Vlbi research in the ”Radioastron” project: structure of the H2O maser in NGC 2071 IRS 1

Within the Radioastron interferometer scientific program, observational data of the H2O maser at a frequency of 22.2280 GHz in the NGC 2071 nebula were processed. A space radio telescope (SRT — 10 m) and three radio telescopes of the ground network: RT — 32 m (Medicina, Italy), RT — 32 m (Torun, Poland) and RT —64 m (Kalyazin, RF) took part in the observations. A map of the maser spot distribution has been obtained, where there are 13 spatial components with VLSR in the range 4.7 — 20.5 km/s. Correlation is observed on ground-space baselines for the component on VLSR = 14.3 km/s. Based on the visibility function dependence analysis from the baseline projection values, there was proposed a twocomponent model of this component spatial structure with the dimensions of the extended and compact constituent of 4 and 0.06 msec, i.e. 1.56 and 0.023 au, respectively.

Overall variation of the H2O masers around W Hydrae in 28 years

In this paper, we present the distribution of H$_2$O masers associated with the semi-regular variable star W Hydrae (W Hya). We have collected the radio interferometric data of the maser distribution taken with the Very Large Array (VLA), the Kashima–Nobeyama InterFErometer (KNIFE), the Multi-Element Radio Link Network (MERLIN), the VLBI Exploration of Radio Astrometry (VERA), and the combined array of the Korean VLBI Network (KVN) and VERA (KaVA) in order to trace the maser distribution variation in two decades. Even though differences in the sensitivities and angular resolutions of the interferometric observations should be taken into account, we attempt to find possible correlation of the maser distribution with the stellar light curve. Our failure in the measurement of the annual parallax of the masers with VERA is likely caused by the properties of the maser features, which have been spatially resolved by the synthesized beam and survived for only half a year or less. No dependence of the maser spot flux density on its size is found in the KNIFE data, suggesting that maser spot size is determined by the physical boundary, as is expected for a clump affected by outward propagation of a stellar pulsation shock wave, rather than the (spherical) geometry of maser beaming in the maser gas clump.

VLBI astrometry of the semi-regular variable star RX Bootis

We present a distance measurement to the semi-regular variable star RX Bootis (RX Boo). Using the VLBI Exploration of Radio Astrometry (VERA) telescope, we conducted astrometric observations of a water maser spot associated with RX Boo, as well as of the continuum reference source J1419+2706. Based on monitoring observations covering a full year, the annual parallax of RX Boo was measured at 7.31 ± 0.50 mas, corresponding to a distance of 136+10−9 pc. This distance uncertainty is smaller by a factor of two than those previously published, allowing us to determine the object's stellar properties more accurately. Using our distance, we can determine the absolute magnitude and discuss more precisely the locus of RX Boo on the period–luminosity (PL) relation. RX Boo exhibits two simultaneous pulsation periods and is located on the fundamental and first overtone Mira sequences of the PL relation. In addition, we calculated the radius and mass of the star.

VLBI Observations and NH3 Mapping of the Star-forming Region NGC2264

We have measured the annual parallax of the water maser source associated with star forming region NGC2264 from observations with VLBI Exploration of Radio Astrometry (VERA). We detected masers at VLSR = 7.2 km s−1. We discussed its driving sources of detected maser spots. One of the maser spots was associated with a centimeter continuum source observed with VLA. Neither optical, infrared nor X-ray sources is catalogued near the spot. The other maser spot is located close to an X-ray source, although there is no optical or infrared counterpart. The proper motion of the former spot was (μα, μδ) = (23.91 ± 4.29, −29.81 ± 4.27) and the proper motion of latter spot was (μα, μδ) = (−0.96 ± 0.58, −6.05 ± 3.06). For the latter spot, the peculiar motion is ∼ 150 km s−1 and it has the high velocity and this may be a jet or an outflow from a young star. The observed parallax is 1.365 ± 0.098 mas, corresponding to the distance of 738+57−50 pc. This value is constant with the photometric distance of NGC2264 previously measured. The fitting result of the parallax is shown in figure 1. We also observed in NH3 (1,1), (2,2), (3,3) lines of NGC2264 with the Kashima 34m telescope. We estimated the star formation efficiency (SFE) of NGC2264 from the dense molecular mass of NH3 and the stellar mass calculated by Teixeira et al. (2012). The SFE is 9 – 12% which is consistent with previous results.

OH (1720 MHz) masers: signposts of SNR/molecular cloud interactions

Over the last decade it has been demonstrated that supernova remnant (SNR) OH (1720 MHz) masers are unique tracers of SNR/molecular cloud interactions. Here I briefly review the current state of our observational understanding of these masers including results from recent MERLIN and VLBA full polarization studies of the masers in W28, W44, and W51C. Some of our findings include that (1) in accordance with theory, the linear polarization position angles are either parallel or perpendicular to the plane-of-sky magnetic field determined through other observations; (2) the maser spot sizes are fairly large (~ 1014 cm) and exhibit a core/halo morphology; and (3) while the magnetic field strengths do increase slightly with higher resolution, this effect can be completely explained by spectral/spatial blending.

Six years of astrometric monitoring of the OH maser in U Herculis

Using VLBI phase referencing, we have been registering the position of the most blue-shifted OH 1667 MHz maser spot with respect to nearby extragalactic reference sources for almost seven years. The data for these 10 epochs allow us to determine the proper motion and parallax of U Her. It also yields the opportunity to compare the maser position to the position of the star, as measured by the Hipparcos satellite. Results support the theory that the compact bright spot is the amplified stellar image.

Magnetic fields in supernova remnants from OH (1720 MHz) masers

Supernovae have a profound effect on the morphology, kinematics, and metallicity of galaxies. The impact of supernova shocks on surrounding molecular clouds is also thought to trigger new generations of star formation. A critical ingredient in such interactions and, indeed, all aspects of supernova remnant (SNR) evolution are magnetic fields. In recent years, OH (1720 MHz) masers have been used as signposts for the interaction of SNRs with molecular gas. In addition to tracing SNR/molecular cloud interactions, the OH (1720 MHz) maser line also provides a unique opportunity to measure the strength of the post-shock magnetic field via Zeeman splitting. Recent results from efforts to both detect the magnetic fields and resolve the maser spot sizes of OH (1720 MHz) masers toward W51C using the VLBA and W44 using MERLIN are presented. These observations have yielded magnetic field detections between 0.5 and 2.5 mG and large maser spot sizes of about 1015 cm.

Astrometry of SiO Masers: First detections of circumstellar SiO maser emission at λ = 7 mm using three European VLBI stations

The positions of circumstellar molecular masers relative to each other, to the central star, and to the compact extragalactic sources are of astrophysical and astrometrical interest. The SiO masers at λ = 7 mm are especially interesting as the maser spots are located very close to the photosphere of the star. For astrometry we would like to see a proper distribution of the maser spots and several epochs of observations in order to estimate the position of the central star and eventual motions of the spots.The optical position of this star can be determined with ground-based astrometric facilities, and many of the stars showing SiO emission are in theHipparcoscatalogue. A link between the Optical and Radio Reference Frames is achieved in a second step relating the maser spot positions to the Extragalactic Reference Frame (EGRF) established with VLBI (Baudryet al., 1984).