Black hole head-on collisions and gravitational waves with fixed mesh-refinement and dynamic singularity excision

We present long-term-stable and convergent evolutions of head-on black hole collisions and extraction of gravitational waves generated during the merger and subsequent ring-down. The new ingredients in this work are the use of fixed mesh-refinement and dynamical singularity excision techniques. We are able to carry out head-on collisions with large initial separations and demonstrate that our excision infrastructure is capable of accommodating the motion of the individual black holes across the computational domain as well as their their merger. We extract gravitational waves from these simulations using the Zerilli-Moncrief formalism and find the ring-down radiation to be, as expected, dominated by the l=2, m=0 quasi-normal mode. The total radiated energy is about 0.1 % of the total ADM mass of the system.Comment: Revised version, 1 figure added, accepted for publication in Phys.Rev.D, 15 pages, 10 figures, revtex 4.

Moving black holes via singularity excision

We present a singularity excision algorithm appropriate for numerical simulations of black holes moving throughout the computational domain. The method is an extension of the excision procedure previously used to obtain stable simulations of single, non-moving black holes. The excision procedure also shares elements used in recent work to study the dynamics of a scalarfield in the background of a single, boosted black hole. The robustness of our excision method is tested with single black-hole evolutions using a coordinate system in which the coordinate location of the black hole, and thus the excision boundary, moves throughout the computational domain.Comment: 9 pages and 11 figure

Does individual learning styles influence the choice to use a web-based ECG learning programme in a blended learning setting?

<p>Abstract</p> <p>Background</p> <p>The compressed curriculum in modern knowledge-intensive medicine demands useful tools to achieve approved learning aims in a limited space of time. Web-based learning can be used in different ways to enhance learning. Little is however known regarding its optimal utilisation. Our aim was to investigate if the individual learning styles of medical students influence the choice to use a web-based ECG learning programme in a blended learning setting.</p> <p>Methods</p> <p>The programme, with three types of modules (learning content, self-assessment questions and interactive ECG interpretation training), was offered on a voluntary basis during a face to face ECG learning course for undergraduate medical students. The Index of Learning Styles (ILS) and a general questionnaire including questions about computer and Internet usage, preferred future speciality and prior experience of E-learning were used to explore different factors related to the choice of using the programme or not.</p> <p>Results</p> <p>93 (76%) out of 123 students answered the ILS instrument and 91 the general questionnaire. 55 students (59%) were defined as users of the web-based ECG-interpretation programme. Cronbach's alpha was analysed with coefficients above 0.7 in all of the four dimensions of ILS. There were no significant differences with regard to learning styles, as assessed by ILS, between the user and non-user groups; Active/Reflective; Visual/Verbal; Sensing/Intuitive; and Sequential/Global (p = 0.56-0.96). Neither did gender, prior experience of E-learning or preference for future speciality differ between groups.</p> <p>Conclusion</p> <p>Among medical students, neither learning styles according to ILS, nor a number of other characteristics seem to influence the choice to use a web-based ECG programme. This finding was consistent also when the usage of the different modules in the programme were considered. Thus, the findings suggest that web-based learning may attract a broad variety of medical students.</p

Impact of densitized lapse slicings on evolutions of a wobbling black hole

We present long-term stable and second-order convergent evolutions of an excised wobbling black hole. Our results clearly demonstrate that the use of a densitized lapse function extends the lifetime of simulations dramatically. We also show the improvement in the stability of single static black holes when an algebraic densitized lapse condition is applied. In addition, we introduce a computationally inexpensive approach for tracking the location of the singularity suitable for mildly distorted black holes. The method is based on investigating the fall-off behavior and asymmetry of appropriate grid variables. This simple tracking method allows one to adjust the location of the excision region to follow the coordinate motion of the singularity.Comment: 10 pages, 8 figure

The (Surprising) Efficacy of Academic and Behavioral Intervention with Disadvantaged Youth : Results from a Randomized Experiment in Chicago

