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The accelerated electrons during solar flares produce radio bursts and nonthermal X-ray signatures. The quasi-periodic pulsations (QPPs) and fine structures in spatial–spectral–temporal space in radio bursts depend on the emission mechanism and the local conditions, such as magnetic fields, electron density, and pitch-angle distribution. Radio burst observations with high-frequency time resolution imaging provide excellent diagnostics. In converging magnetic field geometries, the radio bursts can be produced via the electron cyclotron maser (ECM). Recently, using observations made by the Karl G. Jansky Very Large Array (VLA) at 1–2 GHz, Yu et al. reported a discovery of long-lasting auroral-like radio bursts persistent over a sunspot and interpreted them as ECM-generated emission. Here we investigate the detailed second and subsecond temporal variability of this continuous ECM source. We study the association of 5 s period QPPs with a concurrent GOES C1.5-class flare, utilizing VLA's imaging spectroscopy capability with an extremely high temporal resolution (50 ms). We use the density and magnetic field extrapolation model to constrain the ECM emission to the second harmonic O -mode. Using the delay of QPPs from X-ray emission times, combined with X-ray spectroscopy and magnetic extrapolation, we constrain the energies and pitch angles of the ECM-emitting electrons to ≈4–8 keV and >26°. Our analysis shows that the loss-cone diffusion continuously fuels the ECM via Coulomb collisions and magnetic turbulence between a length scale of 5 and 100 Mm. We conclude that the QPP occurs via the Lotka–Volterra system, where the electrons from solar flares saturate the continuously operating ECM and cause temporary oscillations. [ABSTRACT FROM AUTHOR]

This paper investigates the incidence of coherent emission in solar radio bursts, using a revised catalog of 3800 solar radio bursts observed by the Nobeyama Radio Polarimeters from 1988 to 2023. We focus on the 1.0 and 2.0 GHz data, where radio fluxes of order 1010 Jy have been observed. Previous work has suggested that these bursts are due to electron cyclotron maser (ECM) emission. In at least one well-studied case, the bright emission at 1 GHz consists of narrowband spikes of millisecond duration. Coherent emission at 1 GHz can be distinguished from traditional incoherent gyrosynchrotron flare emission based on the radio spectrum: Gyrosynchrotron emission at 1 GHz usually has a spectrum rising with frequency, so bursts in which 1 GHz is stronger than higher-frequency measurements are unlikely to be incoherent gyrosynchrotron. Based on this criterion, it is found that for bursts exceeding 100 sfu, three-quarters of all bursts at 1 GHz and half of all 2 GHz bursts have a dominant coherent emission component, assumed to be ECM. The majority of the very bright bursts at 1 GHz are highly circularly polarized, consistent with a coherent emission mechanism, but not always 100% polarized. The frequency range from 1 to 2 GHz is heavily utilized for terrestrial applications, and these results are relevant for understanding the extreme flux levels that may impact such applications. Further, they provide a reference for comparison with the study of ECM emission from other stars and potentially exoplanets. [ABSTRACT FROM AUTHOR]

Henize 2–10 is a dwarf galaxy experiencing positive black hole (BH) feedback from a radio-detected low-luminosity active galactic nucleus. Previous Green Bank Telescope (GBT) observations detected a H2O "kilomaser" in Henize 2–10, but the low angular resolution (33″) left the location and origin of the maser ambiguous. We present new Karl G. Jansky Very Large Array observations of the H2O maser line at 22.23508 GHz in Henize 2–10 with ∼2″ resolution. These observations reveal two maser sources distinct in position and velocity. The first maser source is spatially coincident with the known BH outflow and the region of triggered star formation ∼70 pc to the east. Combined with the broad width of the maser (W 50 ∼ 66 km s−1), this confirms our hypothesis that part of the maser detected with the GBT is produced by the impact of the BH outflow shocking the dense molecular gas along the flow and at the interface of the eastern star-forming region. The second maser source lies to the southeast, far from the central BH, and has a narrow width (W 50 ∼ 8 km s−1), suggesting a star formation–related origin. This work has revealed the nature of the H2O kilomaser in Henize 2–10 and illustrates the first known connection between outflow-driven H2O masers and positive BH feedback. [ABSTRACT FROM AUTHOR]

Because of detailed monitoring of the 22.2 GHz water maser, carried out from 2021 October to 2023 May, a very powerful flare phenomenon was detected in the galactic object W51 near the radial velocity of 60 km s−1 with the amplitude of 140 kJy. A phenomenon of this magnitude was unprecedented in the entire history of observations of the source. Eleven short flares were recorded. The exponential increase and decrease in the flare flux density while reducing in their spectral line widths indicated that water masers were in an unsaturated state during the flares. All flares were located at the top of the less powerful Flare 0 with the amplitude of 13.5 kJy and the spectral line half-width of 3.0 km s−1. Such a wide line of the water maser, as well as the amplitude, of the flare phenomenon are so far unique discoveries. The water maser of Flare 0 may have been saturated and created a significant input flux density for other flares of this phenomenon. The extremely high density of maser spots in a cluster led to their partial overlap on the observer's line of sight. This also confirmed the hypothesis about the need for a significant length of the path along which the generation of maser radiation occurs. New parameters of water masers and the most important physical conclusions have been obtained. The possibility of detecting gravitational waves from massive binary stars at the stage of evolution close to merging is considered for the case of W51 Main. [ABSTRACT FROM AUTHOR]

We present the first results from the Southern Hemisphere Parallax Interferometric Radio Astrometry Legacy Survey: 10 μ as accurate parallaxes and proper motions for two Southern Hemisphere 6.7 GHz methanol masers obtained using the inverse MultiView calibration method. Using an array of radio telescopes in Australia and New Zealand, we measured trigonometric parallaxes and proper motions for the masers associated with the star formation region G232.62+00.99 of π = 0.610 ± 0.011 mas, μ x = −2.266 ± 0.021 mas yr−1, and μ y = 2.249 ± 0.049 mas yr−1, which implies its distance to be d = 1.637 ± 0.029 kpc. These measurements represent an improvement in accuracy by more than a factor of 3 over the previous measurements obtained through Very Long Baseline Array observations of the 12 GHz methanol masers associated with this region. We also measure the trigonometric parallax and proper motion for G323.74–00.26 as π = 0.364 ± 0.009 mas, μ x = −3.239 ± 0.025 mas yr−1, and μ y = − 3.976 ± 0.039 mas yr−1, which implies a distance of d = 2.747 ± 0.068 kpc. These are the most accurate measurements of trigonometric parallax obtained for 6.7 GHz class II methanol masers to date. We confirm that G232.62+00.99 is in the Local Arm and find that G323.74–00.26 is in the Scutum–Centaurus arm. We also investigate the structure and internal dynamics of both masers. [ABSTRACT FROM AUTHOR]

We report a measurement of the trigonometric parallax of 22 GHz water masers in the massive star-forming region G034.43+0.24 as part of the Bar and Spiral Structure Legacy Survey using the Very Long Baseline Array. The parallax is 0.330 ± 0.018 mas, corresponding to a distance of 3.03 − 0.16 + 0.17 kpc. This locates G034.43+0.24 near the inner edge of the Sagittarius spiral arm and at one end of a linear distribution of massive young stars that cross nearly the full width of the arm. The measured three-dimensional motion of G034.43+0.24 indicates a near-circular Galactic orbit. The water masers display arc-like distributions, possibly bow shocks, associated with winds from one or more massive young stars. [ABSTRACT FROM AUTHOR]

