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Register a new userThe physical parameters of clumps associated with class I methanol masers
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Dmitry Ladeyschikov A
Publication date
2020
fields
Physics
and research's language is
English
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We present a study of the association between class I methanol masers and cold dust clumps from the ATLASGAL survey. It was found that almost 100% of class I methanol masers are associated with objects listed in the ATLASGAL compact source catalog. We find a statistically significant difference in the flux density, luminosity, number and column density and temperature distributions of ATLASGAL sources associated with 95/44 GHz methanol masers compared with those ATLASGAL sources devoid of 95 GHz methanol masers. The masers tend to arise in clumps with higher densities, luminosities and temperatures compared with both the full sample of the ATLASGAL clumps, as well as the sample of ATLASGAL sources that were cross-matched with positions previously searched for methanol masers but with no detections. Comparison between the peak position of ATLASGAL clumps and the interferometric positions of the associated class I and II methanol masers reveals that class I masers are generally located at larger physical distances from the peak submillimetre emission than class II masers. We conclude that the tight association between ATLASGAL sources and class I methanol masers may be used as a link toward understanding the conditions of the pumping of these masers and evolutionary stages at which they appear.
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We have constructed the largest sample of dust-associated class II 6.7 GHz methanol masers yet obtained. New measurements from the the Methanol MultiBeam (MMB) Survey were combined with the 870 $mu$m APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) and the 850 $mu$m JCMT Plane Survey (JPS). Together with two previous studies we have now identified the host clumps for 958 methanol masers across the Galactic Plane, covering approximately 99% of the MMB catalogue and increasing the known sample of dust-associated masers by over 30%. We investigate correlations between the physical properties of the clumps and masers using distances and luminosities drawn from the literature. Clumps hosting methanol masers are significantly more compact and have higher volume densities than the general population of clumps. We determine a minimum volume density threshold of $n$(H$_2$) $geq 10^4$ cm$^{-3}$ for the efficient formation of intermediate- and high-mass stars. We find 6.7 GHz methanol masers are associated with a distinct part of the evolutionary process ($L_{rm bol}$/$M_{rm fwhm}$ ratios of between 10$^{0.6}$ and 10$^{2.2}$) and have well defined turning on and termination points. We estimate the lower limit for the mass of embedded objects to be $geq$ 6 M$_{odot}$ and the statistical lifetime of the methanol maser stage to be $sim$ 3.3$times$10$^{4}$ yrs. This suggests that methanol masers are indeed reliable tracers of high mass star formation, and indicates that the evolutionary period traced by this marker is relatively rapid.
In this paper, we present a database of class I methanol masers. The compiled information from the available literature provides an open and fast access to the data on class I methanol maser emission, including search, analysis and visualization of the extensive maser data set. There is information on individual maser components detected with single-dish observations and maser spots obtained from interferometric data. At the moment the database contains information from ~100 papers, i.e. ~7500 observations and ~650 sites of class I methanol masers. Analysis of the data collected in the database shows that the distribution of class I methanol maser sources is similar to that of class II methanol masers. They are mostly found in the Molecular Ring, where majority of the OB stars are located. The difference between class I and II distributions is the presence of many class I methanol masers in the Nuclear Disk region (Central Molecular Zone). Access to the class I methanol maser database is available online at http://maserdb.net
Class I CH$_3$OH masers trace interstellar shocks. They have received little attention mostly as a consequence of their low luminosities; this situation has changed recently and Class I masers are now routinely used as signposts of outflows. The recent detection of polarisation in Class I lines now makes it possible to obtain information on magnetic fields in shocks. We make use of newly calculated collisional rates to investigate the excitation of Class I masers and to reconcile their observed properties with model results. We performed LVG calculations with a plane-parallel slab geometry to compute the pump and loss rates which regulate the interactions of the different maser systems with the maser reservoir. We study the dependence of the pump rate, the loss rate, and the inversion efficiency of the pumping scheme of Class I masers on the physics of the gas. Bright Class I masers are mainly high-temperature high-density structures with maser emission measures corresponding to high CH$_3$OH abundances close to the limits set by collisional quenching. Our model reproduces reasonably well most of the observed properties of Class I masers. The 25 GHz masers are the most sensitive to the density and mase at higher densities than other lines. Moreover, even at high density and high abundance, their luminosity is lower than that of the 44 GHz and 36 GHz lines. By comparison between observed isotropic photon luminosities and our model, we infer beam solid angles of ~0.001 steradian. Class I masers can be separated into 3 families: the $(J+1)_{-1}-J_{0}$-E type, the $(J+1)_0-J_1$-A type, and the $J_2-J_1$-E lines. The 25 GHz lines behave in a different fashion from the other masers as they are only inverted at densities above $10^6$ cm$^{-3}$ in contrast to other Class I masers. Therefore, the detection of maser activity in all 3 families is a clear indication of high densities.
The Methanol MultiBeam survey (MMB) provides the most complete sample of Galactic massive young stellar objects (MYSOs) hosting 6.7GHz class II methanol masers. We characterise the properties of these maser sources using dust emission detected by the Herschel Infrared Galactic Plane Survey (Hi-GAL) to assess their evolutionary state. Associating 731 (73%) of MMB sources with compact emission at four Hi-GAL wavelengths, we derive clump properties and define the requirements of a MYSO to host a 6.7GHz maser. The median far-infrared (FIR) mass and luminosity are 630M$_{odot}$ and 2500L$_{odot}$ for sources on the near side of Galactic centre and 3200M$_{odot}$ and 10000L$_{odot}$ for more distant sources. The median luminosity-to-mass ratio is similar for both at $sim$4.2L$_{odot}/$M$_{odot}$. We identify an apparent minimum 70$mu$m luminosity required to sustain a methanol maser of a given luminosity (with $L_{70} propto L_{6.7}^{0.6}$). The maser host clumps have higher mass and higher FIR luminosities than the general Galactic population of protostellar MYSOs. Using principal component analysis, we find 896 protostellar clumps satisfy the requirements to host a methanol maser but lack a detection in the MMB. Finding a 70$mu$m flux density deficiency in these objects, we favour the scenario in which these objects are evolved beyond the age where a luminous 6.7GHz maser can be sustained. Finally, segregation by association with secondary maser species identifies evolutionary differences within the population of 6.7GHz sources.
The Australia Telescope Compact Array has been used to search for 22-GHz water masers towards the 119 6.7-GHz methanol masers detected in the Methanol Multi-Beam survey between Galactic longitudes 6 and 20 degrees; we find water masers associated with 55 (~46 per cent). Methanol masers with associated water masers have a higher mean integrated luminosity than those without and there is a general trend for sources with more luminous 6.7-GHz methanol masers to be associated with more luminous water maser emission. We have inspected the GLIMPSE three colour images of the regions surrounding the masers and cross-matched the maser positions with existing catalogues of Extended Green Objects and Infrared Dark Clouds. We find more Extended Green Objects at sites where both methanol and water masers are present than at sites with only methanol masers, but no significant difference in the fraction embedded within Infrared Dark Clouds. Analysis of the 1.1-mm dust emission shows dust clumps associated with masers that have greater flux densities and higher column densities than those without. Dust clumps associated with both water and 6.7-GHz methanol masers are generally the most compact clumps followed by those associated with only methanol then the clumps without associated maser emission. We conclude that protostars with both methanol and water masers are often older than those with only methanol, however, we suggest that the evolutionary phase traced by water masers is not as well defined as for 6.7-GHz methanol masers.
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