17 research outputs found

    A highly variable methanol maser in G111.256-0.770

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    G111.256-0.770 is a high-mass young stellar object associated with a weak 6.7 GHz methanol maser showing strong variability. We present results of a multi-epoch monitoring program of the target, conducted with the Torun 32 m telescope for more than a decade. We found that the isotropic maser luminosity varied by a factor 16 on a time-scale of 5-6 yr and individual features showed small amplitude short-lived (∼0.2 yr) bursts superimposed on higher amplitude slow (>5 yr) variations. The maser integrated flux density appears to be correlated with the near-infrared flux observed with the (NEO)WISE, supporting radiative pumping of the maser line

    Giant burst of methanol maser in S255IR-NIRS3

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    Context.High-mass young stellar objects (HMYSOs) can undergo accretion episodes that strongly affect the star evolution, the dynamics of the disk, and its chemical evolution. Recently reported extraordinary bursts in the methanol maser emission may be the observational signature of accretion events in deeply embedded HMYSOs. Aims.We analyze the light curve of 6.7GHz methanol masers in S255IR-NIRS3 during the 2015-2016 burst. Methods.8.5-yr monitoring data with an average sampling interval of 5 days were obtained with the Torun 32 m radio telescope. Archival data were added, extending the time series to ~27 yr. Results.The maser emission showed moderate (25-30%) variability on timescales of months to years over ~23 yr since its discovery.The main burst was preceded by a 1 yr increase of the total flux density by a factor of 2.5, then it grew by a factor of 10 over ~0.4 yr and declined by a factor of 8 during the consecutive 2.4 yr. The peak maser luminosity was a factor of 24.5 higher than the pre-burst quiescent value. The light curves of individual features showed considerable diversity but indicated a general trend of suppression of the maser emission at blueshifted (5.8km/s. This new emission provided a contribution of about 80% to the maser luminosity around the peak of the burst. The duration of the burst at the extreme redshifted velocities of 7.1–-8.7km/s was from 0.9 to 1.9 yr, and its lower limit for the other features was ~3.9 yr. Conclusions.The onset of the maser burst exactly coincides with that of the infrared burst estimated from the motion of the light echo. This strongly supports the radiative pumping scheme of the maser transition. The growth of the maser luminosity is the result of an increasing volume of gas where the maser inversion is achieved

    6.7 GHz variability characteristics of new periodic methanol maser sources

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    Discovery of periodic maser emission was an unexpected result from monitoring observations of methanol transitions in high-mass young stellar objects. We report on the detection of five new periodic sources from a monitoring program with the Torun 32 m telescope. Variability with a period of 149 to 540 d and different patterns from sinusoidal-like to intermittent was displayed. Three-dimensional structure of G59.633−0.192 determined from the time delays of burst peaks of the spectral features and high angular resolution map implies that the emission traces a disc. For this source the 6.7 GHz light curve followed the infrared variability supporting a radiative scheme of pumping. An unusual time delay of ∼80 d occurred in G30.400−0.296 could not be explained by the light travel time and may suggest a strong differentiation of physical conditions and excitation in this deeply embedded source. Our observations suggest the intermittent variability may present a simple response of maser medium to the underlying variability induced by the accretion luminosity while other variability patterns may reflect more complex changes in the physical conditions

    Monitoring observations of 6.7 GHz methanol masers

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    We report results of 6.7 GHz methanol maser monitoring of 139 star-forming sites with theTorun 32 m radio telescope from June 2009 to February 2013. The targets were observedat least once a month, with higher cadences of 2-4 measurements per week for circumpolarobjects. Nearly 80 per cent of the sources display variability greater than 10 per cent on atime-scale between a week and a few years but about three quarters of the sample have only1-3 spectral features which vary significantly. Irregular intensity fluctuation is the dominanttype of variability and only nine objects show evidence for cyclic variations with periodsof 120 to 416 d. Synchronised and anti-correlated variations of maser features are detectedin four sources with a disc-like morphology. Rapid and high amplitude bursts of individualfeatures are seen on 3-5 occasions in five sources. Long (>50 d to 20 months) lasting burstsare observed mostly for individual or groups of features in 19 sources and only one sourceexperienced a remarkable global flare. A few flaring features display a strong anti-correlationbetween intensity and line-width that is expected for unsaturated amplification. There is aweak anti-correlation between the maser feature luminosity and variability measure, i.e. maserfeatures with low luminosity tend to be more variable than those with high luminosity. Theanalysis of the spectral energy distribution and continuum radio emission reveals that thevariability of the maser features increases when the bolometric luminosity and Lyman fluxof the exciting object decreases. Our results support the concept of a major role for infraredpumping photons in triggering outburst activity of maser emission

