Showing papers in "Astronomy and Astrophysics in 1998"

Role of the scalar field in gravitational lensing

Abstract: A static and circularly symmetric lens character- ized by mass and scalar charge parameters is constructed For the small values of the scalar charge to the mass ratio, the gravitational lensing is qualitatively similar to the case of the Schwarzschild lens; however, for large values of this ratio the lensing characteristics are significantly different The main fea- tures are the existence of two or nil Einstein ring(s) and a radial critical curve, formation of two or four images and possibility of detecting three images near the lens for sources located at relatively large angular positions Such a novel lens may also be treated as a naked singularity lens

Evolutionary models for solar metallicity low - mass stars: Mass - magnitude relationships and color - magnitude diagrams

Abstract: We present evolutionary models for low mass stars from 0.075 to 1 M for solar-type metallicities (M/H)= 0 and -0.5. The calculations include the most recent interior physics and the latest generation of non-grey atmosphere models. We provide mass-age-color-magnitude relationships for both metal- licities. The mass-MV and mass-MK relations are in excellent agreement with the empirical relations derived observationally. The theoretical color-magnitude diagrams are compared with the sequences of globular clusters (47 Tucanae) and open clus- ters (NGC2420 and NGC2477) observed with the Hubble Space Telescope. Comparison is also made with field star sequences in MV -(V I), MK -(I K) and MK -(J K) diagrams. These comparisons show that the most recent improvements performed in low-mass star atmosphere models yield now re- liable stellar models in the near-infrared. These models can be used for metallicity, mass, temperature and luminosity calibra- tions. Uncertainties still remain, however, in the optical spectral region below Te 3700K, where predicted (V-I) colors are too blue by 0.5 mag for a given magnitude. The possible origins for such a discrepancy, most likely a missing source of opacity in the optical and the onset of grain formation are examined in detail.

Making a comet nucleus

Abstract: Thechemicalcompositionofacometnucleuscanbe very strictly constrained by combining the latest results on: the core-mantleinterstellardustmodel,thesolarsystemabundances of the elements, the space observed composition of the dust of comet Halley, and the latest data on the volatile molecules of comet comae. The distribution of the components in the comet nucleusfallnaturallyintotwobasiccategories{refractoriesand volatiles. The refractory components are tightly constrained to consist of about 26% of the mass of a comet as silicates (a generic term for combinations of the elements Si, Mg, Fe), 23% complexorganicrefractorymaterial(dominatedbycarbon),and about 9% in the form of extremely small (attogram) carbona- ceous/large molecule (PAH) particles. The remaining atoms are in an H2O dominated mixture containing of the order of 2 3% each of CO, CO2 ,C H 3OH plus other simple molecules. The H2O abundance itself is very strictly limited to 30% of the total mass of a comet { not much more nor much less. The re- fractory to volatile (dust to gas) ratio is about 1:1, while the dust to H2O ratio is 2 : 1. The maximum mean density of a fully packed nucleus would be 1:65gcm 3 . The morphological structure of the component materials, following the interstel- lar dust into the nal stage of the presolar cloud contraction, is as tenth micron silicate cores with organic refractory inner mantles and outer mantles of \ices" with each grain containing many thousands of the attogram carbonaceous/large molecule particles embedded in the icy and outer organic fraction.

The chemistry of the early Universe

Abstract: The process of molecule formation in the primor- dial gas is considered in the framework of Friedmann cosmo- logical models from redshift z =1 0 4 to z =0 First, a com- prehensive analysis of 87 gas phase reaction rates (both col- lisional and radiative) relevant in the physical environment of the expanding universe is presented and critically discussed On this basis, calculations are carried out of the abundance of 21 molecular species as function of redshift, for different values of the cosmological parameters 0, and H0, evaluating con- sistently the molecular heating and cooling due to H2 ,H D and LiH molecules One of the major improvements of this work is the use of a better treatment of H recombination that leads to a reduction of a factor 2-3 in the abundance of electrons and H + at freeze-out, with respect to previous studies The lower residual ionization has a negative effect on the chemistry of the primordial gas in which electrons and protons act as cat- alysts in the formation of the first molecules We find that in the standard model (h =0 :67, 10 =4 :5, 0 =1 and (D/H) =4 :3 10 5 ), the residual fractional ionization at z =1 is (e= H) =3 :0210 4 , and the main molecular species fractional abundances (H2= H) =1 :1 10 6 , (HD=H 2 )=1 :1 10 3 , (HeH + = H) =6 :2 10 13 , (LiH + = H) =9 :4 10 18 and (LiH=LiH + )=7 :6 10 3 We devise a reduced chemical network that reproduces with excellent accuracy the numerical results of the complete model and allows to follow the chemical compositions and the thermal properties of a primordial gas in the presence of an external radiation field Finally, we provide accurate cooling functions of H2, HD and LiH in a wide range of density and temperature that can be conveniently used in a variety of cosmological applications

