Integral stellar-nebular model of NGC 7009

Stellar evolution models predict that the chemical composition in the atmosphere of stars in the post- AGB has been enhanced in C and N. The aim of this work is to use the planetary nebula NGC 7009 as a test of the stellar evolution models, by making simultaneous models of the nebula and its central star; the parameters obtained through models are supported by the semi-analytical study of the nebula. This type of work yields a self-consistent model of the whole object imposing more observational constraints to models.

Astronomy and Astrophysics, 2009

Aims. We present a coherent stellar and nebular model that reproduces observations of the planetary nebula IC 418. We aim to test whether a stellar model found to provide an optimal description of the stellar observations is able to satisfactory ionize the nebula and reproduce the nebular observations, a finding that is by no mean evident. This allows us to determine all the physical parameters of both the star and the nebula, including chemical abundances and the distance. Methods. We used all the observational material available (FUSE, IUE, STIS and optical spectra) to constrain the stellar atmosphere model performed using the CMFGEN code. The photoionization model is developed by comparing solutions provided by Cloudy_3D, with results from CTIO, Lick, SPM, IUE, and ISO spectra as well as HST images. The aperture sizes and positions of the different observations are taken into account. More than 140 model nebular emission lines are compared to the observed intensities. The distance is determined using evolutionary tracks. Results. We reproduce all the observations for the star and the nebula. The 3D morphology of the gas distribution is determined. The effective temperature of the star is 36.7 ± 0.5 kK. Its luminosity is 7700 L . No clumping factor is needed to reproduce the age-luminosity relation. We describe an original method for determining the distance of the nebula using evolutionary tracks. The distance of 1.25 kpc is found to be in very good agreement with recent determination using parallax method, and the age of the nebula is estimated to be 1400 years. The chemical composition of both the star and the nebula are determined. Both are carbon-rich. The nebula exhibits evidence of the depletion of elements Mg, Si, S, Cl (0.5 dex lower than solar), and Fe (2.9 dex lower than solar), which is indicative of a depletion of these elements onto grains. Conclusions. We develop the first self-consistent stellar and nebular model of a planetary nebula that reproduces all the available observations ranging from IR to UV, showing that the combined approach to the modeling process leads to more restrictive constraints and, in principle, more trustworthy results.

2008

We analyze the chemical composition of the central star of the planetary nebula NGC 6543 based upon a detailed NLTE model of its stellar wind. The logarithmic abundances by number are H=12.00, He=11.00, C=9.03, N=8.36, O=9.02, Si=8.19, P=5.53, S=7.57 and Fe=7.24. Compared with the solar abundances, most of the elements have solar composition with respect to hydrogen except C which is overabundant by 0.28 dex and Fe which is depleted by $\sim 0.2$ dex. Contrary to most previous work, we find that the star is not H-poor and has a normal He composition. These abundances are compared with those found in the diffuse X-ray plasma and the nebular gas. Compared to the plasma emitting in diffuse X-rays, the stellar wind is much less depleted in iron. Since the iron depletions in the nebular gas and X-ray plasma are similar, we conclude that the plasma emitting diffuse X-rays is derived from the nebular gas rather than the stellar wind. Excellent agreement is obtained between the abundances in the stellar wind and the nebular recombination line abundances for He, C, and O relative to H. On the other hand, the derived stellar N abundance is smaller than the nebular N abundance derived from recombination lines and agrees with the abundance found from collisionally-excited lines. The mean temperature variation determined by five different methods indicates that the difference in the nebular abundances between the recombination lines and collisionally excited lines can be explained as due to the temperature variations in a chemically homogeneous medium.

Astronomy and Astrophysics, 2006

An observational study of chemical abundances in the galactic planetary nebulae NGC 1535, NGC 2438, NGC 2440, NGC 3132, NGC 3242, NGC 6302, and NGC 7009 based on long-slit spectra of high signal-to-noise ratio in the 3100 to 6900 Å range is presented. We determined the N, O, Ne, S, and Cl abundances from collisionally excited lines and the He and O ++ abundances from recombination lines. The O ++ /H + estimates derived from recombination lines are about a factor of four and two higher than those derived from forbidden lines for NGC 7009 and NGC 3242, respectively. Spatial profiles of O ++ /H + abundance from O ii permitted lines and from [O iii] forbidden lines were obtained for the planetary nebula NGC 7009. The differences between O ++ /H + derived from recombination and from forbidden lines present smooth variations along the nebular surface of NGC 7009, with the differences decreasing from the center to the edges of the nebula. If these abundance differences are explained by the presence of electron temperature fluctuations, quantified by the parameter t 2 , a value of about t 2 = 0.09 is required for NGC 3242 and NGC 7009.

