On Random Walks with a General Moving Barrier

Random walks with a general, nonlinear barrier have found recent applications ranging from reionization topology to refinements in the excursion set theory of halos. Here, we derive the first-crossing distribution of random walks with a moving barrier of an arbitrary shape. Such a distribution is shown to satisfy an integral equation that can be solved by a simple matrix inversion, without the need for Monte Carlo simulations, making this useful for exploring a large parameter space. We discuss examples in which common analytic approximations fail, a failure which can be remedied using the method described here.Comment: 6 pages, 7 figures, submitted to Ap

A dynamical interpretation of the radio jet in 3C 31

The radio source 3C 31, which is positionally identified with the galaxy NGC 383, has recently been shown to contain two curved radio jets emanating from the galaxy's nucleus. In this paper, we demonstrate that the projected shape of these jets may be produced by a dynamical interaction between NGC 383 and its neighbour NGC 382. On the basis of this hypothesis, the orbital elements of the NGC383/NGC382 system are estimated and a mean V magnitude mass to light ratio of about 11 solar units is derived for the system. The inferred value for the jet velocity is approximately 500km/s. Some consequences for theoretical models of other extragalactic radio sources are briefly outlined

3D Radiative Transfer in η\eta Carinae: Application of the SimpleX Algorithm to 3D SPH Simulations of Binary Colliding Winds

Eta Carinae is an ideal astrophysical laboratory for studying massive binary interactions and evolution, and stellar wind-wind collisions. Recent three-dimensional (3D) simulations set the stage for understanding the highly complex 3D flows in $\eta$ Car. Observations of different broad high- and low-ionization forbidden emission lines provide an excellent tool to constrain the orientation of the system, the primary's mass-loss rate, and the ionizing flux of the hot secondary. In this work we present the first steps towards generating synthetic observations to compare with available and future HST/STIS data. We present initial results from full 3D radiative transfer simulations of the interacting winds in $\eta$ Car. We use the SimpleX algorithm to post-process the output from 3D SPH simulations and obtain the ionization fractions of hydrogen and helium assuming three different mass-loss rates for the primary star. The resultant ionization maps of both species constrain the regions where the observed forbidden emission lines can form. Including collisional ionization is necessary to achieve a better description of the ionization states, especially in the areas shielded from the secondary's radiation. We find that reducing the primary's mass-loss rate increases the volume of ionized gas, creating larger areas where the forbidden emission lines can form. We conclude that post processing 3D SPH data with SimpleX is a viable tool to create ionization maps for $\eta$ Car.Comment: 18 pages, 11 figures, accepted for publication in MNRA

Hydrodynamical Models of Outflow Collimation in YSOs

We explore the physics of time-dependent hydrodynamic collimation of jets from Young Stellar Objects (YSOs). Using parameters appropriate to YSOs we have carried out high resolution hydrodynamic simulations modeling the interaction of a central wind with an environment characterized by a moderate opening angle toroidal density distribution. The results show that the the wind/environment interaction produces strongly collimated supersonic jets. The jet is composed of shocked wind gas. Using analytical models of wind blown bubble evolution we show that the scenario studied here should be applicable to YSOs and can, in principle, initiate collimation on the correct scales (R ~ 100 AU). The simulations reveal a number of time-dependent non-linear features not anticipated in previous analytical studies including: a prolate wind shock; a chimney of cold swept-up ambient material dragged into the bubble cavity; a plug of dense material between the jet and bow shocks. We find that the collimation of the jet occurs through both de Laval nozzles and focusing of the wind via the prolate wind shock. Using an analytical model for shock focusing we demonstrate that a prolate wind shock can, by itself, produce highly collimated supersonic jets.Comment: Accepted by ApJ, 31 pages with 12 figures (3 JPEG's) now included, using aasms.sty, Also available in postscript via a gzipped tar file at ftp://s1.msi.umn.edu/pub/afrank/SFIC1/SFIC.tar.g

