On the abundance discrepancy problem in HII regions

The origin of the abundance discrepancy is one of the key problems in the physics of photoionized nebula. In this work, we analize and discuss data for a sample of Galactic and extragalactic HII regions where this abundance discrepancy has been determined. We find that the abundance discrepancy factor (ADF) is fairly constant and of the order of 2 in all the available sample of HII regions. This is a rather different behaviour than that observed in planetary nebulae, where the ADF shows a much wider range of values. We do not find correlations between the ADF and the O/H, O++/H+ ratios, the ionization degree, Te(High), Te(Low)/ Te(High), FWHM, and the effective temperature of the main ionizing stars within the observational uncertainties. These results indicate that whatever mechanism is producing the abundance discrepancy in HII regions it does not substantially depend on those nebular parameters. On the contrary, the ADF seems to be slightly dependent on the excitation energy, a fact that is consistent with the predictions of the classical temperature fluctuations paradigm. Finally, we obtain that Te values obtained from OII recombination lines in HII regions are in agreement with those obtained from collisionally excited line ratios, a behaviour that is again different from that observed in planetary nebulae. These similar temperature determinations are in contradiction with the predictions of the model based on the presence of chemically inhomogeneous clumps but are consistent with the temperature fluctuations paradigm. We conclude that all the indications suggest that the physical mechanism responsible of the abundance discrepancy in HII regions and planetary nebulae are different.Comment: 14 pages, 8 figures, 9 tables. Accepted for publication in the Ap

Las Relaciones Semánticas Predicen la Desambiguación Estructural de las Unidades Terminológicas Poliléxicas con Tres Formantes

For English multiword terms (MWTs) of three or more constituents (e.g., sea level rise), a semantic analysis, based on linguistic and domain knowledge, is necessary to resolve the dependency between components. This structural disambiguation, often known as bracketing, involves the grouping of the dependent components so that the MWT is reduced to its basic form of modifier+head, as in [sea level] [rise]. Knowledge of these dependencies facilitates the comprehension of an MWT and its accurate translation into other languages. Moreover, the resolution of MWT bracketing provides a higher overall accuracy in machine translation systems and sentence parsers. This paper thus presents a pilot study that explored whether the bracketing of a ternary compound, when used as an argument in a sentence, can be predicted from the semantic information encoded in that sentence. It is shown that, with a random forest model, the semantic relation of the MWT to another argument in the same sentence, the lexical domain of the predicate, and the semantic role of the MWT were able to predict the bracketing of the 190 ternary compounds used as arguments in a sample of 188 semantically annotated sentences from a Coastal Engineering corpus (100% F1-score). Furthermore, only the semantic relation of an MWT to another argument in the same sentence proved enormous capability to predict ternary compound bracketing with a binary decision-tree model (94.12%F1-score).En unidades terminológicas poliléxicas (UTP) con tres o más formantes en lengua inglesa (p.ej., sea level rise), establecer la dependencia entre dichos formantes requiere de un análisis lingüístico y de conocimiento especializado del área concreta en que se emplean las UTP. Esta desambiguación estructural, o bracketing, implica el agrupamiento de los formantes para reducir la UTP a su estructura básica de modificador+núcleo, como en [sea level] [rise]. Conocer el bracketing de una UTP no solo facilita su comprensión y traducción a otras lenguas, sino que también mejora el desempeño de los sistemas de traducción automática y de los analizadores sintácticos. Por tanto, en este artículo presentamos un estudio piloto que explora si el bracketing de una UTP con tres formantes, al emplearse como argumento en una oración, puede predecirse a partir de la información semántica codificada en dicha oración. Se muestra que, con un modelo random forest, la relación semántica de la UTP con otro argumento en la misma oración, el dominio léxico del verbo y el rol semántico de la UTP son capaces de predecir el bracketing de las 190 UTP ternarias que se usan como argumento en una muestra de 188 oraciones, anotadas semánticamente y extraídas de un corpus sobre ingeniería de costas (con un valor de F1 del 100%). Además, únicamente la relación semántica que mantiene una UTP ternaria con otro argumento en la misma oración posee una enorme capacidad para predecir su bracketing mediante un árbol de decisión binario (con un valor de F1 del 94,12%).This research was carried out as part of projects PID2020-118369GB-I00, "Transversal Integration of Culture in a Terminological Knowledge Base on Environment" (TRANSCULTURE), funded by the Spanish Ministry of Science and Innovation; and A-HUM-600-UGR20, "Culture as Transversal Module in a Terminological Knowledge Base on the Environment" (CULTURAMA), funded by the Andalusian Ministry of Economy, Knowledge, Business, and University