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## Age distribution of the central stars of galactic disk planetary nebulae

MACIEL, W. J.; COSTA, R. D. D.; IDIART, T. E. P.
Fonte: EDP SCIENCES S A Publicador: EDP SCIENCES S A
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
45.99%
Context. Determination of the 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 of estimating the ages of CSPN, based on both the observed nebular properties and some properties of the stars themselves. Aims. 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. Methods. We considered a sample of planetary nebulae in the galactic disk, most of which (similar to 69%) are located in the solar neighbourhood, within 3 kpc from the Sun. We discuss several methods of deriving 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. Results. We estimated the age distribution of CSPN with average uncertainties of 1-2 Gyr, and compared 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. Based on our derived age distributions...

## Age Distribution of Influenza Like Illness Cases during Post-Pandemic A(H3N2): Comparison with the Twelve Previous Seasons, in France

Turbelin, Clément; Souty, Cécile; Pelat, Camille; Hanslik, Thomas; Sarazin, Marianne; Blanchon, Thierry; Falchi, Alessandra
Fonte: Public Library of Science Publicador: Public Library of Science
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.88%
In France, the 2011–2012 influenza epidemic was characterized by the circulation of antigenically drifted influenza A(H3N2) viruses and by an increased disease severity and mortality among the elderly, with respect to the A(H1N1)pdm09 pandemic and post-pandemic outbreaks. Whether the epidemiology of influenza in France differed between the 2011–2012 epidemic and the previous outbreaks is unclear. Here, we analyse the age distribution of influenza like illness (ILI) cases attended in general practice during the 2011–2012 epidemic, and compare it with that of the twelve previous epidemic seasons. Influenza like illness data were obtained through a nationwide surveillance system based on sentinel general practitioners. Vaccine effectiveness was also estimated. The estimated number of ILI cases attended in general practice during the 2011–2012 was lower than that of the past twelve epidemics. The age distribution was characteristic of previous A(H3N2)-dominated outbreaks: school-age children were relatively spared compared to epidemics (co-)dominated by A(H1N1) and/or B viruses (including the 2009 pandemic and post-pandemic outbreaks), while the proportion of adults over 30 year-old was higher. The estimated vaccine effectiveness (54%...

## Characteristic Age Distribution of Plasmodium vivax Infections after Malaria Elimination on Aneityum Island, Vanuatu

Kaneko, Akira; Chaves, Luis F.; Taleo, George; Kalkoa, Morris; Isozumi, Rie; Wickremasinghe, Renu; Perlmann, Hedvig; Takeo, Satoru; Tsuboi, Takafumi; Tachibana, Shin-Ichiro; Kimura, Masatsugu; Björkman, Anders; Troye-Blomberg, Marita; Tanabe, Kazuyuki; D
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.87%
Resurgence is a major concern after malaria elimination. After the initiation of the elimination program on Aneityum Island in 1991, microscopy showed that Plasmodium falciparum disappeared immediately, whereas P. vivax disappeared from 1996 onward, until P. vivax cases were reported in January 2002. By conducting malariometric surveys of the entire population of Aneityum, we investigated the age distribution of individuals with parasites during this epidemic in the context of antimalarial antibody levels and parasite antigen diversity. In July 2002, P. vivax infections were detected by microscopy in 22/759 individuals: 20/298 born after the beginning of the elimination program in 1991, 2/126 born between 1982 and 1991, and none of 335 born before 1982. PCR increased the number of infections detected to 77, distributed among all age groups. Prevalences were 12.1%, 16.7%, and 6.0%, respectively (P < 0.001). In November, a similar age pattern was found, but with fewer infections: 6/746 and 39/741 individuals were found to be infected by microscopy and PCR, respectively. The frequencies of antibody responses to P. vivax were significantly higher in individuals born before 1991 than in younger age groups and were similar to those on Malakula Island...

