arXiv subjects

D. R. Goncalves

Publications and source records attributed to D. R. Goncalves.

13 recordsLinked to original sources

arXiv2024

Confirmation of the planetary nebula nature of HaTr 5. Not the remnant of Nova Sco 1437

The identification of the nebula HaTr 5 with the shell remnant of the historic Nova Sco 1437 around the low-accretion rate cataclysmic variable 2MASS J17022815-4306123 has been used in the framework of the hibernation scenario to set an upper limit of <580 yr to the transition time from a nova-like binary to a dwarf nova. This work aims at clarifying the nature of HaTr 5, which has also previously been proposed to be a possible planetary nebula. Intermediate- and high-dispersion long-slit spectra of HaTr\,5 have been obtained and analyzed in conjunction with archival optical and infrared images to investigate its spectral properties using photoionization models, to derive its H-alpha flux and ionized mass, and to determine its spatio-kinematic by means of 3D models to clarify its true nature. The H-alpha flux of HaTr 5 implies an ionized mass of 0.059 M_Sun at the 0.99 kpc distance of J170228, i.e., about 1000 times the typical ejecta of a nova. If HaTr\,5 were actually an unrelated planetary nebula, its H-alpha flux implies a distance of 2.25 kpc and an ionized mass of 0.47 M_Sun. The expansion velocity of HaTr 5 is found to be of 27 km/s, with a heliocentric radial velocity of -1 km/. The ionized mass of HaTr 5 and its expansion velocity (and associated kinematic age) are clearly inconsistent with those expected for a nova remnant, which all strongly support a planetary nebula nature. The association of J170228 with HaTr 5 is further called into question by their differing radial velocities and almost orthogonal motions on the plane of the sky. It is concluded that HaTr 5 is an old, evolved planetary nebula unrelated to the remnant of Nova Sco 1437 and to the cataclysmic variable J170228, the latter being by chance projected onto HaTr 5.

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arXiv2011

When Shape Matters: correcting the ICFs to derive the chemical abundances of bipolar and elliptical PNe

The extraction of chemical abundances of ionised nebulae from a limited spectral range is usually hampered by the lack of emission lines corresponding to certain ionic stages. So far, the missing emission lines have been accounted for by the ionisation correction factors (ICFs), constructed under simplistic assumptions like spherical geometry by using 1-D photoionisation modelling. In this contribution we discuss the results (Goncalves et al. 2011, in prep.) of our ongoing project to find a new set of ICFs to determine total abundances of N, O, Ne, Ar, and S, with optical spectra, in the case of non-spherical PNe. These results are based on a grid of 3-D photoionisation modelling of round, elliptical and bipolar shaped PNe, spanning the typical PN luminosities, effective temperatures and densities. We show that the additional corrections --to the widely used Kingsburgh and Barlow (1994) ICFs-- are always higher for bipolars than for ellipticals. Moreover, these additional corrections are, for bipolars, up to: 17% for oxygen, 33% for nitrogen, 40% for neon, 28% for argon and 50% for sulphur. Finally, on top of the fact that corrections change greatly with shape, they vary also greatly with the central star temperature, while the luminosity is a less important parameter.

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arXiv2009

Low-ionization pairs of knots in planetary nebulae: physical properties and excitation

We obtained optical long-slit spectra of four planetary nebulae (PNe) with low-ionization pair of knots, namely He 1-1, IC 2149, KjPn 8 and NGC 7662. These data allow us to derive the physical parameters and excitation of the pairs of knots, and those of higher ionization inner components of the nebulae, separately. Our results are as follows. 1) The electron temperatures of the knots are within the range 9500 to 14500 K, similar to the temperatures of the higher ionization rims/shells. 2) Typical knots' densities are 500 to 2000 cm^{-3}. 3) Empirical densities of the inner rims/shells are higher than those of the pairs of knots, by up to a factor of 10. Theoretical predictions, at variance with the empirical results, suggest that knots should be denser than the inner regions, by at least a factor of 10. 4) Empirical and theoretical density contrasts can be reconciled if we assume that at least 90% of the knots' gas is neutral (likely composed of dust and molecules). 5) By using Raga et al. (2008) shock modeling and diagnostic diagrams appropriated for spatially resolved PNe, we suggest that high-velocity shocked knots traveling in the photoionized outer regions of PNe can explain the emission of the pairs of knots analysed in this paper.

