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arXiv 1302.0863astro-ph.SR
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从光谱和HST测光研究到大质量星系团Westerlund 2的距离

The Distance to the Massive Galactic Cluster Westerlund 2 from a Spectroscopic and HST Photometric Study

Carlos A. Vargas Alvarez, Henry A. Kobulnicky, David R. Bradley, Sheila J. Kannappan, Mark A. Norris, Richard J. Cool, Brendan P. Miller

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58 pages, 20 figures
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摘要：

我们提出了一个光谱和光度测定的距离，年轻的银河系疏散星团Westerlund 2使用WFPC 2成像从哈勃太空望远镜和地基光学光谱。使用F336 W、F439 W、F555 W和F814 W滤光片的HST成像解决了许多以前在地面观测中未被发现的源，并为1136颗恒星提供了测光。我们发现了15颗新的O型星，以及两个可能的双星系统，包括MSP 188（O3 + O5.5）。我们适合红SED的基础上的帕多瓦等时线的光度数据，以确定个人的红化参数$R_{V}$和$A_{V}$的O型星在Wd 2。我们发现平均值$\langle R_{V} \rangle = 3.77 \pm 0.09$和$\langle A_{V} \rangle = 6.51 \pm 0.38$ mag，这导致比大多数其他光谱和光度研究更小的距离。经过统计距离校正后，我们对29颗O型星的光谱和光度数据得出，Westerlund 2的距离为$\langle d \rangle = 4.16 \pm 0.07$（随机）$+0.26$（系统）kpc。星团的年龄限制仍然很差，上限为300万年。最后，我们报告了一个微弱的中红外PAH环周围的著名的二元候选MSP~18，这似乎是在Westerlund 2内的次级恒星群的中心。

英文摘要：

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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