## Implications of Differential Age Distribution of Disease-Associated Meningococcal Lineages for Vaccine Development

Brehony, Carina; Trotter, Caroline L.; Ramsay, Mary E.; Chandra, Manosree; Jolley, Keith A.; van der Ende, Arie; Carion, Françoise; Berthelsen, Lene; Hoffmann, Steen; Harðardóttir, Hjördís; Vazquez, Julio A.; Murphy, Karen; Toropainen, Maija; Caniça
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.88%
New vaccines targeting meningococci expressing serogroup B polysaccharide have been developed, with some being licensed in Europe. Coverage depends on the distribution of disease-associated genotypes, which may vary by age. It is well established that a small number of hyperinvasive lineages account for most disease, and these lineages are associated with particular antigens, including vaccine candidates. A collection of 4,048 representative meningococcal disease isolates from 18 European countries, collected over a 3-year period, were characterized by multilocus sequence typing (MLST). Age data were available for 3,147 isolates. The proportions of hyperinvasive lineages, identified as particular clonal complexes (ccs) by MLST, differed among age groups. Subjects <1 year of age experienced lower risk of sequence type 11 (ST-11) cc, ST-32 cc, and ST-269 cc disease and higher risk of disease due to unassigned STs, 1- to 4-year-olds experienced lower risk of ST-11 cc and ST-32 cc disease, 5- to 14-year-olds were less likely to experience ST-11 cc and ST-269 cc disease, and ≥25-year-olds were more likely to experience disease due to less common ccs and unassigned STs. Younger and older subjects were vulnerable to a more diverse set of genotypes...

## Age-dependent changes of the normal human spine during adulthood

Ruhli, F.; Muntener, M.; Henneberg, M.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.85%
The impact of aging on the morphology of the osseous spine is still debated. Clinical studies usually record combined aging effects, as well as age-related degenerative changes. The aim of this study was to determine the impact of (degeneration-independent) aging on the morphology of the osseous human spine during adulthood. Various osseous dimensions of human spinal landmarks at all major vertebral levels have been assessed in macroscopically normal Swiss skeletons (N = 71), with historically known sex and age at death, as well as in larger Central European skeletal samples (N = 277) with anthropologically determined individual age and sex. All measurements were correlated with individual age (or age group) by linear regression and analyzed separately for each sex. Only few osseous spinal dimensions, and only in men, correlate significantly with individual age. Generally, the significant dimensions show an increase in size during adulthood. Similar tendencies, but with significant alterations of spinal measurements in women as well, can be found in the larger samples with anthropologically determined sex and age group. Increase of certain spinal dimensions found in this study may be a reflection of an increase in the robustness of individuals with age. Because of the absence of a significant secular alteration of stature within the well-recorded sample...

## Quantitative analyses of long-run human capital development: age heaping as an indicator for numeracy; Quantitative Analyse langfristiger Humankapitalentwicklung: Age Heaping als Indikator für numerische Fähigkeiten

Crayen, Dorothee
Tipo: Dissertação
EN
Relevância na Pesquisa
45.9%
For researchers studying the economic development of nations, the availability of human capital data is crucial. Economists making efforts to examine the impact of human resources on economic development before the end of World War II often face massive problems. Methods of collecting data, i.e., registration systems, population censuses or surveys did neither acquire sufficiently detailed data nor was the recorded data standardized in any consistent form and little attention was paid to direct information about a population’s level of education. For early time spans, more than any other data, demographic records are available. This thesis proposes to use irregularities in the reporting of age to estimate the people’s level of education. Single year age data, which enables researchers to depict a population’s detailed age structure, almost always exhibits irregularities in the form of heaped data, i.e., the age distribution does not run smoothly but exhibits sharp jumps and clustering at certain ages. This phenomenon is attributed to age heaping, a term which describes people’s ignorance to their age or people’s tendency to round their ages off. To a varying degree, age heaping exists in nearly all historical age statistics when people were asked for their age (as opposed to age records that were calculated from birth certificates or alike). By measuring the degree of age heaping one is able to derive a simple proxy for human capital covering a greater number of countries and regions as well as earlier periods than for instance signature ability rates. Being a proxy of numerical comprehension and diligence when responding to age questions...

