The gas and dust coma of Comet C/1999 H1 (Lee)

The final version is available at: http://www.aanda.orgComet Lee (C/1999 H1) was observed on June 6, 1999 when it was at rh = 0.98 AU and Δ = 1.195 AU. The spectrophotometric observations, between 0.6 and 1 μm, were aimed at the detection of the CI(1D) doublet λλ 9823/9850 Å. The non-detection of these lines, with a 3σ flux upper limit of the order of 4.6 × 10−17 erg cm−2 s−1, confirms the CO depletion already noted by other authors. Several CN and NH2 emission bands lie in that spectral range, making it possible to derive production rates for both species as ∼3.1 × 1026 s−1 and 1.2 × 1027 s−1, respectively. The oxygen forbidden line at 6300 Å was used to obtain QH2O = (1.22 ± 0.7) × 1029 s−1. Assuming that CN and NH2 are directly produced by HCN and NH3, Comet Lee has a HCN/H2O ≈ 0.25% and NH3/H2O ≈ 1% at rh = 0.98 AU, in reasonable agreement with what has been found in other long-period comets. The structural analysis carried out on cometary images acquired with broad band R Bessel filter clearly displays two pairs of ion rays likely produced by the H2O+ doublet at 6198 and 6200 Å, wavelengths covered by the bandpass filter. Identical features are found in the images acquired with the Gunn i filter. The dust brightness profiles in the east-west direction do not deviate from a ρ−m law (with 0.7 < m ≤ 1.2) as expected for a steady state model coma with a constant dust production rate and expanding at constant velocity. The dust production rate, as obtained from the A f ρ parameter, is ∼500 cm, which compared with the gas production rate classifies this comet as a dust poor one with relatively high (6.5–11.7) gas-to-dust mass ratio. Analysis of the normalized reflectivity gradient (i.e. continuum color) as a function of ρ indicates a slight reddening of the solid component in the coma at large cometocentric distances, whereas the average dust color within an aperture of 20 000 km, centered at the nucleus, is ∼9% per 1000 Å. Mie scattering computations applied to an ensemble of particles indicate that the dust coma is haracterised by a relative broad size distribution with a typical mean size of 1 μm. These grains might be composed of a mixture of silicates and icy material.Peer reviewe

Space Telescope Imaging Spectrograph slitless observations of Small Magellanic Cloud Planetary Nebulae: a study on morphology, emission line intensity, and evolution

A sample of 27 Planetary Nebulae (PNs) in the Small Magellanic Clouds (SMC) have been observed with the Hubble Space Telescope Imaging Spectrograph (HST/STIS) to determine their morphology, size, and the spatial variation of the ratios of bright emission lines. The morphologies of SMC PNs are similar to those of LMC and Galactic PNs. However, only a third of the resolved SMC PNs are asymmetric, compared to half in the LMC. The low metallicity environment of the SMC seems to discourage the onset of bipolarity in PNs. We measured the line intensity, average surface brightness (SB), and photometric radius of each nebula in halpha, hbeta, [O III] lambda4959 and 5007, [NII] 6548 and 6584, [S II] lambda6716 and 5731, He I 6678, and [OI] 6300 and 6363. We show that the surface brightness to radius relationship is the same as in LMC PNs, indicating its possible use as a distance scale indicator for Galactic PNs. We determine the electron densities and the ionized masses of the nebulae where the [S II] lines were measured accurately, and we find that the SMC PNs are denser than the LMC PNs by a factor of 1.5. The average ionized mass of the SMC PNs is 0.3 Msun. We also found that the median [O III]/hbeta intensity ratio in the SMC is about half than the corresponding LMC median. We use Cloudy to model the dependence of the [O III]/hbeta ratio on the oxygen abundance. Our models encompass very well the average observed physical quantities. We suggest that the SMC PNs are principally cooled by the carbon lines, making it hard to study their excitation based on the optical lines at our disposal.Comment: Accepted for publication in the Astrophysical Journal, 30 pages, 13 figures, 6 tables. For high resolution version of Figs 1 to 6, see http://archive.stsci.edu/hst/mcpn/home.htm

The chemistry of planetary nebulae and HII regions in the dwarf galaxies Sextans A and B from deep VLT spectra

Spectroscopic observations obtained with the VLT of one planetary nebula (PN) in Sextans A and of five PNe in Sextans B and of several HII regions (HII) in these two dwarf irregular galaxies are presented. The extended spectral coverage, from 320.0 to 1000.0nm, and the large telescope aperture allowed us to detect a number of emission lines, covering more than one ionization stage for several elements (He, O, S, Ar). The electron temperature (Te) diagnostic [OIII] line at 436.3 nm was measured in all six PNe and in several HII allowing for an accurate determination of the ionic and total chemical abundances by means of the Ionization Correction Factors method. For the time being, these PNe are the farthest ones where such a direct measurement of the Te is obtained. In addition, all PNe and HII were also modelled using the photoionization code CLOUDY. The physico-chemical properties of PNe and HII are presented and discussed. A small dispersion in the oxygen abundance of HII was found in both galaxies: 12 + $\log$(O/H)=7.6$\pm$0.2 in SextansA, and 7.8$\pm$0.2 in SextansB. For the five PNe of SextansA, we find that 12 + $\log$(O/H)=8.0$\pm$0.3, with a mean abundance consistent with that of HII. The only PN known in SextansA appears to have been produced by a quite massive progenitor, and has a significant nitrogen overabundance. In addition, its oxygen abundance is 0.4 dex larger than the mean abundance of HII, possibly indicating an efficient third dredge-up for massive, low-metallicity PN progenitors. The metal enrichment of both galaxies is analyzed using these new data.Comment: 19 pages, 11 figures, Accepted by A&

