Prebiotic Chemical Evolution in the Astrophysical ...

Orig Life Evol Biosph DOI 10.1007/s11084-015-9431-8 ORIGINS 2014

Prebiotic Chemical Evolution in the Astrophysical Context L. M. Ziurys & G. R. Adande & J. L. Edwards & D. R. Schmidt & D. T. Halfen & N. J. Woolf

Received: 29 October 2014 / Accepted: 18 January 2015 # Springer Science+Business Media Dordrecht 2015

Abstract An ever increasing amount of molecular material is being discovered in the interstellar medium, associated with the birth and death of stars and planetary systems. Radio and millimeter-wave astronomical observations, made possible by high-resolution laboratory spectroscopy, uniquely trace the history of gas-phase molecules with biogenic elements. Using a combination of both disciplines, the full extent of the cycling of molecular matter, from circumstellar ejecta of dying stars – objects which expel large amounts of carbon to nascent solar systems, has been investigated. Such stellar ejecta have been found to exhibit a rich and varied chemical content. Observations demonstrate that this molecular material is passed onto planetary nebulae, the final phase of stellar evolution. Here the star sheds almost its entire original mass, becoming an ultraviolet-emitting white dwarf. Molecules such as H2CO, HCN, HCO+, and CCH are present in significant concentrations across the entire age span of such nebulae. These data suggest that gas-phase polyatomic, carbon-containing molecules survive the planetary nebula phase and subsequently are transported into the interstellar medium, seeding the chemistry of diffuse and then dense clouds. The extent of the chemical complexity in dense clouds is unknown, hindered by the high spectral line density. Organic species such as acetamide and methyl amine are present in such objects, and NH2CHO has a wide Galactic distribution. However, organophosphorus compounds have not yet been detected in dense clouds. Based on carbon and nitrogen isotope ratios, molecular material from the ISM appears to become incorporated into solar system planetesimals. It is therefore likely that interstellar synthesis influences prebiotic chemistry on planet surfaces. Keywords Interstellar molecules . Radio astronomy . Circumstellar material . Molecular clouds . Prebiotic gas-phase chemistry . Isotope ratios

Paper presented at ORIGINS 2014, Nara Japan, July 6-11 2014.

L. M. Ziurys : G. R. Adande : J. L. Edwards Department of Chemistry, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721-0065, USA L. M. Ziurys : D. R. Schmidt : D. T. Halfen : N. J. Woolf Department of Astronomy and Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721-0065, USA L. M. Ziurys (*) : G. R. Adande : J. L. Edwards : D. R. Schmidt Arizona Radio Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721-0065, USA e-mail: [email protected]

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Introduction: Where Did “Prebiotic” Chemistry Originate? Certain chemical substances are thought to be essential for life. From the Terran viewpoint, this list includes sugars, amino acids, nucleobases, accessible phosphorus, and polymer material. A key question is the role of interstellar chemistry in the creation of these critical compounds. It is thought that early Earth lost most of its original carbon in the gravitational escape of atmospheric gases (Harper and Jacobsen 1996). If so, the carbon had to be returned to Earth, but in what form? Some clues to this question are found in meteorites, comets, and interplanetary dust particles. Amino acids and other organic compounds have been extracted from meteoritic material, for example, along with pre-solar grains that have an obvious origin in stars (e.g., Pizzarello 2007). But these results are really part of a much larger story. Only in evaluating the complete history of organic material can proper perspective be achieved. By following the salient signposts of chemical functionality, molecular and atomic abundances, and isotopic compositions through the molecular life cycle of the interstellar medium (ISM), a better picture should emerge of how interstellar synthesis set the stage for living systems, both here on Earth and in other solar systems. The molecular life cycle in the interstellar medium (ISM) begins in stars. It is in such objects that most of the elements are created, including biogenic carbon, oxygen, nitrogen, sulfur, and phosphorus. As shown in Fig. 1, stars eject most of their original matter into the ISM by mass loss, sometimes followed by supernovae explosions for the most massive objects. The majority of stars (~1–8 Msol) do not explode, but transition into planetary nebulae, or PNe, the final phase of their evolution (e.g., Kwok 2000). At this juncture, the star has shed almost all of its original mass and becomes a very hot, ultraviolet-emitting white dwarf that ionizes its surrounding envelope, which creates a bright-colored nebula. The material from this

