THE NITRILES PROJECT: MOLECULE FORMATION IN THE GALACTIC CENTER

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1 THE NITRILES PROJECT: MOLECULE FORMATION IN THE GALACTIC CENTER JOANNA CORBY ( University of Virginia, Department of Astronomy Advised by Anthony Remijan, National Radio Astronomy Observatory ABSTRACT Nitriles contain a triple bonded CN which acts as a functional group. They account for roughly 15% of the molecules known in space, making them the most prolific family of molecules known in the interstellar medium (ISM). "The Nitriles Project" is among the first coordinated efforts to understand the formation of a family of organic molecules in the ISM. The project is motivated by a laboratory experiment at the University of Virginia that suggests radical-driven chemistry can account for the diversity of nitriles observed in space. To determine whether this mechanism may be important in a source in the Galactic center, we conducted a high spatial resolution mapping campaign of transitions from nine target nitriles and related species with the Very Large Array, a broadband radio interferometer. We report a chemically unique environment associated with a high energy shell of material and preliminary evidence for the presence of nitriles in high UV environments. Molecules in Space: A Historical Context Through the first two thirds of the 20th century, it was widely accepted that polyatomic chemistry could not occur in the interstellar medium due to the hostile conditions including low densities, temperature extremes, and ultraviolet and cosmic ray irradiation. The 1968 discovery of ammonia in space by Charles H. Townes and collaborators disproved this (1) and launched the field of astrochemistry. Since this seminal detection, new interstellar molecules have been discovered at a steady rate of four per year (Figure 1 (2) ) to compile a molecular inventory of >170 molecules. Detected molecules include tens of pre-biotic molecules (3, 4, 5), the seeds of our cellular functions. However, astrochemists do not yet understand how (by what formation routes) most of the molecules were formed. The family of molecules known as nitriles, which have a CN triple bond, are the most prolific molecular family known in space, accounting for approximately 15% of the known interstellar inventory (6). If we include the chemically related Figure 1: Number of molecules detected in the interstellar medium. After the initial detection of NH3, molecules have been detected at a steady rate of 4 per year. Figure adapted from Thaddeus & McCarthy molecules isonitriles and imines, this fraction grows to ~20%. Nitriles, isonitriles, and imines are particularly important for pre-biotic chemistry, as they are believed to be involved in

2 the formation of amino acids and nucleobases (7, 8). The Nitriles Project: Motivation & Rationale The Nitriles Project is among the first coordinated observational campaigns to test a proposed formation route for a family of molecules in the interstellar medium (ISM). It integrates newly available broad-bandwidth methods of laboratory microwave spectroscopy, single dish radio astronomy, and radio interferometry (Figure 2). The research was motivated by a laboratory chemistry experiment that may explain the formation of a wide diversity of nitriles, isonitriles, and imines. Figure 2: The Green Bank Telescope (top left), a broadband FTMW spectrometer at the University of Virginia (top right), and the Karl G. Jansky Very Large Array (bottom) were used in the The Nitriles Project. The majority of the >170 molecules have been detected with single dish radio telescopes operating in the microwave to sub-millimeter frequency regimes. Under the low density (n < 10 6 ) and low temperature (T ~ 10K) conditions of many interstellar molecular clouds, low energy levels are significantly populated. Particularly, the microwave frequency regime (6 < ν < 40 GHz) is advantageous for detecting complex organic molecules, as transitions between their lowest rotational energy levels occur at microwave frequencies. A Laboratory Nitriles Experiment A laboratory experiment conducted by the Pate Group at the University of Virginia performed a supersonic expansion of a single reactant, CH 3 CN through a high voltage DC discharge (9), producing 14 product molecules of which 12 are known interstellar molecules (Table 1). The chemistry, believed to be driven by highly reactive species with unpaired electrons (i.e. radicals), can be explained by the following chemical processes: 1. Dissociation of CH 3 CN by CH 3 CN CH 2 CN + H and CH 3 CN CH 3 + CN 2. Highly exo-energetic radical-radical recombination e.g. CH 2 CN + CH 3 CH 3 CH 2 CN 3. Structural rearrangement (Figure 3). This simple sequence of chemical processes, Figure 3: Cartoon illustrations of three forms of structural rearrangement that can account for the formation of product molecules in the laboratory. Because of the high exo-energeticity of radical-radical recombination, these processes are energetically feasible.

