1,721,105 research outputs found
Rotation in molecular clouds
No full textWe have investigated a broad range of evidence concerning rotation in molecular clouds. As a consequence, we show that trends in specific angular momentum J/M and angular velocity ? are inconsistent with certain models of isothermal, non-magnetic cloud rotation. Similarly, models of rotation which invoke turbulent vorticity may have only limited applicability to clumps and condensations. There is evidence to favour an important r�le for rotation in maintaining the stability of disks, larger cloud structures, and perhaps a large fraction of intermediate sized clouds, whilst rotation may also be implicated in maintaining observed departures from cloud sphericity. Although it is conceivable that magnetic braking is responsible for the radial decrement in specific angular momentum, it appears that observed gradients dln(J/M)/dln(R) are significantly shallower than is normally anticipated through this mechanism. The variation of angular momentum with cloud mass M (viz. J ? M1.7) appears to be highly correlated, and is consistent with models of clump merging in isothermal rotating clouds. Similarly, the orientations of the angular velocity vectors for clumps and condensations appear broadly random, suggesting a turbulent origin for observed components of ? or, alternatively, a process of randomisation through magnetic and/or dynamic clump interactions. By contrast, isolated clouds (and perhaps also disks) are shown to have angular velocity vectors oriented predominantly towards the north and south galactic poles; a distribution which would be anticipated were components of J to arise from galactic shear. We show, finally, that most of the cloud subgroups appear to follow similar functional trends in J, J/M, and ?. Disks and rings, on the other hand, appear to depart from these variations to a significant degree; a difference which presumably derives from their distinct spatio-kinematic structures
The problem with reddening distances to planetary nebulae
No full textIt is noted that the vast majority of Galactic PNe are located outside of the interstellar reddening layer, to the extent that local gradients in E B-V are low to undetectable. This is likely to invalidate certain previous estimates of their reddening distances DRED. It also means that larger scale analyses, such as those of Pottasch (1984) and Acker (1973), lead to values of DRED which are significantly too small. A similar critique applies to analyses based on measures of Na D-line absorption, and it seems likely that most of the distances of Napiwotzki & Schönberner (1995) are similarly in error
Searching for balmer self-absorption in planetary nebulae
No full textWe analyse the distribution of 482 planetary nebulae (PNe) within the H?/H?-H?/H? and H?/H?-H?/H? planes. Whilst most sources appear to possess case B line ratios, and normal reddening decrements, certain of them also appear to be affected by Balmer self-absorption. We are able to identify 9 possible cases, and 11 probable examples where such absorption is important. Approximately half of these nebulae are compact, and probably quite young. Most previous identifications of self-absorption are shown to be open to doubt, and there is no overlap at all between our present candidates, and those of previous analyses
Using planetary nebulae to probe the structure of the galactic thin disk
No full textMost planetary nebulae appear to arise from progenitors with mass M PG ? 1 M?. One would therefore expect them to be closely associated with stars located within the galactic thin disk. This has been confirmed through a series of recent estimates of scale height, which imply values z0(PNe) of order ? 220 pc. We shall show, in the following, that it is also possible to evaluate the variation of z 0(PNe) with galactocentric distance R. Although such an analysis is subject to several uncertainties, including those associated with the distances to these outflows, it is nevertheless possible to demonstrate that Z 0 (PNe) increases with increasing R. This spatial "flaring" is consistent with what is known of various other disk components
Searching for balmer self-absorption in planetary nebulae
No full textWe analyse the distribution of 482 planetary nebulae (PNe) within the Hα/Hβ-Hγ/Hβ and Hδ/Hγ-Hγ/Hβ planes. Whilst most sources appear to possess case B line ratios, and normal reddening decrements, certain of them also appear to be affected by Balmer self-absorption. We are able to identify 9 possible cases, and 11 probable examples where such absorption is important. Approximately half of these nebulae are compact, and probably quite young. Most previous identifications of self-absorption are shown to be open to doubt, and there is no overlap at all between our present candidates, and those of previous analyses