Fig. 1 The history of interstellar organic material, starting with its ejection from dying stars, transport through the interstellar medium, and arrival on planet surfaces

Prebiotic Chemical Evolution in the Astrophysical Context

nebula flows into the diffuse interstellar medium, forming clouds of low density (n~1–100 particles per cc). These “diffuse clouds” eventually collapse into denser clouds, also called molecular clouds, in which stars and solar systems originate. As young stars form out of the cloud material, protoplanetary disks and then solar systems are generated. Some of this material is heavily processed, but pristine cloud matter also survives, partly in the form of comets (e.g., Gibb et al. 2007). With the creation of new stars, the cycle is repeated.

Defining the Tools: Radio Astronomy and High Resolution Laboratory Spectroscopy Radio and millimeter astronomy offer a powerful avenue for the study of the life cycle of interstellar matter through observations of gas-phase molecules. In the colder temperatures typical of the dense ISM (T~10–100 K), the rotational energy levels of interstellar compounds are chiefly populated, principally by collisions. Heterodyne receivers employed on radio telescopes allow for measurements at very high spectral resolution, typically one part in 106–108. Using such receivers, the rotational spectra of molecules can be detected, thus offering a “fingerprint” method for identification and study. Therefore, millimeter-wave telescopes can be viewed as sensitive spectrometers for remote sensing of molecules. The Arizona Radio Observatory (ARO) operates two such facilities, geared toward molecular spectroscopy: the Sub-millimeter Telescope (SMT) on Mt. Graham, AZ, and the new 12 m at Kitt Peak; see Fig. 2. The 12 m dish is the European prototype antenna originally built for the Atacama Large Millimeter Array (ALMA) project. For interstellar studies to be successful, the rotational spectrum of a given chemical species must be well-characterized, which is usually done by gas-phase laboratory measurements. Laboratory high-resolution (i.e., rotational) spectroscopy consequently plays a critical role in molecular astronomy. Two types of experimental techniques are usually employed for pure rotational measurements: millimeter/sub-mm direct absorption and Fourier transform microwave (FTMW) spectroscopy. Both techniques are widely used in the Ziurys lab. Direct absorption is a relatively simple process in which mm-wave radiation is scanned in frequency

Fig. 2 The new 12 m radio telescope of the Arizona Radio Observatory, located at Kitt Peak, AZ

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and directed through a cell containing the species of interest. A detector placed oppositely from the radiation source monitors any absorption in power by the molecules at their rotational resonances (e.g., Ziurys et al. 1994). FTMW spectroscopy, in contrast, is conducted in the time domain. In this case, molecules are pulsed in a supersonic jet expansion into a vacuum chamber containing a two-mirror, resonant cavity. After a short delay, a sharp burst of microwave radiation is launched into the cavity, which sustains the radiation over a narrow frequency range for a limited time interval. If there is a rotational transition in that range, the molecules in the jet will absorb the radiation, and then spontaneously decay. A low noise amplifier detects these emitted molecular signals as a function of time, the so-called “free induction decay,” or FID. A Fourier transform of the FID results in a spectrum. This time sequence is repeated usually at a 5–10 Hz rate and signals then averaged (e.g., Sun et al. 2009). There are multiple challenges associated with both methods. First, the spectrometer systems are not commercially available and must be designed and built by the user. Secondly, many molecules of interest are highly reactive and cannot be purchased but rather are created in situ using unusual synthetic methods. Such syntheses involve electrical discharges, laser ablation, and special oven devices, among other schemes. Additionally, geometries and even electronic ground states of many interesting molecules are not exactly known, such that a significant amount of “guesswork” and iteration are required in obtaining rotational spectra.

