Navegando por Autor "Armah, Mark"

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    Chemical abundances in Seyfert galaxies – X. Sulphur abundance estimates
    (Royal Astronomical Society) Dors Junior, Oli Luiz; Valerdi, Mabel; Riffel, Rogemar A.; Riffel, Rogério; Cardaci, Mónica Viviana; Hägele, Guilhermo F.; Armah, Mark; Revalski, Mitchell; Flury, Sophia; Lemes, Priscila Freitas; Amôres, Eduardo; Krabbe, Angela Cristina; Binette, Luc; Feltre, Anna; Bergmann, Thaisa Storchi
    For the first time, the sulphur abundance relative to hydrogen (S/H) in the narrow-line regions of a sample of Seyfert 2 nuclei (Sy 2s) has been derived via direct estimation of the electron temperature. Narrow emission-line intensities from the Sloan Digital Sky Survey (SDSS) Data Release 17 (DR17) [in the wavelength range 3000 < λ(Å) < 9100] and from the literature for a sample of 45 nearby (z < 0.08) Sy 2s were considered. Our direct estimates indicate that Sy 2s have similar temperatures in the gas region where most of the S+ ions are located in comparison with that of star-forming regions (SFs). However, Sy 2s present higher temperature values (∼10 000 K) in the region where most of the S2+ ions are located relative to that of SFs. We derive the total sulphur abundance in the range of 6.2 􏰀 12 + log(S/H) 􏰀 7.5, corresponding to 0.1–1.8 times the solar value. These sulphur abundance values are lower by ∼0.4 dex than those derived in SFs with similar metallicity, indicating a distinct chemical enrichment of the interstellar medium (ISM) for these object classes. The sulphur abundance relative to oxygen (S/O) values for our Sy 2 sample present an abrupt (∼0.5 dex) decrease with increasing oxygen abundance relative to hydrogen (O/H) for the high-metallicity regime [12 + log(O/H) 􏰁 8.7)], what is not seen for the SFs. However, when our Sy 2 estimates are combined with those from a large sample of SFs, we did not find any dependence between S/O and O/H.
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    Doubly ionized neon ionic abundance of seyfert 2 nuclei based on infrared and optical emission lines
    (2020-10-19) Dors Junior, Oli Luiz; Krabbe, Angela Cristina; Hagelle, Guillermo Frederico; Feltre, Anna; Armah, Mark; São José dos Campos
    One of the most reliable method to determine the chemical abundance of heavy elements in gaseous nebulae is the (tau)e-method, which is based on the measurements of auroral emission lines (e.g., [O III] (lambda)4363 Å). However, this method yields unreal and subsolar abundances in AGNs. This phenomenon is customarily referred to as “temperature problem”, and its origin is an open question in nebular astrophysics. Comparison between optical and infrared abundances can be used to obtain the level of electron temperature fluctuations in AGNs, generally attributed to the origin of the temperature problem. In this work, optical and infrared emission-line intensities of neon from a sample of 36 Seyfert 2 nuclei compiled from the literature and used to calculate the ionic abundance of the neon twice ionized in relation to the hydrogen one ion (Ne2+/H+). This methodology makes it possible to obtain the level of electron temperature fluctuation necessary to conciliate the optical and infrared abundance values. We investigated the use of the Balmer decrement observed ratio of intensities of the 3 (seta) 2 (H(alfa) (lambda)6563 Å) and 4 (seta) 2 (H(beta) (lambda)4861 Å) transitions of the hydrogen atom compared to their intrinsic intensity ratio so as to yield a relative extinction in the Narrow Line Region (NLR) of Seyfert 2 nuclei and find that the use of (iota)(H(alfa)/H(beta)) = 2.85 gives (tau)e values of 700 ± 30 K higher than the (tau)e values derived from (iota)(H(alfa)/H(beta)) = 3.10. Our analysis show that, differences (D) between abundance values from optical and infrared lines range from 0.1334 ± 0.0219 to 2.0636 ± 0.0151 dex, with an averaged value of 0.6931 ± 0.0052 dex. This averaged value is approximately ~ 0.01 ± 0.01 dex higher than the one derived in H ii regions studies. We did not find any relation between the ionic abundance difference (D) and the ionization parameter (U), which implies D is independent from U. We estimated the level of temperature fluctuation in terms of the t2 parameter in the range from 0.0006 to 0.4365 ± 0.0053 with an average value of 0.1859 ± 0.0011. We conclude that, if electron temperature fluctuations are present in AGNs, they are somewhat more significant than in H ii regions.