## Implications of differential age distribution of disease-associated meningococcal lineages for vaccine development

Brehony, Carina; Trotter, Caroline L.; Ramsay, Mary E.; Jolley, Keith A.; van der Ende, Arie; Carion, Françoise; Berthelsen, Lene; Hoffmann, Steen; Harðardóttir, Hjördís; Vazquez, Julio A.; Murphy, Karen; Toropainen, Maija; Caniça, Manuela; Ferreira
Fonte: American Society for Microbiology Publicador: American Society for Microbiology
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.88%
New vaccines targeting meningococci expressing serogroup B polysaccharide have been developed, with some being licensed in Europe. Coverage depends on the distribution of disease-associated genotypes, which may vary by age. It is well established that a small number of hyperinvasive lineages account for most disease, and these lineages are associated with particular antigens, including vaccine candidates. A collection of 4,048 representative meningococcal disease isolates from 18 European countries, collected over a 3-year period, were characterized by multilocus sequence typing (MLST). Age data were available for 3,147 isolates. The proportions of hyperinvasive lineages, identified as particular clonal complexes (ccs) by MLST, differed among age groups. Subjects <1 year of age experienced lower risk of sequence type 11 (ST-11) cc, ST-32 cc, and ST-269 cc disease and higher risk of disease due to unassigned STs, 1- to 4-year-olds experienced lower risk of ST-11 cc and ST-32 cc disease, 5- to 14-year-olds were less likely to experience ST-11 cc and ST-269 cc disease, and ≥25-year-olds were more likely to experience disease due to less common ccs and unassigned STs. Younger and older subjects were vulnerable to a more diverse set of genotypes...

## Chile - Health Insurance Issues : Old Age and Catastrophic Health Costs

World Bank
Fonte: Washington, DC Publicador: Washington, DC
ENGLISH; EN_US
Relevância na Pesquisa
55.8%
The study offers an analytic approach to fundamental questions concerning the effect of the aging population on the Chilean health system, and, to the prospects for, and the extent of financing health care for the elderly, as presumably catastrophic costs are linked to this effect. However, the study reveals that catastrophic care is not a problem primarily affecting the elderly, since interventions show that, rather the highest costly episodes occur among infants. It is specified that technological change introduced the possibility of delivering care, at very high cost, to infants who would otherwise die, or suffer congenital disorders, and it is this change which shaped the current age distribution of catastrophic care. Under the country's health insurance system, the largest group is covered by the public system, while a smaller group is privately insured. However, the study reveals that significant numbers of catastrophic infant events are assisted in public facilities, regardless of patient's affiliation. This implies that apparently the public system does assume disproportionate catastrophic burdens...

## Orphans and Ebola : Estimating the Secondary Impact of a Public Health Crisis

Evans, David K.; Popova, Anna
Fonte: World Bank Group, Washington, DC Publicador: World Bank Group, Washington, DC
Tipo: Publications & Research; Publications & Research :: Policy Research Working Paper
ENGLISH; EN_US
Relevância na Pesquisa
45.89%
The 2014 Ebola Virus Disease outbreak in West Africa is the largest to date by far. Ebola Virus Disease causes disproportionate mortality among the working-age population, resulting in far more mortality for parents of young children than other health crises. This paper combines data on the age distribution of current and projected mortality from Ebola with the fertility distribution of adults in Guinea, Liberia, and Sierra Leone, to estimate the likely impact of the epidemic on the number of orphans in these three countries. Using the latest mortality estimates (from February 11, 2015), it is estimated that more than 9,600 children have lost one or both parents to Ebola Virus Disease. The absolute numbers of orphans created by the Ebola epidemic are significant, but represent a small fraction (1.4 percent) of the existing orphan burden in the affected countries. Ebola is unlikely to increase the numbers of orphans beyond extended family networks' capacities to absorb them. Nonetheless, the pressures of caring for increased numbers of orphans may result in lower quality of care. These estimates should be used to guide policy to support family networks to improve the capacity to provide high quality care to orphans.