Material Generated from Stellar Mass Loss About 85 % of the material in the ISM originates with mass loss from conventional stars that do not become supernovae (e.g., Dorschner and Henning 1995). This mass loss becomes substantial when stars have converted all the hydrogen in their core to helium, and thus leave the main sequence and enter the Red Giant (RG) and Asymptotic Giant Branch (AGB) stages. During these phases, helium is converted into carbon, resulting in energy generation that makes the star unstable, causing convective mixing in the stellar atmosphere and mass ejection. Although a complex process, the main results are that a circumstellar envelope is created around the star, and that this envelope has an elemental composition that reflects the interior nucleosynthesis. As a consequence, circumstellar envelopes can be either slightly carbon or slightly oxygen rich. The C/O ratio highly influences the rich chemistry that takes place in these envelopes. As illustrated by Tenenbaum et al. (2010), C-rich envelopes contain a variety of hydrocarbons, including long acetylenic chains, such as C3H, C4H, C3N, and HC5N, as well as silicon and metal cyanide species. The oxygen-rich counterparts are abundant in SO, SO2, and H2O, among other compounds. The carbon-based chemistry is particularly rich, with numerous polyatomic molecules and large polymer-type species (see Kwok, this volume, for further details). The planetary nebula stage, which lasts about 10,000 years, follows the AGB. Ultraviolet radiation from the star, now a white dwarf, ionizes the remnant AGB shell material as it flows into the ISM. The typical mass of a white dwarf is~0.5 Msol, such that between ~0.5 and 7.5 Msol is overall ejected into the diffuse interstellar medium by the PNe progenitor star. What is the fate of the gas-phase circumstellar molecules from the AGB phase? Chemical models predict that the UV radiation field from the central star is sufficiently intense such that the overall molecular content of planetary nebulae will decrease steadily with time (e.g., Ali et al. 2001; Redman et al. 2003). After 10,000 years, at the end of the PNe phase, molecular abundances are expected to be negligible. Astronomical observations, however, have never really explored the molecular content of these nebulae across their entire lifetime. Although CO has been observed in many such objects (Huggins et al. 2005), the few molecular studies


of PNe have focused principally on the younger sources, in particular NGC 7027 (Zhang et al. 2008). A variety of polyatomic molecules have been found in this object, but it is only ~700 years old. As a test of theoretical models, Edwards et al. (2014) studied the abundances of HCO+ and CS in five PNe that varied in age from~900 to 10,000 years, using the ARO telescopes. Both molecules, along with CO, were detected via multiple transitions in all five objects: K4-47, NGC 6537 (Red Spider Nebula), M2-48, NGC 6720 (Ring), and NGC 6853 (Dumbbell). As shown in Fig. 3, the measured abundances of CO, CS, and HCO+ do not appear to vary significantly with age. There is some variation in the abundances, typically by factors of 5–10, but not the large decreases (>1000–10,000) predicted by chemical models (e.g., Redman et al. 2003). Furthermore, Schmidt and Ziurys (2015, Polyatomic molecules in planetary nebulae: A Survey of HCN and HCO. Astrophys J, submitted) searched for HCO+ and HCN in seventeen additional PNe of different ages, sizes, and morphologies, with a 75 % detection rate. Sample spectra of HCN, measured with the ARO SMT, are shown in Fig. 4, and additional abundances are plotted in Fig. 3. As the figure illustrates, HCN shows no obvious decline in abundance with nebula age, although the Redman et al. (2003) model suggests it diminishes by a factor of 107. The abundance of CS appears to undergo a systematic decrease by~5 across the 10,000 year period, but models calculate a much larger reduction factor near 1000. Edwards and Ziurys (2013, 2014) conducted further observations towards NGC 6537 and M2-48, identifying a number of other species, including HNC, N2H+, SO, and CN, as well as some chemical variations. H2CO and CCH were detected in NGC 6537, but not in M2-48, while SiO and SO2 were found only in the latter source. On closer examination, PNe are more chemically complex and diverse than might be expected. Their chemical evolution must involve significant molecule production or at least preservation that balances destruction mechanisms. Perhaps the most stringent test of theory is to evaluate the chemistry in the oldest known planetary nebula, the Helix, estimated to be 12,000 years old. This object has a large spatial distribution on the sky such that detailed images can be made of a given atomic or molecular

