Rochester Institute of Technology RIT Scholar Works Theses 12-18-2017 A New Radio Spectral Line Survey of Planetary Nebulae: Exploring Radiatively Driven Heating and Chemistry of Molecular Gas Jesse Bublitz [email protected] Follow this and additional works at: https://scholarworks.rit.edu/theses Recommended Citation Bublitz, Jesse, "A New Radio Spectral Line Survey of Planetary Nebulae: Exploring Radiatively Driven Heating and Chemistry of Molecular Gas" (2017). Thesis. Rochester Institute of Technology. Accessed from This Thesis is brought to you for free and open access by RIT Scholar Works. It has been accepted for inclusion in Theses by an authorized administrator of RIT Scholar Works. For more information, please contact [email protected]. A New Radio Spectral Line Survey of Planetary Nebulae: Exploring Radiatively Driven Heating and Chemistry of Molecular Gas Jesse Bublitz AThesisSubmittedinPartialFulfillmentofthe Requirements for the Degree of Master of Science in in Astrophysical Sciences & Technology School of Physics and Astronomy College of Science Rochester Institute of Technology Rochester, NY December 18, 2017 Abstract 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 (<1.5 kpc) planetary nebulae using the 30 m telescope at the Insti- tut de Radioastronomie Millim´etrique. Rotational transitions of nine molecules, including the well-studied 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. Flux ratios were analyzed to identify correla- tions between the central star and/or nebular ultraviolet/X-ray luminosities and the molecular chemistries of the nebulae. Analysis reveals the apparent 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 hints at 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. i Contents 1 Introduction 1 1.1 PNe Formation . 2 1.2 MolecularChemistry.................................... 4 2 Data 7 2.1 SampleSelection ...................................... 7 2.2 Observations ........................................ 9 3Results 10 3.1 Molecular Spectra and Spectral Line Parameters . 10 3.2 IndividualObjects ..................................... 14 3.2.1 NGC 7027 . 14 3.2.2 NGC 6720 . 17 3.2.3 NGC 7293 . 17 3.2.4 NGC 6781 . 21 3.2.5 BD+30◦3639 . 24 3.2.6 NGC 6445 . 25 3.2.7 NGC 7008 . 27 3.2.8 NGC 6853 . 29 3.2.9 NGC 6772 . 33 4 Discussion 33 4.1 Molecules in PNe: New Detections . 35 4.2 Investigating X-ray-induced Molecular Chemistry . 35 4.3 HNC/HCN as a Probe of UV-induced Gas Heating in PNe . 36 4.3.1 Previous Studies of the HNC/HCN Ratio . 36 4.3.2 HNC/HCN Ratio vs. LUV for Sample PNe . 38 ii 5 Conclusions 41 AAppendix 46 iii 1 Introduction Molecular line observations provide keen insight into the envelopes of neutral gas that surround certain planetary nebulae (hereafter PNe). Thrown o↵by low mass (0.8-8 M ) stars in their post- asymptotic giant branch stage, the shells of cold material are then bombarded by hot, ionizing UV and X-rays from the newly unveiled PN central star, which was the inert C/O core that remains from the former AGB star. In recent decades, surveys have yielded a growing list of molecules detected in PNe and these detections reveal crucial information on the structure and chemical reactions within the nebulae. Millimeter CO and later infrared H2 lines gave the first views of the molecular gas 30 years ago and have remained commonplace probes into the shells of ejected mass since then (Huggins & Healy, 1989; Bachiller et al., 1991; Huggins et al., 1996, and references therein). Due to its large abundance and low critical density of excitation, CO is the most commonly observed molecular species found in PN (Huggins et al. 1996). Infrared lines of H2 are also observed from certain PNe; the presence of H2-emitting regions correlates strongly with the bipolar morphology of a PN and indicates an origin in a more massive progenitor star (>1.5 M ); (Kastner et al., 1996). More recent spectroscopic surveys have uncovered other molecular species, such as HCN, HNC, and HCO+ (Bachiller et al., 1997). Analysis of these molecule-rich envelopes is necessary to provide continuity with the previous stage of stellar evolution, i.e. the post-AGB stage (Bl¨ocker, 2001). An understanding of how the observed abundances arise in cold gas is fundamental to the physics of the PN (Phillips, 2003). In this thesis, we present new detections of molecules across nine observed