21 research outputs found
The Spectral Energy Distributions of White Dwarfs in 47 Tucanae: The Distance to the Cluster
We present a new distance determination to the Galactic globular cluster 47
Tucanae by fitting the spectral energy distributions of its white dwarfs to
pure hydrogen atmosphere white dwarf models. Our photometric dataset is
obtained from a 121 orbit Hubble Space Telescope program using the Wide Field
Camera 3 UVIS/IR channels, capturing F390W, F606W, F110W, and F160W images.
These images cover more than 60 square arcmins and extend over a radial range
of 5-13.7 arcmin (6.5-17.9 pc) within the globular cluster. Using a likelihood
analysis, we obtain a best fitting unreddened distance modulus of (m -
M)o=13.36+/-0.02+/-0.06 corresponding to a distance of 4.70+/-0.04+/-0.13 kpc,
where the first error is random and the second is systematic. We also search
the white dwarf photometry for infrared excess in the F160W filter, indicative
of debris disks or low mass companions, and find no convincing cases within our
sample.Comment: Accepted to The Astronomical Journal, 13 Figures, 2 Tables. Figures 3
and 6 are figure sets, each composed of 59 subfigures (to appear in the
electronic journal). This is a Companion paper to the article ID:
submit/037561
Barium and Carbon fluxes in the Canadian Arctic Archipelago
Seasonal and spatial variability of dissolved Barium (Ba) in Amundsen Gulf, southeastern Beaufort Sea, was monitored over a full year from September 2007 to September 2008. Dissolved Ba displays a nutrient-type behavior: the maximum water column concentration is located below the surface layer. Highest Ba concentrations are typically observed at river mouths, the lowest concentrations are found in water masses of Atlantic origin. Barium concentrations decrease eastward through the Canadian Arctic Archipelago. Barite (BaSO4) saturation is reached at the maximum concentrations of dissolved Ba in the subsurface layer, whereas the remaining water column is undersaturated. A three end-member mixing model comprising freshwater from sea-ice melt and rivers, as well as upper halocline water, was used to establish their relative contributions to the Ba concentrations in the upper water column of the Amundsen Gulf. Based on water column and riverine Ba contributions, we assess the depletion of dissolved Ba by formation and concomitant sinking of biologically bound Ba (bio-Ba), from which we derive an estimate of the carbon export production. In the upper 50 m of the water column of Amundsen Gulf, riverine Ba accounts for up to 15% of the available dissolved Ba inventory, of which up to 20% is depleted by bio-Ba formation and export. Since riverine inputs and Ba export occur concurrently, the seasonal variability of dissolved Ba in the upper water column is moderate. Assuming a fixed organic carbon to bio-Ba flux ratio, carbon export out of the surface layer is estimated at 1.8{plus minus}0.45 mol C mâ2 yrâ1. We propose a climatological carbon budget for the Amundsen Gulf
The white dwarf cooling sequence of NGC 6791: a unique tool for stellar evolution
NGC 6791 is a well-studied, metal-rich open cluster that is so close to us
that can be imaged down to luminosities fainter than that of the termination of
its white dwarf cooling sequence, thus allowing for an in-depth study of its
white dwarf population. We use a Monte Carlo simulator that employs up-to-date
evolutionary cooling sequences for white dwarfs with hydrogen-rich and
hydrogen-deficient atmospheres, with carbon-oxygen and helium cores. The
cooling sequences for carbon-oxygen cores account for the delays introduced by
both Ne^22 sedimentation in the liquid phase and by carbon-oxygen phase
separation upon crystallization. We do not find evidence for a substantial
fraction of helium-core white dwarfs, and hence our results support the
suggestion that the origin of the bright peak of the white dwarf luminosity
function can only be attributed to a population of unresolved binary white
dwarfs. Moreover, our results indicate that the number distribution of
secondary masses of the population of unresolved binaries has to increase with
increasing mass ratio between the secondary and primary components of the
progenitor system. We also find that the observed cooling sequence appears to
be able to constrain the presence of progenitor sub-populations with different
chemical compositions and the fraction of non-DA white dwarfs. Our simulations
place interesting constraints on important characteristics of the stellar
populations of NGC 6791. In particular, we find that the fraction of single
helium-core white dwarfs must be smaller than 5%, that a sub-population of
stars with zero metallicity must be <12%, while if the adopted metallicity of
the sub-population is solar the upper limit is ~8%. Finally, we also find that
the fraction of non-DA white dwarfs in this particular cluster is surprinsingly
small <6%.Comment: 9 pages, 14 figures, accepted for publication in Astronomy &
Astrophysic
Carbon to nitrogen (C:N) stoichiometry of the spring-summer phytoplankton bloom in the North Water Polynya (NOW)
