3,847 research outputs found
Two New Tests of the Metallicity Sensitivity of the Cepheid Period-Luminosity Relation (The Leavitt Law)
We undertake a new test of the metallicity sensitivity of the Leavitt Law for
Classical Cepheids. We derive an empirical calibration of the apparent
luminosities of Cepheids as measured from the optical through the mid-infrared
(0.45-8.0um) as a function of spectroscopic [Fe/H] abundances of individual
Cepheids in the Large Magellanic Cloud from Romaniello et al. (2008). The
cumulative trend over the entire wavelength range shows a nearly monotonic
behavior. The sense of the trend is consistent with differential
line-blanketing in the optical, leading to stars of high metallicity being
fainter in the optical. This is followed by a reversal in the trend at longer
wavelengths, with the cross-over occurring near the K band at about 2.2um,
consistent with a subsequent redistribution of energy resulting in a mild
brightening of Cepheids (with increased metallicity) at mid-infrared
wavelengths. This conclusion agrees with that of Romaniello et al. based on a
differential comparison of the mean V- and K-band Leavitt Laws for the Galaxy,
SMC and LMC, but is opposite in sign to most other empirical tests of the
sensitivity of Cepheid distances to mean [O/H] HII region abundances. We also
search for a correlation of Cepheid host-galaxy metallicity with deviations of
the galaxy's Cepheid distance from that predicted from a pure Hubble flow.
Based on Cepheid distances to 26 nearby galaxies in the local flow, only a very
weak signal is detected giving Dmu_o = -0.17 (+/- 0.31) ([O/H] - 8.80) - 0.21
(+/-0.10). This is in agreement with previous determinations, but statistically
inconclusive.Comment: Accepted to the Astrophysical Journal. 8 pages, 5 figures, 1 tabl
Theoretical insights into the RR Lyrae K-band Period-Luminosity relation
Based on updated nonlinear, convective pulsation models computed for several
values of stellar mass, luminosity and metallicity, theoretical constraints on
the K-band Period-Luminosity (PLK) relation of RR Lyrae stars are presented. We
show that for each given metal content the predicted PLK is marginally
dependent on uncertainties of the stellar mass and/or luminosity. Then, by
considering the RR Lyrae masses suggested by evolutionary computations for the
various metallicities, we obtain that the predicted infrared magnitude M_K over
the range 0.0001< Z <0.02 is given by the relation
MK=0.568-2.071logP+0.087logZ-0.778logL/Lo, with a rms scatter of 0.032 mag.
Therefore, by allowing the luminosities of RR Lyrae stars to vary within the
range covered by current evolutionary predictions for metal-deficient (0.0001<
Z <0.006) horizontal branch models, we eventually find that the infrared
Period-Luminosity- Metallicity (PLZK) relation is
MK=0.139-2.071(logP+0.30)+0.167logZ, with a total intrinsic dispersion of 0.037
mag. As a consequence, the use of such a PLZK relation should constrain within
+-0.04 mag the infrared distance modulus of field and cluster RR Lyrae
variables, provided that accurate observations and reliable estimates of the
metal content are available. Moreover, we show that the combination of K and V
measurements can supply independent information on the average luminosity of RR
Lyrae stars, thus yielding tight constraints on the input physics of stellar
evolution computations. Finally, for globular clusters with a sizable sample of
first overtone variables, the reddening can be estimated by using the PLZK
relation together with the predicted MV-logP relation at the blue edge of the
instability strip (Caputo et al. 2000).Comment: 8 pages, including 5 postscript figures, accepted for publication on
MNRA
Distance and reddening of the Local Group dwarf irregular galaxy NGC 6822
On the basis of a new photometric analysis of the Local Group dwarf irregular
galaxy NCG 6822 based on observations obtained with the Advanced Camera for
Surveys onboard the Hubble Space Telescope, we have obtained a new estimate of
the extinction of two fields located in the southeast region of the galaxy.
Because of significant differences in the distance estimates to NGC 6822
available in literature, we decided to provide an independent determination of
the distance to this galaxy based on an updated and self-consistent theoretical
calibration of the tip of the red giant branch brightness. As a result we newly
determined the distance to NGC 6822 to be equal to , and compared our measurement with the most recent determinations of this
distance.Comment: 5 pages, 5 figures, Astronomy & Astrophysics (Research Note), in
press (proof correction included
mRNA localization in metazoans: A structural perspective
This is the final version. Available on open access from Taylor & Francis via the DOI in this recordAsymmetric localization of mRNAs is a widespread gene regulatory mechanism that is crucial for many cellular processes. The localization of a transcript involves multiple steps and requires several protein factors to mediate transport, anchoring and translational repression of the mRNA. Specific recognition of the localizing transcript is a key step that depends on linear or structured localization signals, which are bound by RNA-binding proteins. Genetic studies have identified many components involved in mRNA localization. However, mechanistic aspects of the pathway are still poorly understood. Here we provide an overview of structural studies that contributed to our understanding of the mechanisms underlying mRNA localization, highlighting open questions and future challenges.The authors are funded by the Max Planck Gesellshaft, the European Research Council under the European Union's Seventh Framework Program (FP7/2007-2013), ERC grant agreement no. 310957 and the Deutsche Forschungsgemeinschaft (BO3588/2-1 to F.B.)
