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تسجيل مستخدم جديدPolarisation of Class II Methanol Masers
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P.D. Stack
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We have used the University of Tasmania Mt Pleasant 26m radio telescope to investigate the polarisation characteristics of a sample of strong 6.7 GHz methanol masers, the first spectral line polarisation observations to be undertaken with this instrument. As part of this process we have developed a new technique for calibrating linear polarisation spectral line observations. This calibration method gives results consistent with more traditional techniques, but requires much less observing time on the telescope. We have made the first polarisation measurements of a number of 6.7 GHz methanol masers and find linear polarisation at levels of a few - 10% in most of the sources we observed, consistent with previous results. We also investigated the circular polarisation produced by Zeeman splitting in the 6.7 GHz methanol maser G9.62+0.20 to get an estimate of the line of sight magnetic field strength of 35+/-7 mG.
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We present the light curves of the 6.7 and 12.2 GHz methanol masers in the star forming region G9.62+0.20E for a time span of more than 2600 days. The earlier reported period of 244 days is confirmed. The results of monitoring the 107 GHz methanol ma
ser for two flares are also presented. The results show that flaring occurs in all three masing transitions. It is shown that the average flare profiles of the three masing transitions are similar. The 12.2 GHz masers are the most variable of the three masers with the largest relative amplitude having a value of 2.4. The flux densities for the different masing transitions are found to return to the same level during the low phase of the masers, suggesting that the source of the periodic flaring is situated outside the masing region, and that the physical conditions in the masing region are relatively stable. On the basis of the shape of the light curve we excluded stellar pulsations as the underlying mechanism for the periodicity. It is argued that a colliding wind binary can account for the observed periodicity and provide a mechanism to qualitatively explain periodicity in the seed photon flux and/or the pumping radiation field. It is also argued that the dust cooling time is too short to explain the decay time of about 100 days of the maser flare. A further analysis has shown that for the intervals from days 48 to 66 and from days 67 to 135 the decay of the maser light curve can be interpreted as due to the recombination of a thermal hydrogen plasma with densities of approximately $1.6 times 10^6 mathrm{cm^{-3}}$ and $6.0 times 10^5 mathrm{cm^{-3}}$ respectively.
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.
We report the results of a search for class II methanol masers at 37.7, 38.3 and 38.5 GHz towards a sample of 70 high-mass star formation regions. We primarily searched towards regions known to show emission either from the 107 GHz class II methanol
maser transition, or from the 6.035 GHz excited OH transition. We detected maser emission from 13 sources in the 37.7 GHz transition, eight of these being new detections. We detected maser emission from three sources in the 38 GHz transitions, one of which is a new detection. We find that 37.7 GHz methanol masers are only associated with the most luminous 6.7 and 12.2 GHz methanol maser sources, which in turn are hypothesised to be the oldest class II methanol sources. We suggest that the 37.7 GHz methanol masers are associated with a brief evolutionary phase (of 1000-4000 years) prior to the cessation of class II methanol maser activity in the associated high-mass star formation region.
We present a simultaneous single-dish survey of 22 GHz water maser and 44 GHz and 95 GHz class I methanol masers toward 77 6.7 GHz class II methanol maser sources, which were selected from the Arecibo methanol maser Galactic plane survey (AMGPS) cata
log.Water maser emission is detected in 39 (51%) sources, of which 15 are new detections. Methanol maser emission at 44 GHz and 95 GHz is found in 25 (32%) and 19 (25%) sources, of which 21 and 13 sources are newly detected, respectively. We find 4 high-velocity (> 30 km/s) water maser sources, including 3 dominant blue- or redshifted outflows.The 95 GHz masers always appear with the 44 GHz maser emission. They are strongly correlated with 44 GHz masers in velocity, flux density, and luminosity, while they are not correlated with either water or 6.7 GHz class II methanol masers. The average peak flux density ratio of 95 GHz to 44 GHz masers is close to unity, which is two times higher than previous estimates. The flux densities of class I methanol masers are more closely correlated with the associated BGPS core mass than those of water or class II methanol masers. Using the large velocity gradient (LVG) model and assuming unsaturated class I methanol maser emission, we derive the fractional abundance of methanol to be in a range of 4.2*10^-8 to 2.3*10^-6, with a median value of 3.3pm2.7*10^-7.
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 rece
nt 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.
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