Precise Positions of Methanol Masers

Keywords: ISM: molecules — masers — methanol — stars: formation

Over the past decade, methanol maser emission at the 6668-MHz transition has become recognised as a valuable tracer of young stellar objects — the sites where massive stars have recently formed but are not directly detectable owing to their obscuring mantle of dust and molecules.

Existing work has discovered a large number of methanol masers in our Galaxy by an inhomogeneous mixture of targeted searches (especially towards OH masers, and IR sources, e.g. Caswell et al. 1995a), and unbiased surveys towards some portions of the Galactic plane (e.g. Ellingsen et al. 1996). However, there is a need to consolidate this work and provide accurate positions for the known sources in preparation for a new sensitive search for methanol masers that is currently being conducted with the Parkes Radio Telescope (Green et al. 2009a).

The methanol maser observations described here were obtained with the ATCA in many sessions since 1993 February, chiefly in any of the four standard ‘6-km’ configurations (instantaneously yielding 15 baselines ranging from 76 to 6000 m). The correlator was configured to give a 2048-channel spectrum across a 4-MHz bandwidth for each of the 2 orthogonal linear polarizations. Typically, a target was observed for at least four periods of several minutes each, within a 10-hour timespan. Targets selected for study included methanol sites with positions known only approximately from single dish observations, and the positions of some new OH masers (chiefly from Caswell 1998) not previously searched for methanol. Some new serendiptious discoveries, made while investigating the chosen targets, are also reported. The masers selected for observation lie primarily in the Galactic longitude range 232° through 360° to 16° which is the region covered by extensive southern observations of OH masers (Caswell 1998). Indeed, a major objective was to ensure that the precise position was obtained for all methanol masers that appeared to have a nearby maser counterpart of OH at either (or both) 1665 (or 1667) MHz and 6035 MHz. The region investigated was expanded to include some additional targets that lie between longitudes 16° and 50°, and between 188° and 232°; these extensions were prompted by the absence until recently of a northern hemisphere instrument able to efficiently perform such measurements. The procedures for observing and data reduction closely follow those of Caswell (1996a, 1996b, 1997).

The synthesised beamsize of approximately 2 arcsec enables not only an accurate position measurement for the brightest maser feature in the spectrum of the target, but also enables mapping of the maser spot distribution. Past studies have shown that the maser spots are generally confined to groups with typical maximum extent of less than 1 arcsec (Caswell 1997; Forster & Caswell 1989). Where distances are known, the calculated linear extent rarely exceeds 30 mpc (= 6000 au). Sometimes the maps of maser spot positions show a cluster of two or more maser groups, with separations clearly exceeding group sizes. It is likely that each compact maser group within such a cluster has an embedded embryonic massive star as its source of excitation.

For the current analysis. we do not list each detected maser spot, but we do list, as separate, the site of each compact group of maser spots that is separated more than a few arcsec from any other group in the same cluster. In a few rare cases there is a cluster of three or more groups; somewhat more commonly there is a pair, and the majority of sites are single.

The emphasis of the current work is to provide a catalogue of precise positions. In order to increase its usefulness, Table 1 lists not only our new previously unpublished positions, but also: the 19 sources earlier listed only with 1950 coordinates by Caswell, Vaile & Forster (1995b), results from Caswell (1996a,b, 80 sources), and results from Caswell (1997, 42 sources). In addition, some of our new positions, although not published in tabular form, have been referred to in source notes concerning related OH masers at 1665 MHz (Caswell 1998) and at 6035 MHz (Caswell 2003). For some sources, we made new measurements confirming or replacing the previous published values. The present positions are recommended for future studies and, unless otherwise noted, have RMS uncertainties of 0.4 arcsec. These errors arise chiefly from residual atmospheric phase instabilities between calibration measurements as discussed by Caswell (1997).

Table 1.  Positions of methanol masers

The peak intensity and its velocity at our observing epoch is shown in Columns 4 and 5 of Table 1. However, variability occurs in many sources, primarily on timescales of months to years (see e.g. Caswell, Vaile & Ellingsen 1995c). Consequently, the peak intensity at our observing epoch is often different from that of published spectra (and sometimes a different velocity peak is stronger). Earlier published spectra (e.g. Caswell et al. 1995a) sometimes have a better signal-to-noise ratio than our measurements, and show features over a larger velocity range. In Column 6 we quote the larger ranges in such cases if there is no evidence that any emission comes from an offset position. The resulting velocity ranges are approximate, and are generally underestimated for weak sources with low signal-to-noise ratio, but occasionally overestimated if two sources are blended. A better assessment of velocity ranges will be possible from spectra being obtained in a new methanol multibeam survey, which is now well under way (Green et al. 2009a).

