Maria Rioja

Methanol and water masers in IRAS 20126+4104: the distance, the disk, and the jet

Context. Knowledge of the distance to high-mass star forming regions is crucial to obtain accurate luminosity and mass estimates of young OB-type (proto)stars and thus better constrain their nature and age. IRAS20126+4104 is a special case, being the best candidate of a high-mass (proto)star surrounded by an accretion disk. Such a fact may be used to set constraints on theories of highmass star formation, but requires confirmation that the mass and luminosity of IRAS20126+4104 are indeed typical of a B0.5 star, which in turn requires an accurate estimate of the distance. Aims. The goal of our study is twofold: to determine the distance to IRAS20126+4104, using the parallax of H2O masers associated with the source, and unveil the 3D velocity field of the disk, through proper motion measurements of the 6.7 GHz CH3OH masers. At the same time, we can also obtain an estimate of the systemic velocity in the plane of the sky of the disk+star system. Methods. We used the Very Long Baseline Array and the European VLBI Network to observe the 22.2 GHz H2O and 6.7 GHz CH3OH masers in IRAS20126+4104 at a number of epochs suitably distributed in time. The absolute positions of the maser features were established with respect to reference quasars, which allowed us to derive absolute proper motions. Results. From the parallax of the H2O masers we obtain a distance of 1.64 ± 0.05 kpc, which is very similar to the value adopted so far in the literature (1.7 kpc) and confirms that IRAS20126+4104 is a high-mass (proto)star. From the CH3OH masers we derive the component in the plane of the sky of the systemic velocity of the disk+star system (−16 km s −1 in right-ascension and +7.6 km s −1 in declination). Accurate knowledge of the distance and systemic velocity allows us to improve on the model fit to the H2O maser jet presented in a previous study. Finally, we identify two groups of CH3OH maser features, one undergoing rotation in the disk and possibly distributed along a narrow ring centered on the star, the other characterised by relative proper motions indicating that the features are moving away from the disk, perpendicular to it. We speculate that the latter group might be tracing the disk material marginally entrained by the jet. Conclusions. VLBImulti-epoch observations with phase referencing are confirmed to be an excellent tool for distance determinations and for the investigation of the structure and 3D velocity field within a few 100 AU from newly born high-mass stars.

Water masers in the massive protostar IRAS 20126+4104: ejection and deceleration

Received date; accepted date Abstract. We report on the first multi-epoch, phase referenced VLBI observations of the H2O maser emission in a high-mass protostar associated with a disk-jet system. The source under study, IRAS20126+4104, has been extensively investigated in a large variety of tracers, including H2O maser VLBA data acquired by us three years before the present observations. The new findings fully confirm the interpretation proposed in our previous study, namely that the maser spots are expanding from a common origin coincident with the protostar. We also demonstrate that the observed 3-D velocities of the maser spots can be fitted with a model assuming that the spots are moving along the surface of a conical jet, with speed increasing for increasing distance from the cone vertex. We also present the results of single-dish monitoring of the H2O maser spectra in IRAS20126+4104. These reveal that the peak velocity of some maser lines decreases linearly with time. We speculate that such a deceleration could be due to braking of the shocks from which the maser emission originates, due to mass loading at the shock front or dissipation of the shock energy.

HIGH-PRECISION ASTROMETRIC MILLIMETER VERY LONG BASELINE INTERFEROMETRY USING A NEW METHOD FOR ATMOSPHERIC CALIBRATION

We describe a new method which achieves high-precision very long baseline interferometry (VLBI) astrometry in observations at millimeter (mm) wavelengths. It combines fast frequency-switching observations, to correct for the dominant non-dispersive tropospheric fluctuations, with slow source-switching observations, for the remaining ionospheric dispersive terms. We call this method source-frequency phase referencing. Provided that the switching cycles match the properties of the propagation media, one can recover the source astrometry. We present an analytic description of the two-step calibration strategy, along with an error analysis to characterize its performance. Also, we provide observational demonstrations of a successful application with observations using the Very Long Baseline Array at 86 GHz of the pairs of sources 3C274 and 3C273 and 1308+326 and 1308+328 under various conditions. We conclude that this method is widely applicable to mm-VLBI observations of many target sources, and unique in providing bona fide astrometrically registered images and high-precision relative astrometric measurements in mm-VLBI using existing and newly built instruments, including space VLBI.

