Resolving the distance controversy for Sharpless 269: A possible kink in the outer arm

Abstract

Sharpless 269 (S269) is one of a few HII regions in the outer spiral arm of the Milky Way with strong water maser emission. Based on data from the Very Long Baseline Interferometry (VLBI) Exploration of Radio Astrometry (VERA) array, two parallax measurements have been published, which differ by nearly $2\sigma$. Each distance estimate supports a different structure for the outer arm. Moreover, given its large Galactocentric radii, S269 has special relevance as its proper motion and parallax have been used to constrain the Galactic rotation curve at large radii. Using recent Very Long Baseline Array (VLBA) observations, we accurately measure the parallax and proper motion of the water masers in S269. We interpret the position and motion of S269 in the context of Galactic structure, and possible optical counterparts. S269's 22 GHz water masers and two close-by quasars were observed at 16 epochs between 2015 and 2016 using the VLBA. We measure an annual parallax for S269 of 0.241 $\pm$ 0.012 mas corresponding to a distance from the Sun of $4.15_{-0.20}^{+0.22}$ kpc by fitting four maser spots. The mean proper motion for S269 was estimated as $0.16\pm 0.26$ mas $\mathrm{y}{\mathrm{r}}^{-1}$ and $-0.51\pm 0.26$ mas $\mathrm{y}{\mathrm{r}}^{-1}$ for ${\mu }_{\alpha }cos\delta$ and ${\mu }_{\delta }$ respectively, which corresponds to the motion expected for a flat Galactic rotation curve at large radius. This distance estimate, Galactic kinematic simulations and observations of other massive young stars in the outer region support the existence of a kink in the outer arm at $l\approx$ 140 degrees. Additionally, we find more than 2,000 optical sources in the Gaia DR2 catalog within 125 pc radius around the 3D position of the water maser emission; from those only three sources are likely members of the same stellar association that contains the young massive star responsible for the maser emission (S269 IRS 2w).

Accepted for publication in A&A

References in corpus (10)

• The local spiral structure of the Milky Way
• V838 Monocerotis: A Geometric Distance from Hubble Space Telescope Polarimetric Imaging of its Light Echo
• Mapping Spiral Structure on the far side of the Milky Way
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• Characteristics of Spiral Arms in Late-type Galaxies
• Tracing the Spiral Structure of the Outer Milky Way with Dense Atomic Hydrogen Gas
• The Lifetimes of Phases in High-Mass Star-Forming Regions
• Distance and proper motion measurement of water masers in Shapless 269 IRS 2w
• Simulated Galactic methanol maser distribution to constrain Milky Way parameters

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• Trigonometric Parallaxes Of High-Mass Star Forming Regions: Our View Of The Milky Way
• The First VERA Astrometry Catalog
• SiO maser astrometry of the red transient V838 Monocerotis
• A New Multi-Tracer Approach to Defining the Spiral arm width in the Milky Way
• Interarm islands in the Milky Way -- the one near the Cygnus spiral arm
• Characterizing the Evolved Stellar Population in the Galactic Foreground I: Bolometric Magnitudes, Spatial Distribution and P-L Relations