Valentina Luridiana

Temperature Bias and the Primordial Helium Abundance Determination

We study the effect that the temperature structure has on the determination of the primordial helium abundance, Yp. We provide an equation linking T(O III), the temperature derived from the [O III] lines, and T(He II), the temperature of the He I lines, both for H II regions with O++ only and for H II regions where a fraction of O+ is present. By means of T(He II), which is always smaller than T(O III), we derive the helium abundances of five objects with low and very low metallicity (NGC 346, NGC 2363, Haro 29, SBS 0335-052, and I Zw 18); these objects were selected from the literature because they include the three low-metallicity objects with the best line determinations and the two objects with the lowest metallicity. From these abundances we obtain that Yp(nHc) = 0.2356 ± 0.0020, a value 0.0088 lower than that derived by using T(O III). We call this determination Yp(nHc) because the collisional contribution to the Balmer-line intensities has not been taken into account. All the recent Yp determinations in the literature have not taken into account the collisional contribution to the Balmer-line intensities. By considering the collisional contribution to the Balmer-line intensities of these five objects, we derive that Yp(+Hc) = 0.2384 ± 0.0025.

Photoionization Models of NGC 2363 and Their Implications for the Ionizing Star Cluster

Using the photoionization code CLOUDY, we compute photoionization models for the giant extragalactic H II region NGC 2363 and compare them with optical observational data. We mainly focus on F(Hβ), Ne, EW(Hβ), and the ratios of I(λ5007), I(λ4363), I(λ3727), I(λ6300), I(λ6720), and I(λ4686) to I(Hβ). We discuss the variations of the emission spectra obtained with different input parameters. With low-metallicity models (Z = 0.10 Z☉) we were not able to reproduce the observed features of the spectrum. We review the implications of the λ4686 feature on the stellar population of NGC 2363, showing that it might indicate the presence of Wolf-Rayet (W-R) stars, a fact that would conflict with the metallicity of the region. We suggest several possible solutions to this contradiction, such as inadequate stellar evolutionary tracks, a nonstandard star formation process, and a revised metallicity. Focusing on the last possibility, we further show that the disagreement can be satisfactorily overcome by allowing for spatial temperature fluctuations in the nebula. The presence of temperature fluctuations allows a self-consistent scenario, which naturally accounts for the origin of the fluctuations themselves as a result of injection of mechanical energy by W-R winds and supernova explosions. Accordingly, we show that the metallicity of NGC 2363 has most probably been underestimated and that a value of Z 0.25 Z☉ is in better agreement with the observational data than the usually adopted value Z 0.10 Z☉. We further find that a star formation episode extended over a time interval of ~1.6 Myr gives a better fit than a strictly instantaneous burst. We also derive values for the slope and the high-mass end of the initial mass function, the age of the stellar cluster, and the total gaseous mass of the H II region.

The structure and star-formation history of ngc 5461

We compute photoionization models for the giant extragalactic H II region NGC 5461 and compare their predictions to several observational constraints. Since we aim to reproduce not only the global properties of the region but also its local structure, the models are constrained to reproduce the observed density profile, and our analysis takes into consideration the bias introduced by the shapes and sizes of the slits used by different observers. We find that an asymmetric nebula with a Gaussian density distribution, powered by a young burst of 3.1 Myr, satisfactorily reproduces most of the constraints, and that the star formation efficiency inferred from the model agrees with current estimates. Our results strongly depend on the assumed density law, since constant-density models overestimate the hardness of the ionizing field, affecting the deduced properties of the central stellar cluster. We illustrate the features of our best model, and discuss the possible sources of errors and uncertainties affecting the outcome of this type of studies.

Can stellar winds account for temperature fluctuations in h II regions? the case of ngc 2363

We compare the rate of kinetic energy injected by stellar winds into the extragalactic Hi iregion NGC 2363 to the luminosity needed to feed the observed temperature fluctuations. The kinetic luminosity asso- ciated to the winds is estimated by means of two dierent evolutionary synthesis codes, one of which takes into account the statistical fluctuations expected in the Initial Mass Function. We nd that, even in the most favorable conditions considered by our model, such luminosity is much smaller than the luminosity needed to account for the observed temperature fluctuations. The assumptions underlying our study are emphasized as possible sources of uncertainty aecting our results.

Late Stages of Stellar Evolution: Numerical Tests on AGB Models

We have calculated numerical models for intermediate mass stars, following the evolution from the MS to the AGB phase. The sequences have been obtained with the Gottingen hydrodynamical code. The Schwarzschild criterion for convection is used. Mass loss is modeled using the formula proposed by Blocker: where M is Reimers’ mass loss law with η = 1. The strong luminosity dependence leads to a very high mass loss rate on the AGB, to obtain results in agreement with the high observed values and with Weidemann’s relation between initial and final mass. Thermal pulses in the AGB phase are explicitly calculated, with the aim of obtaining realistic values for chemical yields and planetary nebulae composition.