Search

Planetary Nebulae (PNs) in the Magellanic Clouds offer the unique opportunity to study both the population and evolution of low- and intermediate-mass stars, in an environment that is free of the distance scale bias and the differential reddening that hinder the observations of the Galactic sample. The study of LMC and SMC PNs also offers the direct comparison of stellar populations with different metallicity. The relative proximity of the Magellanic Clouds allows detailed spectroscopic analysis of the PNs therein, while the Hubble Space Telescope (HST) is necessary to obtain their spatially-resolved images. In this paper we discuss the history and evolution of this relatively recent branch of stellar astrophysics by reviewing the pioneering studies, and the most recent ground- and space-based achievements. In particular, we present the results from our recent HST surveys, including the metallicity dependence of PN identification (and, ultimately, the metallicity dependence of PN counts in galaxies); the morphological analysis of Magellanic PNs, and the correlations between morphology and other nebular properties; the relations between morphology and progenitor mass and age; and the direct analysis of Magellanic central stars and their importance to stellar evolution. Our morphological results are broadly consistent with the predictions of stellar evolution if the progenitors of asymmetric PNs have on average larger masses than the progenitors of symmetric PNs, without any assumption or relation to binarity of the stellar progenitors.

The stellar population stripped from galaxies in clusters evolve under the extreme conditions imposed by the intracluster (IC) medium. Intracluster stars generally suffer very high systemic velocities, and evolve within a rarefied and extremely hot IC medium. We present numerical simulations which aim to explore the evolution and survival of IC Asymptotic Giant Branch (AGB) envelopes and Planetary Nebula (PN) shells. Our models reflect the evolution of a low-mass star under the observed conditions in the Virgo IC medium. We find that the integrated hydrogen-recombination line emission of a PN is dominated by the inner dense shell, whose evolution is unaffected by the environment. Ram pressure stripping affects mainly the outermost IC PN shell, which hardly influences the emission when the PN is observed as a point source. More importantly, we find that a PN with progenitor mass of 1 Msun fades to ~30% and 10% of its maximum emission, in 5,000 and 10,000 yr respectively, disclosing an actual PN lifetime t_PN several times shorter to what is usually adopted (25,000 yr). This result affects the theoretical calculation of the luminosity-specific density of IC PNe, which scales with t_PN. For t_PN=10,000 yr, our more conservative estimate, we obtain that the luminosity-specific density of PNe is in fair agreement with the value obtained from Red Giants. With our more realistic PN lifetime we infer a higher fraction (above 15%) of IC starlight in the Virgo core than current estimates., Accepted for publication in the Astrophysical Journal 14 pages, including 2 figures

We present a study on the central stars (CSs) of Planetary Nebulae (PNe) observed in the Small Magellanic Cloud (SMC) with the Space Telescope Imaging Spectrograph instrument on-board the HST. The stellar magnitudes have been measured using broad-band photometry, and Zanstra analysis of the nebulae provided the stellar temperatures. From the location of the CSs on the HR diagram, and by comparing the observed CSs with current models of stellar evolution, we infer the CSs masses. We examine closely the possibility of light contamination in the bandpass from an unrecognized stellar companion, and we establish strong constraints on the existence and nature of any binary companion. We find an average mass of 0.63 Msun, which is similar to the mass obtained for a sample of CSs in the LMC (0.65 Msun). However, the SMC and LMC CS mass distributions differ slightly, the SMC sample lacking an intermediate-mass stellar population (0.65 to 0.75 Msun). We discuss the significance and possible reasons for the difference between the two mass distributions. In particular, we consider the differences in the star formation history between the clouds and the mass-loss rate dependence on metallicity., Comment: 30 pages, 6 figures, 5 tables. To be published in ApJ (October 20)

In some eyes, the Phase I proposal selection process is the most important activity handled by the Space Telescope Science Institute (STScI). Proposing for HST and other missions consists of requesting observing time and/or archival research funding. This step is called Phase I, where the scientific merit of a proposal is considered by a community based peer-review process. Accepted proposals then proceed thru Phase II, where the observations are specified in sufficient detail to enable scheduling on the telescope. Each cycle the Hubble Space Telescope (HST) Telescope Allocation Committee (TAC) reviews proposals and awards observing time that is valued at $0.5B, when the total expenditures for HST over its lifetime are figured on an annual basis. This is in fact a very important endeavor that we continue to fine-tune and tweak. This process is open to the science community and we constantly receive comments and praise for this process. Several cycles ago we instituted several significant changes to the process to address concerns such as: Fewer, broader panels, with redundancy to avoid conflicts of interest; Redefinition of the TAC role, to focus on Larger programs; and incentives for the panels to award time to medium sized proposals. In the last cycle, we offered new initiatives to try to enhance the scientific output of the telescope. Some of these initiatives were: Hubble Treasury Program; AR Legacy Program; and the AR Theory Program. This paper will outline the current HST Peer review process. We will discuss why we made changes and how we made changes from our original system. We will also discuss some ideas as to where we may go in the future to generate a stronger science program for HST and to reduce the burden on the science community. This paper is an update of the status of the HST Peer Review Process that was described in the published paper "Evolution of the HST Proposal Selection Process".

