1,364 research outputs found
Seismology Of White Dwarfs: The ZZ Ceti Stars
We calculate an extensive adiabatic model grid for pulsating white dwarfs with H dominated atmospheres, the ZZ Ceti stars. We developed a new approach for asteroseismology, using the relative observed amplitudes as weights, and compared the computed modes with the observed ones for the class of ZZ Ceti stars. We measure the H layer mass for 83 stars and found an average of M(H)/M(*) = 10(-6.3), which is thinner than the predicted value of M(H)/M(*) = 10(-4). Our results indicate that the stars lose more mass during their evolution than previously expected.Astronom
Search for sdB/WD pulsators in the Kepler FOV
In this article we present the preliminary results of an observational search
for subdwarf B and white dwarf pulsators in the Kepler field of view, performed
using the DOLORES camera attached to the 3.6m Telescopio Nazionale Galileo
(TNG).Comment: Communications in Asteroseismology, in press; 2 pages, 1 figur
Gemini spectra of 12000K white dwarf stars
We report signal-to-noise ratio SNR ~ 100 optical spectra for four DA white
dwarf stars acquired with the GMOS spectrograph of the 8m Gemini north
telescope. These stars have 18<g<19 and are around Teff ~ 12000 K, were the
hydrogen lines are close to maximum. Our purpose is to test if the effective
temperatures and surface gravities derived from the relatively low
signal-to-noise ratio ( ~ 21) optical spectra acquired by the Sloan
Digital Sky Survey through model atmosphere fitting are trustworthy. Our
spectra range from 3800A to 6000A, therefore including H beta to H9. The H8
line was only marginally present in the SDSS spectra, but is crucial to
determine the gravity. When we compare the values published by Kleinman et al.
(2004) and Eisenstein et al. (2006) with our line-profile (LPT) fits, the
average differences are: Delta Teff ~ 320 K, systematically lower in SDSS, and
Delta log g ~ 0.24 dex, systematically larger in SDSS. The correlation between
gravity and effective temperature can only be broken at wavelengths bluer than
3800 A. The uncertainties in Teff are 60% larger, and in log g larger by a
factor of 4, than the Kleinman et al. (2004) and Eisenstein et al. (2006)
internal uncertainties.Comment: 11 pages and 8 figure
The sdA problem - II. Photometric and Spectroscopic Follow-up
Subdwarf A star (sdA) is a spectral classification given to objects showing
H-rich spectra and sub-main sequence surface gravities, but effective
temperature lower than the zero-age horizontal branch. Their evolutionary
origin is an enigma. In this work, we discuss the results of follow-up
observations of selected sdAs. We obtained time resolved spectroscopy for 24
objects, and time-series photometry for another 19 objects. For two targets, we
report both spectroscopy and photometry observations. We confirm seven objects
to be new extremely-low mass white dwarfs (ELMs), one of which is a known
eclipsing star. We also find the eighth member of the pulsating ELM class.Comment: Accepted for publication in MNRAS. 19 pages, 30 figures, 6 table
LifeâCycle Assessment of olive oil addressing alternative production systems: how to deal with olive pomace valorization
Olive oil is an important product of the so-called âMediterranean Dietâ. In Portugal, about 90 000 tonnes of olive oil were produced yearly in the last agricultural campaigns. The main objective of this paper is to present a comparative life-cycle assessment (LCA) of olive oil produced from four types of cultivation systems (familiar, traditional, intensive and organic) and two olive oil extraction processes (three-phase and two-phase extraction), addressing the valorization of olive pomace. The most remarkable difference between three- and two-phase extraction is related to the co-products and residues produced: the three-phase process results in three fractions (olive oil, olive pomace and olive mill wastewater), whereas the two-phase extraction (a more recent and increasingly used process in order to avoid the production of olive mill wastewater) consumes less water and generates, together with olive oil, a suspension called olive wet pomace.
A life-cycle model and inventory was implemented for the entire olive oil chain, including olive cultivation and olive oil extraction (about 5-7 kg of olives are required to produce one liter of olive oil), as well as valorization of olive pomace (three-phase extraction) and olive wet pomace (two-phase) to produce olive pomace oil and extracted pomace. Two approaches for dealing with multifunctionality were analyzed: i) allocation based on market prices of coproducts (olive oil and olive pomace) and ii) substitution (âavoided burdensâ approach) of extracted pomace (displacing conventional fuel in energy intensive processes) and olive pomace oil (displacing virgin oil in biodiesel production). The environmental impacts were calculated for four impact categories (ReCiPe method): greenhouse gas (GHG) intensity, terrestrial acidification, freshwater and marine eutrophication. The cumulative energy demand (CED) method was used to calculate non-renewable primary energy (NRPE).