High-energy gamma-ray emission from the Galactic Center

Abstract: The EGRET instrument on the Compton Gamma- Ray Observatory has observed the Galactic Center (GC) region with good coverage at a number of epochs. A strong excess of emission is observed, peaking at energies > 500 MeV in an error circle of 0.2 degree radius including the position l =0 and b =0 . The close coincidence of this excess with the GC direction and the fact that it is the strongest emission maximum within 15 degrees from the GC is taken as compelling evidence for the source's location in the GC region. The history of the emission intensity, observed over 5 years, leaves room for pos- sible time variation; however, it does not provide evidence. The angular extent of the excess appears only marginally compati- ble with the signature expected for a single compact object. The emission therefore may stem from one or more compact objects or may originate from diffuse interactions within 85 pc from the center of the Galaxy at 8.5 kpc distance. The spatial distri- bution of the emission does not correlate with the details of the CO-line surveys. Thus, in spite of the existence of a strong emis- sion peak, earlier conclusions based on an apparent 'gamma-ray deficit', postulating the masses of the 'wide-line' clouds in the GC area to be an order of magnitude lower than indicated by naive CO interpretation, are supported. However, the total gas mass in the Nuclear Bulge (NB) derived from the gamma-ray emission is found to be in agreement with the mass which in recent studies has been derived from molecular-line and FIR surveys. The -ray emission spectrum is peculiar and different from the spectrum of the large-scale galactic diffuse emission. A diffuse emission scenario requires an enhanced and peculiar Cosmic Ray (CR) spectrum as suggested for the electrons in the 'Radio Arc'. A compact sources model hints at an origin in pulsars. While the spectrum suggests middle-aged pulsars like Vela, too many are required to produce the observed flux.

Steps toward interstellar silicate mineralogy. IV The crystalline revolution

Abstract: Midand far-infrared spectra gained by the Short Wavelength Spectrometer (SWS) of the Infrared Space Observatory (ISO) satellite have provided striking evidence for the presence of crystalline silicates in comets, circumstellar envelopes around young stars and, most of all, evolved stars and planetary nebulae. Since optical properties of astrophysically relevant crystalline silicates are lacking in the literature, in this paper mass absorption coefficients (MACs) of olivines and pyroxenes for a wide range of Mg/Fe ratios are presented, which cover the whole ISO wavelength range . The MAC have been derived from transmission spectra of small grains embedded in potassium bromide and polyethylene pellets. Only in the case of natural enstatite (MgSiO3), was a monocrystalline sample available, which allowed the measurement of optical constants for the different crystallographic orientations of this anisotropic silicate. Since not all Mg/Fe ratios are represented among the natural minerals, we supplemented the series by synthetic products prepared in our lab. We also included two inhomogeneous synthetic materials, one of olivine and the other one of pyroxene composition, which are expected to be similar to the primary condensate in cosmic environments. For all samples the chemical composition, the purity, and the homogeneity have been determined by energy-dispersive X-ray analysis and by scanning electron microscopy. Especially for the minerals, it is important to exclude the spectral influence of differently composed inclusions. The peak positions of the samples are influenced by different factors which are discussed: chemical composition (FeO content), size and shape distribution of the grains, and the matrix in which the grains are embedded for spectroscopy. The continuum-subtracted ISO SWS spectrum of the source AFGL 4106 has been compared with simple optically thin model spectra calculated for our olivine and pyroxene samples. The main result was that a combination of the pure magnesium silicates (forsterite and enstatite) gives a good agreement between observations and laboratory measurements.