Fig. 3 Abundances relative to H2 of CO, CS, HCN, HCO+, and H2CO, measured in various planetary nebulae, plotted against nebular age. This result suggests that molecular abundances do not significantly vary with age over a 10,000 year period, the typical lifetime of PNe (from Edwards et al. 2014; Schmidt & Ziurys, 2015, submitted)

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Fig. 4 Spectra of the J = 3→2 transitions of HCN, observed towards a random sample of planetary nebulae using the ARO SMT (from Schmidt and Ziurys 2015, submitted). The intensity scale is in terms of uncorrected antenna temperature

species. Many atomic lines have been studied across the face of the nebula, as well as emission from H2 and CO (O’Dell et al. 2004; Meaburn et al. 2005; Speck et al. 2002; Young et al. 1999). At one position in the nebula, HCN, HNC, CN, and HCO+ have been detected (Bachiller et al. 1997), and, more recently, CCH, c-C3H2 and H2CO (Tenenbaum et al. 2009). To establish a more global view of the polyatomic molecular content, Zack and Ziurys (2013) searched for HCO+ and H2CO at 8 other positions located across the Helix, and successfully detected both species at every location. Zeigler et al. (2013) then studied HCO+ across the entire nebula, observing over 200 separate positions and creating a full image of the Helix in this molecule, as shown in Fig. 5. HCO+ appears to trace the classic optical image of the nebula, dispelling the notion that PNe are principally atomic in nature at the end of their lifetime.


Fig. 5 A view of the Helix Nebula in the molecule HCO+ (white contours), overlaid on the optical image. The contours trace the emission of the J = 1→0 transition of HCO+ at 89 GHz, measured with the ARO 12 m telescope (Zeigler et al. 2013). Polyatomic material is thus present throughout the nebula, which is one of the oldest planetary nebula known

Transfer of Molecular Material from Dying Star to Diffuse Clouds Given the widespread distribution of HCO+ and H2CO in the Helix Nebula, it appears that the material ejected into the diffuse ISM from PNe must be partly molecular in content. The molecules survive because they are imbedded in cold, dense clumps that are self-shielding (e.g., Zack and Ziurys 2013). Further evidence of this phenomenon arises from observations of diffuse clouds themselves. Recently, numerous polyatomic molecules have been discovered in these objects by Liszt and collaborators (e.g., Liszt et al. 2006). This molecular material remained undetected for years because the densities in the diffuse clouds are so low that rotational levels cannot be collisionally excited, as they are in PNe. The molecular transitions had to be observed in absorption against background quasars. The molecular species thus far identified by Liszt et al. include CO, HCN, HNC, HCO+, CN, SiO, CCH, SO, CS, c-C3H2, and H2CO – an almost identical list to that found in PNe. As shown in Table 1, the abundances in diffuse clouds are typically a factor of 10–100 less than those in older PNe, consistent with the injection and slow dispersion of dense clumps into the diffuse ISM. Furthermore, many molecular ratios appear to be similar. The HCN/HNC ratio is~1–2 in PNe, and in diffuse clouds~5. The CO/HCO+ ratio is about 103 in both types of sources, while the CN/HCN is comparable as well, with CN/HCN~4–9 in PNe and CN/HCN~7 in diffuse clouds. As noted by Snow and McCall (2006), the presence of polyatomic molecules in diffuse clouds cannot be accounted for by in-situ gas-phase formation, because there is simply not sufficient density. Molecular material must consequently be recycled from circumstellar ejecta into the diffuse ISM in the form of gas-phase molecules as well as grains. This result also can account for what appears to be “missing mass” in PNe. From a 1–8 Msol star, ~ 0.5 Msol is incorporated into the

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Table 1 Abundances in older PNe vs. diffuse clouds

Molecule

Older PNe a

Diffuse clouds b

H2CO

0.3–2×10−7

4×10−9

1×10 3×10−8

CO

1–2×10−4

3×10−6

−7

2×10−8

4–9×10

HCN HNC +

HCO

Edwards and Ziurys (2014); Tenenbaum et al. (2009); Zack and Ziurys (2013) b Liszt et al. (2006)

3×10−8 1×10−9

CCH c-C3H2 CN

a

−6

SO CS SiO

−7

3×10−9

1×10

−8

5–8×10

−7

0.5–1×10

6×10−10 2–3×10−9

−7

1×10−9

−8

3×10−9 1×10−10

2×10

3×10 3×10−8

white dwarf, and~0.1 Msol in the ionized gas. The remaining 0.2–7.2 Msol must therefore be in molecules and grains.

