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Linear polarisation of Class I methanol masers in massive star formation regions
摘要： We present the results of the linear polarisation observations of methanol masers at 44 and 95 GHz towards 39 massive star forming regions (Kang et al. 2016). These two lines are observed simultaneously with the 21-m Korean VLBI Network (KVN) telescope in single dish mode. About 60% of the observed showed fractional polarisation of a few percents at least at one of the two transition lines. We note that the linear polarisation of the 44 GHz methanol maser is ?rst detected in this study including single dish and interferometer observations. We ?nd the polarisation properties of these two lines are similar as expected, since they trace similar regions. As a follow-up study, we have carried out the VLBI polarisation observations toward some 44 GHz maser targets using the KVN telescope. We present preliminary VLBI polarisation results of G10.34-0.14, which show consistent polarisation properties in multiple epoch observations.
关键词： polarisation,masers,magnetic ?elds,stars: formation
更新于2025-09-23 15:22:29
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SiO masers from AGB stars in the vibrationally excited <i>v</i> = 1, <i>v</i> = 2, and <i>v</i> = 3 states
摘要： Context. The v = 1 and v = 2 J = 1–0 (43 GHz), and v = 1 J = 2–1 (86 GHz) SiO masers are intense in asymptotic giant branch (AGB) stars and have been mapped using very long baseline interferometry (VLBI) showing ring-like distributions. Those of the v = 1, v = 2 J = 1–0 masers are similar, but the spots are rarely coincident, while the v = 1 J = 2–1 maser arises from a well-separated region farther out. These relative locations can be explained by models tools that include the overlap of two IR lines of SiO and H2O. The v = 3 J = 1–0 line is not directly affected by any line overlap and its spot structure and position, relative to the other lines, is a good test to the standard pumping models. Aims. The aim of this project are to gain insight into the properties and the general theoretical considerations of the different SiO masers that can help to understand them. Methods. We present single-dish and simultaneous VLBI observations of the v = 1, v = 2, and v = 3 J = 1–0 maser transitions of 28SiO in several AGB stars. The results are compared to the predictions of radiative models of SiO masers that both include and not include the effect of IR line overlap. Results. The spatial distribution of the SiO maser emission in the v = 3 J = 1–0 transition from AGB stars is systematically composed of a series of spots that occupy a ring-like structure (as often found in SiO masers). The overall ring structure is extremely similar to that found in the other 43 GHz transitions and is very different from the structure of the v = 1 J = 2–1 maser. The positions of the individual spots of the different 43 GHz lines are, however, very rarely coincident, which in general is separated by about 0.3 AU (between 1 and 5 mas). These results are very difficult to reconcile with standard pumping models, which predict that the masers of rotational transitions within a given vibrational state require very similar excitation conditions (since the levels are placed practically at the same energy from the ground), while the transitions of different vibrational states (which are separated by an energy of 1800 K) should appear in different positions. However, models including line overlap tend to predict v = 1, v = 2, v = 3 J = 1–0 population inversion to occur under very similar conditions, while the requirements for v = 1 J = 2–1 appear clearly different, and are compatible with the observational results.
关键词： masers,stars: AGB and post-AGB,techniques: interferometric,circumstellar matter,radio lines: stars
更新于2025-09-23 15:21:21
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Detection of 84 GHz Class I Methanol Maser Emission toward NGC 253
摘要： We have investigated the central region of NGC 253 for the presence of 84.5 GHz (5?1 → 40E) methanol emission using the Australia Telescope Compact Array. We present the second detection of 84.5 GHz class I methanol maser emission outside of the Milky Way. This maser emission is offset from the dynamical center of NGC 253, in a region with previously detected emission from class I maser transitions (36.2 GHz 4?1 → 30E and 44.1 GHz 70 → 61A+ methanol lines). The emission features a narrow linewidth (～12 km s?1) with a luminosity approximately that is 5 orders of magnitude higher than typical Galactic sources. We determine an integrated line intensity ratio of 1.2 ± 0.4 between the 36.2 and 84.5 GHz class I methanol maser emission, which is similar to the ratio observed toward Galactic sources. The three methanol maser transitions observed toward NGC 253 each show a different distribution, suggesting differing physical conditions between the maser sites, and also that observations of additional class I methanol transitions will facilitate investigations of the maser pumping regime.