We present the spectral and spatial evolution of H2O masers associated with the water fountain source IRAS 18043−2116, found in observations with the Nobeyama 45 m Telescope and the Australia Telescope Compact Array. We have found new highest-velocity components of the H2O masers (at the redshifted side V LSR ≃ 376 km s−1 and at the blueshifted side V LSR ≃ −165 km s−1), and the resulting velocity spread of ≃540 km s−1 breaks the speed record of fast jets/outflows in this type of sources. The locations of those components have offsets from the axis joining the two major maser clusters, indicating a large opening angle of the outflow (∼60°). The evolution of the maser cluster separation of ∼2.9 mas yr−1 and the compact (∼0.″2) CO emission source mapped with the Atacama Large Millimeter-submillimeter Array suggest a very short (∼30 yr) timescale of the outflow. We also confirmed an increase in the flux density of the 22 GHz continuum source. The properties of the jet and the continuum sources and their possible evolution in the transition to the planetary nebula phase are further discussed. [ABSTRACT FROM AUTHOR]

We report the proper motions of 22 GHz water masers toward W49 N that were observed by the KVN and VERA Array (KaVA) during 2017 February–May. We found 263, 268, and 310 features in three successive epochs; they were distributed in a region of size 4 × 4 arcsec2. The strongest flux density was in the third epoch, and its averaged value was 18,090 Jy at V LSR +0.47 km s−1. For 102 H2O maser features, proper motion was detected across all three epochs. The average proper motions in R.A. and decl. offset were −0.352 and +0.890 mas yr−1, respectively. The morphology of the distribution of the H2O maser features was found to be a bipolar outflow structure with an inclination angle of 37° ± 13° to the line of sight, and the features were expanding from a well-defined outflow center. A model of the source combining expansion and rotation yielded a distance to W49 of 11.12 ± 0.96 kpc that is consistent with the results from trigonometric parallax. A redshifted lobe was situated in the northeast direction and a blueshifted lobe in the southwest direction. We also discussed the location of the powerful flaring H2O maser feature at V LSR = + 6 km s−1 and its possible mechanisms on the basis of spatial structures for the maser feature in VLBI maps observed with the KaVA, timed just before and during the rebrightening phase. [ABSTRACT FROM AUTHOR]

The nitrogen-vacancy (NV) centre in diamond is regarded as a promising micro-nano scale thermometer due to its high stability, good temperature resolution and its scope for integration. The relation between the zero-field splitting parameter (D) and temperature (T) is fundamental for its applications. In this work, an optical system that uses continuous-wave optically detected magnetic resonance technique for the measurement of D-T was built. The temperature measured by a home-made thermostatic system and two calibrated platinum resistance thermometers was regarded as reference. Before D was measured, microwave and laser heating effect was investigated quantitatively. The drift of D at different microwave and laser power in this system was observed. Although the dips’ contrast could be improved by higher microwave power, the heating effect was significant if the power was higher thanâ€"5 dBm. The heating effect from laser power showed a linear trend which could be corrected by an average factor of âˆ'3.69 Ă— 10âˆ'5 GHz mWâˆ'1. By using the optimized parameters, D was measured from 298 to 383 K. The reliability of our experimental method can be demonstrated by the good agreement between the present data and those obtained from similar samples. Our work provides a reference for similar experiments that the heating effect could not be neglected, which would be essential to obtain a more precise D-T relation when considering the NV centre as a thermometer. The inconsistency existing in the results from different groups indicates that further metrological investigations are still needed for the application of the NV centre as a practical and accurate thermometer in the near future. [ABSTRACT FROM AUTHOR]

We report the detection of the millimeter CH _3 OH masers including a new detection of class I (11 _0,11 –10 _1,10 A) and class II (6 _1,5 –5 _2,4 E) maser transitions toward the high-mass protostar S255IR NIRS3 in postburst phase. The CH _3 OH emissions were detected as a mixture of maser and thermal characteristics. We examine the detected transitions using an excitation diagram and local thermodynamic equilibrium model spectra and compare the observed properties with those of thermal lines. Class II CH _3 OH maser transitions showed distinctive intensity and velocity distributions from those of thermal transitions. Bright, distinct emission components in addition to the fragmented and arc-shaped emissions are only detected in class I CH _3 OH maser transitions toward southern and western directions from the protostellar position, implying the presence of the slow outflow shocks.

We present the discovery of the simultaneous flux variation of a massive young stellar object (MYSO) G036.70+00.09 (G036.70) both in the maser emission and mid-infrared (MIR; λ = 3â€"5 ÎĽ m) bands. Using the ALLWISE and NEOWISE archival databases that cover a long time span of approximately 10 yr with a cadence of 6 months, we confirm that G036.70 indicates a stochastic year-long MIR variability with no signs of a WISE band color change of W1 (3.4 ÎĽ m) âˆ'W2 (4.6 ÎĽ m). Cross-matching the MIR data set with the high-cadence 6.7 GHz class II methanol maser flux using the Hitachi 32 m radio telescope that discovered its periodicity in the methanol maser of 53.0â€"53.2 days, we also determine the flux correlations between the two bands at two different timescales, year-long and day-long, both of which have never been reported in MYSOs, except when they are in the accretion burst phase. The results of our study support the scenario that a class II methanol maser is pumped up by infrared emission from accreting disks of MYSOs. We also discuss the possible origins of MIR and maser variability. To explain the two observed phenomena, a stochastic year-long MIR variability with no signs of significant color change and maser-MIR variability correlation or a change in mass accretion rate and line-of-sight extinction because of the nonaxisymmetric dust density distribution in a rotating accretion disk are possible origins. Observations through spectroscopic monitoring of accretion-related emission lines are essential for determining the origin of the observed variability in G036.70. [ABSTRACT FROM AUTHOR]

We have used the Shanghai Tianma Radio Telescope to search for three OH transitions at 4.7 GHz toward 155 northern star formation regions. We detect 4.7 GHz OH masers in 18 star formation regions, 8 of which are reported here for the first time. From these 18 sources, we detect 6 4660 MHz masers, 13 4765 MHz masers, and no 4750 MHz masers. A further 1 source (Sgr B2N) has been re-detected with broad quasi-thermal emission in all three OH lines. W49SW was re-detected with broad quasi-thermal emission at both 4660 and 4750 MHz. One source (W31, G010.626âˆ'0.387) was re-detected with quasi-thermal emission at both 4750 and 4765 MHz. One source (G005.885âˆ'0.392) was first detected with quasi-thermal emission at 4660 MHz. We have investigated the associations between the detected 4.7 GHz OH masers with ground-state OH masers near 1.7 GHz, 6.7 GHz methanol masers and 22 GHz water masers reported in the literature. We find that the presence of 1665 MHz OH masers is a better indicator of the presence of 4.7 GHz OH masers than 1720 MHz OH masers. The majority of the 4.7 GHz OH masers are associated with 6.7 GHz methanol and/or 22 GHz water masers. We have compared the characteristics of our detections with those reported previously in the literature and found that only five sources are fairly stable. [ABSTRACT FROM AUTHOR]

