arXiv subjects

W. J. Maciel

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

At least 37 records · Page 2Linked to original sources

arXiv2011

Kinematic ages of central stars of planetary nebulae

The age distribution of the central stars of planetary nebulae (CSPN) is estimated using two methods based on their kinematic properties. First, the expected rotation velocities of the nebulae at their Galactocentric distances are compared with the predicted values for the rotation curve, and the differences are attributed to the different ages of the evolved stars. Adopting the relation between the ages and the velocity dispersions determined by the Geneva-Copenhagen survey, the age distribution can be derived. Second, the U, V, W, velocity components of the stars are determined, and the corresponding age-velocity dispersion relations are used to infer the age distribution. These methods have been applied to two samples of PN in the Galaxy. The results are similar for both samples, and show that the age distribution of the PN central stars concentrates in ages lower than 5 Gyr, peaking at about 1 to 3 Gyr.

astro-ph.SR↗

arXiv2011

Planetary nebulae in the inner Milky Way II: the Bulge-Disk transition

In this work, a sample of planetary nebulae located in the inner-disk and bulge of the Galaxy is used in order to find the galactocentric distance which better separates these two populations, from the point of view of abundances. Statistical distance scales were used to study the distribution of abundances across the disk-bulge interface. A Kolmogorov-Smirnov test was used to find the distance in which the chemical properties of these regions better separate.The results of the statistical analysis indicate that, on the average, the inner population has lower abundances than the outer. Additionally, for the $α$-elements abundances, the inner population does not follow the disk radial gradient towards the galactic center. Based on our results, we suggest a bulge-disk interface at 1.5 kpc, marking the transition between the bulge and inner-disk of the Galaxy, as defined by the intermediate mass population.

astro-ph.GA↗

arXiv2010

Nucleosynthesis and chemical evolution of intermediate-mass stars: results from planetary nebulae

Planetary nebulae (PN) are an excellent laboratory to investigate the nucleosynthesis and chemical evolution of intermediate mass stars. In these objects accurate abundances can be obtained for several chemical elements that are manufactured or contaminated by the PN progenitor stars, such as He, N, C, and also elements that were originally produced by more massive stars of previous generations, namely O, Ne, Ar, and S. Some of these elements are difficult to study in stars, so that PN can be used in order to complement results obtained from stellar data. In the past few years, we have obtained a large sample of PN with accurately derived abundances, including objects of different populations, namely the solar neighbourhood, the galactic disk and anticentre, the galactic bulge and the Magellanic Clouds. In this work, we present the results of our recent analysis of the chemical abundances of He, O, N, S, Ar and Ne in galactic and Magellanic Cloud PN. Average abundances and abundance distributions of all elements are determined, as well as distance-independent correlations. These correlations are particularly important, as they can be directly compared with the predictions of recent theoretical evolutionary models for intermediate mass stars.

astro-ph.SR↗

arXiv2010

Symmetric vs. asymmetric planetary nebulae: morphology and chemical abundances

We analyse a large sample of galactic planetary nebulae based on their chemical composition and morphology. A recent morphological classification system is adopted, and several elements are considered, namely He, N, O, S, Ar, Ne, and C in order to investigate the correlations involving these elements and the different PN types. Special emphasis is given to the differences between symmetric (round or elliptical) nebulae and those that present some degree of asymmetry (bipolars or bipolar core objects). The results are compared with previous findings both for PN in the Galaxy and in the Magellanic Clouds.

astro-ph.GA↗

arXiv2010

Planetary nebulae in the inner Milky Way: new abundances

The study of planetary nebulae in the inner-disk and bulge gives important information on the chemical abundances of elements such as He, N, O, Ar, Ne, and on the evolution of these abundances, which is associated with the evolution of intermediate-mass stars and the chemical evolution of the Galaxy. We present accurate abundances of the elements He, N, S, O, Ar, and Ne for a sample of 54 planetary nebulae located towards the bulge of the Galaxy, for which 33 have the abundances derived for the first time. The abundances are derived based on observations in the optical domain made at the National Laboratory for Astrophysics (LNA, Brazil). The data show a good agreement with other results in the literature, in the sense that the distribution of the abundances is similar to those works.

