[1] Earth's bow shock changes its three-dimensional (3-D) location in response to changes in the solar wind ram pressure P ram , Alfvén Mach number M A , magnetic field orientation, fast mode Mach number M ms , and sonic Mach number M S . Using shock locations from global 3-D ideal MHD simulations [Cairns and Lyon, 1995], empirical models are derived for the 3-D shape and location of Earth's bow shock in the near-Earth regime as a function of solar wind conditions. Multiple simulations with different M A and P ram but two orientations of the interplanetary magnetic field B IMF are analyzed: q IMF = 45°and 90°with respect to the solar wind direction v sw . Models for the (paraboloid) flaring parameter b s as a function of M A , azimuthal angle f, and q IMF = 45°or 90°, show b s decreasing with M A , corresponding to the shock becoming blunter and less swept back (with a larger cross section), as expected. Together with models for the shock's standoff distance (which increases with decreasing M A ) the models for b s (M A , f) predict the shock's 3-D location. Variations of b s with f represent eccentricities in the shock's cross section (i.e., a departure from circularity), with the shock extending further perpendicular to v ms (the fast mode speed) than parallel, as M A ! 1. An additional effect is observed in which the shock shape is ''skewed'' for q IMF = 45°(but not for q IMF = 90°) in the plane containing B IMF and v sw . These latter two effects are consistent with the fast mode velocity varying with propagation direction relative to B IMF .

We present results from a time-dependent gas-phase chemical model of a hot core based on the physical conditions of G305.2+0.2. While the cyanopolyyne HC 3 N has been observed in hot cores, the longer chained species, HC 5 N, HC 7 N and HC 9 N, have not been considered as the typical hot-core species. We present results which show that these species can be formed under hot core conditions. We discuss the important chemical reactions in this process and, in particular, show that their abundances are linked to the parent species acetylene which is evaporated from icy grain mantles. The cyanopolyynes show promise as 'chemical clocks' which may aid future observations in determining the age of hot core sources. The abundance of the larger cyanopolyynes increases and decreases over relatively short time-scales, ∼10 2.5 yr. We present results from a non-local thermodynamic equilibrium statistical equilibrium excitation model as a series of density, temperature and column density dependent contour plots which show both the line intensities and several line ratios. These aid in the interpretation of spectral-line data, even when there is limited line information available. In particular, non-detections of HC 5 N and HC 7 N in Walsh et al. are analysed and discussed.

[1] The location and geometry of Earth's bow shock vary considerably with the solar wind conditions. More specifically, Earth's bow shock is formed by the steepening of fast mode waves, whose speed v ms depends upon the angle q bn between the local shock normal n and the magnetic field vector B IMF , as well as the Alfvén and sound speeds (v A and c S ). Since v ms is a minimum for q bn = 0°and low Alfvén Mach number M A , and maximum for q bn = 90°and high M A , this implies that as q IMF (the angle between B IMF and v sw ) varies, the magnitude of v ms should vary also across the shock, leading to changes in shape. This paper presents 3-D MHD simulation data which illustrate the changes in shock location and geometry in response to changes in q IMF and M A , for 1.4 M A 9.7 and 0° q IMF 90°. Specifically, for oblique IMF the shock's geometry is shown to become skewed in planes containing B IMF (e.g., the x À z plane). This is also emphasized in the terminator plane data, where the shock is best represented by ellipses, with centers translated along the z axis. For the q IMF = 90°simulations the shock is symmetric about the x axis in both the x À y and x À z planes. Simulations for fieldaligned flow (q IMF = 0°) show a dimpling of the nose of the shock as M A ! 1. The simulations also illustrate the general movement of the shock in response to changes in M A ; high M A shocks are found closer to Earth than low M A shocks. Farris et al. 's [1991] magnetopause model is used in the simulations, and we discuss the limitations of this, as well as the expected results using a self-consistent model.

We report on Australia Telescope Compact Array observations of the massive star‐forming region G305.2+0.2 at 1.2 cm. We detected emission in five molecules towards G305A, confirming its hot core nature. We determined a rotational temperature of 26 K for methanol. A non‐local thermodynamic equilibrium excitation calculation suggests a kinematic temperature of the order of 200 K. A time‐dependent chemical model is also used to model the gas‐phase chemistry of the hot core associated with G305A. A comparison with the observations suggest an age of between 2 × 104 and 1.5 × 105 yr. We also report on a feature to the south‐east of G305A which may show weak Class I methanol maser emission in the line at 24.933 GHz. The more evolved source G305B does not show emission in any of the line tracers, but strong Class I methanol maser emission at 24.933 GHz is found 3 arcsec to the east. Radio continuum emission at 18.496 GHz is detected towards two H ii regions. The implications of the non‐detection of radio continuum emission towards G305A and G305B are also discussed.

We investigate the role and behaviour of dust grains in fast C‐type magnetohydrodynamic (MHD) shock waves in weakly ionized, dense molecular clouds. We calculate the structure of steady, oblique, C‐type shocks with shock speed vs= 18 km s−1, propagating in a medium with number density nH= 105 cm, and magnetic field 0.3 mG. The angle between the pre‐shock magnetic field and the shock normal is varied from 90° to ∼40° when the shocks become J‐type. The grain population is represented by either a single‐grain size or an MRN (Mathis, Rumpl and Nordsieck) grain size distribution with and without polycyclic aromatic hydrocarbons (PAHs). The grain charge is assumed to vary, consistent with an approximate electron temperature profile within the shock. Grain inertia is neglected. Charged particle drifts, fluid velocities and the magnetic field are all permitted to have components perpendicular to the ‘shock plane’ containing the pre‐shock (or post‐shock) magnetic field and the shock normal. For the shock parameters considered here, small grains remain coupled to the magnetic field, while large grains are partially decoupled by collisions with the neutrals. The increase in grain charge within the shock front, due to the sticking of electrons, increases the magnetic coupling of the large grains, which acts to suppress the magnetic field and neutral velocity rotation out of the shock plane. Increasing the grain size increases the grain–neutral collisional heating leading to hotter, thinner shocks. The presence of PAHs reduces the electron abundance and grain charging is insignificant; the grain coupling to the magnetic field is therefore reduced and the rotation of the magnetic field out of the shock plane increases. Even in this case, the rotation is less than 5°. The primary effect of including the charged particle drift components perpendicular to the shock plane is to increase the dissipation rate and reduce the shock thickness.