The Table assigns a name to each maser based on its Galactic coordinates (to the nearest millidegree). Where separations of approximately 2 arcsec occur, it is not clear whether the features represent distinctly separate maser sites, or an unusually extended one. The difficult cases are discussed for individual sources in the next section.

The methanol masers listed here include the results of searches towards a comprehensive list of southern OH masers (Caswell 1998) in the Galactic longitude range 232° through 360° to 16°. Some of the methanol masers detected towards OH targets are coincident with the OH and others are not. Coincidences are identified in the column ‘OH?’ by citing a reference to the OH data; most references are to Caswell (1998), but there are others positioned more recently. Some associations require more extensive comment, and ‘text’ in the ‘OH?’ column refers to notes in section 4. The resulting detection statistics of methanol masers towards 1665-MHz masers are discussed later in the paper.

The final column of the Table identifies the epoch of our methanol position measurement or, where the measurement has been discussed in an earlier publication, a reference is given. Published positions have sometimes been improved by additional data at a later epoch.

There are, of course, multiple individual publications on many previously reported sources but we have not attempted to cite these since they are of varying quality and are mostly included in a comprehensive compilation of older methanol maser data (Pestalozzi, Minier & Booth 2005). Many of the positions listed here supersede the earlier approximate positions, and others confirm independently obtained positions of high accuracy. Amongst the positions in the Pestalozzi compilation with accuracy comparable to ours, those obtained by Walsh et al. (1998) were derived from ATCA observations in 1994 and 1995, and used a strategy similar to the present one (but with lower spectral resolution). For most of those sources, the strongest features, tabulated by Pestalozzi et al. (2005) from the full Walsh et al. (1998) dataset, are in agreement with our values to within 0.4 arcsec and provide a useful corroboration of both datasets.

Some northern sources have recently been measured with the Arecibo telescope, with an RMS position uncertainty of 7 arcsec (Pandian, Goldsmith & Deshpande 2007). Comparison with our data confirms this precision, but also reveals a bias in the Arecibo Right Ascensions, suggesting that the values should be reduced by an average of 0.6 s (9 arcsec).

We first draw attention to corrections needed for earlier published data. A source listed by Caswell (1996a) as 335.603–0.078 is now believed to be spurious and is accounted for as a weak distant side-lobe of another maser. The site listed here as 0.475–0.010 is the corrected value for a source listed by Caswell (1996b) as 0.393–0.034.

Problems of this type can occur when sparse antenna arrays are used to observe weak sources for only short periods, but such errors are rare and it is expected that no similar examples remain in the current catalogue.

The remaining notes draw attention to some anomalies regarding the information on a few of the sites, and draw attention to sites with neighbours less than 20 arcsec away, which mostly represent individual stars within a cluster. A few of the sites are as close as a few arcsec and the alternative interpretations of two close separate sites, or a single site of larger extent, are discussed.

291.579–0.431 and 291.582–0.435. As noted previously (Caswell 2004a) these two sites, with separation nearly 20 arcsec, are sufficiently close to lie within the same star cluster, but are quite distinct, with one site accompanied by water, and the other accompanied by both water and OH.

311.947+0.472. This has a possibly associated OH maser but the OH position still has an uncertainty larger than 10 arcsec (Caswell 1998).

312.598+0.045 and 312.597+0.045. The first of these sites is stronger and coincides with an OH maser. The second methanol site is offset 6 arcsec and appears to be distinct, and has no detected OH counterpart.

319.836–0.197. The apparent OH counterpart is offset 3 arcsec, but is weak, and its position uncertainty may account for the discrepancy; we provisionally regard the two species of maser as coincident.

321.030–0.485 and 321.033–0.483. The separation is more than 10 arcsec and only the first, weaker, site has a detected OH counterpart.

327.392+0.199 and 327.395+0.197. The separation of these sources is 14 arcsec and neither has a detected OH counterpart.

328.808+0.633 and 328.809+0.633. These were treated by Caswell (1997) as possibly separate sites, partly on the basis of an overlay with continuum emission, and despite their small separation of only 2.4 arcsec. The spectral features of the second source lie within the velocity range of those from the first source, but we retain them as distinct sources pending more evidence.

329.339+0.148. The discovery of the 1665-MHz maser at this site was reported by Caswell (2001) and it turns out to be an especially interesting distant site with OH maser emission also at 1720 MHz (Caswell 2004b) and 13441 MHz (Caswell 2004c).

