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We present observations of the infrared fine-structure lines of [Si II] (34.8 mum), [O I] (63.2 and 145.5 mum) and [C II] (157.7 mum) obtained with the ISO SWS and LWS spectrographs of nine Planetary Nebulae (PNe). These lines originate in the Photo-Dissociation Regions (PDRs) associated with the nebulae and provide useful information on the evolution and excitation conditions of the ejected material in these regions. In order to interpret the observations, the measured line intensities have been compared with those predicted by photo-dissociation models. This comparison has been done taking into account the C/O content in the nebulae. The densities derived with this comparison show a large scatter for some nebulae, probably because the density is higher than the critical density. Therefore, they are no longer sensitive to this parameter implying that transitions from other species with higher critical density should be used. The possible contribution of shocks to the observed emission characteristics of these PNe is briefly discussed and it is shown that the radiation field is the main driving force responsible for the atomic lines in the PNe that have been studied. In addition, data on the pure rotational lines of H-2 in three nebulae (NGC7027, NGC6302 and Hb 5) are also presented. Assuming local thermal equilibrium the rotational temperature and densities have been derived. We have derived the mass of atomic gas in the PDR associated with these PNe and compared those to ionic masses derived from H and molecular masses derived from low J CO observations. This comparison shows that for these nebulae, the PDR is the main reservoir of gas surrounding these objects. A comparison of the results of these evolved PNe with very young PNe from the literature suggests that as the nebula ages the relative amount of ionic gas increases at the expense of the atomic and molecular mass.

We present a study of the stellar parameters, distances and spectral energy distributions (SEDs) of HD 34282 and HD 141569, two pre-main sequence Herbig AeBe stars. Both objects have been reported to show "anomalous positions" in the HR diagram in the sense that they appear below the main sequence. A significant result of this work is that both stars arc metal-deficient. The Hipparcos distance of HD 34282 is very uncertain and the current study places the star at the expected evolutionary position in the HR diagram, i.e. as a PMS star. The distance for HD 141569 found in this work matches the Hipparcos distance, and the problem of its anomalous position is solved as a result of the low metallicity of the object: using the right metallicity tracks, the star is in the PMS region. The SEDs are constructed using data covering ultraviolet to millimetre wavelengths. Physical, non-parametric models, have been applied in order to extract some properties of the disks surrounding the stars. The disk around HD 34282 is accreting actively, it is massive and presents large grains in the mid-plane and small grains in the surface. HD 141569 has a very low mass disk, which is in an intermediate stage towards a debris-type disk.

We present a study of the stellar parameters, distances and spectral energy distributions (SEDs) of HD 34282 and HD 141569, two pre-main sequence Herbig AeBe stars. Both objects have been reported to show "anomalous positions" in the HR diagram in the sense that they appear below the main sequence. A significant result of this work is that both stars arc metal-deficient. The Hipparcos distance of HD 34282 is very uncertain and the current study places the star at the expected evolutionary position in the HR diagram, i.e. as a PMS star. The distance for HD 141569 found in this work matches the Hipparcos distance, and the problem of its anomalous position is solved as a result of the low metallicity of the object: using the right metallicity tracks, the star is in the PMS region. The SEDs are constructed using data covering ultraviolet to millimetre wavelengths. Physical, non-parametric models, have been applied in order to extract some properties of the disks surrounding the stars. The disk around HD 34282 is accreting actively, it is massive and presents large grains in the mid-plane and small grains in the surface. HD 141569 has a very low mass disk, which is in an intermediate stage towards a debris-type disk.

We present a study of the stellar parameters, distances and spectral energy distributions (SEDs) of HD 34282 and HD 141569, two pre-main sequence Herbig AeBe stars. Both objects have been reported to show `anomalous positions' in the HR diagram in the sense that they appear below the main sequence. A significant result of this work is that both stars are metal-deficient. The {\it Hipparcos} distance of HD 34282 is very uncertain and the current study places the star at the expected evolutionary position in the HR diagram, i.e. as a PMS star. The distance for HD 141569 found in this work matches the {\it Hipparcos} distance, and the problem of its anomalous position is solved as a result of the low metallicity of the object: using the right metallicity tracks, the star is in the PMS region. The SEDs are constructed using data covering ultraviolet to millimetre wavelengths. Physical, non-parametric models, have been applied in order to extract some properties of the disks surrounding the stars. The disk around HD 34282 is accreting actively, it is massive and presents large grains in the mid-plane and small grains in the surface. HD 141569 has a very low mass disk, which is in an intermediate stage towards a debris-type disk., 19 pages, 10 figures. Accepted by A&A

