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Astron. Astrophys. 361, 303-320 (2000)


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Origin of H I clouds in the Local Bubble

I. A hydromagnetic Rayleigh-Taylor instability caused by the interaction between the Loop I and the Local Bubble *

D. Breitschwerdt, M.J. Freyberg and R. Egger

Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85740 Garching, Germany (breitsch,mjf,roe@mpe.mpg.de)

Received 4 January 1999 / Accepted 4 July 2000

Abstract

We present a model to explain the origin of local clouds, including the so-called Local Fluff surrounding the solar system. We present observational evidence from ROSAT PSPC data, that manifest the existence of an interaction shell between our local interstellar bubble and the adjacent Loop I superbubble. A linear stability analysis of this compressed interaction shell has been performed. We show that due to the overpressure in Loop I, a hydromagnetic Rayleigh-Taylor instability will operate, even in the presence of an obstructing and stabilizing tangential magnetic field. The field acts like surface tension in an ordinary fluid, thus inhibiting the growth of the smallest wavelengths. It is shown that the most unstable mode has a growth time of the order of [FORMULA] years (depending on the magnetic field strength), in agreement with the interaction time between the two bubbles being several times longer. The wavelength of the fastest growing mode is of the order of the thickness [FORMULA] of the interaction shell ([FORMULA] pc), or smaller depending on (i) the field strength, (ii) the angle [FORMULA] between the wave vector of the perturbations and and the direction of the unperturbed magnetic field, and (iii) the pressure gradient across the shell. Once the instability becomes fully non-linear, neutral blobs of size [FORMULA] are predicted to detach from the interaction shell, aided by magnetic reconnection, and travel ballistically towards the direction of the Sun. We are thus able to identify the local clouds with products of such instabilities. Our calculations show that we can best reproduce the observed characteristics of the Local Fluff (e.g. its radius, mass, temperature, density and velocity with respect to the Sun), for a range of typical parameters of field strength [FORMULA]G, density [FORMULA] in the interaction shell and a perturbation angle [FORMULA].

We have also analyzed ROSAT PSPC data to show that there is a corresponding depression of absorbing HI column density in the region where the bubbles interact and the blobs detach. Moreover it is shown that this is accompanied by spectral variations in the ROSAT R1 and R2 bands, which indicate depressions or holes in the interaction region between the two bubbles.

Key words: instabilities – Magnetohydrodynamics (MHD) – ISM: bubbles – ISM: clouds – ISM: magnetic fields – X-rays: ISM

* Paper dedicated to the memory of Franz D. Kahn

Present address: Department of Astrophysical Sciences, 108 Peyton Hall, Princeton University, Princeton, NJ 08544, USA (breitsch@astro.princeton.edu)

Send offprint requests to: D. Breitschwerdt

© European Southern Observatory (ESO) 2000

Online publication: September 5, 2000

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