    Long-term multi-frequency maser observations of the intermediate-mass young stellar object G107.298+5.639

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    Context: Periodic flares of maser emission are thought to be induced either by variations of the seed photon flux in young binary systems or the pump rate regulated by stellar and accretion luminosities. Aims: We seek to study the variability of four maser transitions of three different species in G107.298+5.639 to constrain the dominant mechanism of periodic flares. Methods: Light curves of the 6.7 GHz methanol and 22.2 GHz water vapour maser were obtained with the Torun 32 m radio telescope over 39 and 34 cycles, respectively. The target was also monitored at the 1.6 GHz hydroxyl transitions with the Nançay radio telescope over 13 cycles. All these maser lines were imaged using VLBI arrays. Results: The study confirms alternating flares of the methanol and water masers with a period of 34.4 d and reveals the synchronised behaviour of the methanol and hydroxyl masers in this source. The observed spatial distribution of the methanol maser cloudlets and the measured time delays of the flares of individual features imply a ring-like structure of radius 240 au and thickness 30 au. Internal proper motions indicate that the velocity of methanol cloudlets is dominated by a disc-wind component of about 5 km s −1 . The methanol emission detected during only one VLBI observation is located in a region about 550 au from a central star, which also exhibits OH maser flares. The erratic appearance of methanol features can be related to a powering object of relatively low luminosity which, during some variability cycles, can excite molecules only in the nearest part of the disc. A careful analysis of the maser and infrared light curves reveal a strong correlation between the 6.7 GHz line and the infrared flux densities supporting a radiative pumping of the maser. Conclusions: The synchronised behaviour of the hydroxyl 1665/1667 MHz and 6.7 GHz methanol transitions indicates a common pumping mechanism for the periodic flares of G107.298+5.639

    New evidence for Dicke's superradiance in the 6.7 GHz methanol spectral line in the interstellar medium

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    We present new evidence for superradiance (SR) in the methanol 6.7 GHz spectral line for three different star-forming regions: S255IR-NIRS3, G24.329+0.144, and Cepheus A. Our analysis shows that some of the flux–density flares exhibiting fast rise times and asymmetric light curves reported in these sources can naturally be explained within the context of SR. When a threshold for the inverted population column density is exceeded in a maser-hosting region, the radiation mode switches from one regulated by stimulated emission (maser) to SR. Superradiance, as a more efficient energy release mechanism, manifests itself through strong bursts of radiation emanating from spatially compact regions. Elevated inverted population densities and the initiation of SR can be due to a change in radiative pumping. Here, we show that an increase in the pump rate and the inverted population density of only a factor of a few results in a significant increase in radiation. While the changes in the pump rate can take place over a few hundred days, the rise in radiation flux density when SR is initiated is drastic and happens over a much shorter time-scale

    Multi-epoch VLBI of a double maser super burst

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    In a rare and spectacular display, two well-known massive star forming regions, W49N and G25.65+1.05, recently underwent maser 'super burst' - their fluxes suddenly increasing above 30,000 and 18,000 Jy, respectively, reaching several orders of magnitude above their usual values. In quick-response, ToO observations with the EVN, VLBA and KaVA were obtained constituting a 4 week campaign - producing a high-cadence multi-epoch VLBI investigation of the maser emission. The combination of high-resolution, polarisation and flux monitoring during the burst provides one of the best accounts, to date, of the maser super burst phenomenon, aiding their use as astrophysical tools. These proceedings contain the preliminary results of our campaign

    Evidence for Cold Plasma in Planetary Nebulae From Radio Observations With the LOw Frequency ARray (LOFAR)