The elemental abundances of ten planetary nebulae, derived with high accuracy including ISO and IUE spectra, are analysed with the aid of synthetic evolutionary models for the TP-AGB phase. Model prescriptions are varied until we achieve the simultaneous reproduction of all elemental features, which allows placing important constraints on the characteristic masses and nucleosynthetic processes experienced by the stellar progenitors. First of all, it is possible to separate the sample into two groups of PNe, one indicating the occurrence of only the third dredge-up during the TP-AGB phase, and the other showing also the chemical signature of hot-bottom burning. The former group is reproduced by stellar models with variable molecular opacities (see Marigo 2002), adopting initial solar metallicity, and typical efficiency of the third dredge-up 0.3-0.4. The latter group of PNe, with extremely high He content 0.15 4.5-5.0 Mo) with LMC composition have suffered a number of very efficient, carbon-poor, dredge-up events. Finally, the neon abundances of the He-rich PNe can be recovered by invoking a significant production of 22Ne during thermal pulses, which would imply a reduced role of the 22Ne(alpha,n)25Mg reaction as neutron source to the s-process nucleosynthesis in these stars., 22 pages, accepted for publication in Astronomy & Astrophysics

A new method, AMORE - based on a genetic algorithm optimizer, is presented for the automated study of colour-magnitude diagrams. The method combines several stellar population synthesis tools developed in the last decade by or in collaboration with the Padova group. Our method is able to recover, within the uncertainties, the parameters -- distance, extinction, age, metallicity, index of a power-law initial mass function and the index of an exponential star formation rate -- from a reference synthetic stellar population. No a priori information is inserted to recover the parameters, which is done simultaneously and not one at a time. Examples are given to demonstrate and to better understand biases in the results, if one of the input parameters is deliberately set fixed to a non-optimum value., Comment: 26 pages LaTeX, 7 figures, 11 tables, to appear in A&A

IR spectroscopy provides a valuable tool for the characterisation and identification of interstellar molecular species. Here, we present 6-9 mum spectra of a sample of reflection nebulae, HII regions, YSOs, evolved stars and galaxies that show strong unidentified infrared bands, obtained with the SWS spectrograph on board ISO. The IR emission features in this wavelength region show pronounced variations. 1) The 6.2 mum feature shifts from 6.22 to 6.3 mum and clearly shows profile variations. 2) The 7.7 mum complex is comprised of at least two subpeaks peaking at 7.6 and one longwards of 7.7 mum. In some cases the main peak can apparently shift up to 8 mum. Two sources do not exhibit a 7.7 mum complex but instead show a broad emission feature at 8.22 mum. 3) The 8.6 mum feature has a symmetric profile in all sources and some sources exhibit this band at slightly longer wavelengths. For the 6.2, 7.7 and 8.6 mum features, the sources have been classified independently based on their profile and peak position. The classes derived for these features are directly linked with each other. Sources with a 6.2 mum feature peaking at similar to6.22 mum exhibit a 7.7 mum complex dominated by the 7.6 mum component. In contrast, sources with a 6.2 mum profile peaking longwards of 6.24 mum show a 7.7 mum complex with a dominant peak longwards of 7.7 mum and a 8.6 mum feature shifted toward the red. Furthermore, the observed 6-9 mum spectrum depends on the type of object. All ISM-like sources and a few PNe and Post-AGB stars belong to the first group while isolated Herbig AeBe stars, a few Post-AGB stars and most PNe belong to the second group. We summarise existing laboratory data and theoretical quantum chemical calculations of the modes emitting in this wavelength region of PAH molecules. We discuss the variations in peak position and profile in view of the exact nature of the carrier. We attribute the observed 6.2 mum profile and peak position to the combined effect of a PAH family and anharmonicity with pure PAHs representing the 6.3 mum component and substituted/complexed PAHs representing the 6.2 mum component. The 7.6 mum component is well reproduced by both pure and substituted/complexed PAHs but the 7.8 mum component remains an enigma. In addition, the exact identification of the 8.22 mum feature remains unknown. The observed variations in the characteristics of the IR emission bands are linked to the local physical conditions. Possible formation and evolution processes that may influence the interstellar PAH class are highlighted.