astro-ph.GA↗

arXiv2009

Age distribution of the central stars of galactic disk planetary nebulae

The determination of ages of central stars of planetary nebulae (CSPN) is a complex problem, and there is presently no single method that can be generally applied. We have developed several methods to estimate the ages of CSPN, based both on the observed nebular properties and in some properties of the stars themselves.Our aim is to estimate the ages and the age distribution of CSPN and to compare the derived results with mass and age determinations of CSPN and white dwarfs based on empirical determinations of these quantities. We discuss several methods to derive the age distribution of CSPN, namely, (i) the use of an age-metallicity relation that also depends on the galactocentric distance, (ii) the use of an age-metallicity relation obtained for the galactic disk, and (iii) the determination of ages from the central star masses obtained from the observed nitrogen abundances. We consider a sample of planetary nebulae in the galactic disk, most of which ($\sim$ 69%) are located in the solar neighbourhood, within 3 kpc from the Sun. We estimate the age distribution of CSPN with average uncertainties of 1-2 Gyr, and compare our results with the expected distribution based both on the observed mass distribution of white dwarfs and on the age distribution derived from available mass distributions of CSPN. We conclude most CSPN in the galactic disk have ages under 6 Gyr, and that the age distribution is peaked around 2-4 Gyr.

astro-ph.GA↗

arXiv2009

Planetary nebulae in the inner Milky Way

New abundances of planetary nebulae located towards the bulge of the Galaxy are derived based on observations made at LNA (Brazil). We present accurate abundances of the elements He, N, S, O, Ar, and Ne for 56 PNe located towards the galactic bulge. The data shows a good agreement with other results in the literature, in the sense that the distribution of the abundances is similar to those works. From the statistical analysis performed, we can suggest a bulge-disk interface at 2.2 kpc for the intermediate mass population, marking therefore the outer border of the bulge and inner border of the disk.

astro-ph.GA↗

arXiv2009

Metallicity gradients in the Milky Way

Radial metallicity gradients are observed in the disks of the Milky Way and in several other spiral galaxies. In the case of the Milky Way, many objects can be used to determine the gradients, such as HII regions, B stars, Cepheids, open clusters and planetary nebulae. Several elements can be studied, such as oxygen, sulphur, neon, and argon in photoionized nebulae, and iron and other elements in cepheids, open clusters and stars. As a consequence, the number of observational characteristics inferred from the study of abundance gradients is very large, so that in the past few years they have become one of the main observational constraints of chemical evolution models. In this paper, we present some recent observational evidences of abundance gradients based on several classes of objects. We will focus on (i) the magnitude of the gradients, (ii) the space variations, and (iii) the evidences of a time variation of the abundance gradients. Some comments on recent theoretical models are also given, in an effort to highlight their predictions concerning abundance gradients and their variations.

astro-ph.GA↗

arXiv2009

Planetary nebulae and the chemical evolution of the Magellanic Clouds

The determination of accurate chemical abundances of planetary nebulae (PN) in different galaxies allows us to obtain important constraints of chemical evolution models for these systems. We have a long term program to derive abundances in the galaxies of the Local Group, particularly the Large and Small Magellanic Clouds. In this work, we present our new results on these objects and discuss their implications in view of recent abundance determinations the literature. In particular, we obtain distance-independent correlations involving He, N, O, Ne, S, and Ar, and compare the results with data from our own Galaxy and other galaxies in the Local Group. As a result of our observational program, we have a large database of PN in the Galaxy and the Magellanic Clouds, so that we can obtain reliable constraints to the nucleosynthesis processes in the progenitor stars in galaxies of different metallicities.

astro-ph.CO↗

arXiv2009

Chemical evolution of the galactic bulge: single and double infall models

Recent work has produced a wealth of data concerning the chemical evolution of the galactic bulge, both for stars and nebulae. Present theoretical models generally adopt a limited range of such constraints, frequently using a single chemical element (usually iron), which is not enough to describe it unambiguously. In this work, we take into account constraints involving as many chemical elements as possible, basically obtained from bulge nebulae and stars. Our main goal is to show that different scenarios can describe, at least partially, the abundance distribution and several distance-independent correlationss for these objects. Three classes of models were developed. The first is a one-zone, single-infall model, the second is a one-zone, double-infall model and the third is a multizone, double infall model. We show that a one-zone model with a single infall episode is able to reproduce some of the observational data, but the best results are achieved using a multizone, double infall model.

