Conclusions

This may very well be the only thing that a Reader will ever see of your paper. Most of us do not read entire papers from front to back unless the TITLE, ABSTRACT and CONCLUSIONS sections have really intrigued us in some way. In any given month there are hundreds of papers published in the astronomical literature, so each of us only bothers to skim titles and abstracts.

The Conclusion section should summarize your findings, and provide a counterbalance to the Introduction section. The first few sentences are critical, so make sure you prominently can succintly state what it is you did, and what MAJOR NEW DISCOVERY you made from it.

A detailed study of the far-IR and optical properties of 15 comet-like, high-latitude clouds including the Draco cloud, reveals that they are remarkably similar in size and mass, though they vary enormously in morphology and star forming activity. Two of the clouds, G110-13 and G315+21, have a distinctly comet-like shape which can be explained in terms of their supersonic motion through the interstellar medium. These clouds are also sites of star forming activity, possibly indicative of the affects of ram-pressure induced collapse of gravitationally unstable regions within the cloud nuclei.

G359-17, which is associated with R CrA, not only shows star formation activity, but the possible effects of magnetic fields acting on an ionized component of the cloud tail leading to fluted, filamentary optical streamers.

The remaining clouds in this survey show a wide range of morphologies that can be loosely categorized as 'filamentary' or 'cometary'. Little data are currently available for these clouds to ascertain the nature of the mechanism responsible for their morphologies. The dramatic variation in the morphologies of these clouds in spite of their similarity in size and mass, together with their similarity to the Draco cloud, suggests that a hydrodynamical mechanism may be involved in producing their distinctive and unusual shapes.

Consider a population of low-mass molecular cloudlets with sizes of about 2 pc, masses near 50 M(sun) and V ~ 10 km/s. Depending on the properties of the ambient ISM, namely its temperature and density, the cloud shapes will either be cometary where V > Cs or filamentary if V < Cs. If we were to follow a particular cloud as it passes from the low-density, high temperature environment of the galactic halo ( n ~ 0.001 per cc and T ~ 10^5 - 10^6 K) to the high-density, low temperature environment of the HI intercloud medium in the galactic plane ( n ~ 0.17 per cc and T ~ 100 K ) the cloud will move through the changeing ISM at increasing Mach and Reynolds numbers.

Initially, if the ambient medium has T near 1 million K, the cloud moves sub-sonically. The mass lost by ablation in this stage, will form long sinuous filaments since Re < 10 for n < 0.1 /cc. The Draco cloud appears to be such a cloud, and for it, Re ~ 15. As the cloud nears the galactic plane, the ambient medium becomes dominated by the cold HI component. Since T ~ 100 K, Cs ~ 1 km/sec, and the cloud's motion is supersonic. Under these circumstances, the morphology of the cloud will assume a shape defined by the deYoung-Axford model, and a Mach cone may be apparent. The cloud will appear comet-like, as in the case of G110-13 and G315+21. If the mass in the cloud exceeds the Jeans mass, star formation in the dense nuclear region may occur due to shock-induced overpressure. This may have occurred in both G110-13 and G315+21.

If a magnetic field is present within the gas being ablated from the cloud core, this field could have a considerable effect on the dynamics of the ablating material if the material is ionized and has an energy density comparable to that of the magnetic field. As in the case of solar system Type-1 comet tails, the coupling between the magnetic field and the ionized gas in the tail may lead to a variety of instabilities including the growth of sinuous or fluted tails. An example of a cloud in such a state is G359-17.

This highly qualitative scenario leads to several predictions:

1- Filamentary clouds should be more prominent at high galactic latitudes since clouds with z > 100 pc will be in regions where the Reynolds number will be low, and where the cloud velocities will be, largely, sub-sonic.

2 - Within the galactic plane, the dense, cold ISM leads to a correspondingly higher Reynolds number. Since the clouds are also moving supersonically they should have a comet-like appearance and be associated with shocked gas or diffuse, soft X-ray sources for particularly violent interactions.

3 - If the cloud contains an embedded magnetic field and a component of ionized gas, Type-1 tails may also appear with long sinuous filaments due to MHD instabilities.

HI and CO studies of the clouds in this survey should assist in determining their kinematical distances and help to obtain evidence for velocity gradients within their filaments and tails due to ablation outflows. IR and optical polarimetry may also determine whether dust grain alignment in the filaments is present, thereby suggesting the influence of magnetic fields and defining their orientation with respect to the tail material.

This may very well be the only thing that a Reader will ever see of your paper. Most of us do not read entire papers from front to back unless the TITLE, ABSTRACT and CONCLUSIONS sections have really intrigued us in some way. In any given month there are hundreds of papers published in the astronomical literature, so each of us only bothers to skim titles and abstracts.

The Conclusion section should summarize your findings, and provide a counterbalance to the Introduction section. The first few sentences are critical, so make sure you prominently can succintly state what it is you did, and what MAJOR NEW DISCOVERY you made from it.

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