The phase diagram of ice: a quasi-harmonic study based on a flexible water model

The phase diagram of ice is studied by a quasi-harmonic approximation. The free energy of all experimentally known ice phases has been calculated with the flexible q-TIP4P/F model of water. The only exception is the high pressure ice X, in which the presence of symmetric O-H-O bonds prevents its modeling with this empirical interatomic potential. The simplicity of our approach allows us to study ice phases at state points of the T-P plane that have been omitted in previous simulations using free energy methods based on thermodynamic integration. The effect in the phase diagram of averaging the proton disorder that appears in several ice phases has been studied. It is found particularly relevant for ice III, at least for cell sizes typically used in phase coexistence simulations. New insight into the capability of the employed water model to describe the coexistence of ice phases is presented. We find that the H-ordered ices IX and XIV, as well as the H-disordered ice XII, are particularly stable for this water model. This fact disagrees with experimental data. The unexpected large stability of ice IX is a property related to the TIP4P-character of the water model. Only after omission of these three stable ice phases, the calculated phase diagram becomes in reasonable qualitative agreement to the experimental one in the T-P region corresponding to ice Ih, II, III, V, and VI. The calculation of the phase diagram in the quantum and classical limits shows that the most important quantum effect is the stabilization of ice II due to its lower zero-point energy when compared to that one of ices Ih, III, and V.Comment: 13 pages, 8 figures, 5 table

Dyes removal from water using low cost absorbents

In this study, the removal capacity of low cost adsorbents during the adsorption of Methylene Blue (MB) and Congo Red (CR) at different concentrations (50 and 100mg•L-1) was evaluated. These adsorbents were produced from wood wastes (cedar and teak) by chemical activation (ZnCl2). Both studied materials, Activated Cedar (AC) and activated teak (AT) showed a good fit of their experimental data to the pseudo second order kinetic model and Langmuir isotherms. The maximum adsorption capacities for AC were 2000.0 and 444.4mg•g-1 for MB and CR, respectively, while for AT, maximum adsorption capacities of 1052.6 and 86.4mg•g-1 were found for MB and CR, respectively. © Published under licence by IOP Publishing Ltd

Expectations of Mentoring: Novice Teachers’ Voices

Mentoring, as an avenue to support and retain new teachers, has received a renewed interest. As Trubowitz suggests, “School systems are finding that beginning teachers who have access to intensive mentoring are less likely to leave teaching” (2004, p. 59). While several factors may cause teachers to leave, alienation has been identified as one of the major forces. According to previous research, teachers experience “a combination of feelings of isolation, normlessness, powerlessness, and meaninglessness” (Benham & O’Brien, 2002, p. 20). Such feelings of isolation are compounded by the current accountability demands and the professional pressure teachers’ experience. Thus, it is imperative to consider alternative strategies aimed at providing the kind of support congruent with beginning teacher’s needs in order to be successful (Breaux & Wong, 2003, p. iii). A goal of such strategies should be the effective socialization of teachers, and providing on-going support for growth, through different approaches including mentoring (Darling-Hammond, 2003; Brennan, Thames, & Roberts, 1999). Although mentoring can be an effective means to enhance teacher efficacy and help beginning teachers (Breaux & Wong, 2003; Delgado, 1999; Yost, 2002), limited research focuses on teachers’ perspectives associated with their own expectations of mentoring, particularly in diverse school settings (Wang & Odell, 2002). Further, researchers suggest, “There has been limited evidence that points to the expectations of new teachers relative to mentoring” (Tillman, 2005, p. 616). Thus, it is essential that teachers’ voices be illuminated to better understand their needs so that school leaders may “consider the benefits of consulting with novice teachers about their expectations in the mentoring arrangement” (Tillman, 2005, p. 626). Much of the current literature on teacher mentoring is based on experiences of mentors (Ganser, 1996; Trubowitz, 2004), and mentoring internship program descriptions (Brennan, Thames, & Roberts, 1999) however, novice teachers’ voices tend to be absent from the discourse. While few studies have focused on teachers’ perceptions (Rowley, 1999; Olebe, Jackson, & Danielson, 1999), additional research is needed so that beginning teachers’ voices contribute to a better understanding of mentoring as a vehicle to reduce isolation, successfully socialize new teachers into the demands of the profession, provide culturally responsive support to novice teachers, and reduce teacher turnover. Such inquiry could also be useful to avoid the common pitfalls that might have a detrimental effect on teachers and students. Therefore, the purpose of this paper is to present the results of a study conducted to examine teachers’ expectations of mentoring

Completely dark galaxies: their existence, properties, and strategies for finding them

There are a number of theoretical and observational hints that large numbers of low-mass galaxies composed entirely of dark matter exist in the field. The theoretical considerations follow from the prediction of cold dark matter theory that there exist many low-mass galaxies for every massive one. The observational considerations follow from the observed paucity of these low-mass galaxies in the field but not in dense clusters of galaxies; this suggests that the lack of small galaxies in the field is due to the inhibition of star formation in the galaxies as opposed to the fact that their small dark matter halos do not exist. In this work we outline the likely properties of low-mass dark galaxies, and describe observational strategies for finding them, and where in the sky to search. The results are presented as a function of the global properties of dark matter, in particular the presence or absence of a substantial baryonic dark matter component. If the dark matter is purely cold and has a Navarro, Frenk and White density profile, directly detecting dark galaxies will only be feasible with present technology if the galaxy has a maximum velocity dispersion in excess of 70 km/s, in which case the dark galaxies could strongly lens background objects. This is much higher than the maximum velocity dispersions in most dwarf galaxies. If the dark matter in galaxy halos has a baryonic component close to the cosmic ratio, the possibility of directly detecting dark galaxies is much more realistic; the optimal method of detection will depend on the nature of the dark matter. A number of more indirect methods are also discussed.Comment: 12 pages, 4 figures, MNRAS in pres

The Stars in M15 Were Born with the r-process

High-resolution spectroscopy of stars on the red giant branch (RGB) of the globular cluster M15 has revealed a large (~1 dex) dispersion in the abundances of r-process elements such as Ba and Eu. Neutron star mergers (NSMs) have been proposed as a major source of the r-process. However, most NSM models predict a delay time longer than the timescale for cluster formation. One possibility is that a NSM polluted the surfaces of stars in M15 long after the cluster finished forming. In this case, the abundances of the polluting elements would decrease in the first dredge-up as stars turn on to the RGB. We present Keck/DEIMOS abundances of Ba in 66 stars along the entire RGB and the top of the main sequence. The Ba abundances have no trend with stellar luminosity (evolutionary phase). Therefore, the stars were born with the Ba that they have today, and Ba did not originate in a source with a delay time longer than the timescale for cluster formation. In particular, if the source of Ba was a NSM, it would have had a very short delay time. Alternatively, if Ba enrichment took place before the formation of the cluster, an inhomogeneity of a factor of 30 in Ba abundance needs to be able to persist over the length scale of the gas cloud that formed M15, which is unlikely