## A snapshot of the age distribution of psychological well-being in the United States

Stone, Arthur A.; Schwartz, Joseph E.; Broderick, Joan E.; Deaton, Angus
Tipo: Artigo de Revista Científica
EN
Relevância na Pesquisa
45.85%
Psychological well-being (WB) includes a person's overall appraisal of his or her life (Global WB) and affective state (Hedonic WB), and it is considered a key aspect of the health of individuals and groups. Several cross-sectional studies have documented a relation between Global WB and age. Little is known, however, about the age distribution of Hedonic WB. It may yield a different view of aging because it is less influenced by the cognitive reconstruction inherent in Global WB measures and because it includes both positive and negative components of WB. In this study we report on both Global and Hedonic WB assessed in a 2008 telephone survey of 340,847 people in the United States. Consistent with prior studies, Global WB and positive Hedonic WB generally had U-shaped age profiles showing increased WB after the age of 50 years. However, negative Hedonic WB variables showed distinctly different and stronger patterns: Stress and Anger steeply declined from the early 20s, Worry was elevated through middle age and then declined, and Sadness was essentially flat. Unlike a prior study, men and women had very similar age profiles of WB. Several measures that could plausibly covary with the age-WB association (e.g., having children at home) did not alter the age-WB patterns. Global and Hedonic WB measures appear to index different aspects of WB over the lifespan...

## Age distribution of lmc clusters from their integrated ubv colors : history of star formation

Girardi, Leo Alberto; Chiosi, Cesare; Bertelli, Gianpaolo; Bressan, Alessandro
Tipo: Artigo de Revista Científica Formato: application/pdf
ENG
Relevância na Pesquisa
45.98%
In this paper we revise the relationship between ages and metallicities of LMC star clusters and their integrated UBV colors. The study stands on the catalog of UBV colors of the Large Magellanic Cloud (LMC) clusters by Bica et al. (1994; BCDSP) and the photometric models of single stellar populations (SSP) calculated by Bertelli et al. (1994). These photometric models nicely describe the color distribution of LMC clusters in the (U – B) vs. (B – V) plane together with the observed dispersion of the colors and the existence of a gap in a certain region of this diagram. In the case of blue clusters, most of the dispersion in the colors can be accounted for by the presence of stochastic effects on the mass distribution of stars, whereas for the red ones additional dispersion's of ~ 0.2 dex in metallicity and of ~ 0.05 mag in color excess are needed. From comparing the observed distribution of integrated colors in the (U – B) vs. (B – V) diagram with the theoretical models, it turns out that: 1) The data are consistent with the presence of a gap (period of quiescence) in the history of cluster formation. If the age-metallicity relation (AMR) for the LMC obeys the simple model of chemical evolution, the gap is well evident and corresponds to the age interval ~ 3 Gyr to (12- 15) Gyr. On the contrary...

## The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way

Lineweaver, Charles; Fenner, Y; Gibson, Bradley K
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
55.75%
We modeled the evolution of the Milky Way Galaxy to trace the distribution in space and time of four prerequisites for complex life: the presence of a host star, enough heavy elements to form terrestrial planets, sufficient time for biological evolution,

## Searching for Earth analogues around the nearest stars: the disk age-metallicity relation and the age distribution in the Solar Neighbourhood

Reid, I. Neill; Turner, Edwin L.; Turnbull, Margaret C.; Mountain, M.; Valenti, Jeff A.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.92%
The chemical composition of Earth's atmosphere has undergone substantial evolution over the course of its history. It is possible, even likely, that terrestrial planets in other planetary systems have undergone similar changes; consequently, the age distribution of nearby stars is an important consideration in designing surveys for Earth-analogues. Valenti & Fischer (2005) provide age and metallicity estimates for 1039 FGK dwarfs in the Solar Neighbourhood. Using the Hipparcos catalogue as a reference to calibrate potential biases, we have extracted volume-limited samples of nearby stars from the Valenti-Fischer dataset. Unlike other recent investigations, our analysis shows clear evidence for an age-metallicity relation in the local disk, albeit with substantial dispersion at any epoch. The mean metallicity increases from -0.3 dex at a lookback time of ~10 Gyrs to +0.15 dex at the present day. Supplementing the Valenti-Fischer measurements with literature data to give a complete volume-limited sample, the age distribution of nearby FGK dwarfs is broadly consistent with a uniform star-formation rate over the history of the Galactic disk. In striking contrast, most stars known to have planetary companions are younger than 5 Gyrs; however...

