Post-AGB objects and planetary nebulae

Planetary nebulae and post-AGB objects are luminous dust emitters in the infrared. Indeed it is by virtue of their strong dust emission that most post-AGB objects (proto-planetary nebulae) have usually been recognised as such, given that the strong optical line emission from photoionized gas that is typical of PNe is absent from the spectra of post-AGB objects, whose central stars occupy the intermediate temperature range between the cool AGB stars and the hot central stars of PNe. Figure 1 shows the characteristic double-peaked optical-IR spectrum of the post-AGB object HD 161796, from Hoogzaad et al. [12]. At optical wavelengths the spectrum of the F3Ib central star dominates, but at IR wavelengths a second peak, due to emission by dust ejected during the earlier AGB phase, is very prominent in its ISO spectrum. Hoogzaad et al. found that contributions from both amorphous and crystalline silicate particles, as well as crystalline water ice were needed to fit the IR spectrum of this oxygen-rich object. Figure 2, from Molster et al. [15], shows the ISO SWS+LWS infrared spectrum of a much more evolved object, the massive bipolar PN NGC 6302. As highlighted in the caption to Fig. 2, a wide variety of C-rich and O-rich particles contribute to its exceedingly rich spectrum

The destruction and growth of dust grains in interstellar space – III. Surface recombination, heavy element depletion and mantle growth

It is shown that metallic elements in interstellar clouds will be trapped on the surfaces of graphite and iron grains, leading to their differential gas-phase depletion, whereas non-metal elements will be ejected as saturated hydrides from such grains (and as monohydrides from other grains), with no consequent depletion. A consideration of surface reaction pathways leads to the prediction that Na, K and Zn should undergo less depletion than other metals, in accordance with observations. The factors governing the return of depleted metallic species to the gas phase are discussed. In diffuse clouds the growth of mantles is prevented by photo-desorption and by the ejection of non-metal atoms during surface recombination, but in dense clouds which have sufficient ultraviolet shielding, and where the non-metals are in saturated molecules, mantle growth will take place. Upon exposure to the unshielded interstellar ultraviolet radiation field such mantles will rapidly be destroyed. Thus mantles should be found only in dense, shielded regions of interstellar space

Prospects for Studies of Stellar Evolution and Stellar Death in the JWST Era

I review the prospects for studies of the advanced evolutionary stages of low-, intermediate- and high-mass stars by the JWST and concurrent facilities, with particular emphasis on how they may help elucidate the dominant contributors to the interstellar dust component of galaxies. Observations extending from the mid-infrared to the submillimeter can help quantify the heavy element and dust species inputs to galaxies from AGB stars. JWST’s MIRI mid-infrared instrument will be so sensitive that observations of the dust emission from individual intergalactic AGB stars and planetary nebulae in the Virgo Cluster will be feasible. The Herschel Space Observatory will enable the last largely unexplored spectral region, from the far-IR to the submm, to be surveyed for new lines and dust features, while SOFIA will cover the wavelength gap between JWST and Herschel, a spectral region containing important fine structure lines, together with key water-ice and crystalline silicate bands. Spitzer has significantly increased the number of Type II supernovae that have been surveyed for early-epoch dust formation but reliable quantification of the dust contributions from massive star supernovae of Type II, Type Ib and Type Ic to low- and high-redshift galaxies should come from JWST MIRI observations, which will be able to probe a volume over 1000 times larger than Spitzer

The destruction and growth of dust grains in interstellar space – I. Destruction by sputtering

The processes governing the destruction and growth of dust grains in interstellar space are investigated with a view to establishing the conditions required for the existence of ice mantles. In this paper sputtering by particles with energies in the eV to GeV range is considered. Previous sputtering yield estimates which were based on theoretical considerations are shown to be greatly in error for incident particle energies less than 1 keV. Empirical formulae for the sputtering threshold energy and the sputtering yield are derived from the extensive experimental data available. The sputtering of grains in H II regions, in the inter-cloud medium, and in shock waves produced by cloud–cloud collisions and by supernova remnants is investigated. Of these, supernova remnants are shown to be the most important, leading to lifetimes ∼ 2 × 108 yr for ice grains and between 5–20 × 108 yr for refractory grains. Destruction rates are estimated for grains bombarded by MeV and GeV cosmic rays. It is shown that collision cascade sputtering dominates evaporative sputtering produced by thermal spikes. It is also shown that even if all the electron excitation energy loss in a grain material could be transferred to the lattice particles, the observed cosmic ray flux spectrum could not cause significant destruction of ice grains

