Marco Antonio Martos

The Collisions of HVCs with a Magnetized Gaseous Galactic Disk

Resumen en: We discuss 2-D MHD numerical simulations for the interaction of high-velocit y clouds with a magnetized Galactic disk. The initial magnetic field is orie...

Nonlinear Effects in Models of the Galaxy. I. Midplane Stellar Orbits in the Presence of Three-dimensional Spiral Arms

With the aim of studying the nonlinear stellar and gaseous response to the gravitational potential of a galaxy such as the Milky Way, we have modeled three-dimensional Galactic spiral arms as a superposition of inhomogeneous oblate spheroids and added their contribution to an axisymmetric model of the Galactic mass distribution. Three spiral loci are proposed here, based in different sets of observations. A comparison of our model with a tight-winding approximation shows important differences in the middle and outer Galactic regions. A preliminary self-consistency analysis taking Ωp = 15 and 20 km s-1 kpc-1 for the angular speed of the spiral pattern seems to favor the value Ωp = 20 km s-1 kpc-1. As a first step to the full three-dimensional calculations for which the model is suitable, we have explored the stellar orbital structure in the midplane of the Galaxy. We present the standard analysis in the pattern rotating frame and complement this analysis with orbital information from the Galactic inertial frame. Prograde and retrograde orbits are defined unambiguously in the inertial frame, then labeled as such in the Poincare diagrams of the noninertial frame. In this manner, we found a sharp separatrix between the two classes of orbits. Chaos is restricted to the prograde orbits, and its onset occurs for the higher spiral perturbation considered plausible in our Galaxy. An unrealistically high spiral perturbation tends to destroy the separatrix and make chaos pervasive. This might be relevant in other spiral galaxies.

MODELS FOR THE GRAVITATIONAL FIELD OF THE GALACTIC BAR: AN APPLICATION TO STELLAR ORBITS IN THE GALACTIC PLANE AND ORBITS OF SOME GLOBULAR CLUSTERS

We built three models for the gravitational field of the Galactic bar. These models are an inhomogeneous ellipsoid, an inhomogeneous prolate spheroid, and a superposition of four inhomogeneous ellipsoids. Among the three models, the superposition provides our best approximation to the observed boxy mass distribution of the Galactic bar. Adding the bar component to an axisymmetric Galactic model, we have calculated stellar midplane orbits and orbits of some globular clusters with known kinematical data. For all models we find a secular dispersion effect on the orbital energy and angular momentum, as measured in the Galactic inertial frame. This effect might be relevant to explain the orbital prograde-retrograde distribution of globular clusters. For the stellar kinematics, we study the connection between the sense of orbital motion in the midplane and the onset of chaos in the presence of the bar. In the inner region of the bar, chaos is induced by an axisymmetric central component (bulge), and it arises in orbits that change its orbital sense from prograde to retrograde and vice versa as seen from an inertial reference frame. Outside the bar region, chaos appears only in prograde orbits. Our results concerning such a connection are consistent and extend those obtained for midplane orbits in the presence of only a spiral pattern in the axisymmetric Galactic model.

A plausible Galactic spiral pattern and its rotation speed

We report calculations of the stellar and gaseous response to a Milky Way mass distribution model including a spiral pattern with a locus as traced by K-band observations, superimposed on the axisymmetric components in the plane of the disc. The stellar study extends calculations from previous work concerning the self-consistency of the pattern. The stellar response to the imposed spiral mass is studied via computations of the central family of periodic and nearby orbits as a function of the pattern rotation speed, Q p , among other parameters. A fine grid of values of Q p was explored, ranging from 12 to 25 km s -1 kpc -1 . Dynamical self-consistency is highly sensitive to Ω p , with the best fit appearing at 20 km s -1 kpc -1 . We give an account of recent independent pieces of theoretical and observational work that are dependent on the value of Ω p , all of which are consistent with the value found here: the recent star formation history of the Milky Way, local inferences of cosmic ray flux variations and Galactic abundance patterns. The gaseous response, which is also a function of Ω p , was calculated via 2D hydrodynamic simulations with the ZEUS code. For Ω p = 20 km s -1 kpc -1 , the response to a two-armed pattern is a structured pattern of four arms, with bifurcations along the arms and interarm features. The pattern qualitatively resembles the optical arms observed in our Galaxy and other galaxies. The complex gaseous pattern appears to be linked to resonances in stellar orbits. Among these, the 4:1 resonance plays an important role, as it determines the extent of the stellar spiral pattern in the self-consistency study presented here. Our findings seemingly confirm predictions by Drimmel & Spergel (2001), based on K-band data.

