The Potential Usefulness of Proposed Large Telescopes

Although the JCMT is probably the best submillimetre telescope available today it by no means represents the best that is possible and in future years better telescopes will undoubtedly be built. One such proposed telescope is the Large Milimetre Telescope. This is a 50 metre telescope that is to be built through a collaboration between the Mexican Institute of Astronomy and the University of Massachusetts. The surface accuracy of the dish is to be accurate enough to work at frequencies up to arround 300 GHz which will enable the first 3 transitions of CO and its isotopes to be observed. Figures 6.16 & 6.17 therefore present the predicted position velocity diagrams for the observable transitions of CO, $^{13}$CO, C$^{18}$O and C$^{17}$O, which from the results from the JCMT simulations are the only isotopes abundant enough to produce usefully strong emission lines. Note that as before the lowest contours are in some positions close to the centre of the cloud rather strange in shape due to the effects of insufficient gridding points, these are not real effects.

It is immediately clear comparing the position velocity diagrams that the LMT would produce with those from the JCMT that the improved resolution makes a significant difference to the amount of detail that can be gleaned from the data. The main difference is in how well the rotation curve can be determined. In the JCMT diagrams, whilst it is clear that rotation is taking place due to the obvious asymmetric wings, the variation of that speed with position in the cloud is lost due to insufficient resolution. For the higher transitions the resolution is good enough to start to see the rotation curve but the emission is at such a low level that it would be very difficult to detect. For the LMT, however, the factor of three improvement in resolution makes the curve quite easily detectable even in the lower transitions. For the $3 \rightarrow 2$ transition the velocity variation is just detectable at the 2K contour and easily detectable at the 0.5K contour. It is even quite obvious where the disk stops in this model as there is a clear step that is easily missed in the JCMT diagrams. As for the JCMT diagrams the lower abundance transitions may be more suitable for studying the lower velocity material as optical depth effects do not alter the line profiles. It is also noticeable that the large dish significantly increases the temperatures seen making detection of some of the weaker features much easier. Of course, even the LMT does not represent the best resolution that is possible. Other, larger single dish telescopes will probably be built in the future (although dishes much larger than the LMT with the required surface accuracy will probably not be possible on the Earth). There already exist interferometers capable of working at the necessary frequencies and more are being planned. These will be able to obtain vastly improved resolution images. In order to show how differing resolution affects the position velocity diagrams, figure 6.18 shows the C$^{18}$O, $6\rightarrow 5$ transition at a variety of different resolutions. Note that this cloud was assumed to be 140 pc away. This is the closest that such objects are likely to be found. The majority of such objects will of course be further away so these position velocity diagrams can also be viewed as showing the effect that distance has on the resolution that an particular telescope may have. In figure 6.18 the diagrams are labelled as being caused by a telescope of a given resolution, table 6.3 shows the equivalent distance that would produce each diagram for the JCMT, LMT, a 1km diameter telescope and a 10km diameter telescope. These last two are of course only likely to be interferometers for the near future. Note that some standard distances are: Taurus molecular cloud 140 pc, Orion molecular cloud 450 pc, Galactic centre 8.5 Kpc, Large Magellanic Cloud 50 Kpc, Andromeda galaxy 675 Kpc (Pasachoff [26]).

 

Table 6.3: Distances that would produce the position velocity diagrams shown in figure 6.18

Diagram	Distance to object that would
label	produce equivalent diagrams for
in fig 6.18	JCMT (pc)	LMT (pc)	1 Km (Kpc)	10 Km (Kpc)
1 $^{\prime \prime}$	17.5	58	1.2	12
2 $^{\prime \prime}$	35	120	2.3	23
4 $^{\prime \prime}$	70	230	4.7	47
8 $^{\prime \prime}$	140	470	5.3	53
16 $^{\prime \prime}$	280	930	19	190
32 $^{\prime \prime}$	560	1870	37	370

 

1 $^{\prime \prime}$ 2 $^{\prime \prime}$

4 $^{\prime \prime}$ 8 $^{\prime \prime}$

16 $^{\prime \prime}$ 32 $^{\prime \prime}$

Position velocity diagrams for C$^{18}$O $6\rightarrow 5$ using simulated beam sizes of 1 $^{\prime \prime}$ (top left), 2 $^{\prime \prime}$ (top right), 4 $^{\prime \prime}$ (middle left), 8 $^{\prime \prime}$ (middle right), 16 $^{\prime \prime}$ (bottom left) and 32 $^{\prime \prime}$ (bottom right). See text for details.

These diagrams show once again how important resolution is to studying such objects. For an object in the Taurus molecular cloud the absolute minimum beam size that is able to detect the rotation curve is 8 $^{\prime \prime}$ and to view it in any detail 4 $^{\prime \prime}$ is needed. This is on the limit of what a telescope like the JCMT is capable of providing. The LMT, however, will be able to provide 2 $^{\prime \prime}$ resolution for the $3 \rightarrow 2$ transition. The limitations of a single dish telescope show up when considering table 6.3, as even the LMT will only produce a diagram similar to that labelled as 8 $^{\prime \prime}$ at the distance of the Orion molecular cloud (450pc). It is clear then that millimetre wave interferometers are needed to study anything other than the very closest of such objects. The MMA (MilliMeter Array) has a proposed maximum baseline of around 10km which is sufficient to produce diagrams similar to that labelled 1 $^{\prime \prime}$ for objects anywhere in our galaxy^6.2. The massive improvement in the amount of detail that can be observed using such a telescope is immediately obvious.