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Low mass ratio contact binary systems are more likely to have unstable orbits and potentially merge. In addition, such systems exhibit characteristics such as starspots and high energy emissions (UV) suggestive of chromospheric and magnetic activity. Light curve modelling of ten contact binary systems is reported. All were found to be of extreme low mass ratio ranging from 0.122 to 0.24 and three were found to be potentially unstable and possible merger candidates. Filling of the infrared Calcium absorption lines is a marker of increased chromospheric activity. We use the available LAMOST spectra along with matched standard spectra (broadened for rotation) to measure the excess filling of the central core depression flux of the two main infrared Calcium absorption lines at 8542 and 8662 angstroms. We find that all reported contact binaries have excess filling of the core flux in the infrared Calcium lines. Three of the systems reported were also observed by the GALEX mission and we find that all three have features of excess ultraviolet emissions further adding evidence for increased chromospheric activity in low mass ratio contact binaries. Analysis of both orbital stability and absorption line filling is dependent on the determination of geometric and absolute parameters from light curve modelling. Not an insignificant number of contact binary light curves exhibit the O'Connell effect, usually attributed to starspots. We discuss the inclusion of starspots in light curve solutions and how they influence the geometric and absolute parameters, Comment: 17 Pages 2 Figures 5 Tables Accepted - Research in Astronomy and Astrophysics (RAA)

We present time-series linear-polarization observations of the bright O4 supergiant $\zeta$ Puppis. The star is found to show polarization variation on timescales of around an hour and longer. Many of the observations were obtained contemporaneously with Transiting Exoplanet Survey Satellite (TESS) photometry. We find that the polarization varies on similar timescales to those seen in the TESS light-curve. The previously reported 1.78-day photometric periodicity is seen in both the TESS and polarization data. The amplitude ratio of photometry to polarization is ~9 for the periodic component and the polarization variation is oriented along position angle ~70 deg-160 deg. Higher-frequency stochastic variability is also seen in both datasets with an amplitude ratio of ~19 and no preferred direction. We model the polarization expected for a rotating star with bright photospheric spots and find that models that fit the photometric variation produce too little polarization variation to explain the observations. We suggest that the variable polarization is more likely the result of scattering from the wind, with corotating interaction regions producing the periodic variation and a clumpy outflow producing the stochastic component. The H$\alpha$ emission line strength was seen to increase by 10% in 2021 with subsequent observations showing a return to the pre-2018 level., Comment: 19 pages, 16 figures, Accepted by MNRAS

The orbital stability of contact binary systems has been receiving considerable attention recently. Theoretical studies indicate that merger is likely to occur at very low mass ratios, but the actual mass ratio at which merger may take place is likely to be variable and dependent on the mass of the primary. We consider the effects of metal content on the orbital stability of contact binary systems by modelling the gyration radius of a rotating and tidally distorted primary component at various values of metallicity in the range -1.25 to +0.5. We determine the instability mass ratio range for contact binary systems with a low mass primary in the range 0.6M(sun) to 1.4M(sun) at various metallicity levels and show that systems with low metallicity have an instability mass ratio lower than those with higher metal content and therefore are likely to be more stable. We illustrate the effect through light curve analysis of two otherwise very similar contact binary systems, except for different metallicity. While both would be considered unstable if metallicity was not taken into consideration, only one remains in that category after appropriate adjustments based on metallicity have been made., Comment: 9 pages, 7 Figures and 5 Tables Accepted Monthly Notices of Royal Astronomical Society (MNRAS)

With increasing number of contact binary discoveries and the recognition that luminous red novae are the result of contact binary merger events, there has been a significant increase in the number of light curve solutions appearing in the literature. One of the key elements of such solutions is the assignment and fixing of the effective temperature of the primary component (T1). Sometimes much is made of the assigned value with expectation of significantly different light curve solutions even though theoretical considerations suggest that absolute value of T1 has little influence on the geometric elements of the light curve solution. In this study we show that assigning T1 over a range of 1000K has no significant influence on the light curve solutions of two extreme low mass ratio contact binary systems. In addition, we explore the use of photometric spectral energy distribution as a potential standard for assigning T1 in the absence of spectroscopic observations., Comment: 7 pages, 4 tables and 2 figures. Accepted Serbian Astronomical Journal

