Relativistic description of electron scattering on the deuteron

Within a quasipotential framework a relativistic analysis is presented of the deuteron current. Assuming that the singularities from the nucleon propagators are important, a so-called equal time approximation of the current is constructed. This is applied to both elastic and inelastic electron scattering. As dynamical model the relativistic one boson exchange model is used. Reasonable agreement is found with a previous relativistic calculation of the elastic electromagnetic form factors of the deuteron. For the unpolarized inelastic electron scattering effects of final state interactions and relativistic corrections to the structure functions are considered in the impulse approximation. Two specific kinematic situations are studied as examples.Comment: (19 pages in revtex + 15 figures not included, available upon request.) report THU-93-10

The Interaction of Asymptotic Giant Branch Stars with the Interstellar Medium

We study the hydrodynamical behavior of the gas expelled by moving Asymptotic Giant Branch Stars interacting with the ISM. Our models follow the wind modulations prescribed by stellar evolution calculations, and we cover a range of expected relative velocities (10 to 100 km/s), ISM densities (between 0.01 and 1 cm-3), and stellar progenitor masses (1 and 3.5 Msun). We show how and when bow-shocks, and cometary-like structures form, and in which regime the shells are subject to instabilities. Finally, we analyze the results of the simulations in terms of the different kinematical stellar populations expected in the Galaxy.Comment: ApJ in press, 42 pages, 12 figures, movies of the simulations will be available in the published electronic version of the pape

The dynamical evolution of the circumstellar gas around low-and intermediate-mass stars I: the AGB

We have investigated the dynamical interaction of low- and-intermediate mass stars (from 1 to 5 Msun) with their interstellar medium (ISM). In this first paper, we examine the structures generated by the stellar winds during the Asymptotic Giant Branch (AGB) phase, using a numerical code and the wind history predicted by stellar evolution. The influence of the external ISM is also taken into account. We find that the wind variations associated with the thermal pulses lead to the formation of transient shells with an average lifetime of 20,000 yr, and consequently do not remain recorded in the density or velocity structure of the gas. The formation of shells that survive at the end of the AGB occurs via two main processes: shocks between the shells formed by two consecutive enhancements of the mass-loss or via continuous accumulation of the material ejected by the star in the interaction region with the ISM. Our models show that the mass of the circumstellar envelope increases appreciably due to the ISM material swept up by the wind (up to 70 % for the 1 Msun stellar model). We also point out the importance of the ISM on the deceleration and compression of the external shells. According to our simulations, large regions (up to 2.5 pc) of neutral gas surrounding the molecular envelopes of AGB stars are expected. These large regions of gas are formed from the mass-loss experienced by the star during the AGB evolution.Comment: 43 pages, 15 figures. Accepted for publication in the Astrophysical Journa

Measuring The Mass Loss Evolution at The Tip of The Asymptotic Giant Branch

In the final stages of stellar evolution low- to intermediate-mass stars lose their envelope in increasingly massive stellar winds. Such winds affect the interstellar medium and the galactic chemical evolution as well as the circumstellar envelope where planetary nebulae form subsequently. Characteristics of this mass loss depend on both stellar properties and properties of gas and dust in the wind formation region. In this paper we present an approach towards studies of mass loss using both observations and models, focusing on the stage where the stellar envelope is nearly empty of mass. In a recent study we measure the mass-loss evolution, and other properties, of four planetary nebulae in the Galactic Disk. Specifically we use the method of integral field spectroscopy on faint halos, which are found outside the much brighter central parts of a planetary nebula. We begin with a brief comparison between our and other observational methods to determine mass-loss rates in order to illustrate how they differ and complement each other. An advantage of our method is that it measures the gas component directly requiring no assumptions of properties of dust in the wind. Thereafter we present our observational approach in more detail in terms of its validity and its assumptions. In the second part of this paper we discuss capabilities and assumptions of current models of stellar winds. We propose and discuss improvements to such models that will allow meaningful comparisons with our observations. Currently the physically most complete models include too little mass in the model domain to permit a formation of winds with as high mass-loss rates as our observations show.Comment: 7 pages, workshop in honour of Agnes Acker, Legacies of the Macquarie/AAO/Strasbourg Halpha Planetary Nebula project, ed. Q.Parker and D.Frew, PASA, in press; clarified some parts and added some additional reference

