The GLIMPSE and MSX surveys have been used to examine the mid-infrared properties of a statistically complete sample of 6.7 GHz methanol masers. The GLIMPSE point sources associated with methanol ...masers are clearly distinguished from the majority, typically having extremely red mid-infrared colors similar to those expected of low-mass Class 0 young stellar objects. The intensity of the GLIMPSE sources associated with methanol masers is typically 4 mag brighter at 8.0 km than at 3.6 km. Targeted searches toward GLIMPSE point sources with 3.6 - 4.5 > 1.3 and an 8.0 km magnitude less than 10 will detect more than 80% of Class II methanol masers. Many of the methanol masers are associated with sources within infrared dark clouds (IRDCs), which are believed to mark regions where high-mass star formation is in its very early stages. The presence of Class II methanol masers in a significant fraction of IRDC suggests that high-mass star formation is common in these regions. Different maser species are thought to trace different evolutionary phases of the high-mass star formation process. Comparison of the properties of the GLIMPSE sources associated with Class II methanol masers and other maser species shows interesting trends, consistent with Class I methanol masers tracing a generally earlier evolutionary phase and OH masers tracing a later evolutionary phase.
Recently, a comparison between the locations of 6.7-GHz methanol masers and dust continuum emission has renewed speculation that these masers can be associated with evolved stars. The implication of ...such a scenario would be profound, especially for the interpretation of large surveys for 6.7-GHz masers, individual studies where high-mass star formation has been inferred from the presence of 6.7-GHz methanol masers and for the pumping mechanisms of these masers. We have investigated the two instances where 6.7-GHz methanol masers have been explicitly suggested to be associated with evolved stars, and we find the first to be associated with a standard high-mass star formation region, and the second to be a spurious detection. We also find no evidence to suggest that the methanol maser action can be supported in the environments of evolved stars. We thereby confirm their exclusive association with high-mass star formation regions.
Southern class I methanol masers at 36 and 44 GHz Voronkov, M. A; Caswell, J. L; Ellingsen, S. P ...
Monthly notices of the Royal Astronomical Society,
04/2014, Letnik:
439, Številka:
3
Journal Article
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The Australia Telescope Compact Array (ATCA) has been used for high angular resolution imaging of 71 southern class I methanol maser sources quasi-simultaneously at 36 and 44 GHz. The data reveal a ...high level of morphological and kinematical complexity, and allow us to demonstrate associations, at arcsecond precision, of the class I maser emission with outflows, expanding H ii regions, dark clouds, shocks traced by the 4.5-μm emission and 8.0-μm filaments. More than 700 maser component features were found at each of the two methanol transitions, but with only 23 per cent recognizable at both transitions; the morphology of class I emission is much better revealed by our survey of both transitions, compared with either one alone. We found that the number of masers falls exponentially with the projected linear distance from the associated class II 6.7-GHz methanol maser. This distribution has a scale of 263±15 mpc, irrespective of the transition. The class I masers associated with OH masers were found to have a tendency to be more spread out, both spatially and in the velocity domain. This is consistent with the expectation that such sources are more evolved. Apart from a small number of high-velocity components (which are largely blueshifted and predominantly seen at 36 GHz), the velocity distribution was found to be Gaussian, peaking near the systemic velocity of the region, which had been estimated as the middle of the velocity interval of the associated class II methanol maser at 6.7 GHz. The mean indicated a small, but significant blueshift asymmetry of −0.57 km s−1 (uncertainties are 0.06 and 0.07 km s−1 for the 36- and 44-GHz masers, respectively) with respect to the 6.7-GHz masers. The standard deviation of the velocity distribution was found to be 3.65 ± 0.05 and 3.32 ± 0.07 km s−1 for the 36- and 44-GHz masers, respectively. We also suggest a refined rest frequency value of 36 169.238 ± 0.011 MHz for the 4− 1-30 E methanol transition.