The study reports photometric and spectroscopic observations of two recently recognised contact binary systems. Both systems show total eclipses and analysis of the light curves indicate both have a very low mass ratios of less than 0.3. We derive absolute parameters from colour and distance based calibrations and show that although both have low mass ratios they are likely to be in a stable orbit and unlikely to merge. In other respects both systems have characteristics similar to other contact binaries with the secondary larger and brighter than main sequence counterparts and we also find that the secondary is considerably denser than the primary in both systems., Comment: 10 pages. 3 Figures. 1 Table Accepted - Research in Astronomy and Astrophysics (RAA)

Multi-band (B, V and R) photometric and spectroscopic observations of six poorly studied contact binaries carried out at the Western Sydney University and Las Cumbres Observatory were analysed using a recent version of the Wilson-Devenney code. All six were found to be of extreme low mass ratio ranging from 0.073 to 0.149. All are of F spectral class with the mass of the primary component ranging from 1.05Msun to 1.48Msun. None show light curve features of enhanced choromospheric activity (O'Connell Effect) however five of the six do have significant ultraviolet excess indicating presence of increased magnetic and chromospheric activity. Period analysis based on available survey data suggests two systems have a slowly increasing period suggesting mass transfer from the secondary to the primary, two have a slow declining period with likely mass transfer from primary to the secondary while one shows a steady period and one undergoing transition from a declining to increasing period suggesting possible mass transfer reversal. We also compare light curve solutions against theoretical markers of orbital stability and show that three of six systems have mass ratios within the theoretical instability limit and maybe regarded as potential merger candidates., Comment: 10 Pages. 5 Tables. 4 Figures. Accepted Publications of the Astronomical Society of the Pacific

Photometric observations and analysis of twelve previously poorly studied contact binary systems is presented. All show total eclipses and have extremely low mass ratios ranging from 0.072 to 0.15. Also, all show characteristics of orbital instability with mass ratios within the theoretical orbital instability range. Although none demonstrate a significant O'Connell effect at least nine of the systems have other indicators of increased chromospheric and magnetic activity., Comment: 11 Pages, 2 Figures Accepted Publications of the Astronomical Society of the Pacific (PASP)

We have built and tested a compact, low-cost, but very-high-performance astronomical polarimeter based on a continuously rotating half-wave plate and a high-speed imaging detector. The polarimeter is suitable for small telescopes up to ~1 m in aperture. The optical system provides very high transmission over a wide wavelength range from the atmospheric UV cutoff to ~1000 nm. The high-quantum-efficiency, low-noise and high-speed of the detectors enable bright stars to be observed with high-precision as well as polarization imaging of extended sources. We have measured the performance of the instrument on 20 cm and 60 cm aperture telescopes. We show some examples of the type of science possible with this instrument. The polarimeter is particularly suited to studies of the wavelength dependence and time variability of the polarization of stars and planets., Comment: MNRAS, accepted

The study presents a simplified method to identify potential bright red nova progenitors based on the amplitude of the light curve and infrared (J-H) colour of a contact binary system. We employ published criteria for contact binary orbital instability to show that the amplitude of the light curve for a given contact system with a low mass (< 1.4Msun) primary must be less than a specified value for it to be potentially unstable. Using this we search the photometric data of a large survey to identify about 50 potential bright red nova progenitors. We analyse the survey photometry of each to determine the mass ratio and from the estimated mass of the primary other physical parameters of the systems. We show that each system has physical characteristics indicating potential orbital instability. Using the absolute parameters from our sample we model the expected instability separation and period for low mass contact binary systems, Comment: Accepted - Journal of Astrophysics and Astronomy

