On the Properties of the HI shells in the Small Magellanic CLoud

There are 509 expanding neutral hydrogen shells catalogued in the Small Magellanic Cloud (SMC), all apparently very young, with dynamical ages of a few Myr. To examine their relationship with young stellar objects we cross-correlated the shell catalogue with various catalogues of OB associations, super giants, Cepheids, WR stars, supernova remnants, and star clusters. The incidence of chance line-ups was estimated via Monte-Carlo simulations, and found to be high. However, it is important that there are 1.5 times more shells that are {\em not} spatially correlated to an OB association, than shells that are. Moreover, 59 of the 509 shells lie mainly in low stellar density fields and have no young stellar objects associated with them, and therefore no obvious energy source. It is shown that, on the whole, the properties of these "empty" shells are very similar to the properties of the rest of the shells, once selection biases are taken into account. Theoretical implications of the results are discussed.Comment: 15 pages, 8 figures accepted for publication in Monthly Notices of the Royal Astronomical Societ

An analysis of the FIR/RADIO Continuum Correlation in the Small Magellanic Cloud

The local correlation between far-infrared (FIR) emission and radio-continuum (RC) emission for the Small Magellanic Cloud (SMC) is investigated over scales from 3 kpc to 0.01 kpc. Here, we report good FIR/RC correlation down to ~15 pc. The reciprocal slope of the FIR/RC emission correlation (RC/FIR) in the SMC is shown to be greatest in the most active star forming regions with a power law slope of ~1.14 indicating that the RC emission increases faster than the FIR emission. The slope of the other regions and the SMC are much flatter and in the range of 0.63-0.85. The slopes tend to follow the thermal fractions of the regions which range from 0.5 to 0.95. The thermal fraction of the RC emission alone can provide the expected FIR/RC correlation. The results are consistent with a common source for ultraviolet (UV) photons heating dust and Cosmic Ray electrons (CRe-s) diffusing away from the star forming regions. Since the CRe-s appear to escape the SMC so readily, the results here may not provide support for coupling between the local gas density and the magnetic field intensity.Comment: 19 pages, 7 Figure

The recent structural evolution of the SMC

We investigate the spatial distribution of stars of different ages towards the main body of the SMC, in an attempt to further understand the nature of the complex structure of the SMC. The old stellar population of the galaxy shows a rather regular and smooth distribution which is typical for a spheroidal body. On the contrary, the distribution of the younger stellar component is highly asymmetric and irregular giving evidence for the severe impact of the SMC during its close encounter with the LMC some 0.2 to 0.4 Gyr ago. In a series of isodensity contour maps of stars within selected ages, the recent structural evolution of the SMC is presented

The recent structural evolution of the SMC

We investigate the spatial distribution of stars of different ages towards the main body of the SMC, in an attempt to further understand the nature of the complex structure of the SMC. The old stellar population of the galaxy shows a rather regular and smooth distribution which is typical for a spheroidal body. On the contrary, the distribution of the younger stellar component is highly asymmetric and irregular giving evidence for the severe impact of the SMC during its close encounter with the LMC some 0.2 to 0.4 Gyr ago. In a series of isodensity contour maps of stars within selected ages, the recent structural evolution of the SMC is presented

On the properties of H I shells in the Small Magellanic Cloud

There are 509 expanding neutral hydrogen shells catalogued in the Small Magellanic Cloud (SMC), all apparently very young, with dynamical ages of a few Myr. To examine their relationship with young stellar objects, we cross-correlated the shell catalogue with various catalogues of OB associations, supergiants, Cepheids, Wolf-Rayet stars, supernova remnants and star clusters. The incidence of chance line-ups was estimated via Monte Carlo simulations, and found to be high. However, it is important that there are 1.5 times more shells that are not spatially correlated to an OB association, than shells that are. Moreover, 59 of the 509 shells lie mainly in low stellar density fields and have no young stellar objects associated with them, and therefore no obvious energy source. It is shown that, on the whole, the properties of these 'empty' shells are very similar to the properties of the rest of the shells, once selection biases are taken into account. In both cases, the shell radius and expansion velocity distribution functions are consistent with the standard model, according to which shells are created by stellar winds and supernova explosions, as long as all shells were created in a single burst and with a power-law distribution of the input mechanical luminosity. This would indicate a burst of star formation. This interpretation, however, cannot explain why the 59 shells, with no young stellar counterparts, show almost exactly the same behaviour as shells with OB associations within their radius. Gamma-ray bursts could account for some but certainly not for the majority of the 'empty' shells. Many 'empty' shells, including most of the high-luminosity ones, are located in the north-western outer regions of the SMC, and may be associated with a chimney-like feature that is known to exist in that area. Finally, it is noted that turbulence is a promising mechanism for the formation of the shell-like structures, but direct comparison with the observations was not possible at this stage, due to lack of detailed models. © 2005 RAS

Automatic extraction and classification of low-dispersion objective-prism stellar spectra

An automatic method of spectral classification from low-dispersion objective prism images is developed. The distribution of early- and late-type stars inside stellar complexes and their surrounding regions can lead to important clues on the star formation mechanism. The method is applied to one SMC region