The Dawes Review 8: Measuring the Stellar Initial Mass Function

The birth of stars and the formation of galaxies are cornerstones of modern astrophysics. While much is known about how galaxies globally and their stars individually form and evolve, one fundamental property that affects both remains elusive. This is problematic because this key property, the birth mass distribution of stars, referred to as the stellar initial mass function (IMF), is a key tracer of the physics of star formation that underpins almost all of the unknowns in galaxy and stellar evolution. It is perhaps the greatest source of systematic uncertainty in star and galaxy evolution. The past decade has seen a growing number and variety of methods for measuring or inferring the shape of the IMF, along with progressively more detailed simulations, paralleled by refinements in the way the concept of the IMF is applied or conceptualised on different physical scales. This range of approaches and evolving definitions of the quantity being measured has in turn led to conflicting conclusions regarding whether or not the IMF is universal. Here I review and compare the growing wealth of approaches to our understanding of this fundamental property that defines so much of astrophysics. I summarise the observational measurements from stellar analyses, extragalactic studies and cosmic constraints, and highlight the importance of considering potential IMF variations, reinforcing the need for measurements to quantify their scope and uncertainties carefully, in order for this field to progress. I present a new framework to aid the discussion of the IMF and promote clarity in the further development of this fundamental field.Comment: Accepted for publication in PASA. 52 pages, 10 figures. A bug in pasa-mnras.bst causes references beginning with lower-case letters (e.g., "de", "van") to be placed at the end of the reference list, rather than alphabetically. Kindly and skilled people are encouraged to correct this and share with the PASA editor

Unraveling the rapid radiation of crested newts, Triturus cristatus superspecies, using complete mitogenomic sequences

Background - The rapid radiation of crested newts (Triturus cristatus superspecies) comprises four morphotypes: 1) the T. karelinii group, 2) T. carnifex - T. macedonicus, 3) T. cristatus and 4) T. dobrogicus. These vary in body build and the number of rib-bearing pre-sacral vertebrae (NRBV). The phylogenetic relationships of the morphotypes have not yet been settled, despite several previous attempts, employing a variety of molecular markers. We here resolve the crested newt phylogeny by using complete mitochondrial genome sequences. Results - Bayesian inference based on the mitogenomic data yields a fully bifurcating, significantly supported tree, though Maximum Likelihood inference yields low support values. The internal branches connecting the morphotypes are short relative to the terminal branches. Seen from the root of Triturus (NRBV = 13), a basal dichotomy separates the T. karelinii group (NRBV = 13) from the remaining crested newts. The next split divides the latter assortment into T. carnifex - T. macedonicus (NRBV = 14) versus T. cristatus (NRBV = 15) and T. dobrogicus (NRBV = 16 or 17). Conclusions - We argue that the Bayesian full mitochondrial DNA phylogeny is superior to previous attempts aiming to recover the crested newt species tree. Furthermore, our new phylogeny involves a maximally parsimonious interpretation of NRBV evolution. Calibrating the phylogeny allows us to evaluate potential drivers for crested newt cladogenesis. The split between the T. karelinii group and the three other morphotypes, at ca. 10.4 Ma, is associated with the separation of the Balkan and Anatolian landmasses (12-9 Ma). No currently known vicariant events can be ascribed to the other two splits, first at ca. 9.3 Ma, separating T. carnifex - T. macedonicus, and second at ca. 8.8 Ma, splitting T. cristatus and T. dobrogicus. The crested newt morphotypes differ in the duration of their annual aquatic period. We speculate on the role that this ecological differentiation could have played during speciatio