High-resolution historical records from Pettaquamscutt River basin sediments: 1. Pb and varve chronologies validate record of Cs released by the Chernobyl accident

Cesium-137 derived from the explosion of the Chernobyl reactor in 1986 was preserved in anoxic sediments from a coastal environment in southern Rhode Island. Although the radioactive plume was detected in surface air samples at several locations in the United States, this is the first known record of a Chernobyl ¹³⁷Cs peak in sediments from North America. The inventory of Chernobyl ¹³⁷Cs that was preserved in the Pettaquamscutt River is small compared to European counterparts and should only be detectable for the next 15-20 yr. However, the presence of two ¹³⁷Cs peaks (1963 and 1987) identifies a well-dated segment of the sediment column that could be exploited in understanding the decomposition and preservation of terrestrial and aquatic organic matter. Different methods for calculating the ²¹⁰Pb chronology were also evaluated in this study and checked against independent varve counting. The end result is a detailed chronology of a site well suited for reconstruction of historical records of environmental change.     

Effects of mass loss for highly-irradiated giant planets

We present calculations for the evolution and surviving mass of highly-irradiated extrasolar giant planets (EGPs) at orbital semimajor axes ranging from 0.023 to 0.057 AU using a generalized scaled theory for mass loss, together with new surface-condition grids for hot EGPs and a consistent treatment of tidal truncation. Theoretical estimates for the rate of energy-limited hydrogen escape from giant-planet atmospheres differ by two orders of magnitude, when one holds planetary mass, composition, and irradiation constant. Baraffe et al. [Baraffe, I., Selsis, F., Chabrier, G., Barman, T.S., Allard, F., Hauschildt, P.H., Lammer, H., 2004. Astron. Astrophys. 419, L13-L16] predict the highest rate, based on the theory of Lammer et al. [Lammer, H., Selsis, F., Ribas, I., Guinan, E.F., Bauer, S.J., Weiss, W.W., 2003. Astrophys. J. 598, L121-L124]. Scaling the theory of Watson et al. [Watson, A.J., Donahue, T.M., Walker, J.C.G., 1981. Icarus 48, 150-166] to parameters for a highly-irradiated exoplanet, we find an escape rate ∼¹⁰² lower than Baraffe's. With the scaled Watson theory we find modest mass loss, occurring early in the history of a hot EGP. In this theory, mass loss including the effect of Roche-lobe overflow becomes significant primarily for masses below a Saturn mass, for semimajor axes ?0.023 AU. This contrasts with the Baraffe model, where hot EGPs are claimed to be remnants of much more massive bodies, originally several times Jupiter and still losing substantial mass fractions at present.     

Evidence for the stratification of Fe in the photosphere of G191-B2B

The presence of heavy elements in the atmospheres of the hottest H-rich DA white dwarfs has been the subject of considerable interest. While theoretical calculations can demonstrate that radiative forces, counteracting the effects of gravitational settling, can explain the detections of individual species, the predicted abundances do not accord well with observation. However, accurate abundance measurements can only be based on a thorough understanding of the physical structure of the white dwarf photospheres, which has proved elusive. Recently, the availability of new non-local thermodynamic equilibrium model atmospheres with improved atomic data has allowed self-consistent analysis of the extreme ultraviolet (EUV), far UV and optical spectra of the prototypical object G191-B2B. Even so, the predicted and observed stellar fluxes remain in serious disagreement at the shortest wavelengths (below ≈190 Å), while the inferred abundances remain largely unaltered. We show here that the complete spectrum of G191-B2B can be explained by a model atmosphere where Fe is stratified, with increasing abundance at greater depth. This abundance profile may explain the difficulties in matching observed photospheric abundances, usually obtained by analyses using homogeneous model atmospheres, to the detailed radiative levitation predictions, particularly as the latter are only strictly valid for regions deeper than where the EUV/far UV lines and continua are formed. Furthermore, the relative depletion of Fe in the outer layers of the atmosphere may be evidence for radiatively driven mass-loss in G191-B2B.     

The Stanley unconformity in Lake Huron basin: evidence for a climate-driven closed lowstand about 7900 <Superscript>14</Superscript>C BP,with similar implications for the Chippewa lowstand in Lake Michigan basin

J.L. Hough in 1962 recognized an erosional unconformity in the upper section of early postglacial lake sediments in northwestern Lake Huron. Low-level Lake Stanley was defined at 70 m below present water surface on the basis of this observation, and was inferred to follow the Main Algonquin highstand and Post-Algonquin lake phases about 10 ¹⁴C ka, a seminal contribution to the understanding of Great Lakes history. Lake Stanley was thought to have overflowed from the Huron basin through the Georgian Bay basin and the glacio-isostatically depressed North Bay outlet to Ottawa and St. Lawrence rivers. For this overflow to have occurred, Hough assumed that post-Algonquin glacial rebound was delayed until after the Lake Stanley phase. A re-examination of sediment stratigraphy in northwestern Lake Huron using seismic reflection and new core data corroborates the sedimentological evidence of Hough’s Stanley unconformity, but not its inferred chronology or the level of the associated lowstand. Erosion of previously deposited sediment, causing the gap in the sediment sequence down to 70 m present depth, is attributed to wave erosion in the shoreface of the Lake Stanley lowstand. Allowing for non-deposition of muddy sediment in the upper 20 m approximately of water depth as occurs in the present Great Lakes, the inferred water level of the Stanley lowstand is repositioned at 50 m below present in northwestern Lake Huron. The age of this lowstand is about 7.9 ± 0.3¹⁴C ka, determined from the inferred ¹⁴C age of the unconformity by radiocarbon-dated geomagnetic secular variation in six new cores. This relatively young age shows that the lowstand defined by Hough’s Stanley unconformity is the late Lake Stanley phase of the northern Huron basin, youngest of three lowstands following the Algonquin lake phases. Reconstruction of uplift histories for lake level and outlets shows that late Lake Stanley was about 25–30 m below the North Bay outlet, and about 10 m below the sill of the Huron basin. The late Stanley lowstand was hydrologically closed, consistent with independent evidence for dry regional climate at this time. A similar analysis of the Chippewa unconformity shows that the Lake Michigan basin also hosted a hydrologically closed lowstand, late Lake Chippewa. This phase of closed lowstands is new to the geological history of the Great Lakes. This is the ninth in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M Lewis were guest editors of this special issue.     