IR spectroscopy provides a valuable tool for the characterisation and identification of interstellar molecular species. Here, we present 6-9 mum spectra of a sample of reflection nebulae, HII regions, YSOs, evolved stars and galaxies that show strong unidentified infrared bands, obtained with the SWS spectrograph on board ISO. The IR emission features in this wavelength region show pronounced variations. 1) The 6.2 mum feature shifts from 6.22 to 6.3 mum and clearly shows profile variations. 2) The 7.7 mum complex is comprised of at least two subpeaks peaking at 7.6 and one longwards of 7.7 mum. In some cases the main peak can apparently shift up to 8 mum. Two sources do not exhibit a 7.7 mum complex but instead show a broad emission feature at 8.22 mum. 3) The 8.6 mum feature has a symmetric profile in all sources and some sources exhibit this band at slightly longer wavelengths. For the 6.2, 7.7 and 8.6 mum features, the sources have been classified independently based on their profile and peak position. The classes derived for these features are directly linked with each other. Sources with a 6.2 mum feature peaking at similar to6.22 mum exhibit a 7.7 mum complex dominated by the 7.6 mum component. In contrast, sources with a 6.2 mum profile peaking longwards of 6.24 mum show a 7.7 mum complex with a dominant peak longwards of 7.7 mum and a 8.6 mum feature shifted toward the red. Furthermore, the observed 6-9 mum spectrum depends on the type of object. All ISM-like sources and a few PNe and Post-AGB stars belong to the first group while isolated Herbig AeBe stars, a few Post-AGB stars and most PNe belong to the second group. We summarise existing laboratory data and theoretical quantum chemical calculations of the modes emitting in this wavelength region of PAH molecules. We discuss the variations in peak position and profile in view of the exact nature of the carrier. We attribute the observed 6.2 mum profile and peak position to the combined effect of a PAH family and anharmonicity with pure PAHs representing the 6.3 mum component and substituted/complexed PAHs representing the 6.2 mum component. The 7.6 mum component is well reproduced by both pure and substituted/complexed PAHs but the 7.8 mum component remains an enigma. In addition, the exact identification of the 8.22 mum feature remains unknown. The observed variations in the characteristics of the IR emission bands are linked to the local physical conditions. Possible formation and evolution processes that may influence the interstellar PAH class are highlighted.

The fact that Space Telescope WFPC2 images of the planetary nebula NGC 6537 fail to show the central star is used to derive a limit to its magnitude: it is fainter than a magnitude of 22.4 in the visible. This is used to derive a lower limit to the temperature of the star. The Zanstra temperature is at least 500 000 K. The Energy Balance temperature is found to be consistent with this value, as is the ionization state of the nebula. Assuming a reasonable range of distances for the nebula, the radius of the star can be found. It is consistent with the mass-radius relation of a white dwarf of 0.9 M. or higher.

The fact that Space Telescope WFPC2 images of the planetary nebula NGC 6537 fail to show the central star is used to derive a limit to its magnitude: it is fainter than a magnitude of 22.4 in the visible. This is used to derive a lower limit to the temperature of the star. The Zanstra temperature is at least 500 000 K. The Energy Balance temperature is found to be consistent with this value, as is the ionization state of the nebula. Assuming a reasonable range of distances for the nebula, the radius of the star can be found. It is consistent with the mass-radius relation of a white dwarf of 0.9 M. or higher.

We report the serendipitous discovery of seven luminous irregular variables in the LMC by the EROS project. The stars have V similar or equal to 15(m) to 17(m), (B(E) - R(E)) similar or equal to 0(m), and vary by about 0.4(m) on timescales of 10 to 40 days. The variations in B(E) and R(E) are about equal but the stars are slightly bluer when they an fainter. Medium resolution optical spectra show strong H alpha emission, weak H beta emission and absorption lines of the higher Balmer lines. Absorption lines of He I, C III, O II and Si IV are present in the spectra of some of the stars. The spectral types derived from the presence or absence of the lines are between late-O and late-B. Effective temperatures and luminosities were derived from the energy distributions and independently from the EROS photometry combined with the approximate spectral types. The luminosity, derived from V, E(B - V), and the distance of the LMC, are in the range of 10(3) to 10(4) L.. The strong Hey emission, the spectral types, the luminosities and the irregular variations are similar (but not the same) to those of the Galactic pre-main sequence HAeBe stars (i.e. the Herbig Ae/Be stars). However the stars are about a factor 10 more luminous than the luminosity upper limit for Galactic HAeBe stars. This might be due to a shorter accretion timescale (tau(accr) = M(*) /(M) over dot), or to the smaller dust-to-gas ratio in the LMC.