We report the results of a search for 12.2-GHz methanol maser emission, targeted towards 113 known 6.7-GHz methanol masers associated with 1.2-mm dust continuum emission. Observations were carried ...out with the Australia Telescope National Facility (ATNF) Parkes 64-m radio telescope in the period 2008 June 20–25. We detect 68 12.2-GHz methanol masers with flux densities in excess of our 5σ detection limit of 0.55 Jy, 30 of which are new discoveries. This equates to a detection rate of 60 per cent, similar to previous searches of comparable sensitivity. We have made a statistical investigation of the properties of the 1.2-mm dust clumps with and without associated 6.7-GHz methanol maser and find that 6.7-GHz methanol masers are associated with 1.2-mm dust clumps with high-flux densities, masses and radii. We additionally find that 6.7-GHz methanol masers with higher peak luminosities are associated with less dense 1.2-mm dust clumps than those 6.7-GHz methanol masers with lower luminosities. We suggest that this indicates that more luminous 6.7-GHz methanol masers are generally associated with a later evolutionary phase of massive star formation than less luminous 6.7-GHz methanol maser sources. Analysis of the 6.7-GHz associated 1.2-mm dust clumps with and without associated 12.2-GHz methanol maser emission shows that clumps associated with both class II methanol maser transitions are less dense than those with no associated 12.2-GHz methanol maser emission. Furthermore, 12.2-GHz methanol masers are preferentially detected towards 6.7-GHz methanol masers with associated OH masers, suggesting that 12.2-GHz methanol masers are associated with a later evolutionary phase of massive star formation. We have compared the colours of the Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) point sources associated with the maser sources in the following two subgroups: 6.7-GHz methanol masers with and without associated 12.2-GHz methanol masers; and 6.7-GHz methanol masers with high- and those with low-peak luminosities. There is little difference in the nature of the associated GLIMPSE point sources in any of these subgroups, and we propose that the masers themselves are probably much more sensitive than mid-infrared data to evolutionary changes in the massive star formation regions that they are associated with. We present an evolutionary sequence for masers in high-mass star formation regions, placing quantitative estimates on the relative lifetimes for the first time.
The Australia Telescope Compact Array has been used to measure positions with arcsecond accuracy for 379 masers at the 22-GHz transition of water. The principal observation targets were 202 OH masers ...of the variety associated with star formation regions (SFRs) in the Southern Galactic plane. At a second epoch, most of these targets were observed again, and new targets of methanol masers were added. Many of the water masers reported here are new discoveries and others had been reported, with position uncertainties exceeding 10 arcsec, from Parkes telescope single-dish observations many years ago. Variability in the masers is often acute, with very few features directly corresponding to those discovered two decades ago. Within our current observations, less than a year apart, spectra are often dissimilar, but positions at the later epoch, even when measured for slightly different features, mostly correspond to the detected maser site measured earlier, to within the typical extent of the whole site, of a few arcseconds. The precise water positions show that approximately 79 per cent (160 of 202) of the OH maser sites show coincident water maser emission, the best estimate yet obtained for this statistic; however, there are many instances where additional water sites are present offset from the OH target, and consequently less than half of the water masers coincide with a 1665-MHz ground-state OH maser counterpart. Our less uniform sample of methanol targets is not suitable for a full investigation of their association with water masers, but we are able to explore differences between the velocities of peak emission from the three species and quantify the typically larger deviations shown by water maser peaks from systemic velocities. Clusters of two or three distinct but nearby sites, each showing one or several of the principal molecular masing transitions, are found to be common. We also report the detection of ultracompact H ii regions towards some of the sites. In combination with an investigation of correlations with IR sources from the Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) catalogue, these comparative studies allow further progress in the use of the maser properties to assign relative evolutionary stages in star formation to individual sites.
The Southern Parkes Large-Area Survey in Hydroxyl (SPLASH) is a sensitive, unbiased, and fully sampled survey of the southern Galactic plane and Galactic Centre in all four ground-state transitions ...of the hydroxyl (OH) radical. The survey provides a deep census of 1612-, 1665-, 1667-, and 1720-MHz OH absorption and emission from the Galactic interstellar medium, and is also an unbiased search for maser sources in these transitions. We present here first results from the SPLASH pilot region, which covers Galactic longitudes 334° to 344° and latitudes ±2°. Diffuse OH is widely detected in all four transitions, with optical depths that are always small (averaged over the Parkes beam), and with departures from local thermodynamic equilibrium common even in the 1665- and 1667-MHz main lines. To a 3σ sensitivity of ∼30 mK, we find no evidence of OH envelopes extending beyond the CO-bright regions of molecular cloud complexes, and conclude that the similarity of the OH excitation temperature and the level of the continuum background is at least partly responsible for this. We detect masers and maser candidates in all four transitions, approximately 50 per cent of which are new detections. This implies that SPLASH will produce a substantial increase in the known population of ground-state OH masers in the southern Galactic plane.
