Arturo Manchado

The Dynamical Evolution of the Circumstellar Gas around Low- and Intermediate-Mass Stars. II. The Planetary Nebula Formation

We have studied the effect of the mass of the central star (CS) on the gas evolution during the planetary nebula (PN) phase. We have performed numerical simulations of PN formation using CS tracks for six stellar core masses corresponding to initial masses from 1 to 5 M☉. The gas structure resulting from the previous asymptotic giant branch (AGB) evolution is used as the starting configuration. The formation of multiple shells is discussed in the light of our models, and the density, velocity, and Hα emission brightness profiles are shown for each stellar mass considered. We have computed the evolution of the different shells in terms of radius, expansion velocity, and Hα peak emissivity. We find that the evolution of the main shell is controlled by the ionization front rather than by the thermal pressure provided by the hot bubble during the early PN stages. This effect explains why the kinematical ages overestimate the age in young CSs. At later stages in the evolution and for low-mass progenitors the kinematical ages severely underestimate the CS age. Large (up to 2.3 pc), low surface brightness shells (less than 2000 times the brightness of the main shell) are formed in all of our models (with the exception of the 5 M☉ model). These PN halos contain most of the ionized mass in PNe, which we find is greatly underestimated by the observations because of the low surface brightness of the halos.

Evidence for the Naphthalene Cation in a Region of the Interstellar Medium with Anomalous Microwave Emission

We report high-resolution spectroscopy of the moderately reddened ( -->AV = 3) early-type star Cernis 52 located in a region of the Perseus molecular cloud complex with anomalous microwave emission. In addition to the presence of the most common diffuse interstellar bands (DIBs) we detect two new interstellar or circumstellar bands coincident to within 0.01% in wavelength with the two strongest bands of the naphthalene cation (C10H -->8+) as measured in gas-phase laboratory spectroscopy at low temperatures and find marginal evidence for the third strongest band. Assuming these features are caused by the naphthalene cation, from the measured intensity and available oscillator strengths we find that 0.008% of the carbon in the cloud could be in the form of this molecule. We expect hydrogen additions to cause hydronaphthalene cations to be abundant in the cloud and to contribute via electric dipole radiation to the anomalous microwave emission. The identification of new interstellar features consistent with transitions of the simplest polycyclic aromatic hydrocarbon adds support to the hypothesis that this type of molecules are the carriers of both diffuse interstellar bands and anomalous microwave emission.

Planetary Nebula Abundances and Morphology: Probing the Chemical Evolution of the Milky Way

This paper presents a homogeneous study of abundances in a sample of 79 northern Galactic planetary nebulae (PNe) whose morphological classes have been uniformly determined. Ionic abundances and plasma diagnostics were derived from selected optical line strengths in the literature, and elemental abundances were estimated with the ionization correction factor developed by Kingsbourgh & Barlow in 1994. We compare the elemental abundances to the final yields obtained from stellar evolution models of low- and intermediate-mass stars, and we confirm that most bipolar PNe have high nitrogen and helium abundance and are the likely progeny of stars with main-sequence mass greater than 3 M☉. We derive = 0.27 and discuss the implication of such a high ratio in connection with the solar neon abundance. We determine the Galactic gradients of oxygen and neon and found Δ log(O/H)/ΔR = -0.01 dex kpc-1 and Δ log(Ne/H)/ΔR = -0.01 dex kpc-1. These flat PN gradients are irreconcilable with Galactic metallicity gradients flattening with time.

Spitzer Infrared Spectrograph Observations of Magellanic Cloud Planetary Nebulae: The Nature of Dust in Low-Metallicity Circumstellar Ejecta*

We present 5-40 ?m 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 four 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 all cases but three PNe, which show 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.

The Dynamical Evolution of the Circumstellar Gas around Low- and Intermediate-Mass Stars. I. The Asymptotic Giant Branch

We have investigated the dynamical interaction of low- and intermediate-mass stars (from 1 to 5 M☉) with their interstellar medium (ISM). In this first paper, we examine the structures generated by the stellar winds during the asymptotic giant branch (AGB) phase using a numerical code and the wind history predicted by stellar evolution. The influence of the external ISM is also taken into account. We find that the wind variations associated with the thermal pulses lead to the formation of transient shells with an average lifetime of ~20,000 yr and, consequently, do not remain recorded in the density or velocity structure of the gas. The formation of shells that survive at the end of the AGB phase occurs via two main processes: shocks between the shells formed by two consecutive enhancements of the mass loss or continuous accumulation of the material ejected by the star in the interaction region with the ISM. Our models show that the mass of the circumstellar envelope increases appreciably because of the ISM material swept up by the wind (up to ~70% for the 1 M☉ stellar model). We also point out the importance of the ISM on the deceleration and compression of the external shells. According to our simulations, large regions (up to 2.5 pc) of neutral gas surrounding the molecular envelopes of AGB stars are expected. These large regions of gas are formed from the mass loss experienced by the star during the AGB evolution.