Physical properties of high-mass clumps in different stages of evolution

Context. The details of the process of massive star formation are still elusive. A complete characterization of the first stages of the process from an observational point of view is needed to constrain theories on the subject. In the past 20 years we have made a thorough investigation of colour-selected IRAS sources over the whole sky. The sources in the northern hemisphere were studied in detail and used to derive an evolutionary sequence based on their spectral energy distribution. Aims. To investigate the first stages of the process of high-mass star formation, we selected a sample of massive clumps previously observed with the Swedish-ESO Submillimetre Telescope at 1.2 mm and with the ATNF Australia Telescope Compact Array at 1.3 cm. We want to characterize the physical conditions in such sources, and test whether their properties depend on the evolutionary stage of the clump. Methods. With ATCA we observed the selected sources in the NH3(1,1) and (2,2) transitions and in the H2O(616 − 523) maser line. Ammonia lines are a very good temperature probe that allow us to accurately determine the mass and the column, volume, and surface densities of the clumps. We also collected all data available to construct the spectral energy distribution of the individual clumps and to determine if star formation is already occurring through observations of its most common signposts, thus putting constraints on the evolutionary stage of the source. We fitted the spectral energy distribution between 1.2 mm and 70 μm with a modified black body to derive the dust temperature and independently determine the mass. Results. We find that the clumps are cold (T ∼ 10 − 30 K), massive (M ∼ 102 − 103 M ), and dense (n(H2) & 105 cm−3) and that they have high column densities (N(H2) ∼ 1023 cm−2). All clumps appear to be potentially able to form high-mass stars. The most massive clumps appear to be gravitationally unstable, if the only sources of support against collapse are turbulence and thermal pressure, which possibly indicates that the magnetic field is important in stabilizing them. Conclusions. After investigating how the average properties depend on the evolutionary phase of the source, we find that the temperature and central density progressively increase with time. Sources likely hosting a ZAMS star show a steeper radial dependence of the volume density and tend to be more compact than starless clumps.

The VSOP 5 GHz active galactic nucleus survey. V. Imaging results for the remaining 140 sources

In 1997 February, the Japanese radio astronomy satellite HALCA was launched to provide the space-bourne element for the VLBI Space Observatory Program (VSOP) mission. Approximately 25% of the mission time was dedicated to the VSOP survey of bright compact active galactic nuclei (AGNs) at 5 GHz. This paper, the fifth in the series, presents images and models for the remaining 140 sources not included in the third paper in the series, which contained 102 sources. For most sources, the plots of the (u, v) coverage, the visibility amplitude versus (u, v) distance, and the high-resolution image are presented. Model fit parameters to the major radio components are determined, and the brightness temperature of the core component for each source is calculated. The brightness temperature distributions for all of the sources in the VSOP AGN survey are discussed.

FIRST PARALLAX MEASUREMENTS TOWARD A 6.7 GHz METHANOL MASER WITH THE AUSTRALIAN LONG BASELINE ARRAY—DISTANCE TO G 339.884−1.259.

We have conducted the first parallax and proper motion measurements of 6.7 GHz methanol maser emission using the Australian Long Baseline Array. The parallax of G 339.884-1.259 measured from five epochs of observations is 0.48 +/- 0.08 mas, corresponding to a distance of 2.1(-0.3)(+0.4) kpc, placing it in the Scutum spiral arm. This is consistent (within the combined uncertainty) with the kinematic distance estimate for this source at 2.5 +/- 0.5 kpc using the latest Solar and Galactic rotation parameters. We find from the Lyman continuum photon flux that the embedded core of the young star is of spectral type B1, demonstrating that luminous 6.7 GHz methanol masers can be associated with high-mass stars toward the lower end of the mass range.

Giant radio galaxies - I. Intergalactic barometers

We present new wideband radio observations with the Australia Telescope Compact Array of a sample of 12 giant radio galaxies. The radio observations are part of a larger radio-optical study aimed at relating the radio structures with the ambient medium on large scales. With projected linear sizes larger than 0.7 Mpc, these objects are ideal candidates for the study of the Warm-Hot Intergalactic Medium (WHIM). The sample includes sources with sizes spanning 0.8 to 3.2 Mpc and total powers of 1.2*10^24 to 4.0*10^26 W Hz^-1 at 2.1 GHz. Redshifts were limited to z<0.15 to permit spectroscopic observations of the hosts and neighbouring galaxies, which were obtained using the AAOmega spectrograph on the Anglo-Australian Telescope. We derive lobe energy densities from the radio observations via equipartition arguments. The inferred pressures in the lobes of the giant radio sources, which range from 1.1*10^-15 to 2.0*10^-14 Pa (80 to 1500 cm^-3 K), are lower than previously inferred from X-ray observations of dense filaments. Comparison with the OverWhelmingly Large Simulations (OWLS) suggests that the WHIM in pressure balance with the radio lobes has a temperature in excess of ~10^6.5 K or a particle overdensity in the range 50 to 500. This study highlights the capability of next generation surveys, such as the Evolutionary Map of the Universe (EMU) survey with the Australian Square Kilometre Array Pathfinder (ASKAP), to study populations of giant radio sources at lower surface brightness and thereby discriminate between models for the cosmological evolution of the intergalactic medium and examine the validity of cosmological hydrodynamical simulations.