Biases in the kinematic parallaxes of galactic planetary nebulae
No full textIt has recently been noted that the kinematic parallaxes of planetary nebulae (PNe) may be significantly in error. The pattern velocities of the shells, which determine the lateral expansion of the outflows, are likely to differ from line-of-sight velocities determined through visual spectroscopy. It is usually assumed that the mean distances to large ensembles of PNe should be reasonably secure, however. Given that sources are randomly oriented with respect to the line-of-sight, then individual distance biases should wash-out in the mean. We point out that this is unlikely to be the case where sources possess non-spherical structures, however. If one measures lateral expansion velocities V⊥ along the minimum axes of the sources, then distances will (in the mean) tend to be somewhat too large. Alternatively, if one consistently determines V⊥ along the largest axes of the outflows, then values of 〈D〉 will tend to be low. Although the size of error is difficult to assess, it may approach of order ΔD/D ∼ 10%
Using planetary nebulae to probe the structure of the galactic thin disk
No full textMost planetary nebulae appear to arise from progenitors with mass M PG ≈ 1 M⊙. One would therefore expect them to be closely associated with stars located within the galactic thin disk. This has been confirmed through a series of recent estimates of scale height, which imply values z0(PNe) of order ∼ 220 pc. We shall show, in the following, that it is also possible to evaluate the variation of z 0(PNe) with galactocentric distance R. Although such an analysis is subject to several uncertainties, including those associated with the distances to these outflows, it is nevertheless possible to demonstrate that Z 0 (PNe) increases with increasing R. This spatial "flaring" is consistent with what is known of various other disk components
Biases in the kinematic parallaxes of galactic planetary nebulae
No full textIt has recently been noted that the kinematic parallaxes of planetary nebulae (PNe) may be significantly in error. The pattern velocities of the shells, which determine the lateral expansion of the outflows, are likely to differ from line-of-sight velocities determined through visual spectroscopy. It is usually assumed that the mean distances to large ensembles of PNe should be reasonably secure, however. Given that sources are randomly oriented with respect to the line-of-sight, then individual distance biases should wash-out in the mean. We point out that this is unlikely to be the case where sources possess non-spherical structures, however. If one measures lateral expansion velocities V? along the minimum axes of the sources, then distances will (in the mean) tend to be somewhat too large. Alternatively, if one consistently determines V? along the largest axes of the outflows, then values of ?D? will tend to be low. Although the size of error is difficult to assess, it may approach of order ?D/D ? 10%
The problem with reddening distances to planetary nebulae
No full textIt is noted that the vast majority of Galactic PNe are located outside of the interstellar reddening layer, to the extent that local gradients in E B-V are low to undetectable. This is likely to invalidate certain previous estimates of their reddening distances DRED. It also means that larger scale analyses, such as those of Pottasch (1984) and Acker (1973), lead to values of DRED which are significantly too small. A similar critique applies to analyses based on measures of Na D-line absorption, and it seems likely that most of the distances of Napiwotzki & Schönberner (1995) are similarly in error
A 1.4 GHZ and 14.7 GHZ analysis of density gradients in galactic planetary nebulae
No full textThere has been a considerable level of uncertainty as to whether planetary nebulae (PNe) contain radial density gradients. Whilst the work of Taylor et al. (1987) implies that gradients exist in most PNe, that of Si dmiak & Tylenda (2001) suggests precisely the reverse. The work of Phillips (2007) suggests that both of these analyses are suspect, however, and that at least ? 10 ? 20% of PNe probably do contain gradients. We now extend this latter analysis to include significantly higher frequency fluxes, and use these to undertake a more sensitive analysis of the effects of radial density gradients. We conclude than in excess of 85% of PNe appear to contain gradients, of which ? 21% are likely to have density exponents > 1.5, and central cavity sizes which are small (i.e. have radii < 20% of the outer radius of the shell)
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