5 Discussion

5.1 Dust in the pre supernvova CSM

The modelling of the ISOCAM measurements (Sect. 3), combined with the calculations of grain destruction due to evaporation and sputtering (Sect. 4) allow us to infer grain abundances in the pre supernova CSM as well as to make crude estimates of maximum grain sizes and grain composition.

   

Table 4: Solutions for dust in the pre-supernova CSM.

model	comp.	mixturea			$10^{-4}~M_{\rm d}/M_{\rm gas}$
		0.25b	0.10	0.06	0.25	0.10	0.06
I	gra.	-			1.1	1.2	1.3
I	si.+iron	2.15	3.13	0.44	2.3	5.4	152
II	si.+iron	2.45	-	-	2.8	-	-
I	si.+gra.	0.47	2.02	25.4	1.4	2.4	20.
II	si.+gra.	1.42	4.01	62.3	1.8	3.6	48.

^a Relative mass of silicate to iron or silicate to graphite.
^b Maximum grain size in microns.

This information was derived for the best solutions ( $\chi^2_{\nu}<3$, Table 2) using the mass loss estimates given in Table 3 and is tabulated in Table 4. As expected the main parameter determining the composition and abundance of grains in the pre-supernova CSM is the maximum grain size. It is interesting to compare these quantities with observed and derived properties of circumstellar dust in stellar outflows.

We are not aware of direct measurements in the IR and submm regimes of dust abundances in the winds of LMC stars. We will simply estimate it by assuming that the dust abundance in the winds is proportional to the metallicity of the ISM. This is supported by Woods et al. (1992) who found that the outflow velocity of oxygen rich stars in the LMC is significantly lower than for oxygen rich stars in our galaxy and suggested that the lower velocity is due to the lower dust-to-gas ratio caused by the lower metallicity of the LMC. The same has also been proposed for high luminosity stars in the galactic anticentre which show only a modest outflow velocity (Habing et al. 1994). These works were confirmed by van Loon (2000), who found by comparing obscured Asymptotic Giant Branch stars of our galaxy, the Large and the Small Magellanic Clouds that the inferred dust-to-gas ratio of both carbon and oxygen rich stars is approximately proportional to the initial metallicity.

The winds of evolved carbon rich stars in our galaxy have generally been found to have dust-to-gas ratios in the range $\sim$0.1 to $\sim$$1\%$ (Jura 1986; Martin & Rogers 1987; Griffin 1990; Knapp et al. 1993; Bagnulo et al. 1995; and Olofsson et al. 1993; as quoted by Hiriart & Kwan 2000). On the other hand Hiriart & Kwan (2000) estimated from a subsample of Olofsson et al. a maximum dust-to-gas ratio of only $\sim$$0.1\%$. For the galactic oxygen rich star OH 231.8+4.2, Knapp et al. (1993) obtained a dust abundance of $\sim$0.7%.

Scaling by the factor of 2 between the metallicity of the LMC and the metallicity of the solar vicinity (Russell & Dopita 1992) we would expect on this basis dust to gas ratios for LMC carbon stars in the range $\sim$ $0.05{-}0.5\%$ and for LMC oxygen stars of order $\sim$0.3%. Although clearly very uncertain, these numbers are nevertheless only consistent with a subset of the solutions of the pre-supernova gas-to-dust ratios in the CSM of SN 1987A given in Table 4. In particular, all the pure graphite solutions appear underabundant in dust by an order of magnitude compared with expectations for the winds of carbon stars. This confirms the expectation that the dust should in fact be silicate rich on the basis of the gas phase abundances in the inner ring (Lundqvist & Fransson 1996; Sonneborn et al. 1997).

The solutions for silicate-iron and silicate-graphite mixtures in Table 4 are consistent with the expected dust to gas ratios of $\sim$$0.3\%$ for oxygen-rich stars provided the maximum grain size in the CSM of SN 1987A was smaller than $0.1~\mu{\rm m}$. This is in accordance with findings, both theoretical and observational, that cool stars eject mainly small grains, irrespective of composition. An upper limit of $0.14~\mu{\rm m}$ on the maximum sizes of grains around oxygen rich mass-losing stars was found by (Jura 1996). A maximum grain smaller than $\sim$ $0.1~\mu{\rm m}$ is also consistent with the sizes found for the dust ejected from the carbon rich star IRC+10216 (Martin & Rogers 1987; Griffin 1990; Jura 1994; Bagnulo et al. 1995) or the typical sizes around evolved carbon stars derived by Hiriart & Kwan (2000).