Northern Galactic Planetary nebulae (PNs) are studied to disclose possible correlations between the morphology of the nebulae and the evolution of the central stars (CSs). To this end, we have built the best database available to date, accounting for homogeneity and completeness. We use updated statistical distances, an updated morphological classification scheme, and we calculate Zanstra temperatures for a large sample of PNs. With our study we confirm that round, elliptical, and bipolar PNs have different spatial distributions within the Galaxy, with average absolute distances to the Galactic plane 0.73, 0.38, and 0.21 kpc respectively. We also find evidence that the distributions of the central star's masses are different across these morphological groups, although we do not find that CSs hosted by bipolar PNs are hotter, on average, than CSs within round and elliptical PNs. Our results are in broad agreement with the previous analyses, indicating that round, elliptical, and bipolar PNs evolve from progenitors in different mass ranges, and may belong to different stellar populations, as also indicated by the helium and nitrogen abundances of PNs of different morphology., 31 pages, 6 figures, 4 tables. Accepted for publication in ApJ

We determine elemental abundances of He, N, O, Ne, S, and Ar in Magellanic Cloud planetary nebulae (PNe) using direct methods and a large set of observed ions, minimizing the need for ionization correction factors. In contrast to prior studies, we find a clear separation between Type I and non-Type I PNe in these low-metallicity environments, and no evidence that the O-N nucleosynthesis cycle is active in low-mass progenitors. We find that the S/O abundance ratio is anomalously low compared to H ii regions, confirming the “sulfur anomaly” found for Galactic PNe. We also found that Ne/O is elevated in some cases, raising the possibility that Ne yields in low-mass AGB stars may be enhanced at low metallicity.

We explore the realm of post-Asymptotic Giant Branch (post-AGB) stars from a theoretical viewpoint, by constructing synthetic population of transition objects, proto-Planetary Nebulae, Planetary Nebulae Nuclei, and post-Planetary Nebulae objects. We use the Montecarlo procedure to filter out the populations accordingly to a given set of assumptions. We explore the parameter space by studying the effects of the Initial Mass Function (IMF), the Initial Mass-Final Mass Relation (IMFMR), the transition time (t_tr), the envelope mass at the end of the envelope ejection (Me_r), the planetary nebula lifetime t_PN, the hydrogen- and helium-burning phases of the central stars. The results are discussed on the basis of the HR diagram distributions, on the Mv-t plane, and with mass histograms. We found that: (1) the dependence of the synthetic populations on the assumed IMF and IMFMR is generally mild; (2) the Me_r indetermination produces very high indeterminations on the t_tr and thus on the resulting post-AGB populations; (3) the synthetic models give a test check for the ratio of He- to H-burning PNNi. In this paper, disk post-AGB populations are considered. Future applications will include Magellanic Clouds PNe, and populations of bulges and elliptical galaxies., 2 tables, 27 figures (gif format) The Astrophysical Journal, accepted

Planetary Nebulae (PNe) have been used satisfactory to test the effects of stellar evolution on the Galactic chemical environment. Moreover, a link exists between nebular morphology and stellar populations and evolution. We present the latest results on Galactic PN morphology, and an extension to a distance unbiased and homogeneous sample of Large Magellanic Cloud PNe. We show that PNe and their morphology may be successfully used as probes of stellar evolution and populations.

