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Register a new userVLA Survey of Dense Gas in Extended Green Objects: Prevalence of 25 GHz Methanol Masers
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We present $sim1-4$ resolution Very Large Array (VLA) observations of four CH$_3$OH $J_2-J_1$-$E$ 25~GHz transitions ($J$=3, 5, 8, 10) along with 1.3~cm continuum toward 20 regions of active massive star formation containing Extended Green Objects (EGOs), 14 of which we have previously studied with the VLA in the Class~I 44~GHz and Class~II 6.7~GHz maser lines (Cyganowski et al. 2009). Sixteen regions are detected in at least one 25~GHz line ($J$=5), with 13 of 16 exhibiting maser emission. In total, we report 34 new sites of CH$_3$OH maser emission and ten new sites of thermal CH$_3$OH emission, significantly increasing the number of 25~GHz Class I CH$_3$OH masers observed at high angular resolution. We identify probable or likely maser counterparts at 44~GHz for all 15 of the 25~GHz masers for which we have complementary data, providing further evidence that these masers trace similar physical conditions despite uncorrelated flux densities. The sites of thermal and maser emission of CH$_3$OH are both predominantly associated with the 4.5 $mu$m emission from the EGO, and the presence of thermal CH$_3$OH emission is accompanied by 1.3~cm continuum emission in 9 out of 10 cases. Of the 19 regions that exhibit 1.3~cm continuum emission, it is associated with the EGO in 16 cases (out of a total of 20 sites), 13 of which are new detections at 1.3~cm. Twelve of the 1.3~cm continuum sources are associated with 6.7~GHz maser emission and likely trace deeply-embedded massive protostars.
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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.
Methanol masers at 6.7 GHz are associated with high-mass star-forming regions (HMSFRs) and often have mid-infrared (MIR) counterparts characterized by extended emission at 4.5 $mu$m, which likely traces outflows from massive young stellar objects (MYSOs). Our objectives are to determine the milliarcsecond (mas) morphology of the maser emission and to examine if it comes from one or several candidate MIR counterparts in the clusters of MYSOs. The European VLBI Network (EVN) was used to image the 6.7 GHz maser line with ~2.1 field of view toward 14 maser sites from the Torun catalog. Quasi-simultaneous observations were carried out with the Torun 32 m telescope. We obtained maps with mas angular resolution that showed diversity of methanol emission morphology: a linear distribution (e.g., G37.753-00.189), a ring-like (G40.425+00.700), and a complex one (e.g., G45.467+00.053). The maser emission is usually associated with the strongest MIR counterpart in the clusters; no maser emission was detected from other MIR sources in the fields of view of 2.1 in diameter. The maser source luminosity seems to correlate with the total luminosity of the central MYSO. Although the Very Long Baseline Interferometry (VLBI) technique resolves a significant part of the maser emission, the morphology is still well determined. This indicates that the majority of maser components have compact cores.
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}$, a value comparable to those found recently by similar studies.
Using the 870-$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.
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