3 driven by the exo-energicity of radical-radical reactions, formed a significant fraction of the molecules known in space. molecules, we ask the question can radicaldriven chemistry explain the formation of nitriles, isonitriles, and imines in the ISM? Because of the low densities in the ISM, gas phase chemistry is unable to explain the high abundances of many of the interstellar molecules (10). However, there is laboratory evidence that radical-driven chemistry proceeds on ice layers that surround ~0.1 micron sized particulates (dust grains) in the ISM. Experiments have produced the key radical in the Nitriles Project, namely CH 2 CN on ice under UV irradiation (11), and shown that radical-driven chemistry is efficient in high UV and cosmic ray (CR) radiation fields (12, 13). We thus hypothesized that nitriles, isonitriles, and imines form in regions of high UV or CR flux. To test this, we analyzed single dish data on a highly complex source in the Galactic Center and conducted a high spatial resolution mapping campaign with a newly available broadband interferometer. Table 1: Species that were directly detected in the laboratory using Fourier Transform Microwave Spectroscopy. The third column indicates whether each molecule has been observed in the ISM, and stars mark new interstellar detections resulting from this project. A Proposed Interstellar Formation Mechanism Because this chemistry proves effective for forming a large set of known interstellar organic The Nitriles Project: Astronomical Observations Nitriles in PRIMOS The PRebiotic Interstellar MOlecular Survey (PRIMOS) (Figure 4) is a Green Bank Telescope Key Science Project that provides a full microwave spectrum from 1 50 GHz of the high mass star-forming region in the Galactic Center, namely Sagittarius B2(N) (Sgr B2(N)) (14). Figure 4: PRIMOS reveals a rich spectrum towards Sgr B2(N). Many of the notable absorption lines are from nitriles.

4 laboratory and detected in PRIMOS. The observation showcases the flexibility of the Karl G. Jansky Very Large Array and was only possible with newly available capabilities of broadband interferometry. Besides transitions of the nine target molecules, ~10 additional spectral lines were within the observing bands. We discuss the line emission and absorption with respect to previously named components, including the K1 K6 continuum components (labeled in Figure 6) and the hot core region known as the LMH. Figure 5: A VLA map of continuum emission from Sgr B2(N), shows that the source contains significant structure on spatial scales smaller than the GBT beam (white circle), indicating that high resolution data is necessary to resolve distinct chemical environments. Sgr B2(N) has a complex physical structure (Figure 5) and is host to the most complex chemistry documented in any interstellar environment (15). Analyses of spectral line transitions of nitriles and imines towards Sgr B2(N) reveal that these species are anomalous. Whereas >75% of all transitions from other molecules appear in emission, nitriles and imines are typically in absorption. Additionally, the lowest energy states of nitriles and imines are preferentially populated, with typical rotational excitation temperatures of 3 < T ex < 10K. These results provide evidence that many of the nitriles and imines are located in regions of low density material that would produce subthermal excitation. As most emission from complex organic molecules in Sgr B2(N) is associated with the Large Molecule Heimat (LMH) (16), a hot core which hosts high densities (n ~ 10 7 cm -3 ) and warm temperatures (100 < T < 300K), PRIMOS provides evidence that nitriles have a distinct distribution. Nitriles & Imines at High Resolution We conducted high spatial resolution observation to map spectral transitions from GHz from nine of the molecules produced in the Figure 6: Continuum image of Sgr B2(N) at 18 GHz with the VLA, labeled to show the K1 - K4 compact continuum cores, extended K5 and K6 shells (Gaume et al. 1995) and the methanol maser source h (Mehringer & Menten 1995). Figure 7 shows emission and absorption from the five strongest target lines, revealing that many of the nitriles appear in absorption against the compact cores K1, K3 & K4 and against the extended shells K5 & K6, and in emission in a ring of material between the K1 & K3 cores known as the LMH. Table 2 quantifies the absorption towards each component as the equivalent width, a measurement explained by Figure 8. While the common nitriles HC 3 N and CH 3 CN appear in all of these localities, CH 2 CHCN and CH 3 CH 2 CN are not detected in the K1 & K4 cores. CH 3 CH 2 CN is particularly enhanced towards the K3 core and in the LMH

5 compared to all other observed molecules (Table 2). Finally, the radical CH 2 CN, critical for the proposed chemistry, is not present in emission towards the LMH, but is present in absorption towards the K1, K3 & K4 cores and towards the K5 & K6 shells. In comparing the distributions of nitriles with other molecules towards Sgr B2(N), it is clear that nitriles have a highly distinct distribution as indicated by the non-target transitions present in the observing band. One notable exception exists, however: the distribution of c-h 13 CCCH, an isotopologue of the molecule cc 3 H 2 nearly exactly traces the distribution of CH 2 CN. cc 3 H 2 is known to persist in environments with high UV radiation fields (17, 18, 19). Figure 9 shows the distribution of the new interstellar imine, HNCHCN (cyanomethanimine). Lines of HNCHCN appeared in both the laboratory and PRIMOS spectra, yet they remained unidentified for nearly four years, as the microwave spectrum of this terrestrially unstable species was initially poorly constrained. The recent detection of HNCHCN (20), possible only through this project, generated excitement as HNCHCN may be a direct precursor to adenine, vital for cellular respiration in terrestrial organisms. HNCHCN is only detected towards Figure 7 (Left) : Continuum image of Sgr B2(N) at 18 GHz, overlaid with integrated line contours of HC 3 N, CH 3 CN, CH 2 CHCN, CH 3 CH 2 CN, and CH 2 CN. Notice the strong emission from the LMH in the bottom right corner, and absorption along the K5 & K6 shells Table 2 (Bottom Left): Detected absorption (pink) and emission (green) towards physical components in Sgr B2(N). For absorption components, the equivalent width is provided. Figure 8 (Bottom Right): Diagram and formula illustrating equivalent width.