Quasars and galaxy formation

Abstract: The formation of massive black holes may precede the epoch that characterises the peak of galaxy formation, as characterizedbythestarformationhistoryinluminousgalaxies. Hence protogalactic star formation may be profoundly affected by quasar-like nuclei and their associated extensive energetic outflows. We derive a relation between the mass of the cen- tral supermassive black hole and that of the galaxy spheroidal component, and comment on other implications for galaxy for- mation scenarios.

Model atmospheres broad-band colors, bolometric corrections and temperature calibrations for O-M stars

Abstract: Broad band colors and bolometric corrections in the Johnson-Cousins-Glass system (Bessell, 1990; Bessell & Brett, 1988) have been computed from synthetic spectra from new model atmospheres of Kurucz (1995a), Castelli (1997), Plez, Brett & Nordlund (1992), Plez (1995-97), and Brett (1995a,b). These atmospheres are representative of larger grids that are cur- rently being completed. We discuss differences between the dif- ferent grids and compare theoretical color-temperature relations and the fundamental color temperature relations derived from: (a) the infrared-flux method (IRFM) for A-K stars (Blackwell & Lynas-Gray 1994; Alonso et al. 1996) and M dwarfs (Tsuji et al. 1996a); (b) lunar occultations (Ridgway et al. 1980) and (c) Michelson interferometry (Di Benedetto & Rabbia 1987; Dyck et al. 1996; Perrin et al. 1997) for K-M giants, and (d) eclipsing binaries for M dwarfs. We also compare color - color relations and color - bolometric correction relations and find good agree- ment except for a few colors. The more realistic fluxes and spec- tra of the new model grids should enable accurate population synthesis models to be derived and permit the ready calibration of non-standard photometric passbands. As well, the theoreti- cal bolometric corrections and temperature - color relations will permit reliable transformation from observed color magnitude diagrams to theoretical HR diagrams.

The initial conditions of star formation in the rho ophiuchi main cloud :wide-field millimeter continuum mapping

Abstract: We present the results of an extensive 1.3 mm con- tinuum mosaicing study of the Ophiuchi central region ob- tained at the IRAM 30-m telescope with the MPIfR 19-channel bolometer array. The mosaiced field covers a total area of 480 arcmin 2 , corresponding to 1p c 2 at a distance of 160 pc, and includes the DCO + dense cores Oph-A, Oph-B1, Oph-B2, Oph-C, Oph-D, Oph-E, and Oph-F. Our mosaic is sen- sitive to features down to N H2 10 22 cm 2 in column density. It is consistent with, but goes significantly deeper than, previous dust continuum studies of the cloud. For the first time, compact circumstellar dusty structures around young stellar objects are detected simultaneously with more extended emission from the dense cores and the ambient cloud. Thus, it becomes possible to directly study the genetic link between dense cores and young stars. The diffuse cloud emission is itself fragmented in at least 58 small-scale, starless clumps harboring no infrared or radio con- tinuum sources in their centers. Most of these starless fragments are probably gravitationally bound and pre-stellar in nature. Nineteen of them exhibit a relatively flat inner intensity profile, indicating they are not as centrally condensed as the envelopes seen around the embedded (Class I and Class 0) protostars of the cloud. Ten other clumps appear to be sharply peaked, how- ever, and may represent candidate 'isothermal protostars', i.e., collapsing cloud fragments which have not yet developed a cen- tral hydrostatic core. The 6000 AU fragmentation sizescale estimated from our Oph 1.3 mm mosaic is consistent with the typical Jeans length in the DCO + cores and is at least five times smaller than the diameter of isolated dense cores in the Taurus cloud. In agreement with this short lengthscale for fragmenta- tion, the circumstellar envelopes surrounding Oph Class I and Class 0 protostars are observed to have finite sizes and to be significantly more compact than their Taurus counterparts. The measured frequency distribution of pre-stellar clump masses is relatively shallow below 0.5 M, being consistent with N=m / m 1:5 , but steepens to N=m / m 2:5 above 0.5 M. This is reminiscent of the stellar initial mass

