Low-excitation atomic gas around evolved stars: II. ISO observations of O-rich nebulae

We have observed atomic fine-structure lines in the far-infrared (FIR) from 12 oxygen-rich evolved stars. The sample is composed of mostly proto-planetary nebulae (PPNe) and some planetary nebulae (PNe) and asymptotic giant branch (AGB) stars. ISO LWS and SWS observations of [O I], [C II], [N II], [Si I], [Si II], [S I], [Fe I], and [Fe II] lines were obtained. Taking into account also the sample presented by Fong et al. (Paper I) of carbon-rich evolved stars, we find that PPNe emit in these low-excitation atomic transitions only when the central star is hotter than ∼10 000 K. This result suggests that such lines predominantly arise from photodissociation regions (PDRs), and not from shocked regions. The line widths determined from our Fabry-Perot data also suggest that the FIR lines arise from relatively quiescent PDR gas, as opposed to shocked gas. Our results are in reasonable agreement with predictions from PDR emission models, allowing the estimation of the density of the emitting layers by comparison with the model results. On the other hand, the comparison with predictions of the emission from J-type and C-type shocked regions suggests that detected lines do not come from shocks. The [C II] line flux has been used to measure the mass of the low-excitation atomic component in PPNe, since this transition has been found to be a useful model-independent probe to estimate the total mass of these PDRs. The derivation of the mass formula and assumptions made are also discussed

Hubble Space Telescope/NICMOS near-infrared imaging of the proto planetary nebula OH 231.8+4.2

We present observations of the bipolar nebula OH 231.8+4.2, made with the Hubble Space Telescope (HST) NICMOS camera, in three wide filter bands (F205W, F160W, and F110W). The images have excellent dynamic range after removal of low-level instrumental artifacts. In the F205W filter (lambda approximate to 2.04 mum), we achieve a peak/rms of greater than 8000 with an angular resolution of 0."20 (FWHM). The combination of high dynamic range and angular resolution confirms previous observations but also reveals new features in the near-IR morphology of the nebula, which at these wavelengths is dominated by scattered light. The northern (approaching) lobe shows well-defined, limb-brightened edges. The central jet splits into two laments and exhibits some curvature between the center of the nebula and the end of the northern lobe. The southern (receding) lobe has a diffuse, occulent appearance without a sharply defined central jet, in contrast to the northern lobe. A sharpened version of the F205W image shows indications of turbulent structures both in the northern lobe and jet and in the southern lobe. A faint cylindrical halo of scattered light shows a sharp increase in surface brightness inside a radius of similar to4" from the center, possibly resulting from a transition from a spherical wind to a disk- or torus-like mass ejection, on the same timescale as the formation of the collimated fast wind seen in CO and HCO(+) images. We calculate the extinction through the nebula from the measured near-IR colors and a silicate grain model. For a simple geometric model of a dense central disk, we estimate a disk mass (gas and dust) in the range 0.03-0.06 M., relatively insensitive to grain size. The circumstance of an asymptotic giant branch (AGB) star with a massive, highly collimated, high-velocity bipolar flow may be understood in terms of the model of Soker & Rappaport if the central star underwent a change in mass-loss properties from a (roughly) spherical AGB wind to equatorially enhanced mass loss beginning similar to(1-3) X 10(3) yr ago. Supposing the presence of a dwarf companion in a suitable orbit, the bipolar nebula then is a consequence of a strong increase in mass loss as the central star evolves close to the tip of the AGB and will soon evolve to higher T(eff) and appear as a more typical proto planetary nebula

A SEARCH FOR DUST EMISSION IN THE LEO INTERGALACTIC CLOUD

We present a search for infrared dust emission associated with the Leo cloud, a large intergalactic cloud in the M96 group. Mid-infrared and far-infrared images were obtained with IRAC and MIPS on the Spitzer Space Telescope. Our analysis of these maps is done at each wavelength relative to the Hi spatial distribution. We observe a probable detection at 8 µm and a marginal detection at 24µm associated with the highest Hi column densities in the cloud. At 70 and 160µm, upper limits on the dust emission are deduced. The level of the detection is low so that the possibility of a fortuitous cirrus clump or of an overdensity of extragalactic sources along the line of sight can not be excluded. If this detection is confirmed, the quantities of dust inferred imply a dust to gas ratio in the intergalactic cloud up to a few times solar but no less than 1/20 solar. A confirmed detection would therefore exclude the possibility that the intergalactic cloud has a primordial origin. Instead, this large intergalactic cloud could therefore have been formed through interactions between galaxies in the group