Solar-Like Cycle in Asymptotic Giant Branch Stars

I propose that the mechanism behind the formation of concentric semi-periodic shells found in several planetary nebulae (PNs) and proto-PNs, and around one asymptotic giant branch (AGB) star, is a solar-like magnetic activity cycle in the progenitor AGB stars. The time intervals between consecutive ejection events is about 200-1,000 years, which is assumed to be the cycle period (the full magnetic cycle can be twice as long, as is the 22-year period in the sun). The magnetic field has no dynamical effects; it regulates the mass loss rate by the formation of magnetic cool spots. The enhanced magnetic activity at the cycle maximum results in more magnetic cool spots, which facilitate the formation of dust, hence increasing the mass loss rate. The strong magnetic activity implies that the AGB star is spun up by a companion, via a tidal or common envelope interaction. The strong interaction with a stellar companion explains the observations that the concentric semi-periodic shells are found mainly in bipolar PNs.Comment: 10 pages, submitted to Ap

ZOBOV: a parameter-free void-finding algorithm

ZOBOV (ZOnes Bordering On Voidness) is an algorithm that finds density depressions in a set of points, without any free parameters, or assumptions about shape. It uses the Voronoi tessellation to estimate densities, which it uses to find both voids and subvoids. It also measures probabilities that each void or subvoid arises from Poisson fluctuations. This paper describes the ZOBOV algorithm, and the results from its application to the dark-matter particles in a region of the Millennium Simulation. Additionally, the paper points out an interesting high-density peak in the probability distribution of dark-matter particle densities.Comment: 10 pages, 8 figures, MNRAS, accepted. Added explanatory figures, and better edge-detection methods. ZOBOV code available at http://www.ifa.hawaii.edu/~neyrinck/vobo

Periodic bursts of Star Formation in Irregular Galaxies

We present N-body/SPH simulations of the evolution of an isolated dwarf galaxy including a detailed model for the ISM, star formation and stellar feedback. Depending on the strength of the feedback, the modelled dwarf galaxy shows periodic or quasi-periodic bursts of star formation of moderate strength. The period of the variations is related to the dynamical timescale, of the order of $1.5~10^8$ yr. We show that the results of these simulations are in good agreement with recent detailed observations of dwarf irregulars (dIrr) and that the peculiar kinematic and morphological properties of these objects,as revealed by high resolution HI studies, are fully reproduced. We discuss these results in the context of recent surveys of dwarf galaxies and point out that if the star formation pattern of our model galaxy is typical for dwarf irregulars this could explain the scatter of observed properties of dwarf galaxies. Specifically, we show that the time sampled distribution of the ratio between the instanteneous star formation rate (SFR) and the mean SFR is similar to that distribution in observed sample of dwarf galaxies.Comment: 11 pages, 6 figures, accepted for A&

A G1-like globular cluster in NGC 1023

The structure of a very bright (MV = -10.9) globular cluster in NGC 1023 is analyzed on two sets of images taken with the Hubble Space Telescope. From careful modeling of King profile fits to the cluster image, a core radius of 0.55+/-0.1 pc, effective radius 3.7+/-0.3 pc and a central V-band surface brightness of 12.9+/-0.5 mag / square arcsec are derived. This makes the cluster much more compact than Omega Cen, but very similar to the brightest globular cluster in M31, G1 = Mayall II. The cluster in NGC 1023 appears to be very highly flattened with an ellipticity of about 0.37, even higher than for Omega Cen and G1, and similar to the most flattened clusters in the Large Magellanic Cloud.Comment: 14 pages, 3 figures, 1 table. Accepted for AJ, Oct 200

Unequal-mass galaxy mergers and the creation of cluster S0 galaxies

It is a longstanding and remarkable problem when and how red S0 galaxies were formed in clusters of galaxies. We here propose that the major mechanism for the S0 creation is galaxy merging between two spirals with unequal mass. Our numerical simulations demonstrate that galaxy merging exhausts a large amount of interstellar medium of two gas-rich spirals owing to the moderately enhanced star formation, and subsequently transforms the two into one gas-poor S0 galaxy with structure and kinematics strikingly similar to the observed ones. This secondary S0 formation with enhanced star formation explains a smaller fraction of S0 population recently observed in some distant clusters of galaxies. Unequal-mass galaxy mergers thus provide an evolutionary link between a larger number of blue spirals observed in intermediate redshift clusters and red S0s prevalent in the present-day ones.Comment: 14 pages 3 figures (ps file), ApJL in pres