329.405–0.459 and 329.407–0.459. The separation of the sites is 5.7 arcsec and only the first site coincides with an OH maser.

330.953–0.182. As noted by Caswell (2001), the methanol coincides with an OH 6035-MHz maser site, but the major OH 1665- and 1667-MHz masers are offset 3 arcsec.

331.542–0.066 and 331.543–0.066. The separation is only 3 arcsec, but at a likely distance of 6 kpc this correponds to 90 mpc, and there is further evidence that they represent two distinct sites since there is no obvious velocity overlap, and each has an OH counterpart (Caswell 1997, 1998).

333.126–0.440 and 333.128–0.440. The separation is 7 arcsec and OH emission has not been detected at either site.

333.135–0.431s. The suffix denoting south was added by Caswell (1997) to distinguish this site from another site, offset nearly 3 arcsec, which has OH without methanol.

333.128–0.560 and 333.130–0.560. The sites are distinct with separation more than 6 arcsec and neither has a detected OH counterpart.

335.585–0.289 and 335.585–0.290. These appear to represent two distinct sites, but with a separation of barely 3 arcsec, this is uncertain. The first site has coincident OH. The second is a single strong feature at an offset velocity and without OH emission.

337.703–0.053 and 337.705–0.053. Only the second, stronger, maser of this pair has a detected OH counterpart.

338.075+0.012 and 338.075+0.009. Only the first, stronger, maser of this pair has a detected OH counterpart.

339.681–1.208 and 339.682–1.207. As noted by Caswell (1998) there is an OH counterpart to the second site straddling the methanol position. The first methanol site which lies more than 3 arcsec south is stronger, has a wide velocity range encompassing the small range of the second source and although it seems to be spatially distinct, this is not certain.

344.419+0.044 and 344.421+0.045. These are clearly distinct and only the first, weaker, methanol site has an OH counterpart.

345.003–0.223 and 345.003–0.224. The second of these agrees well in position with an OH counterpart at both the 1665- and 6035-MHz transitions. The velocity ranges of the two sources do not noticeably overlap and an overlay on continuum emission argues strongly that the two sites are quite distinct despite their small separation of only 3 arcsec (Caswell 1997).

345.010+1.792 and 345.012+1.797. These are clearly distinct and only the first has a detected OH counterpart.

348.550–0.979 and 348.550–0.979n. The first of these has a continuum and OH counterpart at both the 1665- and 6035-MHz transitions. There is evidence (Caswell 1997) that the second source (offset 2.2 arcsec to the north), detected only on the methanol transition, is quite likely a site with its own source of excitation. The sites have overlapping velocity ranges.

349.092+0.105 and 349.092+0.106. The second source coincides with an OH maser. The first source, with no OH, has a clearly distinct velocity range and seems to be a separate site despite the smallness of its offset, slightly more than 2 arcsec, from the second.

350.105+0.083 and 350.104+0.084. Separated nearly 4 arcsec, neither site has an OH counterpart. The small velocity range of the second source lies wholly within the range of the first.

351.417+0.645 and 351.417+0.646. The separation of these sites is more than 3 arcsec. The first one coincides with the well-known H ii region NGC6334F and its OH maser emission. The second lies clearly offset and appears to be a distinct separate site (Caswell 1997).

351.581–0.353. Caswell (1997) queried whether there was an additional distinct source to the north (351.581–0.353n). The separation of slightly less than 1.8 arcsec leaves this unclear, and the extra maser spots have velocities within the range of the main site. We list the positions of both sites, but note that the 1665-MHz OH counterpart and a compact H ii region lie between the positions, which hints that we are more likely dealing with an extended single site. Recent recognition that the site is most likely in the near portion of the expanding 3-kpc arm (Green et al. 2009b) would imply a distance of 5.3 kpc, and thus a linear extent of 45 mpc which, for a single site, is large but not exceptional.

353.273+0.641. This strong methanol maser was first detected 1993 but was reported only recently (Caswell & Phillips 2008) when an association with a remarkable water maser was confirmed. No OH maser has been detected at the site.