Chemical Complexity in Dense Molecular Clouds If molecular material is being passed on to diffuse clouds, which then collapse into dense clouds, greater overall chemical complexity can be achieved. The chemistry of dense clouds, which is highly organic in nature, can then be initiated from polyatomic molecules, as opposed to simpler species. A subsequent question is the degree of complexity actually present in such objects. Evaluating this question has become increasingly difficult because dense clouds have extremely rich spectra, as shown in Fig. 6. For example, in the ARO survey of the cloud Sgr B2(N), which covers the 1, 2, and 3 mm atmospheric “windows,” over 15,000 individual spectra features were detected (Halfen and Ziurys 2015, A confusion-limited spectral-line survey of Sgr B2(N) at 1, 2, and 3 mm, in preparation). The density of lines is partly attributable

Fig. 6 Spectrum measured towards the molecular cloud Sgr B2(N) at 161.5 GHz, using the ARO 12 m telescope. The identity of known spectra lines is indicated, while unidentified features are marked with BU^. This highly-congested spectrum is typical for Sgr B2(N) and other dense molecular clouds


to the polyatomic organic species that typically exist in such clouds, such as the simple alcohols CH3OH and CH3CH2OH, as well as ethers (CH3OCH3) and esters (CH3OCHO). These species, along with their various isotopologues, are so-called “asymmetric tops,” and exhibit quite complicated rotational spectra. Spectral-line confusion therefore makes molecule identification challenging. One approach to the problem is to conduct spectral-line surveys with broad frequency coverage, such as that of Halfen and Ziurys. Such broadband coverage encompasses hundreds of favorable transitions of a given molecule. Thus, even though a large percentage of lines may be masked by unrelated features, there is sufficient data to readily ascertain if there are ominous “missing” transitions for a given species and make a reliable assessment. Careful studies have clearly demonstrated the presence of simple sugars (CHOCH2OH), amines and imines (CH2NH, CH3NH2), “peptides” (NH2CHO, CH3CONH2), and carboxylic acids (HCOOH, CH3COOH). Typical abundances of such molecules, relative to H2, is~10−10– 10−9. The case of CH3CONH2, acetamide, illustrates the importance of board-band surveys in securing an identification. In the Sgr B2(N) survey of Halfen and Ziurys, 483 favorable transitions of this molecule are present. Some form of emission is actually present at all 483 frequencies, but only 133 of these had distinguishable spectral lines (Halfen et al. 2011). These features have internally-consistent intensities and line shapes, and there were no “missing” transitions. Therefore, acetamide is clearly present in Sgr B2(N). For CH3CH2NH2, ethyl amine, 72 favorable transitions have frequencies in the survey. In this case, however, there are no spectral features present at 20 of these, making this identification negative (Apponi et al. 2008). The general Galactic distribution of complex organic species is also of interest. To gain some insight into this question, Adande et al. (2013) searched for formamide, NH2CHO, in various dense clouds across the Galaxy. Recently, formamide has been proposed as an alternative to HCN in the prebiotic synthesis of purines and pyrimidines (Costanzo et al. 2007); thus, it is of clear biological interest. Adande et al. (2013) found NH2CHO to be a common constituent of dense clouds with star-forming regions within the Galactic Habitable Zone, with abundances comparable to that found in comet Hale-Bopp. The presence of formamide in comets and molecular clouds suggests that the compound could have been brought to Earth by exogenous delivery, perhaps with an infall flux as high as~0.1 mol/km2/yr or 0.18 mmol/m2 in a single impact. Establishing the Galactic distribution of other organic species in addition to formamide would be enlightening.

