The magnetic field of the evolved star W43A

The majority of the observed planetary nebulae exhibit elliptical or bipolar structures. Theoretical modeling has indicated that magnetically collimated jets may be responsible for the formation of the non-spherical planetary nebulae. The aim of this project is to measure the Zeeman splitting caused by the magnetic field in the OH and H2O maser regions occurring in the circumstellar envelope and bipolar outflow of the evolved star W43A. We report a measured magnetic field of approximately 100 micro-gauss in the OH maser region of the circumstellar envelope around W43A. The GBT observations reveal a magnetic field strength B|| of ~30 mG changing sign across the H2O masers at the tip of the red-shifted lobe of the bipolar outflow. We also find that the OH maser shell shows no sign of non-spherical expansion and that it probably has an expansion velocity that is typical for the shells of regular OH/IR stars. The GBT observations confirm that the magnetic field collimates the H2O maser jet, while the OH maser observations show that a strong large scale magnetic field is present in the envelope surrounding the W43A central star. The magnetic field in the OH maser envelope is consistent with the one extrapolated from the H2O measurements, confirming that magnetic fields play an important role in the entire circumstellar environment of W43A.Comment: 6 pages, 5 figure

A Third Star in the T Tauri System

New speckle-holographic images of the T Tauri Infrared Companion (T Tauri IRC; T Tauri S) reveal it to be a double system with a sky-projected separation of 0".05, corresponding to a linear distance of 7 AU. The presence of this third star may account for the relative paucity of dust surrounding the IRC.Comment: 5 pages in AASTeX preprint form, including one grayscale figur

From the ashes: JVLA observations of water fountain nebula candidates show the rebirth of IRAS 18455+0448

[abridged] The class of water fountain nebulae is thought to represent the stage of the earliest onset of collimated bipolar outflows during the post-Asymptotic Giant Branch phase. They thus play a crucial role in the study of the formation of bipolar Planetary Nebulae (PNe). To date, 14 water fountain nebulae have been identified. The identification of more sources in this unique stage of stellar evolution will enable us to study the origin of bipolar PNe morphologies in more detail. We present the results of seven sources observed with the JVLA that were identified as water fountain candidates in an Effelsberg 100m telescope survey of 74 AGB and early post-AGB stars. We find that our sample of water fountain candidates displays strong variability in their 22 GHz H2O maser spectra. The JVLA observations show an extended bipolar H2O maser outflow for one source, the OH/IR star IRAS 18455+0448. This source was previously classified as a 'dying' OH/IR star based on the exponential decrease of its 1612 MHz OH maser and the lack of H2O masers. We therefore also re-observed the 1612, 1665, and 1667 MHz OH masers. We confirm that the 1612 MHz masers have not reappeared and find that the 1665/1667 MHz masers have decreased in strength by several orders of magnitude during the last decade. The OH/IR star IRAS 18455+0448 is confirmed to be a new addition to the class of water fountain nebulae. Its kinematic age is approximately 70 yr, but could be lower, depending on the distance and inclination. Previous observations indicate, with significant uncertainty, that IRAS 18455+0448 has a surprisingly low mass compared to available estimates for other water fountain nebulae. The available historical OH maser observations make IRAS 18455+0448 unique for the study of water fountain nebulae and the launch of post-AGB bipolar outflows...Comment: 8 pages, 5 figures, accepted for publication in A&A (revised minor typos

Simulated Galactic methanol maser distribution to constrain Milky Way parameters

Using trigonometric parallaxes and proper motions of masers associated with massive young stars, the Bar and Spiral Structure Legacy (BeSSeL) survey has reported the most accurate values of the Galactic parameters so far. The determination of these parameters with high accuracy has a widespread impact on Galactic and extragalactic measurements. This research is aimed at establishing the confidence with which such parameters can be determined. This is relevant for the data published in the context of the BeSSeL survey collaboration, but also for future observations, in particular from the Southern Hemisphere. In addition, some astrophysical properties of the masers can be constrained, notably the luminosity function. We have simulated the population of maser-bearing young stars associated with Galactic spiral structure, generating several samples and comparing them with the observed samples used in the BeSSeL survey. Consequently, we checked the determination of Galactic parameters for observational biases introduced by the sample selection. Galactic parameters obtained by the BeSSeL survey do not seem to be biased by the sample selection used. In fact, the published error estimates appear to be conservative for most of the parameters. We show that future BeSSeL data and future observations with Southern arrays will improve the Galactic parameters estimates and smoothly reduce their mutual correlation. Moreover, by modeling future parallax data with larger distance and, thus, greater relative uncertainties for a larger numbers of sources, we found that parallax-distance biasing is an important issue. Hence, using fractional parallax uncertainty in the weighting of the motion data is imperative. Finally, the luminosity function for 6.7 GHz methanol masers was determined, allowing us to estimate the number of Galactic methanol masers.Comment: Accepted for publication in A&A. Language edition include

