bluuebunny/crossref_metadata_2025_split huggingface dataset:
A Circumstellar Disk toward the High-mass Star-forming Region IRAS 23033+5951
Authors: Trinidad M. A. , Rodríguez T. , Migenes V. 
Published: July 2012

AbstractWe present water maser observations toward IRAS 23033+5951 carried out with the VLA-EVLA in the A configuration. In order to study the spatio-kinematical distribution of the water masers detected in the region, we made a simple geometrical and kinematical model based on the conical equation. We find that the water masers are tracing a rotating and contracting circumstellar disk of about 110 AU around a very young source of 18 M⊙, which has not enough ionizing photons to be detected at centimeter wavelengths.
Radio and IR interferometry of SiO maser stars
Authors: Wittkowski Markus , Boboltz David A. , Gray Malcolm D. , Humphreys Elizabeth M. L. , Karovicova Iva , Scholz Michael 
Published: July 2012

AbstractRadio and infrared interferometry of SiO maser stars provide complementary information on the atmosphere and circumstellar environment at comparable spatial resolution. Here, we present the latest results on the atmospheric structure and the dust condensation region of AGB stars based on our recent infrared spectro-interferometric observations, which represent the environment of SiO masers. We discuss, as an example, new results from simultaneous VLTI and VLBA observations of the Mira variable AGB star R Cnc, including VLTI near- and mid-infrared interferometry, as well as VLBA observations of the SiO maser emission toward this source. We present preliminary results from a monitoring campaign of high-frequency SiO maser emission toward evolved stars obtained with the APEX telescope, which also serves as a precursor of ALMA images of the SiO emitting region. We speculate that large-scale long-period chaotic motion in the extended molecular atmosphere may be the physical reason for observed deviations from point symmetry of atmospheric molecular layers, and for the observed erratic variability of high-frequency SiO maser emission.
The first water fountain in a planetary nebula
Authors: Suárez Olga , Francisco Gómez José , Bendjoya Philippe , Miranda Luis. F. , Guerrero Martín. A. , Ramos-Larios Gerardo , Ricardo Rizzo J. , Uscanga Lucero 
Published: July 2012

AbstractWater fountains are evolved stars showing water masers with velocity spanning more than ~100 km/s. They usually appear at the end of the Asymptotic Giant Branch (AGB) phase or at the beginning of the post-AGB phase, and their masers trace the first manifestation of axisymmetric collimated mass-loss. For the first time, masers with water fountain characteristics have been detected towards a PN (IRAS 15103–5754), which might require a revision of the current theories about jet formation and survival times. IRAS 15103-5754 was observed using the ATCA interferometer at 22 GHz (both continuum and water maser). The main results of these observations are summarized here. The evolutionary classification of this object is also discussed.
Polarization properties of R Cas SiO masers
Authors: Assaf K. A. , Diamond P. J. , Richards A. M. S. , Gray M. D. 
Published: July 2012

AbstractSilicon monoxide maser emission has been detected in the circumstellar envelopes of many evolved stars. It is a good tracer of the wind dynamics within a few stellar radii of the central star. We investigated the polarization morphology in the circumstellar envelope of an AGB star, R Cas, by using the VLBA to map the linear and circular polarization of the v=1, J=1-0 SiO maser transition during 23 epochs over two stellar cycles. The average fractional circular polarization is a few percent. The average fractional linear polarization per epoch is 11–58%, but some isolated features exceed 100%, probably because the total intensity emission is smoother and more resolved-out. The maser electric polarization vector angle has a preferrential tendency to be either parallel or perpendicular to the radial direction to the star.
High Resolution Radio and IR Observations of AGB Stars
Authors: Cotton W. , Perrin G. , Millan-Gabet R. , Delaa O. , Mennesson B. 
Published: July 2012

AbstractAsymptotic Giant Branch Stars (AGB) are evolved, mass losing red giants with tenuous molecular envelopes which have been the subject of much recent study using infrared and radio interferometers. In oxygen rich stars, radio SiO masers form in the outer regions of the molecular envelopes and are powerful diagnostics of the extent of these envelopes. Spectroscopically resolved infrared interferometry helps constrain the extent of various species in the molecular layer. We made VLBA 7 mm SiO maser, Keck Interferometer near IR and VLTI/MIDI mid IR high resolution observations of the stars U Ari, W Cnc, RX Tau, RT Aql, S Ser and V Mon. This paper presents evidence that the SiO is depleted from the gas phase and speculate that it is frozen onto Al2O3 grains and that radiation pressure on these grains help drive the outflow.
Water Masers Toward Star-Forming Regions in the Bolocam Galactic Plane Survey
Authors: Dunham Miranda K. ,   The BGPS Team
Published: July 2012

