RCAAP - Repositório Científico de Acesso Aberto de Portugal
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The O [iota] line emission in active galactic nuclei revisited
No full textUV, visible, and near-infrared spectroscopy is used to study the transitions of neutral oxygen leading to the emission of broad O i λ8446, λ11287, and λ1304 in active galactic nuclei. From the strength of the former two lines, contrary to the general belief, we found that in six of seven galaxies, Lyβ fluorescence is not the only mechanism responsible for the formation of these three lines. Because O i λ13165 is almost reduced to noise level, continuum fluorescence is ruled out as an additional excitation mechanism, but the presence of O i λ7774 in one of the objects suggests that collisional ionization may have an important role in the formation of O i λ8446. The usefulness of the O i lines as a reliable reddening indicator for the broad-line region is discussed. The values of E(B−V) derived from the λ1304/ λ8446 ratio agree with those obtained using other reddening indicators. The observations point toward a break in the one-to-one photon relation between λ8446 and λ1304, attributable to several destruction mechanisms that may affect the latter line
Infrared Fe II emission in narrow-line Seyfert 1 galaxies
No full textWe obtained 0.8-2.4 μm spectra at a resolution of 320 km s-ˡ of four narrow-line Seyfert 1 galaxies in order to study the near-infrared properties of these objects. We focus on the analysis of the Fe II emission in that region and the kinematics of the low-ionization broad lines. We show that the 1 μm Fe II lines (λ9997, λ10501, λ10863, and λ11126) are the strongest Fe II lines in the observed interval. For the first time, primary cascade lines of Fe II arising from the decay of upper levels pumped by Lyα fluorescence are resolved and identified in active galactic nuclei. Excitation mechanisms leading to the emission of the 1 μm Fe II features are discussed. A combination of Lyα fluorescence and collisional excitation is found to be the main contributor. The flux ratio between near-IR Fe II lines varies from object to object, in contrast to what is observed in the optical region. A good correlation between the 1 μm and optical Fe II emission is found. This suggests that the upper z 4 F 0 and z 4 D 0 levels from which the bulk of the optical Fe II lines descend are mainly populated by the transitions leading to the 1 μm lines. The width and profile shape of Fe II λ11127, Ca II λ8642, and O I λ8446 are very similar but significantly narrower than Paβ, giving strong observational support to the hypothesis that the regions where Fe II, Ca II, and O I are produced are cospatial, interrelated kinematically, and most probably located in the outermost portion of the broad-line region
Radiatively inefficient accretion flow in the nucleus of NGC 1097
No full textWe present a model for the accretion flow around the supermassive black hole in the LINER nucleus of NGC 1097 that fits the optical to X-ray spectral energy distribution (SED). The X-ray segment of the SED is based on observations with the Chandra X-Ray Observatory, which are reported here for the first time. The inner part of the flow is modeled as a radiatively inefficient accretion flow (RIAF), and the outer part as a standard thin disk. The value of the transition radius (rtr ≈ 225RS, where RS = 2GM/c²) between the RIAF and the outer thin disk was obtained from our previous fitting of the double-peaked Balmer emission line profile, which originates in the thin disk. The black hole mass was inferred from measurements of the stellar velocity dispersion in the host galaxy. When these parameters are used in the accretion flow model, the SED can be successfully reproduced, which shows that the line profile model and the accretion flow model are consistent with each other. A small remaining excess in the near-UV is accounted for by the contribution of an obscured starburst located within 9 pc from the nucleus, as we reported in an earlier paper. The radio flux is consistent with synchrotron emission of a relativistic jet modeled by means of the internal shock scenario. In an appendixwe also analyze the Chandra X-ray observations of the ~1 kpc circumnuclear star-forming ring and of an ultraluminous compact X-ray source located outside the ring
