986 research outputs found
Intense Mass Loss from C-rich AGB Stars at low Metallicity?
We argue that the energy injection of pulsations may be of greater importance
to the mass-loss rate of AGB stars than metallicity, and that the mass-loss
trend with metallicity is not as simple as sometimes assumed. Using our
detailed radiation hydrodynamical models that include dust formation, we
illustrate the effects of pulsation energy on wind properties. We find that the
mass-loss rate scales with the kinetic energy input by pulsations as long as a
dust-saturated wind does not occur, and all other stellar parameters are kept
constant. This includes the absolute abundance of condensible carbon (not bound
in CO), which is more relevant than keeping the C/O-ratio constant when
comparing stars of different metallicity. The pressure and temperature
gradients in the atmospheres of stars, become steeper and flatter,
respectively, when the metallicity is reduced, while the radius where the
atmosphere becomes opaque is typically associated with a higher gas pressure.
This effect can be compensated for by adjusting the velocity amplitude of the
variable inner boundary (piston), which is used to simulate the effects of
pulsation, to obtain models with comparable kinetic-energy input. Hence, it is
more relevant to compare models with similar energy-injections than of similar
velocity amplitude. Since there is no evidence for weaker pulsations in
low-metallicity AGB stars, we conclude that it is unlikely that low-metallicity
C-stars have a lower mass-loss rate, than their more metal-rich counterparts
with similar stellar parameters, as long as they have a comparable amount of
condensible carbon.Comment: 4 pages, 3 figures. Accepted for publication in A&A. Updated after
language editing. Additional typos fixe
Four-colour photometry of eclipsing binaries. XL, uvby light curves for the B-type systems DW Carinae, BF Centauri, AC Velorum, and NSV 5783
Aims. In order to increase the limited number of B-stars with accurately known dimensions, and also the number of well studied eclipsing binaries in open clusters, we have undertaken observations and studies of four southern double-lined eclipsing B-type binaries; DWCar, BF Cen, ACVel, and NSV 5783.
Methods. Complete uvby light curves were observed between January 1982 and April 1991 at the Danish 0.5 m telescope at ESO La Silla, since 1985 known as the Strömgren Automatic Telescope (SAT). Standard indices for the systems and the comparison stars,as well as additional minima observations for ACVel, have been obtained later at SAT. For DWCar and ACVel, high-resolution spectra for definitive spectroscopic orbits have also been obtained; they are presented as part of the detailed analyses of these systems.
A few spectra of NSV 5783 are included in the present paper.
Results. For all four systems, the first modern accurate light curves have been established. DWCar is a detached system consisting of two nearly identical components. It is member of the young open cluster Cr228. A detailed analysis, based on the new light curves and 29 high-resolution spectra, is published separately. BFCen is semidetached and is member of NGC 3766. Modern spectra are
needed for a detailed study. ACVel is a detached system with at least one more star. A full analysis, based on the new light curves and 18 high-resolution spectra, is published separately. NSV 5783 is discovered to be an eclipsing binary consisting of two well-detached
components in an 11-day period eccentric (e = 0.18) orbit. Secondary eclipse is practically total. From the light curves and a few high-resolution spectra, accurate photometric elements and preliminary absolute dimensions have been determined. The quite similar components have masses of about 5 M and radii of about 3.5 R, and they seem to have evolved just slightly off the ZAMS. The
measured rotational velocities (≈150 km s−1) are about 6 times those corresponding to pseudosynchronization
Four-colour photometry of eclipsing binaries. XLI uvby light curves for AD Bootis, HW Canis Majoris, SW Canis Majoris, V636 Centauri, VZ Hydrae, and WZ Ophiuchi
CONTEXT: Accurate mass, radius, and abundance determinations from binaries
provide important information on stellar evolution, fundamental to central
fields in modern astrophysics and cosmology.