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arXiv2006

The chemical content of nearby galaxies from planetary nebulae: NGC 147

We report the results of spectroscopic observations, obtained with the GEMINI Multi-Object Spectrograph, of 8 planetary nebulae (PNe) in the dwarf spheroidal (dSph) galaxy NGC147, a companion of M31. The physico-chemical properties of the six brightest PNe (Corradi et al. 2005) were derived using both the empirical ICF method and photoionization modelling with CLOUDY. Different aspects of the evolution of low and intermediate mass stars in a low-metallicity environment are analysed using relationships between chemical abundances. In addition, certain features of the chemical evolution of NGC147 were examined. In particular, the mean metallicity of PNe, O/H=8.06 (corresponding to [Fe/H](PNe)~-0.97), is close to the metallicity of the old stellar population, [Fe/H]=-1.0 (Butler & Martinez-Delgado), suggesting a negligible chemical enrichment during a substantial amount of time. Finally, the luminosity-metallicity relationship for the dwarf galaxies of the Local Group is discussed. The location in the luminosity-metallicity diagram of dSphs does not exclude their formation from old dwarf irregular (dIrs) galaxies, but it does exclude their formation from the present time dIrs, since the differences between their metallicities are already present in their older populations. The offset in the luminosity-metallicity relationship indicates a faster enrichment of dSphs, and together with the different average abundance ratio [O/Fe] demonstrates the different star formation histories for these two types of galaxies.

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arXiv2004

K 4-47: a planetary nebula excited by photons and shocks

K 4-47 is an unusual planetary nebula composed of a compact high-ionization core and a pair of low-ionization knots. Long-slit medium-resolution spectra of the knots and core are analyzed in this paper. Assuming photoionization from the central star, we have derived physical parameters for all the nebular components, and the (icf) chemical abundances of the core, which appear similar to Type-I PNe for He and N/O but significantly deficient in oxygen. The nebula has been further modelled using both photoionization (CLOUDY) and shock (MAPPINGS) codes. From the photoionization modelling of the core, we find that both the strong auroral [O III] 4363A and [N II] 5755A emission lines observed and the optical size of the core cannot be accounted for if a homogeneus density is adopted. We suggest that a strong density stratification, matching the high-density core detected at radio wavelengths and the much lower density of the optical core, might solve the problem. From the bow-shock modelling of the knots, on the other hand, we find that knots' chemistry is also represented by Type-I PN abundances, and that they would move with velocities of 250 - 300 km/s.

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arXiv2003

New Results on R Aquarii

The first results on new optical data for R Aquarii (based on observations obtained with the 2.5m INT, the 2.5m NOT and the 3.5m NTT) are presented. The morphology and kinematics of the nebula, based on data obtained with the NTT from 1991 to 2000, are discussed. Physical parameters of the outer nebula and the knotty jet are derived using spectra obtained with the INT in 2001. From the analysis of all these data we propose that the spectacular knotty inner structure of R Aqr could result from the interaction of a highly collimated pulsating young jet with the older hourglass inner nebula.

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arXiv2003

The Physical Parameters and Excitation of Jets and Knots in PNe

We are carrying out a study of the physical parameters and excitation of low-ionization structures (LIS) in planetary nebulae (PNe). Since the optical morphology and kinematics of the LIS and the main components (rims, shells and haloes) of our sample were studied previously, our main goal now is to search for: i) the density contrasts between jets/knots and the main nebular components; ii) their main excitation processes; and iii) their chemical abundances. The first results of this survey -- based on the analysis of NGC7009, NGC6543, NGC6891 and K4-47 -- are that there is no significant density contrast between LIS and their surroundings, and that most of the LIS studied (but not all) are mainly photoionized, rather than shock excited.

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arXiv2001

Evolution of active galactic nuclei broad-line region clouds: low- and high-ionization lines

The formation of quasar broad-line region (BLR) clouds via thermal instability in the presence of Alfven heating has been discussed by Goncalves et al. (1993a, 1996). In particular, these studies showed the relevance of Alfven heating in establishing the stability of BLR clouds in the intercloud medium. The present paper shows the results of time-dependent calculations (we use a time-dependent hydrodynamic code) following the evolution of BLR clouds, since their formation from the 10^7 K intercloud medium. We also calculate the UV and optical line emission associated with the clouds in order to compare with observations. Our results are compared with those of UV and optical monitoring of well-studied AGN, which suggest that the BLR is most probably composed of at least two different regions, each one giving rise to a kind of line variability, since low- and high-ionization lines present different patterns of variability. We discuss the alternative scenario in which lines of different ionization could be formed at the same place but heated/excited by distinct mechanisms, considering as a non-radiative mechanism the Alfven heating.