Cosmic magnetic fields in large scale filaments and sheets

Abstract: We consider the possibility that cosmic magnetic field, instead of being uniformly distributed, is strongly cor- related with the large scale structure of the universe. Then, the observed rotational measure of extra-galactic radio sources would be caused mostly by the clumpy magnetic field in cos- mological filaments/sheets rather than by a uniform magnetic field, which was often assumed in previous studies. As a model for the inhomogeneity of the cosmological magnetic field, we adopt a cosmological hydrodynamic simulation, where the field is passively included, and can approximately represent the real field distribution with an arbitrary normalization for the field strength. Then, we derive an upper limit of the magnetic field strength by comparing the observed limit of rotational measure with the rotational measure expected from the magnetic field geometry in the simulated model universe. The resulting upper limit to the magnetic field in filaments and sheets is Bfs < 1G which is 10 3 times higher than the previously quoted values. This value is close to, but larger than, the equipartition magnetic field strength in filaments and sheets. The amplification mech- anism of the magnetic field to the above strength is uncertain. The implications of such a strength of the cosmic magnetic field are discussed.

Evidence for water vapor in Titan's atmosphere from ISO/SWS data ?

Abstract: The infrared spectrum of Titan around 40µ mw as recorded in the grating mode of the Short Wavelength Spectrom- eter (SWS) of ISO, with a resolving power of about 1900. Two emission features appear at 43.9 and 39.4µm, where pure rota- tional water lines are expected. Line strengths are about 8 times the 1� statistical noise level. The H2O vertical profile for wa- ter suggested by the photochemical model of Lara et al. (1996), rescaled by a factor of about 0.4 +0.3 0.2, is compatible with the data. The associated water mole fraction is about 8 +6 4 × 10 9 at an altitude of 400 km (column density of 2.6 +1.9 1.6 ×10 14 mol cm 2 above the surface). The inferred water influx at 700 km in Titan's atmosphere is in the range (0.8-2.8) ×10 6 mol cm 2 s 1 .

Construction of Highly Accurate Models of Rotating Neutron Stars : Comparison of Three Different Numerical Schemes

Abstract: We conduct a direct comparison of three dier- ent representative numerical codes for constructing mod- els of rapidly rotating neutron stars in general relativity. Our aim is to evaluate the accuracy of the codes and to investigate how the accuracy is aected by the choice of interpolation, domain of integration and equation of state. In all three codes, the same physical parameters, equations of state and interpolation method are used. We construct 25 selected models for polytropic equations of state and 22 models with realistic neutron star matter equations of state. The three codes agree well with each other (typical agreement is better than 0.1% to 0.01%) for most mod- els, except for the extreme assumption of uniform density stars. We conclude that the codes can be used for the con- struction of highly accurate initial data congurations for polytropes of index N> 0 :5 (which typically correspond to realistic neutron stars), when the domain of integra- tion includes all space and for realistic equations with no phase transitions. With the exception of the uniform den- sity case, the obtained values of physical parameters for the models considered in this paper can be regarded as \\standard\" and we display them in detail for all models.

Introducing the Gaussian shape hypothesis for asteroids and comets

Abstract: A hypothesis is presented that the irregular shapes of asteroids and cometary nuclei can be modeled by using lognor- mal statistics (Gaussian random sphere). The Gaussian sphere is fully described by the mean and covariance function of the radius. A suitable covariance function is devised here for the generation of sample Gaussian spheres that closely resemble the shapes observed for asteroids. To collect more evidence for the Gaussian hypothesis, assuming simple Lommel-Seeliger and Lambert scattering laws, lightcurves are computed for ro- tating Gaussian spheres. The results show striking similarities to asteroid lightcurves. For example, the observed increase of lightcurve amplitude with increasing solar phase angle appears to be at least partly explained by the numerical simulations. Making further use of the Gaussian random sphere, a statistical model is developed for albedo variegations on asteroids, and for characterizing active regions on cometary nuclei.

Ultracompact jets in active galactic nuclei

Abstract: We study the properties of ultracompact jets in sev- eral prominent radio sources (Cygnus A, 3C 309.1, 3C 345, 3C 395, 4C 39.25, and 1038+528 A), using the frequency de- pendence of observed position of the optically thick jet core. Frequency dependent offsets of the core positions are used for calculating the luminosities, magnetic fields, and geomet- rical properties of the jets. Opacity effects are studied, for both synchrotron self-absorption and external free-free absorption. Pressure and density gradients in the jets and in surrounding ambient medium are shown to be a primary factor determining the observed properties of ultracompact jets. We discuss pos- sible applications of opacity effects to studying the conditions existing in central regions of active galactic nuclei.