The discrepancy between the observed abundances of 3He in the ISM and those predicted by stellar and galactic chemical evolution remains largely unexplained. In this paper, we attempt to shed some light on this unsolved problem by presenting a quantitative comparison of the 3He abundances recently measured in six planetary nebulae (PNe) with the corresponding predictions of stellar evolution theory. The determination of the mass of the PNe progenitors allows us to dismiss, to a good degree of confidence, the hypothesis that the abundance of 3He in the envelope of all low-mass stars is strongly reduced with respect to the standard theoretical values by some mixing mechanism acting in the latest phases of stellar evolution. The abundance versus mass correlation, allowance made for the limitation of the sample, is in fact found to be fully consistent with the classical prediction of stellar evolution. We examine the implications of this result on the galactic evolution of 3He with the help of a series of models with standard and non-standard nucleosynthesis prescriptions. The results are found to be consistent with the observed galactic abundances only if the vast majority of low-mass stars follows non-standard prescriptions. This implies that either the sample of PNe nebulae under exam is highly biased, or the solution to the 3He problem lies elsewhere., 13 pages, LaTeX, 4 Figures. To be published in The Astrophysical Journal

We show how the different assumptions on the transition time (the time lag between the superwind quenching at the AGB and the illumination of the Planetary Nebula [PN]) reflect into very different theoretical outcomes, both in the characteristics of the stellar remnants, and in the evolution of the nebular shells.We use a Monte Carlo simulation of post-AGB stars with a set of assumptions on the transition time, to show the effect on the resulting location of the stars on the HR diagram, and on the derived core mass distributions.We have also performed numerical simulations of the PN formation process, and investigated the effects of the transition time on the resulting PN structure. We found that the transition time determines not only the size of the PN shell, but also its dynamical evolution.We show the important implications that the transition time has on the observable parameters during the PN stage.

The Astrophysical Journal 753.2 (2012): 172 reproduced by permission of the AAS, We present the Spitzer/Infrared Spectrograph (IRS) spectra of 157 compact Galactic planetary nebulae (PNe). These young PNe provide insight on the effects of dust in early post-asymptotic giant branch evolution, before much of the dust is altered or destroyed by the hardening stellar radiation field. Most of the selected targets have PN-type IRS spectra, while a few turned out to be misclassified stars. We inspected the group properties of the PN spectra and classified them based on the different dust classes (featureless or F, carbon-rich dust or CRD, oxygen-rich dust or ORD, mixed-chemistry dust or MCD) and subclasses (aromatic and aliphatic, and crystalline and amorphous). All PNe are characterized by dust continuum and more than 80% of the sample shows solid-state features above the continuum, in contrast with the Magellanic Cloud sample where only ∼40% of the entire sample displays solid-state features; this is an indication of the strong link between dust properties and metallicity. The Galactic PNe that show solid-state features are almost equally divided among the CRD, ORD, and MCD. We analyzed dust properties together with other PN properties and found that (1) there is an enhancement of MCD PNe toward the Galactic center, in agreement with studies of Galactic bulge PNe; (2) CRD PNe could be seen as defining an evolutionary sequence, contrary to the ORD and MCD PNe, which are scattered in all evolutionary diagrams; (3) carbon-rich and oxygen-rich grains retain different equilibrium temperatures, as expected from models; and (4) ORD PNe are highly asymmetric, i.e., bipolar or bipolar core, and CRD PNe highly symmetric, i.e., round or elliptical; point symmetry is statistically more common in MCD than in other dust class PNe. By comparing the sample of this paper to that of Magellanic Cloud PNe, we find that the latter sample does not include MCD PNe, and the other dust classes are differently populated, with continuity of the fraction of F, CRD, ORD, and MCD populations from high to low metallicity environments. We also find similar sequences for CRD PNe in the Galactic disk and the Magellanic Clouds, except that the Magellanic Cloud PNe seem to attain higher dust temperatures at similar evolutionary stages, in agreement with the observational findings of smaller dust grains (i.e., lower radiation efficiency) in low metallicity interstellar environments., Support for this work was provided by NASA through a grant issued by JPL/Caltech for Spitzer Program GO 50261. We acknowledge support from the Faculty of the European Space Astronomy Centre (ESAC). D.A.G.H. and A.M. also acknowledge support provided by the Spanish Ministry of Science and Innovation (MICINN) under a 2008 JdC grant and under grant AYA-2007-64748. E.V. acknowledges support provided by the Spanish Ministry of Science and Innovation (MICINN) under grant AYA2010-20630 and to the Marie Curie FP7-People-RG268111.

Using the data from the catalogue of PNe images of Schwarz et al. (1992), we have classified 255 nebulae into 5 morphological groups. The images were taken through a filter centred on the Ha line and included the [NII]6583 line. Most of the observations were made with the 3.5m ESO NTT during commissioning time. The main groups are called: stellar (st), irregular (i), elliptical (e), bipolar (b), and point-symmetric (p). The ellipticals are subdivided into: simple (e), multiple event (em), and inner structure (es) types. The bipolars into: simple (b), and multiple event (bm). The point-symmetric objects into: simple (p), and multiple events (pm).