By using self-consistent 2.5-dimensional particle-in-cell simulations, we study the excitation efficiency of electromagnetic waves by power-law energetic electrons with an anisotropic pitch-angle velocity distribution, which can simultaneously trigger the Langmuir and electron cyclotron maser instabilities, in differently magnetized coronal plasmas. It is found that the (transverse) electromagnetic waves can be excited much more efficiently in the case of strongly magnetized plasmas with ω ce > ω pe than that of weakly magnetized plasmas with ω ce < ω pe, where ω ce and ω pe are the electron cyclotron frequency and the electron plasma frequency, respectively. In particular, in a weakly magnetized plasma the electromagnetic wave is hardly excited effectively via the nonlinear coupling of Langmuir waves; although the Langmuir waves can be generated by the power-law energetic electrons, implying that the so-called plasma emission does not effectively work. These results can be helpful for us to better understand the physical mechanism of solar radio bursts. [ABSTRACT FROM AUTHOR]

In this work, we propose an accreting stellar binary model for understanding the active periodic fast radio bursts (FRBs). The system consists of a stellar compact object (CO) and a donor star (DS) companion in an eccentric orbit, where the DS fills its own Roche lobe near the periastron. The CO accretes the material from the DS and then drives relativistic magnetic blobs. The interaction between the magnetic blobs and the stellar wind of the DS produces a pair of shocks. We find that both the reverse shock and the forward shock are likely to produce FRBs via the synchrotron maser mechanism. We show that this system can in principle sufficiently produce highly active FRBs with a long lifetime, and also can naturally explain the periodicity and the duty cycle of the activity that appeared in FRBs 180916 and 121102. The radio nebula excited by the long-term injection of magnetic blobs into the surrounding environment may account for the associated persistent radio source. In addiction, we discuss the possible multiwavelength counterparts of FRB 180916 in the context of this model. Finally, we encourage the search for FRBs in ultraluminous X-ray sources. [ABSTRACT FROM AUTHOR]

We consider the problem of testing the Einstein equivalence principle (EEP) by measuring the gravitational redshift with two Earth-orbiting stable atomic clocks. For a reasonably restricted class of orbits we find an optimal experiment configuration that provides for the maximum accuracy of measuring the relevant EEP violation parameter. The perigee height of such orbits is ∼1000 km and the period is 3–5 h, depending on the clock type. For the two of the current best space-qualified clocks, the VCH-1010 hydrogen maser and the PHARAO cesium fountain clock, the achievable experiment accuracy is, respectively, 1 × 10−7 and 5 × 10−8 after 3 years of data accumulation. This is more than 2 orders of magnitude better than achieved in Gravity Probe A and GREAT missions as well as expected for the RadioAstron gravitational redshift experiment. Using an anticipated future space-qualified clock with a performance of the current laboratory optical clocks, an accuracy of 3 × 10−10 is reachable. [ABSTRACT FROM AUTHOR]

We have investigated the stellar and interstellar content of the distant star formation region IRAS 17591-2228 (WISE H ii region GAL 007.47+0.06). It is associated with a water maser, whose parallax distance is kpc, supported by independent measurements of proper motion and radial velocity. It is projected in the same direction as an extremely red (J−KS ∼ 6 mag) group of stars, and a shell of mid-infrared emission. We qualify the group of stars as a cluster candidate, VVV CL177. Its radius spans between 0.′45 and 1′ and contains at least two young stellar objects with an extreme extinction near mag. Yet more analysis will be required to determine if it is a real single cluster associated with the water maser. The 13CO emissions at the radial velocity of the maser corresponds to the mid-infrared emission. [ABSTRACT FROM AUTHOR]

OH megamasers (OHMs) are rare, luminous masers found in gas-rich major galaxy mergers. In untargeted neutral hydrogen (H i) emission-line surveys, spectroscopic redshifts are necessary to differentiate the λrest = 18 cm masing lines produced by OHMs from H i 21 cm lines. Next-generation H i surveys will detect an unprecedented number of galaxies, most of which will not have spectroscopic redshifts. We present predictions for the numbers of OHMs that will be detected and the potential "contamination" they will impose on H i surveys. We examine the Looking at the Distant Universe with the MeerKAT Array (LADUMA), a single-pointing deep-field survey reaching redshift zH I = 1.45, as well as potential future surveys with the Square Kilometre Array (SKA) that would observe large portions of the sky out to redshift zH I = 1.37. We predict that LADUMA will potentially double the number of known OHMs, creating an expected contamination of 1.0% of the survey's H i sample. Future SKA H i surveys are expected to see up to 7.2% OH contamination. To mitigate this contamination, we present methods to distinguish H i and OHM host populations without spectroscopic redshifts using near- to mid-IR photometry and a k-Nearest Neighbors algorithm. Using our methods, nearly 99% of OHMs out to redshift zOH ∼ 1.0 can be correctly identified. At redshifts out to zOH ∼ 2.0, 97% of OHMs can be identified. The discovery of these high-redshift OHMs will be valuable for understanding the connection between extreme star formation and galaxy evolution. [ABSTRACT FROM AUTHOR]

A complete understanding of solar radio bursts requires developing numerical techniques that can connect large-scale activities with kinetic plasma processes. As a starting point, this study presents a numerical scheme combining three different techniques: (1) extrapolation of the magnetic field overlying a specific active region in order to derive the background field, (2) guiding-center simulation of the dynamics of millions of particles within a selected loop to reveal the integral velocity distribution function (VDF) around certain sections of the loop, and (3) particle-in-cell simulation of kinetic instabilities driven by energetic electrons initiated by the obtained distributions. Scattering effects at various levels (weak, moderate, and strong) due to wave turbulence-particle interaction are considered using prescribed timescales of scattering. It was found that the obtained VDFs contain strip-like and loss-cone features with positive gradient, and both features are capable of driving electron cyclotron maser emission, which is a viable radiation mechanism for some solar radio bursts, in particular, solar radio spikes. The strip-like feature is important in driving the harmonic X mode, while the loss-cone feature can be important in driving the fundamental X mode. In the weak-scattering case, the rate of energy conversion from energetic electrons to X2 can reach up to , where is the initial kinetic energy of energetic electrons. The study demonstrates a novel way of exciting the X2 mode in the corona during solar flares and provides new sight into how escaping radiation can be generated within a coronal loop. [ABSTRACT FROM AUTHOR]

Following an eruptive accretion event in NGC 6334I-MM1, flares in the various maser species, including water masers, were triggered. We report the observed relative proper motion of the highly variable water masers associated with the massive star-forming region, NGC 6334I. High velocity H2O maser proper motions were detected in five maser clusters, CM2-W2 (bow-shock structure), MM1-W1, MM1-W3, UCHII-W1, and UCHII-W3. The overall average of the derived relative proper motion is 85 km s−1. This mean proper motion is in agreement with the previous results from VLA multiepoch observations. Our position as well as the velocity variance and covariance matrix analyses of the maser proper motions show its major axis to have a position angle of −79.°4, cutting through the dust cavity around MM1B and being aligned in the northwest–southeast direction. We interpret this as the axis of the jet driving the CM2 shock and the maser motion. The complicated proper motions in MM1-W1 can be explained by the combined influence of the MM1 northeast–southwest bipolar outflow, the CS(6−5) north–south collimated bipolar outflow, and the radio jet. The relative proper motions of the H2O masers in UCHII-W1 are likely not driven by the jets of the MM1B protostar but by MM3-UCHII. Overall, the post-accretion burst relative proper motions of the H2O masers trace shocks of jet motion. [ABSTRACT FROM AUTHOR]

