A Precise Distance to IRAS 00420+5530 via H2O Maser Parallax with the VLBA

📝 Abstract

We have used the VLBA to measure the annual parallax of the H2O masers in the star-forming region IRAS 00420+5530. This measurement yields a direct distance estimate of 2.17 +/- 0.05 kpc (<3%), which disagrees substantially with the standard kinematic distance estimate of ~4.6 kpc (according to the rotation curve of Brand and Blitz 1993), as well as most of the broad range of distances (1.7-7.7 kpc) used in various astrophysical analyses in the literature. The 3-dimensional space velocity of IRAS 00420+5530 at this new, more accurate distance implies a substantial non-circular and anomalously slow Galactic orbit, consistent with similar observations of W3(OH) (Xu et al., 2006; Hachisuka et al. 2006), as well as line-of-sight velocity residuals in the rotation curve analysis of Brand and Blitz (1993). The Perseus spiral arm of the Galaxy is thus more than a factor of two closer than previously presumed, and exhibits motions substantially at odds with axisymmetric models of the rotating Galaxy.

💡 Analysis

We have used the VLBA to measure the annual parallax of the H2O masers in the star-forming region IRAS 00420+5530. This measurement yields a direct distance estimate of 2.17 +/- 0.05 kpc (<3%), which disagrees substantially with the standard kinematic distance estimate of ~4.6 kpc (according to the rotation curve of Brand and Blitz 1993), as well as most of the broad range of distances (1.7-7.7 kpc) used in various astrophysical analyses in the literature. The 3-dimensional space velocity of IRAS 00420+5530 at this new, more accurate distance implies a substantial non-circular and anomalously slow Galactic orbit, consistent with similar observations of W3(OH) (Xu et al., 2006; Hachisuka et al. 2006), as well as line-of-sight velocity residuals in the rotation curve analysis of Brand and Blitz (1993). The Perseus spiral arm of the Galaxy is thus more than a factor of two closer than previously presumed, and exhibits motions substantially at odds with axisymmetric models of the rotating Galaxy.

📄 Content

arXiv:0901.0517v1 [astro-ph.GA] 5 Jan 2009 To appear in the Astrophysical Journal, March 2009 A Precise Distance to IRAS 00420+5530 via H2O Maser Parallax with the VLBA G. A. Moellenbrock, M. J. Claussen, and W. M. Goss National Radio Astronomy Observatory, Socorro, NM 87801 gmoellen@nrao.edu ABSTRACT We have used the VLBA to measure the annual parallax of the H2O masers in the star-forming region IRAS 00420+5530. This measurement yields a direct distance estimate of 2.17±0.05 kpc (<3%), which disagrees substantially with the standard kinematic distance estimate of ∼4.6 kpc (according to the rotation curve of Brand & Blitz 1993), as well as most of the broad range of distances (1.7−7.7 kpc) used in various astrophysical analyses in the literature. The 3-dimensional space velocity of IRAS 00420+5530 at this new, more accurate distance implies a substantial non-circular and anomalously slow Galactic orbit, consistent with similar observations of W3(OH) (Xu et al., 2006; Hachisuka et al., 2006), as well as line-of-sight velocity residuals in the rotation curve analysis of Brand & Blitz (1993). The Perseus spiral arm of the Galaxy is thus more than a factor of two closer than previously presumed, and exhibits motions substantially at odds with axisymmetric models of the rotating Galaxy. Subject headings: astrometry— Galaxy: structure— masers— stars: distances— stars: individual (IRAS 00420+5530)— techniques: interferometric 1. Introduction Distance estimates to celestial objects are one of the most fundamental measurements in astronomy and astrophysics. Knowledge of the distance to astronomical sources in the sky are needed in order to estimate their physical properties, such as luminosities, masses, kinematics, and dynamics. In recent years, the technique of Very Long Baseline Interferom- etry (VLBI) has been used to make precise astrometric measurements (to a precision of of a – 2 – few tens of microarcseconds for a single observation) and thus holds the promise to extend the range of the direct distance measurements of annual parallax up to at least 10 kpc with 10% accuracy using radio telescopes such as the Very Long Baseline Array (VLBA) of the National Radio Astronomy Obsevatory (NRAO). Compact, bright radio-emitting objects such as pulsars and masers are choice bea- cons for VLBI distance measurements across the Galaxy (e.g., van Langevelde et al., 2000; Brisken et al., 2002; Chatterjee et al., 2004; Xu et al., 2006). Such distance measurements enable enormous improvements in our understanding of Galactic structure and also the physics of individual objects. Reid (2008) reviewed the importance of measuring parallaxes and proper motions to help delineate Galactic structure and showed, in a survey for parallaxes and proper motions of young, high-mass stars, that kinematic distances are systematically too large. In this paper, we report on using the VLBA to measure the annual parallax of the star- forming region IRAS 00420+5530, using the bright water masers associated with the IRAS source. IRAS 00420+5530 (l = 122.0◦, b = −7.1◦) is a star formation region exhibiting a molecular outflow (Zhang et al., 2005), 3.6cm and 3mm continuum emission (Molinari et al., 2002), and H2O masers (e.g., Brand et al., 1994). It is also coincident with dense gas traced by HCO+(1-0), indicating that the cluster of young stars remains deeply embedded in its natal cloud of molecular gas (Molinari et al., 2002). The systemic velocity of the gas around IRAS 00420+5530 is ∼−51 km s−1 with respect to the local standard of rest (LSR) based on the measurements of ammonia radial velocities (Molinari et al., 1996) and the HCO+(1- 0) (Molinari et al., 2002). The water masers occur at similar line-of-sight velocities as the surrounding gas, but with a somewhat broader range (LSR velocities −52 to −40 km s−1). Distances to IRAS 00420+5530 of 4.3 to 7.7 kpc (presumably kinematic, in most cases) have been used in recent papers, despite a much closer photometric distance of 1.7 kpc (Neckel & Staude 1984). Reflecting this broad range of distance estimates, luminosities of the IRAS source (presumably the exciting star) between 1.2 × 104 and 5.2 × 104L⊙have been reported (e.g., Molinari et al., 2002; Zhang et al., 2005). Clearly, a better distance estimate is desirable for this object. In addition, IRAS 00420+5530 lies in the direction of the Perseus arm, which has been the topic of a recent distance study by Xu et al., (2006). Xu et al., (2006) addressed the large discrepancy between the luminosity and kinematic distance estimates in the Perseus arm by accurately measuring the distance (using VLBI techniques) of the methanol masers in the compact HII region W3(OH). It is important that distances to other objects in the direction of the Perseus arm be precisely measured to compare to the work presented by Xu et al., (2006), and to determine small-scale deviations from the distances and kinematics in this region which could be due to peculiar mot

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