A New Radio Spectral Line Survey of Planetary Nebulae: Exploring Radiatively Driven Heating and Chemistry of Molecular Gas

Planetary nebulae contain shells of cold gas and dust whose heating and chemistry is likely driven by UV and X-ray emission from their central stars and from wind-collision-generated shocks. We present the results of a survey of molecular line emissions in the 88 - 235 GHz range from nine nearby

The Fate of Stellar Material: Radio Molecular Line Studies of Nearby Planetary Nebulae

Planetary nebulae constitute the near-end stages of low-to-intermediate-mass stars, when their ejected envelopes of gas and dust become ionized by their unveiled stellar cores. As essential components of our understanding of late-stage stellar evolution and the enrichment of the interstellar medium in the products of stellar nucleosynthesis, planetary nebulae serve as laboratories for the study of plasma physics and shock processes in astrophysical environments. While known mainly for their optical emission lines and high ionization states due to high-energy radiation from their hot central stars, certain planetary nebulae also contain large masses of molecular gas and dust that surround or even lie embedded within their ionized interiors. The resulting regions of photoionization and photodissociation thus represent geometrically straightforward analogues for highly complex or poorly resolved systems, such as protoplanetary disks, cold cloud cores, and active galactic nuclei. The abundance of emission features in the radio-wavelength spectra of planetary nebulae enables state-of-the-art single-dish radio telescopes and interferometers to obtain high-quality measurements of the molecular chemistry present in the ejected gas. Ultimately, the stellar ejecta that constitute planetary nebulae are incorporated into the next generation of stars and planetary systems such that, by studying this material, we strive to understand the origins of our solar system and even life itself. Through radio molecular line observations carried out with the 30 m telescope and interferometer operated by the Institut de Radioastronomie Millimétrique (IRAM), as well as the Atacama Large Millimeter Array (ALMA), we seek to explore and identify the role that UV and X-ray irradiation plays in driving the molecular chemistry within the envelopes of planetary nebulae. In our 30 m survey of nine nearby planetary nebulae, we have made new detections of molecular species in four objects, and established an anticorrelation between the HNC/HCN line intensity ratio and central star UV luminosity that suggests this high-energy radiation continues to drive the chemistry within PNe as they age. Our detailed studies of the nearby, bright NGC 7293 and NGC 7027 probe deeper into the relationships between central star high-energy radiation fields and molecular chemistry. Radiative transfer codes have been applied to model the dense globule structures in NGC 7293, and comparisons of these models to our IRAM 30 m and APEX 12 m observations suggest that, in addition to gradients in UV irradiation, gradients in gas pressure and density may be required to explain the observed variations in HNC/HCN. We further investigate the spatial gradients of this and other emission line diagnostics within individual parcels of molecular gas with arcsecond-resolution ALMA spectral line mapping of two prominent globules in NGC 7293. Finally, we present NOEMA interferometric maps of NGC 7027 in the molecular ions CO+, which is imaged for the first time in any planetary nebula, and HCO+. Via comparison with archival near-IR H2 imaging, we diagnose the chemical pathways, and in particular the dependence on UV and X-ray irradiation, that drives production of CO+ and HCO+ in NGC 7027

A New Radio Molecular Line Survey of Planetary Nebulae: HNC/HCN as a Diagnostic of Ultraviolet Irradiation

Certain planetary nebulae contain shells, filaments, or globules of cold gas and dust whose heating and chemistry are likely driven by UV and X-ray emission from their central stars and from wind-collision-generated shocks. We present the results of a survey of molecular line emission in the 88-236 GHz range from nine nearby (<1.5 kpc) planetary nebulae spanning a range of UV and X-ray luminosities, using the 30 m telescope of the Institut de Radioastronomie Millimetrique. Rotational transitions of thirteen molecules, including CO isotopologues and chemically important trace species, were observed and the results compared with and augmented by previous studies of molecular gas in PNe. Lines of the molecules HCO+, HNC, HCN, and CN, which were detected in most objects, represent new detections for five planetary nebulae in our study. Specifically, we present the first detections of 13CO (1-0, 2-1), HCO+, CN, HCN, and HNC in NGC 6445; HCO+ in BD+303639; 13CO (2-1), CN, HCN, and HNC in NGC 6853; and 13CO (2-1) and CN in NGC 6772. Flux ratios were analyzed to identify correlations between the central star and/or nebular UV and X-ray luminosities and the molecular chemistries of the nebulae. This analysis reveals a surprisingly robust dependence of the HNC/HCN line ratio on PN central star UV luminosity. There exists no such clear correlation between PN X-rays and various diagnostics of PN molecular chemistry. The correlation between HNC/HCN ratio and central star UV luminosity demonstrates the potential of molecular emission line studies of PNe for improving our understanding of the role that high-energy radiation plays in the heating and chemistry of photodissociation regions.Comment: 17 pages, 17 figures, 6 tables, accepted for publication in Astronomy & Astrophysic

