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Register a new userTrigonometric Parallaxes of 6.7 GHz Methanol Masers
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K.L.J. Rygl
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Emission from the 6.7 GHz methanol maser transition is very strong, is relatively stable, has small internal motions, and is observed toward numerous massive star-forming regions in the Galaxy. Our goal is to perform high-precision astrometry using this maser transition to obtain accurate distances to their host regions. Eight strong masers were observed during five epochs of VLBI observations with the European VLBI Network between 2006 June, and 2008 March. We report trigonometric parallaxes for five star-forming regions, with accuracies as good as $sim22 mathrm{mu}$as. Distances to these sources are $2.57^{+0.34}_{-0.27}$ kpc for ON 1, $0.776^{+0.104}_{-0.083}$ kpc for L 1206, $0.929^{+0.034}_{-0.033}$ kpc for L 1287, $2.38^{+0.13}_{-0.12}$ kpc for NGC 281-W, and $1.59^{+0.07}_{-0.06}$ kpc for S 255. The distances and proper motions yield the full space motions of the star-forming regions hosting the masers, and we find that these regions lag circular rotation on average by $sim$17 km s$^{-1}$, a value comparable to those found recently by similar studies.
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Using the 870-$mu$m APEX Telescope Large Area Survey of the Galaxy (ATLASGAL), we have identified 577 submillimetre continuum sources with masers from the methanol multibeam (MMB) survey in the region $280degr < ell < 20degr$; $|,b,| < 1.5degr$. 94,per,cent of methanol masers in the region are associated with sub-millimetre dust emission. We estimate masses for ~450 maser-associated sources and find that methanol masers are preferentially associated with massive clumps. These clumps are centrally condensed, with envelope structures that appear to be scale-free, the mean maser position being offset from the peak column density by 0.0 pm 4. Assuming a Kroupa initial mass function and a star-formation efficiency of ~30,per,cent, we find that over two thirds of the clumps are likely to form clusters with masses >20,msun. Furthermore, almost all clumps satisfy the empirical mass-size criterion for massive star formation. Bolometric luminosities taken from the literature for ~100 clumps range between ~100 and 10$^6$,lsun. This confirms the link between methanol masers and massive young stars for 90,per,cent of our sample. The Galactic distribution of sources suggests that the star-formation efficiency is significantly reduced in the Galactic-centre region, compared to the rest of the survey area, where it is broadly constant, and shows a significant drop in the massive star-formation rate density in the outer Galaxy. We find no enhancement in source counts towards the southern Scutum-Centaurus arm tangent at $ell ~ 315degr$, which suggests that this arm is not actively forming stars.
Methanol masers at 6.7 GHz are well known tracers of high-mass star-forming regions. However, their origin is still not clearly understood. We aimed to determine the morphology and velocity structure for a large sample of the maser emission with generally lower peak flux densities than those in previous surveys. Using the European VLBI Network we imaged the remaining sources (17) from a sample of sources that were selected from the unbiased survey using the Torun 32 m dish. Together they form a database of a total of 63 source images with high sensitivity, milliarcsecond angular resolution and very good spectral resolution for detailed studies. We studied in detail the properties of the maser clouds and calculated the mean and median values of the projected size (17.4 au and 5.5 au, respectively) as well as the FWHM of the line (0.373 km s$^{-1}$ and 0.315 km s$^{-1}$ for the mean and median values, respectively), testing whether it was consistent with Gaussian profile. We also found maser clouds with velocity gradients (71 per cent) that ranged from 0.005 km s$^{-1}$ au$^{-1}$ to 0.210 km s$^{-1}$ au$^{-1}$. We tested the kinematic models to explain the observed structures of the 6.7 GHz emission. There were targets where the morphology supported the scenario of a rotating and expanding disk or a bipolar outflow. Comparing the interferometric and single-dish spectra we found that, typically, 50-70 per cent of the flux was missing. This phenomena is not strongly related to the distance of the source. The EVN imaging reveals that in the complete sample of 63 sources the ring-like morphology appeared in 17 per cent of sources, arcs were seen in a further 8 per cent, and the structures were complex in 46 per cent cases. The UC HII regions coincide in position in the sky for 13 per cent of the sources. They are related both to extremely high and low luminosity masers from the sample.
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.
To investigate whether distinctions exist between low and high-luminosity Class II 6.7-GHz methanol masers, we have undertaken multi-line mapping observations of various molecular lines, including the NH3(1,1), (2,2), (3,3), (4,4) and 12CO(1-0) transitions, towards a sample of 9 low-luminosity 6.7-GHz masers, and 12CO (1-0) observations towards a sample of 8 high-luminosity 6.7-GHz masers, for which we already had NH3 spectral line data. Emission in the NH3 (1,1), (2,2) and (3,3) transitions was detected in 8 out of 9 low-luminosity maser sources, in which 14 cores were identified. We derive densities, column densities, temperatures, core sizes and masses of both low and high-luminosity maser regions. Comparative analysis of the physical quantities reveals marked distinctions between the low-luminosity and high-luminosity groups: in general, cores associated with high-luminosity 6.7-GHz masers are larger and more massive than those traced by low-luminosity 6.7-GHz masers; regions traced by the high-luminosity masers have larger column densities but lower densities than those of the low-luminosity maser regions. Further, strong correlations between 6.7-GHz maser luminosity and NH3(1,1) and (2,2) line widths are found, indicating that internal motions in high-luminosity maser regions are more energetic than those in low-luminosity maser regions. A 12CO (1-0) outflow analysis also shows distinctions in that outflows associated with high-luminosity masers have wider line wings and larger sizes than those associated with low-luminosity masers.
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