International audienceThe carbon to nitrogen (C:N) stoichiometry of phytoplankton production varied significantly during the spring-summer bloom in the North Water Polynya (NOW), from April through July 1998. The molar ratio of particulate organic carbon (POC) to nitrogen (PON) production by phytoplankton (ÎPOC:ÎPON) increased from 5.8 during April through early June to 8.9 in late June and July. The molar dissolved inorganic carbon (DIC) to nitrate+nitrite (NO 3) drawdown ratio (ÎDIC: ÎNO 3) increased from 6.7 in April and May, to 11.9 in June (no estimate for July because of ice melting). The discrepancy between ÎPOC:ÎPON and ÎDIC:ÎNO 3 was likely due to dissolved organic carbon (DOC) production. Increased ÎPOC:ÎPON of phytoplankton and surface water ÎDIC:ÎNO 3 throughout the phytoplankton blooms resulted from changes in physical properties of the upper water column, such as reduced thickness of the surface mixed layer that exposed phytoplankton to increased photosynthetically available radiation (PAR), accompanied by NO 3 depletion. This is expected to have significant effects on the cycling of carbon (C) and nitrogen (N) in pelagic ecosystems, as the increased C:N ratio of organic matter decreases its quality as substrate for grazers and microbial communities. Based on ÎPOC:ÎPON, the ratio of POC to chlorophyll a (Chl) production (ÎPOC:ÎChl) and the relationship between Chl yields and NO 3 depletion, we estimate that 71±17% and 46±20% of the depleted NO 3 went to PON production in the euphotic zone over the polynya from April to early June, and late June to July, respectively. The remaining NO 3 was likely channelled to dissolved organic nitrogen (DON) and heterotrophic bacteria, which were not returned to the dissolved inorganic nitrogen (DIN) pool through recycling during the course of the study. Hence, the autotrophic production of organic N and its recycling by the microbial food web were not coupled temporally
Synoptic evaluation of carbon cycling in the Beaufort Sea during summer: contrasting river inputs, ecosystem metabolism and airâsea CO2 fluxes
International audienceThe accelerated decline in Arctic sea ice and an ongoing trend toward more energetic atmospheric and oceanic forcings are modifying carbon cycling in the Arctic Ocean. A critical issue is to understand how net community production (NCP; the balance between gross primary production and community respiration) responds to changes and modulates airâsea CO2 fluxes. Using data collected as part of the ArcticNetâMalina 2009 expedition in the southeastern Beaufort Sea (Arctic Ocean), we synthesize information on sea ice, wind, river, water column properties, metabolism of the planktonic food web, organic carbon fluxes and pools, as well as airâsea CO2 exchange, with the aim of documenting the ecosystem response to environmental changes. Data were analyzed to develop a non-steady-state carbon budget and an assessment of NCP against airâsea CO2 fluxes. During the field campaign, the mean wind field was a mild upwelling-favorable wind (⌠5 km hâ1) from the NE. A decaying ice cover ( 600 mg C m â2 d â1) over the shelf prior to our survey, (2) freshwater dilution by river runoff and ice melt, and (3) the presence of cold surface waters offshore. Only the Mackenzie River delta and localized shelf areas directly affected by upwelling were identified as substantial sources of CO2 to the atmosphere (> 10 mmol C mâ2 dâ1). Daily PP rates were generally < 100 mg C mâ2 dâ1 and cumu-lated to a total PP of ⌠437.6 Ă 10 3 t C for the region over a 35-day period. This amount was about twice the organic carbon delivery by river inputs (⌠241.2 Ă 10 3 t C). Subsurface PP represented 37.4 % of total PP for the whole area and as much as ⌠72.0 % seaward of the shelf break. In the upper 100 m, bacteria dominated (54 %) total community respiration (⌠250 mg C mâ2 dâ1), whereas protozoans, metazoans, and benthos, contributed to 24, 10, and 12 %, respectively. The range of production-to-biomass Published by Copernicus Publications on behalf of the European Geosciences Union. 2828 A. Forest et al.: Synoptic evaluation of carbon cycling ratios of bacteria was wide (1â27 % dâ1), while we estimated a narrower range for protozoans (6â11 % d â1) and metazoans (1â3 % dâ1). Over the shelf, benthic biomass was twofold (⌠5.9 g C mâ2) the biomass of pelagic heterotrophs (⌠2.4 g C mâ2), in accord with high vertical carbon fluxes on the shelf (956 ± 129 mg C mâ2 dâ1). Threshold PP (PP at which NCP becomes positive) in the surface layer oscillated from 20 to 152 mg C mâ2 dâ1 , with a pattern from low-to-high values as the distance from the Mackenzie River decreased. We conclude that (1) climate change is exacerbating the already extreme biological gradient across the Beaufort shelfâbasin system; (2) the Mackenzie Shelf acts as a weak sink for atmospheric CO2 , suggesting that PP might exceed the respiration of terrigenous and marine organic matter in the surface layer; and (3) shelf break upwelling can transfer CO2 to the atmosphere, but CO2 outgassing can be attenuated if nutrients brought also by upwelling support diatom production. Our study underscores that cross-shelf exchange of waters, nutrients and particles is a key mechanism that needs to be properly monitored as the Arctic transits to a new state