Ruprecht 55: an OB association at the edge of our Galaxy

We present new spectroscopy in the optical range and 21-cm H  i data covering the Ruprecht 55 (Ru 55) field in the Puppis window where several authors have proposed the existence of one (or two) clusters.
We have determined new MK spectral types for about 50 stars in the region, finding 43 OB-type stars among them. LS 985 was found to be an O9 V + O9.5 III binary and it is the earliest type of star in our observed sample.
We have identified a stellar OB association (Ru 55), which is most likely related to a depletion detected in our H  i data, as: (i) they are located at the same distance (6 kpc), within observational errors; (ii) both have similar radial velocities (∼67 km s⁻¹); (iii) current OB stars could have provided the energy needed to blow the cavity; (iv) the dynamical time-scale for the hole buildup matches the age estimated for the earliest OB stars; and (v) LS 985 might be responsible for ionizing the H  i cavity inner walls close to it.     

Geochronological data from TTG-type rock associations of the Arroio dos Ratos Complex and implications for crustal evolution of southernmost Brazil in Paleoproterozoic times

U–Pb isotope analyses by LA-MC-ICPMS (Laser Ablation – Multi Collector – Inductively Coupled Plasma Mass Spectrometry) in zircon crystals from metatonalites, tonalites and granodiorite gneiss from the Arroio dos Ratos Complex (ARC) early magmatism in southernmost Brazil are presented. The ARC is located in the eastern portion of the Sul-rio-grandense Shield, occurring as septa and roof pendants on granitoids emplaced along the Southern Brazilian Shear Belt (SBSB). The SBSB corresponds to a translithospheric structure composed of several anastomosed shear zones of dominantly transcurrent kinematics whose syntectonic magmatism, of Neoproterozoic age, is characteristic of post-collisional environments. The studied rocks comprise TTG-type associations with coeval mafic magmatism, deformed and metamorphosed within a ductile shear zone. Zircon crystals obtained from six samples are interpreted as igneous given that the crystals are subhedral to euhedral, bipyramidal, with concentric zonation, have ratios Th/U between 0.13 and 0.81 and have restricted evidence of overgrowth. The oldest Association 1 (A1) has structures compatible with recrystallization under conditions of high temperature and an igneous age of 2148 ± 33 Ma, obtained in a metatonalite. The rocks of Association 2 (A2) have similar compositions, although with a more significant coeval mafic fraction. They are intrusive into A1 and also show high-temperature recrystallization features. However, they are less deformed and partly preserve their primary, igneous fabric. The igneous ages obtained from two A2 tonalites are 2150 ± 28 Ma and 2136 ± 27 Ma. Association 3 (A3) is represented by tonalitic to granodioritic gneisses whose structure, composition and metamorphic features are similar to those of A1 rocks, except for the absence of coeval mafic magmas in the former. Local features resulting from partial melting are present in A3 rocks. Three samples from A3 were dated. A tonalitic gneiss gives igneous age of 2099 ± 10 Ma and two granodioritic gneisses give igneous ages of 2081 ± 7 Ma and 2077 ± 13 Ma. Restricted to A1, inheritance is represented by one subhedral, zoned, gently rounded zircon crystal interpreted as igneous, of 2732 ± 40 Ma (²⁰⁷Pb/²⁰⁶Pb age), with discordance of 9% and ²³²Th/²³⁸U ratio of 1.17. A single Neoproteozoic metamorphic date value was obtained from the rim of a zircon crystal of Paleoproterozoic core. The age of 635 ± 6 Ma (²⁰⁷Pb/²⁰⁶Pb age), with Th/U ratio < 0.1 and 1% discordance, is interpreted as compatible with adjacent SBSB magmatism. The three associations are interpreted to represent the record of successive magmatic pulses that mark the evolution of a Paleoproterozoic continental magmatic arc. In the study area, these magmatic arc associations represent relict areas partly reworked and relatively well-preserved from Neoproterozoic tectono-magmatic post-collisional events during the construction of the Southern Brazilian Shear Belt.     

Participants or customers in water governance? Community-public partnerships for peri-urban water supply

We examine the performance of water user associations (WUAs) and the role of actors, power relations, socio-institutional dynamics, and context in supplying water to poor urban and peri-urban neighborhoods of Malawi’s two major cities. Using a preliminary survey, key-informant interviews, focus groups, secondary data, and insights from the community-based natural resources management (CBNRM) literature and common-pool resources (CPR) theory, we argue that while a business-based WUA model can enhance water supply and access, the urban/peri-urban and historical context alters the nature of water and social actors and power relations involved, causing tradeoffs between water-supply and social goals of ownership, participation, and empowerment. The ensuing tradeoffs demonstrate that water supply to the urban/peri-urban landscape through community-based initiatives require flexibility in CBNRM expectations.     