Planetary Nebulae as Probes of Stellar Evolution and Populations

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.Comment: to appear in: Chemical Evolution of the Milky Way: stars versus clusters, ed. F. Giovannelli and F. Matteucci, Kluwer (2000), in pres

Large Magellanic Cloud Planetary Nebula Morphology: Probing Stellar Populations and Evolution

Planetary Nebulae (PNe) in the Large Magellanic Cloud (LMC) offer the unique opportunity to study both the Population and evolution of low- and intermediate-mass stars, by means of the morphological type of the nebula. Using observations from our LMC PN morphological survey, and including images available in the HST Data Archive, and published chemical abundances, we find that asymmetry in PNe is strongly correlated with a younger stellar Population, as indicated by the abundance of elements that are unaltered by stellar evolution (Ne, Ar, S). While similar results have been obtained for Galactic PNe, this is the first demonstration of the relationship for extra-galactic PNe. We also examine the relation between morphology and abundance of the products of stellar evolution. We found that asymmetric PNe have higher nitrogen and lower carbon abundances than symmetric PNe. Our two main results are broadly consistent with the predictions of stellar evolution if the progenitors of asymmetric PNe have on average larger masses than the progenitors of symmetric PNe. The results bear on the question of formation mechanisms for asymmetric PNe, specifically, that the genesis of PNe structure should relate strongly to the Population type, and by inference the mass, of the progenitor star, and less strongly on whether the central star is a member of a close binary system.Comment: The Astrophysical Journal Letters, in press 4 figure

The Local Group Census: planetary nebulae in IC 10, Leo A and Sextans A

In the framework of our narrow-band survey of the Local Group galaxies, we present the results of the search for planetary nebulae (PNe) in the dwarf irregular galaxies IC10, LeoA and SextansA. Using the standard on-band/off-band technique, sixteen new candidate PNe have been discovered in the closest starburst galaxy, IC10. The optical size of this galaxy is estimated to be much larger than previously thought, considering the location of the new PNe in an area of 3.6 kpc X 2.7 kpc. We also confirm the results of previous studies for the other two dwarf irregular galaxies, with the detection of one candidate PN in LeoA and another one in SextansA. We review the number of planetary nebulae discovered in the Local Group to date and their behaviour with metallicity. We suggest a possible fall in the observed number of PNe when [Fe/H]$<<$-1.0, which might indicate that below this point the formation rate of PNe is much lower than for stellar populations of near Solar abundances. We also find non-negligible metallicity effects on the [OIII] luminosity of the brightest PN of a galaxy.Comment: 10 pages, 4 figures, A&A accepte

Chemical evolution of the Small Magellanic Cloud based on planetary nebulae

We investigate the chemical evolution of the Small Magellanic Cloud (SMC) based on abundance data of planetary nebulae (PNe). The main goal is to investigate the time evolution of the oxygen abundance in this galaxy by deriving an age-metallicity relation. Such a relation is of fundamental importance as an observational constraint of chemical evolution models of the SMC. We have used high quality PNe data in order to derive the properties of the progenitor stars, so that the stellar ages could be estimated. We collected a large number of measured spectral fluxes for each nebula, and derived accurate physical parameters and nebular abundances. New spectral data for a sample of SMC PNe obtained between 1999 and 2002 are also presented. These data are used together with data available in the literature to improve the accuracy of the fluxes for each spectral line. We obtained accurate chemical abundances for PNe in the Small Magellanic Cloud, which can be useful as tools in the study of the chemical evolution of this galaxy and of Local Group galaxies. We present the resulting oxygen versus age diagram and a similar relation involving the [Fe/H] metallicity based on a correlation with stellar data. We discuss the implications of the derived age-metallicity relation for the SMC formation, in particular by suggesting a star formation burst in the last 2-3 Gyr.Comment: 11 pages, 6 figures, accepted for publication in Astronomy and Astrophysic

Spitzer Infrared Spectrograph Observations of Magellanic Cloud Planetary Nebulae: the nature of dust in low metallicity circumstellar ejecta

We present 5 - 40 micron spectroscopy of 41 planetary nebulae (PNe) in the Magellanic Clouds, observed with the Infrared Spectrograph on board the Spitzer Space Telescope. The spectra show the presence of a combination of nebular emission lines and solid-state features from dust, superimposed on the thermal IR continuum. By analyzing the 25 LMC and 16 SMC PNe in our sample we found that the IR spectra of 14 LMC and 4 SMC PNe are dominated by nebular emission lines, while the other spectra show solid-state features. We observed that the solid-state features are compatible with carbon-rich dust grains (SiC, polycyclic aromatic hydrocarbons (PAHs), etc.) in most cases, except in three PNe showing oxygen-rich dust features. The frequency of carbonaceous dust features is generally higher in LMC than in SMC PNe. The spectral analysis allowed the correlations of the dust characteristics with the gas composition and morphology, and the properties of the central stars. We found that: 1) all PNe with carbonaceous dust features have C/O>1, none of these being bipolar or otherwise highly asymmetric; 2) all PNe with oxygen-rich dust features have C/O<1, with probable high mass progenitors if derived from single-star evolution (these PNe are either bipolar or highly asymmetric); 3) the dust temperature tracks the nebular and stellar evolution; and 4) the dust production efficiency depends on metallicity, with low metallicity environments not favoring dust production.Comment: The Astrophysical Journal, in pres