We analyze the expansion of hydrogen-poor knots and filaments in the born-again planetary nebulae A30 and A78 based on Hubble Space Telescope (HST ) images obtained almost 20 yr apart. The proper motion of these features generally increases with distance to the central star, but the fractional expansion decreases, i.e., the expansion is not homologous. As a result, there is not a unique expansion age, which is estimated to be 610-950 yr for A30 and 600-1140 yr for A78. The knots and filaments have experienced complex dynamical processes: the current fast stellar wind is mass loaded by the material ablated from the inner knots; the ablated material is then swept up until it shocks the inner edges of the outer, hydrogen-rich nebula. The angular expansion of the outer filaments shows a clear dependence on position angle, indicating that the interaction of the stellar wind with the innermost knots channels the wind along preferred directions. The apparent angular expansion of the innermost knots seems to be dominated by the rocket effect of evaporating gas and by the propagation of the ionization front inside them. Radiation-hydrodynamical simulations show that a single ejection of material followed by a rapid onset of the stellar wind and ionizing flux can reproduce the variety of clumps and filaments at different distances from the central star found in A30 and A78. Subject headings: ISM: kinematics and dynamics -planetary nebulae: individual (A30 and A78) 2. OBSERVATIONS AND DATA ANALYSIS New HST WFC3 F502N narrowband images of A30 and A78 (PI: M.A. Guerrero, Prop. ID 12935) were obtained on 2013 March 21 and 2012 November 22, respectively. The images were reduced following stan-arXiv:1410.3872v1 [astro-ph.SR]

Mid-IR observations of planetary nebulae (PNe) have revealed diffuse emission associated to their main nebular shells and outer envelopes or haloes. The interpretation of this emission is uncertain because the broad-band mid-IR images may include contributions of different components. In particular, the Spitzer IRAC 8 µm images, that best reveal these nebular features, can include contributions not only of H 2 lines, but also those of ionic species, PAH features, and thermal dust continuum emission. To investigate the nature of the emission detected in mid-IR observations of a sample of 10 PNe, we have obtained narrow-band near-IR H 2 λ2.122 µm and optical [N ii] λ6584Å images. The comparison between these images confirm that a significant fraction of the emission detected in the IRAC 8 µm images can be attributed to molecular hydrogen, thus confirming the utility of these mid-IR images to investigate the molecular component of PNe. We have also detected H 2 emission from PNe whose physical structure cannot be described as bipolar, but rather as ellipsoidal or barrel-like. These detections suggest that, as more sensitive observations of PNe in the H 2 λ2.122 line are acquired, the detection of H 2 emission is not exclusive of bipolar PNe, although objects with this morphology are still the brightest H 2 emitters. Finally, we remark that the bright H 2 emission from the equatorial ring of a bipolar PN does not arise from a photo-dissociation region shielded from the UV stellar radiation by the ring itself, but from dense knots and clumps embedded within the ionized material of the ring.

We present near-IR K-band intermediate-dispersion spatially-resolved spectroscopic observations of a limited sample of bipolar planetary nebulae (PNe). The spectra have been used to determine the excitation mechanism of the H 2 molecule using standard line ratios diagnostics. The H 2 molecule is predominantly shock-excited in bipolar PNe with broad equatorial rings, whereas bipolar PNe with narrow equatorial waists present either UV excitation at their cores (e.g., Hb 12) or shock-excitation at their bipolar lobes (e.g., M 1-92). The shock-excitation among bipolar PNe with ring is found to be correlated with emission in the H 2 1-0 S(1) line brighter than Brγ. We have extended this investigation to other PNe with available near-IR spectroscopic observations. This confirms that bipolar PNe with equatorial rings are in average brighter in H 2 than in Brγ and show dominant shock excitation.

We present the XMM-Newton discovery of X-ray emission from the planetary nebula (PN) A78, the second born-again PN detected in X-rays apart from A30. These two PNe share similar spectral and morphological characteristics: They harbor diffuse soft X-ray emission associated with the interaction between the H-poor ejecta and the current fast stellar wind, and a point-like source at the position of the central star (CSPN). We present the spectral analysis of the CSPN, using for the first time a NLTE code for expanding atmospheres which takes line blanketing into account for the UV and optical spectra. The wind abundances are used for the X-ray spectral analysis of the CSPN and the diffuse emission. The X-ray emission from the CSPN in A78 can be modeled by a single C vi emission line, while the X-ray emission from its diffuse component is better described by an optically thin plasma emission model with temperature kT =0.088 keV (T ≈1.0×10 6 K). We estimate X-ray luminosities in the 0.2-2.0 keV energy band of L X,CSPN =(1.2±0.3)×10 31 erg s −1 and L X,DIFF =(9.2±2.3)×10 30 erg s −1 for the CSPN and diffuse components, respectively. Subject headings: planetary nebulae: general -planetary nebulae: individual (A78) -stars: winds, outflows -X-rays: ISM

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