arXiv subjects

T. W. Hartquist

Publications and source records attributed to T. W. Hartquist.

47 records · Page 3Linked to original sources

arXiv2005

The Shocking Properties of Supersonic Flows: Dependence of the Thermal Overstability on M, alpha, and T_c/T_0

We investigate the Mach number dependence of the radiative overstability of shocks. We find that the stability of radiative shocks increases with decreasing Mach number, with the result that M=2 shocks require cooling exponents < -1.2 to be overstable. The frequency of oscillation of the fundamental mode has a strong Mach number dependence. We find that feedback between the cooling region and the cold dense layer (CDL) further downstream is a function of Mach number, with stronger feedback and oscillation of the boundary between the CDL and the cooling region occuring at lower Mach numbers. This feedback can be quantified in terms of the reflection coefficient of sound waves. An interesting finding is that the stability properties of low Mach number shocks can be dramatically altered if the shocked gas is able to cool to temperatures less than the pre-shock value, and it is probable that low Mach number astrophysical shocks will be overstable in a variety of situations. The results of this work are relevant to astrophysical shocks with low Mach numbers, such as supernova remnants (SNRs) immersed in a hot interstellar medium (e.g., within a starburst region), and shocks in molecular clouds, where time-dependent chemistry can lead to overstability. (Abridged)

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arXiv2004

Molecular abundance ratios as a tracer of accelerated collapse in regions of high mass star formation?

Recent observations suggest that the behaviour of tracer species such as N_2H+ and CS is significantly different in regions of high and low mass star formation. In the latter, N_2H+ is a good tracer of mass, while CS is not. Observations show the reverse to be true in high-mass star formation regions. We use a computational chemical model to show that the abundances of these and other species may be significantly altered by a period of accelerated collapse in high mass star forming regions. We suggest these results provide a potential explanation of the observations, and make predictions for the behaviour of other species.

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arXiv2003

The Formation of Broad Emission Line Regions in Supernova-QSO Wind Interactions II. 2D Calculations

One aspect of supernova remnant evolution that is relatively unstudied is the influence of an AGN environment. A high density ambient medium and a nearby powerful continuum source will assist the cooling of shocked ejecta and swept-up gas. Motion of the surrounding medium relative to the remnant will also affect the remnant morphology. In an extension to previous work we have performed 2D hydrodynamical calculations of SNR evolution in an AGN environment, and have determined the evolutionary behaviour of cold gas in the remnant. The cold gas will contribute to the observed broad line emission in AGNs, and we present preliminary theoretical line profiles from our calculations. A more detailed comparison with observations will be performed in future work. The SNR-AGN interaction may be useful as a diagnostic of the AGN wind.

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arXiv2003

The Thermal Stability of Mass-Loaded Flows

We present a linear stability analysis of a flow undergoing conductively-driven mass-loading from embedded clouds. We find that mass-loading damps isobaric and isentropic perturbations, and in this regard is similar to the effect of thermal conduction, but is much more pronounced where many embedded clumps exist. The stabilizing influence of mass-loading is wavelength independent against isobaric (condensing) perturbations, but wavelength dependent against isentropic (wave-like) perturbations. We derive equations for the degree of mass-loading needed to stabilize such perturbations. We have also made 1D numerical simulations of a mass-loaded radiative shock and demonstrated the damping of the overstability when mass-loading is rapid enough.

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arXiv2003

Diffusive grain-surface chemistry involving the atoms and diatomic molecules of two elements

A model of the grain surface chemistry involving the accretion of atoms of two different elements, X and Y, and their reactions to form species X_2, XY, and Y_2 was examined for a wide range of choices for the values of its three free parameters - the accretion rate of X and Y, the desorption rate of X and the grain surface sweeping time of Y, all considered relative to the grain surface sweeping rate of X. Relative production rates of the diatomics were calculated with five methods involving, respectively, a high-order truncation of the master equation, a low-order truncation of the master equation, the standard deterministic rate equation approach, a modified rate equation approach and a set of approximations which are in some cases appropriate for accretion dominated chemistry. The accuracies of the relative production rates calculated with the different methods were assessed for the wide range of model parameters. The more accurate of the low-truncation master equation calculations and the standard deterministic rate equation approach gives results which are in most cases within ten or twenty per cent of the results given by the high-truncation master equation calculations. For many cases, the more accurate of the low order truncation and the standard deterministic rate equation approaches is indicated by a consideration of the average number of atoms of the two species on the grain's surface.