Magnetohydrodynamic Modeling of a Galactic Spiral Arm as a Combination Shock and Hydraulic Jump

We consider the interarm-to-arm transition for gas flow in the Galactic disk, modeled as a thick, magnetized, cloudless layer of gas in hydrostatic equilibrium with external gravity from stars, and having parameters appropriate to the solar neighborhood. We neglect the self-gravity of the gas and shear, and radial variations in gravity. We show that such a transition, if supersonic, must present characteristics of both a hydraulic jump (or bore) and a shock. Our numerical simulations confirm this prediction. Modeling the spiral perturbation as local, we find that flow passing through it builds dense, long-lived vertical structures with high velocity flow over the top, followed by a downstream shock, and sometimes secondary jumps. In addition, gravity waves generated in the thick disk appear to promote the formation of marked density enhancements in the midplane.

The Density Structure of Highly Compact H II Regions

We report the density structure of the ultracompact (UC) H ii regions G35.2021.74, G9.6210.19-E, and G75.7810.34-H O. The density profiles are derived from radio continuum emission at wavelengths from 6 to 2 0.3 cm. In the case of G35.2021.74, a cometary UC H ii region with a core and a tail, the spectrum of the core varies as , implying that the density structure is . The emission from the tail has a flatter spectrum, 0.6 22 S / n n / r n e indicating that the density gradient is also negative but shallower. For the case of G9.62 10.19, which is an H ii region complex with several components, the spectrum of the region designated component E is , cor0.95 S / n n responding to . The steepest spectral index, , is for the super UC H ii region G75.7810.34-H O; 22.5 1.4 n / rS / n e n 2 its density stratification may be as steep as . The actual density gradient may be smaller, owing to an 24 n / r e exponential (rather than a power-law) density distribution or to the effects of finite spatial extent. The contribution from dust emission and some of the possible implications of these density distributions are briefly discussed. Stellar groups form in the dense cores of molecular clouds. The structure of these clouds has been unveiled with optical, infrared, and radio observations over the last three decades, and a great effort has been made to understand the connections between cloud properties and star-forming activity. In particular, knowledge of the gas density structure is required to determine fundamental properties, such as the mass and stability of star-forming cores. Extinction studies can be used to derive the density profiles of clouds, but given the large opacities involved, they can only probe the outermost gas layers. Tracers occurring at radio frequencies, on the other hand, can penetrate deeper into the cloud and can reveal the density stratification of dark clouds and cloud cores. The information obtained from extinction and molecular-line studies shows that molecular clouds are centrally condensed. For instance, moderate-density (10 3 ‐10 5 cm ) envelopes sur23 rounding higher density (10 7 cm ) cores suggest the existence 23

The Parker Instability in a Thick Galactic Gaseous Disk. I. Linear Stability Analysis and Nonlinear Final Equilibria

A linear stability analysis of a multicomponent and magnetized Galactic disk model is presented. The disk model uses the observed stratifications for the gas density and gravitational acceleration at the solar neighborhood, and in this sense it can be called a realistic model. The distribution of the total gas pressure is defined by these observed stratifications, and the gaseous disk is assumed isothermal. The initial magnetic field is taken parallel to the disk, with a midplane value of 5 μG, and its stratification along the z-axis is derived from the condition of magnetohydrostatic equilibrium in an isothermal atmosphere. The resulting isothermal sound speed is ~8.4 km s-1, similar to the velocity dispersion of the main gas components within 1.5 kpc from the midplane. The thermal-to-magnetic pressure ratio decreases with [z], and the warm model is Parker unstable. The dispersion relations show that the fastest growing mode has a wavelength of about 3 kpc, for both symmetric and antisymmetric perturbations, and the corresponding growth timescales are of about 3 × 107 yr. The structure of the final equilibrium stage is also derived, and we find that the midplane antisymmetric (MA) mode gathers more gas in the magnetic valleys. The resulting MA gas condensations have larger densities, and the column density enhancement is a factor of about 3 larger than the value of the initial stage. The unstable wavelengths and growth times for the multicomponent disk model are substantially larger than those of a thin-disk model, and some of the implications of these results are discussed.