355.344+0.147, 355.343+0.148 and 355.346+0.149. The third maser is quite distinct spatially, with an offset of 10 arcsec from the other sites. The first two sites have no overlap in velocity but are separated spatially only 3.7 arcsec, and the likelihood that they are distinct sites depends on an estimate of their distance. Crovisier, Fillit & Kazes (1973) argue, on the basis of intervening absorption features near +100 km s^–1, that the radio continuum emission and the OH 1665-MHz maser emission are at a distance beyond the Galactic Centre. This evidence subsequently passed unnoticed in the literature, attracting no relevant citations, and in particular was overlooked by Caswell (1997) and Forster & Caswell (1989, 1999, 2000) who assigned the complex to a distance of only 2 kpc. If we reject the nearby location, the alternative distances then include: outside the solar circle and thus beyond 17 kpc; near the Galactic Centre at 8.5 kpc which might account for unusual velocities; or (perhaps most likely) a location in the far-side counterpart to the 3-kpc expanding arm at 11.5 kpc (Dame & Thaddeus 2008; Green et al. 2009b). At any distance beyond 8.5 kpc, the separation of 3.7 arcsec then corresponds to more than 150 mpc, indicative of clearly distinct sites for all three methanol masers.

357.967–0.163 and 357.965–0.164. Two clearly distinct sites, separated 7.6 arcsec, of which only the first has an OH counterpart.

359.436–0.104 and 359.436–0.102. Clearly distinct and separated 6 arcsec, the first has a well-known OH counterpart (Caswell 1998) and the second also now has a more recently reported OH maser counterpart (Argon, Reid & Menten 2000).

0.315–0.201 and 0.316–0.201. These are separated by 2.5 arcsec and listed as distinct sites by Caswell (1996b), despite the weak features of the second lying wholly within the range of the first source. At neither position is there any detected OH counterpart. Pending further evidence, we list both sites, but caution that they may in fact be components of a more than usually extended single site.

0.475–0.010. This source was discovered as a new source in the Galactic centre survey by Caswell (1996b) but was incorrectly reported as 0.393–0.034. Re-analysis of the data showed that the position error arose because the sparse uv-coverage caused a side-lobe to be of comparable amplitude to the main lobe and was incorrectly interpreted as the source position. The source is closer to the target pointing than first estimated, and so its flux density correction for the offset is not as large, and the new estimate of peak flux density is therefore lower, 2.9 Jy.

0.645–0.042 to 0.695–0.038. These nine sites within the Sgr B2 complex were distinguished by both Houghton and Whiteoak (1995) and Caswell (1996b); they are all clearly distinct sites. Existing OH 1665 and 1667-MHz observations towards Sgr B2 remain incomplete, and those by Argon et al. (2000) are some of the best currently available. The detailed information in their datasets show counterparts at 1665 and 1667 MHz for 0.657–0.041 and 0.672–0.031, and an OH 1720 MHz counterpart for 0.665–0.036. One of the other methanol sites, 0.666–0.029, is accompanied by a 6035-MHz maser (Caswell 1997).

Two weak additional methanol sites in the Sgr B2 complex were reported by Houghton and Whiteoak (1995) and are believed reliable but were too weak to confirm in the present observations. They are omitted from the present listing which is intended to present only the results of our independent observations.

9.621+0.196 and 9.619+0.193. The first of these is the strongest known methanol maser, and the second is a clearly distinct site offset more than 10 arcsec. Both have OH counterparts and ucH ii regions (Forster & Caswell 2000).

11.034+0.062. Note that the weak feature seen on the spectrum of Caswell et al. (1995a) at velocity 24.4 km s^–1 is a side-lobe of 10.958+0.022.

12.025–0.031. There is an OH 1665–MHz maser counterpart at 18^h12^m01.88^s, –18°31′55.6″ (Caswell unpublished; this is a precise position for a source previously listed as 12.03–0.04 by Caswell 1998) and it is thus now confirmed to coincide with the methanol.

12.209–0.102. A 1665-MHz OH maser counterpart lies at this position (Argon et al. 2000; Caswell unpublished).

12.889+0.489. Spectral features over the velocity range 28 to 42 km s^–1 mostly lie within 0.5 arcsec of the tabulated position, but a single strong feature at velocity +33.5 km s^–1 is offset 2 arcsec northeast, at 18^h11^m51.49^s, –17°31′28.0″. The OH counterpart is offset from both methanol features by slightly more than 1 arcsec and we treat this as a single site.

13.657–0.609. This source was first reported by MacLeod et al. (1998) but is not in the compilation of Pestalozzi et al. (2005). As also noted by MacLeod et al., there is associated OH emission at 1665 and 1667 MHz which new observations (Caswell in preparation) show to be at 18^h17^m24.27^s, –17°22′13.4″, effectively coincident with the methanol.

