Developmental Education Repeaters: Stories About Repetition

Developmental education students make up almost half of the community college population in the United States (Bettinger & Long, 2005). Approximately 42% of first-time freshmen at community colleges must enroll in at least one developmental education course in English, reading and/or math (NCES, 2010). Many developmental education students are unsuccessful in passing a developmental education course in their first and second attempts and retake the course sometimes five times before passing. There is substantial research on persistence among college students, but the research fails to link persistence to developmental education repeaters. My study sought to explore community college developmental education repeaters’ experiences with and stories about repetition in a reading course. My study was framed around developmental education and its students, course repeaters, and persistence. I used qualitative research methods with a narrative research design. Two methods of data collection included multiple one-on-one interviews and document collection. Four participants were selected from one community college in the New Orleans area, two who repeated and completed developmental reading upon their third attempt and two who were in the process of completing developmental reading a third time. Data analysis revealed six themes. The information gleaned from the inquiry may inform community college faculty practice with regard to not only reducing and preventing course repetition but also increasing persistence and retention of developmental education students

Developmental Education Repeaters: Stories About Repetition

Developmental education students make up almost half of the community college population in the United States (Bettinger & Long, 2005). Approximately 42% of first-time freshmen at community colleges must enroll in at least one developmental education course in English, reading and/or math (NCES, 2010). Many developmental education students are unsuccessful in passing a developmental education course in their first and second attempts and retake the course sometimes five times before passing. There is substantial research on persistence among college students, but the research fails to link persistence to developmental education repeaters. My study sought to explore community college developmental education repeaters’ experiences with and stories about repetition in a reading course. My study was framed around developmental education and its students, course repeaters, and persistence. I used qualitative research methods with a narrative research design. Two methods of data collection included multiple one-on-one interviews and document collection. Four participants were selected from one community college in the New Orleans area, two who repeated and completed developmental reading upon their third attempt and two who were in the process of completing developmental reading a third time. Data analysis revealed six themes. The information gleaned from the inquiry may inform community college faculty practice with regard to not only reducing and preventing course repetition but also increasing persistence and retention of developmental education students

Development of Electroanalytical Techniques to Quantify Silver Nanoparticle Dissolution, Aggregation, and Release Kinetics

Recently, the manufacture of engineered nanomaterials has seen an increase worldwide. This is due to the desirable properties of materials at the nanoscale rather than the bulk scale, such as improved optical, electronic and magnetic properties. Silver nanoparticles (AgNPs) are one of the fastest growing nanomaterials to be incorporated into consumer products due to silver’s well known antibacterial and antimicrobial properties. AgNP-enhanced products represent the largest proportion of engineered nanomaterial products on the consumer market, despite questions regarding the life cycle of such products. AgNPs can undergo a number of transformations during their life cycle including dissolution, aggregation, and protein corona formation. Moreover, when incorporated into consumer products, silver can be released in a number of ways, all of which depend on how the nanoparticulate silver was originally incorporated into the product. The release of silver species can have impacts on human and environmental health. Thus, the development of affordable, reliable, and efficient methods of detecting AgNP transformations and release mechanisms is required and was the primary goal of this work. Electrochemical techniques including linear sweep stripping voltammetry (LSSV) and particle impact voltammetry coupled with UV-vis spectroscopy (PIV/UV-vis) were used to measure Ag(I) and AgNPs in solution, respectively. Specifically, LSSV was used to quantify the dissolution kinetics of AgNPs (release of Ag(I)), while PIV/UV-vis was used to quantify aggregation kinetics and determine colloidal parameters like the critical coagulation concentration (CCC). The optimization of each technique and proof of concept experiments are presented and show that both techniques provide rapid, reproducible quantitative data that is well-supported by other studies in the literature. Finally, these two techniques were coupled to quantify the release kinetics of Ag(I) and in-tact AgNPs from AgNP-enabled cotton fabrics, in an effort to gain insight into silver release mechanisms. Preliminary data suggest that the combined LSSV-PIV/UV-vis technique has significant promise for in situ quantification and speciation of released silver and provides several advantages over current techniques. Overall, the work presented herein demonstrates the successful development and application of rapid, affordable and quantitative electroanalytical techniques to evaluate AgNP transformations in situ