6 tthe K6 shell of Sgr B2(N), isolating this region as a unique chemical environment. Figure 9: Contours of integrated emission from HNCHCN overlaid on 18GHz continuum emission shows that it is present in absorption against the K6 shell of continuum emission. Implications of the Astronomical Distributions A very complex picture of the chemical environments of Sgr B(2) emerges from the high resolution data set. No obvious yes or no answer to our initial question exists. Yet the data set provides evidence that HC 3 N, CH 3 CN, CH 2 CHCN, and CH 2 CN persist in low density, high UV environments in which many other molecular species are absent. The correlation with c-c 3 H 2 absorption is consistent with this hypothesis. Further, the data set provides some evidence for imine formation from nitriles in the presence of a strong UV or CR source. A discussion of the results is best conducted in the context of previous studies that have constrained the physics of the distinct components towards Sgr B2(N). Studies of continuum emission and hydrogen and helium line emission have shown that continuum emission in K1, K3, K4, K5, & K6 is generated by free-free scattering in regions in which hydrogen is entirely ionized (HII regions) (21, 22, 23). Except for K3, ionized helium is observed (21), making the K1, K4, K5 & K6 regions particularly high energy. Authors discuss the nature of the K5 & K6 shells (21, 22), arguing that they are formed either by an expanding ionization front or by a stellar wind of cosmic rays from a very massive star. Particularly, authors have argued that the observed twocomponent emission profile of hydrogen and helium lines toward the K6 shell favors an expanding ionization front rather than a stellar wind. While little has been done to characterize the chemistry on the surface of HII regions in this source particularly, theoretical and observational studies exist on the chemistry and physics of regions called photodissociation regions (PDRs) (17, 18, 19, 24, 25). These regions are at lower density and higher temperatures than dense molecular clouds and are characterized by an exotic chemistry activated by a high UV flux. While the research task of teasing out the chemical picture from the data is yet incomplete, it is reasonable to conclude that the molecular line absorption associated with K1, K4, K5 and K6 arise within PDRs. Emission associated with the LMH in contrast occurs in a very dense environment (n ~ 10 7 ) shielded from UV radiation by dust extinction. The physical and chemical picture towards K3 is less obvious; while K3 is an HII region and should presumably have a PDR at its edge, its close proximity to the LMH emission region makes it difficult to determine if these are distinct chemical regions or if the absorption towards K3 is the result of a projection effect. However, it is likely that the distribution of absorption of CH 2 CN towards K3 originates in gas that is distinct from that associated with the LMH, evidenced by the nondetection of CH 2 CN in the LMH. A full analysis of the kinematics in this source will better inform our understanding of this region. The observed distribution of HNCHCN points to

7 a chemically unique environment towards K6. Because of the high energy ionized helium and the double peaked emission lines associated with K6, it has been a subject of great interest for previous authors. Three possible physical inputs may drive the formation of HNCHCN in this environment: 1. A high UV flux associated with the edge of an ionization front 2. A high CR flux from a stellar wind 3. Shocks associated with the expanding ionization front Currently it is difficult to determine which of these three scenarios is most probable. The physical and chemical picture in the Galactic Center's resident high mass star-forming region proves highly complex. From this data set, it is clear that it will yield no obvious answer as to whether the large family of interstellar nitriles and imines is formed via radical-driven chemistry. Yet the Nitriles Project provides unprecedented detail on the relationship between chemical structure (molecular abundances) and physical structure (e.g. densities and UV radiation fields) in the interstellar medium. References (1) Cheung et al. (1968), Phys.Review L21, (2) Thaddeus & McCarthy (2001), Spectrochimica:Mol&Bio, 57,757. (3) Nummelin et al. (1998), A&A, 337, 275. (4) Lovas et al. (2006), ApJ, 643, L29. (5) Snyder et al. (2006), ApJ, 647, 412. (6) Woon, astrochymist_ism.html. (7) Schwartz et al. (1982), BioSystems,15,191. (8) Roy et al. (2007), PNAS, 104, (9) McCarthy et al. (2000), ApJS, 129, 611. (10) Smith et al. (2011), A&A, 369, 611. (11) Svejda & Dolman (1970), JphCh, 74, (12) Hudson et al. (2008), AsBio, 8, 771. (13) Thuele, Borget & Mispelaer (2011), A&A, 534, A64. (14) Remijan et al. (2013), AAS, 221, (15) Belloche et al. (2008), A&A, 482, 179. (16) Puletti et al. (2010), MNRAS, 402,1667. (17) Fuente et al. (2003), A&A, 406, 899. (18) Pety et al. (2005), A&A, 435, 885. (19) Pilleri et al. (2013), a4rxiv: (20) Zaleski et al. (2013), ApJ, 765, L10. (21) depree et al. (1995), ApJ, 451, 284. (22) Gaume & Claussen (1990), ApJ, 351, 538. (23) Gaume et al. (1995), ApJ, 449, 663. (24) le Petit et al. (2006), ApJS, 160, 506. (25) Godard et al. (2009), A&A, 495, 847.