Atmospheric parameters and carbon abundance for hot DB white dwarfs

Atmospheric parameters for hot DB (helium atmosphere) white dwarfs near effective temperatures of 25000K are extremely difficult to determine from optical spectroscopy. This is particularly unfortunate, because this is the range of variable DBV or V777 Her stars. Accurate atmospheric parameters are needed to help or confirm the asteroseismic analysis of these objects. Another important aspect is the new class of white dwarfs - the hot DQ - detected by Dufour et al. (2007), with spectra dominated by carbon lines. The analysis shows that their atmospheres are pure carbon. The origin of these stars is not yet understood, but they may have an evolutionary link with the hotter DBs as studied here. Our aim is to determine accurate atmospheric parameters and element abundances and study the implications for the evolution white dwarfs of spectral classes DB and hot DQ. High resolution UV spectra of five DBs are studied with model atmospheres. We determine stellar parameters and abundances or upper limits of C and Si. These objects are compared with cooler DBs below 20000K. We find photospheric C and no other heavy elements - with extremely high limits on the C/Si ratio - in two of the five hot DBs. We compare various explanations for this unusual composition, which have been proposed in the literature: accretion of interstellar or circumstellar matter, radiative levitation, carbon dredge-up from deeper interior below the helium layer, and a residual stellar wind. None of these explanations is completely satisfactory, and the problem of the origin of the hot DQ remains an open question

Understanding Experience: Reflections on the Empowering Nature of Story

Technological growth has changed our relationships and interactions within society and theatre artists are calling into question the future of our art form. Are we still essential? And if so, how do we renovate our form in order to relate to our changing society? In my experience, I’ve found that all renovations of our art have one thing in common: the empowering nature of story. Story helps us to understand our experiences in life. It is not the self, the cause, or the goal that is behind the wheel, but the story itself. This thesis explores three instances of the empowering nature of story during my graduate studies

Sleeping with Herodotus in The English Patient

Michael Ondaatje's narratological view of history places him in the proximity of postcolonial/postmodern historicism that treats "history" as "text." But while The English Patient offers a postcolonial/postmodern critique of The Histories, Herodotus cannot simply be dismissed; he is literally bound up with the English patient's life. Ondaatje bifurcates Herodotus into main and supplemental arguments, corresponding to imperial and existential histories — a bifurcation that does not exist in The Histories. Such an interpretation is a result of Ondaatje's ongoing reflection on the "text" of Western history in light of its postcolonial/postmodern deconstruction

Social Sciences and Humanities Research and the Public Good: A Synthesis of Presentations and Discussions

In May 2010, with the support of funds from the Social Sciences and Humanities Research Council (SSHRC) of Canada, a one-day workshop, entitled, “Social Sciences and Humanities Research as a Public Good: Identifying Research Prospects for Advancing Research Among Academic and Non-Academic Discourse Communities” was held in Montreal, Québec. The workshop brought together Canadian stakeholders involved in extending the reach of research (for the public good), including those involved in open access and knowledge mobilization, as well as organizations linked to the research community, and non-academic organizations with a clear mandate to include research in their activities or to extend the reach of research. This article presents a summary of the workshop presentations and a synthesis of the workshop discussions. The article also provides a discussion of the emergent issues arising from the workshop (such as the sustainability of open access journal publishing, the challenges of knowledge mobilization, and the limited media uptake of social sciences and humanities research), areas of inquiry that these issues open up (engaged scholarship and the engaged university, faculty reward structures, and public knowledge/knowledge mobilization as areas of scholarly inquiry), and collaborative next steps for stakeholders to take, to address concerns raised and to seize opportunities to advance shared interests.&nbsp

Seven-Period Asteroseismic Fit of the Kepler DBV

We present a new, better-constrained asteroseismic analysis of the helium-atmosphere (DB) white dwarf discovered in the field of view of the original Kepler mission. Observations obtained over the course of two years yield at least seven independent modes, two more than were found in the discovery paper for the object. With several triplets and doublets, we are able to fix the $\ell$ and $\rm{m}$ identification of several modes before performing the fitting, greatly reducing the number of assumptions we must make about mode identification. We find a very thin helium layer for this relatively hot DB, which adds evidence to the hypothesis that helium diffuses outward during DB cooling. At least a few of the modes appear to be stable on evolutionary timescales and could allow us to obtain a measurement of the rate of cooling with monitoring of the star over the course of the next few years with ground-based follow-up.Comment: 24 pages, 4 figures. 2 tables. Published fall 2014 in the Astrophysical Journa

Discovery of the Eclipsing Detached Double White Dwarf Binary NLTT 11748

We report the discovery of the first eclipsing detached double white dwarf (WD) binary. In a pulsation search, the low-mass helium core WD NLTT 11748 was targeted for fast (approx 1 minute) differential photometry with the Las Cumbres Observatory's Faulkes Telescope North. Rather than pulsations, we discovered approx 180 s 3%-6% dips in the photometry. Subsequent radial velocity measurements of the primary white dwarf from the Keck telescope found variations with a semi-amplitude K_1 = 271 +/- 3 km/s, and confirmed the dips as eclipses caused by an orbiting WD with a mass M_2 = 0.648-0.771 M_sun for M_1 = 0.1-0.2 M_sun. We detect both the primary and secondary eclipses during the P_orb = 5.64 hr orbit and measure the secondary's brightness to be 3.5% +/- 0.3% of the primary at SDSS-g'. Assuming that the secondary follows the mass-radius relation of a cold C/O WD and including the effects of microlensing in the binary, the primary eclipse yields a primary radius of R_1 = 0.043-0.039 R_sun for M_1 = 0.1-0.2 M_sun, consistent with the theoretically expected values for a helium core WD with a thick, stably burning hydrogen envelope. Though nearby (at approx 150 pc), the gravitational wave strain from NLTT 11748 is likely not adequate for direct detection by the Laser Interferometer Space Antenna. Future observational efforts will determine M_1, yielding accurate WD mass-radius measurement of both components, as well as a clearer indication of the binary's fate once contact is reached.Comment: 5 pages, 3 figures: Accepted to ApJ Letters, 2010 May 11. v2 corrects a typo in ephemeris and impliments redline correction