We present photometric analysis of three bright red nova progenitor contact binary systems: ASAS J082151-0612.6, TYC 7281-269-1 and TYC 7275-1968-1. The primary components in all three systems are solar-type low mass stars with radii somewhat larger than their Zero Age Main Sequence counterparts. The secondaries, as in most contact binary systems, have radii and luminosities well above their main sequence counterparts. All three have extreme low mass ratios ranging from 0.075 to 0.097 and two have high degrees of contact, in excess of 75%. All three have mass ratios and separations below the theoretical values for orbital stability. Chromospheric activity, a hallmark of magnetic activity and magnetic braking, considered important in mediating angular momentum loss, is also explored. All three systems demonstrate the O'Connell effect, and all systems require the introduction of star spots for a better light curve solution. In addition, we show that ASAS J082151-0612.6 and TYC 7281-269-1 have a UV colour excess in the range indicating high chromospheric activity. Another measure of potential significant magnetic activity is X-Ray luminosity; TYC 7275-1968-1, and probably also TYC 7281-269-1, have X-Ray luminosity well above other contact binary systems. We conclude that it is likely that all three are unstable and hence are potential merger candidates., Comment: Accepted in Research in astronomy and Astrophysics

Here we report the detection of polarization variations due to nonradial modes in the beta Cephei star beta Crucis. In so doing we confirm 40-year-old predictions of pulsation-induced polarization variability and its utility in asteroseismology for mode identification. In an approach suited to other beta Cep stars, we combine polarimetry with space-based photometry and archival spectroscopy to identify the dominant nonradial mode in polarimetry, f2, as l = 3, m = -3 (in the m-convention of Dziembowski) and determine the stellar axis position angle as 25 (or 205) +/- 8 deg. The rotation axis inclination to the line of sight was derived as approx. 46 deg. from combined polarimetry and spectroscopy, facilitating identification of additional modes and allowing for asteroseismic modelling. This reveals a star of 14.5 +/- 0.5 Solar masses and a convective core containing approx. 28% of its mass -- making beta Crucis the most massive star with an asteroseismic age., Comment: 33 pages, 11 figures, 8 tables (incl. Supplementary Information). Accepted version; published as Cotton et al. (2022), Nature Astro., Vol 6, pp 154-164. Data is available through VizieR as: J/other/NatAs/6.154. Two explainer articles are available: Cotton & Buzasi (2022), Nature Astro., Vol 6, pp 24-25; Baade (2022), Nature Astro., Vol 6, pp 20-21

The study presents photometric analysis of the completely eclipsing contact binary systems TYC 8351-1081-1 and ASAS J210406-0522.3. TYC 8351-1081-1 is an extremely low mass ratio (q=0.086) system with high degree of contact (f=0.66) while ASAS J210406-0522.3 is found to be in marginal contact (f=0.08) with a relatively low mass ratio of 0.272. There is good thermal contact in both systems with only a small difference in the temperature of the components. The systems have been observed by a number sky surveys over the past 20 years. We compare the light curve solutions from up to three of these surveys and find that survey photometric data manually analysed is robust and yields results comparable to dedicated ground based photometry. There is evidence of significant luminosity transfer from the primary to the secondary, on the order of 0.5LJ for TYC 8351-1081-1 and 0.06LJ for ASAS J210406-0522.3. There appears to be no change in period of either system over the past 20 years and theoretical angular momentum loss is below current measurement threshold in both cases. We also show that the mass ratio and separation are well above the theoretical values for orbital instability in both cases. As would be expected, the density of the secondary components is significantly higher relative to the primary., Comment: 9 Pages, 3 Tables and 4 Figures Accepted Journal of Astrophysics and Astronomy

ZZ PsA is a neglected bright southern contact binary system with maximum V magnitude of 9.26. We present the first multi-band photometric analysis and find the system to be in deep contact (>95%) with an extremely low mass ratio of 0.078. The primary has a mass of 1.213 M? in keeping with its reported spectral class of F6. In order to determine if ZZ PsA is a merger candidate we outline the current status regarding the instability mass ratio and develop new relationship linking the mass of the primary to the instability mass ratio of the system and the degree of contact. We find that ZZ PsA along with two other examples from the literature to be merger candidates while an additional three require further observations to be confirmed as potential merger candidates., Comment: 7 pages, 6 figures Accepted MNRAS