We report the serendipitous discovery of seven luminous irregular variables in the LMC by the EROS project. The stars have V similar or equal to 15(m) to 17(m), (B(E) - R(E)) similar or equal to 0(m), and vary by about 0.4(m) on timescales of 10 to 40 days. The variations in B(E) and R(E) are about equal but the stars are slightly bluer when they an fainter. Medium resolution optical spectra show strong H alpha emission, weak H beta emission and absorption lines of the higher Balmer lines. Absorption lines of He I, C III, O II and Si IV are present in the spectra of some of the stars. The spectral types derived from the presence or absence of the lines are between late-O and late-B. Effective temperatures and luminosities were derived from the energy distributions and independently from the EROS photometry combined with the approximate spectral types. The luminosity, derived from V, E(B - V), and the distance of the LMC, are in the range of 10(3) to 10(4) L.. The strong Hey emission, the spectral types, the luminosities and the irregular variations are similar (but not the same) to those of the Galactic pre-main sequence HAeBe stars (i.e. the Herbig Ae/Be stars). However the stars are about a factor 10 more luminous than the luminosity upper limit for Galactic HAeBe stars. This might be due to a shorter accretion timescale (tau(accr) = M(*) /(M) over dot), or to the smaller dust-to-gas ratio in the LMC.

Astronomy and Astrophysics 544 (2012): A78 Reproduced with permission from Astronomy & Astrophysics, We observed a sample of 20 representative Herbig Ae/Be stars and 5 A-type debris discs with PACS onboard Herschel, as part of the GAS in Protoplanetary Systems (GASPS) project. The observations were done in spectroscopic mode, and cover the far-infrared lines of [O i], [C ii], CO, CH+, H2O, and OH. We have a [Oi] 63 μm detection rate of 100% for the Herbig Ae/Be and 0% for the debris discs. The [Oi] 145 μm line is only detected in 25% and CO J = 18–17 in 45% (and fewer cases for higher J transitions) of the Herbig Ae/Be stars, while for [Cii] 157 μm, we often find spatially variable background contamination. We show the first detection of water in a Herbig Ae disc, HD 163296, which has a settled disc. Hydroxyl is detected as well in this disc. First seen in HD 100546, CH+ emission is now detected for the second time in a Herbig Ae star, HD 97048. We report fluxes for each line and use the observations as line diagnostics of the gas properties. Furthermore, we look for correlations between the strength of the emission lines and either the stellar or disc parameters, such as stellar luminosity, ultraviolet and X-ray flux, accretion rate, polycyclic aromatic hydrocarbon (PAH) band strength, and flaring. We find that the stellar ultraviolet flux is the dominant excitation mechanism of [Oi] 63 μm, with the highest line fluxes being found in objects with a large amount of flaring and among the largest PAH strengths. Neither the amount of accretion nor the X-ray luminosity has an influence on the line strength. We find correlations between the line flux of [Oi] 63 μm and [Oi] 145 μm, CO J = 18–17 and [Oi] 6300 Å, and between the continuum flux at 63 μm and at 1.3 mm, while we find weak correlations between the line flux of [O i] 63 μm and the PAH luminosity, the line flux of CO J = 3–2, the continuum flux at 63 μm, the stellar effective temperature, and the Brγ luminosity. Finally, we use a combination of the [O i] 63 μm and 12CO J = 2–1 line fluxes to obtain order of magnitude estimates of the disc gas masses, in agreement with the values that we find from detailed modelling of two Herbig Ae/Be stars, HD 163296 and HD 169142, G. Meeus, C. Eiroa, I. Mendigutía, and B. Montesinos are partly supported by AYA-2008- 1727 and AYA-2011-26202. G. Meeus is supported by RYC-2011-07920. C.A.G. and S.D.B. acknowledge NASA/JPL for funding support. W.F.T. thanks CNES for financial support. F.M. thanks the Millennium Science Initiative (ICM) of the Chilean ministry of Economy (Nucleus P10-022-F). F.M., I.K., and W.F.T. acknowledge support from the EU FP7-2011 under Grant Agreement No. 284405. C.P. acknowledges funding from the EU FP7 under contract PERG06-GA-2009-256513 and from ANR of France under ontract ANR-2010-JCJC-0504-01. PACS has been developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KUL, CSL, IMEC (Belgium); CEA, OAMP (France); MPIA (Germany); IFSI, OAP/AOT, OAA/CAISMI, LENS, SISSA (Italy); IAC (Spain). This development has been supported by the funding agencies BMVIT (Austria), ESA-PRODEX (Belgium), CEA/CNES (France), DLR (Germany), ASI (Italy), and CICT/MCT (Spain). This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France

