotic binaries IV. Optical light curves from the near-ultraviolet

The aim of this note is to reconstruct optical light curves (LC) from the near-UV fluxes of the ultraviolet spectra of symbiotic binaries during their quiescent phases. The method is based on the fact that the nebular component of radiation dominates the near-UV during quiescent phases and represents the main source of the light variability in the optical. We demonstrate this approach on example of two quiet symbiotic stars, SY Mus and RW Hya. Using their IUE spectra we determined U and B magnitudes in the standard Johnson system. Values derived from the near-ultraviolet are fainter then those measured photometrically by $\mathrm{\Delta }U\approx 0.2$ and $\mathrm{\Delta }B\approx 0.4$. This difference is due to emission lines.     

Searches for HH objects in star formation regions. VII. Herbig-Haro objects in the region of the nebula GM 2–41

Five new Herbig-Haro objects (HH 1036–1040) have been discovered in the neighborhood of the nebula GM 2–41 in a region with an area of 14′ × 14′, at the center of the HII region DR 15 located in the southern periphery of the Cyg OB2 association. Four of them have a complex structure typical of HH flows. Hydrogen molecular emission is detected in the object HH 1036 using archived images from the Spitzer telescope. Two new infrared nebulae illuminated by very red young stellar objects are also found.     

Observations of the December 6, 2006 solar flare: Electron acceleration and plasma heating

The flare plasma temperature calculated from GOES-11 (1.5–12.4 and 3.1–24.8 keV) data is compared with the solar nonthermal fluxes in various energy ranges in the December 6, 2006 event. Particle acceleration and plasma heating episodes took place in the pre-flare and impulsive phases; a hard (ACS SPI > 150 keV) X-ray emission was observed 5 min before the onset of the GOES X-ray flare and was not accompanied by a temperature rise. A close correlation has been found between the flare plasma temperature and the hard X-ray intensity. The temperature delayed by 0.4 min turned out to be directly proportional to the logarithm of the ACS SPI count rate within the first 3 min of the impulsive phase. This shows that the accelerated electrons responsible for the X-ray emission were the main plasma heating source in the pre-flare and impulsive phases. The correlation between the temperature and the hard X-ray intensity disappears after the observation of a resonance peak at a frequency of 245 MHz. Significant electron fluxes may no longer be able to effectively heat the expanding plasma when its density in the interaction region reaches ∼10⁹ cm⁻³. The observations of the July 23, 2002 and December 5, 2006 events confirm the trends found.     

Changes in the spectrum of the recombining planetary nebula He 1–5

In 2008, we observed the spectrum of the planetary nebula He 1–5, which has had no ionization source for more than 50 years. Comparison with the observations performed in 1972–1976 and 1988 shows that the line intensity ratio I(λ5007[O III])/I(Hβ) decreased by a factor of 2.5, the intensity ratio of the red [N II] doublet to Hα was almost constant, and the [S II]-to-Hα intensity ratio may have slightly increased. The observed changes in the spectrum agree, within the limits of the observational errors, with the calculations of gas recombination in a medium-excitation nebula performed by Tylenda in 1980, but the nitrogen line intensities are higher than the calculated ones by 10–15%. This is most likely related to the slightly reduced nitrogen abundance adopted in the model. The onset of nebular recombination dates from the end of the 19th century and is consistent with its estimate from the photometric history of the central star.     

Optical and X-Ray Observations of the Symbiotic System Ag Draconis During Quiescence and Outburst

We review the main characteristics of the symbiotic system AG Draconis, with special emphasis on its optical and X-ray variations. We also discuss the X-ray to visual energy distribution during quiescence and outburst and describe our spectroscopic and X-ray observations during the 2003 outburst. Based on X-ray observations collected with the XMM–Newton Observatory, on INES data from the IUE satellite, and on optical spectra collected with the Asiago–Cima Ekar, Bologna–Loiano and La Palma–Galileo Italian telescopes.     