astro-ph.GA↗

arXiv2008

Chemical evolution of the Magellanic Clouds based on planetary nebulae

Planetary nebulae (PN) are an essential tool in the study of the chemical evolution of the Milky Way and galaxies of the Local Group, particularly the Magellanic Clouds. In this work, we present some recent results on the determination of chemical abundances from PN in the Large and Small Magellanic clouds, and compare these results with data from our own Galaxy and other galaxies in the Local Group. As a result of our continuing long term program, we have a large database comprising about 300 objects for which reliable abundances of several elements from He to Ar have been obtained. Such data can be used to derive constraints to the nucleosynthesis processes in the progenitor stars in galaxies of different metallicities. We also investigate the time evolution of the oxygen abundances in the SMC by deriving the properties of the PN progenitor stars, which include their masses and ages. We have then obtained an age-metallicity relation taking into account both oxygen and [Fe/H] abundances. We show that these results have an important consequence on the star formation rate of the SMC, in particular by suggesting a star formation burst in the last 2-3 Gyr.

astro-ph↗

arXiv2008

Abundance gradients in the galactic disk: space and time variations

Recent work on abundance gradients have focussed not only on their magnitudes, but also on their spatial and temporal variations. In this work, we analyze the behaviour of radial abundance gradients in the galactic disk giving special emphasis on these variations. The data used includes planetary nebulae and objects in different age brackets, namely open clusters, HII regions, cepheid variables and stars in OB associations. We find evidences for a space variation of the radial gradients as measured for element ratios such as O/H, S/H, Ne/H, Ar/H and [Fe/H], in the sense that the gradients tend to flatten out at large galactocentric distances. Moreover, near the bulge-disk interface a steep decrease in the abundances is observed. The time evolution of the gradients is also evaluated on the basis of approximate ages attributed to the central stars of planetary nebulae and open cluster stars. It is concluded that the available data is consistent with a time flattening of the gradients during the last 6 to 8 Gyr, a time interval in which the age determinations are probably more accurate.

astro-ph↗

arXiv2008

Metallicity effects on the modified wind momentum of CSPN

Recent investigations on the central stars of planetary nebulae (CSPN) indicate that the masses based on model atmospheres can be much larger than the masses derived from theoretical mass-luminosity relations. Also, the dispersion in the relation between the modified wind momentum and the luminosity depends on the mass spread of the CSPN, and is larger than observed in massive hot stars. Since the wind characteristics probably depend on the metallicity, we analyze the effects on the modified wind momentum by considering the dispersion in this quantity caused by the stellar metallicity. Our CSPN masses are based on a relation between the core mass and the nebular abundances. We conclude that these masses agree with the known mass distribution both for CSPN and white dwarfs, and that the spread in the modified wind momentum can be explained by the observed metallicity variations.

astro-ph↗

arXiv2007

Bayesian posterior classification of planetary nebulae according to the Peimbert types

In this paper we present a re-analysis of the criteria used to characterize the Peimbert classes I, IIa, IIb, III and IV, through a statistical study of a large sample of planetary nebulae previously classified according to these groups. In the original classification, it is usual to find planetary nebulae that cannot be associated with a single type; these most likely have dubious classifications into two or three types. Statistical methods can greatly contribute in providing a better characterization of planetary nebulae groups. We use the Bayes Theorem to calculate the posterior probabilities for an object to be member of each of the types I, IIa, IIb, III and IV. This calculation is particularly important for planetary nebulae that are ambiguously classified in the traditional method. The posterior probabilities are defined from the probability density function of classificatory parameters of a well-defined sample, composed only by planetary nebulae unambiguously fitted into the Peimbert types. Because the probabilities depend on the available observational data, they are conditional probabilities, and, as new observational data are added to the sample, the classification of the nebula can be improved, to take into account this new information. This method differs from the original classificatory scheme, because it provides a quantitative result of the representativity of the object within its group. Also, through the use of marginal distributions it is possible to extend the Peimbert classification even to those objects for which only a few classificatory parameters are known.

