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We present Murchison Widefield Array observations of the supernova remnant (SNR) 1987A between 72 and 230 MHz, representing the lowest frequency observations of the source to date. This large lever arm in frequency space constrains the properties of the circumstellar medium created by the progenitor of SNR 1987A when it was in its red supergiant phase. As of late 2013, the radio spectrum of SNR 1987A between 72 MHz and 8.64 GHz does not show any deviation from a non-thermal power law with a spectral index of -0.74 ± 0.02. This spectral index is consistent with that derived at higher frequencies, beneath 100 GHz, and with a shock in its adiabatic phase. A spectral turnover due to free-free absorption by the circumstellar medium has to occur below 72 MHz, which places upper limits on the optical depth of ≤0.1 at a reference frequency of 72 MHz, emission measure of≲13 000 cm-6 pc, and an electron density of ≲110 cm-3. This upper limit on the electron density is consistent with the detection of prompt radio emission and models of the X-ray emission from the supernova. The electron density upper limit implies that some hydrodynamic simulations derived a red supergiant mass-loss rate that is too high, or a wind velocity that is too low. The mass-loss rate of ~5 × 10-6 M⊙ yr-1 and wind velocity of 10 km s-1 obtained from optical observations are consistent with our upper limits, predicting a current turnover frequency due to free-free absorption between 5 and 60 MHz. [ABSTRACT FROM AUTHOR]

ABSTRACT We report late-time spectroscopic observations of the Type IIn supernova (SN) 1998S, taken 14 years after explosion using the Large Binocular Telescope. The optical spectrum exhibits strong, broad emission features of [O i], [O ii] and Hα, in addition to weaker features of [O iii], Hβ and [Fe ii]. The last decade of evolution has exhibited a strengthening of the oxygen transitions relative to Hα, evidence that the late-time emission is powered by increasingly metal-rich SN ejecta crossing the reverse shock. The Hα luminosity of ≈8000 L⊙ requires that SN 1998S is still interacting with relatively dense circumstellar material (CSM), probably produced by the strong wind of a red supergiant progenitor at least ∼103 years before explosion. The emission lines exhibit asymmetric blueshifted profiles, which implies that the receding hemisphere of the SN is obscured by dust. The [O iii] λ5007 line, in particular, exhibits a complete suppression of its red wing. This could be the result of the expected wavelength dependence for dust extinction or a smaller radial distribution for [O iii]. In the latter case, the red wing of [O iii] could be absorbed by core dust, while both the blue and red wings are absorbed by dust within the cool dense shell between the forward and reverse shocks; this interpretation could explain why late-time [O iii] emission from SNe is often weaker than models predict. The [O i] line exhibits double-peaked structure on top of the broader underlying profile, possibly due to emission from individual clumps of ejecta or ring-like structures of metal-rich debris. The centroids of the peaks are blueshifted and lack a red counterpart. However, an archival spectrum obtained on day 1093 exhibits a third, redshifted peak, which we suspect has become extinguished by dust that formed over the last decade, after day 1093. This implies that the 'missing' red components of multi-peaked oxygen profiles observed in other SNe might be obscured by varying degrees of dust extinction. [ABSTRACT FROM AUTHOR]

ABSTRACT We present new MIDI interferometric and VISIR spectroscopic observations of HD 113766 and HD 172555. Additionally, we present VISIR 11-m and 18-m imaging observations of HD 113766. These sources represent the youngest (16 and 12 Myr old, respectively) debris disc hosts with emission on ≪10 au scales. We find that the disc of HD 113766 is partially resolved on baselines of 42-102 m, with variations in resolution with baseline length consistent with a Gaussian model for the disc with a full width at half-maximum (FWHM) of 1.2-1.6 au (9-12 mas). This is consistent with the VISIR observations which place an upper limit of 0.14 arcsec (17 au) on the emission, with no evidence for extended emission at larger distances. For HD 172555, the MIDI observations are consistent with complete resolution of the disc emission on all baselines of lengths 56-93 m, putting the dust at a distance of >1 au (>35 mas). When combined with limits from TReCS imaging, the dust at ∼10 m is constrained to lie somewhere in the region of 1-8 au. Observations at ∼18 m reveal extended disc emission which could originate from the outer edge of a broad disc, the inner parts of which are also detected but not resolved at 10 m, or from a spatially distinct component. These observations provide the most accurate direct measurements of the location of the dust at 1-8 au that might originate from the collisions expected during terrestrial planet formation. These observations provide valuable constraints for models of the composition of discs at this epoch and provide a foundation for future studies to examine in more detail the morphology of debris discs. [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]