A cost function for similarity-based hierarchical clustering

The development of algorithms for hierarchical clustering has been hampered by a shortage of precise objective functions. To help address this situation, we introduce a simple cost function on hierarchies over a set of points, given pairwise similarities between those points. We show that this criterion behaves sensibly in canonical instances and that it admits a top-down construction procedure with a provably good approximation ratio

Interpreting high [O III]/H β ratios with maturing starbursts

Star-forming galaxies at high redshift show ubiquitously high-ionization parameters, as measured by the ratio of optical emission lines. We demonstrate that local (z < 0.2) sources selected as Lyman break analogues also manifest high line ratios with a typical [O III]/Hβ=3.36+0.14−0.04 – comparable to all but the highest ratios seen in star-forming galaxies at z ∼ 2–4. We argue that the stellar population synthesis code BPASS can explain the high-ionization parameters required through the ageing of rapidly formed star populations, without invoking any AGN contribution. Binary stellar evolution pathways prolong the age interval over which a starburst is likely to show elevated line ratios, relative to those predicted by single stellar evolution codes. As a result, model galaxies at near-solar metallicities and with ages of up to ∼100 Myr after a starburst typically have a line ratio [O III]/Hβ ∼ 3, consistent with those seen in Lyman break galaxies and local sources with similar star formation densities. This emphasises the importance of including binary evolution pathways when simulating the nebular line emission of young or bursty stellar populations

A consistent estimate for gravitational wave and electromagnetic transient rates

Gravitational wave transients, resulting from the merger of two stellar remnants, are now detectable. The properties and rates of these directly relates to the stellar population which gave rise to their progenitors, and thus to other, electromagnetic transients which result from stellar death. We aim to estimate simultaneously the event rates and delay time distribution of gravitational wave-driven compact object mergers together with the rates of core collapse and thermonuclear supernovae within a single consistent stellar population synthesis paradigm. We combine event delay-time distributions at different metallicities from the Binary Population and Spectral Synthesis (BPASS) models with an analytic model of the volume-averaged cosmic star formation rate density and chemical evolution to determine the volume-averaged rates of each event rate at the current time. We estimate rates in excellent agreement with extant observational constraints on core-collapse supernovae, thermonuclear supernovae and long GRBs. We predict rates for gravitational wave mergers based on the same stellar populations, and find rates consistent with current LIGO estimates. We note that tighter constraints on the rates of these events will be required before it is possible to determine their redshift evolution, progenitor metallicity dependence or constrain uncertain aspects of stellar evolution

Controlled Waterfowl Hunting At Lake Odessa, Louisa County, Iowa

The Lake Odessa Public Hunting Area supports two systems of regulated waterfowl hunting. Control Area A consists of 55 marked blind sites which are chosen randomly by hunters during a daily drawing. Hunters using the second area, Control Area B, are not restricted to blind sites and a daily fee is not required, but all parties must possess a valid permit. Hunter use of Control Area A was uniform throughout the 1972 and 1973 hunting seasons due to the better mallard (Anas platy rhynchos) shooting on that area. Control Area B hunter use decreased as the season progressed, reflecting the early migration of wood ducks (Aix sponsa) which were more prevalent. In 1973, a year of poor mallard production but good wood duck production, hunter use of Control Area B increased over the previous year as hunters sought wood ducks. Hunters using Control Area A belonged to higher income, education, and occupation brackets, spent more money on equipment, and drove further to hunt than hunters in Control Area B. Hunter success was positively related to increasing values of vegetation parameters, but the dominant influence was not apparent. A heavy zone of annual emergent vegetation appeared to influence hunter success, but a lack of this zone could be compensated for with a strong representation of buttonbush (Cephalanthus occidentalis). Sites on medium-sized water areas (14-20 ac.) with a strong zone of annual emergent vegetation produced the highest success rates in 1972, but the same or similar sites produced low success rates in 1973 after severe loss of vegetation. Because of reduced annual emergent vegetation on the area and the receding zones of bottonbush, a summer drawdown of water level was recommended