AIMS: Within the long-term Copenhagen Binary Project, we aim to obtain
high-quality light curves and standard photometry for double-lined detached
eclipsing binaries with late A, F, and G type main-sequence components, needed
for the determination of accurate absolute dimensions and abundances, and for
detailed comparisons with results from recent stellar evolutionary models.
METHODS: Between March 1985 and July 2007, we carried out photometric
observations of AD Boo, HW CMA, SW CMa, V636 Cen, VZ Hya, and WZ Oph at the
Str"omgren Automatic Telescope at ESO, La Silla.
RESULTS: We obtained complete uvby light curves, ephemerides, and standard
uvby\beta indices for all six systems.For V636 Cen and HW CMa, we present the
first modern light curves, whereas for AD Boo, SW CMa, VZ Hya, and WZ Oph, they
are both more accurate and more complete than earlier data. Due to a high
orbital eccentricity (e = 0.50), combined with a low orbital inclination (i =
84.7), only one eclipse, close to periastron, occurs for HW CMa. For the two
other eccentric systems, V636 Cen (e = 0.134) and SW CMa (e = 0.316), apsidal
motion has been detected with periods of 5270 +/- 335 and 14900 +/- 3600 years,
respectively.Comment: Only change is: Bottom lines (hopefully) not truncated anymore.
Accepted for publication in Astonomy & Astrophysic
Experimental implications of quantum phase fluctuations in layered superconductors
I study the effect of quantum and thermal phase fluctuations on the in-plane
and c-axis superfluid stiffness of layered d-wave superconductors. First, I
show that quantum phase fluctuations in the superconductor can be damped in the
presence of external screening of Coulomb interactions, and suggest an
experiment to test the importance of these fluctuations, by placing a metal in
close proximity to the superconductor to induce such screening. Second, I show
that a combination of quantum phase fluctuations and the linear temperature
dependence of the in-plane superfluid stiffness leads to a linear temperature
dependence of the c-axis penetration depth, below a temperature scale
determined by the magnitude of in-plane dissipation.Comment: 6 pgs, 1 figure, minor changes in comparison with c-axis expt, final
published versio
Recommended from our members
Osmolarity-regulated swelling initiates egg activation in Drosophila.
Egg activation is a series of highly coordinated processes that prepare the mature oocyte for embryogenesis. Typically associated with fertilization, egg activation results in many downstream outcomes, including the resumption of the meiotic cell cycle, translation of maternal mRNAs and cross-linking of the vitelline membrane. While some aspects of egg activation, such as initiation factors in mammals and environmental cues in sea animals, have been well-documented, the mechanics of egg activation in insects are less well-understood. For many insects, egg activation can be triggered independently of fertilization. In Drosophila melanogaster, egg activation occurs in the oviduct resulting in a single calcium wave propagating from the posterior pole of the oocyte. Here we use physical manipulations, genetics and live imaging to demonstrate the requirement of a volume increase for calcium entry at egg activation in ex vivo mature Drosophila oocytes. The addition of water, modified with sucrose to a specific osmolarity, is sufficient to trigger the calcium wave in the mature oocyte and the downstream events associated with egg activation. We show that the swelling process is regulated by the conserved osmoregulatory channels, aquaporins and DEGenerin/Epithelial Na+ channels. Furthermore, through pharmacological and genetic disruption, we reveal a concentration-dependent requirement of transient receptor potential M channels to transport calcium, most probably from the perivitelline space, across the plasma membrane into the mature oocyte. Our data establish osmotic pressure as a mechanism that initiates egg activation in Drosophila and are consistent with previous work from evolutionarily distant insects, including dragonflies and mosquitos, and show remarkable similarities to the mechanism of egg activation in some plants
Ab initio Pseudopotential Plane-wave Calculations of the Electronic Structure of YBa_2Cu_3O_7
We present an ab initio pseudopotential local density functional calculation
for stoichiometric high-Tc cuprate YBa_2Cu_3O_7 using the plane-wave basis set.
