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There is much evidence that planet formation is occurring in the disc around the Herbig Be star HD100546. To learn more about the processes occurring in this disc, we conducted high-resolution imaging at 43/45 GHz with the Australia Telescope Compact Array. Multiple array configurations were used, providing a best spatial resolution of ∼0.15 arcsec, or 15 au at HD100546's distance of ~100 pc. Significant structure is revealed, but its precise form is dependent on the u − v plane sampling used for the image reconstruction. At a resolution of ≤ 30 au, we detected an inner gap in the disc with a radius of ~25 au and a position angle approximately along the known disc major axis. With different weighting, and an achieved resolution of ~15 au, emission appears at the centre and the disc takes on the shape of an incomplete ring, much like a horseshoe, again with a gap radius of ~25 au. The position angle of the disc major axis and its inclination from face-on are determined to be 140° ± 5° and 40° ± 5°, respectively. The ~25 au gap radius is confirmed by a null in the real part of the binned visibilities at 320 ± 10 kλ, whilst the non-axisymmetric nature is also confirmed through significant structure in the imaginary component. The emission mechanism at the central peak is most likely to be free–free emission from a stellar or disc wind. Overall our data support the picture of at least one, but probably several, giant planets orbiting HD100546 within 25 au. [ABSTRACT FROM AUTHOR]

This paper presents a study of the envelope of the young stellar object (YSO) GGD30IR. What distinguishes this from most other YSOs is the elongated absorption feature seen in silhouette against the background emission in the Spitzer Galactic Legacy Infrared Midplane Survey Extraordinaire 8 μm Infrared Array Camera image of the region. The size and the symmetrical placement of GGD30IR in the centre of this feature suggest that it is an extended envelope, perhaps the remnant of the collapse of the GGD30 core. We have used the extinction in the envelope measured from (i) the reduction in the 8 μm background intensity and (ii) field star colour excesses, to estimate the envelope mass, obtaining values of 0.6 ± 0.2 and , respectively. To investigate the envelope further, we have obtained Australia Telescope Compact Array 3 mm continuum and HCO+ line observations of the region. The continuum emission at 3 mm arises from both a compact (unresolved; ≤730 au) core embedded in an extended envelope ∼18 000 au × 38 000 au in extent. We estimate the core mass to be 0.11 ± 0.02 . The HCO+ emission is extended in a direction perpendicular to the long axis of the envelope, suggesting it comes from an outflow. The spectral energy distribution (SED) provides a 2–24 μm spectral index, α= 1.0, which places GGD30IR in the Class I YSO category. Integrating the SED provides a luminosity of . [ABSTRACT FROM AUTHOR]

The phase of H2O ice and the librational ice band detected toward the post-asymptotic giant branch (post-AGB) star OH231.8+4.2 have been examined using radiative transfer modelling. The results indicate that the ice is largely crystalline and not amorphous as previously reported in the literature. The earlier result is shown to be due to the assumption of unrealistically thick ice mantles. It has also been shown that, in comparison with radiative transfer modelling, Mie theory modelling of the 3-μm ice band detected towards moderately optically thick shells leads to an underestimate of the ice mantle thickness. The modelling results also suggest that the mid-infrared (MIR) feature previously identified with the librational band of H2O ice could be the result of amorphous Al2O3. This dust component can also account for the observed enhanced opacity between the 10- and 18-μm silicate bands. [ABSTRACT FROM AUTHOR]