Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy

We review HB stars in a broad astrophysical context, including both variable and non-variable stars. A reassessment of the Oosterhoff dichotomy is presented, which provides unprecedented detail regarding its origin and systematics. We show that the Oosterhoff dichotomy and the distribution of globular clusters (GCs) in the HB morphology-metallicity plane both exclude, with high statistical significance, the possibility that the Galactic halo may have formed from the accretion of dwarf galaxies resembling present-day Milky Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the second-parameter problem is presented. A technique is proposed to estimate the HB types of extragalactic GCs on the basis of integrated far-UV photometry. The relationship between the absolute V magnitude of the HB at the RR Lyrae level and metallicity, as obtained on the basis of trigonometric parallax measurements for the star RR Lyrae, is also revisited, giving a distance modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are studied. Finally, the conductive opacities used in evolutionary calculations of low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and Space Scienc

Engineering-in sustainability through the application of SUSOP®

There are considerable challenges in developing new resource projects that meet the ideals of sustainability. The high-level principles of corporate sustainability policies cannot be easily integrated into project management systems or the everyday activities at operations. In addition, existing project management systems do not readily deliver the innovative solutions needed to address key sustainability issues, such as significantly reduced carbon emissions, minimal environmental impacts, and maintaining the societal licence to operate. SUSOP® (SUStainable OPerations) is an approach for the integration of sustainable development principles into the design and operation of industrial processes, which is being developed through the Co-operative Research Centre for Sustainable Resource Processing (CSRP). Somewhat analogous to HAZOP (Hazard and Operability Studies), the key aim of SUSOP® is to produce a holistic, systematic and rigorous set of processes for identifying, evaluating, and implementing Sustainable Development opportunities within the organising architecture of a sustainability framework. This approach is currently being tested and enhanced through application to ‘live’ case studies in the minerals industry. The results of applying selected elements of SUSOP® to these case studies, both at the conceptual and pre-feasibility phases, have led to significant insights into the process of identifying and evaluating options for enhancing an operations contribution to sustainability and its long-term business case. This paper discusses the need for an approach such as SUSOP®, presents the key outcomes from the development of SUSOP® to date with particular reference to the case study learnings, and highlights the practical ways for incorporating sustainability into project management systems

APOGEE-2S Discovery of Light- And Heavy-element Abundance Correlations in the Bulge Globular Cluster NGC 6380

We derive abundance ratios for nine stars in the relatively high-metallicity bulge globular cluster NGC 6380. We find a mean cluster metallicity between [Fe/H] = -0.80 and -0.73, with no clear evidence for a variation in iron abundances beyond the observational errors. Stars with strongly enhanced [N/Fe] abundance ratios populate the cluster and are anticorrelated with [C/Fe], trends that are considered a signal of the multiple-population phenomenon in this cluster. We detect an apparent intrinsic star-to-star spread (⪆0.27 dex) in the slow neutron-capture process element (s-element) Ce ii. Moreover, the [Ce/Fe] abundance ratio exhibits a likely correlation with [N/Fe], and a somewhat weaker correlation with [Al/Fe]. If confirmed, NGC 6380 could be the first high-metallicity globular cluster where a N-Ce correlation is detected. Furthermore, this correlation suggests that Ce may also be an element involved in the multiple-population phenomenon. Currently, a consensus interpretation for the origin of the this apparent N-Ce correlation in high-metallicity clusters is lacking. We tentatively suggest that it could be reproduced by different channels - low-mass asymptotic giant branch stars in the high-metallicity regime or fast-rotating massive stars ("spinstars"), due to the rotational mixing. It may also be the cumulative effect of several pollution events including the occurrence of peculiar stars. Our findings should guide stellar nucleosynthesis models, in order to understand the reasons for its apparent exclusivity in relatively high-metallicity globular clusters. © 2021. The Author(s). Published by the American Astronomical Society.Open access articleThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

The contribution of N-rich stars to the Galactic stellar halo using APOGEE red giants

The contribution of dissolved globular clusters (GCs) to the stellar content of the Galactic halo is a key constraint on models for GC formation and destruction, and the mass assembly history of the Milky Way. Earlier results from APOGEE pointed to a large contribution of destroyed GCs to the stellar content of the inner halo, by as much as 25 percent, which is an order of magnitude larger than previous estimates for more distant regions of the halo. We set out to measure the ratio between nitrogen-rich (N-rich) and normal halo field stars, as a function of distance, by performing density modelling of halo field populations in APOGEE DR16. Our results show that at 1.5 kpc from the Galactic Centre, N-rich stars contribute a much higher 16.8+10.0-7.0, percent fraction to the total stellar halo mass budget than the 2.7 +1.0-0.8percent ratio contributed at 10 kpc. Under the assumption that N-rich stars are former GC members that now reside in the stellar halo field, and assuming the ratio between first and second population GC stars being 1:2, we estimate a total contribution from disrupted GC stars of the order of 27.5+15.4-11.5 percent at r = 1.5 kpc and 4.2+1.5-1.3 percent at r = 10 kpc. Furthermore, since our methodology requires fitting a density model to the stellar halo, we integrate such density within a spherical shell from 1.5 to 15 kpc in radius, and find a total stellar mass arising from dissolved and/or evaporated GCs of MGC,total = 9.6+4.0-2.6 × 107 M·. © 2021 2020 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.Immediate accessThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]