AbstractWe present preliminary results of a search for 22 GHz water masers toward 1400 star-forming regions seen in the Bolocam Galactic Plane Survey (BGPS) using the Green Bank Telescope (GBT). The BGPS is a blind survey of the Northern Galactic plane in 1.1 mm thermal dust emission that has cataloged star-forming regions at all evolutionary stages. Further information is required to determine the stage of each BGPS source. Since water masers are produced by outflows from low and high-mass star forming regions, their presence is a key component of determining whether the BGPS sources are forming stars and which evolutionary stage they are in. We present preliminary detection statistics, basic properties of the water masers, and correlations with physical properties determined from the 1.1 mm emission and ammonia observations obtained concurrently with the water masers on the GBT.
Simultaneous observations of SiO and H2O masers toward known stellar SiO and/or H2O maser sources
Authors: Kim Jaeheon , Cho Se-Hyung , Kim Sang Joon 
Published: July 2012

AbstractWe present the results of simultaneous observations of SiO v = 1, 2, 29SiO v = 0, J = 1−0 and H2O 616-523 maser lines toward 318 known stellar SiO and/or H2O maser sources using the Yonsei 21-m radio telescope of the Korean VLBI Network. Toward 166 known SiO and H2O maser sources, both SiO and H2O maser emissions were detected from 112 sources giving a detection rate of 67.5 %. On the other hand, toward 152 known H2O-only maser sources, both SiO and H2O maser emissions were detected from 62 sources, giving a detection rate of 40.8 %. Characteristics of all observed sources in the IRAS two-color diagram is investigated including their evolutionary sequence and mutual relations between SiO and H2O maser properties.
Cosmology and the Hubble Constant: On the Megamaser Cosmology Project (MCP)
Authors: Henkel C. , Braatz J. A. , Reid M. J. , Condon J. J. , Lo K. Y. , Impellizzeri C. M. Violette , Kuo C. Y. 
Published: July 2012

AbstractThe Hubble constant H0 describes not only the expansion of local space at redshift z ~ 0, but is also a fundamental parameter determining the evolution of the universe. Recent measurements of H0 anchored on Cepheid observations have reached a precision of several percent. However, this problem is so important that confirmation from several methods is needed to better constrain H0 and, with it, dark energy and the curvature of space. A particularly direct method involves the determination of distances to local galaxies far enough to be part of the Hubble flow through water vapor (H2O) masers orbiting nuclear supermassive black holes. The goal of this article is to describe the relevance of H0 with respect to fundamental cosmological questions and to summarize recent progress of the ‘Megamaser Cosmology Project’ (MCP) related to the Hubble constant.
AGN and Megamasers
Authors: Tarchi Andrea 
Published: July 2012

AbstractLuminous extragalactic masers are traditionally referred to as the ‘megamasers’. Those produced by water molecules are associated with accretion disks, radio jets, or outflows in the nuclear regions of active galactic nuclei (AGN). The majority of OH maser sources are instead driven by intense star formation in ultra-luminous infrared galaxies, although in a few cases the OH maser emission traces rotating (toroidal or disk) structures around the nuclear engines of AGN. Thus, detailed maser studies provide a fundamental contribution to our knowledge of the main nuclear components of AGN, constitute unique tools to measure geometric distances of host galaxies, and have a great impact on probing the, so far, paradigmatic Unified Model of AGN.
Maser Astrometry: from Galactic Structure to Local Group Cosmology
Authors: Reid Mark J. 
Published: July 2012

AbstractThis review summarizes current advances in astrometry of masers as they pertain to large-scale Galactic structure and dynamics and Local Group cosmology. Parallaxes and proper motions have now been measured for more than 60 massive star forming regions using the Japanese VERA array, the EVN and the VLBA. These results provide “gold standard” distances and 3-dimensional velocities for sources across the Milky Way, revealing its spiral structure. Modeling these data tightly constrains the fundamental parameters of the Milky Way: R0and Θ0. Proper motions of Local Group galaxies have been measured, improving our understanding of the history and fate of the Group.
3D velocity fields from methanol and water masers in an intermediate-mass protostar
Authors: Goddi C. , Moscadelli L. , Sanna A. 
Published: July 2012