PNe with data obtained at the IRAM 30m telescope. We also provide updated analysis of previously reported lines in various papers. Finally, we compare the integrated intensities of observed molecular lines to study correlations between the molecular chemistry and properties of the central star of the planetary nebula. Specifically, we strive to find tracers of non-LTE chemistry due to X-irradiation of molecular gas. The remainder of Section 1 provides background to the study of molecular lines in PNe. Obser- vational details are laid out in Section 2, with analysis of the new observations in Section 3. Finally, 1 the results are presented with discussion on line comparisons in Section 4. 1.1 PNe Formation Planetary nebulae arise from the outflowing mass of late stage low-mass stars. Entering the main sequence over the range 0.8-8.0 M (Bl¨ocker, 2001), the star is set on a course that will see it ⇠ convert its core hydrogen into helium, carbon, oxygen, nitrogen, and various heavier elements. The initial phase of fusion converts 90% of the hydrogen in the core of the star to helium until insufficient photon pressure causes the core to gravitationally contract. A shell undergoing H-burning surrounds the contracting He core as the star leaves the main sequence. As the core contracts, the surrounding H-rich envelope expands and cools, producing the titular Red Giant Branch (RGB) (Herwig, 2005). Pressure and temperature increase until He begins fusing into carbon and oxygen. For stars of initial mass below 1.8 M , the He core will become degenerate and unable to regulate the temperature. ⇠ This runaway process creates an explosive He flash that pushes the star onto the zero-age horizontal branch. Ultimately these stars rapidly deplete their He as the H layer grows in luminosity and size. Eventually, the core He is exhausted as well, and the carbon/oxygen core collapses into electron degeneracy (Iben Jr., 1995). With the onset of C/O core degeneracy, the star has now reached the Asymptotic Giant Branch (AGB) evolutionary stage. The AGB stage begins with a dormant H shell around a He-burning shell, which continues to contribute material to the C/O core until only a thin layer of He remains (Iben Jr., 1995). Without significant material to separate them, the hydrogen shell reignites, which moves the star to the second half of the AGB where phases of H-shell burning, He-shell burning (with the resulting thermal pulses), and dredge-up mix the contents of the convective interior, bringing nuclear processed elements to the surface (Phillips, 2003). Stellar winds, due mainly to shocks and radiation pressure, can now expel the star’s H-rich envelope. The H envelope cools and contracts as its luminosity drops, allowing dust grains to form in the outer atmosphere of the star. This process is enhanced by shock waves from stellar pulsations that inflate the surface layers and deposit dust grains into the atmosphere (Liljegren et al., 2017). During the AGB phase, He-shell flashes occur when sufficient He is accreted from 2 the H-burning shell onto the layers below (Iben Jr., 1995). When He ignites, it produces sufficient energy to surpass the H-burning phase in luminosity. Radiation pressure then pushes the dust grains away from the star, dragging envelope gas along for the ride. The He shell again contracts and the H shell reignites, restarting the thermal pulse cycle. This process leads to an increasing 7 4 rate of mass ejection of 10− -10− M /yr as the star progresses along the last stages of the AGB (Bl¨ocker, 2001). The atomic and molecular composition of the ejected mass is dependent on the surface temperature of the thermally pulsing star, as well as the mass of ejected material capable of insulating globules from the radiation below (Iben Jr., 1995). 2 When the H envelope is depleted to a mass of 10− M , mass loss ceases and the star progresses ⇠ from the AGB to the post-AGB and, soon after, to the central star of a PN (hereafter CSPN). Due to the ejection of the star’s outer layers of mass, the much hotter post-fusion core is exposed and produces high energy UV and, sometimes, point-like X-ray emission that can ionize the nebular gas. The fast winds from the pre-white dwarf star may also slam into the slower moving ejected envelope, shocking the gas up to temperatures 106 K (Kastner et al., 2012, and references therein).