arXiv subjects

W. J. Maciel

Publications and source records attributed to W. J. Maciel.

At least 55 records · Page 3Linked to original sources

arXiv2006

New results on the time variation of the radial abundance gradients from planetary nebulae

New results on the time variation of the radial abundance gradients in the galactic disk are presented on the basis of four different samples of planetary nebulae. These comprise both smaller, homogeneous sets of data, and larger but non-homogeneous samples. Four different chemical elements are considered, namely, O, S, Ar, and Ne. Other objects such as open clusters, cepheids and HII regions are also taken into account. Our analysis support our earlier conclusions in the sense that, on the average, the radial abundance gradients have flattened out during the last 6 to 8 Gyr, with important consequences for models of the chemical evolution of the Galaxy.

astro-ph↗

arXiv2006

An estimate of the time variation of the abundance gradient from planetary nebulae III. O, S, Ar, and Ne: A comparison of PN samples

The time behaviour of the radial abundance gradients in the galactic disk is investigated on the basis of four different samples of planetary nebulae, comprising both smaller, homogeneous sets of data, and larger, albeit non-homogeneous samples. Four different chemical elements are considered, namely, oxygen, sulphur, argon and neon. Our analysis support our earlier conclusions that, on the average, the radial abundance gradients have flattened out in the last 6 to 8 Gyr.

astro-ph↗

arXiv2005

Planetary Nebulae as a Chemical Evolution Tool: Abundance Gradients

We have studied the time variation of the radial abundance gradients using samples of planetary nebulae, open clusters, cepheids and other young objects. Based on the analysis of O/H and S/H abundances for planetary nebulae and metallicities of the remaining objects, we concluded that the gradients have been flattening out in the last 8 Gyr with an average rate of the order of 0.005 - 0.010 dex/(kpc Gyr). We have estimated the errors involved in the determination of the gradients, and concluded that the existence of systematic abundance variations is more likely than a simple statistical dispersion around a mean value.

astro-ph↗

arXiv2004

An estimate of the time variation of the abundance gradient from planetary nebulae II. Comparison with open clusters, cepheids and young objects

The temporal behaviour of the radial abundance gradients has important consequences for models of the chemical evolution of the Galaxy. We present a comparison of the time variation of the abundance gradients in the Milky Way disk as determined from a sample of planetary nebulae, open clusters, cepheids and young objects, such as stars in OB associations and HII regions. We conclude that the [Fe/H] gradients as measured in open cluster stars strongly support the time flattening of the abundance gradient as determined from O/H and S/H measurements in planetary nebulae. This conclusion is also supported by the cepheid variables, for which very accurate gradients and ages can be determined, and also by some recent estimates for OB stars and HII regions. It is estimated that the average flattening rate for the last 8 Gyr is in the range 0.005-0.010 dex/kpc/Gyr.

astro-ph↗

arXiv2004

Evolution from AGB to planetary nebula in the MSX survey

We investigate the evolution of oxygen- and carbon-rich AGB stars, post-AGB objects, and planetary nebulae using data collected mainly from the MSX catalogue. Magnitudes and colour indices are compared with those calculated from a grid of synthetic spectra that describe the post-AGB evolution beginning at the onset of the superwind. We find that carbon stars and OH/IR objects form two distinct sequences in the (K-[8.3])x([8.3]-[14.7]) MSX colour diagram. OH/IR objects are distributed in two groups: the bluest ones are crowded near [14.7]-[21.3]=1 and [8.3]-[14.7]=2, and a second, redder group is spread over a large area in the diagram, where post-AGB objects and planetary nebulae are also found. High mass-loss rate OH/IR objects, post-AGB stars, and planetary nebulae share the same region in the (K-[8.3])x([8.3]-[14.7]) and [14.7]-[21.3]x([8.3]-[14.7]) colour-colour diagrams. This region in the diagram is clearly separated from a bluer one where most OH/IR stars are found. We use a grid of models of post-AGB evolution, which are compared with the data. The gap in the colour-colour diagrams is interpreted as the result of the rapid trajectory in the diagram of the stars that have just left the AGB.