The Astrophysical Journal, 2000

In response to the recent molecular observations by ISO and millimeter-wave telescopes, we have developed a spherically symmetric, steady state chemical model for the planetary nebula NGC 7027. In our model, the neutral envelope consists of a geometrically thin, high-density shell of constant density and an outer stellar wind region with an inverse-square law density profile. The neutral envelope is subjected to ultraviolet (UV) radiation both from the central star and from the external interstellar field. Under an assumed visual extinction of 4 mag for the neutral envelope, the gas in our model is half molecular and half atomic. Simple molecules such as CH, CH+, OH are abundantly formed at an assumed gas temperature of 800 K in the partially molecular shell. The presence of molecular ions such as HCO+ and CO+ is well explained by photochemistry at high temperatures. The model CN/HCN ratio is almost constant at about 30 throughout the wind region and is 1-10 in the dense shell, comparable to observed values in planetary nebulae. Our model suggests that molecules are as efficiently formed with timescales shorter than 100 yr as they are photodissociated in the neutral envelope of NGC 7027 and probably in other young planetary nebulae.

Monthly Notices of the Royal Astronomical Society, 2001

We have studied the chemistry of the molecular gas in evolved planetary nebulae. Three pseudo-time-dependent gas-phase models have been constructed for dense ð10 4 -10 5 cm 23 Þ and cool ðT , 15 KÞ clumpy envelopes of the evolved nebulae NGC 6781, M4-9 and NGC 7293. The three nebulae are modelled as carbon-rich stars evolved from the asymptotic giant branch to the late planetary nebula phase. The clumpy neutral envelopes are subjected to ultraviolet radiation from the central star and X-rays that enhance the rate of ionization in the clumps. With the ionization rate enhanced by four orders of magnitude over that of the ISM, we find that resultant abundances of the species HCN, HNC, HC 3 N and SiC 2 are in good agreement with observations, while those of CN, HCO 1 , CS and SiO are in rough agreement.

Astronomy & Astrophysics, 2011

Aims. Chemical evolution models are useful for understanding the formation and evolution of stars and galaxies. Model predictions will be more robust when more observational constraints are used. We present chemical evolution models for the dwarf irregular galaxy NGC 6822 using chemical abundances of old and young planetary nebulae (PNe) and H ii regions as observational constraints. We use two sets of chemical abundances, one derived from collisionally excited lines (CELs) and one from recombination lines (RLs). We use our models as a tool to distinguish between both procedures for abundance determinations. Methods. In our chemical evolution code the chemical contribution of low and intermediate mass stars is time-delayed, while for the massive stars the chemical contribution follows the instantaneous recycling approximation. Our models have two main free parameters: the mass-loss rate of a well-mixed outflow and the upper mass limit, M up , of the initial mass function (IMF). To reproduce the gaseous mass and the present-day O/H value we need to vary the outflow rate and the M up value. Results. We calculate two models with different M up values that reproduce the constraints adequately. The abundances of old PNe agree with our models and support the star-formation history derived independently from photometric data. Both require an early wellmixed wind, lasting 5.3 Gyr, to reproduce the observed gaseous mass in the galaxy. In addition, by assuming a fraction of binaries producing SNIa of 1%, the models fit the Fe/H abundance ratio as derived from A supergiants. The first model (M4C), which assumes M up = 40 M , fits within errors smaller than 2σ the O/H, Ne/H, S/H, Ar/H and Cl/H abundances obtained from CELs for old and young PNe and H ii regions. The second model (M1R), which adopts M up = 80 M , reproduces within 2σ errors the O/H, C/H, Ne/H and S/H abundances adopted from RLs. Both models reproduce the increase of the O, Ne, S, and Ar elements during the last 6 Gyr. We are not able to match the observed N/O ratios in either case, which suggests that the N yields of LIMS need to be improved. Model M1R does not provide a good fit to the Cl/H and Ar/H ratios, because the SN yields of those elements for m > 40 M are not adequate and need to be improved (two sets of yields were tried). From these results we are unable to conclude which set of abundances (the one from CELs or the one from RLS) represents the real abundances in the ISM better. We discuss the predicted ΔY/ΔO values, finding that the value from model M1R agrees better with data for other galaxies from the literature than the value from model M4C.