Collaborative links

We report results from a Hubble Space Telescope (HST) and Near-Infrared Camera and Multiobject Spectrometer (NICMOS) program to study the distribution of hot neutral (molecular hydrogen) and ionized circumstellar material in the young planetary nebulae NGC 7027. HST/NICMOS provided very high spatial resolution imaging in line and continuum emission, and the stability and large dynamic range needed for investigating detailed structures in the circumstellar material. We present dramatic new images of NGC 7027 that have led to a new understanding of the structure in this important planetary nebula. The central star is clearly revealed, providing near-infrared fluxes that are used to directly determine the stellar temperature very accurately (T* = 198,000 K). It is found that the photodissociation layer as revealed by near-infrared molecular hydrogen emission is very thin (Delta R similar to 6 x 10(15) cm) and is biconical in shape. The interface region is structured and filamentary, suggesting the existence of hydrodynamic instabilities. We discuss evidence for the presence of one or more highly collimated, off-axis jets that might be present in NGC 7027. The NICMOS data are combined with earlier Hubble Space Telescope data to provide a complete picture of NGC 7027 using the highest spatial resolution data to date. The evolutionary future of NGC 7027 is discussed

Low-excitation atomic gas around evolved stars II. ISO observations of O-rich nebulae

We have observed atomic fine-structure lines in the far-infrared (FIR) from 12 oxygen-rich evolved stars. The sample is composed of mostly proto-planetary nebulae (PPNe) and some planetary nebulae (PNe) and asymptotic giant branch (AGB) stars. ISO LWS and SWS observations of [O I]. [C II], [N II], [Si I]: [Si II], [S I], [Fe I], and [Fe II] lines were obtained. Taking into account also the sample presented by Fong et al. (Paper I) of carbon-rich evolved stars, we find that PPNe emit in these low-excitation atomic transitions only when the central star is hotter than similar to 10 000 K. This result suggests that such lines predominantly arise from photodissociation regions (PDRs), and not from shocked regions. The line widths determined from our Fabry-Perot data also suggest that the FIR lines arise from relatively quiescent PDR gas, as opposed to shocked gas. Our results are in reasonable agreement with predictions from PDR emission models, allowing the estimation of the density of the emitting layers by comparison with the model results. On the other hand, the comparison with predictions of the emission from J-type and C-type shocked regions suggests that detected lines do not come from shocks. The [C II] line flux has been used to measure the mass of the low-excitation atomic component in PPNe, since this transition has been found to be a useful model-independent probe to estimate the total mass of these PDRs. The derivation of the mass formula and assumptions made are also discussed

ISO-LWS observations of rotational CO lines from C-rich objects: AFGL 2688, AFGL 618 and NGC 7027

We present ISO-LWS full scan observations of CO rotational emission lines (J=14-13 up to J=37-36) from two C-rich post-AGE objects, AFGL 2688; AFGL 618, and one C-rich PN, NGC 7027. The presence of high rotational lines cannot be explained by a spherical, constant velocity wind during the previous AGE phase, but indicates a layer of warm, dense gas with a substantial beam filling factor(Ohm > 10(-10) sr). By simple optically thin calculations, we estimate the total mass of gas cooled through CO rotational lines to be typically similar to 0.1 M.. We also consider the physical processes responsible for heating and cooling the warm gas in these objects. Energy sources for the gas could include FUV photons, resulting in a PhotoDissociation Region (PDR), as well as shocks driven by the interacting winds during this evolutionary phase. We have calculated detailed models for the heating and cooling in C-rich PDRs and shocks in order to explain the CO spectra observed. In both models, the gas is cooled radiatively by molecular rotational lines as well as by atomic fine structure lines. Both models can produce warm gas of >500 K at a density of 10(6) - 10(7) cm(-3). We conclude that the source of heating for the young PN, NGC 7027, is FUV photons from the central star For the least evolved post-AGE star, AFGL 2688, the effective temperature is too cool to produce many FUV photons to heat the gas. However, there is evidence of a fast wind developing and hence the most likely heating mechanism for the envelope is shocks. For AFGL 618, the central star is hot enough to provide copious amounts of FUV photons but fast outflows have also been observed and both processes may be operational. However, in view of [O I] 63 mu m high resolution observations, the heating mechanism is more likely to be a PDR