The Jeans mass and the origin of the knee in the IMF

We use numerical simulations of the fragmentation of a  1000 M_⊙  molecular cloud and the formation of a stellar cluster to study how the initial conditions for star formation affect the resulting initial mass function (IMF). In particular, we are interested in the relation between the thermal Jeans mass in a cloud and the knee of the IMF, i.e. the mass separating the region with a flat IMF slope from that typified by a steeper, Salpeter-like, slope. In three isothermal simulations with   M _Jeans= 1, 2  and  5 M_⊙  , the number of stars formed, at comparable dynamical times, scales roughly with the number of initial Jeans masses in the cloud. The mean stellar mass also increases (though less than linearly) with the initial Jeans mass in the cloud. It is found that the IMF in each case displays a prominent knee, located roughly at the mass scale of the initial Jeans mass. Thus clouds with higher initial Jeans masses produce IMFs which are shallow to higher masses. This implies that a universal IMF requires a physical mechanism that sets the Jeans mass to be near  1 M_⊙  . Simulations including a barotropic equation of state as suggested by Larson, with cooling at low densities followed by gentle heating at higher densities, are able to produce realistic IMFs with the knee located at  ≈1 M_⊙  , even with an initial   M _Jeans= 5 M_⊙  . We therefore suggest that the observed universality of the IMF in the local Universe does not require any fine tuning of the initial conditions in star forming clouds but is instead imprinted by details of the cooling physics of the collapsing gas.     

Contamination and exclusion in the σ Orionis young group

We present radial velocities for 38 low-mass candidate members of the σ Orionis young group. We have measured their radial velocities by cross-correlation of high-resolution  ( R ≈ 6000) AF2/Wide  Field Fibre Optical Spectrograph (WYFFOS) spectra of the gravity-sensitive Na  i doublet at 8183, 8195 Å. The total sample contained 117 objects, of which 54 have sufficient signal-to-noise ratio to detect Na  i at an equivalent width of 3 Å; however, we only detect Na  i in 38 of these. This implies that very low-mass members of this young group display weaker Na  i absorption than similarly aged objects in the Upper Scorpius OB association. We develop a technique to assess membership using radial velocities with a range of uncertainties that does not bias the selection when large uncertainties are present. The resulting membership probabilities are used to assess the issue of exclusion in photometric selections, and we find that very few members are likely to be excluded by such techniques. We also assess the level of contamination in the expected pre-main-sequence region of colour–magnitude space brighter than   I = 17  . We find that contamination by non-members in the expected pre-main-sequence region of the colour–magnitude diagram is small. We conclude that although radial velocity alone is insufficient to confirm membership, high signal-to-noise ratio observations of the Na  i doublet provide the opportunity to use the strength of Na  i absorption in concert with radial velocities to asses membership down to the lowest masses, where lithium absorption no longer distinguishes youth.     

Modelling the components of binaries in the Hyades: the dependence of the mixing-length parameter on stellar mass

We present our findings based on a detailed analysis of the binaries of the Hyades, in which the masses of the components are well known. We fit the models of the components of a binary system to observations so as to give the observed total V and B − V of that system and the observed slope of the main sequence in the corresponding parts. According to our findings, there is a very definite relationship between the mixing-length parameter and the stellar mass. The fitting formula for this relationship can be given as  α= 9.19( M /M_⊙− 0.74)^0.053− 6.65  , which is valid for stellar masses greater than  0.77 M_⊙  . While no strict information is gathered for the chemical composition of the cluster, as a result of degeneracy in the colour–magnitude diagram, by adopting   Z = 0.033  and using models for the components of 70 Tau and θ² Tau we find the hydrogen abundance to be   X = 0.676  and the age to be 670 Myr. If we assume that   Z = 0.024  , then   X = 0.718  and the age is 720 Myr. Our findings concerning the mixing-length parameter are valid for both sets of the solution. For both components of the active binary system V818 Tau, the differences between radii of the models with   Z = 0.024  and the observed radii are only about 4 per cent. More generally, the effective temperatures of the models of low-mass stars in the binary systems studied are in good agreement with those determined by spectroscopic methods.