Retinoic acid as a modulator of T cell immunity
Indexación: Scopus. DOAJ.Vitamin A, a generic designation for an array of organic molecules that includes retinal, retinol and retinoic acid, is an essential nutrient needed in a wide array of aspects including the proper functioning of the visual system, maintenance of cell function and differentiation, epithelial surface integrity, erythrocyte production, reproduction, and normal immune function. Vitamin A deficiency is one of the most common micronutrient deficiencies worldwide and is associated with defects in adaptive immunity. Reports from epidemiological studies, clinical trials and experimental studies have clearly demonstrated that vitamin A plays a central role in immunity and that its deficiency is the cause of broad immune alterations including decreased humoral and cellular responses, inadequate immune regulation, weak response to vaccines and poor lymphoid organ development. In this review, we will examine the role of vitamin A in immunity and focus on several aspects of T cell biology such as T helper cell differentiation, function and homing, as well as lymphoid organ development. Further, we will provide an overview of the effects of vitamin A deficiency in the adaptive immune responses and how retinoic acid, through its effect on T cells can fine-tune the balance between tolerance and immunity.http://www.mdpi.com/2072-6643/8/6/34
Blue Straggler masses from pulsation properties. I. The case of NGC6541
We used high-spatial resolution images acquired with the WFC3 on board HST to
probe the population of variable blue straggler stars in the central region of
the poorly studied Galactic globular cluster NGC 6541. The time sampling of the
acquired multi wavelength (F390W, F555W and F814W) data allowed us to discover
three WUma stars and nine SX Phoenicis. Periods, mean magnitudes and pulsation
modes have been derived for the nine SX Phoenicis and their masses have been
estimated by using pulsation equations obtained from linear non adiabatic
models. We found masses in the range 1.0-1.1Mo, with an average value of
1.06+-0.09 Mo (sigma = 0.04), significantly in excess of the cluster Main
Sequence Turn Off mass (~ 0.75Mo). A mild trend between mass and luminosity
seems also to be present. The computed pulsation masses turn out to be in very
good agreement with the predictions of evolutionary tracks for single stars,
indicating values in the range ~ 1.0-1.2 Mo for most of the BSS population, in
agreement with what discussed in a number of previous studies.Comment: 8 pages, 9 figures, ApJ, accepte
Concreciones carbonatadas en margas del Eoceno: consecuencias geomorfológicas
[Resumen] Las margas del Eoceno de la cuenca de Jaca (Pirineo Central) presentan, en numerosos lugares, una topografÃa de detalle muy caracterÃstica, en la que destacan un conjunto de núcleos más resistentes de aspecto frecuentemente esférico y de orden métrico, que permiten hablar de un paisaje de "margas con bolas". Se trata en realidad de concreciones dolomÃticas interestratificadas, menos susceptibles a los procesos erosivos que las propias margas. En este trabajo hemos intentado establecer si las diferencias mineralógicas entre concreciones carbonatadas y margas encajantes permiten explicar esta meteorización diferencial; para ello se ha estudiado la mineralogÃa de las concreciones, las margas y las margas alteradas. Se concluye que los procesos quÃmicos de alteración resultan insuficientes para explicar las diferentes tasas erosivas, debiendo por tanto atribuirse a causas fÃsicas (susceptibilidad ante la helada, procesos de humectación-desecación) la mayor erosionabilidad de las margas frente a las concreciones carbonatadas y areniscas interestratificadas.[Abstract] The Eocene marls from the Jaca basin (Central Pyrenees) show a conspicuous morphologic feature consisting of disperse hard cores, often with an spherical shape and metric size, surrounded by the marly matrix. These hard cores are dolomitic concretions isolated from the marls because of less intense weathering. In this paper we study the mineralogical composition of dolomitic concretionsand marls, in order to better define the differences in weathering processes. We conclude that chemical weathering processes do not fully explain the differences of erosion rates. We propose physical processes (frost susceptibility, dry-moist alternations) as the main erosion factors in the Eocene marls and, consequently, they are responsible for the greater erodibility of the marls compared to the dolomite concretions and interlayered sandstones
Classical Cepheid Pulsation Models. XI. Effects of convection and chemical composition on the Period-Luminosity and Period-Wesenheit relations
In spite of the relevance of Classical Cepheids as primary distance
indicators, a general consensus on the dependence of the Period-Luminosity (PL)
relation on the Cepheid chemical composition has not been achieved yet. From
the theoretical point of view, our previous investigations were able to
reproduce some empirical tests for suitable assumptions on the helium to metal
relative enrichment, but those results relied on specific assumptions
concerning the Mass-Luminosity relation and the efficiency of the convective
transfer in the pulsating envelopes. In this paper, we investigate the effects
of the assumed value of the mixing length parameter l/Hp on the pulsation
properties and we release the assumption of a fixed Mass-Luminosity relation.
As a whole, we show that our pulsation relations appear fully consistent with
the observed properties of Galactic and Magellanic Cloud Cepheids, supporting
the predicted steepening and brightening of the PL relations when moving from
metal-rich to metal-poor variables. Moreover, we show that the distances
inferred by the predicted PW relations agree with recently measured
trigonometric parallaxes, whereas they suggest a correction to the values based
on the Infrared Surface Brightness technique, as already found from an
independent method. Finally, also the pulsation metal contents suggested by the
predicted PW relations appear in statistical agreement with spectroscopic
[Fe/H] measurements.Comment: 20 pages, 15 figure
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