We have overcome well-known difficulties in applying pseudopotential methods to
first-row elements, transition metals, and rare-earth materials by carefully
generating norm-conserving pseudopotentials with excellent transferability and
employing an extremely efficient iterative diagonalization scheme optimized for
our purpose. The self-consistent band structures, the total and site-projected
densities of states, the partial charges and their symmetry-decompositions, and
some characteristic charge densities near E_f are presented. We compare our
results with various existing (F)LAPW and (F)LMTO calculations and establish
that the ab initio pseudopotential method is competitive with other methods in
studying the electronic structure of such complicated materials as high-Tc
cuprates. [8 postscript files in uuencoded compressed form]Comment: 14 pages, RevTeX v3.0, 8 figures (appended in postscript file), SNUTP
94-8
Eclipsing binary stars in the Large Magellanic Cloud : results from the EROS-2, OGLE and VMC surveys
We present a catalogue of 1768 eclipsing binary stars (EBs) detected in the Large Magellanic Cloud (LMC) by the second generation of the EROS survey (hereinafter EROS-2); 493 of them are new discoveries located in outer regions (out of the central bar) of the LMC. These sources were originally included in a list of candidate classical Cepheids (CCs) extracted from the EROS-2 catalogue on the basis of the period (0.89 < 17.82 mag] diagram. After visual inspection of the light curves we reclassified them as eclipsing binaries. They have blue colours (B_EROS - R_EROS < 0.2 mag) hence we classed them as hot eclipsing binaries (HEBs) containing hot massive components: main sequence (MS) stars or blue giants. We present Ks-band light curves for 999 binaries from our sample that have a counterpart in the VISTA near-infrared ESO public survey of the Magellanic Clouds system (VMC). We provide spectral classifications of 13 HEBs with existing spectroscopy. We divided our sample into contact-like binaries and detached/semi-detached systems based on both visual inspection and the parameters of the Fourier decomposition of the light curves and analysed the period-luminosity (PL) relations of the contact-like systems using the R_EROS and Ks magnitudes at maximum light. The contact-like binaries in our sample do not follow PL relations. We analysed the sample of contact binaries from the OGLE III catalogue and confirmed that PL_I and PL_Ks sequences are defined only by eclipsing binaries containing a red giant component.Peer reviewe
European Sea Bass (Dicentrarchus labrax) immune status and disease resistance are impaired by arginine dietary supplementation
Infectious diseases and fish feeds management are probably the major expenses in the aquaculture business. Hence, it is a priority to define sustainable strategies which simultaneously avoid therapeutic procedures and reinforce fish immunity. Currently, one preferred approach is the use of immunostimulants which can be supplemented to the fish diets. Arginine is a versatile amino acid with important mechanisms closely related to the immune response. Aiming at finding out how arginine affects the innate immune status or improve disease resistance of European seabass (Dicentrarchus labrax) against vibriosis, fish were fed two arginine-supplemented diets (1% and 2% arginine supplementation). A third diet meeting arginine requirement level for seabass served as control diet. Following 15 or 29 days of feeding, fish were sampled for blood, spleen and gut to assess cell-mediated immune parameters and immune-related gene expression. At the same time, fish from each dietary group were challenged against Vibrio anguillarum and survival was monitored. Cell-mediated immune parameters such as the extracellular superoxide and nitric oxide decreased in fish fed arginine-supplemented diets. Interleukins and immune-cell marker transcripts were down-regulated by the highest supplementation level. Disease resistance data were in accordance with a generally depressed immune status, with increased susceptibility to vibriosis in fish fed arginine supplemented diets. Altogether, these results suggest a general inhibitory effect of arginine on