Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: UBVRI photometry and spectroscopy of the young open cluster Haffner 18   UBVRI CCD photometry of a $2.^{\prime}1 \times 3.^{\prime}3$ field centered
on the young open cluster Haffner~18 is presented and discussed. Spectroscopic
classification of seven stars is also provided. 44 clusters members are
identified, the earliest type being O6. The distance to the cluster is found to
be 6.3 kpc, corresponding to a galactocentric distance of 12.7 kpc (for a Sun
galactocentric distance of 8.5 kpc). Excellent fit to the observed main
sequence is achieved by a solar composition isochrone of 2$\times 10^6$ years
reddened by E$_{B-V}$=0.62 mag. Differential reddening of intra-cluster origin
is present. Pre-main sequence members are likely to be present over the 6 mag
range explored by our observations (reaching down to earliest A spectral
types). The presence of differential reddening and pre-main sequence members
agrees with the evidence for a bright parent nebulosity embedding the whole
cluster. The radial velocity of the cluster is consistent with the Hron (1987)
model of galactic rotation.

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The categories of this paper are: q-fin
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The categories of this paper are: alg-geom
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The categories of this paper are: patt-sol
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The categories of this paper are: q-bio
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The categories of this paper are: math
18
The categories of this paper are: econ
31
The categories of this paper are: ao-sci
1
The categories of this paper are: cs
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The categories of this paper are: math-ph
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The categories of this paper are: eess
4
The categories of this paper are: cond-mat
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The categories of this paper are: nucl-ex
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The categories of this paper are: plasm-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: adap-org
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The categories of this paper are: atom-ph