We report the detection of phase-locked polarization in the bright ($m_V$=2.98-3.24) semidetached eclipsing binary $\mu^1$ Sco (HD 151890). The phenomenon was observed in multiple photometric bands using two different HIPPI-class (HIgh Precision Polarimetric Instrument)polarimeters with telescopes ranging in size from 35-cm to 3.9-m. The peak-to-trough amplitude of the polarization is wavelength dependent and large, $\sim$700 parts-per-million in green light, and is easily seen with even the smallest telescope. We fit the polarization phase curve with a SYNSPEC/VLIDORT polarized radiative transfer model and a Wilson-Devinney geometric formalism, which we describe in detail. Light from each star reflected by the photosphere of the other, together with a much smaller contribution from tidal distortion and eclipse effects, wholly accounts for the polarization amplitude. In the past polarization in semidetached binaries has been attributed mostly to scattering from extra-stellar gas. Our new interpretation facilitates determining masses of such stars in non-eclipsing systems., Comment: 16 pages, 8 figures, 8 tables. Accepted to MNRAS

We present 1 to 10GHz radio continuum flux density, spectral index, polarisation and Rotation Measure (RM) images of the youngest known Galactic Supernova Remnant (SNR) G1.9+0.3, using observations from the Australia Telescope Compact Array (ATCA). We have conducted an expansion study spanning 8 epochs between 1984 and 2017, yielding results consistent with previous expansion studies of G1.9+0.3. We find a mean radio continuum expansion rate of ($0.78 \pm 0.09$) per cent year$^{-1}$ (or $\sim8900$ km s$^{-1}$ at an assumed distance of 8.5 kpc), although the expansion rate varies across the SNR perimeter. In the case of the most recent epoch between 2016 and 2017, we observe faster-than-expected expansion of the northern region. We find a global spectral index for G1.9+0.3 of $-0.81\pm0.02$ (76 MHz$-$10 GHz). Towards the northern region, however, the radio spectrum is observed to steepen significantly ($\sim -$1). Towards the two so called (east & west) "ears" of G1.9+0.3, we find very different RM values of 400-600 rad m$^{2}$ and 100-200 rad m$^{2}$ respectively. The fractional polarisation of the radio continuum emission reaches (19 $\pm$ 2)~per~cent, consistent with other, slightly older, SNRs such as Cas~A., Comment: 17 Pages, 13 Figures

We describe the High-Precision Polarimetric Instrument-2 (HIPPI-2) a highly versatile stellar polarimeter developed at the University of New South Wales (UNSW). Two copies of HIPPI-2 have been built and used on the 60-cm telescope at Western Sydney University's (WSU) Penrith Observatory, the 8.1-m Gemini North Telescope at Mauna Kea and extensively on the 3.9-m Anglo-Australian Telescope (AAT). The precision of polarimetry, measured from repeat observations of bright stars in the SDSS g' band, is better than 3.5 ppm (parts per million) on the 3.9-m AAT and better than 11 ppm on the 60-cm WSU telescope. The precision is better at redder wavelengths and poorer in the blue. On the Gemini North 8-m telescope the performance is limited by a very large and strongly wavelength dependent telescope polarization that reached 1000's of ppm at blue wavelengths and is much larger than we have seen on any other telescope., Comment: 25 pages, 14 Figures, Accepted by Publcations of the Astronomical Society of Australia

Close binary systems often show linear polarization varying over the binary period, usually attributed to light scattered from electrons in circumstellar clouds. One of the brightest close binary systems is Spica (Alpha Virginis) consisting of two B type stars orbiting with a period of just over 4 days. Past observations of Spica have shown low polarization with no evidence for variability. Here we report new high-precision polarization observations of Spica that show variation with an amplitude ~200 parts-per-million (ppm). Using a new modelling approach we show that the phase-dependent polarization is primarily due to reflected light from the primary off the secondary and vice versa. The stars reflect only a few per-cent of the incident light, but the reflected light is very highly polarized. The polarization results show that the binary orbit is clockwise and the position angle of the line of nodes is 130.4 +/- 6.8 degrees in agreement with Intensity Interferometer results. We suggest that reflected light polarization may be much more important in binary systems than has previously been recognized and may be a way of detecting previously unrecognized close binaries., Comment: Published in Nature Astronomy - published version at: https://www.nature.com/articles/s41550-019-0738-7 with title "Polarized reflected light from the Spica binary system". Preprint is original submitted version. Behind the Paper post at: https://astronomycommunity.nature.com/channels/1490-behind-the-paper/posts/47387-polarized-reflected-light-from-the-spica-binary-system