AbstractWe report multi-epoch VLBI observations of molecular masers towards the high-mass star forming region AFGL 5142, leading to the determination of the 3D velocity field of circumstellar molecular gas at radii <0.″23 (or 400 AU) from the protostar MM–1. Our observations of CH3OH maser emission enabled, for the first time, a direct measurement of infall of a molecular envelope on to an intermediate-mass protostar (radius of 300 AU, velocity of 5 km s−1, and infall rate of 6 × 10−4n8M⊙ yr−1, where n8 is the ambient volume density in units of 108 cm−3). In addition, our measurements of H2O maser (and radio continuum) emission revealed a collimated bipolar molecular outflow (and ionized jet) from MM–1. The evidence of simultaneous accretion and outflow at small spatial scales, makes AFGL 5142 an extremely compelling target for high-angular resolution studies of high-mass star formation.
The bar effect in the galactic gas motions traced by 6.7 GHz methanol maser sources with VERA
Authors: Matsumoto Naoko , Honma Mareki ,   VERA project members
Published: July 2012

AbstractTo establish the existence of the galactic bar, ten methanol maser sources around the starting points of the spiral arms were observed with VERA (VLBI Exploration of radio astrometry) using the phase-referencing technique at 6.7 GHz band. For six out of ten sources, absolute proper motions were obtained with better than 3σ accuracy. Using VLBI 3-D data of eight sources, including our five sources, we compared the observed data with three galactic models and found that the model including the bar effect is better to explain the 3-D data, than a flat circular rotation model. A non-flat circular rotation model is also consistent with the VLBI data. Based on a dynamical model with a bar, we estimate an inclination angle of the bar around 35°, which is consistent with previous studies.
Distance and Size of the Red Hypergiant NML Cyg
Authors: Zhang B. , Reid M. J. , Menten K. M. , Zheng X. W. , Brunthaler A. 
Published: July 2012

AbstractWe present astrometric results of phase-referencing VLBI observations of 22 GHz H2O maser and 43 GHz SiO maser emission towards the red hypergiant NML Cyg using VLBA. We obtained an annual parallax of 0.62 ± 0.04 mas, corresponding to a distance of 1.61+0.13−0.11 kpc. With a VLA observation in its largest (A) configuration at 43 GHz, we barely resolve the radio photosphere of NML Cyg, and find a uniform-disk diameter of 44 ± 16 mas.
Class I Methanol Masers in the Galactic Center
Authors: Sjouwerman Loránt O. , Pihlström Ylva M. 
Published: July 2012

AbstractWe report on 36 and 44 GHz Class I methanol (CH3OH) maser emission in the Sagittarius A (Sgr A) region with the Expanded Very Large Array (EVLA). At least three different maser transitions tracing shocked regions in the cm-wave radio regime can be found in Sgr A. 44 GHz masers correlate with the positions and velocities of 36 GHz CH3OH masers, but the methanol masers correlate less with 1720 MHz OH masers. Our results agree with theoretical predictions that the densities and temperatures conducive for 1720 MHz OH masers may also produce 36 and 44 GHz CH3OH maser emission. However, many 44 GHz masers do not overlap with 36 GHz methanol masers, suggesting that 44 GHz masers also arise in regions too hot and too dense for 36 GHz masers to form. This agrees with the non-detection of 1720 MHz OH masers in the same area, which are thought to be excited under cooler or denser conditions. We speculate that the geometry of the bright 36 GHz masers in Sgr A East outlines the location of a SNR shock front.
Maser observations with new instruments
Authors: Wootten Alwyn 
Published: July 2012

AbstractThe Atacama Large Millimeter/submillimeter Array (ALMA)‡, and the Jansky Very Large Array (JVLA) have recently begun probing the Universe. Both provide the largest collecting area available at locations on a high dry site, endowing them with unparalleled potential for sensitive spectral line observations. Over the next few years, these telescopes will be joined by other telescopes to provide advances in maser science, including NOEMA and the LMT. Other instruments of note for maser science which may commence construction include the North American Array, the CCAT, and an enlarged worldwide VLB network outfitted to operate into the millimeter wavelength regime.
OH maser observations using the Russian interferometric network “Quasar” in preparation for scientific observations with the space mission RadioAstron.
Authors: Litovchenko I. D. , Alakoz A. V. , Kostenko V. I. , Lihachev S. F. , Finkelstein A. M. , Ipatov A. V. 
Published: July 2012