There is growing concern that improving the academic skills of disadvantaged youth is too difficult and costly, so policymakers should instead focus either on vocationally oriented instruction for teens or else on early childhood education. Yet this conclusion may be premature given that so few previous interventions have targeted a potential fundamental barrier to school success: "mismatch" between what schools deliver and the needs of disadvantaged youth who have fallen behind in their academic or non-academic development. This paper reports on a randomized controlled trial of a two-pronged intervention that provides disadvantaged youth with non-academic supports that try to teach youth social-cognitive skills based on the principles of cognitive behavioral therapy (CBT), and intensive individualized academic remediation. The study sample consists of 106 male 9th and 10th graders in a public high school on the south side of Chicago, of whom 95% are black and 99% are free or reduced price lunch eligible. Participation increased math test scores by 0.65 of a control group standard deviation (SD) and 0.48 SD in the national distribution, increased math grades by 0.67 SD, and seems to have increased expected graduation rates by 14 percentage points (46%). While some questions remain about the intervention, given these effects and a cost per participant of around $4,400 (with a range of$3,000 to $6,000), this intervention seems to yield larger gains in adolescent outcomes per dollar spent than many other intervention strategies

Does peer learning or higher levels of e-learning improve learning abilities? A randomized controlled trial

Background and aims : The fast development of e-learning and social forums demands us to update our understanding of e-learning and peer learning. We aimed to investigate if higher, pre-defined levels of e-learning or social interaction in web forums improved students&#x2019; learning ability. Methods : One hundred and twenty Danish medical students were randomized to six groups all with 20 students (eCases level 1, eCases level 2, eCases level 2+, eTextbook level 1, eTextbook level 2, and eTextbook level 2+). All students participated in a pre-test, Group 1 participated in an interactive case-based e-learning program, while Group 2 was presented with textbook material electronically. The 2+ groups were able to discuss the material between themselves in a web forum. The subject was head injury and associated treatment and observation guidelines in the emergency room. Following the e-learning, all students completed a post-test. Pre- and post-tests both consisted of 25 questions randomly chosen from a pool of 50 different questions. Results : All students concluded the study with comparable pre-test results. Students at Level 2 (in both groups) improved statistically significant compared to students at level 1 (p&#x003E;0.05). There was no statistically significant difference between level 2 and level 2+. However, level 2+ was associated with statistically significant greater student&#x0027;s satisfaction than the rest of the students (p&#x003E;0.05). Conclusions : This study applies a new way of comparing different types of e-learning using a pre-defined level division and the possibility of peer learning. Our findings show that higher levels of e-learning does in fact provide better results when compared with the same type of e-learning at lower levels. While social interaction in web forums increase student satisfaction, learning ability does not seem to change. Both findings are relevant when designing new e-learning materials

Molecular and Atomic Excitation Stratification in the Outflow of the Planetary Nebula M27

High resolution spectroscopy with FUSE and STIS of atomic and molecular velocity stratification in the nebular outflow of M27 challenge models for the abundance kinematics in planetary nebulae. The simple picture of a very high speed (~ 1000 km/s), high ionization, radiation driven stellar wind surrounded by a slower (~ 10 km/s) mostly molecular outflow, with low ionization and neutral atomic species residing at the wind interaction interface, is not supported... We find ...there is a fast (33 -- 65 km/s) low ionization zone, surrounding a slower (<~ 33 km/s) high ionization zone and, at the transition velocity (33 km/s), vibrationally excited H_2 is intermixed with a predominately neutral atomic medium... Far-UV continuum fluorescence of H_2 is not detected, but Lyman alpha (Lya) fluorescence is present. The diffuse nebular medium is inhospitable to molecules and dust. Maintaining the modest equilibrium abundance of H_2 (N(H_2)/N(HI) << 1) in the diffuse nebular medium requires a source of H_2, mostly likely the clumpy nebular medium. The stellar SED shows no signs of reddening (E(B-V) < 0.01), but paradoxically measurements of Ha/Hb ... indicate E(B-V) ~ 0.1. ...the apparent enhancement of Ha/Hb in the absence of dust may result from a two step process of H_2 ionization by Lyman continuum (Lyc) photons followed by dissociative recombination (H_2 + gamma -> H_2^+ + e -> H(1s) + H (nl)), which ultimately produces fluorescence of Ha and Lya. In the optically thin limit at the inferred radius of the velocity transition we find dissociation of H_2 by stellar Lyc photons is an order of magnitude more efficient than spontaneous dissociation by far-UV photons. We suggest that the importance of this H_2 destruction process in HII regions has been overlooked.Comment: In emulateapj 2 column, 28 pages total, 18 figures. Accepted for publication in the Astrophysical Journal on 5 January 2007. Abstract abridge