Dust-driven Winds and Mass Loss of C-rich AGB Stars with subsolar Metallicities

We investigate the mass loss of highly evolved, low- and intermediate mass stars and stellar samples with subsolar metallicity. We give a qualitative as well as quantitative description which can be applied to LMC/SMC-type stellar populations. For that purpose we apply the same approach as we did for solar metallicity stars and calculate hydrodynamical wind models including dust formation with LMC and SMC abundances under consideration of an adapted model assumption. In particular, we improved the treatment of the radiative transfer problem in order to accommodate larger non-local contributions occurring with smaller opacities. For each wind model we determine an averaged mass-loss rate. The resulting, approximate mass-loss formulae are then applied to well-tested and calibrated stellar evolution calculations in order to quantify the stellar mass loss. The dynamical models for LMC and SMC metallicity result in mass-loss rates of the same order of magnitude as the solar metallicity models which is in this basic approach in agreement with observations. The hydrodynamical properties like e.g. the outflow velocity differ (for fixed C/O abundance ratio) noticeably, though. While critical luminosities of LMC and solar metallicity models fairly coincide, the SMC models need higher luminosities to develop dust-driven winds.Comment: 8 pages, 4 figures. Accepted for publication in A&

Evolution of the dust mass loss with luminosity along the giant branch of the globular cluster 47 Tuc

The paper investigates the properties of the dust mass loss in stars populating the giant branch of the globular cluster 47 Tuc, by combining ISOCAM and DENIS data. Raster maps of 5 fields covering areas ranging from 4 x 4 to 15 x 15 arcmin2 at different distances from the center of the cluster have been obtained with ISOCAM at 11.5 mum (LW10 filter). The covered fields include most of the red variables known in this cluster. A detection threshold of about 0.2 mJy is achieved, allowing to detect giant stars at 11.5 mum all the way down to the horizontal branch. No dust-enshrouded asymptotic giant branch stars have been found in the observed fields, contrary to the situation encountered in LMC/SMC globular clusters with larger turnoff masses. The color index [12]-[2] (based on the ISO 11.5 mum flux and on the DENIS Ks magnitude) is used as a diagnostic of dust emission (and hence dust mass loss). Its evolution with luminosity along the giant branch reveals that dust mass loss is only present in V3 (the only cluster Mira variable observed in the present study) and in V18, a star presenting intermittent variability. This conclusion confirms the importance of stellar pulsations in the dust formation and ensuing mass loss.Comment: 16 pages, accepted in Astronomy & Astrophysic

Bispectrum speckle interferometry of IRC+10216: the dynamic evolution of the innermost circumstellar environment from 1995 to 2001

We present new near-infrared (JHK) bispectrum speckle-interferometry monitoring of the carbon star IRC+10216 obtained between 1999 and 2001 with the SAO 6m telescope. The J-, H-, and K-band resolutions are 50mas, 56mas, and 73mas, resp. The total sequence of K-band observations covers now 8 epochs from 1995 to 2001 and shows the dynamic evolution of the inner dust shell. The present observations show that the appearance of the dust shell has considerably changed compared to the epochs of 1995 to 1998. Four main components within a 0.2" radius can be identified in the K-band images. The apparent separation of the two initially brightest components A and B increased from ~191mas in 1995 to ~351mas in 2001. Simultaneously, component B has been fading and almost disappeared in 2000 whereas the initially faint components C and D became brighter (relative to peak intensity). These changes can be related to changes of the optical depth caused, e.g., by mass-loss variations or new dust condensation in the wind. Our 2D radiative transfer model suggests that the observed relative motion of A and B is not consistent with the known terminal wind velocity of 15 km/s. The apparent motion with a deprojected velocity of 19 km/s on average and of recently 27 km/s appears to be caused by adisplacement of the dust density peak due to dust evaporation in the optically thicker and hotter environment. Our monitoring, covering more than 3 pulsation periods, shows that the structural variations are not related to the stellar pulsation cycle in a simple way. This is consistent with the predictions of hydrodynamical models that enhanced dust formation takes place on a timescale of several pulsation periods. The timescale of the fading of component B can well be explained by the formation of new dust in the circumstellar envelope.Comment: 11 pages including 6 PostScript figures; also available from http://www.mpifr-bonn.mpg.de/div/ir-interferometry/publications.html; Astronomy & Astrophysics, in pres

