Showing papers in "Astronomy and Astrophysics in 1996"

The heliospheric magnetic field at solar minimum: ULYSSES observations from pole to pole.

Abstract: The fast transit of the Ulysses spacecraft from the south polar regions of the heliosphere to the north polar regions has allowed us to obtain a unique perspective on the configuration and properties of the heliospheric magnetic field at a time of solar minimum activity. We compare the magnetic field in the northern and southern hemispheres and find no evidence of asymmetry in the radial component of the field or in the variances of the field. We find that the magnetic field polarity corresponds to that observed at the Sun. The only difference between the hemispheres is that, while magnetic field lines in the southern hemisphere have a most probable azimuth angle approximately 24◦ more tightly wound than predicted by the Parker model, those in the northern hemisphere are in good agreement with the model.

Ulysses plasma parameters : latitudinal, radial, and temporal variations

Abstract: Observations by the Ulysses SWOOPS plasma ex- periment are used to investigate spatial and temporal gradients during the mission, with emphasis on more recent high lati- tude observations including the recent South Pole to North Pole passage during solar minimum. Compared to lower latitudes, the high latitude solar wind had higher average speed, proton temperature, and momentum flux, and lower number flux den- sity. As the average momentum flux observed in the high speed windwas21%greaterthanattheequator,duringsolarminimum the distance to the heliopause will be comparatively less in the solar equatorial plane than over the poles. The long term tem- poral gradients of momentum flux over the life of the mission are considerably larger than the latitudinal gradient observed by Ulysses during solar minimum. A modest North-South high latitude asymmetry is observed in the plasma parameters; the velocity is on the average 13 km/s to 24 km/s greater at North- ern latitudes than at Southern, and temperature is also higher. The North-South temperature asymmetry is greater than can be explained by the North-South velocity difference and the de- pendence of solar wind temperature upon speed. The power law dependence of temperature on heliocentric distance, r, at high latitudes is in range r 0:81to 1:03 , where r 0:81 is the Southern latitude result and r 1:03 the Northern. The parameter T=n 1=2 , where T is temperature and n is proton number density, can be better predicted from speed than can temperature alone. Com- parison with calculations based on source models and magneto- graphdataindicatethattheexpansionofopencoronaleldlines close to the Sun was greater in the Southern hemisphere than in the Northern; this anticorrelation with the expansion factor is consistent with previous observational and theoretical work.

Spatial variation of >40 MeV/n nuclei fluxes observed during the Ulysses rapid latitude scan

Abstract: In this paper we present fluxes of galactic cosmic ray nuclei with energies above 40 MeV/n observed by the Kiel Electron Telescope on board the Ulysses spacecraft during the fast scan from the South Pole (September 1994) to the North Pole (July 1995). This part of the Ulysses orbit gives us the unique opportunity to investigate spatial modulation of cosmic rays under solar minimum conditions. We show that during this time period temporal variations are well ordered with particle rigidity using 1 AU data from the University of Chicago particle instrument on board IMP8. The latitudinal variation is partic- ularly strong for 38-125 MeV/n helium nuclei and decreases with increasing energy. In contrast, the spatial variation for pro- tons is most dominant in the energy range of a few hundred MeV. Spatial effects are very small for some ten to one hundred MeV protons. The proton results are compared with predictions of a steady-state modulation model which takes into account modifications of the large scale heliospheric magnetic field and an increased level of cosmic ray scattering over the poles. We find that the model is in excellent agreement with the latitudinal variations of >2 GeV protons, whereas it does not reproduce the behaviour at low rigidities. We conclude that either the as- sumed rigidity dependence for the diffusion coefficients at low energies has to be modified or that time-dependent modulation effects dominate at low energies.

Observing with the ISO Short-Wavelength Spectrometer.

Abstract: The Short-Wavelength Spectrometer (SWS) is one of the four instruments on-board ESA's Infrared Space Observatory (ISO), launched on November 17, 1995. The spectrometer covers the wavelength range of 2.38 to 45.2 μm with a spectral resolution ranging from 1000 to 2000. By inserting Fabry-Perot filters the resolution can be enhanced by a factor 20 for the wavelength range from 11.4 to 44.5 μm. An overview is given of the instrument, its in-orbit calibration, performance, observing modes and off-line processing software.