## The Age Distribution of Clusters in M83

Silva-Villa, E.; Adamo, A.; Bastian, N.; Fouesneau, M.; Zackrisson, E.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.88%
In order to empirically determine the timescale and environmental dependence of stellar cluster disruption, we have undertaken an analysis of the unprecedented multi-pointing (seven), multi-wavelength (U, B, V, H$\alpha$, and I) Hubble Space Telescope imaging survey of the nearby, face-on spiral galaxy M83. The images are used to locate stellar clusters and stellar associations throughout the galaxy. Estimation of cluster properties (age, mass, and extinction) was done through a comparison of their spectral energy distributions with simple stellar population models. We constructed the largest catalog of stellar clusters and associations in this galaxy to-date, with ~1800 sources with masses above ~5000 M$_{\odot}$ and ages younger than ~300 Myr. In the present letter, we focus on the age distribution of the resulting clusters and associations. In particular, we explicitly test whether the age distributions are related with the ambient environment. Our results are in excellent agreement with previous studies of age distributions in the centre of the galaxy, which gives us confidence to expand out to search for similarities or differences in the other fields which sample different environments. We find that the age distribution of the clusters inside M83 varies strongly as a function of position within the galaxy...

## On the Interpretation of the Age Distribution of Star Clusters in the Small Magellanic Cloud

Gieles, M.; Lamers, H. J. G. L. M.; Zwart, S. F. Portegies
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.92%
We re-analyze the age distribution (dN/dt) of star clusters in the Small Magellanic Cloud (SMC) using age determinations based on the Magellanic Cloud Photometric Survey. For ages younger than 3x10^9 yr the dN/dt distribution can be approximated by a power-law distribution, dN/dt propto t^-beta, with -beta=-0.70+/-0.05 or -beta=-0.84+/-0.04, depending on the model used to derive the ages. Predictions for a cluster population without dissolution limited by a V-band detection result in a power-law dN/dt distribution with an index of ~-0.7. This is because the limiting cluster mass increases with age, due to evolutionary fading of clusters, reducing the number of observed clusters at old ages. When a mass cut well above the limiting cluster mass is applied, the dN/dt distribution is flat up to 1 Gyr. We conclude that cluster dissolution is of small importance in shaping the dN/dt distribution and incompleteness causes dN/dt to decline. The reason that no (mass independent) infant mortality of star clusters in the first ~10-20 Myr is found is explained by a detection bias towards clusters without nebular emission, i.e. cluster that have survived the infant mortality phase. The reason we find no evidence for tidal (mass dependent) cluster dissolution in the first Gyr is explained by the weak tidal field of the SMC. Our results are in sharp contrast to the interpretation of Chandar et al. (2006)...

## Age distribution of young clusters and field stars in the SMC

Chiosi, E.; Vallenari, A.; Held, E. V.; Rizzi, L.; Moretti, A.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.91%
In this paper we discuss the cluster and field star formation in the central part of the Small Magellanic Cloud. The main goal is to study the correlation between young objects and their interstellar environment. The ages of about 164 associations and 311 clusters younger than 1 Gyr are determined using isochrone fitting. The spatial distribution of the clusters is compared with the HI maps, with the HI velocity dispersion field, with the location of the CO clouds and with the distribution of young field stars. The cluster age distribution supports the idea that clusters formed in the last 1 Gyr of the SMC history in a roughly continuous way with periods of enhancements. The two super-shells 37A and 304A detected in the HI distribution are clearly visible in the age distribution of the clusters: an enhancement in the cluster formation rate has taken place from the epoch of the shell formation. A tight correlation between young clusters and the HI intensity is found. The degree of correlation is decreasing with the age of the clusters. Clusters older than 300 Myr are located away from the HI peaks. Clusters and associations younger than 10 Myr are related to the CO clouds in the SW region of the SMC disk. A positive correlation between the location of the young clusters and the velocity dispersion field of the atomic gas is derived only for the shell 304A...