Observational properties of massive black hole binary progenitors

The first directly detected gravitational waves (GW 150914) were emitted by two coalescing black holes (BHs) with masses of ~36Msun and ~29Msun. Several scenarios have been proposed to put this detection into an astrophysical context. The evolution of an isolated massive binary system is among commonly considered models. Various groups have performed detailed binary-evolution calculations that lead to BH merger events. However, the question remains open as to whether binary systems with the predicted properties really exist. The aim of this paper is to help observers to close this gap by providing spectral characteristics of massive binary BH progenitors during a phase where at least one of the companions is still non-degenerate. Stellar evolution models predict fundamental stellar parameters. Using these as input for our stellar atmosphere code (PoWR), we compute a set of models for selected evolutionary stages of massive merging BH progenitors at different metallicities. The synthetic spectra obtained from our atmosphere calculations reveal that progenitors of massive BH merger events start their lives as O2-3V stars that evolve to early-type blue supergiants before they undergo core-collapse during the Wolf-Rayet phase. When the primary has collapsed, the remaining system will appear as a wind-fed high-mass X-ray binary. We provide feedback parameters, broad band magnitudes, and spectral templates that should help to identify such binaries in the future. Comparisons of empirically determined mass-loss rates with those assumed by evolution calculations reveal significant differences. The consideration of the empirical mass-loss rates in evolution calculations will possibly entail a shift of the maximum in the predicted binary-BH merger rate to higher metallicities, that is, more candidates should be expected in our cosmic neighborhood than previously assumed.Comment: 64 pages, 30 figures, accepted for publication in Astronomy & Astrophysics, v2: typos correcte

Gravitational wave spectral synthesis

We study the LISA sources that arise from isolated binary evolution, and how these depend on age and metallicity, using model stellar populations from BPASS. We calculate the combined GW spectrum of all the binaries within these populations, including all types of compact binaries as well as those with living stars, and use these results to investigate the detectability of star clusters with LISA. We find at late times the dominant sources are WD-WD binaries, but at times before approx. $10^9$ years we find a significant population of NS-WD and BH-WD binaries, which is related to the treatment of mass transfer and common envelope events in BPASS, wherein mass transfer is relatively likely to be stable. Metallicity also has an effect on the GW spectrum and on the relative dominance of different types of binaries. Our results suggest that nearby star clusters might produce GWs detectable by LISA or future missions throughout most of their evolution.Comment: 16 pages, 9 figures. Submitted to MNRA

Moderate velocity oblique impact sliding: Production of shocked meteorite textures and palaeomagnetically important metallic spherules in planetary regoliths

We detail the production of metallic spherules in laboratory oblique shock impact experiments, and their applicability (1) to textures in a partly shock-melted chondritic meteorite and (2) to the occurrence of palaeomagnetically important fine iron or iron alloy particles in the lunar regolith. Samples recovered from 29–44 GPa, 800 ns, experiments revealed melting and textures reminiscent of metallic spherules in the Yanzhuang H-chondrite, including “dumbbell” forms and other more complex morphologies. Our experiments demonstrate that metallic spherules can be produced via oblique impact sliding at lower velocities (1.85 km s^(−1)) than are generally assumed in previous work associated with bulk-shock melting, and that oblique impact sliding is a viable mechanism for producing spherules in shock-induced veins in moderately shocked meteorites. Significantly, our experiments also produced fine metallic (iron alloy) spherules within the theoretical narrow size range (a few tens of nanometers for slightly ellipsoidal particles) for stable single-domain (SSD) particles, which are the most important palaeomagnetically, since they can record lunar and planetary magnetic fields over geological time periods. The experiments also produced spherules consistent with superparamagnetic (SP) and multidomain (MD) particle sizes. The fine SSD and SP particles on the lunar surface are currently thought to have been formed predominantly by space weathering processes. Our experiments suggest that oblique shock impact sliding may be a further means of producing the SSD and SP iron or iron alloy particles observed in the lunar regolith, and which are likely to occur in the regoliths of Mercury and other planetary bodies