astro-ph↗

arXiv2004

Chemical Composition of Planetary Nebulae: The Galaxy and the Magellanic Clouds

An analysis is made of the abundances of planetary nebulae (PN) in the Galaxy (MW), the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). The data has been gathered by the IAG/USP group in a homogeneous procedure, involving observational techniques, data acquisition and reduction. Emphasis is placed on distance-independent correlations, which are particularly important for PN in the galactic disk. It is shown that (i) our abundances are comparable to the accurate abundances of nebulae with ISO data, (ii) most abundance correlations hold for all three systems, taking into account the metallicity differences between the Galaxy and the Magellanic Clouds, and (iii) there is a good general agreement of our data with the predictions of recent theoretical models for PN progenitors.

astro-ph↗

arXiv2004

Chemical enrichment and star formation in the Milky Way disk III. Chemodynamical constraints

In this paper, we investigate some chemokinematical properties of the Milky Way disk, by using a sample composed by 424 late-type dwarfs. We show that the velocity dispersion of a stellar group correlates with the age of this group, according to a law proportional to t^0.26, where t is the age of the stellar group. The temporal evolution of the vertex deviation is considered in detail. It is shown that the vertex deviation does not seem to depend strongly on the age of the stellar group. Previous studies in the literature seem to not have found it due to the use of statistical ages for stellar groups, rather than individual ages. The possibility to use the orbital parameters of a star to derive information about its birthplace is investigated, and we show that the mean galactocentric radius is likely to be the most reliable stellar birthplace indicator. However, this information cannot be presently used to derive radial evolutionary constraints, due to an intrinsic bias present in all samples constructed from nearby stars. An extensive discussion of the secular and stochastic heating mechanisms commonly invoked to explain the age-velocity dispersion relation is presented. We suggest that the age-velocity dispersion relation could reflect the gradual decrease in the turbulent velocity dispersion from which disk stars form, a suggestion originally made by Tinsley and Larson (1978) and supported by several more recent disk evolution calculations. A test to distinguish between the two types of models using high-redshift galaxies is proposed.

astro-ph↗

arXiv2004

Chemical abundances of planetary nebulae towards the direction of the galactic anticenter

In this paper we report new observations and derive chemical abundances for a sample of 26 planetary nebulae (PN) located in the anticenter direction. Most of these nebulae are far away objects, located at galactocentric distances greater than about 8 kpc, so that they are particularly useful for the determination of the radial gradients at large distances from the galactic center. A comparison of the present results with previously determined abundances suggests that the radial abundance gradients flatten out at distances larger than about 10 kpc from the center.

astro-ph↗

arXiv2004

A comparison of the time variation of radial abundance gradients from planetary nebulae and open clusters

The temporal behaviour of the radial abundance gradients has important consequences on models for the chemical evolution of the Galaxy. We present a comparison of the time variation of the abundance gradients in the Milky Way disk as determined from a sample of planetary nebulae and open clusters. We conclude that the [Fe/H] gradients as measured in open cluster stars strongly support the time flattening of the abundance gradient as recently determined from O/H measurements in planetary nebulae. We estimate the average flattening rate in the time interval 10 Myr < t < 5 Gyr as roughly 0.005 dex kpc-1 Gyr-1.

astro-ph↗

arXiv2003

New abundances of planetary nebulae in the Galactic Bulge

New observations and derived chemical abundances are reported for a sample of 57 bulge planetary nebulae (PN). Together with our previous results, a total of over a hundred objects have been analyzed, which constitute one of the largest samples of bulge nebulae studied under homogeneous conditions, including equipment and reduction procedures. In general, our data show a good agreement with some recent results in the literature, in the sense that the average abundances of bulge PN are similar to those from disk objects, however showing a larger dispersion.

astro-ph↗

arXiv2002

An estimate of the time variation of the O/H radial gradient from planetary nebulae

Radial abundance gradients are a common feature of spiral galaxies, and in the case of the Galaxy both the magnitude of the gradients and their variations are among the most important constraints of chemical evolution models. Planetary nebulae (PN) are particularly interesting objects to study the gradients and their variations. Owing to their bright emission spectra, they can be observed even at large galactocentric distances, and the derived abundances are relatively accurate, with uncertainties of about 0.1 to 0.2 dex, particularly for the elements that are not synthesized in their progenitor stars. On the other hand, as the offspring of intermediate mass stars, with main sequence masses in the interval of 1 to 8 solar masses, they are representative of objects with a reasonable age span. In this paper, we present an estimate of the time variation of the O/H radial gradient in a sample containing over 200 nebulae with accurate abundances. Our results are consistent with a flattening of the O/H gradient roughly from -0.11 dex/kpc to -0.06 dex/kpc during the last 9 Gyr, or from -0.08 dex/kpc to -0.06 dex/kpc during the last 5 Gyr.