We have carried out observations of CCH (N = 1 − 0), CH3CN (J = 5 − 4), and three 13C isotopologues of HC3N (J = 10 − 9) toward three massive young stellar objects (MYSOs), G12.89+0.49, G16.86−2.16, and G28.28−0.36, with the Nobeyama 45 m radio telescope. Combined with previous results on HC5N, the column density ratios of N(CCH)/N(HC5N), hereafter the CCH/HC5N ratios, in the MYSOs are derived to be ∼15. This value is lower than that in a low-mass warm carbon chain chemistry (WCCC) source by more than one order of magnitude. We compare the observed CCH/HC5N ratios with hot-core model calculations. The observed ratios in the MYSOs can be best reproduced by models when the gas temperature is ∼85 K, which is higher than in L1527, a low-mass WCCC source (∼35 K). These results suggest that carbon-chain molecules detected around the MYSOs exist at least partially in higher temperature regions than those in low-mass WCCC sources. There is no significant difference in column density among the three 13C isotopologues of HC3N in G12.89+0.49 and G16.86-2.16, while HCC13CN is more abundant than the others in G28.28–0.36. We discuss carbon-chain chemistry around the three MYSOs based on the CCH/HC5N ratio and the 13C isotopic fractionation of HC3N. [ABSTRACT FROM AUTHOR]

We have performed a multiwavelength study of supernova remnant (SNR) G57.2+0.8 and its environment. The SNR hosts the magnetar SGR 1935+2154, which emitted an extremely bright millisecond-duration radio burst on 2020 April 28. We used the 12CO and 13CO J = 1–0 data from the Milky Way Image Scroll Painting CO line survey to search for molecular gas associated with G57.2+0.8, in order to constrain the physical parameters (e.g., the distance) of the SNR and its magnetar. We report that SNR G57.2+0.8 is likely impacting the molecular clouds (MCs) at the local standard of rest (LSR) velocity and excites a weak 1720 MHz OH maser with a peak flux density of 47 mJy beam−1. The chance coincidence of a random OH spot falling in the SNR is ≤12%, and the OH–CO correspondence chance is 7% at the maser spot. This combines to give <1% false probability of the OH maser, suggesting a real maser detection. The LSR velocity of the MCs places the SNR and magnetar at a kinematic distance of 6.6 ± 0.7 kpc. The nondetection of thermal X-ray emission from the SNR and the relatively dense environment suggests G57.2+0.8 be an evolved SNR with an age. The explosion energy of G57.2+0.8 is lower than , which is not very energetic even assuming a high ambient density. This reinforces the opinion that magnetars do not necessarily result from very energetic supernova explosions. [ABSTRACT FROM AUTHOR]

We present 43 GHz VLA spectra for 51 AGB sources with the goal of verifying an infrared (IR) color cut intended to separate carbon-rich (C) and oxygen-rich (O) AGB sources throughout the Galaxy. The color cut is a simple line in the [Ks] − [A] versus [A] − [E] color–color diagram based on 2MASS and MSX photometry, and was originally derived from SiO detection rates in the Bulge Asymmetries and Dynamical Evolution (BAaDE) sample. The division is fully supported by the spectra presented here, which show that SiO maser detections lie on the O-rich side, and SiO nondetections and a single HC3N detection are found on the C-rich side of the division. We further compare the color cut with classifications of the sources based on Low-Resolution Spectra from the Infrared Astronomical Satellite, and find good agreement, verifying that the division is a reliable and efficient method for differentiating O- and C-rich AGB sources. These observations also demonstrate that single lines detected in the BAaDE survey around 42.9 GHz are almost certainly the 29SiO v = 0 line. SiO maser sources where this rare isotopologue transition is brighter than the dominant 28SiO lines have not been reported before, and our observations show that these sources can reverse their behavior such that the typical ratios of 28SiO and 29SiO are restored within a few years. [ABSTRACT FROM AUTHOR]

Recently, coherent radio emission has been discovered from a number of hot magnetic stars, via the process of electron cyclotron maser emission (ECME). This emission, observed in the form of highly circularly polarized pulses, has interesting properties that contain information about the host star. One of the important properties of ECME is the frequency dependence of the pulse arrival time. This has been attributed to the propagation effect by Trigilio et al. and could explain the sequence observed for CU Vir qualitatively. However, no quantitative treatment exists for this phenomenon despite it being a promising tool to estimate the density in the stellar magnetosphere. Additionally, the effect of propagation through the magnetosphere on ECME has been thought to be limited to giving rise to a particular sequence of arrival of pulses, and in some cases producing the upper cutoff frequency for ECME. Here, we present a framework to deal with the propagation effect by considering continuous refraction in the inner magnetosphere of the star. This framework is capable of incorporating any type of density distribution, and in principle any type of magnetic field, though we limit ourselves to a dipolar magnetic field for this work. We show by simulation that for stars with high obliquity, the propagation effect can influence not only the sequence of arrival of pulses drastically, but also the pulse shapes, and the observability of a pulse from a particular magnetosphere. [ABSTRACT FROM AUTHOR]

We report a systemic search for nonmetastable NH3 masers toward a high-mass star-forming region Sgr B2(N) using the Shanghai 65 m Tianma radio telescope with its Ku-band (12–18 GHz) and K-band (18–26.5 GHz) receivers. A total of 18 maser emission transitions were detected, and eight of these are newly identified NH3 maser transitions in interstellar space. Most (five) of the new maser transitions were found from the Ku-band observations which were absent from previous searches. Rotation diagram analysis for the 18 quasi-thermal transitions derived a high gas kinematic temperature (∼400 K) and an extremely high H2 column density (∼5 × 1025 cm−2). Such conditions suggest that the NH3 masers are likely pumped by vibrational excitation. Compared with quiet maser activity observed more than 20 yr ago, NH3 masers may be bursting in our observation time when the gas kinematic temperature might increase from ∼150 to ∼400 K. As extreme conditions (either high gas kinematic temperature and/or high gas column density) are required to excite nonmetastable NH3 masers, it can be argued that the detected NH3 maser outburst may be linked to a more intensive accretion-burst event from an high-mass young stellar object in the Sgr B2(N) region. [ABSTRACT FROM AUTHOR]

