Follow-up Observations of the Second and Third Known Pulsating Hot DQ White Dwarfs

We present follow-up time-series photometric observations that confirm and extend the results of the significant discovery made by Barlow et al.(2008) that the Hot DQ white dwarfs SDSS J220029.08-074121.5 and SDSS J234843.30-094245.3 are luminosity variable. These are the second and third known members of a new class of pulsating white dwarfs, after the prototype SDSS J142625.71+575218.3 (Montgomery et al. 2008). We find that the light curve of SDSS J220029.08-074121.5 is dominated by an oscillation at 654.397+-0.056 s, and that the light pulse folded on that period is highly nonlinear due to the presence of the first and second harmonic of the main pulsation. We also present evidence for the possible detection of two additional pulsation modes with low amplitudes and periods of 577.576+-0.226 s and 254.732+-0.048 s in that star. Likewise, we find that the light curve of SDSS J234843.30-094245.3 is dominated by a pulsation with a period of 1044.168+-0.012 s, but with no sign of harmonic components. A new oscillation, with a low amplitude and a period of 416.919+-0.004 s, is also probably detected in that second star. We argue, on the basis of the very different folded pulse shapes, that SDSS J220029.08-074121.5 is likely magnetic, while SDSS J234843.30-094245.3 is probably not.Comment: 12 pages, 19 figures, accepted for publication in Ap

Follow-up Studies of the Pulsating Magnetic White Dwarf SDSS J142625.71+575218.3

We present a follow-up analysis of the unique magnetic luminosity-variable carbon-atmosphere white dwarf SDSS J142625.71+575218.3. This includes the results of some 106.4 h of integrated light photometry which have revealed, among other things, the presence of a new periodicity at 319.720 s which is not harmonically related to the dominant oscillation (417.707 s) previously known in that star. Using our photometry and available spectroscopy, we consider the suggestion made by Montgomery et al. (2008) that the luminosity variations in SDSS J142625.71+575218.3 may not be caused by pulsational instabilities, but rather by photometric activity in a carbon-transferring analog of AM CVn. This includes a detailed search for possible radial velocity variations due to rapid orbital motion on the basis of MMT spectroscopy. At the end of the exercise, we unequivocally rule out the interacting binary hypothesis and conclude instead that, indeed, the luminosity variations are caused by g-mode pulsations as in other pulsating white dwarfs. This is in line with the preferred possibility put forward by Montgomery et al. (2008).Comment: 11 pages in emulateApJ, 12 figures, accepted for publication in Ap

Separation of the monopole contribution to the nuclear Hamiltonian

It is shown that the nuclear Hamiltonian can be separated rigorously as ${\cal H}={\cal H}_m+{\cal H}_M.$ For sufficiently smooth forces the monopole part ${\cal H}_m$is entirely responsible for Hartree-Fock selfconsistency and hence saturation properties. The multipole part ${\cal H}_M$ contains the ``residual'' terms - pairing, quadrupole, etc. - that will be analyzed in a companion paper. We include a review of the basic results often needed when working with multipole decompositions and average monopole properties.Comment: 20 Pages, RevTeX 3.

Might Carbon-Atmosphere White Dwarfs Harbour a New Type of Pulsating Star?

In the light of the recent and unexpected discovery of a brand new type of white dwarfs, those with carbon-dominated atmospheres, we examine the asteroseismological potential of such stars. The motivation behind this is based on the observation that past models of carbon-atmosphere white dwarfs have partially ionized outer layers that bear strong resemblance with those responsible for mode excitation in models of pulsating DB (helium-atmosphere) and pulsating DA (hydrogen-atmosphere) white dwarfs. Our exciting main result is that, given the right location in parameter space, some carbon-atmosphere white dwarfs are predicted to show pulsational instability against gravity modes. We are eagerly waiting the results of observational searches for luminosity variations in these stars.Comment: 4-page letter + 4 figure

Causes and biophysical consequences of cellulose production by Pseudomonas fluorescens SBW25 at the air-liquid interface

Cellulose over-producing wrinkly spreader mutants of Pseudomonas fluorescens SBW25 have been the focus of much investigation, but conditions promoting the production of cellulose in ancestral SBW25, its effects and consequences have escaped in-depth investigation through lack of in vitro phenotype. Here, using a custom built device, we reveal that in static broth microcosms ancestral SBW25 encounters environmental signals at the air-liquid interface that activate, via three diguanylate cyclase-encoding pathways (Wsp, Aws and Mws), production of cellulose. Secretion of the polymer at the meniscus leads to modification of the environment and growth of numerous micro-colonies that extend from the surface. Accumulation of cellulose and associated microbial growth leads to Rayleigh-Taylor instability resulting in bioconvection and rapid transport of water-soluble products over tens of millimetres. Drawing upon data we build a mathematical model that recapitulates experimental results and captures the interactions between biological, chemical and physical processes.IMPORTANCE This work reveals a hitherto unrecognized behaviour that manifests at the air-liquid interface, which depends on production of cellulose, and hints to undiscovered dimensions to bacterial life at surfaces. Additionally, the study links activation of known diguanylate cyclase-encoding pathways to cellulose expression and to signals encountered at the meniscus. Further significance stems from recognition of the consequences of fluid instabilities arising from surface production of cellulose for transport of water-soluble products over large distances