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arXiv2000

Knots in the outer shells of the planetary nebulae IC 2553 and NGC 5882

We present images and high-resolution spectra of the planetary nebulae IC 2553 and NGC 5882. Spatio-kinematic modeling of the nebulae shows that they are composed of a markedly elongated inner shell, and of a less aspherical outer shell expanding at a considerably higher velocity than the inner one. Embedded in the outer shells of both nebulae are found several low-ionization knots. In IC 2553, the knots show a point-symmetric distribution with respect to the central star: one possible explanation for their formation is that they are the survivors of pre-existing point-symmetric condensations in the AGB wind, a fact which would imply a quite peculiar mass-loss geometry from the giant progenitor. In the case of NGC 5882, the lack of symmetry in the distribution of the observed low-ionization structures makes it possible that they are the result of in situ instabilities.

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arXiv1999

Low-ionization structures in planetary nebulae

We present new results of a program aimed at studying the physical properties, origin and evolution of those phenomena which go under the somewhat generic definition of "low-ionization, small-scale structures in PNe". We have obtained morphological and kinematical data for 10 PNe, finding low-ionization structures with very different properties relative to each other, in terms of expansion velocities, shapes, sizes and locations relatively to the main nebular components. It is clear that several physical processes have to be considered in order to account for the formation and evolution of the different structures observed. We present here some results that are illustrative of our work - on IC 4593, NGC 3918, K 1-2, Wray 17-1, NGC 6337, He 2-186 and K 4-47 - and some of the questions that we try to address.

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arXiv1998

Magnetic Fields on the Dynamics of the ICM

Could the discrepancies found in the determination of mass in clusters of galaxies, from gravitational lensing data and from X-rays observations, be consequence of the standard description of the ICM, in which it is assumed hydrostatic equilibrium maintained by thermal pressure? In analogy to the interstellar medium of the Galaxy, it is expected a non-thermal term of pressure, which contains contributions of magnetic fields. We follow the evolution of the ICM, considering a term of magnetic pressure, aiming at answering the question whether or not these discrepancies can be explained via non-thermal terms of pressure. Our results suggest that the magnetic pressure could only affect the dynamics of the ICM on scales as small as $\la 1 {\rm kpc}$. These results are compared to the observations of large and small scale magnetic fields and we are successful at reproducing the available data.

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arXiv1998

The Possibility of Thermal Instability in Early-Type Stars Due to Alfven Waves

It was shown by dos Santos et al. the importance of Alfvén waves to explain the winds of Wolf-Rayet stars. We investigate here the possible importance of Alfvén waves in the creation of inhomogeneities in the winds of early-type stars. The observed infrared emission (at the base of the wind) of early-type stars is often larger than expected. The clumping explains this characteristic in the wind, increasing the mean density and hence the emission measure, making possible to understand the observed infrared, as well as the observed enhancement in the blue wing of the $H_α$ line. In this study, we investigate the formation of these clumps a via thermal instability. The heat-loss function used, $H(T,n)$, includes physical processes such as: emission of (continuous and line) recombination radiation; resonance line emission excited by electron collisions; thermal bremsstrahlung; Compton heating and cooling; and damping of Alfvén waves. As a result of this heat-loss function we show the existence of two stable equilibrium regions. The stable equilibrium region at high temperature is the diffuse medium and at low temperature the clumps. Using this reasonable heat-loss function, we show that the two stable equilibrium regions can coexist over a narrow range of pressures describing the diffuse medium and the clumps.

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arXiv1996

Alfven heating in optical filaments in cooling flows

One of the major problems concerning cooling flows is the nature of the mechanism powering the emission lines of the optical filaments seen in the inner regions of cooling flows. In this work we investigate the plausibility of Alfvén heating (AH) as a heating/excitation mechanism of optical filaments in cooling flows. We use a time-dependent hydrodynamical code to follow the evolution of cooling condensations arising from the $10^7$ K cooling flow. The filaments contain magnetic fields and AH is at work at several degrees of efficiency (including no AH at all). We consider two damping mechanisms of Alfvén waves: nonlinear and turbulent. We calculate the optical line emission associated with the filaments and compare our results to the observations. We find that AH can be an important ionizing and heating source for class I filaments in the line-ratio scheme of Heckman et al. In addition, AH can be an important contributor to [OI]$λ6300$ emission even for the more luminous class II systems.

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