From observations of LPVs (long period variables) in the LMC we derive that the ratio of the number of C-rich LPVs to the total number of carbon stars is approximately 0.05 and that the ratio of the number of oxygen-rich LPVs to the total number of oxygen-rich AGB stars is between 0.05 and 0.10. The lifetime of the LPV phase in the LMC is only a few 10(4) yr, considerable below estimates for the Galaxy, where the duration of the LPV phase (about 2 10(5) yr) is similar to the total AGB lifetime. If the possible incompleteness of the surveys for LPVs is invoked to explain this discrepancy in the lifetimes then the ratio of small amplitude variables to large amplitude variables must be about 30, considerably larger than the ratio of Semi Regular to Mira variables in the Galaxy (1-3). We present a simple model to explain the observed properties of LPVs in the LMC. It is assumed that pulsation only occurs in an instability strip in the HR diagram. The instability strip is characterised by three parameters: the temperature at some reference luminosity, the width of the instability strip and its slope dT(eff)/dM(bol). The first two are free parameters in the model. Based on observations we use dT(eff)/dM(bol) = 275 K mag-1 for M(bol) > -5 and 100 K mag-1 for M(bol)

From observations of LPVs (long period variables) in the LMC we derive that the ratio of the number of C-rich LPVs to the total number of carbon stars is approximately 0.05 and that the ratio of the number of oxygen-rich LPVs to the total number of oxygen-rich AGB stars is between 0.05 and 0.10. The lifetime of the LPV phase in the LMC is only a few 10(4) yr, considerable below estimates for the Galaxy, where the duration of the LPV phase (about 2 10(5) yr) is similar to the total AGB lifetime. If the possible incompleteness of the surveys for LPVs is invoked to explain this discrepancy in the lifetimes then the ratio of small amplitude variables to large amplitude variables must be about 30, considerably larger than the ratio of Semi Regular to Mira variables in the Galaxy (1-3).We present a simple model to explain the observed properties of LPVs in the LMC. It is assumed that pulsation only occurs in an instability strip in the HR diagram. The instability strip is characterised by three parameters: the temperature at some reference luminosity, the width of the instability strip and its slope dT(eff)/dM(bol). The first two are free parameters in the model. Based on observations we use dT(eff)/dM(bol) = 275 K mag-1 for M(bol) > -5 and 100 K mag-1 for M(bol) Both a model with a Reimers mass loss law inside and outside the instability strip, and a model with the mass loss in the instability strip given by a scaled version of the Blocker & Schonberner (1993) mass loss law, fit the observational constraints equally well.We conclude that first harmonic pulsation can be excluded unless the canonical relation between (J - K) color and effective temperature (based on lunar occultation observations) gives temperatures which are too high by approximately 20%, much larger than the estimated uncertainty of approximately 8% or possible systematic effects (less-than-or-similar-to 10%). Fundamental mode pulsation is therefore probably the dominant pulsation mode among LPVs in the LMC.A second conclusion is that for most stars the instability strip is not the final phase of AGB evolution. Based on our calculations for individual stars we find that AGB stars more massive than about 1.2M. spend a considerable amount of time in the phase between the end of pulsation and the end of the AGB.We propose an alternative explanation for (some of) the non-variable OH/IR stars in the Galaxy.