We observed polarization of the SiO rotational transitions from Orion Source I (SrcI) to probe the magnetic field in bipolar outflows from this high-mass protostar. Both 43 GHz J = 1–0 and 86 GHz J = 2–1 lines were mapped with ∼20 au resolution, using the Very Large Array (VLA) and Atacama Large Millimeter/Submillimeter Array (ALMA), respectively. The 28SiO transitions in the ground vibrational state are a mixture of thermal and maser emission. Comparison of the polarization position angles in the J = 1–0 and J = 2–1 transitions allows us to set an upper limit on possible Faraday rotation of 104 rad m−2, which would twist the J = 2–1 position angles typically by less than 10°. The smooth, systematic polarization structure in the outflow lobes suggests a well-ordered magnetic field on scales of a few hundred au. The uniformity of the polarization suggests a field strength of ∼30 mG. It is strong enough to shape the bipolar outflow and possibly lead to sub-Keplerian rotation of gas at the base of the outflow. The strikingly high fractional linear polarizations of 80%–90% in the 28SiO v = 0 masers require anisotropic pumping. We measured circular polarizations of 60% toward the strongest maser feature in the v = 0 J = 1–0 peak. Anisotropic resonant scattering is likely to be responsible for this circular polarization. We also present maps of the 29SiO v = 0 J = 2–1 maser and several other SiO transitions at higher vibrational levels and isotopologues. [ABSTRACT FROM AUTHOR]

We present results of continuum and spectral line observations with Atacama Large Millimeter/submillimeter Array (ALMA) and 22 GHz water (H2O) maser observations using the KVN (Korean VLBI Network) and VERA (VLBI Exploration of Radio Astrometry) array (KaVA) toward a high-mass star-forming region, G25.82–0.17. Multiple 1.3 mm continuum sources are revealed, indicating the presence of young stellar objects (YSOs) at different evolutionary stages, namely an ultracompact H ii region, G25.82–E, a high-mass young stellar object (HM-YSO), G25.82–W1, and starless cores, G25.82–W2 and G25.82–W3. Two SiO outflows, at N–S and SE–NW orientations, are identified. The CH3OH 8−1–70 E line, known to be a Class I CH3OH maser at 229 GHz, is also detected, showing a mixture of thermal and maser emission. Moreover, the H2O masers are distributed in a region ∼0.″25 shifted from G25.82–W1. The CH3OH 224–215 E line shows a compact ringlike structure at the position of G25.82–W1 with a velocity gradient, indicating a rotating disk or envelope. Assuming Keplerian rotation, the dynamical mass of G25.82–W1 is estimated to be >25 M⊙ and the total mass of 20–84 M⊙ is derived from the 1.3 mm continuum emission. The driving source of the N–S SiO outflow is G25.82–W1 while that of the SE–NW SiO outflow is uncertain. Detection of multiple high-mass starless/protostellar cores and candidates without low-mass cores implies that HM-YSOs could form in individual high-mass cores as predicted by the turbulent core accretion model. If this is the case, the high-mass star formation process in G25.82 would be consistent with a scaled-up version of low-mass star formation. [ABSTRACT FROM AUTHOR]

The middle-aged supernova remnant (SNR) CTB 37A is known to interact with several dense molecular clouds through the detection of shocked H2 and OH 1720 MHz maser emission. In the present work, we use eight years of Fermi-LAT Pass 8 data, with an improved point-spread function and an increased acceptance, to perform detailed morphological and spectral studies of the γ-ray emission toward CTB 37A from 200 MeV to 200 GeV. The best fit of the source extension is obtained for a very compact Gaussian model with a significance of 5.75σ and a 68% containment radius of 0.°116 ± 0.°014stat ± 0.°017sys above 1 GeV, which is larger than the TeV emission size. The energy spectrum is modeled as a LogParabola, resulting in a spectral index α = 1.92 ± 0.19 at 1 GeV and a curvature β = 0.18 ± 0.05, which becomes softer than the TeV spectrum above 10 GeV. The SNR properties, including a dynamical age of 6000 yr, are derived assuming the Sedov phase. From the multiwavelength modeling of emission toward the remnant, we conclude that the nonthermal radio and GeV emission is mostly due to the reacceleration of preexisting cosmic rays (CRs) by radiative shocks in the adjacent clouds. Furthermore, the observational data allow us to constrain the total kinetic energy transferred to the trapped CRs in the clouds. Based on these facts, we infer a composite nature for CTB 37A to explain the broadband spectrum and to elucidate the nature of the observed γ-ray emission. [ABSTRACT FROM AUTHOR]

We analyze recent observations of the spiral arm width in the Milky Way, as a function of the galactic radius, and we compare this relation with the prediction from the density wave theory. We use the following method: in each spiral arm, we concentrate on the separation (or offset) between the star-forming region (radio masers) near the shock front of a density wave, and the aged star region (diffuse CO gas) near the potential minimum of a density wave; we take this separation between these two tracers as the arm width. New results: we find a typical separation (maser to diffuse CO gas) near 250 ± 50 pc and an increase of this separation with galactic radius of about 25 ± 5 pc per kpc. We note that, as expected, this separation is somewhat smaller than that found earlier between the dust lane and the aged star region. Overall, these results support the basics of a density wave. [ABSTRACT FROM AUTHOR]

The size reduction of atomic clocks is a long-standing research issue. Many atomic clocks such as passive hydrogen masers (PHMs) and compact rubidium masers (CRMs) use iris-loaded resonance cavities (IRCs) as their microwave cavities because they can dramatically reduce the radical sizes of the atomic clocks. In this paper, the electromagnetic characteristic of the IRC is investigated by a theoretical model based on electromagnetic field theory. The formulas to calculate the resonance frequency, quality factor, and magnetic energy filling factor are presented. The relationship between the IRC structure and its electromagnetic characteristic is clarified. The theoretical calculation results accord well with the electromagnetic software simulations and experimental results. The results in this paper should be helpful in understanding the physical mechanism of the IRC and designing the atomic clocks. [ABSTRACT FROM AUTHOR]

We present the very long baseline interferometry H2O maser monitoring observations of the red supergiant, PZ Cas, at 12 epochs from 2006 April to 2008 May. We fitted maser motions to a simple model composed of a common annual parallax and linear motions of the individual masers. The maser motions with the parallax subtracted were well modeled by a combination of a common stellar proper motion and a radial expansion motion of the circumstellar envelope. We obtained an annual parallax of 0.356 ± 0.026 mas and a stellar proper motion of μ"α cos δ = -3.7 ± 0.2 and μ " δ = -2.0 ± 0.3 mas yr-1 eastward and northward, respectively. The annual parallax corresponds to a trigonometric parallax of 2.81+0.22 -0.19 kpc. By rescaling the luminosity of PZ Cas in any previous studies using our trigonometric parallax, we estimated the location of PZ Cas on a Hertzsprung-Russell diagram and found that it approaches a theoretically evolutionary track around an initial mass of ~25 M☉ . The sky position and the distance to PZ Cas are consistent with the OB association, Cas OB5, which is located in a molecular gas super shell. The proper motion of PZ Cas is close to that of the OB stars and other red supergiants in Cas OB5 measured by the Hipparcos satellite.We derived the peculiar motion of PZ Cas of Us = 22.8 ± 1.5, Vs = 7.1 ± 4.4, andWs = -5.7 ± 4.4 kms-1. This peculiar motion has rather a large Us component, unlike those of near high-mass star-forming regions with negatively large Vs motions. The uniform proper motions of the Cas OB5 member stars suggest random motions of giant molecular clouds moving into local potential minima in a time-dependent spiral arm, rather than a velocity field caused by the spiral arm density wave. [ABSTRACT FROM AUTHOR]