Quadrupolar Planetary Nebulae: A New Morphological Class

In the context of the Instituto de Astrofisica de Canarias (IAC) morphological survey of Galactic planetary nebulae (PNs), a new morphological class has been found, and we define their members as quadrupolar PNs. We have found five quadrupolar objects whose lobes are, in pairs, symmetric with respect to two different axes. Among these PNs, three (M2-46, K3-24, and M1-75) have well-defined pairs of lobes; another two (M3-28 and M4-14) are irregular and very possibly quadrupolar. For M2-46, we have measured the extension and the angle between the lobes, and the expansion velocities of the lobes by means of spectroscopic analysis. We propose that these nebulae have been formed by precession of the rotation axis of the central stars, possibly in the presence of a binary companion, associated with multiple shell ejection at the asymptotic giant branch. A simple binary mechanism not associated with precession cannot produce such a morphology.

The Correlations between Planetary Nebula Morphology and Central Star Evolution: Analysis of the Northern Galactic Sample

The morphology of 111 Galactic planetary nebulae has been studied in relation to the evolutionary stage of their central stars. In order to locate these stars on the logT eff -logL/L ○ . plane, we have calculated the Zanstra temperatures with the most up to date fluxes and magnitudes available in the literature. Distances to the nebulae were estimated with statistical methods. The different nature and evolutionary stages of central stars have been related to the morphologies of the surrounding nebulae in a statistical sense. We found that multiple shell nebulae contain stars that are at a different evolutionary stage than those of single shell nebulae; we also found that bipolar and elliptical planetary nebulae very likely contain central stars with a different mass distribution; furthermore, we build an optical thickness sequence of morphological types

Ram Pressure Stripping in Planetary Nebulae

We present two-dimensional numerical simulations of the evolution of a low-mass star moving supersonically through its surrounding interstellar medium (ISM). We show that the ejecta of a moving star with a systemic velocity of 20 km s-1 will interact with the ISM and will form bow shock structures qualitatively similar to what is observed. We find that, owing to ram pressure stripping, most of the mass ejected during the asymptotic giant branch (AGB) phase is left downstream of the moving star. As a consequence, the formation of the planetary nebula is highly influenced, even at the low relative velocity of the star. The models are based on the predictions of stellar evolution calculations. Therefore, the density and velocity of the AGB and post-AGB winds are time dependent and give rise to the formation of shock regions inside the cavity formed by the previous winds. As a result, the stand-off distance is also time dependent and cannot be determined by simple analytical arguments.

On The Kinematics of Multiple-Shell Planetary Nebulae. I. Data and Expansion Velocities

We present spatially resolved echelle spectroscopy, obtained at high spectral resolution, for 15 multiple-shell planetary nebulae. Most exhibit faint detached halos (IC 1295, MA 3, M 2-2, M 2-40, NGC 6804, NGC 6826, NGC 6884, NGC 6891, NGC 7662, PM 1-295, and Vy 2-3). Furthermore, we have included some with attached shells (IC 1454, K 1-20, K 3-73, and PM 1-276) to allow comparison of the kinematic properties of the two subclasses of multiple-shell planetary nebulae. In addition, some of the nebulae in our sample show a triple-shell structure, composed of the bright main nebula and a combination of two attached shells (PB 9), one attached shell and one detached halo (NGC 6826, NGC 6891, NGC 7662, and Vy 2-3), or two detached shells (NGC 6804). A new method for computing the expansion velocities of those shells that do not show line splitting has been developed. This method assumes a thick-shell model and uses the observed Hα emission brightness profile to compute the volume emissivity dependence, (r), with the distance from the center of the nebula. The expansion velocity is then worked out by modeling how much the width of a the Hα line decreases with the radius of the shell. The radial velocity, expansion velocities of each shell, and turbulence contribution to the line width are presented. The expansion velocity of the detached halos spans from 12 to 30 km s-1. It is worth noting that the expansion velocities obtained by this method are greater than if they were computed with a thin-shell model, as has previously been done. In relation to the attached shells, their expansion velocities span from 10 to 30 km s-1. When the expansion velocities of the outer attached shells are related to the ellipticity of the inner shells, a trend toward faster expansion of the outer than the inner shells at higher ellipticities is found. The turbulent contribution to the line width has also been established. It is smaller for halos (0 km s-1 ≤ σtur ≤ 6 km s-1) than for attached shells (0 km s-1 ≤ σtur ≤ 15 km s-1). This suggests that large-scale hydrodynamic processes are more important in attached shells than in detached halos. We have also studied the kinematics of the detached halos whose morphology is perturbed from a round shape to a dipole asymmetry, indicating its interaction with the surrounding interstellar medium. We found systematic differences between the kinematical behavior of the enhanced edge of the halo and the opposite side in these cases, thus revealing the kinematic effect of the interaction of the halos with the interstellar medium.

H2 and Brγ Narrowband Imaging of Bipolar Planetary Nebulae

We present near-IR narrowband continuum-subtracted images in the H2 2.122 μm, and Brγ 2.166 μm emission lines for a sample of 15 bipolar planetary nebulae (PNe). H2 emission was definitely detected for most of the objects in this sample (13 of 15). The very high H2 detection rate supports the idea that bipolar PNe have important reservoirs of molecular material and offer suitable physical conditions for the excitation of H2. The strength of the H2 emission and the H2/Brγ flux ratio are found to correlate with the morphology of the bipolar PNe observed. Bipolar PNe with broad and bright rings exhibit stronger H2 emission than bipolar PNe with narrow twists. High-quality (subarcsec) [N II] and Hα optical images have been used to compare the distribution of the ionized and molecular material. The H2 emission lies just outside the optical [N II] emission zone.