Analysis of in-situ observations in the Strait of Gibraltar

During the EU-project CANIGO intensive ship-board observations were carried out in April 1996 and October 1997 in order to observe the spatial and temporal variability of the flow, of the internal bore and of the water mass structure in the Strait of Gibraltar. An inverse model for the current and interfacefluctuations was developed to remove tidal currents from the measurement and to calculate the volume transport for the in and outflow seperately. In addition traveltime measurements across the strait have been analysed to test the suitability of acoustical instruments for a longterm monitoring of the exchange through the strait

Beyond the iron group: heavy metals in hot subdwarfs

We report the discovery of strong photospheric resonance lines of Ga III, Ge IV, Sn IV and Pb IV in the UV spectra of more than two dozen sdB and sdOB stars at temperatures ranging from 22000 K to 40000 K. Lines of other heavy elements are also detected, however in these cases more atomic data are needed. Based on these discoveries, we present a hypothesis to explain the apparent lack of silicon in sdB stars hotter than ~32000 K. The existence of triply ionised Ge, Sn, and Pb suggests that rather than silicon sinking deep into the photosphere, it is removed from the star in a fractionated stellar wind. This hypothesis provides a challenge to diffusion models of sdB stars.Comment: 5 pages, 3 figures, to appear in A&A Letter

Intensive ship-board observations of the flow through the Strait of Gibraltar

During the European project Canary Island Azores Gibraltar Experiment (CANIGO), intensive shipboard observations were carried out in April 1996 and October 1997 in order to observe the spatial and temporal variability of the flow and of the water mass structure in the Strait of Gibraltar. At the sill and the eastern and western entrances to the strait, repeated cross-strait sections and station time series of the flow and of T–S profiles were measured using vessel-mounted and lowered acoustic Doppler current profilers (ADCP) and conductivity-temperature-depth probes (CTD)/expendable bathythermographs (XBT), yielding new views of the rapid changes over tidal cycles and of approximate tidal means. It is argued that transport observations might be easier to carry out away from the sill, in the eastern part of the strait, even though maximum resolved shears were comparable in both places, 0.03–0.04 s−1 in the vertical and 0.001–0.016 s−1 in the horizontal. In the east, coordinated changes in the stratification and the flow field are documented via four time series over M 2 tidal cycles, showing a sharpening/diffusing of the vertical gradients in the water masses and the flow. Maximum shear and maximum water mass gradients do not always coincide there, and both are much shallower (50–80 m) than the delimiter between inflow and outflow (120–150 m). The mean salinity of the outflow core decreases from 38.43 in the east to 38.17 west of the sills as a result of the mixing processes. The internal bore was followed and directly observed with rapid CTD-yoyo stations and with XBT/vessel-mounted ADCP measurements. It generates extreme changes in currents and shears on timescales of minutes, with directly measured vertical velocities reaching ±50 cm s−1. Patches of density inversions were observed as the bore passed by, consistent with active turbulent mixing along the interface. The time series of flow and CTD measurements allow the direct calculation of Froude numbers at various locations and over tidal cycles. These and along-strait sections suggest that the exchange through the strait is maximal in April 1996 and submaximal in October 1997, supporting the expectations of Garrett et al. [1990]

Abundance studies of sdB stars using UV echelle HST/STIS spectroscopy

Aims: We test the hypothesis that the pulsations in sdB stars are correlated with the surface abundances of iron-group elements. Any correlation might explain why, when given two spectroscopically similar stars, one will pulsate while the other will not. Methods: We have obtained high-resolution ultraviolet spectra two pulsating and three non-pulsating sdB stars using the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope. We determined abundances for 25 elements including the iron group and even heavier elements such as tin and lead using LTE curve-of-growth and spectrum synthesis techniques. Results: We find no clear correlation between pulsations and metal abundances, and we comment on the resulting implications, including whether it is possible to determine the difference between a pulsating and a non-pulsating sdB spectroscopically. In addition to the main goal of our observations, we have also investigated the effect of supersolar metallicity on fundamental parameter determination, possible trends with iron abundance, and the hypothesis that weak winds may be selectively removing elements from the stellar envelopes. These effects provide challenges to stellar atmosphere modelling and diffusion models for sdB stars.Comment: 14 pages, 12 figures, accepted for publication in A&