We report the results of a full-Stokes survey of all four 18 cm OH lines in 77 OH megamasers (OHMs) using the Arecibo Observatory. This is the first survey of OHMs that included observations of the OH satellite lines; only four of the 77 OHMs have existing satellite line observations in the literature. Satellite line emission is detected in five sources, three of which are redetections of previously published sources. The two sources with new detections of satellite line emission are IRAS F10173+0829, which was detected at 1720 MHz, and IRAS F15107+0724, for which both the 1612 MHz and 1720 MHz lines were detected. In IRAS F15107+0724, the satellite lines are partially conjugate, as 1720 MHz absorption and 1612 MHz emission have the same structure at some velocities within the source, along with additional broader 1612 MHz emission. This is the first observed example of conjugate satellite lines in an OHM. In the remaining sources, no satellite line emission is observed. The detections and upper limits are generally consistent with models of OHM emission in which all of the 18 cm OH lines have the same excitation temperature. There is no evidence for a significant population of strong satellite line emitters among OHMs. [ABSTRACT FROM AUTHOR]

We report the first short term velocity centroid (VC) periodicity derived from SiO maser emission. L2 Puppis, a semi-regular AGB star, was observed using the Mopra radio telescope of the Australia Telescope National Facility in the SiO v = 1, J = 1-0 and v = 1, J = 2-1 transitions. It exhibits a 139 day period in its SiO maser VC based on a period folding analysis and a Lomb Scargle analysis. L2 Pup's SiO maser emission has an unusually large velocity range and an unusual three-peaked spectrum. To create the change in VC the entire spectrum does not shift in velocity, but changes in the relative emission of the peaks generate the variation. The changes in the VC may be due to differential illumination, an asymmetric circumstellar distribution of material, or a mixture of causes. The unusual velocity structure, similar to that observed in Orion source 1, may be due to revolution of the circumstellar material or asymmetries in the circumstellar environment. [ABSTRACT FROM AUTHOR]

Using the Very Long Baseline Array and the European Very Long Baseline Interferometry Network, we have observed 22.2 GHz H2O and 1612 MHz OH masers in the water fountain source IRAS 18460−0151. The H2O maser spectrum has a very wide line-of-sight velocity range (≈310 km s−1) and consists of three groups of emission features at the blueshifted (−68 km s−1≲ VLSR ≲ −17 km s−1) and redshifted (VLSR ≃240 km s−1) edges as well as around the systemic velocity (112 km s−1 ≲ VLSR ≲ 133 km s−1). The first two H2O spectral components exhibit a highly collimated high-velocity bipolar jet on the sky, with an angular separation of ≈120 milliarcsec (mas) (240 AU in linear length) and a three-dimensional flow velocity of ≈160 km s−1. The flow dynamical age is estimated to be only ≈6 yr (at the time of the observation epochs of 2006–2007). Interestingly, the systemic velocity component clearly exhibits a spherically expanding outflow with a radius of ≈36 AU and a flow velocity of ≈9 km s−1. On the other hand, the OH maser spectrum shows double peaks with a velocity separation of ≈25 km s−1 (VLSR = 111–116 and 138–141 km s−1), as typically seen in circumstellar envelopes of OH/IR stars. The angular offset between the velocity-integrated brightness peaks of the two high-velocity H2O components is ≈25 mas (50 AU). The offset direction and the alignment of the redshifted maser spots are roughly perpendicular to the axis of the H2O maser flow. High-accuracy astrometry for the H2O and OH masers demonstrates that the collimated fast jet and the slowly expanding outflow originate from a single or multiple sources which are located within 15 mas (30 AU). On the other hand, the estimated systemic velocity of the collimated jet (Vsys ≈ 87–113 km s−1) has a large uncertainty. This makes it difficult to provide strong constraints on models of the central stellar system of IRAS 18460−0151. [ABSTRACT FROM AUTHOR]

We present a model in which the 22 GHz H2O masers observed in star-forming regions occur behind shocks propagating in dense regions (preshock density n0 ~ 106-108 cm-3). We focus on high-velocity (υs ≳ 30 km s-1) dissociative J shocks in which the heat of H2 re-formation maintains a large column of ~300-400 K gas; at these temperatures the chemistry drives a considerable fraction of the oxygen not in CO to form H2O. The H2O column densities, the hydrogen densities, and thewarm temperatures produced by these shocks are sufficiently high to enable powerful maser action. The observed brightness temperatures (generally ~ 1011-1014 K) are the result of coherent velocity regions that have dimensions in the shock plane that are 10-100 times the shock thickness of ~1013 cm. The masers are therefore beamed toward the observer, who typically views the shock "edge-on," or perpendicular to the shock velocity; the brightest masers are then observed with the lowest line-of-sight velocities with respect to the ambient gas. We present numerical and analytic studies of the dependence of the maser inversion, the resultant brightness temperature, the maser spot size and shape, the isotropic luminosity, and the maser region magnetic field on the shock parameters and the coherence path length; the overall result is that in galactic H2O 22 GHz masers, these observed parameters can be produced in J shocks with n0 ~ 106-108 cm-3 and υs ~ 30-200 km s-1. A number of key observables such as maser shape, brightness temperature, and global isotropic luminosity depend only on the particle flux into the shock, j = n0υs , rather than on n0 and vs separately. [ABSTRACT FROM AUTHOR]

We present a comprehensive view of the W51B Hii region complex and theW51C supernova remnant (SNR) using new radio observations from the VLA, VLBA, MERLIN, JCMT, and CSO along with archival data from Spitzer, ROSAT, ASCA, and Chandra. Our VLA data include the first λ = 400 cm (74 MHz) continuum image of W51 at high resolution (88"). The 400 cm image shows non-thermal emission surrounding the G49.2-0.3 Hii region, and a compact source of non-thermal emission (W51B_NT) coincident with the previously-identified OH (1720 MHz) maser spots, non-thermal 21 and 90 cm emission, and a hard X-ray source. W51B_NT falls within the region of high likelihood for the position of TeV γ -ray emission. Using the VLBA, three OH (1720 MHz) maser spots are detected in the vicinity of W51B_NT with sizes of 60-300 AU and Zeeman effect magnetic field strengths of 1.5-2.2 mG. The multiwavelength data demonstrate that the northern end of the W51B Hii region complex has been partly enveloped by the advancing W51C SNR and this interaction explains the presence of W51B_NT and the OH masers. This interaction also appears in the thermal molecular gas which partially encircles W51B_NT and exhibits narrow pre-shock (Δv ~ 5 km s-1) and broad post-shock (Δv ~ 20 km s-1) velocity components. RADEX radiative transfer modeling of these two components yield physical conditions consistent with the passage of a non-dissociative C-type shock. Confirmation of the W51B/W51C interaction provides additional evidence in favor of this region being one of the best candidates for hadronic particle acceleration known thus far. [ABSTRACT FROM AUTHOR]