19.472+0.170 and 19.472+0.170sw. The second source is offset 3.7 arcsec south-west from the first, and offset to smaller velocity. No other maser is known at these positions so it is not clear whether the sites are distinct, or simply a larger than usual single site.

20.237+0.065 and 20.239+0.065. The two sites are separated more than 7 arcsec. The first coincides with OH maser emission at 1665 MHz, 6035 MHz and 1720 MHz (Caswell 2003, 2004b), whereas the second is solitary.

23.437–0.184 and 23.440–0.182. The clear separation of more than 10 arcsec establishes these as distinct sites with distinct velocity ranges.

28.146–0.005 and 28.201–0.049. The proximity of these sources to declination zero causes the beamsize in declination to be large, and the declinations to have larger than usual uncertainties, estimated to be 1 arcsec. The correspondence in each case with an OH maser (Argon et al. 2000) to better than 2 arcsec suggests that our errors are indeed no greater than 2 arcsec.

43.149+0.013 to 49.161+0.004. These four sites are part of the W49 complex and were noted as distinct in the single dish observations of Caswell et al. (1995a). Pandian et al. (2007) detect all four, plus an additional weak one which has a peak less than 1 Jy.

49.470–0.371 to 49.490–0.388. These five sites are part of the complex W51. Caswell et al. (1995a) recognised that there were at least three sites here and Pandian et al. (2007) recognised four. Our higher resolution now distinguishes five sites with clearly defined separate positions, although the velocity ranges of weak features are uncertain owing to side-lobe confusion.

The precise methanol maser positions reported here allow an improved study of the association of methanol masers and 1665-MHz OH masers in regions of Massive Star Formation. However, beyond the 16 degree Galactic longitude limit of the Caswell (1998) catalogue, the OH information is incomplete, although some individual sources can be studied using the OH positions available in Forster & Caswell (1989, 1999) and in Argon et al. (2000).

Therefore, for statistical purposes in the evaluation of the discovery statistics of methanol towards OH masers, we consider only the Galactic longitude range 232° through 360° to 16°, covered by the OH catalogue of Caswell (1998). A preliminary analysis was performed by Caswell (1998), but accurate positions for some of the methanol masers were not known and some of the possible associations were therefore uncertain. Furthermore, there are several more recent OH results in this region as noted in section 4.1. We find that for the (updated) list of 207 star-formation-region OH masers with precise positions known in this longitude range, 168 (81 percent) possess a methanol counterpart. Of course, the interpretation of methanol and OH associations in terms of common conditions and evolutionary stages for methanol and OH co-existence requires a closer inspection of line ratios and investigation of co-propagation.

However, a practical consequence of this 81 percent statistic is that when a new, deep, unbiased survey for methanol masers has been completed (Green 2009a), and the positions used as targets for an OH search, we may expect the results to be a useful proxy for a deep unbiased survey for OH, and perhaps to recover at least 80 percent of the full OH population.

We have explored the velocity widths of the methanol masers and find that only 10 of our sample have velocity widths exceeding 16 km s^–1. The largest are 24 and 23 km s^–1, shown by 340.785–0.096 and 335.060–0.427. For both sites, red-shifted emission is weaker, most noticeably for 340.785–0.096 (Caswell et al. 1995a; Caswell 1997). The two sources with extent 17 km s^–1 (344.227–0.569 and 340.054–0.244) also have only weak emission at one of their extreme velocities, one blue, the other red. The six sources with velocity ranges of 18 or 19 km s^–1 (339.681–1.208, 330.070 + 1.064, 10.473 + 0.027, 2.536 + 0.198, 6.795– 0.257 and 22.435–0.155) have somewhat stronger emission near both extremes of velocity, the intensity ratio of blue to red ranging from 1.8 to 0.1.

Since velocity extents greater than 16 km s^–1 are rare (less than 3 percent of the total), this lends validity to the practice of using methanol velocities (e.g. the mid-values of the range) as the systemic velocity, with the expectation that the uncertainty is rarely as large as 10 km s^–1, and most commonly less than 5 km s^–1.

The systemic velocity is dominated by Galactic rotation for the Galactic disk population of young massive stars, and is thus suitable for estimating kinematic distances. Velocity ranges of individual sources often match those of OH counterparts quite well, and confirm the likely systemic velocities.

The precise positions reported here confirm that more than 80 percent of OH masers in regions of massive star formation have associated methanol masers.

We have also explored the velocity widths of the methanol masers and find that values greater than 16 km s^–1 are rare (less than three percent of the total), in marked contrast to water masers where more than half the sources have velocity widths exceeding 20 km s^–1.