Striking Photospheric Abundance Anomalies in Blue Horizontal-Branch Stars in Globular Cluster M13

High-resolution optical spectra of thirteen blue horizontal-branch (BHB) stars in the globular cluster M13 show enormous deviations in element abundances from the expected cluster metallicity. In the hotter stars (T_eff > 12000 K), helium is depleted by factors of 10 to 100 below solar, while iron is enhanced to three times the solar abundance, two orders of magnitude above the canonical metallicity [Fe/H] ~= -1.5 dex for this globular cluster. Nitrogen, phosphorus, and chromium exhibit even more pronounced enhancements, and other metals are also mildly overabundant, with the exception of magnesium, which stays very near the expected cluster metallicity. These photospheric anomalies are most likely due to diffusion --- gravitational settling of helium, and radiative levitation of the other elements --- in the stable radiative atmospheres of these hot stars. The effects of these mechanisms may have some impact on the photometric morphology of the cluster's horizontal branch and on estimates of its age and distance.Comment: 11 pages, 1 Postscript figure, uses aaspp4.sty, accepted for publication in ApJ Letter

Rotations and Abundances of Blue Horizontal-Branch Stars in Globular Cluster M15

High-resolution optical spectra of eighteen blue horizontal-branch (BHB) stars in the globular cluster M15 indicate that their stellar rotation rates and photospheric compositions vary strongly as a function of effective temperature. Among the cooler stars in the sample, at Teff ~ 8500 K, metal abundances are in rough agreement with the canonical cluster metallicity, and the v sin i rotations appear to have a bimodal distribution, with eight stars at v sin i < 15 km/s and two stars at v sin i ~ 35 km/s. Most of the stars at Teff > 10000 K, however, are slowly rotating, v sin i < 7 km/s, and their iron and titanium are enhanced by a factor of 300 to solar abundance levels. Magnesium maintains a nearly constant abundance over the entire range of Teff, and helium is depleted by factors of 10 to 30 in three of the hotter stars. Diffusion effects in the stellar atmospheres are the most likely explanation for these large differences in composition. Our results are qualitatively very similar to those previously reported for M13 and NGC 6752, but with even larger enhancement amplitudes, presumably due to the increased efficiency of radiative levitation at lower intrinsic [Fe/H]. We also see evidence for faster stellar rotation explicitly preventing the onset of the diffusion mechanisms among a subset of the hotter stars.Comment: 11 pages, 1 figure, 1 table, accepted to ApJ

The NextGen Model Atmosphere grid: II. Spherically symmetric model atmospheres for giant stars with effective temperatures between 3000 and 6800~K

We present the extension of our NextGen model atmosphere grid to the regime of giant stars. The input physics of the models presented here is nearly identical to the NextGen dwarf atmosphere models, however spherical geometry is used self-consistently in the model calculations (including the radiative transfer). We re-visit the discussion of the effects of spherical geometry on the structure of the atmospheres and the emitted spectra and discuss the results of NLTE calculations for a few selected models.Comment: ApJ, in press (November 1999), 13 pages, also available at http://dilbert.physast.uga.edu/~yeti/PAPERS and at ftp://calvin.physast.uga.edu/pub/preprints/NG-giants.ps.g

Tidal fronts and their role in air-sea gas exchange

Author Posting. © Sears Foundation for Marine Research, 2006. This article is posted here by permission of Sears Foundation for Marine Research for personal use, not for redistribution. The definitive version was published in Journal of Marine Research 64 (2006): 483-515, doi:10.1357/002224006778715766.Tidal fronts are a common feature of many coastal environments. They are characterized by a surface convergence zone that enhances wave breaking and the generation of gas bubbles due to wave-current interaction. The associated downwelling currents carry bubbles to depths of up to 160 m and increase the amount of air that dissolves from them. An energetic tidal front is formed at the entrance to the Strait of Georgia, BC, Canada, by a hydraulically controlled sill flow with vertical velocities of up to 0.75 m s−1. Extensive ship-board measurements during two cruises are interpreted with models of wave-current interaction and gas bubble behavior. The observations suggest that tidal fronts may contribute significantly to the aeration of the subsurface waters in the Fraser Estuary. This process may be also of importance for other coastal environments with plunging sill flows of dense water that deliver aerated surface water to intermediate depths