Discovery of calcite in the solar type protostar NGC 1333-IRAS 4

We present observations, obtained with ISO-LWS, of the continuum between 50–200 μm of the solar type protostar IRAS 4, in the NGC 1333 complex. The continuum presents an excess, around 95 μm, that we demonstrate must be a dust feature. We compared the 95 μm excess with the calcite feature at 92 μm and find that it fits the observations reasonably well. There may be a further contribution from hydrous silicates at ∼100 μm, but this seems a less robust result. The detected calcite mass is ∼8 × 10−5 M and represents about 1% of the warm (∼23 K) dust mass surrounding IRAS 4. This is only the second observation indicating the presence of carbonates outside the solar system, and the first revealing calcite in a young protostar. It is remarkable and intriguing that in all the objects where calcite has been detected so far, namely meteorites, planetary nebulae and IRAS 4, it represents from 0.3 to 1% of the dust mass. This new detection of calcite strengthens the claim by Kemper et al. (2002a) that calcite formation does not necessarely requires liquid water. We suggest that calcite forms at the surface of the grains, where water ice layers may locally have an enhanced mobility caused by heating due to hard X-rays emitted by the central object.