The discovery of the first electromagnetic counterpart to a gravitational wave signal has generated follow-up observations by over 50 facilities world-wide, ushering in the new era of multi-messenger astronomy. In this paper, we present follow-up observations of the gravitational wave event GW170817 and its electromagnetic counterpart SSS17a/DLT17ck (IAU label AT2017gfo) by 14 Australian telescopes and partner observatories as part of Australian-based and Australian-led research programs. We report early-to late-time multi-wavelength observations, including optical imaging and spectroscopy, midinfrared imaging, radio imaging, and searches for fast radio bursts. Our optical spectra reveal that the transient source emission cooled from approximately 6 400 K to 2 100 K over a 7-d period and produced no significant optical emission lines. The spectral profiles, cooling rate, and photometric light curves are consistent with the expected outburst and subsequent processes of a binary neutron star merger. Star formation in the host galaxy probably ceased at least a Gyr ago, although there is evidence for a galaxy merger. Binary pulsars with short (100 Myr) decay times are therefore unlikely progenitors, but pulsars like PSR B1534+12 with its 2.7 Gyr coalescence time could produce such a merger. The displacement (∼2.2 kpc) of the binary star system from the centre of the main galaxy is not unusual for stars in the host galaxy or stars originating in the merging galaxy, and therefore any constraints on the kick velocity imparted to the progenitor are poor.

We report on results from the first solar Fitting at Low‐Angular degree Group (solar FLAG) hare‐and‐hounds exercise. The group is concerned with the development of methods for extracting the parameters of low‐l solar p‐mode data (‘peak bagging’), collected by Sun‐as‐a‐star observations. Accurate and precise estimation of the fundamental parameters of the p modes is a vital pre‐requisite of all subsequent studies. Nine members of the FLAG (the ‘hounds’) fitted an artificial 3456‐d data set. The data set was made by the ‘hare’ (WJC) to simulate full‐disc Doppler velocity observations of the Sun. The rotational frequency splittings of the l= 1, 2 and 3 modes were the first parameter estimates chosen for scrutiny. Significant differences were uncovered at l= 2 and 3 between the fitted splittings of the hounds. Evidence is presented that suggests this unwanted bias had its origins in several effects. The most important came from the different way in which the hounds modelled the visibility ratio of the different rotationally split components. Our results suggest that accurate modelling of the ratios is vital to avoid the introduction of significant bias in the estimated splittings. This is of importance not only for studies of the Sun, but also of the solar analogues that will be targets for asteroseismic campaigns.

This purpose is about a three dimensional study of natural convection within cavities. This problem is receiving more and more research interest due to its practical applications in the engineering and the astrophysical research The turbulent natural convection of air in an enclosed tall cavity with high aspect ratio (AR=H/W=28.6) is examined numerically. Two cases of differential temperature have been considered between the lateral cavity plates corresponding, respectively, to the low and high Rayleigh numbers: Ra=8.6?105 and Ra=1.43?106 [1]. For these two cases, the flow is characterized by a turbulent low Reynolds number. This led us to improve the flow characteristics using two one point closure low-Reynolds number turbulence models: RNG k-e model and SST k-w model, derived from standard k-e model and standard k-w model, respectively. Both turbulence models have provided an excellent agreement with the experimental data. In order to choose the best model, the average Nusselt number is compared to the experiment and other numerical results. The vorticity components surfaces confirm that the flow can be considered two-dimensional with stretched vortex in the cavity core. Finally, a correlation between Nusselt number and Rayleigh number is obtained to predict the heat transfer characteristics.

Abstract. Object images obtained by mean of ground based instrument are degraded by the earth atmosphere. Indeed, the wave-front at the entrance of the instrument pupil, present phase and amplitude random fluctuations depending of the time, the position and the line of sight. The recorded images are consequently filtered leading to bad measurements of the studied object parameters. To qualify the atmospheric degradations, several parameters are commonly defined. For solar diameter measurements performed with an astrolabe, errors due to the atmospheric turbulence are directly related to these parameters. After, a brief recall of the basic properties of the atmospheric turbulence, a method allowing to generate realistic random wave-fronts will be presented. They will be used to study errors on diameter measurements performed with a solar astrolabe. The obtained results will clearly show the necessity to have a seeing monitor observing together with the solar experiment. A method allowing to obtain the observation conditions for solar observation will then be given.

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