Scientific and Technical Aerospace Reports Volume 39 April 6, 2001

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ASTROPHYSICS
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20010022633 Stanford Univ., Center for Turbulence Research, Stanford, CA USA
In Pusuit of Structures in Protoplanetary Disks
Umurhan, O.M., Stanford Univ., USA; Annual Research Briefs - 2000: Center for Turbulence Research; December 2000, pp.
17-30; In English; See also 20010022631; No Copyright; Avail: CASI; A03, Hardcopy; A03, Microfiche
This brief summarizes work devoted to studying the linear stability of baroclinic protoplanetary Keplerian disks. This work
largely builds on the foundation and fundamental results obtained by Barranco, Marcus, and Umurhan (hereafter BMU) which
presented a model to describe structures in weakly baroclinic Keplerian disks. Interest in protoplanetary disks has grown since
the discovery of the first of what are now dozens of extrasolar planets around F and G type stars. The apparent ubiquity of planets
suggests that the nebular disks (or protoplanetary disks), out of which these stars are born, must be susceptible to the development
of structures. However, theoretical work (see references in BMU) argues that purely pressure-supported gaseous barotropic
Keplerian disks do not undergo any known type of hydrodynamic instabilities. The basis for this claim is that the strong rotation
in Keplerian shear flows manages to suppress any of the usual instabilities seen in laboratory shear flows such as the Taylor-
Couette and Kelvin-Helmholtz instabilities. Some investigators have gone so far as to claim that protoplanetary disks are featureless
objects totally bereft of vortical structures or long-lasting stable patterns. to counter this assertion, some investigators have
suggested that the protoplanetary disk is sufficiently magnetized to undergo a magneto-rotational instability as first explored by
Chandresekhar using linear analysis and later demonstrated numerically by Balbus, Hawley, and Stone for fully nonlinear flows.
However, Desch has indicated that disks suspected to be around solar type stars will be too cool and weakly ionized such that local
dust particles in short time will sweep up the ionization in the gas, leaving the fluid nearly neutral. What little ionization is left
in the disk is too little to allow strong coupling to an external magnetic field. Thus this mechanism is unlikely to play an important
role in the formation of planets at the radii where the large gas giants are observed to reside. The point of view taken by BMU
is that a weakly baroclinic Keplerian disk will produce the type of shear flow that will promote hydrodynamical instabilities. The
assumption was that the added component of vertical shear driven by baroclinic effects generates coherent structures in an otherwise
featureless protoplanetary disk. to isolate this effect they derived a number of asymptotic reductions of the 3D Euler equations
in cylindrical geometry with a central mass source.
Author
Baroclinity; Shear Flow; Stability; Mathematical Models
20010025059 NASA Ames Research Center, Moffett Field, CA USA
Identification of Gas Phase PAHs in Absorption Towards Protostellar Sources
Bregman, Jesse D., NASA Ames Research Center, USA; Temi, Pasquale, NASA Ames Research Center, USA; [2000]; 10p; In
English; Sponsored in part by ESA Member States: France, Germany, the Netherlands and the UK
Contract(s)/Grant(s): RTOP 344-02-00-99; No Copyright; Avail: CASI; A02, Hardcopy; A01, Microfiche
The infrared emission bands (also known as the UIR bands.) have recently been observed in absorption at 3.25 micrometers
in the ices surrounding a few proto-stellar objects at 11.2 micrometers in MonR2, and at 6.2 micrometers towards two sources
near the galactic center. The UIR bands have been observed in emission for many years, but identifying these bands has proven
to be both difficult and contentious as no one has yet found a single material that provides a good match to the features. However,
most investigators agree that some form of carbon-based material with aromatic bonds is the most likely candidate, and many
arguments favor free molecules (polycyclic aromatic hydrocarbons, PAHs) as the carriers of at least the narrow emission bands.
Since the emission arises not from a single molecule but from a family of molecules, identifying which PAHs are contributing
to the infrared emission bands is difficult. The identification is further complicated by the fact that the emission at short wavelengths
is dominated by small molecules while at long wavelengths it is dominated by large molecules. Thus, for example, the
emission at 3.3 micrometers is from a different mix of molecules than those which produce the 11.2 micrometer band. to complicate
matters further, the molecular mix includes both neutral and ionic species. In absorption, the same mixture of molecules contributes
at all wavelengths and the molecules should be neutral, potentially simplifying comparisons with lab data. Also,
absorption strengths measured in the lab are directly applicable to interstellar absorption bands without the need to model an emission
spectrum of an unknown mixture of ionized and neutral PAHs. In this paper we show that a mixture of argon matrix isolated
PAH molecules can reproduce the 3.25 micrometers absorption band seen in the ISO SWS spectra of four embedded Infrared
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