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

Abstract

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 $⟨R_V⟩=3.77\pm 0.09$ and $⟨A_V⟩=6.51\pm 0.38$ 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 $⟨d⟩=4.16\pm 0.07$ (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.

58 pages, 20 figures

References in corpus (17)

• An Analysis of the Shapes of Interstellar Extinction Curves. V. The IR-Through-UV Curve Morphology
• UBVJHK synthetic photometry of Galactic O stars
• Molecular clouds towards RCW 49 and Westerlund 2; Evidence for cluster formation triggered by cloud-cloud collision
• The VLT-FLAMES survey of massive stars: Wind properties and evolution of hot massive stars in the LMC
• An Updated Look at Binary Characteristics of Massive Stars in the Cygnus OB2 Association
• Open clusters in dense fields: the importance of field-star decontamination for NGC5715, Lynga4, Lynga9, Trumpler23, Trumpler26 and Czernik37
• An Analysis of the Shapes of Interstellar Extinction Curves. VI. The Near-IR Extinction Law
• The secrets of the nearest starburst cluster: II. The present-day mass function in NGC 3603
• Detection of extended very-high-energy gamma-ray emission towards the young stellar cluster Westerlund 2
• Fermi-LAT Search for Pulsar Wind Nebulae around gamma-ray Pulsars
• Globular Clusters in the Outer Galactic Halo: AM-1 and Palomar 14
• Chandra monitoring of the very massive binary WR20a and the young massive cluster Westerlund2
• On the Distance and Molecular Environment of Westerlund 2 and HESS J1023-575
• A spectroscopic investigation of early-type stars in the young open cluster Westerlund 2
• Investigating the age and structure of the infrared old open clusters LK1, LK10, FSR1521 and FSR1555
• WR 38/38a and the ratio of total-to-selective extinction in Carina
• IRAS03063+5735: A Bowshock Nebula Powered by an Early B Star

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