We have constructed a model to calculate in a synthetic way the evolution of stars on the asymptotic giant branch (AGB). The evolution is started at the first thermal pulse (TP) and is terminated when the envelope mass has been lost due to mass loss or when the core mass reaches the Chandrasekhar mass.Our model is more realistic than previous synthetic evolution models in that more physics has been included. The variation of the luminosity during the interpulse period is taken into account as well as the fact that, initially, the first few pulses are not yet at full amplitude and that the luminosity is lower than given by the standard core-mass-luminosity relations. Most of the relations used are metallicity dependent to be able to make a realistic comparison with stars of different metallicity. The effects of first, second and third dredge-up are taken into account. The effect of hot bottom burning (HBB) is included in an approximate way. Mass loss on the AGB is included through a Reimers Law. We also included mass loss prior to the AGB.The free parameters in our calculations are the minimum core mass for dredge-up (M(c)min), the third dredge-up efficiency (lambda) and three mass loss scaling parameters (eta(RGB), eta(EAGB), eta(AGB).The model has been applied to the LMC using a recent determination of the age-metallicity and star formation rate (SFR) for the LMC. The observed carbon star luminosity function and the observed ratio of oxygen-rich to carbon-rich AGB stars in the LMC acted as constraints to the model.Several models are calculated to demonstrate the effects of the various parameters. A model with M(c)min = 0.58M., lambda = 0.75, eta(RGB) = 0-86, eta(AGB) = eta(EAGB) = 5, including HBB reproduces the observations quite well. It is possible that the amount of carbon formed after a TP is higher than the standard value of X12 = 0.22. As long as lambdaX12=0.165 the model fits the observations. It is difficult to discriminate between a higher X12 and a higher lambda. Third dredge-up needs to be more efficient and must start at lower core masses than commonly predicted to account for the observed carbon star LF. It is suggested that evolutionary calculations have been performed with mixing-length parameters which are too small.The adopted mass loss rate coefficients correspond to a pre-AGB mass loss of 0.20M. for a 1M. and 1.8M. for a 5M. star. The low mass stars lose this on the RGB, the high mass stars in the core helium burning phase when they reach high luminosities before the TP-AGB. The Reimers coefficient on the AGB (eta(AGB) = 5) corresponds to a mass loss rate of 1.0 10(-6) M. yr-1 at the first TP for an initially 1 M. star with LMC abundances.These high mass loss rates are necessary to fit the initial-final mass relation and the high luminosity tail of the carbon star LF. The lifetime of the massive stars with these high mass loss rates are in good agreement with the observed number of massive AGB stars and their progenitors, the Cepheid variables. We suggest that the core mass at the first TP for massive stars has previously been overestimated because their evolution was calculated neglecting pre-AGB mass loss.Observationally the distribution of C-13 enriched carbon stars in the LMC is bimodal. There is a small number (approximately 0.1%) of high-luminosity (M(bol) - 4.75) J-type stars. The difference in relative numbers as well as the gap in luminosity between the two distributions suggst a different evolutionary origin. The low-luminosity J-type stars may be related to the R-stars in the Galactic bulge which have luminosities between 0 less than or similar to M(bol) less than or similar to - 3 indicating an origin at luminosities below the AGB.The small number of high-luminosity J-type carbon stars can be explained by HBB. Given the uncertainty in the observed LF and our approximate treatment of HBB the agreement is good. We predict that about 1 % of M and S stars are enriched in C-13 (and N-14).We considered the effect of ''obscuration'', when stars lose so much mass that they become optically invisible. Based on V-band, I-band and IRAS data of Reid et al. (1990) and a radiative transfer model we find that at most 3% of all carbon stars brighter than M(bol) = - 6 could have been missed in optical surveys. Using our model we derive that the overall effect of obscuration of carbon stars is negligible (approximately 0.1%).The predicted average final mass (M(f) = 0.59M.) is in good agreement with the observed value (0.60 +/- 0.02M.). We predict for our LMC model that stars with initial masses larger than 1.2-1.4M. turn into carbon stars directly and that stars initially more massive than about 1.5M. pass through an intermediate S-star phase before becoming carbon stars. This is consistent with the observation of carbon stars and S-stars in LMC clusters.The predicted birth rate of AGB stars is found to be in agreement with the death rate of Cepheids and the clump stars. The birth rate of planetary nebulae (PN) is a factor of 2 lower than the death rate of AGB stars suggesting that low mass stars (M less than or similar 1.1 M.) may not become PN.