Measuring the evolution of the most stable optical clock G 117-B15A
No full textWe report our measurement of the rate of change of period with time (˙P ) for the 215 s periodicity in the pulsating white dwarf G 117-B15A, the most stable optical clock known. After 31 years of observations, we have finally obtained a 4σ measurement ˙Pobserved = (4.27± 0.80) x 10 15 s s-ˡ. Taking into account the proper-motion effect of ˙P proper = (7.0 ± 2.0) x 10 16 s s-ˡ,we obtain a rate of change of period with time of ˙P =(3.57 ± 0.82) ; 10 15 s s-ˡ. This value is consistent with the cooling rate in our white dwarf models only for cores of C or C/O. With the refinement of the models, the observed rate of period change can be used to accurately measure the ratio of C/O in the core of the white dwarf
Whole Earth Telescope observations of the pulsating subdwarf B star PG 0014+067
No full textPG 0014+067 is one of the most promising pulsating subdwarf B stars for seismic analysis, as it has a rich pulsation spectrum. The richness of its pulsations, however, poses a fundamental challenge to understanding the pulsations of these stars, as the mode density is too complex to be explained only with radial and nonradial low-degree (l< 3) p-modes without rotational splittings. One proposed solution, suggested by Brassard et al. in 2001 for the case of PG 0014+067 in particular, assigns some modes with high degree (l=3). On the other hand, theoretical models of sdB stars suggest that they may retain rapidly rotating cores, and so the high mode density may result from the presence of a few rotationally split triplet (l = 1) and quintuplet (l = 2) modes, along with radial (l = 0) p-modes. To examine alternative theoretical models for these stars, we need better frequency resolution and denser longitude coverage. Therefore, we observed this star with the Whole Earth Telescope for two weeks in 2004 October. In this paper we report the results of Whole Earth Telescope observations of the pulsating subdwarf B star PG 0014+067.We find that the frequencies seen in PG 0014+067 do not appear to fit any theoretical model currently available; however, we find a simple empirical relation that is able to match all of the well-determined frequencies in this star
On the possible existence of short-period g-mode instabilities powered by nuclear-burning shells in post-asymptotic giant branch h-deficient (PG1159-TYPE) stars
No full textWe present a pulsational stability analysis of hot post-asymptotic giant branch (AGB) H-deficient pre-white dwarf stars with active He-burning shells. The stellar models employed are state-of-the-art equilibrium structures representative of PG1159 stars derived from the complete evolution of the progenitor stars, through the thermally pulsing AGB phase and born-again episode. On the basis of fully nonadiabatic pulsation computations, we confirmed theoretical evidence for the existence of a separate PG1159 instability strip in the log Teff–log g diagram characterized by short-period g-modes excited by the -mechanism. This instability strip partially overlaps the already known GW Vir instability strip of intermediate/long-period g-modes destabilized by the classical κ-mechanism acting on the partial ionization of C and/or O in the envelope of PG1159 stars. We found that PG1159 stars characterized by thick He-rich envelopes and located inside this overlapping region could exhibit both short and intermediate/ long periods simultaneously. As a natural application of our results, we study the particular case of VV 47, a pulsating planetary nebula nucleus (PG1159 type) that is particularly interesting because it has been reported to exhibit a rich and complex pulsation spectrum including a series of unusually short pulsation periods.We found that the long periods exhibited byVV47 can be readily explained by the classical κ-mechanism,while the observed shortperiod branch below ≈300 s could correspond to modes triggered by the He-burning shell through the ε-mechanism, although more observational work is needed to confirm the reality of these short-period modes. Were the existence of short-period g-modes in this star convincingly confirmed by future observations, VV 47 could be the first known pulsating star in which both the κ-mechanism and the ε-mechanism of mode driving are simultaneously operating
