Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Distance to the Massive Galactic Cluster Westerlund 2 from a
  Spectroscopic and HST Photometric Study   We present a spectroscopic and photometric determination of the distance to
the young Galactic open cluster Westerlund 2 using WFPC2 imaging from the
Hubble Space Telescope and ground-based optical spectroscopy. HST imaging in
the F336W, F439W, F555W, and F814W filters resolved many sources previously
undetected in ground-based observations and yielded photometry for 1136 stars.
We identified fifteen new O-type stars, along with two probable binary systems,
including MSP 188 (O3 + O5.5). We fit reddened SEDs based on the Padova
isochrones to the photometric data to determine individual reddening parameters
$R_{V}$ and $A_{V}$ for O-type stars in Wd2. We find average values $\langle
R_{V} \rangle = 3.77 \pm 0.09$ and $\langle A_{V} \rangle = 6.51 \pm 0.38$ mag,
which result in a smaller distance than most other spectroscopic and
photometric studies. After a statistical distance correction accounting for
close unresolved binaries (factor of 1.08), our spectroscopic and photometric
data on 29 O-type stars yield that Westerlund 2 has a distance $\langle d
\rangle = 4.16 \pm 0.07$ (random) $+0.26$ (systematic) kpc. The cluster's age
remains poorly constrained, with an upper limit of 3 Myr. Finally, we report
evidence of a faint mid-IR PAH ring surrounding the well-known binary candidate
MSP~18, which appears to lie at the center of a secondary stellar grouping
within Westerlund 2.

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The categories of this paper are: quant-ph
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The categories of this paper are: cs
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The categories of this paper are: adap-org
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The categories of this paper are: q-alg
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The categories of this paper are: math-ph
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The categories of this paper are: econ
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The categories of this paper are: hep-ex
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The categories of this paper are: math
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The categories of this paper are: gr-qc
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The categories of this paper are: acc-phys
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The categories of this paper are: chem-ph
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The categories of this paper are: solv-int
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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: eess
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The categories of this paper are: stat