AbstractWe present results of a VLBI experiment at a wavelength of 18 cm, which simulates the ground-space interferometer with space link to RadioAstron. An array of five antennas was used, four of them are located in the Russian Federation, plus the the 32-m radio telescope in Medicine (Italy). The 22-m radio telescope in Pushchino (Moscow Region) acted in place of the space arm. It has an effective area of 100 square meters. The three other Russian 32-m antennas are operated by the Institute of Applied Astronomy RAS; they are located at Badary, Svetloe and Zelenchukskaya (interferometer network “Quasar”). The maximum base-line, Badary-Svetloe, was about 4402 km, providing an angular resolution of about 0.009 arc seconds at a wavelength of 18 cm. The duration of the experiment was 10 hours on 02/03 February 2011. The program of observations included quasars 3C273, 3C279, 3C286 and the maser source - W3(OH). W3(OH) was observed only by the Russian telescopes and was investigated at the frequency of the 1665 MHz main line. The data were recorded on the MK5 recorder (32-m radio telescopes) and the RDR system (RadioAstron Digital Recorder) in Pushchino. The low SEFD (system equivalence of flux density) of Pushchino emulated the RadioAstron antenna. Correlation was performed with the universal software correlator of the AstroSpace Center of Lebedev Physical Institute. The correlator output format is compatible with that used by the AIPS package, which was used for data analysis. After analyzing the correlated data we obtained relative coordinates of the maser components. The main results are tabulated and presented in the figures. The data quality is sufficient for astrophysical analysis and comparison with previous observations of maser source W3(OH) on VLBI networks EVN and VLBA.
Birth of an ocean in the Red Sea: Initial pangs
Authors: Ligi Marco , Bonatti Enrico , Bortoluzzi Giovanni , Cipriani Anna , Cocchi Luca , Caratori Tontini Fabio , Carminati Eugenio , Ottolini Luisa , Schettino Antonio 
Published: July 2012

We obtained areal variations of crustal thickness, magnetic intensity, and degree of melting of the sub‐axial upwelling mantle at Thetis and Nereus Deeps, the two northernmost axial segments of initial oceanic crustal accretion in the Red Sea, where Arabia is separating from Africa. The initial emplacement of oceanic crust occurred at South Thetis and Central Nereus roughly ∼2.2 and ∼2 Ma, respectively, and is taking place today in the northern Thetis and southern Nereus tips. Basaltic glasses major and trace element composition suggests a rift‐to‐drift transition marked by magmatic activity with typical MORB signature, with no contamination by continental lithosphere, but with slight differences in mantle source composition and/or potential temperature between Thetis and Nereus. Eruption rate, spreading rate, magnetic intensity, crustal thickness and degree of mantle melting were highest at both Thetis and Nereus in the very initial phases of oceanic crust accretion, immediately after continental breakup, probably due to fast mantle upwelling enhanced by an initially strong horizontal thermal gradient. This is consistent with a rift model where the lower continental lithosphere has been replaced by upwelling asthenosphere before continental rupturing, implying depth‐dependent extension due to decoupling between the upper and lower lithosphere with mantle‐lithosphere‐necking breakup before crustal‐necking breakup. Independent along‐axis centers of upwelling form at the rifting stage just before oceanic crust accretion, with buoyancy‐driven convection within a hot, low viscosity asthenosphere. Each initial axial cell taps a different asthenospheric source and serves as nucleus for axial propagation of oceanic accretion, resulting in linear segments of spreading.
Damage in step‐overs may enable large cascading earthquakes
Authors: Finzi Y. , Langer S. 
Published: July 2012

Seismic hazard analysis relies on the ability to predict whether an earthquake will terminate at a fault tip or propagate onto adjacent faults, cascading into a larger, more devastating event. While ruptures are expected to arrest at fault discontinuities larger than 4–5 km, scientists are often puzzled by much larger rupture jumps. Here we show that material properties between faults significantly affect the ability to arrest propagating ruptures. Earthquake simulations accounting for fault step‐over zones weakened by accumulated damage provide new insights into rupture propagation. Revealing that lowered rigidity and material interfaces promote rupture propagation, our models show for the first time that step‐overs as wide as 10 km may not constitute effective earthquake barriers. Our results call for re‐evaluation of seismic hazard analyses that predict rupture length and earthquake magnitude based on historic records and fault segmentation models.
Physical parameterization of Strombolian eruptions via experimentally‐validated modeling of high‐speed observations
Authors: Taddeucci J. , Alatorre‐Ibargüengoitia M. A. , Moroni M. , Tornetta L. , Capponi A. , Scarlato P. , Dingwell D. B. , De Rita D. 
Published: July 2012