Thermal physics of asteroids. i. effects of shape, heat conduction and beaming

Abstract: With new technology and space observatories, IR-astronomy has the possibility to become a powerful tool for the investigation of the smaller bodies in the solar system In order to respond to these expectations it is becoming very important to improve the

ISOPHOT - capabilities and performance

Abstract: ISOPHOT covers the largest wavelength range on ISO from 2.5 to 240 m. Its scientific capabilities include multi- filter and multi-aperture photometry, polarimetry, imaging and spectrophotometry. All modes can optionally include a focal plane chopper. The backbone of the photometric calibration are internal standard sources checked against celestial primary stan- dards. The instrument is in excellent condition with all compo- nents functioning properly. There is increased detector noise due to the ionizing cosmic radiation affecting the detectability of the faintest sources. All lessons learnt from operating the instrument in space are continously being implemented in the standard data reduction software packages available to every observer. Particular strengths of the instrument include the de- tectability of very cold sources, high resolution mapping and fast spectrophotometry.The far infrared serendipity sky survey performed during slews of the satellite has successfully started.

Mineralogy of oxygen-rich dust shells

Abstract: Spectra taken with the Short Wavelength Spectrometer on board of the Infrared Space Observatory of dust shells around evolved oxygen-rich stars reveal the presence of several emission features at wavelengths between 20 and 45 mu m. These features have a range of widths and strengths, but are all narrow compared to the well-known amorphous silicate bands at 9.7 and 18 mu m. The emission peaks are tentatively identified with crystalline forms of silicates such as pyroxenes and olivine. The emission features tend to be more prominent for objects with cooler dust shells (T <300 K). This may be due to an intrinsic change in optical properties of the dust as it cools, or it may be due to an increase in the fraction of crystalline silicates compared to amorphous forms as the mass loss rate increases. The implications for the physics of dust formation in the outflows of cool giants are briefly discussed.

SWS observations of young main-sequence stars with dusty circumstellar disks

Abstract: We present SWS full-scan observations of three ob- jects that are thought to be in a stage of evolution between the youngest, embedded, Herbig Ae/Be stars and Beta Pictoris, a young main-sequence star with a circumstellar disk. The 8-12 m spectra of all three stars cannot be understood in terms of purely amorphous silicates, but require the presence of crys- talline silicates in different amounts. Around two objects both oxygen-rich and carbon-rich dust particles are present: the spec- trum of HD 100546 displays the full set of UIR features; in the spectrum of HD 142527 both the 3.29 and 3.42 m emission features are observed, as well as a strong 3.51 m feature. The spectrum of HD 100546 is extremely rich in silicate features in the spectral range from 20 to 45 m; some of these features strongly suggest the presence of appreciable amounts of crys- talline silicates.

light Scattering by Aggregate Particles

Abstract: We have investigated scattering properties of aggregates, emphasizing the size of constituent monomers comparable with the wavelength of visible light, in order to model the scattering properties of cometary dust. This has differentiated our study from previous investigations of aggregates in which the size of the monomers was much smaller than the wavelength.

An iso sws view of interstellar ices : first results

Abstract: The availability of SWS data from the Infrared Space Observatory is a landmark in the study of interstellar ices. This paper presents a brief review of what was known prior to the launch of ISO, and attempts a synthesis of what has been learned from the ISO observations available to date. Key areas of un- certainty are identified to provide a basis for future research.

Arcs from a universal dark matter halo profile

Abstract: Navarro, Frenk, \& White have recently found numerically that the density profile of dark-matter halos can be described by a universal two-parameter function over a broad range of halo masses. The profile is singular, approaching the halo center with $\rho\propto r^{-1}$. It had been argued previously that radially distorted, gravitationally lensed images of background sources in galaxy clusters, so-called radial arcs, required a flat core in the cluster density profile. Such radial arcs have so far been detected in two galaxy clusters, in apparent contradiction with a singular density profile. I show here that the profile suggested by Navarro et al. can produce radial arcs despite its central singularity, and describe how the two parameters of the profile can be determined in clusters where radial and tangential arcs are observed. I then apply this analysis to the two clusters where radial arcs were detected. In both cases, the redshifts of the radial arcs are yet unknown, hence definitive conclusions on the profile parameters cannot yet be drawn. Numerically determined values for the parameters of cluster-sized halos can, however, be used to predict the range of the unknown arc redshifts, thus providing a direct observational test for the proposed density profile. A potential difficulty with the profile is that the radial magnification of tangential arcs is large, hence tangential arcs should be thick or their sources should be very thin in the radial direction.