关键词： galaxies: starburst,masers,radio lines: galaxies,galaxies: individual (NGC 253)
更新于2025-09-23 15:21:21
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VLBA imaging of the 3 mm SiO maser emission in the disk-wind from the massive protostellar system Orion Source I
摘要： Context. High-mass star formation remains poorly understood due to observational difficulties (e.g. high dust extinction and large distances) hindering the resolution of disk-accretion and outflow-launching regions. Aims. Orion Source I is the closest known massive young stellar object (YSO) and exceptionally powers vibrationally-excited SiO masers at radii within 100 AU, providing a unique probe of gas dynamics and energetics. We seek to observe and image these masers with Very Long Baseline Interferometry (VLBI). Methods. We present the first images of the 28SiO v = 1, J = 2?1 maser emission around Orion Source I observed at 86 GHz (λ3 mm) with the Very Long Baseline Array (VLBA). These images have high spatial (～0.3 mas) and spectral (～0.054 km s?1) resolutions. Results. We find that the λ3 mm masers lie in an X-shaped locus consisting of four arms, with blue-shifted emission in the south and east arms and red-shifted emission in the north and west arms. Comparisons with previous images of the 28SiO v = 1, 2, J = 1?0 transitions at λ7 mm (observed in 2001–2002) show that the bulk of the J = 2?1 transition emission follows the streamlines of the J = 1?0 emission and exhibits an overall velocity gradient consistent with the gradient at λ7 mm. While there is spatial overlap between the λ3 mm and λ7 mm transitions, the λ3 mm emission, on average, lies at larger projected distances from Source I (～44 AU compared with ～35 AU for λ7 mm). The spatial overlap between the v = 1, J = 1?0 and J = 2?1 transitions is suggestive of a range of temperatures and densities where physical conditions are favorable for both transitions of a same vibrational state. However, the observed spatial offset between the bulk of emission at λ3 mm and λ7 mm possibly indicates different ranges of temperatures and densities for optimal excitation of the masers. We discuss different maser pumping models that may explain the observed offset. Conclusions. We interpret the λ3 mm and λ7 mm masers as being part of a single wide-angle outflow arising from the surface of an edge-on disk rotating about a northeast-southwest axis, with a continuous velocity gradient indicative of differential rotation consistent with a Keplerian profile in a high-mass proto-binary.
关键词： stars: formation,masers,ISM: jets and outflows,radio lines: stars,ISM: individual objects: Orion BN/KL (except planetary nebulae)
更新于2025-09-23 15:21:21
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Development of cross-correlation spectrometry and the coherent structures of maser sources
摘要： We have developed a new method of data processing for radio telescope observation data to measure time-dependent temporal coherence, and we have named it 'cross-correlation spectrometry' (XCS). XCS is an autocorrelation procedure that expands time lags over the integration time and is applied to data obtained from a single-dish observation. The temporal coherence property of received signals is enhanced by XCS. We tested the XCS technique using the data of strong H2O masers in W 3 (H2O), W 49 N, and W 75 N. We obtained the temporal coherent lengths of the maser emission to be 17.95 ± 0.33 μs, 26.89 ± 0.49 μs, and 15.95 ± 0.46 μs for W 3 (H2O), W 49 N, and W 75 N, respectively. These results may indicate the existence of a coherent astrophysical maser.
关键词： techniques: interferometric,masers,methods: data analysis
更新于2025-09-19 17:15:36
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High-resolution IR and radio observations of AGB stars
摘要： Aims. We present the results of observations with interferometers of a sample of pulsating asymptotic giant branch (AGB) stars in the infrared and at radio wavelengths. The goal of these observations is to explore the extended stellar atmospheres and to establish links between the spatial scales of molecular envelopes and of the dust shell. This is the key to better understand the process of dust formation and therefore of mass loss. Methods. We used the ESO VLTI/MIDI interferometer in the N band, the Keck Interferometer in the K band, and NRAO VLBA observations of SiO masers at 7 mm wavelength of a sample of AGB stars: U Ari, W Cnc, RX Tau, RT Tau, RT Aql, S Ser, and V Mon. The various instruments probe different altitudes of the atmosphere of the AGB stars. They are sensitive to regions below the silicate dust condensation distance and provide the opportunity of finding hints about how dust and its precursors form in the extended atmosphere of an AGB star. The K-band observations are sensitive to water and carbon-monoxyde vapors. Unfortunately, we were only able to observe S Ser in this wavelength range. Results. We find a ratio of 2.2 between the molecular envelope radius and the photospheric size, which is consistent with previous results. The N-band observations are mostly sensitive to vapors of SiO and water and to dust (alumina and silicate). The silicate dust shell is fully resolved, and no precise parameters can be deduced from the N-band observations other than a spatial extension of at least 12–16 R* for our sample. The sizes found for the SiO region are consistent with the radii of the SiO maser rings provided by the VLBA observations. The sizes of the alumina and water vapor regions are systematically found to be larger. There is clear evidence that SiO is absent from regions farther from the star where silicate dust condenses. Conclusions. These observations support a possible scenario in which SiO is adsorbed by species such as corundum. An alternative explanation could be that SiO has chemically disappeared at this range of distances.