the immune defences and disease resistance of European seabass. Still, further research will certainly clarify arginine immunomodulation pathways thereby allowing the validation of its potential as a prophylactic strategy.European Union's Seventh Framework Programme AQUAEXCEL (Aquaculture Infrastructures for Excellence in European Fish Research) [262336]; AQUAIMPROV [NORTE-07-0124-FEDER-000038]; North Portugal Regional Operational Programme (ON. 2 - O Novo Norte) , under the National Strategic Reference Framework, through the European Regional Development Fund; North Portugal Regional Operational Programme (ON. 2 - O Novo Norte), under the National Strategic Reference Framework through the COMPETE - Operational Competitiveness Programme; Fundacao para a Ciencia e Tecnologia; Fundacao para a Ciencia e Tecnologia [SFRH/BD/89457/2012, SFRH/BPD/77210/2011]; Generalitat Valenciana through the project REVIDPAQUA [ISIC/2012/003]; [PEst-C/MAR/LA0015/2013]; [UID/Multi/04423/2013]info:eu-repo/semantics/publishedVersio
A precision study of two eclipsing white dwarf plus M dwarf binaries
We use a combination of X-shooter spectroscopy, ULTRACAM high-speed
photometry and SOFI near-infrared photometry to measure the masses and radii of
both components of the eclipsing post common envelope binaries SDSS J1212-0123
and GK Vir. For both systems we measure the gravitational redshift of the white
dwarf and combine it with light curve model fits to determine the inclinations,
masses and radii. For SDSS J1212-0123 we find a white dwarf mass and radius of
0.439 +/- 0.002 Msun and 0.0168 +/- 0.0003 Rsun, and a secondary star mass and
radius of 0.273 +/- 0.002 Msun and 0.306 +/- 0.007 Rsun. For GK Vir we find a
white dwarf mass and radius of 0.564 +/- 0.014 Msun and 0.0170 +/- 0.0004 Rsun,
and a secondary star mass and radius of 0.116 +/- 0.003 Msun and 0.155 +/-
0.003 Rsun. The mass and radius of the white dwarf in GK Vir are consistent
with evolutionary models for a 50,000K carbon-oxygen core white dwarf. Although
the mass and radius of the white dwarf in SDSS J1212-0123 are consistent with
carbon-oxygen core models, evolutionary models imply that a white dwarf with
such a low mass and in a short period binary must have a helium core. The mass
and radius measurements are consistent with helium core models but only if the
white dwarf has a very thin hydrogen envelope, which has not been predicted by
evolutionary models. The mass and radius of the secondary star in GK Vir are
consistent with evolutionary models after correcting for the effects of
irradiation by the white dwarf. The secondary star in SDSS J1212-0123 has a
radius ~9 per cent larger than predicted.Comment: 21 pages, 14 Figures and 11 Tables. Accepted for publication in MNRA
Smc5/6 coordinates formation and resolution of joint molecules with chromosome morphology to ensure meiotic divisions
During meiosis, Structural Maintenance of Chromosome (SMC) complexes underpin two fundamental features of meiosis: homologous recombination and chromosome segregation. While meiotic functions of the cohesin and condensin complexes have been delineated, the role of the third SMC complex, Smc5/6, remains enigmatic. Here we identify specific, essential meiotic functions for the Smc5/6 complex in homologous recombination and the regulation of cohesin. We show that Smc5/6 is enriched at centromeres and cohesin-association sites where it regulates sister-chromatid cohesion and the timely removal of cohesin from chromosomal arms, respectively. Smc5/6 also localizes to recombination hotspots, where it promotes normal formation and resolution of a subset of joint-molecule intermediates. In this regard, Smc5/6 functions independently of the major crossover pathway defined by the MutLγ complex. Furthermore, we show that Smc5/6 is required for stable chromosomal localization of the XPF-family endonuclease, Mus81-Mms4Eme1. Our data suggest that the Smc5/6 complex is required for specific recombination and chromosomal processes throughout meiosis and that in its absence, attempts at cell division with unresolved joint molecules and residual cohesin lead to severe recombination-induced meiotic catastroph
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