astro-ph↗

arXiv2002

Using Cepheids to determine the galactic abundance gradient. III.First results for the outer disc

As a continuation of our previous work, which concerned the radial abundance distribution in the galactic disc over the distances 4-10 kpc this paper presents the first results on the metallicicty in the outer disc (RG > 10 kpc). Based on high-resolution spectra obtained for 19 distant Cepheids we sampled galactocentric distances from 10 to 12 kpc. Combined with the results of our previous work on the inner and middle parts of the galactic disc, the present data enable one to study the structure of the radial abundance distribution over a large baseline. In particular, we find indications of a discontinuity in the radial abundance distribution for iron as well as a number of the other elements. The discontinuity is seen at a galactocentric distance RG = 10 kpc. This finding supports the results reported earlier by Twarog et al. (1997).

astro-ph↗

arXiv2001

Using Cepheids to determine the galactic abundance gradient I. The solar neighbourhood

A number of studies of abundance gradients in the galactic disk have been performed in recent years. The results obtained are rather disparate: from no detectable gradient to a rather significant slope of about -0.1 dex kpc -1. The present study concerns the abundance gradient based on the spectroscopic analysis of a sample of classical Cepheids. These stars enable one to obtain reliable abundances of a variety of chemical elements. Additionally, they have well determined distances which allow an accurate determination of abundance distributions in the galactic disc. Using 236 high resolution spectra of 77 galactic Cepheids, the radial elemental distribution in the galactic disc between galactocentric distances in the range 6-11 kpc has been investigated. Gradients for 25 chemical elements (from carbon to gadolinium) are derived...

astro-ph↗

arXiv2001

Using Cepheids to determine the galactic abundance gradient II. Towards the galactic center

Based on spectra obtained at the Anglo-Australian Observatory, we present a discussion of the metallicity of the galactic disc derived using Cepheids at galactocentric distances 4-6 kpc. Our new results together with previous gradient determination (Paper I) show that the overall abundance distribution within the galactocentric distances 4-11 kpc cannot by represented by a single gradient value. The distribution is more likely bimodal: it is flatter in the solar neighbourhood with a small gradient, and steepens towards the galactic center. The steepening begins at a distance of about 6.6 kpc.

astro-ph↗

arXiv2001

Chromospherically young, kinematically old stars

We have investigated a group of stars known to have low chromospheric ages, but high kinematical ages. Isochrone, chemical and lithium ages are estimated for them. The majority of stars in this group show lithium abundances much smaller than expected for their chromospheric ages, which is interpreted as an indication of their old age. Radial velocity measurements in the literature also show that they are not close binaries. The results suggest that they can be formed from the coalescence of short-period binaries. Coalescence rates, calculated taking into account several observational data and a maximum theoretical time scale for contact, in a short-period pair, predict a number of coalesced stars similar to what we have found in the solar neighbourhood.

astro-ph↗

arXiv2001

PN and galactic chemical evolution

Recent applications of PN to the study of galactic chemical evolution are reviewed, such as PN and stellar populations, abundance gradients, including their space and time variations, determination of the He/H radial gradient and of the helium-to-metals enrichment ratio, and the [O/Fe] x [Fe/H] relation in the solar neighbourhood and in the galactic bulge.

astro-ph↗

arXiv2001

The Galactic Bulge: The Stellar and Planetary Nebulae Populations

We compare abundances patterns in the Bulge for elements observed in stars and in planetary nebulae. Some alpha elements, like Mg and Ti, are overabundant respect to Fe, and others are not, like He, O, Si, S, Ar, Ca. The first ones favor a quick evolution of the Galactic Bulge, and the seconds a much slower one.

astro-ph↗

arXiv2000

Radial gradients and metallicities in the galactic disk

Radial O/H abundance gradients derived from HII regions, hot stars and planetary nebulae are combined with [Fe/H] gradients from open cluster stars in order to derive an independent [O/Fe] x [Fe/H] relation for the galactic disk. A comparison of the obtained relation with recent observational data and theoretical models suggests that the [O/Fe] ratio is not higher than [O/Fe] ~ 0.4, at least within the metallicity range of the considered samples.

astro-ph↗