We present a study on the filling factors and ionized masses of four sets of galactic and extra-galactic planetary nebulae (PNe) at known distances. The calculation of filling factors and ionized masses has been pursued as to get a deeper insight on the evolution of this class of objects. We used a galactic set of PNe, another set of nebulae that are averagely located near the galactic center, and two sets of nebulae in the Magellanic Clouds. As input data, we used the electron densities derived from the forbidden line intensity ratios, the Hβ nebular fluxes, the distances of galactic PNe derived from extinction, and the distances of galactic center PNe and of extra-galactic PNe derived from galaxy (or galactic region) memberships. All these quantities, plus the input angular radii, have been selected among the most recent measurements available in the literature. We obtained several interesting results. (1) The calculated filling factors are on average much smaller than what is usually assumed, independently for each set. (2) The ionized masses are all in good agreement with the theoretical predictions, with the possible exception of the Galactic Bulge PNe. (3) Both filling factors and ionized masses cover a wide range of values as it is shown in the cumulative histograms below (filled circles=local PNe; open circles=galactic center PNe; filled squares=LMC PNe; open squares=SMC PNe).

Infrared photometry suggests that IRAS 17516–2525 is in transition from the AGB to a young PN (Van der Veen et al. 1989a,b, Manchado et al. 1989). IR spectra (van der Veen et al. 1989b, Kaufl et al. 1992) revealed that the object has an ionized core. Optical observations show a weak H α emitting unresolved object (Kaufl et al. 1992) at the coordinates (α 1950: 17 h 51 m 37.8 s , δ 1950: —25°25’58”) suggested by van der Veen et al. (1989a). The spectrum between 2.0 and 2.4μm shows e.g. Br γ , 2.0875 μm (unidentified) or the Na I -doublet but no molecular Hydrogen (present in other compact PNs). The ratio of Pf β to Br α (hardly affected by extinction, insensitive to electron density and temperature) has been studied in IRAS 17516–2525 and Hen1044. For Hen1044 we find this ratio to be in accordance with theoretical calculations (Hummer and Storey, 1987). For IRAS 17516–2525, however, Pf β is approximately twice as strong. Hence presumably Hen1044 is optically thin whereas IRAS 17516–2525 is optically thick at λ ≈4–5μm. Line fluxes appear to be constant over a time scale of 4 years. If the visual extinction A V in the object is ≥ 2 mag than the infrared luminosity equals the total luminosity (i.e. \( L = 1400{L_ \odot } * {(\frac{{diatance}}{{kpc}})^2} \) ). A main-sequence object accidentally extincted by an intervening cloud can be excluded because the observed line-width of Br α ( \( \approx 40\frac{{km}}{s} \), Van der Veen et al. 1989a) is too small as compared to that of O-stars ( \( \approx 1000\frac{{km}}{s} \), Kaufl, 1992). Assuming an expansion-velocity of \( \approx 20\frac{{km}}{s} \) the stellar appearance constrains dynamic age and size (\( age \leqslant 360y * \frac{{diatance}}{{kpc}},size \leqslant 1500AU * \frac{{diatance}}{{kpc}} \)) A pre main sequence object can also be excluded. The max. luminosity (L PMS ≤ 5000L ⊙) then requires the object to be closer than 2 kpc where it should be resolvable. In conclusion the observational evidence strongly suggests that IRAS 17516–2525 represents the searched ‘missing link’ between AGB stars and young compact Planetary Nebulae.

We present narrow band images and spectra illustrating the morphological and kinematical characteristics of a new class of planetary nebulae. It consists of all those objects in the catalogue by Schwarz et al. (1992) whose morphology shows no other symmetries than point-reflection about the central source. This peculiar shape is often determined by pairs of ansae or blobs separated from the central body of the nebula. The kinematical data show both low (e.g. IC 4634) and high (He 2–186; V exp > 135 km s-1) velocity outflows.

Extensive Montecarlo simulations of Post-Asymptotic Giant Branch (P-AGB) populations have been constructed, exploring the effects of various assumptions on synthetic H-R diagrams, luminosity functions, and inferred mass distributions. Such assumptions include the IMF, the initial mass-final mass relation, the AGB to PN transition time, the duration of the planetary nebula (PN) stage, etc. We have also investigated how the observational errors in luminosity and temperature propagate into the inferred mass distribution of the P-AGB stars.

