The 2018 RASNZ Conference was held in Christchurch from Friday May 4th to Sunday May 6th. VSS held the 5th Variable Stars South Symposium immediately following the conference on the Sunday evening and Monday May 7th.
Mark Blackford, Neil Butterworth, Tom Richards “V883 Sco Period determination” In 2015 Neil Butterworth attempted to record eclipses of V833 Scorpii however predicted eclipse times using the accepted 1.29484 day period and HJD 2443285.794 epoch were clearly wrong. In order to determine the correct period we needed a large set of homogeneous, high quality observations recorded over several years. The All Sky Automated Survey is an excellent source of such observations for stars between V magnitudes 6 and 12, or so. Data for V833 Sco from the ASAS archive was imported into David Benn’s excellent VStar light curve plotting and analysis software. After rejecting several obviously discrepant data points (possibly due to clouds or image artifacts), the Discrete Fourier Transform algorithm in VStar was employed to find candidate periods. The strongest signal from the DFT analysis was for a period of 2.1706 d, however this produced a folded light curve with two overlapping eclipses. Replotting with period twice that (4.3412 d) worked very well, showing slightly shallower secondary eclipses offset from the midpoint between successive primary eclipses. This period was further refined to 4.341164(11) d using data from INTEGRAL-OMC and Neil’s DSLR observations.
Yael Hillman “Novae – where simulations meet observations” Results of nova simulations show a strong dependence of the white dwarf (WD) mass and mass transfer rate on the observable characteristics – the eruption time and the time between eruptions. Thus, they may be used reversely to determine the WD mass and average mass transfer rate of known systems. Combining these results with Galactic novae databases allows to derive distributions of WD masses and mass transfer rates for classical and recurrent novae, eliminating observational bias. The results of many thousands of consecutive eruptions show how the WD changes on a long-term perspective, e.g., the possibility of the WD to grow and become a SNIa progenitor, the limits on the WD mass and on the mass transfer rate and the consequences of helium flashes on this growth. Including the companion in simulations allows for “live” updating of the binary separation which directly and continually affects the mass transfer rate. I will present a handful of aspects from short-term and long-term nova simulations, that may contribute to the determination of the properties of known nova systems, predict future eruptions and detect potential progenitors of type Ia SN.
Stan Walker, Mark Blackford, Ed Budding “QZ Carinae – 2018 update” The quadruple system of QZ Carinae is interesting as one of the most massive multiple star systems in our galaxy, but also challenging due to the orbital period of the eclipsing component being only 124 seconds short of six days. As a result its phases at one particular longitude repeat only every 11.5 years. Observations of the eclipsing pair began in 1971 and measures of the light time effects (LTE) which have an amplitude of ~12.7 hours show that the two pairs of stars have not yet completed one orbit of each other since then. With the help of observations from the BRITE-Heweliusz (BHr) satellite a better mean light curve has been prepared and the various LTE values recalculated to provide a more accurate picture of this orbit. Measures of the primary eclipse by Variable Stars South members in 2017 and continuing in 2018 seem to indicate that the total phase may be slightly shorter than previously believed. Our present understanding of QZ Carinae is reviewed in this presentation.
Stan Walker & Giorgio di Scala “ R Centauri – An Interesting and Perhaps Unique Mira Star” The first recorded maximum of R Centauri was in 1871 but reasonable coverage dates from only 1891. It displays two clear maxima, which characteristic is shared by less than a dozen others of the thousand or so well studied Mira stars. We present observations of the changing light curve shape and amplitudes with supplementary UBV and BVRI measures which appear to differ dramatically from other Miras. The pulsation period is also changing, believed to be the result of a helium flash, the onset of this period change in 1922 being perhaps the only one observed.
Tom Richards, Neil Butterworth, Mark Blackford “When Homer Nods – reconciling our period analysis of V883 Sco with history” V883 Sco is an eclipsing binary with a V magnitude of 7.3 and widely quoted period of 1.29484 d. Observations at Townsville Observatory by NDB over three observing seasons showed this period was very wrong. MDB analysed NDB’s data and derived an incommensurate period of 4.3412 d, which moreover fitted with analysis of INTEGRAL-OMC and ASAS3 data. This paper will present the original photometric data and provide an analysis method showing how that data, though apparently supporting the original period, are consistent with and support our period.
Ed Budding, Mark Blackford, Roger Butland, (et al.) “The multiple star systems V454 Car and HX Vel” We present combined photometric and spectroscopic analyses of the southern close binary systems V454 Car and HX Vel. High-resolution spectra of these systems were taken at the University of Canterbury Mt. John Observatory in the years 2009-15. Absolute parameters for the triple (at least) system V454 Car confirm its young and near-Main-Sequence nature. We find: M1 = 6.30+/-0.20, M2 = 5.86+/-0.18, R1 = 3.12+/-0.05, R2 = 3.04+/-0.04, (all in solar units); T1 ~19000, T2 ~18500 (K); photometric distance ~ 550 (pc). The close binary is found to be the major component (A) of a multiple star, whose other main component (B) is also probably a binary. The two components are in eccentric orbits about a common centre of gravity with a period of ~ 1900 d. Analysis of the radial velocity curves is strongly suggestive that the mass of component B is greater than that of its visible member, while the measured radial velocities of this visible star show irregularities incompatible with reasonable measurement accuracies. These variations occur on a timescale of days to weeks. For HX Vel we find absolute parameters as follows: M1 = 9.6+/-0.4, M2 = 5.6+/-0.3, R1 = 4.80+/-0.18, R2 = 3.05+/-0.16, (all in solar units); T1 ~25000, T2 ~17000 (K); photometric distance ~ 390+/-30 (pc). HX Vel, the brighter component of the visual pair CCDM J08423-4806 AB is often considered to be the brightest member of the cluster IC 2395. There appears to be an inconsistency in the distance estimates, however. The ~400 pc distance of our determination would make the system a likely member of the Sco-Cen OB2 Association within Gould’s Belt.