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arXiv2003

The Evolution of Mass Loaded Supernova Remnants. II. Temperature Dependent Mass Injection Rates

We investigate the evolution of spherically symmetric supernova remnants in which mass loading takes place due to conductively driven evaporation of embedded clouds. Numerical simulations reveal significant differences between the evolution of conductively mass loaded and the ablatively mass loaded remnants studied in Paper I. A main difference is the way in which conductive mass loading is extinguished at fairly early times, once the interior temperature of the remnant falls below ~10 million K. Thus, at late times remnants that ablatively mass load are dominated by loaded mass and thermal energy, while those that conductively mass load are dominated by swept-up mass and kinetic energy. Simple approximations to the remnant evolution, complementary to those in Paper I, are given.

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arXiv2001

Self-similar evolution of wind-blown bubbles with mass loading by hydrodynamic ablation

We present similarity solutions for adiabatic bubbles that are blown by winds having time independent mechanical luminosities and that are each mass-loaded by the hydrodynamic ablation of distributed clumps. The mass loading is `switched-on' at a specified radius (with free-expansion of the wind interior to this point) and injects mass at a rate per unit volume proportional to M^delta r^lambda where delta = 4/3 (1) if the flow is subsonic (supersonic) with respect to the clumps. In the limit of negligible mass loading a similarity solution found by Dyson (1973) for expansion into a smooth ambient medium is recovered. The presence of mass loading heats the flow, which leads to a reduction in the Mach number of the supersonic freely-expanding flow, and weaker jump conditions across the inner shock. In solutions with large mass loading, it is possible for the wind to connect directly to the contact discontinuity without first passing through an inner shock. For a solution that gives the mass of swept-up ambient gas to be less than the sum of the masses of the wind and ablated material, lambda < -2. Maximum possible values for the ratio of ablated mass to wind mass occur when mass loading starts very close to the bubble center and when the flow is supersonic with respect to the clumps over the entire bubble radius. The maximum temperature in the bubble often occurs near the onset of mass loading, and in some cases can be many times greater than the post-inner-shock temperature. Our solutions are relevant to eg stellar wind-blown bubbles, galactic winds, etc. This work complements Pittard et al (2001) where it was assumed that clumps were evaporated through conductive energy transport.

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arXiv2001

The Formation of Broad Emission Line Regions in Supernova-QSO Wind Interactions

We show that a cooled region of shocked supernova ejecta forms in a type II supernova-QSO wind interaction, and has a density, an ionization parameter, and a column density compatible with those inferred for the high ionization component of the broad emission line regions in QSOs. The calculations are based on the assumption that the ejecta flow is described initially by a similarity solution investigated by Chevalier (1982) and Nadyozhin (1985) and is spherically symmetric. Heating and cooling appropriate for gas irradiated by a nearby powerful continuum source is included in our model, together with reasonable assumptions for the properties of the QSO wind. The model results are also in agreement with observational correlations and imply reasonable supernova rates.

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arXiv2000

Self-Similar Evolution of Wind-Blown Bubbles with Mass-loading by Conductive Evaporation

We present similarity solutions for mass-loaded adiabatic bubbles that are blown by winds having time dependent mechanical luminosities. We consider mass-loading through conductive evaporation of clumps. In the limit of little mass loading a similarity solution found by Dyson (1973) for expansion into a smooth ambient medium is recovered. We find that the Mach number in the shocked mass-loaded wind shows a radial dependence that varies qualitatively from solution to solution. In some cases it is everywhere less than unity in the frame of the clumps being evaporated, while in others it is everywhere greater than unity. In some solutions the mass-loaded shocked wind undergoes one or two sonic transitions in the clump frame. Maximum possible values of the ratio of evaporated mass to stellar wind mass are found as a consequence of the evaporation rates dependence on temperature and the lowering of the temperature by mass-loading. Mass-loading tends to reduce the emissivity in the interior of the bubble relative to its limb, whilst simultaneously increasing the central temperature relative to the limb temperature.

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arXiv2000

Models of Wave Supported Clumps in Giant Molecular Clouds

We present plane-parallel equilibrium models of molecular clumps that are supported by Alfven waves damped by the linear process of ion-neutral friction. We used a WKB approximation to treat the inward propagation of waves and adopted a realistic ionization structure influenced by dissociation and ionization due to photons of external origin. The model clumps are larger and less centrally condensed than those obtained for an assumed ionization structure, used in some previous studies, that is more appropriate for dark regions.

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arXiv2000

Magnetic ionization fronts II: Jump conditions for oblique magnetization

We present the jump conditions for ionization fronts with oblique magnetic fields. The standard nomenclature of R- and D-type fronts can still be applied, but in the case of oblique magnetization there are fronts of each type about each of the fast- and slow-mode speeds. As an ionization front slows, it will drive first a fast- and then a slow-mode shock into the surrounding medium. Even for rather weak upstream magnetic fields, the effect of magnetization on ionization front evolution can be important. [Includes numerical MHD models and an application to observations of S106.]

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