Astrometry of circumstellar masers

The circumstellar masers around evolved stars offer an interesting possibility to measure stellar parameters through VLBI astrometry. In this paper the application of this technique is discussed, including the accuracy and the uncertainties of the method. The different maser species (OH, H_2O, SiO) have slightly different characteristics and applications. This paper does not concern astrometry of maser spots to study the kinematics of the envelope, but concentrates on attempting to measure the motion of the underlying star.Comment: 8 pages, 2 figures, to appear in "Mass-losing Stars and their Circumstellar Matter", eds Y. Nakada & M. Honma, Kluwer ASSL serie

Molecular outflows and 1000 AU structure of low mass YSO envelopes

We present the results of an observational study into the molecular outflows and small scale ( ~ 1000 AU) envelope structure of a sample of nine low mass young stellar objects (YSOs) in Taurus. The characteristics of the outflows are derived from ^(12)CO J = 3 - 2 mapping with the James Clerk Maxwell Telescope, while the envelopes are imaged in the HCO^+ 1-0, ^(13)CO 1- 0 and C^(18)O 1-0 emission lines with the Owens Valley Millimeter Array. Using dust envelope continuum fluxes at 1 mm as the basis for an evolutionary ordering, a picture emerges in which the mass, extent, and collimation of outflows decreases over time as the envelopes become less massive, the opening angle of the outflow cavity increases, and mass accretion through the disk slows down. On 1000 AU scales the HCO+ and ^(13)CO/C^(18)O emission in the envelope is closely related to the outflow cavity, often outlining the cavity walls. In addition, the envelopes are clumpy, and two sources appear surrounded by an incomplete ring or torus, 1500-3000 AU in radius. The role of the outflow in shaping the small scale molecular emission may be passive (creating a low-opacity pathway for heating radiation) rather than, or in addition to, active (compressing and shock-heating the material)

The spatial distribution of excited H_2 in T Tau: a molecular outflow in a young binary system

Strong extended emission from molecular hydrogen in the v = 1 → 0 S(l) transition is mapped around T Tau. In addition, the v = 2 → 1 S(l) line is detected close to the star. The ratio of the two transitions is consistent with an excitation process in which both fluorescence by stellar ultraviolet radiation and collisions in a warm, dense medium play a role. The morphology is interpreted as emission from a molecular outflow which appears to wiggle as a result of the fact that T Tau is a binary system seen almost pole-on. It is shown that an outflow with a small opening angle can reproduce the observed extended emission. From comparison with previous studies it is argued that the molecular outflow originates from T Tau S, the infrared component. The presented model constrains the orientation and geometry of the system

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&

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

Subarcsecond Imaging at 267 GHz of a Young Binary System: Detection of a Dust Disk of Radius Less than 70 AU around T Tauri N

The young binary system T Tauri was observed with the Owens Valley Millimeter Array in the 267 GHz continuum and HCO^+ J = 3-2 emission at 0".8 resolution, with the single-baseline interferometer of the James Clerk Maxwell Telescope-Caltech Submillimeter Observatory in the 357 GHz continuum and with the W. M. Keck Telescope at λ = 4 μm. The 267 GHz emission is unresolved, with a flux of 397±35 mJy, located close to the position of the optical star T Tau N. An upper limit of 100 mJy is obtained toward the infrared companion T Tau S. The 357 GHz continuum emission is unresolved, with a flux of 1.35±0.68 Jy. HCO^+ J = 3-2 was detected from a 2" diameter core surrounding T Tau N and S. Both stars are detected at 4 μm, but there is no evidence of the radio source T Tau R. We propose a model in which T Tau S is intrinsically similar to T Tau N but is obscured by the outer parts of T Tau N's disk. A fit to the spectral energy distribution (SED) between 21 cm and 1.22 μm is constructed on this basis. Adopting an r^(−1) surface density distribution and an exponentially truncated edge, disk masses of 0.04±0.01 and 6×10^(−5) to 3×10^(−3) M_☉ are inferred for T Tau N and T Tau S, respectively. A 0.005-0.03 M_☉ circumbinary envelope is also required to fit the millimeter to mid-infrared SED

