The vertical distribution of Titan's neutral atmosphere compounds is calculated from a new photochemical model extending from 40 to 1432 km. This model makes use of many updated reaction rates, and of the new scheme for methane photolysis proposed by Mordaunt et al. [1993]. The model also includes a realistic treatment of the dissociation of N2, of the deposition of water in the atmosphere from meteoritic ablation, and of condensation processes. The sensitivity of the results to the eddy diffusion coefficient profile is investigated. Fitting the methane thermospheric profile and the stratospheric abundance of the major hydrocarbons requires a methane stratospheric mixing ratio of 1.5-2% rather than 3%. Fitting the HCN stratospheric profile requires an eddy diffusion coefficient at 100-300 km that is 5-20 times larger than that necessary for the hydrocarbons. Most species are reasonably well reproduced, with the exception of CH3C2 H and HC3N. The formation of CH3CN may involve the reaction of CN with either CH 4 or (preferably) C2H 6. The observed CO2 profile can be modeled by assuming an external source of water of---6 x 106 cm -2 s -•. For a nominal CO mixing ratio of 5 x 10 -s, the chemical loss of CO exceeds its production by ---15%, and equilibrium is achieved for CO = 1 x 10 -s. 23,261 23,262 LARA ET AL.: PHOTOCHEMICAL MODELING OF TITAN'S ATMOSPHERE ual species were derived and, consequently, no comparison with observational data was possible. The first detailed photochemical model since Voyager was developed by Yung et al. [1984] (and updated by Yung [1987]). This work made use of a very complete set of chemical reactions, based on the compilation of the earlier studies by Strobel [1974, 1982] andAllen et al. [1980], and adding the photochemistry of oxygen compounds in a mildly reducing atmosphere (investigated by Pinto et al. [1980]), as well as new chemical reactions, mainly those forming long-chain hydrocarbons or polyynes. Vertical profiles for all the constituents observed in Titan's atmosphere were derived, and average mixing ratios were compared to early analyses of Voyager infrared observations [Hanel et al., 1981; Maguire et al., 1981; Kunde et al., 1981; Samuelson et al., 1983]. Implications of the model for the composition of the troposphere, the origin and evolution of the atmosphere, and the geochemistry were also assessed. Despite the qualitative and quantitative importance of this work, there are at least two reasons to reconsider Titan's photochemical models today. First, several new observational constraints have become available. The Voyager infrared imaging spectrometer (IRIS) spectra at the equator have been more fully exploited, resulting in improved determinations of the mixing ratios, in well-understood altitude ranges [Coustenis et al., 1989]. Vertical information is also available for some minor constituents observed in the north polar region [Coustenis et al., 1991]. A reanalysis of the Voyager ultraviolet spectrometer (UVS) data has also been performed, resulting in a new vertical...

We present the results of a high angular resolution study of the BL Lac object Markarian 501 in the radio band. We consider data taken at 14 different epochs, ranging between 1.6 and 22 GHz in frequency, and including new Space VLBI observations obtained on 2001 March 5 and 6 at 1.6 and 5 GHz. We study the kinematics of the parsec-scale jet and estimate its bulk velocity and orientation with respect to the line of sight. Limb-brightened structure in the jet is clearly visible in our data, and we discuss its possible origin in terms of velocity gradients in the jet. Quasi-simultaneous, multiwavelength observations allow us to map the spectral index distribution and to compare it to the jet morphology. Finally, we estimate the physical parameters of the parsec-scale jet.

We studied the growth of the shell-like radio structure of supernova SN 1993J in M 81 from September 1993 to October 2003 with very-long-baseline interferometry (VLBI) observations at the wavelengths of 3.6, 6, and 18 cm. We developed a method to accurately determine the outer radius (R) of any circularly symmetric compact radio structure such as SN 1993J. The source structure of SN 1993J remains circularly symmetric (with deviations from circularity under 2%) over almost 4000 days. We characterize the decelerated expansion of SN 1993J until approximately day 1500 after explosion with an expansion parameter m = 0.845 ± 0.005 (R ∝ t m ). However, from that day onwards the expansion differs when observed at 6 and 18 cm. Indeed, at 18 cm, the expansion can be well characterized by the same m as before day 1500, while at 6 cm the expansion appears more decelerated, and is characterized by another expansion parameter, m 6 = 0.788 ± 0.015. Therefore, since about day 1500 onwards, the radio source size has been progressively smaller at 6 cm than at 18 cm. These findings differ significantly from those of other authors in the details of the expansion. In our interpretation, the supernova expands with a single expansion parameter, m = 0.845 ± 0.005, and the 6 cm results beyond day 1500 are caused by physical effects, perhaps also coupled to instrumental limitations. Two physical effects may be involved: (a) a changing opacity of the ejecta to the 6 cm radiation; and (b) a radial decrease of the magnetic field in the emitting region. We also found that at 6 cm about 80% of the radio emission from the backside of the shell behind the ejecta is absorbed (our average estimate, since we cannot determine any possible evolution of the opacity), and the width of the radio shell is (31 ± 2)% of the outer radius. The shell width at 18 cm depends on the degree of assumed absorption. For 80% absorption, the width is (33.5 ± 1.7)%, and for 100% absorption, it is (37.8 ± 1.3)%. A comparison of our VLBI results with optical spectral line velocities shows that the deceleration is more pronounced in the radio than in the optical. This difference might be due to a progressive penetration of ejecta instabilities into the shocked circumstellar medium, as also suggested by other authors.

We present a new, complete, sample of 95 radio sources selected from the B2 and 3CR catalogues, with z < 0.1. Since no selection effect on the core radio power, jet velocity, or source orientation is present, this sample is well suited for statistical studies. In this first paper we present the observational status of all sources on the parsec (mas) and kiloparsec (arcsec) scale; we give new parsec-scale data for 28 sources and discuss their parsec-scale properties. Combining these data with those in the literature, information on the parsec-scale morphology is available for a total of 53 radio sources with different radio power and kpc-scale morphology. We investigate their properties. We find a dramatically higher fraction of two-sided sources in comparison to previous flux limited VLBI surveys.Comment: 29 pages, 21 figures - ApJ in press (10 Jan 2005 issue