3-D magnetic configurations supporting prominences I. The natural presence of lateral feet

Abstract: It is now commonly accepted that prominence plasma is supported in magnetic dips, in particular in twisted flux-tubes. But present two-dimensional models are unable to explaintheobservedpresenceandstructureofprominencesfeet. This requires three-dimensional models. We modeled thefield using linear force-freefield equations. Combining a small number of harmonics, and using observa- tionalconstraints,wehavefoundtheareaintheparameterspace where prominences are likely to be present. Then, adding 3-D harmonics, we show that feet appear periodically underneath the prominence body. For great helicity, the parameter space is mostly fulfilled by configurations which have feet alternat- ingbetweenbothsidesoftheprominenceaxis,asobserved.The theoreticalphotosphericfieldhasaquasi-bipolarpatternandthe prominence stands above a magnetic corridor containing only small parasitic polarities. The lateral feet are formed by dips in the vicinity of these small polarities. These configurations show in a natural fashion a number of well-established as well as more recent observationnal aspects of prominences, in par- ticular the vector magnetic field measurements in prominences andthechiralitypatterns(thedextral/sinistral,right/leftbearing, skew of the overlying coronal arcade and fibril organization in prominence channels).

A two-parameter luminosity correction for type ia supernovae

Abstract: If, in addition to the usual luminosity correction de- scribed by the light curve slope parameter b, we introduce a color correction parameter R, then an extraordinarily good t is realized for all the 29 distant Type Ia supernovae of the re- cent Calan/Tololo supernova survey. The reduced 2 found is much lower than can be expected. All of the intrinsic disper- sion is thereby removed from the data sample, leaving perfectly standardized candles insofar as one can measure with present techniques. The best-t solution has a mysteriously low value for R (referred to the B-band) of about 2 and a value of b 0.5 which is smaller than previously reported without the color cor- rection. These parameters lead, using pre-Hipparcos distances for Cepheid calibrations, to essentially the same value for the Hubble constant. Our preliminary value, subject to further in- vestigation of Cepheid-calibrated supernovae using this same color correction, is H0 =6 0 6 kms 1 Mpc 1 .

A model of the wavelength dependence of solar irradiance variations

Abstract: Thevariationofthesolarirradianceoverthesolarcy- cle has a strong wavelength dependence, being larger at shorter wavelengths. Here we present simple models of the spectral de- pendence of irradiance variations between solar activity maxi- mum and minimum. We nd that the observations (which con- centrate on the UV) cannot be reproduced by a change in effec- tive temperature of the Sun (or of parts of its surface) alone. We can,however,reproducethedatawitheithera2-componentora 3-component model, of which one component is the quiet Sun, another is a facular component, and the third (in the case of the 3-componentmodel)representsthetemperaturestraticationof sunspots. The facular component is found to be very close to the facular models F or P of Fontenla et al. (1993). The suc- cess of these models supports the assumption underlying many studies of total solar irradiance variations that these are caused mainly by magnetic elds at the solar surface. Our investigation also allows an improved estimate of the relative contribution of the various layers in the solar photosphere and of the different wavelength regions to the total irradiance variations.

Effect of varying strength and orientation of local interstellar magnetic field on configuration of exterior heliosphere : 3d mhd simulations

Abstract: The aim of this paper is to present the effects of varying magnitude and orientation of the local interstellar mag- netic field on the heliospheric boundary region (the region be- tween the termination shock and the bow shock containing the heliopause) Other effects such as interstellar neutrals, cosmic rays and the asymmetry of the solar wind caused by its heli- olatitude dependence are disregarded We calculate the shape and structure of the heliospheric boundary region for different interstellar Alfv ´ enic Mach numbers and various inclination an- gles between Very Local InterStellar Medium (VLISM) velocity and magnetic field vectors using a fully three-dimensional MHD computational analysis The new results show the asymmetry of this region for inclination angles 0 << 90 and are in agreement with the Newtonian approximation theory (Fahr et al 1986, 1988) concerning trends in the heliopause orientation and location Unlike the NA model which only qualitatively indicates the effects of the VLISM magnetic field on the he- liospheric boundary region the present 3D MHD calculations reveal fully the nature of these effects by capturing all discon- tinuities including the termination shock, heliopause and bow shock The numerical scheme employed in this study is fully im- plicit and conservative, using a Roe-type Riemann solver in a generalized coordinate system