Spot activity in the RS CVn binary UX arietis

Photoelectric observations of the RS CVn type non-eclipsing binary UX Arietis obtained at Nizamiah Observatory during the observing seasons of 1975–76, 1981–82 and 1982–83 are presented. The light curve of UX Ari showed a distortion wave with an amplitude inV varying from 0.02 mag during 1975–76 to 0.15 mag during 1982–83. An analysis of the available data shows that the light maximum is almost constant. It is also evident that the light-curve minimum decreases as the wave amplitude increases. The constant light at maximum,V = 6.51 ± 0.03 indicates the unspotted photospheric brightness. It is also suggested that the variation in meanV brightness is mainly due to spot activity and not due to intrinsic variation.     

Spectroscopic study of the variable planetary nebula IC 4997

Based on our spectroscopic observations of the variable planetary nebula IC 4997 in 2003–2009, we have obtained the relative fluxes in optical emission lines. The interstellar extinction c = 0.35 has been found from the Balmer decrement by taking into account the effect of self-absorption in hydrogen lines in dense nebular regions. The variations in the Balmer decrement point to variability of the self-absorption and circumstellar extinction. We have investigated the variations in the relative intensities of some spectral lines and their ratios with time. The drop in the ratios F(λ4363)/F(Hγ) and F(λ363)/F(λ4959) that began back in 1990–1995 has continued, suggesting a decrease in the electron density and temperature in the central nebular region. The ratio F(λ6731)/F(λ6717) has remained constant. It gives an estimate for the electron density in the outer regions of IC 4997, N _e ∼ 10⁴ cm⁻³.     

Optical identification of iras point sources on the basis of low-dispersion FBS spectra. III

Part three of optically identified point sources from the IRAS Point Source Catalog contains 113 objects. The identifications were based on the Digitized Sky Survey (DSS), the First Byurakan Survey, blue and red maps from the Palomar Sky Survey, and infrared fluxes at 12, 25, 60, and 100 μm in the region of +61° ≤ δ ≤ + 65° and 11^h15^m ≤ α ≤ 18^h35^m with an area of 200 sq. deg. Of the 119 sources in this region, 6 could not be identified owing to the absence of optical counterparts with the given coordinates. For the identified objects we determined their optical coordinates, their departures from the IR coordinates, and their stellar V magnitudes, color indices (CI), and preliminary types. The objects have optical magnitudes in the range of 8ⁿ–21^m. Of the 113 objects, 31 turned out to be stars of spectral types K and M, 1 is a planetary nebula, 2 are candidates to be quasi- stellar objects, and 79 are galaxies. A list of the 48 nonstellar objects is given. The identified galaxies include Seyfert candidates, interacting pairs, galaxies with companions, superassociations, etc. The galaxies are in groups in many cases, and the IR emission may be due to heating of intergalactic matter within a group. Finder charts for these objects from the DSS are given. Translated from Astrofizika, Vol. 41, No. 2, pp. 251–262, April-June, 1998.     

A Diatom Model of Dust in the Trapezium Nebula

Measurements are reported of the 5–35 μm infrared spectrum of a mixed diatom culture dispersed in a CsI disc. These data are used to compute the flux from a diatom model of dust in the Trapezium nebula, where dust grain temperatures are assumed to be distributed in the range 230–130 K. Good agreement with the observational data is found for the model. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spectral and photographic studies of the known T association in the neighborhood of Cyg OB7

Spectral and photographic observations of an 11′×11′ region in the center of the known association Cyg OB7 are presented. The observations were made on the 2.6-m telescope of the Byurakan Astrophysical Observatory in 2004. The spectral camera SCORPIO, a multiregime focal reducer for the primary focus, was used. One new HH node and three new emission stars were discovered in the region under study. One of the previously known emission stars is found to have spectral class M0Ve and to lie at a distance of 200 pc. Some variability is detected in the brightness of a known infrared nebula and the cometary nebula CN2. __________ Translated from Astrofizika, Vol. 49, No. 3, pp.375–390 (August 2006).     

Circumstellar material in the symbiotic binary V443 herculis

A quantitative model explaining a periodic wave-like variation in both the optical continuum and the emission line fluxes of the symbiotic binary V443 Her is presented. The model suggests that behaviour of spectrophotometric parameters is caused by a different projection of the circumstellar matter located mostly between the components of the binary, viewing the system at different orbital phases. Our model suggests that the inclination of the orbit is 18° (+12°, –3°) and the mass ratio of the cool to the hot component is 2(+1). It is shown that the previous interpretation in terms of a reflection effect is incorrect.