We have constructed a model to calculate in a synthetic way the evolution of stars on the asymptotic giant branch (AGB). The evolution is started at the first thermal pulse (TP) and is terminated when the envelope mass has been lost due to mass loss or when the core mass reaches the Chandrasekhar mass. Our model is more realistic than previous synthetic evolution models in that more physics has been included. The variation of the luminosity during the interpulse period is taken into account as well as the fact that, initially, the first few pulses are not yet at full amplitude and that the luminosity is lower than given by the standard core-mass-luminosity relations. Most of the relations used are metallicity dependent to be able to make a realistic comparison with stars of different metallicity. The effects of first, second and third dredge-up are taken into account. The effect of hot bottom burning (HBB) is included in an approximate way. Mass loss on the AGB is included through a Reimers Law. We also included mass loss prior to the AGB. The free parameters in our calculations are the minimum core mass for dredge-up (M(c)min), the third dredge-up efficiency (lambda) and three mass loss scaling parameters (eta(RGB), eta(EAGB), eta(AGB). The model has been applied to the LMC using a recent determination of the age-metallicity and star formation rate (SFR) for the LMC. The observed carbon star luminosity function and the observed ratio of oxygen-rich to carbon-rich AGB stars in the LMC acted as constraints to the model. Several models are calculated to demonstrate the effects of the various parameters. A model with M(c)min = 0.58M., lambda = 0.75, eta(RGB) = 0-86, eta(AGB) = eta(EAGB) = 5, including HBB reproduces the observations quite well. It is possible that the amount of carbon formed after a TP is higher than the standard value of X12 = 0.22. As long as lambdaX12=0.165 the model fits the observations. It is difficult to discriminate between a higher X12 and a higher lambda. Third dredge-up needs to be more efficient and must start at lower core masses than commonly predicted to account for the observed carbon star LF. It is suggested that evolutionary calculations have been performed with mixing-length parameters which are too small. The adopted mass loss rate coefficients correspond to a pre-AGB mass loss of 0.20M. for a 1M. and 1.8M. for a 5M. star. The low mass stars lose this on the RGB, the high mass stars in the core helium burning phase when they reach high luminosities before the TP-AGB. The Reimers coefficient on the AGB (eta(AGB) = 5) corresponds to a mass loss rate of 1.0 10(-6) M. yr-1 at the first TP for an initially 1 M. star with LMC abundances. These high mass loss rates are necessary to fit the initial-final mass relation and the high luminosity tail of the carbon star LF. The lifetime of the massive stars with these high mass loss rates are in good agreement with the observed number of massive AGB stars and their progenitors, the Cepheid variables. We suggest that the core mass at the first TP for massive stars has previously been overestimated because their evolution was calculated neglecting pre-AGB mass loss. Observationally the distribution of C-13 enriched carbon stars in the LMC is bimodal. There is a small number (approximately 0.1%) of high-luminosity (M(bol) - 4.75) J-type stars. The difference in relative numbers as well as the gap in luminosity between the two distributions suggst a different evolutionary origin. The low-luminosity J-type stars may be related to the R-stars in the Galactic bulge which have luminosities between 0 less than or similar to M(bol) less than or similar to - 3 indicating an origin at luminosities below the AGB. The small number of high-luminosity J-type carbon stars can be explained by HBB. Given the uncertainty in the observed LF and our approximate treatment of HBB the agreement is good. We predict that about 1 % of M and S stars are enriched in C-13 (and N-14). We considered the effect of ''obscuration'', when stars lose so much mass that they become optically invisible. Based on V-band, I-band and IRAS data of Reid et al. (1990) and a radiative transfer model we find that at most 3% of all carbon stars brighter than M(bol) = - 6 could have been missed in optical surveys. Using our model we derive that the overall effect of obscuration of carbon stars is negligible (approximately 0.1%). The predicted average final mass (M(f) = 0.59M.) is in good agreement with the observed value (0.60 +/- 0.02M.). We predict for our LMC model that stars with initial masses larger than 1.2-1.4M. turn into carbon stars directly and that stars initially more massive than about 1.5M. pass through an intermediate S-star phase before becoming carbon stars. This is consistent with the observation of carbon stars and S-stars in LMC clusters. The predicted birth rate of AGB stars is found to be in agreement with the death rate of Cepheids and the clump stars. The birth rate of planetary nebulae (PN) is a factor of 2 lower than the death rate of AGB stars suggesting that low mass stars (M less than or similar 1.1 M.) may not become PN.

Context: In recent years mid- and far infrared spectra of planetary nebulae have been analysed and lead to more accurate abundances. It may be expected that these better abundances lead to a better understanding of the evolution of these objects. Aims: The observed abundances in planetary nebulae are compared to those predicted by the models of Karakas (2003) in order to predict the progenitor masses of the various PNe used. The morphology of the PNe is included in the comparison. Since the central stars play an important role in the evolution, it is expected that this comparison will yield additional information about them. Methods: First the nitrogen/oxygen ratio is discussed with relation to the helium/hydrogen ratio. The progenitor mass for each PNe can be found by a comparison with the models of Karakas. Then the present luminosity of the central stars is determined in two ways: first by computing the central star effective temperature and radius, and second by computing the nebular luminosity from the hydrogen and helium lines. This luminosity is also a function of the initial mass so that these two values of initial mass can be compared. Results: Six of the seven bipolar nebulae can be identified as descendants of high mass stars (4Msun - 6Msun) while the seventh is ambiguous. Most of the elliptical PNe have central stars which descend from low initial mass stars, although there are a few caveats which are discussed. There is no observational evidence for a higher mass for central stars which have a high carbon/oxygen ratio. The evidence provided by the abundance comparison with the models of Karakas is consistent with the HR diagram to which it is compared. In the course of this discussion it is shown how `optically thin' nebulae can be separated from those which are 'optically thick'., Comment: 12 pages, 4 tables, 4 figures. Accepted for publication in A&A