Structure and Physical Conditions in the Huygens Region of the Orion Nebula

Hubble Space Telescope images, MUSE maps of emission lines, and an atlas of high velocity resolution emission-line spectra have been used to establish for the first time correlations of the electron temperature, electron density, radial velocity, turbulence, and orientation within the main ionization front of the nebula. From the study of the combined properties of multiple features, it is established that variations in the radial velocity are primarily caused by the photoevaporating ionization front being viewed at different angles. There is a progressive increase of the electron temperature and density with decreasing distance from the dominant ionizing star θ1 Ori C. The product of these characteristics (ne × Te) is the most relevant parameter in modelling a blister-type nebula like the Huygens region, where this quantity should vary with the surface brightness in Hα. Several lines of evidence indicate that small-scale structure and turbulence exist down to the level of our resolution of a few arcseconds. Although photoevaporative flow must contribute at some level to the well-known non-thermal broadening of the emission lines, comparison of quantitative predictions with the observed optical line widths indicates that it is not the major additive broadening component. Derivation of Te values for H+ from radio+optical and optical-only ionized hydrogen emission showed that this temperature is close to that derived from [N II] and that the transition from the well-known flat extinction curve which applies in the Huygens region to a more normal steep extinction curve occurs immediately outside of the Bright Bar feature of the nebula

Measurement and Interpretation of Deuterium-Line Emission in the Orion Nebula

We present new observations of the deuterium and hydrogen Balmer lines in the Orion Nebula. There is a real variation in the deuterium-to-hydrogen line ratios across the nebula, being greatest in the emission from the largest proplyd (Orion 244-440). We also present the results of a detailed model for the emission of these lines, the hydrogen lines being the result of photoionization and recombination while the deuterium lines are produced by fluorescent excitation of the upper energy states by the far-UV radiation from θ1 Ori C. Comparison of the observations and predictions of the line intensities shows good agreement, both in the strength of the reference lines at Hβ and also in the differences of the Balmer decrement for the two atoms. The fact that both the deuterium and hydrogen emissions arise from mechanisms that count the near-ultraviolet (deuterium) and photoionizing ultraviolet (hydrogen) photons from the dominant star means that there is little prospect of similar observations being useful for determination of D/H abundances in H II regions. Based in part on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Based in part on observations obtained at the Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the National Science Foundation. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation

A Multi-Instrument Study of the Helix Nebula Knots with the \u3cem\u3eHubble Space Telescope\u3c/em\u3e

We have conducted a combined observational and theoretical investigation of the ubiquitous knots in the Helix Nebula (NGC 7293). We have constructed a combined hydrodynamic + radiation model for the ionized portion of these knots and have accurately calculated a static model for their molecular regions. Imaging observations in optical emission lines were made with the Hubble Space Telescope\u27s STIS, operating in a slitless mode, complemented by WFPC2 images in several of the same lines. The NICMOS camera was used to image the knots in H2. These observations, when combined with other studies of H2 and CO, provide a complete characterization of the knots. They possess dense molecular cores of densities about 106 cm-3 surrounded on the central star side by a zone of hot H2. The temperature of the H2-emitting layer defies explanation either through detailed calculations for radiative equilibrium or through simplistic calculations for shock excitation. Farther away from the core is the ionized zone, whose peculiar distribution of emission lines is explained by the expansion effects of material flowing through this region. The shadowed region behind the core is the source of most of the CO emission from the knot and is of the low temperature expected for a radiatively heated molecular region