New solar models including helioseismological constraints and light-element depletion

Abstract: We have computed new solar models using the same stellar evolution code as described in Charbonnel, Vauclair and Zahn (1992). This code, originating from Geneva, now includes the computation of element segregation for helium and 12 heavier isotopes. It may also include any type of mixing of the stellar gas, provided this mixing can be parametrized with an effective diffusion coefficient as a function of radius. Here we introduced rotationinduced mixing as prescribed by Zahn (1992). We present five solar models: 1) the standard model, computed with heavy element abundances as given by Grevesse (1991); 2) a model including pure element segregation (no mixing outside the convective zone) with Grevesse (1991) as initial abundances; 3) same model as (2), but iterated so that the final abundances are those of Grevesse (1991); 4) a model with both element segregation and rotation-induced mixing, leading to lithium and beryllium depletion consistent with the observations, with Grevesse (1991) as initial abundances; 5) same model as (4) but iterated to obtain Grevesse (1991) as final abundances. This model (5) now represents our best new solar model consistent with the observations. The u = P ρ function computed as a function of radius in these new solar models are compared to the helioseismological results obtained for the same function by Dziembowski et al (1994). Improving the physics of the models leads to a better consistency with helioseismology. In our best model (5), which includes both segregation and mixing, the relative difference in the u function between the model and the helioseismological results is smaller than 0.5 per cent at all radii except at the center and the surface. Meanwhile lithium is depleted by a factor 155 and beryllium by a factor 2.9, which is consistent with the observations. The bottom of the convective zone lies at a fractional radius of 0.716, consistent with helioseismology. The neutrino fluxes are not decreased in any of these models. Send offprint requests to: S. Vauclair The models including the computations of element segregation lead to a present surface helium abundance of: Ysurf between 0.248 and 0.258, which is in satisfactory agreement with the value derived from helioseismology.

Constraints on high-speed solar wind structure near its coronal base : a ulysses perspective

Abstract: Ulysses plasma data at high heliographic latitudes were studied to develop constraints on the structure of the corona at the base of the high-speed solar wind. Salient features of the flow poleward of ±60◦ revealed: 1) low variances of all bulk flow parameters, 2) parameter values that agree with those measured during high-speed conditions in the ecliptic plane when all are scaled to 1 AU, 3) the continuous presence of two interpenetrating proton streams that are not resolved in velocity space, 4) a single alpha-particle beam that travels at a speed that is close to the local Alfvén speed faster than the primary proton beam, 5) a proton temperature that is a factor of 2.4 times that of the electrons, and 6) a constant helium abundance that averages 4.4% , about half that inferred from helioseismic data in the solar convection zone. These data are combined with a host of other remote-sensing solar data and solar wind data to develop support for a model of a well-mixed solar atmosphere that is driven by reconnection-generated plasma-jet transients. In this model, acceleration of the solar wind to its terminal speed is complete within a heliocentric distance of about 5 Rs.

Acceleration of the High Speed Solar Wind

Abstract: A theory is developed for the high speed solar wind based on a simple dissipation length characterization of wave heating of the coronal plasma close to the Sun. It is shown that solutions with the correct particle and energy fluxes and with a realistic magnetic field, match the requirements on the density at the base of the corona provided the dissipation length is relatively small (~ 0.25 – 0.5 solar radii). The significant features of these solutions are that the acceleration is rapid, with the sonic point at about 2 solar radii, and the maximum proton temperatures are high, namely 8−10 × 106 K, in agreement with some recent observations. Such efficient dissipation requires any Alfven waves responsible to have frequencies in the range 0.01 Hz - 10 kHz. This has implications for the nature of the plasma and energy source in the chromospheric network.

Soft X-ray properties of narrow-line Seyfert 1 galaxies.

Abstract: We report on AGN with extremely soft X-ray spectra observed with ROSAT. From their optical emission lines these objects are classified as narrow-line Seyfert 1 galaxies (NLS1), almost all with extremely large Fe II/H-beta ratios and relatively narrow optical lines of hydrogen. Our results are based on a systematic study of 46 NLS1. We find that NLS1 have generally steeper soft X-ray continuum slopes than normal Seyfert 1s, and there may exist an anticorrelation between 0.1-2.4 keV continuum slope and the FWHM of the H-beta line. Objects with steep 0.1-2.4 keV continuum slopes and H-beta FWHM > 3000 km s^{-1} are clearly discriminated against by nature. When simple power-law models are fit to the data, photon indices reach values up to about 5, much higher than is usually seen in Seyfert 1s. We discuss steep ROSAT spectra in light of soft X-ray excess and hard X-ray tail models. We consider models for NLS1 where they are Seyfert 1s with extremal values of pole-on orientation, black hole mass and/or accretion rate, warm absorption and BLR thickness and confront these models with the known properties of NLS1. All simple models appear to have drawbacks, but models with smaller mass black holes and thicker BLRs show some promise. We suggest specific further tests of the models.