Experimental cross section of 144Sm(α, γ)148Gd and implications for the p-process

Abstract: The cross section of the 144 Sm(;) 148 Gd reac- tion has been measured at bombarding energies in the 10.5- 13.4 MeV range using an activation method based on the off-line -activity measurement of the 148 Gd residual nucleus. The long measuring time of the -decay has required the use of etched track detectors. The resulting cross sections and astrophysical S-factors are compared to new statistical model calculations. The measured data at low energies reflect the strong energy dependence of the optical potential which is found to affect significantly our predictions of the -capture rate.

Thermal physics of asteroids. IV. Thermal infrared beaming

Abstract: The so called beaming is an important factor when studying the thermal emission from the atmosphereless bodies of the solar system. The emission is non-Lambertian and has a tendency to be "beamed" in the solar direction. The small scale surface roughness

Cosmic rays. VII. Individual element spectra: prediction and data

Abstract: Based on the earlier papers in this series, we discuss here the errors in the prediction of the model, and compare with the error ranges in the spectra for individual elements. The predictions are spectra of $E^{-2.75 \pm 0.04}$ for hydrogen, and $E^{-2.67 -0.02 \pm 0.02}$ for helium and heavier elements below the knee. For particle energies above $10 Z$ GeV the data give $E^{-2.77 \pm 0.02}$ for hydrogen and $E^{-2.64 \pm 0.02}$ for helium and similar values for all heavier elements combined, where $Z$ is the charge of the nucleus considered. At the higher energy range considered here the secondary elements Li, Be, and B as well as the sub-Fe group have spectra consistent with source-related spallation, such as occurs when a supernova driven explosions runs into a shell around the wind of the predecessor star.

An analytic solar magnetic field model

Abstract: We describe a simple analytic model for the mag- netic field in the solar corona and interplanetary space which is appropriate to solar minimum conditions. The model com- bines an azimuthal current sheet in the equatorial plane with an axisymmetric multipole field representing the internal mag- netic field of the Sun. The radial component of the field filling interplanetary space is approximately monopolar at large helio- centric distances as observed. These open field lines connect to the polar regions of the Sun and define the polar coronal holes which are prevalent at solar minimum and which are the source of the fast solar wind. By including both dipole and a quadrupole terms at the origin it is possible to construct a good representa- tion of the coronal magnetic field in such conditions. We also note that the Parker spiral will be underwound relative to the case of the monopole because the open field lines emanate from solar latitudes in excess of 60.

Cluster radio relics as a tracer of shock waves of the large - scale structure formation

Abstract: Wepresentevidencefortheexistenceofshockwaves caused by the formation of the large-scale structure. In some clusters of galaxies peripherally located sources of extended diffuse radio emission exist, the so-called cluster radio relics. They have steep radio spectra but no apparent cutoff, as old remnants of radio galaxies usually have. Therefore particle ac- celeration has to take place within them. We propose that shock structures of the cosmological large-scale matter flows are re- sponsible for the acceleration of relativistic electrons: cluster accretion shocks and bow shocks of merger events. We develop a theory of radio plasma having traversed these shocks and compare it to observational data of nine radio relics (0038-096, 0917+75, 1140+203, 1253+275, 1712+64, 1706+78, 2006-56, 2010-57, 1401-33) and their host clusters (A85, A786, A1367, Coma, A2255, A2256, A3667, S753). The necessary accretion power, the spectral index of the radio spectrum, the acceler- ation efciency of the shock, the diffusion coefcient in the post-shock region, and the predicted radio polarization in all of our examples t into a coherent interpretation of the observa- tional data. Since polarization measurements are available only for four sources, the predictions of our theory can be indepen- dently checked using other examples. The predicted values of the shock compression ratio, density and temperature of the infalling gas, magnetic eld strength of the shocked and un- shockedradioplasmaarediscussedwithintheframeofstructure formation theory.

