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Register a new userPeriodic Flare of the 6.7 GHz Methanol Maser in IRAS22198+6336
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We have detected periodic flares of the 6.7 GHz methanol maser from an intermediate-mass star-forming region IRAS22198+6336. The maser was monitored daily in 2011, 2012, and 2013. Six flares were observed with a period of 34.6 days. The variation pattern is intermittent, and the flux ratio of the flaring and the quiescent states exceeds 30. Such intermittent variation with the short period uniquely characterizes the variation of the IRAS22198+6336 maser. At least five spectral components were identified. The spectral components varied almost synchronously, but their peak times differed by 1.8 days. These characteristics can be explained by the colliding-wind binary model.
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Intriguing work on observations of 4.83 GHz formaldehyde (H2CO) absorptions and 4.87 GHz H110a radio recombination lines (RRLs) towards 6.7 GHz methanol (CH3OH) maser sources is presented. Methanol masers provide ideal sites to probe the earliest stages of massive star formation, while 4.8 GHz formaldehyde absorptions are accurate probes of physical conditions in dense $(10^{3} - 10^{5} cm^{-3})$ and low temperature molecular clouds towards massive star forming regions. The work is aimed at studying feature similarities between the formaldehyde absorptions and the methanol masers so as to expand knowledge of events and physical conditions in massive star forming regions. A total of 176 methanol maser sources were observed for formaldehyde absorptions, and formaldehyde absorptions were detected 138 of them. 53 of the formaldehyde absorptions were newly detected. We noted a poor correlation between the methanol and formaldehyde intensities, an indication that the signals (though arise from about the same regions) are enhanced by different mechanisms. Our results show higher detection rates of the formaldehyde lines for sources with stronger methanol signals. The strongest formaldehyde absorptions were associated with IRAS sources and IRDCs that have developed HII regions, and that do not have EGOs.
We detected flaring flux variability that regularly occurred with the period of 23.9 days on a 6.7 GHz methanol maser emission at Vlsr = 25.30 km/s in G 014.23-00.50 through highly frequent monitoring using the Hitachi 32-m radio telescope. By analyzing data from 05 January 2013 to 21 January 2016, the periodic variability has persisted in at least 47 cycles, corresponding to approximately 1,100 days. The period of 23.9 days is the shortest one observed in masers at around high-mass young stellar objects so far. The flaring component normally falls below the detection limit (3 sigma) of 0.9 Jy. In the flaring periods, the component rises above the detection limit with the ratio of the peak flux density more than 180 in comparison with a quiescent phase, showing intermittent periodic variability. The time-scale of the flux rise was typically two days or shorter, and both symmetric and asymmetric profiles of flux variability were observed through intraday monitoring. These characteristics might be explained by a change in the flux of seed photons by a colliding-wind binary (CWB) system or a variation of the dust temperature by an extra heating source of a shock formed by the CWB system within a gap region in a circumbinary disk, in which the orbital semi-major axes of the binary are 0.26-0.34 au.
The Central Molecular Zone (CMZ) spans the inner ~450 pc (3 degrees) of our Galaxy. This region is defined by its enhanced molecular emission and contains 5% of the entire Galaxys molecular gas mass. However, the number of detected star forming sites towards the CMZ may be low for the amount of molecular gas that is present, and improved surveys of star formation indicators can help clarify this. With the Karl G Jansky Very Large Array (VLA), we conducted a blind survey of 6.7 GHz methanol masers spanning the inner 3deg x 40arcmin (450 pc x 100 pc) of the Galaxy. We detected 43 methanol masers towards 28 locations, 16 of which are new detections. The velocities of most of these masers are consistent with being located within the CMZ. A majority of the detected methanol masers are distributed towards positive Galactic longitudes, similar to 2/3 of the molecular gas mass distributed at positive Galactic longitudes. The 6.7 GHz methanol maser is an excellent indicator of high mass (>8 solar mass) star formation, with new detections indicating sites of massive star formation in the CMZ.
22 GHz water and 6.7 GHz methanol masers are usually thought as signposts of early stages of high-mass star formation but little is known about their associations and the physical environments they occur in. The aim was to obtain accurate positions and morphologies of the water maser emission and relate them to the methanol maser emission recently mapped with Very Long Baseline Interferometry. A sample of 31 methanol maser sources was searched for 22 GHz water masers using the VLA and observed in the 6.7 GHz methanol maser line with the 32 m Torun dish simultaneously. Water maser clusters were detected towards 27 sites finding 15 new sources. The detection rate of water maser emission associated with methanol sources was as high as 71%. In a large number of objects (18/21) the structure of water maser is well aligned with that of the extended emission at 4.5 $mu$m confirming the origin of water emission from outflows. The sources with methanol emission with ring-like morphologies, which likely trace a circumstellar disk/torus, either do not show associated water masers or the distribution of water maser spots is orthogonal to the major axis of the ring. The two maser species are generally powered by the same high-mass young stellar object but probe different parts of its environment. The morphology of water and methanol maser emission in a minority of sources is consistent with a scenario that 6.7 GHz methanol masers trace a disc/torus around a protostar while the associated 22 GHz water masers arise in outflows. The majority of sources in which methanol maser emission is associated with the water maser appears to trace outflows. The two types of associations might be related to different evolutionary phases.
We present the results of daily monitoring of 6.7 GHz methanol maser in Cepheus A (Cep A) using Yamaguchi 32-m radio telescope as well as the results of imaging observations conducted with the JVN (Japanese VLBI Network). We indentified five spectral features, which are grouped into red-shifted (-1.9 and -2.7 km/s) and blue-shifted (-3.8, -4.2, and -4.9 km/s), and we detected rapid variabilities of these maser features within a monitoring period of 81 days. The red-shifted features decreased in flux density to 50% of its initial value, while the flux density of the blue-shifted features rapidly increased within a 30 days. The time variation of these maser features showed two remarkable properties; synchronization and anti-correlation between the red-shifted and the blue-shifted. The spatial distribution of the maser spots obtained by the JVN observation showed an arclike structure with a scale of $sim$1400 AU, and separations of the five maser features were found to be larger than 100 AU. The absolute position of the methanol maser was also obtained based on the phase-referencing observations, and the arclike structure were found to be associated with the Cep A-HW2 object, with the elongation of the arclike structure nearly perpendicularly to the radio continuum jet from the Cep A-HW2 object. These properties of the masers, namely, the synchronization of flux variation, and the spectral and spatial isolation of features, suggest that the collisional excitation by shock wave from a common exciting source is unlikely. Instead, the synchronized time variation of the masers can be explained if all the maser features are excited by infrared radiation from dust which is heated by a common exciting source with a rapid variability.
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