Hibernation Revived by Weak Magnetic Braking

Cataclysmic variables undergo periodic nova explosions during which a finite mass of material is expelled on a short timescale. The system widens and, as a result, the mass-transfer rate drops. This state of hibernation may account for the variety of cataclysmic variable types observed in systems of similar mass and period. In the light of recent changes to the theory of nova ignition and magnetic braking we investigate whether hibernation remains a viable mechanism for creating cataclysmic variable diversity. We model the ratio of time spent as dwarf novae (DNe) to nova-like systems (NLs). Above a critical mass-transfer rate the system is NL and below it a DN. The dominant loss of angular momentum is by magnetic braking but the rate is uncertain. It is also uncertain what fraction of the mass accreted is expelled during the novae. We compare the models of the ratios against the period of the system for different magnetic braking rates and different ejected masses with the ratio of the number of observed NLs to DNe. We deduce that a rate of angular momentum loss a factor of ten smaller than that traditionally assumed is necessary if hibernation is to account for the observed ratios

Relationship Between Non-Cognitive Skills And GPA In A Rural Maine Community College

The 21st century has altered the workscape, emphasizing non-cognitive skills required for success in the workforce. Community colleges that profess workforce readiness in their workreadiness programs use primarily cognitive assessment to ensure content and curricular learning. This quantitative study of rural Maine community college graduates examined non-cognitive workforce skills from this workforce readiness institution, correlating non-cognitive instrument scores to the standard cognitive score, cumulative grade point average (GPA). The Social-Emotional Health Survey-Higher Education (SEHS-HE) and the Review of Personal Experience with Locus of Control (ROPELOC) data were examined in relation to cumulative grade point average (GPA) to assess for the presence of non-cognitive schemas in graduating respondents. Statistical analysis of this data revealed that only 15% of the non-cognitive skills assessed were found to be correlated with GPA, while 35-40% of students responding to the surveys graduated from the institution with less than average non-cognitive scores, and scored an average of 35 points less than ideal scores, on both instruments. This study suggests there is little relationship between non-cognitive skills and GPA. Community colleges may need to refine assessments and practices to ensure graduates are truly being prepared for the 21st century workplace

Forecasting Seismic Signatures of Stellar Magnetic Activity

For the Sun, a tight correlation between various activity measures and oscillation frequencies is well documented. For other stars, we have abundant data on magnetic activity and its changes but not yet on its seismic signature. A prediction of the activity induced frequency changes in stars based on scaling the solar relations is presented. This seismic signature of the activity should be measurable in the data expected within few years.Comment: 8 pages, 3 figures, to appear in proceedings of "Unsolved Problems in Stellar Physics" conference (Cambridge, UK, July 2007

Mass loss and supernova progenitors

We first discuss the mass range of type IIP SN progenitors and how the upper and lower limits impose interesting constraints on stellar evolution. Then we discuss the possible implications of two SNe, 2002ap and 2006jc, for Wolf-Rayet star mass-loss rates and long Gamma-ray bursts.Comment: 7 pages, 3 figures, to appear in the Conference Proceedings of "Unsolved Problems in Stellar Astrophysics

The coupling between pulsation and mass loss in massive stars

To what extent can pulsational instabilities resolve the mass-loss problem of massive stars? How important is pulsation in structuring and modulating the winds of these stars? What role does pulsation play in redistributing angular momentum in massive stars? Although I cannot offer answers to these questions, I hope at the very least to explain how they come to be asked.Comment: 12 pages, 1 figure, to appear in proceedings of "Unsolved Problems in Stellar Physics" conference (Cambridge, UK, July 2007

A Search for Pulsation in Very Low-mass Stars and Brown Dwarfs

Brown dwarfs and very low-mass stars constitute a crucial link between the intertwined processes of star formation and planet formation. To date, however, observational methods to uncover their formation mechanism or determine important properties such as mass and age have been lacking. Pulsation powered by deuterium burning in brown dwarfs and very low-mass stars is a newly suggested phenomenon that offers unprecedented opportunities to probe the interiors and evolution of these objects. We report on a photometric campaign to search for low-amplitude pulsations among young star-cluster members using a number of telescopes

Disc precession in Be/X-ray binaries drives superorbital variations of outbursts and colour

Superorbital periods that are observed in the brightness of Be/X-ray binaries may be driven by a misaligned and precessing Be star disc. We examine how the precessing disc model explains the superorbital variation of (i) the magnitude of the observed X-ray outbursts and (ii) the observed colour. With hydrodynamical simulations we show that the magnitude of the average accretion rate on to the neutron star, and therefore the X-ray outbursts, can vary by over an order of magnitude over the superorbital period for Be star spin-orbit misalignments $\gtrsim 70^\circ$ as a result of weak tidal truncation. Most Be/X-ray binaries are redder at optical maximum when the disc is viewed closest to face-on since the disc adds a large red component to the emission. However, A0538-66 is redder at optical minimum. This opposite behaviour requires an edge-on disc at optical minimum and a radially narrow disc such that it does not add a large red signature when viewed face-on. For A0538-66, the misalignment of the disc to the binary orbit must be about $70-80^\circ$ and the inclination of the binary orbit to the line of sight must be similarly high, although restricted to $<75^\circ$ by the absence of X-ray eclipses.Comment: Accepted for publication in MNRA

The effect of different opacity data and chemical element mixture on the Petersen diagram

The Petersen diagram is a frequently used tool to constrain model parameters such as metallicity of radial double-mode pulsators. In this diagram the period ratio of the radial first overtone to the fundamental mode, P_1/P_0, is plotted against the period of the fundamental mode. The period ratio is sensitive to the chemical composition as well as to the rotational velocity of a star. In the present study we compute stellar pulsation models to demonstrate the sensitivity of the radial period ratio to the opacity data (OPAL and OP tables) and we also examine the effect of different relative abundances of heavy elements. We conclude that the comparison with observed period ratios could be used successfully to test the opacity data.Comment: 5 pages, 5 figures, 1 table; to be published in the Proceedings of the Conference 'Unsolved Problems in Stellar Physics', Cambridge, 2-6 July 200

A practical model of convective dynamics for stellar evolution calculations

Turbulent motions in the interior of a star play an important role in its evolution, since they transport chemical species, thermal energy and angular momentum. Our overall goal is to construct a practical turbulent closure model for convective transport that can be used in a multi-dimensional stellar evolution calculation including the effects of rotation, shear and magnetic fields. Here, we focus on the first step of this task: capturing the well-known transition from radiative heat transport to turbulent convection with and without rotation, as well as the asymptotic relationship between turbulent and radiative transport in the limit of large Rayleigh number. We extend the closure model developed by Ogilvie (2003) and Garaud and Ogilvie (2005) to include heat transport and compare it with experimental results of Rayleigh-Benard convection.Comment: Conference proceeding for poster at conference "Unsolved problems in Stellar Physics