BEGIN:VCALENDAR VERSION:2.0 CALSCALE:GREGORIAN PRODID:UW-Madison-Physics-Events BEGIN:VEVENT SEQUENCE:0 UID:UW-Physics-Event-4291 DTSTART:20161013T203000Z DTEND:20161013T220000Z DTSTAMP:20260419T060542Z LAST-MODIFIED:20160919T150226Z LOCATION:4421 Sterling Hall\, 3:30 pm Coffee and Cookies\, Talk starts 3:45 SUMMARY:Six Decades of Spiral Density-Wave Theory\, Astronomy Colloqui um\, Frank Shu\, Hilldale Lecturer\, University Professor Emeritus UC Berkely and San Diego DESCRIPTION:
\nThe theory of spiral density waves had its origins about six decades ago in an attempt to reconcile the winding dilemma of material spiral arms in flattened disk galaxies. We begin with the earliest calculations of linear and nonlinear spiral density waves in disk galaxies\, in which the hypothesis of quasi-stationary spiral st ructure (QSSS) plays a central role. We then review the observational implications and tests\, in which the prediction of the nonlinear com pression of the interstellar medium and its embedded magnetic field wa s the earliest success\, and the failure to detect color gradients ass ociated with the migration of OB stars whose formation is triggered do wnstream from the spiral shock front seemed to be the earliest failure . We give the reasons for this apparent failure with an update on the current status of the problem of OB star formation\, including its re lationship to the feathering substructure of galactic spiral arms and giant associations of atomic and molecular gas. Infrared images can s how two-armed grand-design spirals\, even when the optical and UV imag es show flocculent structures. We suggest how the nonlinear response of the interstellar gas\, coupled with overlapping sub-harmonic reson ances\, might introduce chaotic behavior in the dynamics of the inters tellar medium and Population I objects\, even though the underlying fo rces to which they are subject are regular. We then move to a discuss ion of resonantly forced spiral density waves in planetary ring and it s relation to the ideas of disk truncation\, and the shepherding of na rrow rings by satellites orbiting nearby. The back reaction of the ri ngs on the satellites led to the prediction of planet migration in pro toplanetary disks\, which has had widespread application in the explod ing data sets concerning hot Jupiters and extrasolar planetary systems . As our final topic\, we return to the issue of global normal modes in the stellar disk of spiral galaxies and its relationship to the QSS S hypothesis\, where the central theoretical concepts involve waves wi th negative and positive surface densities of energy and angular momen tum in the regions interior and exterior\, respectively\, to the corot ation circle\; the consequent transmission and over-reflection of prop agating spiral density waves incident on the corotation circle\; and t he role of feedback from the central regions. We review self-consiste nt theoretical calculations of slowly growing normal modes in collisio nless stellar disks. N-body simulations show that the growth of such modes can saturate at finite amplitudes without the collisional dampin g of a gaseous interstellar medium as long as N exceeds 3 x107. Lastl y\, we discuss how the amplitude modulation predicted for the destruct ive interference of oppositely propagating waves that form standing wa ve patterns may have been observed in deep infrared images of nearby s piral galaxies. URL:https://www.physics.wisc.edu/events/?id=4291 END:VEVENT END:VCALENDAR