The power of simultaneous multifrequency observations for mm-VLBI: Astrometry up to 130 GHz with the KVN

Simultaneous observations at multiple frequency bands have the potential to overcome the fundamental limitation imposed by the atmospheric propagation in mm-VLBI observations. The propagation effects place a severe limit in the sensitivity achievable in mm-VLBI, reducing the time over which the signals can be coherently combined, and preventing the use of phase referencing and astrometric measurements. We carried out simultaneous observations at 22, 43, 87 and 130 GHz of a group of five AGNs, the weakest of which is ca. 200 mJy at 130 GHz, with angular separations ranging from 3.6 to 11 degrees, using the KVN. We analysed this data using the Frequency Phase Transfer (FPT) and the Source Frequency Phase Referencing (SFPR) techniques, which use the observations at a lower frequency to correct those at a higher frequency. The results of the analysis provide an empirical demonstration of the increase in the coherence times at 130 GHz from a few tens of seconds to about twenty minutes, with FPT, and up to many hours with SFPR. Moreover the astrometric analysis provides high precision relative position measurements between two frequencies, including, for the first time, astrometry at 130 GHz. Finally we demonstrate a method for the generalised decomposition of the relative position measurements into absolute position shifts for bona fide astrometric registration of the maps of the individual sources at multiple frequencies, up to 130 GHz.

Exploration of Source Frequency Phase Referencing Techniques for Astrometry and Observations of Weak Sources with High Frequency Space Very Long Baseline Interferometry

Space Very-Long-Baseline-Interferometry (S-VLBI) observations at high frequencies hold the prospect of achieving the highest angular resolutions and astrometric accuracies, resulting from the long baselines between ground and satellite telescopes. Nevertheless, space-specific issues, such as limited accuracy in the satellite orbit reconstruction and constraints on the satellite antenna pointing operations, limit the application of conventional phase referencing. We investigate the feasibility of an alternative technique, source frequency phase referencing (SFPR), to the S-VLBI domain. With these investigations we aim to contribute to the design of the next-generation of S-VLBI missions. We have used both analytical and simulation studies to characterize the performance of SFPR in S-VLBI observations, applied to astrometry and increased coherence time, and compared these to results obtained using conventional phase referencing. The observing configurations use the specifications of the ASTRO-G mission for their starting point. Our results show that the SFPR technique enables astrometry at 43 GHz, using alternating observations with 22 GHz, regardless of the orbit errors, for most weathers and under a wide variety of conditions. The same applies to the increased coherence time for the detection of weak sources. Our studies show that the capability to carry out simultaneous dual frequency On secondment Observatorio Astronómico Nacional (OAN), Spain.Space very long baseline interferometry (S-VLBI) observations at high frequencies hold the prospect of achieving the highest angular resolutions and astrometric accuracies, resulting from the long baselines between ground and satellite telescopes. Nevertheless, space-specific issues, such as limited accuracy in the satellite orbit reconstruction and constraints on the satellite antenna pointing operations, limit the application of conventional phase referencing. We investigate the feasibility of an alternative technique, source frequency phase referencing (SFPR), to the S-VLBI domain. With these investigations we aim to contribute to the design of the next generation of S-VLBI missions. We have used both analytical and simulation studies to characterize the performance of SFPR in S-VLBI observations, applied to astrometry and increased coherence time, and compared these to results obtained using conventional phase referencing. The observing configurations use the specifications of the ASTRO-G mission for their starting point. Our results show that the SFPR technique enables astrometry at 43 GHz, using alternating observations with 22 GHz, regardless of the orbit errors, for most weathers and under a wide variety of conditions. The same applies to the increased coherence time for the detection of weak sources. Our studies show that the capability to carry out simultaneous dual frequency observations enables application to higher frequencies, and a general improvement of the performance in all cases, hence we recommend its consideration for S-VLBI programs.

THE IMPACT OF FREQUENCY STANDARDS ON COHERENCE IN VLBI AT THE HIGHEST FREQUENCIES

We have carried out full imaging simulation studies to explore the impact of frequency standards in millimeter and submillimeter very long baseline interferometry (VLBI), focusing on the coherence time and sensitivity. In particular, we compare the performance of the H-maser, traditionally used in VLBI, to that of ultra-stable cryocooled sapphire oscillators over a range of observing frequencies, weather conditions, and analysis strategies. Our simulations show that at the highest frequencies, the losses induced by H-maser instabilities are comparable to those from high-quality tropospheric conditions. We find significant benefits in replacing H-masers with cryocooled sapphire oscillator based frequency references in VLBI observations at frequencies above 175 GHz in sites which have the best weather conditions; at 350 GHz we estimate a 20%-40% increase in sensitivity over that obtained when the sites have H-masers, for coherence losses of 20%-10%, respectively. Maximum benefits are to be expected by using co-located Water Vapor Radiometers for atmospheric correction. In this case, we estimate a 60%-120% increase in sensitivity over the H-maser at 350 GHz.