Models of accretion disks around a star in a binary system predict that the disk will have a retrograde precession with a period a factor of ∼10 times the orbital period. If the star+disk system ejects a bipolar outflow, this outflow will be subject to the effects of both the orbital motion and the precession. We present an analytic, ballistic model and a three-dimensional gasdynamical simulation of a bipolar outflow from a source in a circular orbit, and with a precessing outflow axis. We find that this combination results in a jet/counterjet system with a small spatial scale, reflection-symmetric spiral (resulting from the orbital motion) and a larger-scale, point-symmetric spiral (resulting from the longer period precession). These results provide interesting possibilities for modeling specific Herbig-Haro jets and bipolar planetary nebulae.

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@@@--¿ astroph can't handle png figs in pdflatex! better to download pdf file from ABSTRACT We explore the possibility that the steepening observed shortward of 1000Å in the energy distribution of quasars may result from absorption by dust, being either intrinsic to the quasar environment or intergalactic. We find that a dust extinction curve consisting of nanodiamonds, composed of terrestrial cubic diamonds or with surface impurities as found in carbonaceous chondrite meteorites, such as Allende, is successful in reproducing the sharp break observed. The intergalactic dust model is partially successful in explaining the shape of the composite energy distribution, but must be discarded in the end, as the amount of crystalline dust required is unreasonable and would imply an improbable fine tuning among the dust formation processes. The alternative intrinsic dust model requires a mixture of both cubic diamonds and Allende nanodiamonds and provide a better fit of the UV break. The gas column densities implied are of the order 10 20 cm −2 , assuming solar metallicity for carbon and full depletion of carbon into dust. The absorption only occurs in the ultraviolet and is totally negligible in the visible. The minimum dust mass required is of the order ∼ 0.003r 2 pc M ⊙ , where r pc is the distance in parsec between the dust screen and the continuum source. The intrinsic dust model reproduces the flux rise observed around 660Å in key quasar spectra quite well. We present indirect evidence of a shallow continuum break near 670Å (18.5 eV), which would be intrinsic to the quasar continuum. ; anja@nordita.dk 1 In the text, λ rest and λ obs. will indicate whether the wavelength refers to the quasar rest-frame or the observer-frame, respectively; λ rest = (1 + z q ) −1 λ obs.

Using Chandra and HST archival data, we have studied the individual SED of 11 quasars at redshifts 0:3 < z < 1:8. All UV spectra show a spectral break around 1100 8. Five X-ray spectra showed the presence of a ''soft excess,'' and seven spectra showed an intrinsic absorption. We found that for most quasars a simple extrapolation of the far-UV power law into the X-ray domain generally lies below the X-ray data and that the big blue bump and the soft X-ray excess do not share a common physical origin. We explore the issue of whether the observed SED might be dust absorbed in the far-and near-UV. We fit the UV break, assuming a power law that is absorbed by cubic nanodiamond dust grains. We then explore the possibility of a universal SED (with a unique spectral index) by including further absorption from SMC-like extinction. Using this approach, satisfactory fits to the spectra can be obtained. The hydrogen column densities required by either nanodiamonds or amorphous dust models are all consistent, except for one object, with the columns deduced by our X-ray analysis, provided that the C depletion is $0.6. Because dust absorption implies a flux recovery in the EUV (<700 8), our modeling opens the possibility that the intrinsic quasar SED is much harder and more luminous in the EUV than inferred from the near-UV data, as required by photoionization models of the broad emission line region. We conclude that the intrinsic UV SED must undergo a sharp turnover before the X-ray domain.

We have carried out 3D hydrodynamic simulations of precessing, variable ejection velocity bipolar jets in order to model multipolar protoplanetary (or planetary) nebulae. For these nebulae, we assume a binary source, with an asymptotic giant branch primary star which ejects an isotropic wind, and a companion which ejects the bipolar jet system. We find that it is possible to relate the large‐scale morphological characteristics of these nebulae (lobe size, semi‐aperture angle, number of observed lobes) to some of the parameters of the binary system, such as the ratio between the orbital and precession periods, the ratio between the masses of the binary components and the major axis of the elliptical orbit. Our results show that synthetic nebulae with well‐defined lobe morphologies (resembling many of the observed multipolar planetary nebulae) are obtained from our models.