Structure and Physical Conditions in the Huygens Region of the Orion Nebula

Hubble Space Telescope images, MUSE maps of emission lines, and an atlas of high velocity resolution emission-line spectra have been used to establish for the first time correlations of the electron temperature, electron density, radial velocity, turbulence, and orientation within the main ionization front of the nebula. From the study of the combined properties of multiple features, it is established that variations in the radial velocity are primarily caused by the photoevaporating ionization front being viewed at different angles. There is a progressive increase of the electron temperature and density with decreasing distance from the dominant ionizing star θ1 Ori C. The product of these characteristics (ne × Te) is the most relevant parameter in modelling a blister-type nebula like the Huygens region, where this quantity should vary with the surface brightness in Hα. Several lines of evidence indicate that small-scale structure and turbulence exist down to the level of our resolution of a few arcseconds. Although photoevaporative flow must contribute at some level to the well-known non-thermal broadening of the emission lines, comparison of quantitative predictions with the observed optical line widths indicates that it is not the major additive broadening component. Derivation of Te values for H+ from radio+optical and optical-only ionized hydrogen emission showed that this temperature is close to that derived from [N II] and that the transition from the well-known flat extinction curve which applies in the Huygens region to a more normal steep extinction curve occurs immediately outside of the Bright Bar feature of the nebula

Studies of NGC 6720 with Calibrated \u3cem\u3eHST\u3c/em\u3e/WFC3 Emission-line Filter Images. III. Tangential Motions using AstroDrizzle Images

We have been able to compare with astrometric precision AstroDrizzle processed images of NGC 6720 (the Ring Nebula) made using two cameras on the Hubble Space Telescope. The time difference of the observations was 12.925 yr. This large time base allowed the determination of tangential velocities of features within this classic planetary nebula. Individual features were measured in [N II] images as were the dark knots seen in silhouette against background nebular [O III] emission. An image magnification and matching technique was also used to test the accuracy of the usual assumption of homologous expansion. We found that homologous expansion does apply, but the rate of expansion is greater along the major axis of the nebula, which is intrinsically larger than the minor axis. We find that the dark knots expand more slowly than the nebular gas, that the distance to the nebula is 720 pc ±30%, and that the dynamic age of the Ring Nebula is about 4000 yr. The dynamic age is in agreement with the position of the central star on theoretical curves for stars collapsing from the peak of the asymptotic giant branch to being white dwarfs

The Abundance Discrepancy Factor and \u3cem\u3et\u3c/em\u3e\u3csup\u3e2\u3c/sup\u3e in Nebulae: Are Non-Thermal Electrons the Culprits?

We discuss recent claims that the free electrons in ionized nebulae may not have a significantly Maxwellian velocity distribution. Supra-thermal electrons, electrons with much more energy than is encountered at electron temperatures found in nebulae, may solve the t2/ADF puzzle, the observations that abundances obtained from recombination and collisionally excited lines do not agree, and that different temperature indicators give different results. These non-Maxwellian electrons can be designated by the kappa formalism. We show that the distance over which heating rates change are much longer than the distance supra-thermal electrons can travel, and that the timescale to thermalize these electrons are much shorter than the heating or cooling timescales. These estimates show that supra-thermal electrons will have disappeared into the Maxwellian velocity distribution long before they affect the collisionally-excited forbidden and recombination lines, so the electron velocity distribution will be closely thermal

Studies of NGC 6720 with Calibrated \u3cem\u3eHST\u3c/em\u3e/WFC3 Emission-line Filter Images. II. Physical Conditions

We have performed a detailed analysis of the electron temperature and density in the Ring Nebula using the calibrated Hubble Space Telescope WFC3 images described in the preceding paper. The electron temperature (Te) determined from [NII] and [O III] rises slightly and monotonically toward the central star. The observed equivalent width (EW) in the central region indicates that Te rises as high as 13,000 K. In contrast, the low EWs in the outer regions are largely due to scattered diffuse Galactic radiation by dust. The images allowed determination of unprecedented small-scale variations in Te. These variations indicate that the mean square area temperature fluctuations are significantly higher than expected from simple photoionization. The power producing these fluctuations occurs at scales of less than 3.5 × 1015 cm. This scale length provides a strong restriction on the mechanism causing the large t2 values observed