24 research outputs found

    Declining Rotation Curve and Brown Dwarf MACHOs

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    If the Galactic rotation speed at the Solar circle is 200\sim 200 km s1^{-1} or smaller, which is supported by several recent studies, the rotation curve of the Galaxy could be declining in the outermost region. Motivated by this, we investigate the effect of such declining rotation curve on the estimate of the MACHO mass and the fractional contribution of the MACHOs to the Galactic dark halo. Using Hernquist and Plummer halo models instead of the standard halo model, we find that the MACHO mass could be significantly smaller than that for the standard halo case. In particular, there exists a certain set of halo parameters for which the MACHO mass is 0.1MM_\odot or less and at the same time the MACHO contribution to the total mass of the halo is almost 100 %. This result indicates that a halo which consists solely of brown dwarfs can be consistent with both of the observed microlensing properties and the constraints from the rotation curve, provided the outer rotation curve is indeed declining.Comment: 8 pages and 4 figures, accepted for publication in ApJ Letter

    Astrometry of Star Forming Region IRAS 05137+3919 in the far outer Galaxy

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    We present the results of astrometric observations with VERA toward the H2O maser sources in IRAS 05137+3919, which is thought to be located in the far outer Galaxy. We have derived the parallax of \pi = 0.086 +/- 0.027 mas, which corresponds to the source distance of D=11.6+5.3-2.8 kpc. Although the parallax measurement is only 3-sigma level and thus the distance uncertainty is considerably large, we can strongly constrain the minimum distance to this source, locating the source at the distance from the Sun greater than 8.3 kpc (or 16.7 kpc from the Galaxy's center) at 90% confidence level. Our results provide an astrometric confirmation that this source is located in the far outer Galaxy beyond 15 kpc from the Galaxy center, indicating that IRAS 05137+3919 is one of the most distant star-forming regions from the Galaxy center.Comment: 7 pages, 2 figures, to appear in PASJ vol 63. No1 (VERA special issue

    Astrometry of H2_2O masers in the massive star--forming region IRAS 06061+2151 with VERA

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    We measured the trigonometric annual parallax of H2_2O maser source associated with the massive star-forming regions IRAS 06061+2151 with VERA. The annual parallax of 0.496±0.0310.496\pm0.031 mas corresponding to a distance of 2.020.12+0.132.02^{+0.13}_{-0.12} kpc was obtained by 10 epochs of observations from 2007 October to 2009 September. This distance was obtained with a higher accuracy than the photometric distance previously measured, and places IRAS 06061+2151 in the Perseus spiral arm. We found that IRAS 06061+2151 also has a peculiar motion of larger than 15 km s1^{-1} counter to the Galactic rotation. That is similar to five sources in the Perseus spiral arm, whose parallaxes and proper motions have already been measured with higher accuracy. Moreover, these sources move at on average 27 km s1^{-1} toward the Galactic center and counter to the Galactic rotation.Comment: 8 pages, 4 figures, accepted to PAS

    Distance to Orion KL Measured with VERA

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    We present the initial results of multi-epoch VLBI observations of the 22 GHz H2O masers in the Orion KL region with VERA (VLBI Exploration of Radio Astrometry). With the VERA dual-beam receiving system, we have carried out phase-referencing VLBI astrometry and successfully detected an annual parallax of Orion KL to be 2.29+/-0.10 mas, corresponding to the distance of 437+/-19 pc from the Sun. The distance to Orion KL is determined for the first time with the annual parallax method in these observations. Although this value is consistent with that of the previously reported, 480+/-80 pc, which is estimated from the statistical parallax method using proper motions and radial velocities of the H2O maser features, our new results provide the much more accurate value with an uncertainty of only 4%. In addition to the annual parallax, we have detected an absolute proper motion of the maser feature, suggesting an outflow motion powered by the radio source I along with the systematic motion of source I itself.Comment: 7 pages, 3 figures. PASJ, in press (Vol. 59, No. 5, October 25, 2007 issue

    Absolute Proper Motions of H2O Masers Away from the Galactic Plane Measured with VERA in the "Superbubble" Region NGC 281

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    We report on absolute proper-motion measurements of an H2O maser source in the NGC 281 West molecular cloud, which is located ~320 pc above the Galactic plane and is associated with an HI loop extending from the Galactic plane. We have conducted multi-epoch phase-referencing observations of the maser source with VERA (VLBI Exploration of Radio Astrometry) over a monitoring period of 6 months since May 2006. We find that the H2O maser features in NGC 281 West are systematically moving toward the southwest and further away from the Galactic plane with a vertical velocity of ~20-30 km/s at its estimated distance of 2.2-3.5 kpc. Our new results provide the most direct evidence that the gas in the NGC 281 region on the HI loop was blown out from the Galactic plane, most likely in a superbubble driven by multiple or sequential supernova explosions in the Galactic plane.Comment: 10 pages, 5 figures, PASJ in press (Vol. 59, No. 4; August 25, 2007 issue