A&A 486, 779-780 (2008)
DOI: 10.1051/0004-6361:20065436e

Erratum

The shape and composition of interstellar silicate grains

M. Min1 - L. B. F. M. Waters1,2 - A. de Koter1 - J. W. Hovenier1 - L. P. Keller3 - F. Markwick-Kemper4


1 - Astronomical institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
2 - Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200B, 3001 Heverlee, Belgium
3 - Mail Code KR, NASA Johnson Space Center, Houston, TX 77058, USA
4 - University of Virginia, Department of Astronomy, PO Box 400325, Charlottesville VA 22904-4325, USA

A&A, 462, 667-676 (2007), DOI: 10.1051/0004-6361:20065436


Key words: dust, extinction - infrared: ISM - errata, addenda


The paper ``The shape and composition of interstellar silicate grains'' by Min et al. (2007) discusses the composition, shape distribution, and the magnesium and iron content of interstellar silicate grains. These properties of the interstellar grains were derived from infrared spectral observations of absorption by interstellar dust. In Sects. 4 and 5, the implications of the findings of this study are discussed with respect to processing in various environments. In particular the relation with glass with embedded metal and sulfides (GEMS) is discussed. Our survey of the relevant literature on GEMS grains was incomplete and overlooked some important observations that we clarify in detail below. Despite these changes, we would like to stress that the results for the composition, magnesium and iron content, and the shape distribution of interstellar silicates remain correct. As a result, the conclusions unrelated to the GEMS grains remain unaltered.

In addition to the corrections for Sect. 4.3, we would like to point out that the SiC found by Bernatowicz et al. (1987) was not detected in an IDP, as we suggested, but in the Murray carbonaceous chondrite.

We first outline the shortcomings in the discussion in that paper. Second, we discuss the implications of these corrections and clarifications for the conclusions we draw concerning the nature of GEMS.

Corrections and clarifications for Sect. 4.3

The nature of GEMS

We now evaluate the conclusions drawn by Min et al. (2007) concerning the nature of GEMS taking the above corrections to the GEMS data into account. As we already pointed out in the original paper, GEMS with anomalous oxygen isotopes have been reported. This indicates that these GEMS originate in the outflows of AGB stars and red giants. Therefore, they must have travelled through the ISM from their formation region to the molecular cloud from which the Solar System formed. Thus, at some point they have been part of the interstellar dust population. However, they are only a small fraction of the total GEMS population.

Several differences between the grains in the diffuse ISM and GEMS can be pointed out:

On the other hand, similarities between GEMS grains and the diffuse ISM dust grains can be reported: We note that, though the bulk O/Si ratio in GEMS and ISM silicates is similar, our analysis shows that the ISM grains are consistent with stoichiometric silicates, while GEMS grains are, on average, not stoichiometric; instead they are enriched in oxygen (see Ishii et al. 2008; and the comment to Min et al. 2007, by Bradley & Ishii). Currently there are no laboratory spectra available for oxygen-enriched amorphous silicates, so a comparison cannot be made. We thus conclude that a direct comparison between the bulk O/Si in GEMS grains and ISM silicates is, at this point, ambiguous.

These arguments taken together suggest that, although there are interesting similarities between the composition of GEMS and those of the silicates in the diffuse ISM, the overall properties of GEMS are not similar to the bulk grain properties in the diffuse ISM. Though it is clear that the GEMS with anomalous oxygen isotopes have resided in the ISM, therefore have been part of the population of ISM grains, the origin of isotopically solar GEMS remains an open question. We would like to point out that no significant difference is found between the isotopically solar and the anomalous GEMS grains, suggesting a similar formation mechanism, and possibly a similar origin. Our results, on the other hand, suggest that GEMS, with the composition as found in our Solar System, are inconsistent with the bulk composition of the grains in the diffuse ISM. This indicates that diffuse interstellar silicates are not predominantly GEMS. However, we would like to stress that we cannot turn the argument around so that, from our results, we cannot exclude the possibility that a significant fraction of GEMS grains have an interstellar origin, as long as they do not dominate the silicate grain population. They may either be of circumstellar origin, but having resided in the ISM for a long period ($\sim$$10^8\,$yr), or they may have formed in the collapsing solar molecular cloud. In addition, recent work by Keller et al. (2005) and Keller & Messenger (2007) on GEMS grains suggests that most are products of the early solar system.

Acknowledgements

We are grateful to John Bradley for pointing out the above inaccuracies and misquotations in the discussion and for providing us with a more complete picture of this fascinating topic.

References

 

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