A near-infrared spectroscopic search for very-low-mass cool companions to notable DA white dwarfs

We have undertaken a detailed near-infrared spectroscopic analysis of eight notable white dwarfs, predominantly of southern declination. In each case the spectrum failed to reveal compelling evidence for the presence of a spatially unresolved, cool, late-type companion. Therefore, we have placed an approximate limit on the spectral type of a putative companion to each degenerate. From these limits we conclude that if GD659, GD50, GD71 or WD2359−434 possesses an unresolved companion then most probably it is substellar in nature  ( M < 0.072 M_⊙)  . Furthermore, any spatially unresolved late-type companion to RE J0457−280, RE J0623−374, RE J0723−274 or RE J2214−491 most likely has   M < 0.082 M_⊙  . These results imply that if weak accretion from a nearby late-type companion is the cause of the unusual photospheric composition observed in a number of these degenerates then the companions are of very low mass, beyond the detection thresholds of this study. Furthermore, these results do not contradict a previously noted deficit of very-low-mass stellar and brown dwarf companions to main sequence F, G, K and early-M type primaries ( a ≲ 1000 au).     

New faint optical spectrophotometric standards: hot white dwarfs from the Sloan Digital Sky Survey

The spectral energy distributions for pure-hydrogen (DA) hot white dwarfs can be accurately predicted by model atmospheres. This makes it possible to define spectrophotometric calibrators by scaling the theoretical spectral shapes with broad-band photometric observations – a strategy successfully exploited for the spectrographs onboard the Hubble Space Telescope ( HST ) using three primary DA standards. Absolute fluxes for non-DA secondary standards, introduced to increase the density of calibrators in the sky, need to be referred to the primary standards, but a far better solution would be to employ a network of DA stars scattered throughout the sky. We search for blue objects in the sixth data release of the Sloan Digital Sky Survey (SDSS) and fit DA model fluxes to identify suitable candidates. Reddening needs to be considered in the analysis of many of these stars. We propose a list of nine pure-hydrogen white dwarfs with absolute fluxes with estimated uncertainties below 3 per cent, including four objects with estimated errors <2 per cent, as candidates for spectrophotometric standards in the range  14 < g < 18  , and provide model-based fluxes scaled to match the SDSS broad-band fluxes for each. We apply the same method to the three HST DA standards, linking the zero point of their absolute fluxes to ugr magnitudes transformed from photometry obtained with the US Naval Observatory 1-m telescope. For these stars, we estimate uncertainties of <1 per cent in the optical, finding good consistency with the fluxes adopted for HST calibration.     

Photospheric phosphorus in the FUSE spectra of GD71 and two similar DA white dwarfs

We report the detection, from the Far Ultraviolet Spectroscopic Explorer (FUSE) data, of phosphorus in the atmospheres of GD71 and two similar DA white dwarfs. This is the first detection of a trace metal in the photosphere of the spectrophotometric standard star GD71. Collectively, these objects represent the coolest DA white dwarfs in which photospheric phosphorus has been observed. We use a grid of homogeneous non-local thermodynamic equilibrium synthetic spectra to measure abundances of  [P/H]=−8.57^+0.09_−0.13, −8.70^+0.23_−0.37  and  −8.36^+0.14_−0.19  in GD71, RE J1918+595 and RE J0605−482 respectively. At the observed level we find that phosphorus has no significant impact on the overall energy distribution of GD71. We explore possible mechanisms responsible for the presence of this element in these stars, concluding that the most likely is an interplay between radiative levitation and gravitational settling, possibly modified by weak mass loss.     

The effect of photospheric heavy elements on the hot DA white dwarf temperature scale

Using the latest non-local thermodynamic equilibrium (non-LTE) synthetic spectra and stellar model calculations, we have evaluated the potential effect of the presence of heavy elements in the photospheres of hot H-rich DA white dwarfs. In particular, we have examined their influence on the effective temperature and surface gravity perceived from analysis of the Balmer line profiles. It is apparent that both the inclusion of non-LTE effects in the models and significant quantities of heavy elements act independently to lower the value of T _eff determined from a particular spectrum. Hence, the true effective temperatures of the heavy element-rich DA white dwarfs, currently estimated to be above 55 000 K, are apparently lower than previously reported from pure-H LTE analyses, by some 4000–7000 K. We do not see any similar influence on measurements of log g . This work concentrates on a group of relatively bright well-studied objects, for which heavy element abundances are known. As a consequence of this, establishment of correct temperatures for all other hot white dwarfs will require a programme of far-UV spectroscopy in order to obtain the essential compositional information. Since only stars with effective temperatures lying notionally in the range from ≈ 55 000 to 70 000 K (52 000–62 000 K when the non-LTE effects and heavy elements are taken into account) have been considered here, important questions remain regarding the magnitude of any similar effects in even hotter white dwarfs and pre-white dwarfs. The resulting implications for the plausibility of the evolutionary link between the main hot DA population and their proposed precursors, the H-rich central stars of planetary nebulae, need to be investigated.