Grid-based estimates of stellar ages in binary systems: SCEPtER: Stellar CharactEristics Pisa Estimation gRid

Aims. We investigate the performance of grid-based techniques in estimating the age of stars in detached eclipsing binary systems. We evaluate the precision of the estimates due to the uncertainty in the observational constraints-masses, radii, effective temperatures, and [Fe/H]-and the systematic bias caused by the uncertainty in convective core overshooting, element diffusion, mixing-length value, and initial helium content. Methods. We adopted the SCEPtER grid, which includes stars with mass in the range [0.8; 1.6] M⊙ and evolutionary stages from the zero-age main sequence to the central hydrogen depletion. Age estimates have been obtained by a generalisation of the maximum likelihood technique described in our previous work. Results. We showed that the typical 1σ random error in age estimates-due only to the uncertainty affecting the observational constraints-is about ± 7%, which is nearly independent of the masses of the two stars. However, such an error strongly depends on the evolutionary phase and becomes larger and asymmetric for stars near the zero-age main sequence where it ranges from about + 90% to-25%. The systematic bias due to the including convective core overshooting-for mild and strong overshooting scenarios-is about 50% and 120% of the error due to observational uncertainties. A variation of ± 1 in the helium-to-metal enrichment ratio ΔY/ΔZ accounts for about ± 150% of the random error. The neglect of microscopic diffusion accounts for a bias of about 60% of the error due to observational uncertainties. We also introduced a statistical test of the expected difference in the recovered age of two coeval stars in a binary system. We find that random fluctuations within the current observational uncertainties can lead genuine coeval binary components to appear to be non-coeval with a difference in age as high as 60%

Mixing-Length Estimates From Binary Systems. A Theoretical Investigation on the Estimation Errors

We performed a theoretical investigation on the biases and random uncertainties affecting the recovery of the mixing-length parameter αml from an ideal eclipsing double-lined binary system, with well constrained masses and radii. We focused on a test case composed by a primary of mass M = 0.95 M⊙ and a secondary of M = 0.85 M⊙. Synthetic stars were generated coevally and with a common chemical composition by sampling from a dense grid of stellar models. Observational errors were simulated by adding random perturbations to mock data. The αml parameter was then recovered by means of the SCEPtER-binary pipeline. Several Monte Carlo simulations were conducted considering three metallicities, coupled to three different evolutionary stages of the primary. For each configuration, artificial data were sampled assuming an increasing difference between the mixing-length of the two stars. The mixing length values were then reconstructed adopting three alternative set-ups. A first method, which assumes full independence between the two stars, showed a great difficulty to constrain the mixing-length values; the recovered values were nearly unconstrained with a standard deviation of about 0.40. The second technique imposes the constraint of common age and initial chemical composition for the two stars in the fit. We found that αml, 1 values closely match the ones recovered under the previous configuration, but αml, 2 values are much more peaked around unbiased estimates. This occurs because the primary star provides a much tighter age constraint in the joint fit than the secondary, thus leading to the rejection of several extreme solutions for the secondary. Within this second scenario we also explored, for systems sharing a common αml = 2.0, the difference in the mixing-length values of the two stars only due to random fluctuations owing to the observational errors. The posterior distribution of these differences was peaked around zero, with a somewhat large standard deviation of 0.3 (about 15% of the solar-scaled value). Therefore, about 32% of systems with true identical αml are expected to show differences higher than that only owing to random errors. The third technique also imposes the constraint of a common mixing-length value for the two stars. This assumption is generally not true for the sample stars and served as a test for identifying wrong fitting assumptions. In this case, the common mixing-length is mainly dictated by the value of αml, 2. However, an increasing share of systems cannot be fitted by the algorithm as the differences of αml between the two stars in the synthetic systems increases. For Δαml > 0.4, less than half of the systems can be recovered and only 20% at Δαml = 1.0

Evolution of Li, Be and B in the Galaxy

In this paper we study the production of Li, Be and B nuclei by Galactic cosmic ray spallation processes. We include three kinds of processes: (i) spallation by light cosmic rays impinging on interstellar CNO nuclei (direct processes); (ii) spallation by CNO cosmic ray nuclei impinging on interstellar p and 4He (inverse processes); and (iii) alpha-alpha fusion reactions. The latter dominate the production of 6Li and 7Li. We calculate production rates for a closed-box Galactic model, verifying the quadratic dependence of the Be and B abundances for low values of Z. These are quite general results and are known to disagree with observations. We then show that the multi-zone multi-population model we used previously for other aspects of Galactic evolution produces quite good agreement with the linear trend observed at low metallicities without fine tuning. We argue that reported discrepancies between theory and observations do not represent a nucleosynthetic problem, but instead are the consequences of inaccurate treatments of Galactic evolution.Comment: 26 pages, 5 figures, LaTeX. The Astrophysical Journal, in pres

Theoretical uncertainties on classical Cepheid pulsational quantities.

The expected distribution of Cepheids within the instability strip directly reflects upon the predicted Period-Luminosity (PL) relation; however from a theoretical point of view this distribution is affected by the uncertaintes on several model inputs. We evaluated the effects on the theoretical PL relation of current uncertainties on the chemical abundances of Cepheids in the Large Magellanic Cloud (LMC) on the basis of new and updated sets of evolutionary and pulsational models. As a result, we found that current uncertainties on the LMC chemical composition affect significantly the loop extension and also reflect in the predicted topology of the instability strip; however their influence on the predicted pulsational parameters is negligible

The updated Pisa evolutionary model database

We present the updated Pisa evolutionary database (available at the link: http://astro.df.unipi.it/stellar-models) which includes tracks and isochrones for evolutionary phases from the Pre-Main Sequence to the Asympthotic Giant Branch. Models are calculated by adopting a well-tested evolutionary code (FRANEC) implemented with updated physical and chemical inputs. Calculations for all the evolutionary phases have been computed for a grid of 216 chemical compositions with the fractional metal abundance in mass, Z, ranging from 0.0001 to 0.01, and the original helium content, Y, from 0.25 to 0.42 while PMS models includes metallicities up to Z=0.03. Models were computed for both solar-scaled and alpha -enhanced abundances with different external convection efficiencies

Multidisciplinary management of a large pheochromocytoma presenting with cardiogenic shock: a case report

Pheochromocytoma is well-known for sudden initial presentations, particularly in younger patients. Hemodynamic instability may cause serious complications and delay a patient's ability to undergo surgical resection. Larger tumors present a further challenge because of the risk of catecholamine release during manipulations. In the case we present, increases in systemic vascular resistance caused cardiogenic shock, and the size of the lesion prompted surgeons to veer off from their usual approach