Mapping NGC 7027 in New Light: CO+^+ and HCO+^+ Emission Reveal Its Photon- and X-ray-Dominated Regions

The young and well-studied planetary nebula NGC 7027 harbors significant molecular gas that is irradiated by luminous, point-like UV (central star) and diffuse (shocked nebular) X-ray emission. This nebula represents an excellent subject to investigate the molecular chemistry and physical conditions within photon- and X-ray-dominated regions (PDRs and XDRs). As yet, the exact formation routes of CO$^+$ and HCO$^+$ in PN environments remain uncertain. Here, we present $\sim$2$"$ resolution maps of NGC 7027 in the irradiation tracers CO$^+$ and HCO$^+$, obtained with the IRAM NOEMA interferometer, along with SMA CO and HST 2.12~$\mu$m H$_2$ data for context. The CO$^+$ map constitutes the first interferometric map of this molecular ion in any PN. Comparison of CO$^+$ and HCO$^+$ maps reveal strikingly different emission morphologies, as well as a systematic spatial displacement between the two molecules; the regions of brightest HCO$^+$, found along the central waist of the nebula, are radially offset by $\sim$1$"$ ($\sim$900 au) outside the corresponding CO$^+$ emission peaks. The CO$^+$ emission furthermore precisely traces the inner boundaries of the nebula's PDR (as delineated by near-IR H$_2$ emission), suggesting that central star UV emission drives CO$^+$ formation. The displacement of HCO$^+$ radially outward with respect to CO$^+$ is indicative that dust-penetrating soft X-rays are responsible for enhancing the HCO$^+$ abundance in the surrounding molecular envelope, forming an XDR. These interferometric CO$^+$ and HCO$^+$ observations of NGC 7027 thus clearly establish the spatial distinction between the PDR and XDR formed (respectively) by intense UV and X-ray irradiation of molecular gas.Comment: 15 pages, 7 figures, 1 tabl

First Results from a Panchromatic HST/WFC3 Imaging Study of the Young, Rapidly Evolving Planetary Nebulae NGC 7027 and NGC 6302

We present the first results from comprehensive, near-UV-to-near-IR Hubble Space Telescope Wide Field Camera 3 (WFC3) emission-line imaging studies of two young planetary nebulae (PNe), NGC 7027 and NGC 6302. These two objects represent key sources for purposes of understanding PNe shaping processes. Both nebulae feature axisymmetric and point-symmetric (bipolar) structures and, despite hot central stars and high nebular excitation states, both harbor large masses of molecular gas and dust. The sweeping wavelength coverage of our Cycle 27 Hubble Space Telescope (HST)/WFC3 imaging surveys targeting these two rapidly evolving PNe will provide a battery of essential tests for theories describing the structural and chemical evolution of evolved star ejecta. Here, we present initial color overlays for selected images, and we highlight some of the first results gleaned from the surveys

Panchromatic HST/WFC3 Imaging Studies of Young, Rapidly Evolving Planetary Nebulae. II. NGC 7027