Improved VLBI astrometry of OH maser stars

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

The magnetic field at milliarcsecond resolution around IRAS20126+4104

IRAS20126+4104 is a well studied B0.5 protostar that is surrounded by a ~1000 au Keplerian disk and is where a large-scale outflow originates. Both 6.7-GHz CH3OH masers and 22-GHz H2O masers have been detected toward this young stellar object. The CH3OH masers trace the Keplerian disk, while the H2O masers are associated with the surface of the conical jet. Recently, observations of dust polarized emission (350 um) at an angular resolution of 9 arcseconds (~15000 au) have revealed an S-shaped morphology of the magnetic field around IRAS20126+4104. The observations of polarized maser emissions at milliarcsecond resolution (~20 au) can make a crucial contribution to understanding the orientation of the magnetic field close to IRAS20126+4104. This will allow us to determine whether the magnetic field morphology changes from arcsecond resolution to milliarcsecond resolution. The European VLBI Network was used to measure the linear polarization and the Zeeman splitting of the 6.7-GHz CH3OH masers toward IRAS20126+4104. The NRAO Very Long Baseline Array was used to measure the linear polarization and the Zeeman splitting of the 22-GHz H2O masers toward the same region. We detected 26 CH3OH masers and 5 H2O masers at high angular resolution. Linear polarization emission was observed toward three CH3OH masers and toward one H2O maser. Significant Zeeman splitting was measured in one CH3OH maser (\Delta V_{Z}=-9.2 +/- 1.4 m/s). No significant (5 sigma) magnetic field strength was measured using the H2O masers. We found that in IRAS20126+4104 the rotational energy is less than the magnetic energy.Comment: 9 pages, 5 figures, 2 tables, accepted by Astronomy & Astrophysic

EVN observations of 6.7-GHz methanol maser polarization in massive star-forming regions II. First statistical results

Magnetic fields have only recently been included in theoretical simulations of high-mass star formation. The simulations show that magnetic fields play an important role in the formation and dynamics of molecular outflows. Masers, in particular 6.7-GHz CH3OH masers, are the best probes of the magnetic field morphologies around massive young stellar objects on the smallest scales of 10-100 AU. This paper focuses on 4 massive young stellar objects, IRAS06058+2138-NIRS1, IRAS22272+6358A, S255-IR, and S231, which complement our previous 2012 sample (the first EVN group). From all these sources, molecular outflows have been detected in the past. Seven of the European VLBI Network antennas were used to measure the linear polarization and Zeeman-splitting of the 6.7-GHz CH3OH masers in the star-forming regions in this second EVN group. We detected a total of 128 CH3OH masing cloudlets. Fractional linear polarization (0.8%-11.3%) was detected towards 18% of the CH3OH masers in our sample. The linear polarization vectors are well ordered in all the massive young stellar objects. We measured significant Zeeman-splitting in IRAS06058+2138-NIRS1 (DVz=3.8+/-0.6 m/s) and S255-IR (DVz=3.2+/-0.7 m/s). By considering the 20 massive young stellar objects towards which the morphology of magnetic fields was determined by observing 6.7-GHz CH3OH masers in both hemispheres, we find no evident correlation between the linear distributions of CH3OH masers and the outflows or the linear polarization vectors. On the other hand, we present first statistical evidence that the magnetic field (on scales 10-100 AU) is primarily oriented along the large-scale outflow direction. Moreover, we empirically find that the linear polarization fraction of unsaturated CH3OH masers is P_l<4.5%.Comment: 13 pages, 8 figures, 7 tables, accepted by Astronomy & Astrophysic

VLBI Astrometry of the Stellar Image of U Herculis, Amplified by the 1667 OH Maser

The OH 1667 MHz maser in the circumstellar shell around the Mira variable U Herculis has been observed with the NRAO Very Long Baseline Array (VLBA) at 6 epochs, spread over 4 years. Using phase referencing techniques the position of the most blue-shifted maser spot was monitored with respect to two extra-galactic radio sources. The absolute radio positions of the maser can be compared with the stellar optical position measured by the Hipparcos satellite to 15 mas accuracy. This confirms the model in which one of the maser spots corresponds to the stellar continuum, amplified by the maser. The stellar proper motion and the annual parallax (5.3 +/- 2.1 mas) were measured.Comment: 6 pages, 4 figures; to be published in A&