259 research outputs found

    The distance to a star forming region in the Outer arm of the Galaxy

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    We performed astrometric observations with the VLBA of WB89-437, an H2O maser source in the Outer spiral arm of the Galaxy. We measure an annual parallax of 0.167 +/- 0.006 mas, corresponding to a heliocentric distance of 6.0 +/- 0.2 kpc or a Galactocentric distance of 13.4 +/- 0.2 kpc. This value for the heliocentric distance is considerably smaller than the kinematic distance of 8.6 kpc. This confirms the presence of a faint Outer arm toward l = 135 degrees. We also measured the full space motion of the object and find a large peculiar motion of ~20 km/s toward the Galactic center. This peculiar motion explains the large error in the kinematic distance estimate. We also find that WB89-437 has the same rotation speed as the LSR, providing more evidence for a flat rotation curve and thus the presence of dark matter in the outer Galaxy.Comment: The Astrophysical Journal, accepted, 16 pages, 4 Figure

    A model for soap film dynamics with evolving thickness

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    Previous research on animations of soap bubbles, films, and foams largely focuses on the motion and geometric shape of the bubble surface. These works neglect the evolution of the bubble’s thickness, which is normally responsible for visual phenomena like surface vortices, Newton’s interference patterns, capillary waves, and deformation-dependent rupturing of films in a foam. In this paper, we model these natural phenomena by introducing the film thickness as a reduced degree of freedom in the Navier-Stokes equations and deriving their equations of motion. We discretize the equations on a nonmanifold triangle mesh surface and couple it to an existing bubble solver. In doing so, we also introduce an incompressible fluid solver for 2.5D films and a novel advection algorithm for convecting fields across non-manifold surface junctions. Our simulations enhance state-of-the-art bubble solvers with additional effects caused by convection, rippling, draining, and evaporation of the thin film

    Trigonometric Parallax of W51 Main/South

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    We report measurement of the trigonometric parallax of W51 Main/South using the Very Long Baseline Array (VLBA). We measure a value of 0.185 +/- 0.010 mas, corresponding to a distance of 5.41 (+0.31/-0.28) kpc. W51 Main/South is a well-known massive star-forming region near the tangent point of the Sagittarius spiral arm of the Milky Way. Our distance to W51 yields an estimate of the distance to the Galactic center of Ro = 8.3 +/- 0.46 (statistical) +/- 1.0 (systematic) kpc by simple geometry. Combining the parallax and proper motion measurements for W51, we obtained the full-space motion of this massive star forming region. We find W51 is in a nearly circular orbit about the Galactic center. The H2O masers used for our parallax measurements trace four powerful bipolar outflows within a 0.4 pc size region, some of which are associated with dusty molecular hot cores and/or hyper- or ultra-compact HII regions.Comment: Accepted to ApJ; 32 pages; 6 tables; 5 figure

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

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    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.Comment: 33 pages, 12 figures; Accepted by ApJ (to appear March 2009

    Improved VLBI astrometry of OH maser stars

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    Aims: Accurate distances to evolved stars with high mass loss rates are needed for studies of many of their fundamental properties. However, as these stars are heavily obscured and variable, optical and infrared astrometry is unable to provide enough accuracy. Methods: Astrometry using masers in the circumstellar envelopes can be used to overcome this problem. We have observed the OH masers of a number of Asymptotic Giant Branch (AGB) stars for approximately 1 year with the Very Long Baseline Array (VLBA). We have used the technique of phase referencing with in-beam calibrators to test the improvements this technique can provide to Very Long Baseline Interferometry (VLBI) OH maser astrometric observations. Results: We have significantly improved the parallax and proper motion measurements of the Mira variable stars U Her, S CrB and RR Aql. Conclusions: It is shown that both in-beam phase-referencing and a decrease in solar activity during the observations significantly improves the accuracy of the astrometric observations. The improved distances to S CrB (418 +21 -18 pc) and RR Aql (633 +214 -128 pc) are fully consistent with published P-L relations, but the distance to U Her (266 +32 -28 pc) is significantly smaller. We conclude that for sources that are bright and have a nearby in-beam calibrator, VLBI OH maser astrometry can be used to determine distances to OH masing stars of up to ~2 kpc.Comment: 15 pages, 10 figures; accepted for publication in A&A; for a version with high-resolution figures see http://www.astro.uni-bonn.de/~wouter/papers/astrom/astrom.shtm

    A Trigonometric Parallax of Sgr B2

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    We have measured the positions of water masers in Sgr B2, a massive star forming region in the Galactic center, relative to an extragalactic radio source with the Very Long Baseline Array. The positions measured at 12 epochs over a time span of one year yield the trigonometric parallax of Sgr B2 and hence a distance to the Galactic center of Ro=7.9 (+0.8/-0.7) kpc. The proper motion of Sgr B2 relative to Sgr A* suggests that Sgr B2 is about 0.13 kpc nearer than the Galactic center, assuming a low-eccentricity Galactic orbit.Comment: Submitted to ApJ; 4 tables; 3 figures. Version 2 corrects Fig. 2 which was missing some dat

    Trigonometric Parallaxes of Massive Star Forming Regions: III. G59.7+0.1 and W 51 IRS2

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    We report trigonometric parallaxes for G59.7+0.1 and W 51 IRS2, corresponding to distances of 2.16^{+0.10}_{-0.09} kpc and 5.1^{+2.9}_{-1.4} kpc, respectively. The distance to G59.7+0.1 is smaller than its near kinematic distance and places it between the Carina-Sagittarius and Perseus spiral arms, probably in the Local (Orion) spur. The distance to W 51 IRS2, while subject to significant uncertainty, is close to its kinematic distance and places it near the tangent point of the Carina-Sagittarius arm. It also agrees well with a recent estimate based on O-type star spectro/photometry. Combining the distances and proper motions with observed radial velocities gives the full space motions of the star forming regions. We find modest deviations of 5 to 10 km/s from circular Galactic orbits for these sources, both counter to Galactic rotation and toward the Galactic center.Comment: 16 pages, 6 figures; to appear in the Astrophysical Journa

    Trigonometric Parallaxes of 6.7 GHz Methanol Masers

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    Emission from the 6.7 GHz methanol maser transition is very strong, is relatively stable, has small internal motions, and is observed toward numerous massive star-forming regions in the Galaxy. Our goal is to perform high-precision astrometry using this maser transition to obtain accurate distances to their host regions. Eight strong masers were observed during five epochs of VLBI observations with the European VLBI Network between 2006 June, and 2008 March. We report trigonometric parallaxes for five star-forming regions, with accuracies as good as ∼22μ\sim22 \mathrm{\mu}as. Distances to these sources are 2.57−0.27+0.342.57^{+0.34}_{-0.27} kpc for ON 1, 0.776−0.083+0.1040.776^{+0.104}_{-0.083} kpc for L 1206, 0.929−0.033+0.0340.929^{+0.034}_{-0.033} kpc for L 1287, 2.38−0.12+0.132.38^{+0.13}_{-0.12} kpc for NGC 281-W, and 1.59−0.06+0.071.59^{+0.07}_{-0.06} kpc for S 255. The distances and proper motions yield the full space motions of the star-forming regions hosting the masers, and we find that these regions lag circular rotation on average by ∼\sim17 km s−1^{-1}, a value comparable to those found recently by similar studies.Comment: 17 pages, 21 figures, 5 tables, accepted for publication in A&A, corrected typo
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