The water fountain source IRAS 18286-0959 (I18286) was reported as an object exhibiting highly collimated, double-helix stellar jets traced by the H2O maser emission. Using the European VLBI Network, we measured the absolute coordinates of the 1612 MHz OH maser emission in I18286, which is very likely associated with a relic of a circumstellar envelope (CSE) developed in the asymptotic giant branch stage of the central star. The location of the OH maser is near the center of the H2O maser feature cluster, where one of the originating points of the two jets is located. We also mapped 22.2 GHz H2O maser emission in this object using the Very Long Baseline Array in the seasons of 2006-2007 as well as 2008-2009. In the first three epochs in 2006-2007, we detected at least 90 maser features per epoch. In six epochs in 2008-2009, when the observation data had already been published and analyzed to identify the double-helix jets, we newly identified 14 proper motions of H2O maser features which could not be measured in the previous analysis due to their short lifetimes. In this paper, together with the OH maser emission, we focus on "outlier" H2O maser features, which exhibit slow expansion velocities (Vexp ≲ 30 km s-1) and are likely associated with either the relic CSE or an equatorial flow of I18286. They were marginally distinguishable from the maser features associated with the jets in I18286. [ABSTRACT FROM AUTHOR]

Using the Herschel Space Observatory's Heterodyne Instrument for the Far-Infrared, we have performed mapping observations of the 620.701 GHz 532–441 transition of ortho-H2O within a ∼1.5 × 1.5 region encompassing the Kleinmann–Low nebula in Orion (Orion-KL), and pointed observations of that transition toward the Orion South condensation and the W49N region of high-mass star formation. Using the Effelsberg 100 m radio telescope, we obtained ancillary observations of the 22.23508 GHz 616–523 water maser transition; in the case of Orion-KL, the 621 GHz and 22 GHz observations were carried out within 10 days of each other. The 621 GHz water line emission shows clear evidence for strong maser amplification in all three sources, exhibiting narrow (∼1 km s−1 FWHM) emission features that are coincident (kinematically and/or spatially) with observed 22 GHz features. Moreover, in the case of W49N—for which observations were available at three epochs spanning a 2 yr period—the spectra exhibited variability. The observed 621 GHz/22 GHz line ratios are consistent with a maser pumping model in which the population inversions arise from the combined effects of collisional excitation and spontaneous radiative decay, and the inferred physical conditions can plausibly arise in gas heated by either dissociative or non-dissociative shocks. The collisional excitation model also predicts that the 22 GHz population inversion will be quenched at higher densities than that of the 621 GHz transition, providing a natural explanation for the observational fact that 22 GHz maser emission appears to be a necessary but insufficient condition for 621 GHz maser emission. [ABSTRACT FROM AUTHOR]

We present the results of a 22 GHz H2O maser survey toward a new sample of asymptotic giant branch (AGB) and post-AGB star candidates. Most of the objects are selected for the first time based on the AKARI data, which have high flux sensitivity in the mid-infrared ranges.We aim at finding H2O maser sources in the transient phase between the AGB and post-AGB stages of evolution, where the envelopes start to develop large deviations from spherical symmetry. The observations were carried out with the Effelsberg 100 m radio telescope. Among 204 observed objects, 63 detections (36 new) were obtained. We found four objects that may be "water fountain" sources (IRAS 15193+3132, IRAS 18056−1514, OH 16.3−3.0, and IRAS 18455+0448). They possess an H2O maser velocity coverage much smaller than those in other known water fountains. However, the coverage is still larger than that of the 1612 MHz OH maser. It implies that there is an outflow with a higher velocity than the envelope expansion velocity (typically ≦25 km s−1), meeting the criterion of the water fountain class. We suggest that these candidates are possibly oxygen-rich late AGB or early post-AGB stars in a stage of evolution immediately after the spherically symmetric AGB mass loss has ceased. [ABSTRACT FROM AUTHOR]

Trigonometric parallax measurements of nine water masers associated with the Local Arm of the Milky Way were carried out as part of the BeSSeL Survey using the Very Long Baseline Array. When combined with 21 other parallax measurements from the literature, the data allow us to study the distribution and three-dimensional motions of star forming regions in the spiral arm over the entire northern sky. Our results suggest that the Local Arm does not have the large pitch angle characteristic of a short spur. Instead its active star formation, overall length (>5 kpc), and shallow pitch angle (∼10◦) suggest that it is more like the adjacent Perseus and Sagittarius Arms; perhaps it is a branch of one of these arms. Contrary to previous results, we find the Local Arm to be closer to the Perseus than to the Sagittarius Arm, suggesting that a branching from the former may be more likely. An average peculiar motion of near zero toward both the Galactic center and north Galactic pole, and counter rotation of ∼5 km s−1 were observed, indicating that the Local Arm has similar kinematic properties as found for other major spiral arms. [ABSTRACT FROM AUTHOR]

We present the direct measurement of the Hubble constant, yielding the direct measurement of the angular-diameter distance to NGC 6264 using the H2O megamaser technique. Our measurement is based on sensitive observations of the circumnuclear megamaser disk from four observations with the Very Long Baseline Array, the Green Bank Telescope (GBT), and the Effelsberg telescope. We also monitored the maser spectral profile for 2.3 years using the GBT to measure accelerations of maser lines by tracking their line-of-sight velocities as they change with time. The measured accelerations suggest that the systemic maser spots have a significantly wider radial distribution than in the archetypal megamaser in NGC 4258. We model the maser emission as arising from a circumnuclear disk with orbits dominated by the central black hole. The best fit of the data gives a Hubble constant of H0 = 68 ± 9 km s-1Mpc-1, which corresponds to an angular-diameter distance of 144 ± 19 Mpc. In addition, the fit also gives a mass of the central black hole of (3.09 ± 0.42) ×107M☉. The result demonstrates the feasibility of measuring distances to galaxies located well into the Hubble flow by using circumnuclear megamaser disks. [ABSTRACT FROM AUTHOR]

In Papers I and II from the Megamaser Cosmology Project, we reported initial observations of H2O masers in an accretion disk of a supermassive black hole at the center of the galaxy UGC 3789, which gave an angular-diameter distance to the galaxy and an estimate of H0 with 16% uncertainty.We have since conducted more very long baseline interferometric observations of the spatial-velocity structure of these H2O masers, as well as continued monitoring of its spectrum to better measure maser accelerations. These more extensive observations, combined with improved modeling of the masers in the accretion disk of the central supermassive black hole, confirm our previous results, but with significantly improved accuracy. We find H0 = 68.9 ± 7.1 km s-1 Mpc-1 this estimate of H0 is independent of other methods and is accurate to ±10%, including sources of systematic error. This places UGC 3789 at an angular-diameter distance of 49.6 ± 5.1 Mpc, with a central supermassive black hole of (1.16 ± 0.12) ×107 M☉. [ABSTRACT FROM AUTHOR]