Pressurized gas drives explosive volcanic eruptions. Existing models can predict the amount and pressure of gas in erupting magma, but application and testing of such models is currently limited by the accuracy of input parameters from natural systems. Here, we present a new methodology, based on a novel integration of 1) high‐speed imaging and 2) shock‐tube modeling of volcanic activity in order to derive estimates of sub‐second variations in the pressure, mass, and volume of gas that drive the dynamics of unsteady eruptions. First, we validate the method against laboratory‐scale shock‐tube experiments. Having validated the method we then apply it to observations of eruptions at Stromboli volcano (Italy). Finally, we use those results for a parametric study of the weight of input parameters on final outputs. We conclude that Strombolian explosions, with durations of seconds, result from discrete releases of gas with mass and pressure in the 4–714 kg and 0.10–0.56 MPa range, respectively, and which occupy the volcano conduit to a depth of 4–190 m. These variations are present both among and within individual explosions.
Interpretation of vector electric field measurements of bursty Langmuir waves in the cusp
Authors: Dombrowski M. P. , LaBelle J. , Rowland D. E. , Pfaff R. F. , Kletzing C. A. 
Published: July 2012

An analysis of auroral Langmuir waves in the cusp observed by two high‐frequency electric field instruments on the TRICE high‐flyer sounding rocket shows many examples of Langmuir wave bursts modulated at approximately 10 kHz. Previous studies have explained these and similar observations as the result of beating between waves with very close frequencies near the Langmuir cutoff, resulting from either wave‐wave interactions or independent linear excitations. The unique three‐dimensional (3‐D) data set provided by the NASA Goddard Space Flight Center TAEFWD instrument shows that up to 25% of waveforms selected from the most intense bursts exhibit anisotropic modulations, i.e., the beat nulls and peaks are not aligned in time across the three perpendicular electric field components. Modulations of this type arise when superposed wave normal modes possess differing polarizations, and simulations using wave modes calculated with the J‐WHAMP numerical dispersion code show (1) that waves with differing polarizations can exist in conditions like those observed by TRICE‐High and (2) that superpositions of such waves can produce anisotropic modulation. Fourier analysis of the 3‐D waveform data suggests that both linear and elliptically polarized waves are present near the Langmuir cutoff at these times. These observations illustrate how 3‐D measurements can give valuable insight into the nature of wave interactions in the auroral plasma environment.
Estimating fracture orientation from elastic‐wave propagation: An ultrasonic experimental approach
Authors: de Figueiredo J. J. S. , Schleicher J. , Stewart R. R. , Dyaur N. 
Published: July 2012

Elastic‐wave propagation in fractured and cracked media depends on the dominant spatial orientation of the discontinuities. Consequently, compressional and shear‐wave velocities can give valuable information about the orientation of the cracks. The main goal of this work is to estimate the preferential fracture orientation based on an analysis of cross‐correlated S‐wave seismograms and Thomsen parameters. For this purpose, we analyzed ultrasonic measurements of elastic (P and S) waves in a physical‐modeling experiment with an artificially anisotropic cracked model. The solid matrix of the model consisted of epoxy‐resin; small rubber strips simulate cracks with a compliant fill. The anisotropic cracked model consists of three regions, each with a different fracture orientation. We used the rotation of the S‐wave polarizations for a cross‐correlation analysis of the orientations, and P‐ and S‐wave measurements to evaluate the weak anisotropic parametersγandε.The shear and compressional wave sources had dominant frequencies of 90 kHz and 120 kHz. These frequencies correspond to long wavelengths compared to the spacing between layers, indicating a nearly effective‐media behavior. Integrating the results from cross‐correlation with anisotropic parameter analysis, we were able to estimate the fracture orientation in our anisotropic cracked physical model. Theγparameter showed good agreement with the cross‐correlation analysis and, beyond that, provided additional information about the crack orientation that cross‐correlation alone did not fully resolve. Moreover, our results show that the shear waves are much more strongly influenced by, and can thus contain more information about, crack orientation than compressional waves.