Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Physical Characteristics of the Small-Scale Interstellar Structure
  towards Mu Crucis   We present HST/GHRS echelle observations of multiple interstellar lines of
CI, MgI, CrII, and ZnII towards both stars in the mu Cru binary system. Despite
large differences in the profiles of the neutral species, no significant
variations between the stars are seen in the CrII and ZnII line profiles. In
particular, the ZnII absorption observed at -8.6 km/sec towards mu Cru is
constant despite greatly enhanced columns of the neutral species at this
velocity towards mu^1 Cru. An analysis of the fine-structure excitation of CI
in this cloud implies that the density is n_H < 250 cm^{-3}. From the lack of
variation in the (optical) NaI D2 line profiles towards mu^1 and mu^2 Cru in
spectra taken 21 months apart, we can place a lower limit to the size of the
structures of ~10 AU. These results are discussed in the context of recent
radio and optical studies of apparently pervasive high density small-scale
interstellar structure.

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The categories of this paper are: gr-qc
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The categories of this paper are: plasm-ph
36
The categories of this paper are: cmp-lg
28
The categories of this paper are: alg-geom
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The categories of this paper are: hep-lat
25
The categories of this paper are: patt-sol
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The categories of this paper are: q-bio
31
The categories of this paper are: ao-sci
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The categories of this paper are: eess
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The categories of this paper are: nucl-ex
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The categories of this paper are: adap-org
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The categories of this paper are: solv-int
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The categories of this paper are: math
18
The categories of this paper are: econ
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The categories of this paper are: hep-ph
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The categories of this paper are: stat