We present the unambiguous discovery of six new class II methanol maser transitions, three of which are torsionally excited (vt = 1). The newly discovered 6.18 GHz 17−2 → 18−3 E (vt = 1), 7.68 GHz ...124 → 133 A− (vt = 0), 7.83 GHz 124 → 133 A+ (vt = 0), 20.9 GHz 101 → 112 A+ (vt = 1), 44.9 GHz 20 → 31 E (vt = 1), and 45.8 GHz 93 → 102 E (vt = 0) methanol masers were detected toward G 358.931−0.030, where the known 6.68 GHz maser has recently been reported to be undergoing a period flaring. The detection of the vt = 1 torsionally excited lines corroborates one of the missing puzzle pieces in class II maser pumping, but the intensity of the detected emission provides an additional challenge, especially in the case of the very highly excited 6.18 GHz line. Together with the newly detected vt = 0 lines, these observations provide significant new information that can be utilized to improve class II methanol maser modeling. We additionally present detections of 6.68, 19.9, 23.1, and 37.7 GHz class II masers, as well as 36.2 and 44.1 GHz class I methanol masers, and provide upper limits for the 38.3 and 38.5 GHz class II lines. Near simultaneous Australia Telescope Compact Array observations confirm that all 10 of the class II methanol maser detections are co-spatial to ∼0.2 arcsec, which is within the uncertainty of the observations. We find significant levels of linearly polarized emission in the 6.18, 6.67, 7.68, 7.83, 20.9, 37.7, 44.9, and 45.8 GHz transitions, and low levels of circular polarization in the 6.68, 37.7, and 45.8 GHz transitions.
We present the fifth portion of an unbiased survey of the Galactic plane, |b| ≤ 2°, for 6668-MHz methanol masers. This section of the survey completes the Galactic longitude range visible to the ...Parkes radio telescope, incorporating the longitude range 20°–60°. Within this section of the survey we find 265 methanol masers, 64 new to the survey, bringing the total number of methanol masers detected across the full longitude coverage (186°, through 0°, to 60°) to 972 sources.
In this study we analyze 70 radio continuum sources that are associated with dust clumps and which are considered to be candidates for the earliest stages of high-mass star formation. The detection ...of these sources was reported by Rosero et al., who found most of them to show weak ( mJy) and compact ( 0 6) radio emission. Herein, we used the observed parameters of these sources to investigate the origin of the radio continuum emission. We found that at least ∼30% of these radio detections are most likely to be ionized jets associated with high-mass protostars. However, for the most compact sources, we cannot discard the scenario that they represent pressure-confined H ii regions. This result is highly relevant for recent theoretical models that are based on core accretion, which predict the first stages of ionization from high-mass stars to be in the form of jets. Additionally, we found that properties such as the radio luminosity as a function of the bolometric luminosity of ionized jets from low and high-mass stars are extremely well-correlated. Our data improve upon previous studies by providing further evidence of a common origin for jets independently of luminosity.
We have selected the positions of 54 6.7 GHz methanol masers from the Methanol Multibeam Survey catalogue, covering a range of longitudes between 20° and 34° of the Galactic plane. These positions ...were mapped in the J = 3-2 transition of both the 13CO and C18O lines. A total of 58 13CO emission peaks are found in the vicinity of these maser positions. We search for outflows around all 13CO peaks, and find evidence for high-velocity gas in all cases, spatially resolving the red and blue outflow lobes in 55 cases. Of these sources, 44 have resolved kinematic distances, and are closely associated with the 6.7 GHz masers, a subset referred to as Methanol Maser Associated Outflows (MMAOs). We calculate the masses of the clumps associated with each peak using 870 μm continuum emission from the ATLASGAL survey. A strong correlation is seen between the clump mass and both outflow mass and mechanical force, lending support to models in which accretion is strongly linked to outflow. We find that the scaling law between outflow activity and clump masses observed for low-mass objects, is also followed by the MMAOs in this study, indicating a commonality in the formation processes of low-mass and high-mass stars.
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