Extensive Montecarlo simulations of populations of post-Asymptotic Giant Branch (P-AGB) stars have been constructed. The time evolution of luminosity and effective temperature for any stellar mass has been approximated by suitable analytical approximations based on the P-AGB evolutionary tracks of Paczynski (1971, Acta Astron. 21, 417) and Schonberner (1983, A&A 79, 108). By constructing synthetic HR diagrams for P-AGB stars we explore the effects of various assumptions, such as the IMF, the initial mass-final mass relation, the AGB to PN transition time, etc. We have also investigated how the uncertainties in the various assumptions and observational errors in luminosity and temperature propagate into the inferred mass distribution of the P-AGB stars. As an example of the possible applications, we show in the Figure the mass distributions of two P-AGB populations, which differ only on the effective temperatures. We have assumed a systematic error in the observed temperature, such as logT obs = 1.01.log T true + 0.01. Such an error is very small in comparison to usual Zanstra temperature uncertainties for Planetary Nebula Nuclei, yet the two distributions are remarkably different. The broken line represents the observed distribution. The mean mass are respectively = 0.594 for the true distribution (i. e. using T true), and = 0.628 for the observed one (i. e. using Tobs). This simulation emphasizes the need for accurate temperatures in order to obtain useful mass distributions for P-AGB stars, when using an observed distribution of P-AGB stars in the HR diagram. Further applications and details will be included in a forthcoming paper (Stanghellini & Renzini, in preparation).

The observed period decrease of the strongest mode in the post-AGB variable star PG 1159-035 (GW Vir) is investigated with standard evolution models and ones with the surface helium greatly reduced. These latter models were investigated because the presence of even 10% of helium by mass can poison the cyclical CO ionization that produces the k and γ effects pulsational driving. We use the Lagrangian based pulsation program of Pesnell (1990). As found by others before, we theoretically predict period increases, in spite of a radius decrease, rather than the observed period decrease. If mode trapping to decrease the period of this particular 516 second mode is the explanation, it has to be done with only a slight composition gradient at the few percent level of mass into the model. New evolution models with more mass loss and less surface helium are being investigated to see if they can explain the observed period decrease.

In order to compare the evolutionary time of planetary nebula nuclei (PNNs) to the kinematical age of the planetary nebula (PN) one has to specify the AGB to PN transition time, i.e., the time taken by the central star to become hot enough for hydrogen ionization (∼30,000 K). We have shown that, whatever the physical process driving the transition, the transition time depends on the residual envelope mass after the cessation of the AGB superwind, i.e., on a quantity that current theory can hardly predict at all. Given this serious limitation, we have envisaged an empirical approach which can set limits on the transition time, and possibly determine its average value

Using the data from the catalogue of PNe images of Schwarz et al. (1992), we have classified 255 nebulae into 5 morphological groups. The images were taken through a filter centred on the Hα line and included the [NII]6583 line. Most of the observations were made with the 3.5m ESO NTT during commissioning time. The main groups are called: stellar (st), irregular (i), elliptical (e), bipolar (b), and point-symmetric (p). The ellipticals are subdivided into: simple (e), multiple event (em), and inner structure (es) types. The bipolars into: simple (b), and multiple event (bm). The point-symmetric objects into: simple (p), and multiple events (pm).

We present narrow band images and spectra illustrating the morphological and kinematical characteristics of a new class of planetary nebulae. It consists of all those objects in the catalogue by Schwarz et al. (1992) whose morphology shows no other symmetries than point–reflection about the central source. This peculiar shape is often determined by pairs of ansae or blobs separated from the central body of the nebula. The kinematical data show both low (e.g. IC 4634) and high (He 2–186; Vexp > 135 km s−1) velocity outflows.

Stability analyses are performed for nonradial g(+)-mode pulsations of postasymptotic AGB stellar models to determine the location of their pulsational instability strips in the Hertzsprung-Russell diagram. Stellar models are analyzed that are assumed to have undergone a major mass loss event either near the tip of the AGB or shortly thereafter and, therefore, consist of a 50-percent carbon 50-percent oxygen core. Their mass is 0.6 solar mass, which is in good agreement with the peak of the observed distribution of white dwarf masses. The results are compared both to the observed planetary nebula nuclei variables and the hot pulsating DO variables. An analysis is also presented of the stability of DB white dwarfs, which have much deeper and stronger convection zones, and an instability strip is found between about 18,000 and 26,000 K approximately as observed for the known variable stars.