On the other hand, this small maximum grain size contrasts with results for oxygen rich stars derived from their ultraviolet extinction. Rogers et al. (1983) suggested that the grains around $\mu$ Cep should be in the range 0.1  $\mu {\rm m}$ to 0.5  $\mu {\rm m}$. Based on extinction measurements made for $\alpha$ Sco, Seab & Snow (1989) concluded that the grains in the circumstellar environments of cool oxygen rich giants should be larger than $\sim$ $0.08~\mu{\rm m}$ and possibly enrich the ISM with grains as large as $\sim$ $1~\mu{\rm m}$. If this were also the case for the RSG wind of the progenitor of SN 1987A, the low dust abundance found by ISOCAM could neither be explained by evaporation nor by sputtering. It might then have had to have been intrinsically low. Alternatively, the grain abundance could have been reduced by radiation pressure (Turner & Pearce 1992) after the progenitor evolved to its final BSG phase 20 000 years ago (Crotts & Heathcote 1991). However for this, the coupling of the grains to the gas would have had to have been weak. Another scenario for reducing the grain abundance might be a mixing of the material of the BSG wind, with a negligible dust abundance, with that of the RSG wind at a dynamically unstable interface between the winds (García et al. 1996a, 1996b).

5.2 Iron abundance in the shocked CSM

On the basis of a spherically symmetrical hydrodynamical simulation of the interaction of the blast wave with the HII region, Borkowski et al. (1997) derived an upper limit on the gas phase iron abundance in the HII region of only 0.1 of the solar iron abundance from the X-ray spectrum measured with the ROSAT satellite (Hasinger et al. 1996). Comparing Chandra data with a plane parallel shock model Park et al. (2002) found a value of 0.07 of solar for the iron abundance in the X-ray emitting region between epochs 1999 and 2001. These gas phase abundances for iron are lower by a factor of at least $\sim$3compared to the iron abundance in the LMC from Russell & Dopita (1992), which is $36\%$ of the solar abundance (determined from the photosphere; Anders & Grevesse 1989). This prompted Borkowski et al. to suppose that most of the iron was condensed into grains. However, this is not supported by the ISOCAM measurements. An upper limit for the mass of iron condensed in grains can be taken from the calculations for pure iron grains in the shocked CSM tabulated in Table 2. Taking for iron grain masses of $1.5\times 10^{-6}~{M_{\odot}}$ (model I) or $1.1\times 10^{-6}~{M_{\odot}}$ (model II) and the iron abundance in the LMC to be $n_{\rm Fe}/n_{\rm H}=1.7\times 10^{-5}$ (Russell & Dopita 1992), the fraction of iron in solid form is at maximum $\sim$$32\%$ (model I) or $\sim$$45\%$ (model II). For the better fitting silicate/iron mixtures the upper limits will be still lower. On the basis of this evidence, the HII-region would underabundant in iron, whether in gaseous or condensed form. Further X-ray and infrared observations would be valuable to investigate this problem.

5.3 Outlook

In the short term, further observations to follow the MIR light curve will provide information on the dependence on radial position of the dust-to-gas ratios in the shocked HII region for comparison with model predictions. On the one hand there will be a tendency for the overall volume-averaged dust-to-gas ratios to be lowered due to sputtering at later epochs, if the upstream dust abundance is constant. On the other hand, the survived dust abundance after the UV-flash will increase with radius, especially for silicate and iron grains. Another potential reason for increasing dust abundances with radius could be a flushing out of grains from the inner regions of the CSM through radiation pressure, after the RSG turned into a BSG. This might also offer an explanation for the puzzle of the low iron abundance in both grains and gas inferred from the ISOCAM and ROSAT and CHANDRA data, as discussed above. As already stated this may require a weak coupling of the grains to the gas.

Observations of the brightening so-called "hot spots'' seen in the optical (Lawrence et al. 2000) show that the blast wave is already interacting at certain places with dense material of the thick inner ring. The origin of the thick inner ring, is unknown. It is also not clear whether the MIR echo seen after 580 days (Roche et al. 1993) can be attributed to the ring. If the ring is composed of material from the red supergiant phase of the progenitor star (see e.g. Fransson et al. 1989), then one might anticipate a rapid increase in MIR luminosity with time, which would soon dominate the continuum emission from the HII-region. The higher gas densities in the ring might lead to an accompanying increase in grain temperature, which would allow thermal dust emission from the thick inner ring to be distinguished from an increasing contribution from the HII region. If, on the other hand, the thick inner ring is composed of material from the companion star in the putative binary system (e.g. Podsiadlowski 1992) then one might speculate that the dust abundance and composition of the ring might deviate markedly from that of the HII region discussed in this paper.