We present Very Long Baseline Interferometry (VLBI) observations of 22 GHz H2O masers in the high-mass star-forming region of W75N, carried out with VLBI Exploration of Radio Astrometry (VERA) for three epochs in 2007 with an angular resolution of ~1 mas. We detected H2O maser emission toward the radio jet in VAL 1 and the expanding shell-like structure in VAL 2. The spatial distribution of the H2O masers detected with VERA and measured proper motions around VAL 1 and VAL 2 are similar to those found with previous VLBI observations in epochs 1999 and 2005, with the masers in VAL 1 mainly distributed along a linear structure parallel to the radio jet and, on the other hand, forming a shell-like structure around VAL 2. We have made elliptical fits to the VAL 2 H2O maser shell-like structure observed in the different epochs (1999, 2005, and 2007), and found that the shell is still expanding eight years after its discovery. From the difference in the size of the semi-major axes of the fitted ellipses in epochs 1999 (≃71 ± 1 mas), 2005 (≃97 ± 3 mas), and 2007 (≃111 ± 1 mas), we estimate an average expanding velocity of ~5 mas yr-1, similar to the proper motions measured in the individual H2O maser features. A kinematic age of ~20 yr is derived for this structure. In addition, our VERA observations indicate an increase in the ellipticity of the expanding shell around VAL 2 from epochs 1999 to 2007. In fact, the elliptical fit of the VERA data shows a ratio of the minor and major axes of ~0.6, in contrast with an almost circular shape for the shell detected in 1999 and 2005 (b/a ~ 0.9). This suggests that we are probably observing the formation of a jet-driven H2O maser structure in VLA2, evolving from a non-collimated pulsed-outflow event during the first stages of evolution of a massive young stellar object (YSO). This may support predictions made earlier by other authors on this issue, consistent with recent magnetohydrodynamical simulations. We discuss possible implications of our results in the study of the first stages of evolution of massive YSOs. [ABSTRACT FROM AUTHOR]

To rapidly and accurately investigate the performance of the dielectric loaded rectangular Cerenkov maser, a simplified nonlinear theory is proposed, in which the variations of wave amplitude and wave phase are determined by two coupled first-order differential equations. Through combining with the relativistic equation of motion and adopting the forward wave assumption, the evolutions of the forward wave power, the power growth rate, the axial wave number, the accumulated phase offset, and the information of the particle movement can be obtained in a single-pass calculation. For an illustrative example, this method is used to study the influences of the beam current, the gap distance between the beam and the dielectric surface, and the momentum spread on the forward wave. The variations of the saturated power and the saturation length with the working frequency for the beams with different momentum spreads have also been studied. The result shows that the beam-wave interaction is very sensitive to the electron beam state. To further verify this simplified theory, a comparison with the result produced from a rigorous method is also provided, we find that the evolution curves of the forward wave power predicted by the two methods exhibit excellent agreement. In practical applications, the developed theory can be used for the design and analysis of the rectangular Cerenkov maser. [ABSTRACT FROM AUTHOR]

We used the Very Long Baseline Array and the European VLBI Network to perform phase-referenced very long baseline interferometry observations of the three most powerful maser transitions associated with the high-mass star-forming region G28.87 + 0.07: the 22.2 GHz H2O, 6.7 GHz CH3OH, and 1.665 GHz OH lines. We also performed Very Large Array (VLA) observations of the radio continuum emission at 1.3 and 3.6 cm and Subaru observations of the continuum emission at 24.5 µm. Two centimeter-continuum sources are detected and one of them (named hot molecular core (HMC)) is compact and placed at the center of the observed distribution of H2O, CH3OH, and OH masers. The bipolar distribution of line-of-sight velocities and the pattern of the proper motions suggest that the water masers are driven by a (proto)stellar jet interacting with the dense circumstellar gas. The same jet could both excite the centimeter-continuum source named HMC (interpreted as free--free emission from shocked gas) and power the molecular outflow observed at larger scales--although one cannot exclude that the free--free continuum is rather originating from a hypercompact H II region. At 24.5 µm, we identify two objects separated along the north-south direction, whose absolute positions agree with those of the two VLA continuum sources. We establish that ~90% of the luminosity of the region (~2 x 105 L⊙) is coming from the radio source HMC, which confirms the existence of an embedded massive young stellar object exciting the masers and possibly still undergoing heavy accretion from the surrounding envelope. [ABSTRACT FROM AUTHOR]

We present the results of multi-epoch very long baseline interferometry (VLBI) water (H2O) maser observations carried out with the VLBI Exploration of Radio Astrometry toward the Cepheus A HW3d object. We measured for the first time relative proper motions of the H2O maser features, whose spatio-kinematics traces a compact bipolar outflow. This outflow looks highly collimated and expanding through ~280 AU (400 mas) at a mean velocity of ~21 km s-1 (~6 mas yr-1) without taking into account the turbulent central maser cluster. The opening angle of the outflow is estimated to be ~30 °. The dynamical timescale of the outflow is estimated to be ~100 years. Our results provide strong support that HW3d harbors an internal massive young star, and the observed outflow could be tracing a very early phase of star formation. We also have analyzed Very Large Array archive data of 1.3 cm continuum emission obtained in 1995 and 2006 toward Cepheus A. The comparative result of the HW3d continuum emission suggests the possibility of the existence of distinct young stellar objects in HW3d and/or strong variability in one of their radio continuum emission components. [ABSTRACT FROM AUTHOR]

We report on a detailed study of the water maser kinematics and radio continuum emission toward the most massive and young object in the star-forming region AFGL 2591. Our analysis shows at least two spatial scales of multiple star formation, one projected across 0.1 pc on the sky and another one at about 2000 AU from a ZAMS star of about 38 M⊙. This young stellar object drives a powerful jet- and wind-driven outflow system with the water masers associated to the outflow walls, previously detected as a limb-brightened cavity in the NIR band. At about 1300 AU to the north of this object a younger protostar drives two bow shocks, outlined by arc-like water maser emission, at 200 AU either side of the source. We have traced the velocity profile of the gas that expands along these arc-like maser structures and compared it with the jet-driven outflow model. This analysis suggests that the ambient medium around the northern protostar is swept up by a jet-driven shock (>66 km s-1) and perhaps a lower-velocity (~10 km s-1) wind with an opening angle of about 20° from the jet axis. [ABSTRACT FROM AUTHOR]

It is well known that non-thermal electrons, often modeled by the kappa distribution, are ubiquitously observed in space. Kappa-type distributions have been widely discussed in various plasma environments. On the other hand, when these non-thermal electrons are trapped by magnetic fields in space they frequently have a loss-cone distribution in velocity space. In this paper, we investigated the growth rates of electron cyclotron maser (ECM) instability in space plasmas by using a kappa loss-cone distribution. Based on the cold plasma assumption and weakly relativistic approximation, the results show that non-thermal electrons with such a distribution can excite ECM emission efficiently. The growth rates of O1 and X2 modes both increase with the kappa spectral index (κ) but the growth rate of X2 mode increases slightly more rapidly than that of O1 mode. The present analysis also shows that the growth rates increase rapidly with the loss-cone index σ for σ < 10 but increase comparatively slowly for larger σ values. [ABSTRACT FROM AUTHOR]

We report a trigonometric parallax measurement with the Very Long Baseline Array for the water maser in the distant high-mass star-forming region G75.30+1.32. This source has a heliocentric distance of 9.25 Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed kpc, which places it in the Outer arm in the first Galactic quadrant. It lies 200 pc above the Galactic plane and is associated with a substantial H I enhancement at the border of a large molecular cloud. At a Galactocentric radius of 10.7 kpc, G75.30+1.32 is in a region of the Galaxy where the disk is significantly warped toward the North Galactic Pole. While the star-forming region has an instantaneous Galactic orbit that is nearly circular, it displays a significant motion of 18 km s-1 toward the Galactic plane. The present results, when combined with two previous maser studies in the Outer arm, yield a pitch angle of about 12° for a large section of the arm extending from the first quadrant to the third. [ABSTRACT FROM AUTHOR]