Magnetic field measurements at milliarcsecond resolution around massive young stellar objects

Magnetic fields have only recently been included in theoretical simulations of high-mass star formation. The simulations show that magnetic fields can play a crucial role not only in the formation and dynamics of molecular outflows, but also in the evolution of circumstellar disks. Therefore, new measurements of magnetic fields at milliarcsecond resolution close to massive young stellar objects (YSOs) are fundamental for providing new input for numerical simulations and for understanding the formation process of massive stars. The polarized emission of 6.7 GHz CH3OH masers allows us to investigate the magnetic field close to the massive YSO where the outflows and disks are formed. Recently, we have detected with the EVN CH3OH maser polarized emission towards 10 massive YSOs. From a first statistical analysis we have found evidence that magnetic fields are primarily oriented along the molecular outflows. To improve our statistics we are carrying on a large observational EVN campaign for a total of 19 sources, the preliminary results of the first seven sources are presented in this contribution. Furthermore, we also describe our efforts to estimate the Lande' g-factors of the CH3OH maser transition to determine the magnetic field strength from our Zeeman-splitting measurements.Comment: Accepted for publication in the proceeding of the "12th European VLBI Network Symposium and Users Meeting", eds Tarchi et al. PoS(EVN 2014)04

Planar infall of CH3OH gas around Cepheus A HW2

Aims: In order to test the nature of an (accretion) disk in the vicinity of Cepheus A HW2, we measured the three-dimensional velocity field of the CH3OH maser spots, which are projected within 1000au of the HW2 object, with an accuracy of the order of 0.1km/s. Methods: We made use of the European VLBI Network (EVN) to image the 6.7GHz CH3OH maser emission towards Cepheus A HW2 with 4.5 milli-arcsecond resolution (3au). We observed at three epochs spaced by one year between 2013 and 2015. During the last epoch, on mid-march 2015, we benefited from the new deployed Sardinia Radio Telescope. Results: We show that the CH3OH velocity vectors lie on a preferential plane for the gas motion with only small deviations of 12+/-9 degrees away from the plane. This plane is oriented at a position angle of 134 degrees east of north, and inclined by 26 degrees with the line-of-sight, closely matching the orientation of the disk-like structure previously reported by Patel et al.(2005). Knowing the orientation of the equatorial plane, we can reconstruct a face-on view of the CH3OH gas kinematics onto the plane. CH3OH maser emission is detected within a radius of 900au from HW2, and down to a radius of about 300au, the latter coincident with the extent of the dust emission at 0.9mm. The velocity field is dominated by an infall component of about 2km/s down to a radius of 300au, where a rotational component of 4km/s becomes dominant. We discuss the nature of this velocity field and the implications for the enclosed mass. Conclusions: These findings bring direct support to the interpretation that the high-density gas and dust emission, surrounding Cepheus A HW2, trace an accretion disk.Comment: 9 pages, 4 figures, 2 tables, accepted by Astronomy & Astrophysic

Finding evolved stars in the inner Galactic disk with Gaia

The Bulge Asymmetries and Dynamical Evolution (BAaDE) survey will provide positions and line-of-sight velocities of ~20,000 evolved, maser bearing stars in the Galactic plane. Although this Galactic region is affected by optical extinction, BAaDE targets may have Gaia cross-matches, eventually providing additional stellar information. In an initial attempt to cross-match BAaDE targets with Gaia, we have found more than 5,000 candidates. Of these, we may expect half to show SiO emission, which will allow us to obtain velocity information. The cross-match is being refined to avoid false positives using different criteria based on distance analysis, flux variability, and color assessment in the mid- and near-IR. Once the cross-matches can be confirmed, we will have a unique sample to characterize the stellar population of evolved stars in the Galactic bulge, which can be considered fossils of the Milky Way formation.Comment: To appear in the Proceedings of the IAU Symposium No. 330: "Astrometry and Astrophysics in the Gaia sky