Prof. Marco Martos

Instituto de Astronomia - Universidad Nacional Autonoma de Mexico

``Mass Models of the Milky Way: Spiral Pattern, Bar,
 and the Stellar and Gaseous Response to the Mass
 Distribution''


 With the aim of studying the nonlinear stellar
 and gaseous response to the gravitational potential
 of our Galaxy, we have modeled 3D spiral arms and
 bar as superpositions of ellipsoids and spheroids
 using the available data. 
 Calculating stellar orbits, we found a sharp
 separatrix between prograde & retrograde orbits
 with respect to the spiral rotation: chaos is
 restricted to prograde orbits. The separatrix 
 widens in the bar potential, but still chaos is 
 restricted to prograde motion (defined unambiguously
 in the inertial frame). Also, the nature of the
 global (as opposed to the usual local aproximation)
 modeling of the spiral arms utilized shows important
 differences with respect to previous assumptions
 on the large scale shock structure.
 A self consistency analysis of the proposed
 spiral pattern revealed strong constraints on the
 pattern speed (a controversial quantity in the
 literature) and its extent, with consequences suggesting
 new views of cosmic ray flux histories studied previously
 assuming a wide range of spiral pattern speeds.
 Preliminary forays using hydrodynamic simulations
 indicate that the gas response to the two arms
 pattern of the K band gives rise to a complex four
 arms pattern observed in the optical band. This
 response is seemingly related to stellar resonances;
 the 4/1 plays a very important role in truncating
 the spiral and also bifurcating the "stellar" 
 (K band) arms into gaseous arms. A number of
 questions are the subject of current work
 as a follow up of these results: and example is,
 how are the bar and the spiral coupled (if they are
 coupled), when their angular speeds are so different --
 about 60 vs 20 km/(s kpc), respectively?