We have studied the chemistry in dense (10 4 -10 5 cm -3), and cool (T = 25 K) gas clumps, such as observed in the Helix nebula (NGC 7293). Two gas-phase chemical evolutionary models have been constructed for the evolved planetary nebulae NGC 7293. The nebula is modeled as carbon-rich progenitor evolved from asymptotic giant branch to late planetary nebula phase. The clumpy neutral envelope assumed to be subjected to ultraviolet radiation from the central star and reasonable amount of X-rays that enhances the rate of ionization in the clumps. Our results are in good agreement compared with the observed abundances that have been detected in the molecular clumps of NGC 7293 such as, CN, HCN, HNC, HCO + , CS, HC 3 N, SiO and SiC 2 .

Monthly Notices of the Royal Astronomical Society, 2016

We present integral field unit (IFU) spectroscopy and self-consistent photoionization modelling for a sample of four southern Galactic planetary nebulae (PNe) with supposed weak emissionline central stars. The Wide Field Spectrograph on the ANU 2.3 m telescope has been used to provide IFU spectroscopy for NGC 3211, NGC 5979, My 60, and M 4-2 covering the spectral range of 3400-7000 Å. All objects are high-excitation non-Type I PNe, with strong He II emission, strong [Ne V] emission, and weak low-excitation lines. They all appear to be predominantly optically thin nebulae excited by central stars with T eff > 10 5 K. Three PNe of the sample have central stars which have been previously classified as weak emission-line stars (WELS), and the fourth also shows the characteristic recombination lines of a WELS. However, the spatially resolved spectroscopy shows that rather than arising in the central star, the C IV and N III recombination line emission is distributed in the nebula, and in some cases concentrated in discrete nebular knots. This may suggest that the WELS classification is spurious, and that, rather, these lines arise from (possibly chemically enriched) pockets of nebular gas. Indeed, from careful background subtraction we were able to identify three of the sample as being hydrogen rich O(H)-Type. We have constructed fully self-consistent photoionization models for each object. This allows us to independently determine the chemical abundances in the nebulae, to provide new model-dependent distance estimates, and to place the central stars on the Hertzsprung-Russell diagram. All four PNe have similar initial mass (1.5 < M/M < 2.0) and are at a similar evolutionary stage.

The Astrophysical Journal, 2008

We analyze the chemical composition of the central star of the planetary nebula NGC 6543 based on a detailed NLTE model of its stellar wind. The logarithmic abundances by number are H = 12.00, He = 11.00, C = 9.03, N = 8.36, O = 9.02, Si = 8.19, P = 5.53, S = 7.57, and Fe = 7.24. Compared with the solar abundances, most of the elements have solar composition with respect to hydrogen, except C, which is overabundant by 0.28 dex, and Fe, which is depleted by ~0.2 dex. Contrary to most previous work, we find that the star is not H-poor and has a normal He composition. These abundances are compared with those found in the diffuse X-ray plasma and the nebular gas. Compared to the plasma emitting in diffuse X-rays, the stellar wind is much less depleted in iron. Since the iron depletions in the nebular gas and X-ray plasma are similar, we conclude that the plasma emitting diffuse X-rays is derived from the nebular gas rather than the stellar wind. Excellent agreement is obtained between the abundances in the stellar wind and the nebular recombination line abundances for He, C, and O relative to H. On the other hand, the derived stellar N abundance is smaller than the nebular N abundance derived from recombination lines and agrees with the abundance found from collisionally excited lines. The mean temperature variation determined by five different methods indicates that the difference in the nebular abundances between the recombination lines and collisionally excited lines can be explained as due to the temperature variations in a chemically homogeneous medium.