ASCI/Alliances Center for Astrophysical Thermonuclear Flashes

Evaporation of Clouds in Thermally Conducting, Radiative Supernova Remnants

S. Orlando(1), G. Peres(2), F. Reale(2), R. Rosner(3), T. Plewa(3), & A. Siegel(3)

http://www.astropa.unipa.it http://flash.uchicago.edu

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(1) INAF – Osservatorio Astronomico di Palermo “G.S. Vaiana”, Palermo, ITALY(2) Dip. Di Scienze Fisiche ed Astronomiche – Sez. Di Astronomia, Univ. di Palermo, Palermo, ITALY

(3) FLASH Center, The University of Chicago, Chicago, IL, USA

Panel 1

We assume a planar supernova shock front impacting an isolated spherical cloud at time t = t0.

Introduction: It has long been suspected that one of the major reasons for the great morphological complexity of supernova remnants (SNR) is the inhomogeneity of the interstellar medium (ISM) into which the shock wave expands.

One crucial point not yet fully explored is the detailed treatment of the radiative losses together with the thermal conduction in SNR, including the effects of “saturation” (flux-limiting) (Cowie and MacKee 1977, ApJ 211, 135).

In the present study we investigate the role of thermal conduction and of radiative losses in the interaction of the SN shock wave with an interstellar gas cloud. In the following, we show the shock-cloud interaction computed with the FLASH code after including these important physical processes.

The code setup includes thermal conduction with “saturation, and the radiative losses from an optically thin plasma; gravity and magnetic fields are assumed to be negligible.

Using the FLASH code, we can isolate the physical effects of radiation and thermal conduction on the shock-cloud interaction by selectively turning these physical processes “on” and “off”.

The figure on the right shows four different cases of the mass density evolution after 9000 yr from the beginning of the interaction: neither thermal conduction nor radiative losses (panel A); radiative losses but no thermal conduction (panel B); thermal conduction but no radiation (panel C); and both radiation and conduction (panel D).

Panel A: In this case, the cloud is destroyed by the combined action of the Kelvin-Helmholtz (KH) and Rayleigh-Taylor (RT) instabilities. The shock transmitted into the cloud has a temperature of ~ 105 K. The bow shock reflected back into the intercloud medium undergoes so-called conical self-reflection (Tenorio-Tagle & Rozyczka 1984, A&A 141, 351). It is evident that cloud fragmentation is driven by dynamical instabilities.

Panel B: This figure shows that radiative losses strongly affect the dynamics of the

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(A) (B)

(C) (D)

interaction. In fact, the shock transmitted into the cloud is affected by strong cooling due to radiative losses, which causes a mass density increase and a temperature decrease to values where the radiative losses are more efficient: the radiative cooling leads to a thermal instability (which cannot be stabilized in the absence of thermal conduction). As a consequence, the shock transmitted into the cloud is weakened by the radiative losses.

Panel C: Thermal conduction is a diffusive process, which smooths the temperature profile, causing modification of the pressure field and smoothing of density gradients. As a consequence, the KH and RT instabilities are strongly suppressed by thermal conduction: the conduction timescale is much shorter than the KH and RT instability timescales. However, the general evolution of the shock-cloud interaction is similar to that analyzed in panel A: the transmitted shock propagates into the cloud, and is characterized by values of density and temperature comparable with those found in case A.

Panel (D): When radiation losses and thermal conduction are both included, thermal conduction limits the development of the thermal instability due to the radiative losses. For the example studied here, we conclude that the radiative losses have a crucial role in the dynamics of the shock-cloud interaction, strongly affecting the structure of the shock transmitted into the cloud; we also see that in this case the cooling timescale is still shorter than the conduction timescale.

Panel 3The results shown on the right illustrate the resulting evolution of mass density (upper panels) and temperature (lower panels), at three different times; t=0 marks the beginning of the shock-cloud interaction. The simulation takes into account both thermal conduction and radiative losses.

After the SN shock front strikes the cloud, a shock is transmitted into the cloud, and a bow shock is reflected back into the intercloud medium. The flow around the cloud converges on the z axis; and the shock deposits vorticity which in long term promotes cloud destruction (Klein et al. 1995, ApJ 420, 213). Due to the values of density and temperature of the shocked cloud plasma, the radiative losses are very efficient, causing rapid cooling of the plasma (thermal instability) partially limited by thermal conduction.

The result is that no shock propagates into the cloud which is compressed and gradually ablated at the bottom.