## A Comparison of Methods for Determining the Age Distribution of Star Clusters: Application to the Large Magellanic Cloud

Chandar, Rupali; Whitmore, Bradley C.; Fall, S. Michael
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.88%
The age distribution of star clusters in nearby galaxies plays a crucial role in evaluating the lifetimes and disruption mechanisms of the clusters. Two very different results have been found recently for the age distribution chi(t) of clusters in the Large Magellanic Cloud (LMC). We found that chi(t) can be described approximately by a power law chi(t) propto t^{gamma}, with gamma -0.8, by counting clusters in the mass-age plane, i.e., by constructing chi(t) directly from mass-limited samples. Gieles & Bastian inferred a value of gamma~, based on the slope of the relation between the maximum mass of clusters in equal intervals of log t, hereafter the M_max method, an indirect technique that requires additional assumptions about the upper end of the mass function. However, our own analysis shows that the M_max method gives a result consistent with our direct counting method for clusters in the LMC, namely chi(t) propto t^-0.8 for t<10^9 yr. The reason for the apparent discrepancy is that our analysis includes many massive (M>1.5x10^3 M_sol), recently formed (t<10^7 yr) clusters, which are known to exist in the LMC, whereas Gieles & Bastian are missing such clusters. We compile recent results from the literature showing that the age distribution of young star clusters in more than a dozen galaxies...

## The Star Cluster Population of M51: II. Age distribution and relations among the derived parameters

Bastian, N.; Gieles, M.; Lamers, H. J. G. L. M.; de Grijs, R.; Scheepmaker, R. A.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.87%
We use archival {\it Hubble Space Telescope} observations of broad-band images from the ultraviolet (F255W-filter) through the near infrared (NICMOS F160W-filter) to study the star cluster population of the interacting spiral galaxy M51. We obtain age, mass, extinction, and effective radius estimates for 1152 star clusters in a region of $\sim 7.3 \times 8.1$ kpc centered on the nucleus and extending into the outer spiral arms. In this paper we present the data set and exploit it to determine the age distribution and relationships among the fundamental parameters (i.e. age, mass, effective radius). Using this dataset we find: {\it i}) that the cluster formation rate seems to have had a large increase $\sim$ 50-70 Myr ago, which is coincident with the suggested {\it second passage} of its companion, NGC 5195, {\it ii}) a large number of extremely young ($<$ 10 Myr) star clusters, which we interpret as a population of unbound clusters of which a large majority will disrupt within the next $\sim$10 Myr, and {\it iii)} that the distribution of cluster sizescan be well approximated by a power-law with exponent, $-\eta = -2.2 \pm 0.2$, which is very similar to that of Galactic globular clusters, indicating that cluster disruption is largely independent of cluster radius. In addition...

## The Star Clusters of the Small Magellanic Cloud: Age Distribution

Rafelski, Marc; Zaritsky, Dennis
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
45.9%
We present age measurements for 195 star clusters in the Small Magellanic Cloud based on comparison of integrated colors measured from the Magellanic Clouds Photometric Survey with models of simple stellar populations. We find that the modeled nonuniform changes of cluster colors with age can lead to spurious age peaks in the cluster age distribution, that the observed numbers of clusters with age, t, declines smoothly as t^{-2.1}, that for an assumed initial cluster mass function scaling as M^{-2} the dependence of the cluster disruption time on mass is proportional to M^{0.48}, that despite the apparent abundance of young clusters the dominant epoch of cluster formation was the initial one, and that there are significant differences in the spatial distribution of clusters of different ages. Because of limited precision in our age measurements, we cannot address the question of detailed correspondence between the cluster age function and the field star formation history. However, this sample provides an initial guide for which clusters to target in more detailed studies of specific age intervals.; Comment: 11 pages (emulateapj), accepted for publication in the Astronomical Journal. Electronic tables available at: http://ngala.as.arizona.edu/dennis/rafelski.html

## Age distribution of the central stars of galactic disk planetary nebulae

Maciel, W. J.; Costa, R. D. D.; Idiart, T. E. P.
Tipo: Artigo de Revista Científica
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...