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    We present observations of planetary nebulae with the LOw Frequency ARray (LOFAR) between 120 and 168 MHz. The images show thermal free–free emission from the nebular shells. We have determined the electron temperatures for spatially resolved, optically thick nebulae. These temperatures are 20%–60% lower than those estimated from collisionally excited optical emission lines. This strongly supports the existence of a cold plasma component, which co-exists with hot plasma in planetary nebulae. This cold plasma does not contribute to the collisionally excited lines, but does contribute to recombination lines and radio flux. Neither of the plasma components are spatially resolved in our images, although we infer that the cold plasma extends to the outer radii of planetary nebulae. However, more cold plasma appears to exist at smaller radii. The presence of cold plasma should be taken into account in modeling of radio emission of planetary nebulae. Modelling of radio emission usually uses electron temperatures calculated from collisionally excited optical and/or infrared lines. This may lead to an underestimate of the ionized mass and an overestimate of the extinction correction from planetary nebulae when derived from the radio flux alone. The correction improves the consistency of extinction derived from the radio fluxes when compared to estimates from the Balmer decrement flux ratios

    A Giant Water Maser Flare in the Galactic Source IRAS 18316-0602

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    The results of long-term monitoring of the Galactic maser source IRAS 18316-0602 (G25.65+1.05) in the water-vapor line at frequency f = 22.235 GHz (6_16-5_23 transitioin) carried out on the 22-m Simeiz, 26-m HartRAO, and 26-m Torun radio telescopes are reported. The source has been episodically observed on the Simeiz telescope since 2000, with more regular observations beginning in 2017. A double flare was observed beginning in September 2017 and continuing to February 2018, which was the most powerful flare registered over the entire history of observations of this object. Most of the monitoring of the flare was carried out in a daily regime. Detailed analysis of the variations of the flux density, which reached a maximum value P ≈ 1.3 × 10^5 Jy, have led to important scientific conclusions about possible mechanisms for the emission in this water line. The exponential growth in the flux density in this double flare testifies that it was associated with a maser that was unsaturated right up to the maximum flux densities observed. An additional argument suggesting the maser was unsaturated is the relatively moderate degree of linear polarization (≈30%), nearly half the value displayed by the Galactic kilomasers in Orion KL. The accurate distance estimate for IRAS 18316-0602 (12.5 kpc) and the flux density at the flare maximum (≈1.3 × 10^5 Jy) makes this the most powerful Galactic kilomaser known. The double form of the flare with exponential rises in flux density rules out the possibility that the flare is the effect of directivity of a radiation beam relative to the observer. The physical nature of the flare is most likely related to internal parameters of the medium in which the maser clumps radiating in the water line are located. A rapid, exponential growth in the flux density of a kilomaser and associated exponential decays requires the presence of an explosive increase in the density of the medium and the photon flux, leading to an increase in the temperature by 10-40 K above the initial base level. A mechanism for the primary energy release in IRAS 18316-0602 is proposed, which is associated with a multiple massive star system located in a stage of evolution preceding its entry onto the main sequence. A flare in this object could initiate gravitational interaction between the central star and a massive companion at its periastron. The resulting powerful gravitational perturbation could lead to the ejection of the envelope of the central supermassive star, which gives rise to an explosive increase in the density and temperature of the associate gas-dust medium when it reaches the disk, where the maser clumps are located

    A Water-Vapor Maser Flare in a High-Velocity Line toward W49N

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    —Powerful flares in Galactic kilomasers are closely associated with regions of intense star formation. They contribute to the elucidation of physical processes occurring in these structures. We have recorded a superpowerful flare in the high-velocity −81 km s−1 line in the Galactic maser source W49N. As a result of our monitoring at the RT-22 (Simeiz), RT-32 (Torun), RT-100 (Effelsberg), and RT-32 (Medicina) radio telescopes in the period from September 2017 to November 2018, we have obtained the shape of the spectral flux density variations in the source with time. At the peak the flux density reached P ≈ 5 × 10^4 Jy. The flare has a double pattern and different durations of its components. The pattern of spectral flux density variations for the first flare with a considerably shorter duration is apparently related to a sharp increase in the density of the medium and the photon flux and to a significant rise in the temperature to hundreds of kelvins. We propose a mechanism of primary energy release related to the existence of close massive multiple systems in star-forming regions. A powerful gravitational perturbation at the system’s periastron can lead to a partial ejection of the envelope of the central massive star in a direction close to the major axis of the ellipse of the companion’s orbit. This explains the significant asymmetry of high-velocity lines in W49N. The ejected envelope is an energy source more significant than the stellar wind and can explain the giant flares in the object. Further comprehensive studies in this direction, including monitoring VLBI studies, are needed to confirm this assumption
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