关键词： infrared: stars,stars: mass-loss,masers,stars: AGB and post-AGB,techniques: interferometric,stars: atmospheres
更新于2025-09-19 17:15:36
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A new approach to the maser emission in the solar corona
摘要： Aims. The electron plasma frequency ωpe and electron gyrofrequency Ωe are two parameters that allow us to describe the properties of a plasma and to constrain the physical phenomena at play, for instance, whether a maser instability develops. In this paper, we aim to show that the maser instability can exist in the solar corona. Methods. We perform an in-depth analysis of the ωpe/Ωe ratio for simple theoretical and complex solar magnetic ?eld con?gurations. Using the combination of force-free models for the magnetic ?eld and hydrostatic models for the plasma properties, we determine the ratio of the plasma frequency to the gyrofrequency for electrons. For the sake of comparison, we compute the ratio for bipolar magnetic ?elds containing a twisted ?ux bundle, and for four di?erent observed active regions. We also study how ωpe/Ωe is a?ected by the potential and non-linear force-free ?eld models. Results. We demonstrate that the ratio of the plasma frequency to the gyrofrequency for electrons can be estimated by this novel method combining magnetic ?eld extrapolation techniques and hydrodynamic models. Even if statistically not signi?cant, values of ωpe/Ωe ≤ 1 are present in all examples, and are located in the low corona near to photosphere below one pressure scale-height and/or in the vicinity of twisted ?ux bundles. The values of ωpe/Ωe are lower for non-linear force-free ?elds than potential ?elds, thus increasing the possibility of maser instability in the corona. Conclusions. From this new approach for estimating ωpe/Ωe, we conclude that the electron maser instability can exist in the solar corona above active regions. The importance of the maser instability in coronal active regions depends on the complexity and topology of the magnetic ?eld con?gurations.
关键词： magnetohydrodynamics (MHD),masers,Sun: radio radiation,Sun: magnetic ?elds
更新于2025-09-19 17:15:36
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Enhancing the H <sub/>2</sub> O Megamaser Detection Rate Using Optical and Mid-infrared Photometry
摘要： Water megamasers from circumnuclear disks in galaxy centers provide the most accurate measurements of supermassive black hole masses and uniquely probe the subparsec accretion processes. At the same time, these systems offer independent crucial constraints of the Hubble constant in the nearby universe, and thus, the arguably best single constraint on the nature of dark energy. The chances of finding these golden standards are, however, abysmally low, at ～3% overall for any level of water maser emission detected at 22 GHz and ～1% for those exhibiting disk-like configuration. We provide here a thorough summary of the current state of detection of water megamaser disks along with a novel investigation of the likelihood of increasing their detection rates based on a multivariate parameter analysis of the optical and mid-infrared (mid-IR) photometric properties of the largest database of galaxies surveyed for 22 GHz emission. We find that galaxies with water megamaser emission tend to be associated with strong emission in all Wide-field Infrared Survey Explorer mid-IR wavelengths, with the strongest enhancement in the W4 band, at 22 μm, as well as with previously proposed and newly found indicators of active galactic nucleus strength in the mid-IR, such as red W1?W2 and W1?W4 colors, and the integrated mid-IR luminosity of the host galaxy. These trends offer a potential boost of the megamaser detection rates to 6%–15%, or a factor of 2–8 relative to the current rates, depending on the chosen sample selection criteria, while fostering real chances for discovering ～20 new megamaser disks.
关键词： infrared: galaxies,surveys,galaxies: nuclei,galaxies: active,masers
更新于2025-09-10 09:29:36
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Analysis of time series in space maser signals
摘要： Many astrophysical sources that emit narrow radio-frequency spectral lines are believed to be sets of molecular condensation, each of which works as a maser, so that the whole set produces a characteristic spectrum. The forms of these spectra vary with time on various time-scales from months to dozens of minutes. We analysed the ultra-short variations of the separate components of space maser spectra in search for the periodical behaviour. We used a consecutive statistical analysis of the observation data that includes fast Fourier transformations (FFTs), a Lomb-Scargle procedure, and a modi?ed Lomb-Scargle procedure. In at least 8 of the 49 sources we studied, we found that the intensity of one of the components of a space maser spectrum that corresponds to a single condensation changes periodically with a period of dozens of minutes or hours. Three sources had a period of 68 min; in one source the oscillations lasted at least four days.
关键词： binaries: close,gravitational waves,masers
更新于2025-09-04 15:30:14