The abundances of the planetary nebulae (PNe) in the galactic bulge are presented. Comparisons are made with the disk nebulae, the solar values, and the prediction based on the observed gradients in the disk. The mean abundances in the bulge PNe cannot be predicted by the observed gradient in the disk sample. Except for helium and nitrogen, which are systematically higher in the bulge, all other elements show distributions similar to their counterparts in the disk PNe. The relatively older age of the bulge objects suggests a much faster enhancement in the bulge. The significantly higher mean N/O ratio in the bulge PNe compared to the disk PNe, is caused by the much larger fraction of low abundance nebulae with a high N/O ratio in the bulge, rather than by having -on average- more massive progenitors. This is argued since the low abundance is noticed not only in the case of oxygen, which could be largely reduced if we allow for a more intensive ON-cycle to occur, but also in the cases of neon, sulfur, and argon - the elements which are believed to be essentially untouched by the nucleosynthesis in the PN progenitors.Several lines of evidence strongly argue that in the bulge, the production rates of elements and isotopes mostly produced in low and intemediate stars relative to those of elements mainly produced in massive stars, are higher than the corresponding ratios in the solar neighbourhood or in other positions in the disk. This probably indicates an initial mass function in the bulge which is different from that in the rest of the Galaxy.An evolutionary scenario for the bulge, allowing for a more efficient star formation and a much shorter timescale to collapse than in the disk, with an initial mass function such that the fraction of massive stars is lower than in the solar neighbourhood, is then proposed to explain the exisiting data.

Over the last few years, the chemistry of molecules other than CO in the planet-forming zones of disks is starting to be explored with Spitzer and high-resolution ground-based data. However, these studies have focused only on a few simple molecules. The aim of this study is to put observational constraints on the presence of more complex organic and sulfur-bearing molecules predicted to be abundant in chemical models of disks and to simulate high resolution spectra in view of future missions. High S/N Spitzer spectra at 10-30 micron of the near edge-on disks IRS46 and GVTau are used to search for mid-infrared absorption bands of various molecules. These disks are good laboratories because absorption studies do not suffer from low line/continuum ratios that plague emission data. Simple LTE slab models are used to infer column densities (or upper limits) and excitation temperatures. Bands of HCN, C2H2 and CO2 are clearly detected toward both sources. The HCN and C2H2 absorption arises in warm gas with Tex of 400-700 K, whereas the CO2 absorption originates in cooler gas of app. 250 K (as in Lahuis 2006). No other absorption features are detected. Limits of those molecules are determined and compared with disk models. The inferred ratios wrt. C2H2 and HCN are roughly consistent with models of the chemistry in high-T gas. Models of UV irradiated disk surfaces generally agree better than pure X-ray models. The limit on NH3/HCN implies that evaporation of NH3 containing ices is only a minor contributor. The inferred ratios also compare well with those found in comets, suggesting that part of the cometary material may derive from warm inner disk gas. High resolution simulations show that future instruments on JWST, ELTs, SOFIA and SPICA can probe up to an order of magnitude lower ratios and put important new constraints on the models, especially if pushed to high S/N ratios., Comment: accepted for publication in A&A, 39 pages, 29 figures

The abundances of the planetary nebulae (PNe) in the galactic bulge are presented. Comparisons are made with the disk nebulae, the solar values, and the prediction based on the observed gradients in the disk. The mean abundances in the bulge PNe cannot be predicted by the observed gradient in the disk sample. Except for helium and nitrogen, which are systematically higher in the bulge, all other elements show distributions similar to their counterparts in the disk PNe. The relatively older age of the bulge objects suggests a much faster enhancement in the bulge. The significantly higher mean N/O ratio in the bulge PNe compared to the disk PNe, is caused by the much larger fraction of low abundance nebulae with a high N/O ratio in the bulge, rather than by having -on average- more massive progenitors. This is argued since the low abundance is noticed not only in the case of oxygen, which could be largely reduced if we allow for a more intensive ON-cycle to occur, but also in the cases of neon, sulfur, and argon - the elements which are believed to be essentially untouched by the nucleosynthesis in the PN progenitors. Several lines of evidence strongly argue that in the bulge, the production rates of elements and isotopes mostly produced in low and intemediate stars relative to those of elements mainly produced in massive stars, are higher than the corresponding ratios in the solar neighbourhood or in other positions in the disk. This probably indicates an initial mass function in the bulge which is different from that in the rest of the Galaxy. An evolutionary scenario for the bulge, allowing for a more efficient star formation and a much shorter timescale to collapse than in the disk, with an initial mass function such that the fraction of massive stars is lower than in the solar neighbourhood, is then proposed to explain the exisiting data.