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Reference: Giaretta, David Leslie., (1977). Instabilities in interstellar space. DPhil. University of Oxford.Citable link to this page:


Instabilities in interstellar space

Abstract: This thesis is a partial investigation of instabilities inthe interstellar gas which are driven by a coupling between theambient radiation field and the gas, and which do not arise whenthis coupling is missed out. The modes of couplings consideredare, firstly, the attenuation of the radiation with the concomitanteffects on the temperature, density and composition of the gas, invarious combinations. Secondly, velocity dependent effects areexamined in various circumstances and thirdly, radiation pressure,not included in the other two, is looked at in the simple case inwhich temperature and compositional changes are excluded.The explanation of why these instabilities may be of interest,and an outline of the extent to which similar instabilities havebeen investigated, is given in Chapter 1. Chapter 2 gives detailsof the basic equations used in the case in which the absorption lineshape is ignored.Many of the equations are used in the other chapters. Theequations are linearised in perturbations of the density, temperature,radiation field and composition, and the resulting dispersionrelationship is found for a harmonic perturbation. Because of theattenuation term in the radiative transfer equation, the polynomialhas complex coefficients.In Chapter 3 we investigate the properties of the roots of acomplex polynomial by an extension of Routh's methods, and derivea set of criteria to determine the number of roots which have positivereal part. These roots correspond to exponentially growingperturbations, or, in other words, they correspond to instabilities.Later in the chapter we apply these methods to Field's dispersionrelationship for thermal instabilities and derive many of hisconclusions in a fairly simple way. By a slight extension themethod yields estimates of the growth times of the instabilities.Some related situations are also examined in a similar way.After the detour of Chapter 3, Chapter 4 gives details of somemodels of the heating and cooling of the interstellar gas as well asof the reactions to be considered, namely the formation anddestruction of H2 and of carbon ions. Some of the limitations ofthe models are also discussed and the roots of the dispersion relationare given for different values of the parameters. New instabilitiesdo appear; for H2 their timescales of growth are rather too long tobe of interest; for carbon no short timescale instabilities arediscovered. Chapter 5 gives similar details for a system of purehydrogen gas which may be of interest in studies of the formation ofthe first generation of stars. In Chapter 6 there is a criticismof an earlier work by Schatzman on a similar subject, in which it isshown that his analysis was wrong.Chapter 7 deals with a new possibility, namely that, as the gasmoves, photons will be seen to be shifted in frequency and so themolecules will be exposed to a new set of destructive photons atfrequencies which have not been selectively absorbed in theunperturbed gas. First the simplest case, that in which thetemperature is unperturbed, is treated analytically. The attenuationof the radiation field is not considered. The effectiveness ofthis doppler-induced effect depends upon both the absorption profileand the radiation spectrum; these factors as well as temperatureperturbations are included next. Both line absorption andcontinuum absorption are considered. The former is used toinvestigate the stability of the interstellar gas and of purehydrogen gas, where hydrogen molecules are dissociated by lineabsorption; the latter is used in connection with HII regions andalso the interstellar gas where the photodissociated species arehydrogen atoms and neutral carbon respectively.Radiation pressure was not included in the previous chaptersbut in Chapter 8 a modified version of Field's theory of instabilitiesdriven by radiation pressure is presented. The new feature is thatthe frequency dependence of the absorption coefficient is includedin the equations and this, in the case of a flat radiation spectrum,leads to an exact cancellation of the dominant term in Field'sequation. Several restrictive features of Field's conclusions arethus modified and seem to make this instability rather more usefulin the study of instabilities in the interstellar gas than itappeared in Field's work.

Type of Award:DPhil Level of Award:Doctoral Awarding Institution: University of Oxford Notes:This thesis was digitised thanks to the generosity of Dr Leonard Polonsky


Haar, D. terMore by this contributor



Dr. D. ter HaarMore by this contributor


 Bibliographic Details

Issue Date: 1977Identifiers

Urn: uuid:fd9bfec4-ba22-4582-8d84-aad07c8b1bd0

Source identifier: 601870273 Item Description

Type: Thesis;

Language: eng Subjects: Valence fluctuations Coupling constants Gas dynamics Tiny URL: td:601870273


Author: Giaretta, David Leslie. - institutionUniversity of Oxford facultyMathematical and Physical Sciences Division - - - - Contributors



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