Mass-Losing Semiregular Variable Stars in Baade's Windows

By cross-correlating the results of two recent large-scale surveys, the general properties of a well defined sample of semi-regular variable stars have been determined. ISOGAL mid-infrared photometry and MACHO lightcurves are assembled for approximately 300 stars in the Baade's Windows of low extinction towards the Galactic bulge. These stars are mainly giants of late M spectral type, evolving along the asymptotic giant branch (AGB). They are found to possess a wide and continuous distribution of pulsation periods and to obey an approximate log~period -- bolometric magnitude relation or set of such relations. Approximate mass-loss rates in the range of 1e-8 to 5e-7 M_sun per year are derived from ISOGAL mid-infrared photometry and models of stellar spectra adjusted for the presence of optically-thin circumstellar silicate dust. Mass-loss rates depend on luminosity and pulsation period. Some stars lose mass as rapidly as short-period Miras but do not show Mira-like amplitudes. A period of 70 days or longer is a necessary but not a sufficient condition for mass loss to occur. For AGB stars in the mass-loss ranges that we observe, the functional dependence of mass-loss rate on temperature and luminosity is found to be in agreement with recent theoretical predictions. If we include our mass-loss rates with a sample of extreme mass-losing AGB stars in the Large Magellanic Cloud, we get the general result for AGB stars that mass-loss rate is proportional to luminosity^{2.7}, valid for AGB stars with 10^{-8} to 10^{-4} M_sun per year (Abridged).Comment: to appear in The Astrophysical Journal, 51 pages, 9 figures, 3 tables; table 1 will be available in machine-readable format at the electronic Ap

Multi-Jet Event Rates in Deep Inelastic Scattering and Determination of the Strong Coupling Constant

Jet event rates in deep inelastic ep scattering at HERA are investigated applying the modified JADE jet algorithm. The analysis uses data taken with the H1 detector in 1994 and 1995. The data are corrected for detector and hadronization effects and then compared with perturbative QCD predictions using next-to-leading order calculations. The strong coupling constant alpha_S(M_Z^2) is determined evaluating the jet event rates. Values of alpha_S(Q^2) are extracted in four different bins of the negative squared momentum transfer~\qq in the range from 40 GeV2 to 4000 GeV2. A combined fit of the renormalization group equation to these several alpha_S(Q^2) values results in alpha_S(M_Z^2) = 0.117+-0.003(stat)+0.009-0.013(syst)+0.006(jet algorithm).Comment: 17 pages, 4 figures, 3 tables, this version to appear in Eur. Phys. J.; it replaces first posted hep-ex/9807019 which had incorrect figure 4

Low Q^2 Jet Production at HERA and Virtual Photon Structure

The transition between photoproduction and deep-inelastic scattering is investigated in jet production at the HERA ep collider, using data collected by the H1 experiment. Measurements of the differential inclusive jet cross-sections dsigep/dEt* and dsigmep/deta*, where Et* and eta* are the transverse energy and the pseudorapidity of the jets in the virtual photon-proton centre of mass frame, are presented for 0 < Q2 < 49 GeV2 and 0.3 < y < 0.6. The interpretation of the results in terms of the structure of the virtual photon is discussed. The data are best described by QCD calculations which include a partonic structure of the virtual photon that evolves with Q2.Comment: 20 pages, 5 Figure