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Register a new userA Precise Distance to IRAS 00420+5530 via H2O Maser Parallax with the VLBA
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Added by
George Moellenbrock III
Publication date
2009
fields
Physics
and research's language is
English
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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.
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We report the results of VLBI observations of H$_{2}$O masers in the IRAS 20143+3634 star forming region using VERA (VLBI Exploration of Radio Astronomy). By tracking masers for a period of over two years we measured a trigonometric parallax of $pi = 0.367 pm 0.037$ mas, corresponding to a source distance of $D = 2.72 ^{+0.31}_{-0.25}$ kpc and placing it in the Local spiral arm. Our trigonometric distance is just 60% of the previous estimation based on radial velocity, significantly impacting the astrophysics of the source. We measured proper motions of $-2.99 pm 0.16$ mas yr$^{-1}$ and $-4.37 pm 0.43$ mas yr$^{-1}$ in R.A. and Decl. respectively, which were used to estimate the peculiar motion of the source as $(U_{s},V_{s},W_{s}) = (-0.9 pm 2.9, -8.5 pm 1.6, +8.0 pm 4.3)$ km s$^{-1}$ for $R_0=8$ kpc and $Theta_0=221$ km s$^{-1}$, and $(U_{s},V_{s},W_{s}) = (-1.0 pm 2.9, -9.3 pm 1.5, +8.0 pm 4.3)$ km s$^{-1}$ for $R_0=8.5$ kpc and $Theta_0=235$ km s$^{-1}$. IRAS 20143+3634 was found to be located near the tangent point in the Cygnus direction. Using our observations we derived the angular velocity of Galactic rotation of the local standard of rest (LSR), $Omega_{0} = 27.3 pm 1.6$ km s$^{-1}$ kpc$^{-1}$, which is consistent with previous values derived using VLBI astrometry of SFRs at the tangent points and Solar circle. It is higher than the value recommended by the IAU of $Omega_{0} = 25.9$ km s$^{-1}$ kpc$^{-1}$ which was calculated using the Galactocentric distance of the Sun and circular velocity of the LSR.
IRAS 16293-2422 is a very well studied young stellar system seen in projection towards the L1689N cloud in the Ophiuchus complex. However, its distance is still uncertain with a range of values from 120 pc to 180 pc. Our goal is to measure the trigonometric parallax of this young star by means of H$_2$O maser emission. We use archival data from 15 epochs of VLBA observations of the 22.2 GHz water maser line. By modeling the displacement on the sky of the H$_2$O maser spots, we derived a trigonometric parallax of $7.1pm1.3$ mas, corresponding to a distance of $141_{-21}^{+30}$ pc. This new distance is in good agreement with recent values obtained for other magnetically active young stars in the L1689 cloud. We relate the kinematics of these masers with the outflows and the recent ejections powered by source A in the system.
We present the results of astrometic observations of H2O masers associated with the star forming region G192.16-3.84 with the VLBI Exploration of Radio Astrometry (VERA). The H2O masers seem to be associated with two young stellar objects (YSOs) separated by sim1200 AU as reported in previous observations. In the present observations, we successfully detected an annual parallax of 0.66 pm 0.04 mas for the H2 O masers, which corresponds to a distance to G192.16-3.84 of D = 1.52 pm 0.08 kpc from the Sun. The determined distance is shorter than the estimated kinematic distance. Using the annual parallax distance and the estimated parameters of the millimeter continuum emission, we estimate the mass of the disk plus circumstellar cloud in the southern young stellar object to be 10.0+4.3Mcdot. We also estimate the galactocentric distance and the peculiar motion -3.6 of G192.16-3.84, relative to a circular Galactic rotation: Rstar = 9.99 pm 0.08 kpc, Zstar = -0.10 pm 0.01 kpc, and (Ustar,Vstar,Wstar)=(-2.8pm1.0,-10.5pm0.3,4.9pm2.7)[kms-1]respectively. The peculiar motion of G192.16-3.84 is within that typically found in recent VLBI astrometric results. The angular distribution and three-dimensional velocity field of H2O maser features associated with the northern YSO indicate the existence of a bipolar outflow with a major axis along the northeast-southwest direction.
We test two different methods of using near-infrared extinction to estimate distances to dark clouds in the first quadrant of the Galaxy using large near infrared (2MASS and UKIDSS) surveys. VLBI parallax measurements of masers around massive young stars provide the most direct and bias-free measurement of the distance to these dark clouds. We compare the extinction distance estimates to these maser parallax distances. We also compare these distances to kinematic distances, including recent re-calibrations of the Galactic rotation curve. The extinction distance methods agree with the maser parallax distances (within the errors) between 66% and 100% of the time (depending on method and input survey) and between 85% and 100% of the time outside of the crowded Galactic center. Although the sample size is small, extinction distance methods reproduce maser parallax distances better than kinematic distances; furthermore, extinction distance methods do not suffer from the kinematic distance ambiguity. This validation gives us confidence that these extinction methods may be extended to additional dark clouds where maser parallaxes are not available.
Using multi-epoch Very Long Baseline Array observations, we have measured the trigonometric parallax of the weak-line T Tauri star HP Tau/G2 in Taurus. The best fit yields a distance of 161.2 $pm$ 0.9 pc, suggesting that the eastern portion of Taurus (where HP Tau/G2 is located) corresponds to the far side of the complex. Previous VLBA observations have shown that T Tau, to the South of the complex, is at an intermediate distance of about 147 pc, whereas the region around L1495 corresponds to the near side at roughly 130 pc. Our observations of only four sources are still too coarse to enable a reliable determination of the three-dimensional structure of the entire Taurus star-forming complex. They do demonstrate, however, that VLBA observations of multiple sources in a given star-forming region have the potential not only to provide a very accurate estimate of its mean distance, but also to reveal its internal structure. The proper motion measurements obtained simultaneously with the parallax allowed us to study the kinematics of the young stars in Taurus. Combining the four observations available so far, we estimate the peculiar velocity of Taurus to be about 10.6 km s-1 almost completely in a direction parallel to the Galactic plane. Using our improved distance measurement, we have refined the determination of the position on the HR diagram of HP Tau/G2, and of two other members of the HP Tau group (HP Tau itself and HP Tau/G3). Most pre-main sequence evolutionary models predict significantly discrepant ages (by 5 Myr) for those three stars -expected to be coeval. Only in the models of Palla & Stahler (1999) do they fall on a single isochrone (at 3 Myr).
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