We present a chemo-dynamical study conducted with 2dF$$ + $$AAOmega of $$ \sim 6000$$ Gaia DR3 non-variable candidate metal-poor stars that lie in the direction of the Galactic plane. Our spectral analysis reveals 15 new extremely metal-poor (EMP) stars, with the lowest metallicity at $$\left[ {{\rm{Fe/H}}} \right] = - 4.0 \pm 0.2$$ dex. Two of the EMP stars are also carbon enhanced, with the largest enhancement of $$\left[ {{\rm{C/Fe}}} \right] = 1.3 \pm 0.1$$ occurring in a dwarf. Using our $$\left[ {{\rm{C/Fe}}} \right]$$ results, we demonstrate that the number of carbon-depleted stars decreases with lower metallicities, and the fraction of carbon-enhanced stars increases, in agreement with previous studies.

Our dynamical analysis reveals that the fraction of prograde and retrograde disk stars, defined as $${z_{{\rm{max}}}} \lt 3$$ kpc, with $${J_\phi }/{J_{{\rm{tot}}}} \gt 0.75$$ and $${J_\phi }/{J_{{\rm{tot}}}} \lt - 0.75$$ respectively, changes as metallicities decrease. Disk stars on retrograde orbits make up $$ \sim 10$$% of all the stars in our sample with metallicities below $$ - 2.1$$ dex. Interestingly, the portion of retrograde disk stars compared with the number of kinematically classified halo stars is approximately constant at $$4.6$$ % for all metallicities below $$ - 1.5$$ dex. We also see that $${J_\phi }$$ increases from $$380 \pm 50$$ to $$1320 \pm 90$$ km s$${^{ - 1}}$$ kpc across metallicity range $$ - 1.5$$ to $$ - 1.1$$, consistent with the spin-up of the Galactic disk. Over the metallicity range $$ - 3.0 \lt \left[ {{\rm{Fe/H}}} \right] \lt - 2.0$$, the slopes of the metallicity distribution functions for the prograde and retrograde disk stars are similar and comparable to that for the halo population. However, detailed chemical analyses based on high resolution spectra are needed to distinguish the accreted versus in-situ contributions. Finally, we show that our spectroscopic parameters reveal serious systematics in the metallicities published in recent studies that apply various machine learning techniques to Gaia XP spectra.

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, mid-infrared 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.

The recently completed HI mosaic survey of the Large Magellanic Cloud (Kim et al. 1997) reveals complex structure in the interstellar medium, including filaments, arcs, holes and shells. We have catalogued giant and supergiant HI shells and searched for correlations with Hα emission, using a new image taken with a camera lens mounted on the 16-inch telescope at Siding Spring Observatory.

Simultaneous high dispersion photoelectric and photographic spectra have been obtained for the short period variables δ Set, δ Del and p Pup over at least one cycle of variation.

Carbon stars have always been amongst the most intriguing of stars which show grossly non-solar abundances. All such stars raise the questions:

(a) at what stage of stellar evolution does this peculiar abundance phase occur?

(b) do all stars pass through this stage?

(c) is the phase significant in the overall picture of galactic nucleosynthesis?

Many of the important events in the life of a star occur, or are thought to occur, during the red giant or supergiant phase of evolution. For example, in heavy and intermediate mass stars supernova explosions terminate normal evolutionary processes while in lower mass stars the stellar envelope is entirely removed giving rise to planetary nebulae and, subsequently, white dwarfs. Theoretical calculations suggest that before the onset of these rather drastic events, a significant amount of nucleosynthesis occurs, giving rise to enhanced surface abundances of He, C, N and s-process elements (e.g., Iben and Truran 1978; Renzini and Voli 1981); loss of the envelope material by stellar winds, planetary nebula ejection and supernova explosions produce overall galactic enrichment in these elements.