The iconic planetary nebula (PN) NGC 7027 is bright, nearby ( D ∼ 1 kpc), highly ionized, intricately structured, and well observed. This nebula is hence an ideal case study for understanding PN shaping and evolution processes. Accordingly, we have conducted a comprehensive imaging survey of NGC 7027 comprised of 12 HST Wide Field Camera 3 images in narrow-band and continuum filters spanning the wavelength range 0.243–1.67 μ m. The resulting panchromatic image suite reveals the spatial distributions of emission lines covering low-ionization species such as singly ionized Fe, N, and Si, through H recombination lines, to more highly ionized O and Ne. These images, combined with available X-ray and radio data, provide the most extensive view of the structure of NGC 7027 obtained to date. Among other findings, we have traced the ionization structure and dust extinction within the nebula in subarcsecond detail; uncovered multipolar structures actively driven by collimated winds that protrude through and beyond the PN’s bright inner core; compared the ionization patterns in the WFC3 images to X-ray and radio images of its interior hot gas and to its molecular outflows; pinpointed the loci of thin, shocked interfaces deep inside the nebula; and more precisely characterized the central star. We use these results to describe the recent history of this young and rapidly evolving PN in terms of a series of shaping events. This evolutionary sequence involves both thermal and ram pressures, and is far more complex than predicted by extant models of UV photoionization or winds from a single central progenitor star, thereby highlighting the likely influence of an unseen binary companion

Irradiation Investigation: Exploring the Molecular Gas in NGC 7293

International audienceBackground: Many planetary nebulae retain significant quantities of molecular gas and dust despite their signature hostile radiation environments and energetic shocks. Photoionization and dissociation by extreme UV and (often) X-ray emission from their central stars drive the chemical processing of this material. Their well-defined geometries make planetary nebulae ideal testbeds for modeling the effects of radiation-driven heating and chemistry on molecular gas in photodissociation regions. Methods: We have carried out IRAM 30m/APEX 12m/ALMA radio studies of the Helix Nebula and its molecule-rich globules, exploiting the unique properties of the Helix to follow up our discovery of an anti-correlation between HNC/HCN line intensity ratio and central star UV Luminosity. Results: Analysis of HNC/HCN across the Helix Nebula reveals the line ratio increases with distance from the central star, and thus decreasing incident UV flux, indicative of the utility of the HNC/HCN ratio as a tracer of UV irradiation in photodissociation environments. However, modeling of the observed regions suggests HNC/HCN should decrease with greater distance, contrary to the observed trend. Conclusion: HNC/HCN acts as an effective tracer of UV irradiation of cold molecular gas. Further model studies are required

DE-STAR: Phased-Array Laser Technology for Planetary Defense and Other Scientific Purposes

Current strategies for diverting threatening asteroids require dedicated operations for every individual object. We propose a stand-off, Earth-orbiting system capable of vaporizing the surface of asteroids as a futuristic but feasible approach to impact risk mitigation. We call the system DE-STAR (Directed Energy System for Targeting of Asteroids and exploRation). DE-STAR is a modular phased array of laser amplifiers, powered by solar photovoltaic panels. Lowcost development of test systems is possible with existing technology. Larger arrays could be tested in sub-orbital demonstrations, leading eventually to an orbiting system. Design requirements are established by seeking to vaporize the surface of an asteroid, with ejected material creating a reaction force to alter the asteroid’s orbit. A proposed system goal would be to raise the surface spot temperature t

Directed Energy Planetary Defense

Asteroids and comets that cross Earth’s orbit pose a credible risk of impact, with potentially severe disturbances to Earth and society. Numerous risk mitigation strategies have been described, most involving dedicated missions to a threatening object. We propose an orbital planetary defense system capable of heating the surface of potentially hazardous objects to the vaporization point as a feasible approach to impact risk mitigation. We call the system DE-STAR for Directed Energy System for Targeting of Asteroids and exploRation. DE-STAR is a modular phased array of kilowatt class lasers powered by photovoltaic\u27s. Modular design allows for incremental development, test, and initial deployment, lowering cost, minimizing risk, and allowing for technological co-development, leading eventually to an orbiting structure that would be developed in stages with both technological and target milestones. The main objective of DE-STAR is to use the focused directed energy to raise the surface spot temperature to ~3,000K, allowing direct vaporization of all known substances. In the process of heating the surface ejecting evaporated material a large reaction force would alter the asteroid’s orbit. The baseline system is a DE-STAR 3 or 4 (1-10km array) depending on the degree of protection desired. A DE-STAR 4 allows for asteroid engagement starting beyond 1AU with a spot temperature sufficient to completely evaporate up to 500-m diameter asteroids in one year. Small asteroids and comets can be diverted/evaporated with a DESTAR 2 (100m) while space debris is vaporized with a DE-STAR 1 (10m)