astro-ph↗

arXiv2007

Chemical evolution of the Small Magellanic Cloud based on planetary nebulae

We investigate the chemical evolution of the Small Magellanic Cloud (SMC) based on abundance data of planetary nebulae (PNe). The main goal is to investigate the time evolution of the oxygen abundance in this galaxy by deriving an age-metallicity relation. Such a relation is of fundamental importance as an observational constraint of chemical evolution models of the SMC. We have used high quality PNe data in order to derive the properties of the progenitor stars, so that the stellar ages could be estimated. We collected a large number of measured spectral fluxes for each nebula, and derived accurate physical parameters and nebular abundances. New spectral data for a sample of SMC PNe obtained between 1999 and 2002 are also presented. These data are used together with data available in the literature to improve the accuracy of the fluxes for each spectral line. We obtained accurate chemical abundances for PNe in the Small Magellanic Cloud, which can be useful as tools in the study of the chemical evolution of this galaxy and of Local Group galaxies. We present the resulting oxygen versus age diagram and a similar relation involving the [Fe/H] metallicity based on a correlation with stellar data. We discuss the implications of the derived age-metallicity relation for the SMC formation, in particular by suggesting a star formation burst in the last 2-3 Gyr.

astro-ph↗

arXiv2007

Time variation of radial gradients in the galactic disk: electron temperatures and abundances

We investigate the electron temperature gradient in the galactic disk as measured by young HII regions on the basis of radio recombination lines and the corresponding gradient in planetary nebulae (PN) based on [OIII] electron temperatures. The main goal is to investigate the time evolution of the electron temperature gradient and of the radial abundance gradient, which is essentially a mirror image of the temperature gradient. The recently derived electron temperature gradient from radio recombination lines in HII regions is compared with a new determination of the corresponding gradient from planetary nebulae for which the progenitor star ages have been determined. The newly derived electron temperature gradient for PN with progenitor stars with ages in the 4-5 Gyr range is much steeper than the corresponding gradient for HII regions. These electron temperature gradients are converted into O/H gradients in order to make comparisons with previous estimates of the flattening rate of the abundance gradient. It is concluded that the O/H gradient has flattened out in the past 5 Gyr at a rate of about 0.0094 dex kpc-1 Gyr-1, in good agreement with our previous estimates.

astro-ph↗

arXiv2006

The Electron Temperature Gradient in the Galactic Disk

We derive the electron temperature gradient in the Galactic disk using a sample of HII regions that spans Galactocentric distances 0--17 kpc. The electron temperature was calculated using high precision radio recombination line and continuum observations for more than 100 HII regions. Nebular Galactocentric distances were calculated in a consistent manner using the radial velocities measured by our radio recombination line survey. The large number of nebulae widely distributed over the Galactic disk together with the uniformity of our data provide a secure estimate of the present electron temperature gradient in the Milky Way. Because metals are the main coolants in the photoionized gas, the electron temperature along the Galactic disk should be directly related to the distribution of heavy elements in the Milky Way. Our best estimate of the electron temperature gradient is derived from a sample of 76 sources for which we have the highest quality data. The present gradient in electron temperature has a minimum at the Galactic Center and rises at a rate of 287 +/- 46 K/kpc. There are no significant variations in the value of the gradient as a function of Galactocentric radius or azimuth. The scatter we find in the HII region electron temperatures at a given Galactocentric radius is not due to observational error, but rather to intrinsic fluctuations in these temperatures which are almost certainly due to fluctuations in the nebular heavy element abundances. Comparing the HII region gradient with the much steeper gradient found for planetary nebulae suggests that the electron temperature gradient evolves with time, becoming flatter as a consequence of the chemical evolution of the Milky Way's disk.

astro-ph↗

arXiv2006

Dynamical evidence of the age--metallicity relation in the Milky Way disk

We studied the relationship between the average stellar abundance of several elements and the orbital evolution of stars in the neighbourhood of the Sun, using both observational data for 325 late-type dwarfs in a volume-complete sample and simulations of the orbital diffusion. Metallicities, ages, and initial position and velocities for the simulated stars are sampled from empirical distributions of these quantities in the Milky Way. We found that that there is a relationship between the average stellar abundance of Fe, Na, Si, Ca, Ni, and Ba and the mean orbital radius of stars currently passing through the solar neighbourhood. The greater the difference between the mean orbital radius and the solar Galactocentric distance, the more deficient the star is, on average, in these chemical species. The stars that take a longer time to come from their birthplaces to arrive in the present solar neighbourhood are more likely to be more metal-poor than those that were born here. This result is a direct, independent indication that a tightly defined Galactic age-metallicity relation exists.

astro-ph↗