THE PARKER INSTABILITY IN A THICK GASEOUS DISK. II. NUMERICAL SIMULATIONS IN TWO DIMENSIONS

We present 2D, ideal–MHD numerical simulations of the Parker instability in a multi–component warm disk model. The calculations were done using two numerical codes with different algorithms, TVD and ZEUS-3D. The outcome of the numerical experiments performed with both codes is very similar, and confirms the results of the linear analysis for the undular mode derived by Kim et al. (2000): the most unstable wavelength is about 3 kpc and its growth timescale is between 30–50 Myr (the growth rate is sensitive to the position of the upper boundary of the numerical grid). Thus, the time and length scales of this multicomponent disk model are substantially larger than those derived for thin disk models. We use three different types of perturbations, random, symmetric, and antisymmetric, to trigger the instability. The antisymmetric mode is dominant, and determines the minimum time for the onset of the nonlinear regime. The instability generates dense condensations and the final peak column density value in the antisymmetric case, as also derived by Kim et al. (2000), is about a factor of 3 larger than its initial value. These wavelengths and density enhancement factors indicate that the instability alone cannot be the main formation mechanism of giant molecular clouds in the general interstellar medium. The role of the instability in the formation of large-scale corrugations along spiral arms is briefly discussed.

Spiral Density Wave Shock-induced Star Formation at High Galactic Latitudes

We have modeled the gas response to a spiral density wave (SDW) in a thick, magnetized galactic disk. The inclusion in the model of the vertically extended galactic warm ionized gas layer alters the conventional view of the SDW scenario for star formation: whereas marked density enhancements still occur in the midplane, the shock and a prominent high column density structure extend to high z (the height above the galactic midplane) above the arm. We argue that if the SDW mechanism indeed triggers molecular cloud and star formation, it should do so not only at the midplane but also at distances well above the star-forming thin disk of the conventional picture. The resulting structure resembles a hydraulic jump, or bore, in which gas entering the spiral arm rises suddenly on the upstream side of the arm, then accelerates and angles downward, finally landing on a large downfall region downstream of the arm.

ON THE GALACTIC SPIRAL PATTERNS: STELLAR AND GASEOUS

The gas response to a proposed spiral stellar pattern for our Galaxy is presented here as calculated via 2D hydrodynamic calculations utilizing the ZEUS code in the disk plane. The locus is that found by Drimmel (2000) from emission profiles in the K band and at 240 . The self-consistency of the stellar spiral pattern was studied in previous work (see Martos et al. 2004). It is a sensitive function of the pattern rotation speed, p, among other parameters which include the mass in the spiral and its pitch angle. Here we further discuss the complex gaseous response found there for plausible values of p in our Galaxy, and argue that its value must be close to from the strong self-consistency criterion and other recent, independent studies which depend on such parameter. However, other values of p that have been used in the literature are explored to study the gas response to the stellar (K band) 2-armed pattern. For our best fit values, the gaseous response to the 2-armed pattern displayed in the K band is a four-armed pattern with complex features in the interarm regions. This response resembles the optical arms observed in the Milky Way and other galaxies with the smooth underlying two-armed pattern of the old stellar disk populations in our interpretation. The complex gaseous response appears to be related to resonances in stellar orbits. Among them, the 4:1 resonance is paramount for the axisymmetric Galactic model employed, and the set of parameters explored. In the regime seemingly proper to our Galaxy, the spiral forcing appears to be marginally strong in the sense that the 4:1 resonance terminates the stellar pattern, despite its relatively low amplitude. In current work underway, the response for low values of p tends to remove most of the rich structure found for the optimal self-consistent model and the gaseous pattern is ring-like. For higher values than the optimal, more features and a multi-arm structure appears.