The metal content of stars (and galaxies) is of great interest as it relates to the theories of element formation, and through these to the history of our galaxy, the frequency of supernovae, the star formation rate and even the quasar rate. The kinematics and metallicity of stars have been correlated in a theory of formation of the galaxy by Eggen et al. (1962). Alternate theories (Unsold 1969) have also been proposed. Much work has gone into isolating individual metal deficient stars and determining their abundances, and also the abundances for stars in clusters.

On the basis of (R — I) colours it has been suggested that the N stars (the cool carbon stars) occupy a very restricted region in the Mb01 — log Te diagram (Te = 3300 to 3500 K) Mb01 ~ —3), falling on the normal giant sequence, but hotter than the M variables. This appeared somewhat puzzling in view of the variability of many of the carbon stars and their large range of periods, which suggested that the N stars ought perhaps to have similar temperatures to the variable M stars. In order to compare the continua of the M stars and N stars within the range of the R and I filters, and also to determine the continuum between the molecular bands, photometric observations were made of 20 M and N stars at carefully chosen wavelengths with 30Å resolution.

Zeldovich and Guseynov suggested that neutron stars and collapsed stars (‘Black holes’) may be found as highly evolved companions of single lined spectroscopic binaries. They pointed out that the detection of X-rays or γ-rays from such a system would constitute positive evidence for the highly evolved nature of the companion. Trimble and Thorne searched through the catalogue of orbital elements of spectroscopic binaries (Batten) and found a few systems which were single line spectroscopic binaries with companions of mass M2 ≥ (1.4 M⊙, M1), where M1 is the primary mass. None of these has, however been identified with an X-ray source and other explanations such as multiple systems could not be ruled out. The alternative procedure, namely the identification of galactic X-ray sources with spectroscopic binaries is clearly more rewarding from the view point of searching for black holes and neutron stars. In particular, a high mass secondary which could be identified with a black hole is more likely to be discovered as a companion of an X-ray binary with a massive primary such as a blue supergiant.

The nearest star α Centauri is a triple system. The star α Cen, one of the pointers to the Southern Cross, is itself a −0.3 mag visual binary consisting of a α Cen A, a G2V star popularly considered to resemble the Sun, and α Cen B, a KOV star. The orbital period is 80.089 years, the semimajor axis is 23.5 AU, the distance 1.34 pc, the masses are 1.11 and 0.92 (±0.03) M⊙ (Kamper & Wesselink 1977). The third star, Proxima Cen (V645 Cen) is a faint common proper-motion companion separated from α Cen AB by about 2°. It is known to most people only by virtue of being slightly closer to the Sun than is α Cen A, and therefore actually the closest star.. This 11th mag flare star is undoubtedly coeval with a α Cen AB, but being the least massive by a considerable margin, may have been ejected from the system shortly after formation.

Carbon, nitrogen and oxygen are of interest for stellar and galactic evolution for four main reasons. Firstly, they comprise most of the mass of elements heavier than helium, so their abundance reflects the bulk of chemical enrichment. Secondly, all of the oxygen, much of the carbon and perhaps some of the nitrogen is believed to be produced in shorter-lived stars more massive than those responsible for the Fe production (Tinsley 1979), so their abundance relative to Fe in very metal deficient objects should provide key information for modelling the chemical history of galaxies. Thirdly, because C and O comprise the bulk of the metals in stellar material (Fe:C:N:0 = 1:12:2.5:21 in the Sun) it is their abundance as well as that of iron, which is needed to compute evolutionary tracks for different metallicities. Finally, the O abundance will indicate whether CNO material has been mixed to the surface.

Cen X-3 and Her X-1 are the only two X-ray binaries which are known to pulsate periodically (at 4.8s and 1.2s respectively) in the X-ray region. The latter was identified with a fourteenth magnitude light and spectrum variable HZ Her, which was subsequently found to exhibit 1.24 second optical pulsations during certain phases. The possibility that Cen X-3 may prove to be as interesting an object optically has resulted in a wide spread search for its optical counterpart.