A series of new radio-continuum ({\lambda}=20, 13, 6 and 3 cm) mosaic images focused on the NGC55 galactic system were produced using archived observational data from the Australia Telescope Compact Array. These new images are both very sensitive (down to rms=33 {\mu}Jy) and feature high angular resolution (down to <4"). Using these newly created images, 66 previously unidentified discrete sources are identified. Of these sources, 46 were classified as background sources, 11 as HII regions and 6 as supernova remnant candidates. This relatively low number of SNR candidates detected coupled with the low number of large HII regions is consistent with the estimated low star formation rate of the galaxy at 0.06 solar masses per year. Our spectral index map shows that the core of galaxy appears to have a shallow spectral index between {\alpha} = -0.2 and -0.4. This indicates that the core of the galaxy is a region of high thermal radiation output., Comment: 11 pages, 8 figures. Accepted for publication in Astrophysics and Space Science

A series of new radio-continuum (lambda=20 cm) mosaic images focused on the NGC 300 galactic system were produced using archived observational data from the VLA and/or ATCA. These new images are both very sensitive (rms=60 microJy) and feature high angular resolution (<10"). The most prominent new feature is the galaxy's extended radio-continuum emission, which does not match its optical appearance. Using these newly created images a number of previously unidentified discrete sources have been discovered. Furthermore, we demonstrate that a joint deconvolution approach to imaging this complete data-set is inferior when compared to an immerge approach., Comment: 13 pages, 12 figures, accepted to APSS, new version to correct the missing references

We present preliminary results of our deep Australia Telescope Compact Array (ATCA) radio-continuum survey of the Magellanic Clouds Planetary Nebulae., Comment: 2 pages 1 figure, to appear in Planetary Nebulae an Eye to the Future Proceedings IAU Symposium No. 283

We present a detailed spatially-resolved spectroscopic analysis of two X-ray observations (with a total integration time of 73280 seconds) made of the luminous northwestern rim complex of the Galactic supernova remnant (SNR) G266.2-1.2 (RX J0852.0-4622) with the Chandra X-ray Observatory. G266.2-1.2 is a member of a class of Galactic SNRs which feature X-ray spectra dominated by non-thermal emission: in the cases of these SNRs, the emission is believed to have a synchrotron origin and studies of the X-ray spectra of these SNRs can lend insights into how SNRs accelerate cosmic-ray particles. The Chandra observations have clearly revealed fine structure in this rim complex and the spectra of these features are dominated by non-thermal emission. We have measured the length scales of the upstream structures at eight positions along the rim and derive lengths of 0.02-0.08 pc (assuming a distance of 750 pc to G266.2-1.2). We have also extracted spectra from seven regions in the rim complex and fit these spectra with such models as a simple power law as well as the synchrotron models SRCUT and SRESC. We have constrained our fits to the latter two models using estimates for the flux densities of these filaments at 1 GHz as determined from radio observations made with the Australia Telescope Compact Array (ATCA). Statistically-acceptable fits to all seven regions are derived using each model: differences in the fit parameters (such as photon index and cutoff frequency) are seen in the different regions, which may indicate variations in shock conditions and the maximum energies of the cosmic-ray electrons accelerated at each region. Finally, we estimate the maximum energy of cosmic-ray electrons accelerated along this rim complex to be approximately 40 TeV. We include a summary of estimated maximum energies for both Galactic SNRs as well as SNRs located in the Large Magellanic Cloud., Comment: 56 Pages, 11 Figures, Accepted for publication in the Astrophysical Journal