The results (price based allocation) showed that olive cultivation was the life-cycle phase which contributed the most to the overall environmental impacts (55-95% to GHG intensity, 80-98% to acidification and 70-100% to eutrophication), except for the familiar cultivation system with no fertilizers and pesticides being applied. Fertilizers production and application contributed more than 43% to the environmental impacts of traditional, intensive and organic cultivation. Results calculated with the âavoided burdensâ approach are highly dependent on the type of virgin oil displaced by olive pomace oil. This research shows the importance of olive cultivation practices and olive pomace valorization to reduce the life-cycle impacts of olive oil.
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The pulsating DA white dwarf star EC 14012-1446: results from four epochs of time-resolved photometry
The pulsating DA white dwarfs are the coolest degenerate stars that undergo
self-driven oscillations. Understanding their interior structure will help to
understand the previous evolution of the star. To this end, we report the
analysis of more than 200 h of time-resolved CCD photometry of the pulsating DA
white dwarf star EC 14012-1446 acquired during four observing epochs in three
different years, including a coordinated three-site campaign. A total of 19
independent frequencies in the star's light variations together with 148
combination signals up to fifth order could be detected. We are unable to
obtain the period spacing of the normal modes and therefore a mass estimate of
the star, but we infer a fairly short rotation period of 0.61 +/- 0.03 d,
assuming the rotationally split modes are l=1. The pulsation modes of the star
undergo amplitude and frequency variations, in the sense that modes with higher
radial overtone show more pronounced variability and that amplitude changes are
always accompanied by frequency variations. Most of the second-order
combination frequencies detected have amplitudes that are a function of their
parent mode amplitudes, but we found a few cases of possible resonantly excited
modes. We point out the complications in the analysis and interpretation of
data sets of pulsating white dwarfs that are affected by combination
frequencies of the form f_A+f_B-f_C intruding into the frequency range of the
independent modes.Comment: 14 pages, 6 figures, 6 tables. MNRAS, in pres
Discovery of an ultramassive pulsating white dwarf
We announce the discovery of the most massive pulsating hydrogen-atmosphere
(DA) white dwarf (WD) ever discovered, GD 518. Model atmosphere fits to the
optical spectrum of this star show it is a 12,030 +/- 210 K WD with a log(g) =
9.08 +/- 0.06, which corresponds to a mass of 1.20 +/- 0.03 Msun. Stellar
evolution models indicate that the progenitor of such a high-mass WD endured a
stable carbon-burning phase, producing an oxygen-neon-core WD. The discovery of
pulsations in GD 518 thus offers the first opportunity to probe the interior of
a WD with a possible oxygen-neon core. Such a massive WD should also be
significantly crystallized at this temperature. The star exhibits
multi-periodic luminosity variations at timescales ranging from roughly 425-595
s and amplitudes up to 0.7%, consistent in period and amplitude with the
observed variability of typical ZZ Ceti stars, which exhibit non-radial g-mode
pulsations driven by a hydrogen partial ionization zone. Successfully
unraveling both the total mass and core composition of GD 518 provides a unique
opportunity to investigate intermediate-mass stellar evolution, and can
possibly place an upper limit to the mass of a carbon-oxygen-core WD, which in
turn constrains SNe Ia progenitor systems.Comment: 5 pages, 3 figures, Astrophysical Journal Letters, 771, L2 (2013
Tungsten oxide nanoparticles prepared with soft templating methods
Fundação para a CiĂȘncia e a Tecnologia (FCT
Analysis of IUE spectra of helium-rich white dwarf stars
We studied the class of DB white dwarf stars, using re-calibrated UV spectra
for thirty four DBs obtained with the IUE satellite. By comparing the observed
energy distributions with model atmospheres, we simultaneously determine
spectroscopic distances (d), effective temperature (Teff), and surface
gravities (log g). Using parallax measurements and previous determinations of
Teff and log g from optical spectra, we can study whether the atmospheres of
eleven DB stars are consistent with pure He or have a small amount of H
contamination. We also report on our observations of seventeen stars with Teff
close to the DB instability strip through time series photometry and found them
to be non variable within our detection limits.Comment: 8 pages, 5 figure
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