A 35 M0 star

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Using mass-loss data kindly provided to us by Norbert Langer, we have modeled the evolution of the medium around a 35 M0. star, and the further interaction of the shock wave with this medium once the star explodes as a SN. The star goes through the sequence O-Star, Red-Supergiant Star (RSG) and Wolf-Rayet (WR) star. Below we describe, mainly through images of the fluid density, the subsequent evolution of the CSM around the star.

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Fig. 8.4. Time-sequence of images of the formation of the CSM around a 35 M0 O-Star during the main sequence.

Main Sequence (MS) Stage

The wind from the star, with velocity of a few (3-4) thousand km/s and mass loss rate on the order of 10-7 M0/yr expands into a medium with density of about 1 particle/cc, giving rise to a bubble about 74 pc in radius. Fig 8.4 shows (from left to right) a time-sequence of density images of the formation of the MS bubble. Note that the main-sequence shell is unstable to a Vishniac-type thin-shell instability. The density inhomogeneities lead to pressure fluctuations which propagate within the interior, which soon develops into a turbulent state. The evolution of these perturbations distinguishes our results from those of Garcia-Segura et al. (1996), who considered the MS shell to be stable and therefore assumed spherical symmetry. However the 2D structure is quite different, and has significant implications for the succeeding evolution of the bubble.

Red-Supergiant (RSG) Stage

In the RSG stage the wind velocity falls to a low value of about 75 km/s, whereas the mass loss rate jumps up to a few times 10-5 M0/yr. A new pressure equilibrium is established, and a RSG shell is formed in the interior, which is also unstable to thin-shell instabilities. Fig 8.5 (frames 1 and 2) shows images of the density during the RSG evolution.

Wolf-Rayet (WR) Phase

The wind velocity in the WR phase climbs back up to almost 3000 km/s, whereas the mass loss rate drops by only a factor of a few from the RSG stage. The momentum of the WR wind is then about an order of magnitude larger than that of the RSG wind, and the wind pushes the RSG shell outwards, simultaneously causing it to fragment (Fig 8.5, frame 3). The RSG wind material is mixed in with the rest of the MS material (Fig 8.5, frame 4), a key result since the RSG wind velocity was so low that the material by itself could not have gone very far. Out of ~26M0 of material lost in the wind, about 19 M0 is lost in the RSG stage, so much of the material within the nebula is composed of matter lost in the RSG phase.

Fig. 8.5. The first two density images from left show the formation of the inner RSG shell, which is unstable to thin-shell perturbations. The next two display the onset of the WR wind and its collision with the RSG shell, causing it to fragment and the RSG material to be mixed in with the rest of the nebula.

Fig. 8.5. The first two density images from left show the formation of the inner RSG shell, which is unstable to thin-shell perturbations. The next two display the onset of the WR wind and its collision with the RSG shell, causing it to fragment and the RSG material to be mixed in with the rest of the nebula.

Fig. 8.6. Time-sequence of pressure images of the interaction of the SNR shock with the WR bubble. HPR represents the high-pressure region between the inner and outer SNR shocks. Note the rippled structure of the outer shock from frame 2 onwards, and its interaction at different times with various parts of the shell.

Fig. 8.6. Time-sequence of pressure images of the interaction of the SNR shock with the WR bubble. HPR represents the high-pressure region between the inner and outer SNR shocks. Note the rippled structure of the outer shock from frame 2 onwards, and its interaction at different times with various parts of the shell.

SN- CSM Interaction

At the end of the WR phase the stellar mass is about 9.1 M0. We assume that about 1.4 Mq remains as a neutron star, and the remaining mass is ejected in a supernova explosion, with a density profile that goes as a power-law in the outer parts, with a power-law index of 7. The interaction soon forms the usual double-shocked structure. In Figure 8.6 we show images of the fluid pressure. This variable is chosen to clearly illustrate the shocked region between the inner and outer shocks. The shock starts off as a spherical shock (Fig 8.6a), but the pressure within the turbulent interior soon causes it to become rippled (Fig 8.6b). The corrugated shock structure collides with the boundary of the bubble in apiecemeal fashion (Fig 8.6c), and as each small part collides with the outer boundary, a reflected shock arises in that region. There exist many pieces of reflected shock that arise from various interactions, have different velocities, and consequently reach the inner boundary at different times. The thermalization of the material then occurs in different stages, and X-ray images will reveal a very complicated structure which will differ considerably on scales of tens to hundreds of years.

HST images of SN 1987A have revealed the presence of various bright spots around the circumstellar ring, presumably due to the interaction of the SN shock front with the equatorial ring structure (eg. Sugerman et al. 2002). The collision of a highly wrinkled shock with various parts of the circumstellar shell, leading to the different parts brightening up at different times, is very similar to the current situation of the shock front in SN 1987A. The case of SN 1987A however is more complicated in that the region interior to the ring is presumed to be an ionized HII region (Chevalier & Dwarkadas 1995). It is possible though that an aspherical HII region would serve only to accentuate the asphericity in the shock front. The simulation described herein is for a 35 MQ star, whereas in 87A the progenitor star was less massive, and possibly part of a binary system. Nevertheless the similarities are striking, and suggest the existence of such wrinkled shock fronts when SNe evolve in wind-blown bubbles.

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