The compton-thick Seyfert 2 nucleus of NGC 3281 torus constraints from the 9.7μm silicate absorption
No full textWe present mid-infrared (mid-IR) spectra of the Compton-thick Seyfert 2 galaxy NGC 3281, obtained with the Thermal-Region Camera Spectrograph at the Gemini-South telescope. The spectra present a very deep silicate absorption at 9.7μm, and [S IV] 10.5μm and [Ne II] 12.7μm ionic lines, but no evidence of polycyclic aromatic hydrocarbon emission. We find that the nuclear optical extinction is in the range 24 mag ≤ AV ≤ 83 mag. A temperature T = 300 K was found for the blackbody dust continuum component of the unresolved 65 pc nucleus and the region at 130 pc SE, while the region at 130 pc NW reveals a colder temperature (200 K). We describe the nuclear spectrum of NGC 3281 using a clumpy torus model that suggests that the nucleus of this galaxy hosts a dusty toroidal structure. According to this model, the ratio between the inner and outer radius of the torus in NGC 3281 is R0/Rd = 20, with 14 clouds in the equatorial radius with optical depth of τV = 40 mag. We would be looking in the direction of the torus equatorial radius (i = 60º), which has outer radius of R0 ∼ 11 pc. The column density is NH ≈ 1.2 × 10 24 cm-² and the iron Kα equivalent width (≈0.5–1.2 keV) is used to check the torus geometry. Our findings indicate that the X-ray absorbing column density, which classifies NGC 3281 as a Compton-thick source, may also be responsible for the absorption at 9.7μm providing strong evidence that the silicate dust responsible for this absorption can be located in the active galactic nucleus torus
A fossil bulge globular cluster revealed by very large telescope multi-conjugate adaptive optics
No full textThe globular cluster HP 1 is projected on the bulge, very close to the Galactic center. The Multi-Conjugate Adaptive Optics Demonstrator on the Very Large Telescope allowed us to acquire high-resolution deep images that, combined with first epoch New Technology Telescope data, enabled us to derive accurate proper motions. The cluster and bulge fields’ stellar contents were disentangled through this process and produced an unprecedented definition in color–magnitude diagrams of this cluster. The metallicity of [Fe/H] ≈ −1.0 from previous spectroscopic analysis is confirmed, which together with an extended blue horizontal branch imply an age older than the halo average. Orbit reconstruction results suggest that HP 1 is spatially confined within the bulge
Direct measurement of a secular pulsation period change in the pulsating hot pre-white dwarf PG 1159-035
No full textGW Vir (PG 1159[035) is the prototype of the class of multiperiodic, nonradially pulsating hot white dwarfs, and shows a strong pulsation mode at 516 s. All measurements to date of the secular variation of the 516 s pulsation quote as best value P= (-2.49±0.06)]10 11 s s-ˡ. The original measurement gave two best solutions, and a x² analysis indicated that the quoted value was preferred at the level of 0.97 probability. On other hand, the best-developed models for planetary nebula nuclei (PNNs), using models from the asymptotic giant branch as starting points and simulating the observed mass loss, provide positive values for any model with log (L/Lʘ)<~ as PG 1159-035. This conflict between the measurement and the theoretical models has been a challenge to stellar evolution theory. Exploiting a much larger data set and computational techniques previously unavailable, we show that the earlier analysis of the data grossly underestimated the true uncertainties due to interferences between frequencies. Using new data along with the old, and more accurate statistical methods, we calculated the secular period change of the 516 s pulsation, and obtained a positive value : P=(+13.07±0.03)]10 -11 s s-ˡ. We show that three additional methods yield the same solution. This new value was the second best of the original possible solutions ; it was eliminated on the basis of statistical arguments that we show to be invalid. It is an order of magnitude larger than the theoretical predictions. Additionally, from rotational splitting analysis, we were able to estimate, for the first time, a limit to the secular variation of the rotational period P rot=(-1.0±3.5)]10 -11 s s-ˡ, leading to a contraction timescale upper limit of |tr-ˡ|=|R/R| < 48 x 10 11 s-ˡ with 99.5% probability