Diurnal Yarkovsky effect as a source of mobility of meter-sized asteroidal fragments. I. Linear theory

Abstract: A linear theory for the heat conduction in a spherical, solid and rotating body illuminated by solar radiation is developed in detail. The principal aim is to compute the recoil force, due to thermally reemitted radiation, which is commonly known as the “Yarkovsky force”. We concentrate on the thermal effect which depends on the rotational period of a body rather than on the period of revolution around the Sun and deal with the general case of an arbitrary obliquity of the spin axis to the orbital plane. This “diurnal” thermal effect is considered to be an important source of mobility for meter-sized stony asteroidal fragments in the main belt. We compare our results with those of previous authors and show that the results of Peterson (1976) are accurate for meter-sized asteroidal bodies (although he used unrealistically long rotation periods).

Implicit and semi-implicit schemes in the Versatile Advection Code : numerical tests

Abstract: We describe and evaluate various implicit and semi-implicit time integration schemes applied to the numerical simulation of hydrodynamical and magnetohydrodynamical problems. The schemes were implemented recently in the software package Versatile Advection Code, which uses modern shock capturing methods to solve systems of conservation laws with optional source terms. The main advantage of implicit solution strategies over explicit time integration is that the restrictive constraint on the allowed time step can be (partially) eliminated, thus the computational cost is reduced. The test problems cover one and two dimensional, steady state and time accurate computations, and the solutions contain discontinuities. For each test, we confront explicit with implicit solution strategies.

Wavelength dependency of the Solar limb darkening

Abstract: A single parameter model of the mean Solar limb- darkening is presented. This empirical law has the advantage to represent the limb darkening over a large spectrum at least as well as a quadratic or logarithmic law. Since it is less sensi- tive than high degree polynomial laws, it is recommended for subsequent analysis of average limb-darkening variations and comparisons.

Asteroids as far-infrared photometric standards for ISOPHOT

Abstract: Asteroids are used as far-IR calibrators for the imaging photopolarimeter ISOPHOT on board the Infrared Space Observatory ISO. For the 10 selected objects we compiled a large uniform database(1) of 678 individual observations, ranging from 7-2 000 mu m. A

The spectrum of the young star HD 100546 observed with the Infrared Space Observatory

Abstract: It is generally assumed that planets form in the dusty disks that surround young stars (Beckwith & Sargent 1996). The Infrared Space Observatory (Kessler et al. 1996) now enables us to determine the characteristics of these disks with unprecedented spectral resolution and signal-to-noise. We present here ISO spectra of the disk that surrounds the young star HD 100546. A remarkable variety of emission features of carbon- and oxygen-rich dust occurs. Most prominent are a se- ries of emission features that can be attributed to silicates in crystalline form, mostly forsterite. In the interstellar medium and HII regions the silicate dust is mostly amorphous, but crys- talline silicates are found in comets, meteorites and interplan- etary dust particles. The forsterite features of HD 100546 are astonishingly similar to those observed in the ISO spectrum of Comet Hale-Bopp (Crovisier et al. 1997), strengthening the hy- pothesis that the disk around HD 100546 contains a huge swarm of comets (Grady et al. 1997). We argue that the crystallisation process occurs during the early evolution of the circumstellar disks of young stars and speculate about the formation of an Oort cloud around HD 100546.

Broadening of Si VIII lines observed in the solar polar coronal holes

Abstract: We study the variation of the line width and electron density as a function of height above two coronal holes from forbidden spectral lines of Si viii. The spectra were obtained with the Solar Ultraviolet Measurements of Emitted Radiation spectrometer flown on the Solar and Heliospheric Observatory spacecraft. The observations concentrate on the dark regions outside the plumes, which are believed to be the locations, where the fast solar wind originates. The line width data show that the non-thermal line-of-sight velocity increases from 27 km s 1 at 27 arc sec above the limb to 46 km s 1 some 250 arc sec (i.e. 180,000 km) above the limb. The electron density shows a decrease from 1: 11 0 8 cm 3 to 1: 61 0 7 cm 3 over the same distance. This data implies that the non-thermal velocity is in- versely proportional to the quadratic root of the electron density, in excellent agreement with that predicted for undamped radi- ally propagating Alfv´ en waves. We show that the energy flux associated with these hydromagnetic waves is sufficient to drive the high speed solar wind streams.