Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Interstellar Titanium in the Galactic Halo   We present observations of Ti~II~$\lambda 3384$ absorption towards 15 distant
stars in the Galactic halo and the Magellanic Clouds. These new data extend
existing surveys of the distribution of Ti$^+$ to larger distances from the
plane of the Galaxy than sampled previously, allowing the scale height of
titanium to be determined for the first time. We find $h_{Ti^+} = 1.5 \pm
0.2$~kpc, a value which although greater than those of other tracers of neutral
gas, is not as large as had been suspected. We interpret the extended
distribution of Ti$^+$ as an indication that its severe depletion in
interstellar clouds in the disk is reduced at the lower densities prevailing in
the halo. The data are consistent with a simple power-law dependence of the Ti
abundance on the ambient density, with exponent $k \simeq -1$. If the model is
correct, it implies that refractory elements like Ti are fully returned to the
gas phase at distances beyond $\sim 1$ kpc from the plane of the Galaxy.

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The categories of this paper are: gr-qc
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The categories of this paper are: nucl-th
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The categories of this paper are: hep-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: stat
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The categories of this paper are: alg-geom
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The categories of this paper are: acc-phys
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The categories of this paper are: nlin
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The categories of this paper are: chem-ph
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The categories of this paper are: supr-con
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The categories of this paper are: mtrl-th
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The categories of this paper are: cond-mat
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The categories of this paper are: q-bio
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The categories of this paper are: atom-ph
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The categories of this paper are: hep-lat
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The categories of this paper are: patt-sol