Planetary nebulae beyond the milky way - Historical overview

Up to 10% of the total luminosity of a planetary nebula, ~500 L⊙, can be emitted in the dominant cooling line, [O iii] λ5007. This, coupled with the narrowness of the line (~15-25 km s-1), makes it extremely easy to detect PNe in external galaxies using a narrow-band lter tuned to the galaxy redshift. The availability of multiple independent distance indicators for our closest neighbouring galaxies, the Magellanic Clouds and M 31, means that the luminosities of the PNe in these galaxies (in particular the large numbers of PNe in the LMC) can be used to calibrate PN luminosity functions, which have been used over the past 15 years as probes of the Hubble constant. Given the fact that we still do not have accurate distances for most planetary nebulae in the Milky Way, this has been an astonishing development. Unlike H ii regions, which cannot be used to probe elliptical galaxies, planetary nebulae can be used to probe the dynamics and metallicity of any type of galaxy. Today, via accurate radial velocity measurements, extragalactic PNe are being used as dynamical mass probes of galaxies, and even of the stellar mass content of the intracluster regions of galaxy clusters, with dedicated ‘planetary nebula spectrographs’ being built to further such studies. The angular resolution of the Hubble Space Telescope has turned out to be ideally suited to the study of PNe in the LMC and SMC, and the exploitation of the accurately known distances to these two galaxies has allowed reliable luminosities and masses to be derived for the central stars and nebulae, thereby putting PN research on a much more quantitative footing

The impact of future space observatories on planetary nebula research

A comparison between the past four decades of astrophysical space missions and those expected to be launched over the next decade shows a marked decrease in numbers. However, missions such as Gaia and the JWST are expected to have a major impact on planetary nebula research. The capabilities of these and other anticipated space missions are discussed

The determination of the masses of Magellanic Cloud planetary nebulae using [O II] doublet ratio electron densities

Spectrophotometric data, including [O II] 3726, 3729 Å doublet ratios, are presented for 32 planetary nebulae (PN) in the Magellanic Clouds. It is argued that the electron densities derived from these ratios provide a much better diagnostic for the determination of nebular masses than previously assumed. The 32 PN are classified as either Type I or else as optically thick or optically thin in the hydrogen Lyman continuum. The optically thick PN are found to all have electron densities greater than 6000 cm –3 , while the optically thin PN all have electron densities below 5000 cm –3. The optically thin PN show a range of only a factor of 2.0 in their derived masses, and have a mean ionized mass of 0.27±0.06M⊙⁠. The absolute Hβ fluxes of the optically thick nebulae show a range of only a factor of 1.8. The application of these results to Galactic PN would yield distances which are generally larger than those previously estimated. A method of distance determination is proposed for optically thin PN that uses integrated nebular [O II] electron densities rather than angular diameters

The destruction and growth of dust grains in interstellar space – II. Destruction by grain surface reactions, grain–grain collisions and photodesorption

Chemical reactions on the surfaces of ice grains are shown to be unimportant as destruction agents. However, graphite grains can be destroyed by reaction with chemisorbed hydrogen and oxygen atoms, for grain temperatures in excess of ∼ 65 K, leading to an explanation for the weakness of the 2175 Å extinction feature towards several stars embedded in H II regions. It is also shown that such reactions govern the conditions under which graphite grains can condense in stellar atmospheres. The classical Oort–van de Hulst destruction mechanism of grain–grain collisions during cloud collisions is shown to be ineffective, by means of a detailed consideration of the shock wave structure at the interface between the colliding clouds. Magnetic field influenced grain collisions in shock fronts, between grains from the same cloud, do not lead to a significant overall destruction rate for dust grains in the interstellar medium. It is argued that photodesorption is the dominant destruction mechanism for ice grains held together by weak van der Waals dispersion forces. The timescale for destruction of an ice grain of radius 10−5 cm by the interstellar ultraviolet radiation field is derived to be ∼ 5 × 104 yr, much shorter than for other destruction mechanisms