The K giants are the brightest stars in clusters of both intermediate (∽ 108 years) and old (∽ 109-10 years) age. They have strong metal lines and strong bands of CN and CH in the optical spectrum, and are the logical objects in which to measure the metal abundance of such stellar systems and trace the evolution of heavy elements in the universe.

In the five years since the last invited paper on model stellar atmosphere applications there have been many significant advances made on all fronts. The five aspects which I will cover in this paper are:

• (1) the results of white dwarf model atmosphere investigations;

• (2) the results of the inclusion of non LTE phenomena in the atmosphere computations of hot (T > 15,000 K) stars;

• (3) the probable understanding of the cause of peculiar abundance patterns in the Ap and Bp and Am stars;

• (4) the advances in theory and observations of cool star atmospheres; and

• (5) the use of synthetic spectra and colours.

A white dwarf is a body of stellar mass, but of planetary dimensions. It is self luminous with a surface temperature that has been observed as high as 70,000 K or as low as 4,000 K. Because of its low surface area the star is far fainter than a main sequence star of the same colour; equally the light it emits is bluer than that from a main sequence star of similar luminosity. Figure 1 is the HR diagram from data obtained by the US Naval Observatory showing the dwarf and white dwarf sequences.

Angular diameters of Magellanic Cloud planetary nebulae obtained using speckle interferometry on the AAT are presented. The mass of ionized gas in each nebula is derived from the angular diameter and published Hβ line fluxes; the derived masses range from 0.005M⊙ to 0.19M⊙, with a mean value of 0.08M⊙. All the planetary nebulae observed are relatively small (diameter ≲0.13pc), young (age ≲ 2500 years), bright and dense. They are therefore almost certainly only partially ionized, so that the masses derived for the ionized parts of the nebula are lower limits to the total nebula mass.

This is a preliminary report on the first Strömgren–Hβ photometric survey of yellow supergiants in the Magellanic Clouds. Less luminous supergiants have been chosen in preference for this program in order to facilitate calculation of their physical parameters. This will not only provide a basis for future detailed spectral analysis of these stars, but will allow a better calibration of the colours derived from model atmospheres for this type of star. The colours of these stars are more affected by changes in metallicity than the hotter stars more frequently studied, and since they are also young, they reflect the metallicity of their local InterStellar Medium (ISM). If indeed the Strömgren abundance index m1 can be calibrated successfully for these stars, then their observation in any galaxy will provide the metallicity of that particular ISM. Our results show that it is important for future colours to be calculated over the range of turbulent velocities important for supergiants.

One of the major science goals of the SkyMapper survey of the Southern Hemisphere sky is the determination of the shape and extent of the halo of the Galaxy. In this paper, we quantify the likely efficiency and completeness of the survey as regards the detection of RR Lyrae variable stars, which are excellent tracers of the halo stellar population. We have accomplished this via observations of the RR Lyrae-rich globular cluster NGC 3201. We find that for single-epoch uvgri observations followed by two further epochs of g, r imaging, as per the intended three-epoch survey strategy, we recover known RR Lyraes with a completeness exceeding 90%. We also investigate boundaries in the gravity-sensitive single-epoch two-colour diagram that yield high completeness and high efficiency (i.e., minimal contamination by non-RR Lyraes) and the general usefulness of this diagram in separating populations.

This paper presents the design and science goals for the SkyMapper telescope. SkyMapper is a 1.3-m telescope featuring a 5.7-square-degree field-of-view Cassegrain imager commissioned for the Australian National University's Research School of Astronomy and Astrophysics. It is located at Siding Spring Observatory, Coonabarabran, NSW, Australia and will see first light in late 2007.

The imager possesses 16 384 × 16 384 0.5-arcsec pixels. The primary scientific goal of the facility is to perform the Southern Sky Survey, a six-colour and multi-epoch (four-hour, one-day, one-week, one-month and one-year sampling) photometric survey of the southerly 2π sr to g ∼23 mag. The survey will provide photometry to better than 3% global accuracy and astrometry to better than 50 milliarcsec. Data will be supplied to the community as part of the Virtual Observatory effort. The survey will take five years to complete.