Tracing Interstellar Phosphorus The presence and distribution of interstellar phosphorus is another question relevant to the origin of life. The cosmic abundance of this element is P/H~2.8×10−7 (Grevesse and Sauval 1998). For many years, the only known interstellar molecules containing this important element were PN, found in dense molecular clouds (e.g., Ziurys 1987; Turner et al. 1990), and CP, located in the carbon-rich envelope of IRC + 10216 (Guélin et al. 1990). However, this situation has changed recently with the detections of HCP (Agúndez et al. 2007), PO (Tenenbaum et al. 2007), PH3 (Tenenbaum and Ziurys 2008; Agúndez et al. 2014), and CCP (Halfen et al. 2008). These molecules have been identified thus far only in circumstellar shells, with abundances relative to H2 of f~10−9–10−7 (see Table 2). As summarized in Milam et al. (2008) and Agúndez et al. (2014), PO has been found in the O-rich shells of VY CMa, IK Tau, TX Cam, and R Cas, CP and CCP in IRC + 10216, and HCP and PH3 in IRC + 10216 and CRL 2688 (also see De Beck et al. 2013; Halfen et al. ( Halfen DT, Bernal JJ, Ziurys LM (2015) A survey of PO and PN in AGB stars. in preparation)). Figure 7 shows new spectra of

L.M. Ziurys et al. Table 2 Known interstellar phosphorus-bearing molecules Molecule f(X/H2)

Sources

References

PN

0.001–3× 10−7

IRC + 10216, CRL 2688, VY CMa, IK Tau, TX Cam, R Cas, various dense clouds

Milam et al. 2008; Halfen et al. 2008; De Beck et al. 2013; Turner et al. 1990

CP

5×10−9

IRC + 10216

Halfen et al. 2008

PO

0.5–6×10−7

VY CMa, IK Tau, TX Cam, R Cas Halfen et al. 2008, 2015; De Beck et al. 2013

HCP

0.3–2×10−7

IRC + 10216, CRL 2688

Milam et al. 2008

CCP

1×10−9

IRC + 10216

Halfen et al. 2008

PH3

0.1–4×10−7

IRC + 10216, CRL 2688

Tenenbaum and Ziurys 2008; Agúndez et al. 2008, 2014

PO measured towards TX Cam. None of these species, however, have been observed to date in dense molecular clouds. Phosphorus in dense clouds may be present in more complex molecules. Phosphorus analogs of abundant nitrogen-bearing organic compounds are good candidate carriers, such as PH2CN and CH3PH2. Their N-containing counterparts, cyanamide and methyl amine, are

Fig. 7 Spectra of the molecule PO observed towards the envelope of the oxygen-rich star, TX Cam, using the ARO SMT. The J = 5.5→4.5 and J = 6.5→5.5 transitions of PO are shown, observed near 240 GHz and 283 GHz, respectively; each transition consists of lambda-doublets, indicated by the lines under the spectra


both abundant molecules in Sgr B2(N), for example (Halfen et al. 2013, 2014). However, the millimeter-wave spectrum of neither species is well-characterized, necessitating new laboratory measurements for astronomical searches. Both molecules are not commercially available, and must be created by exotic synthetic techniques. Furthermore, spectroscopic studies of CH3PH2 are particularly challenging, as the methyl group rotates internally with respect to the molecular frame. This “internal rotation” creates additional splittings in the energy levels and produces a far more complex spectrum. Halfen et al. (2014) have recently measured the millimeter-wave spectra of both PH2CN and CH3PH2. The molecules were synthesized in an AC discharge of vaporized red phosphorus and either (CN)2 or CH4. Between 8 and 12 rotational transitions were recorded over the region 210–470 GHz for PH2CN and CH3PH2, respectively, each consisting of multiple asymmetry components. Searches for both molecules were subsequently conducted towards Sgr B2(N), using the ARO 12 m telescope. Neither molecule was identified, with abundance upper limits, relative to H2, of f(PH2CN/H2) < 7.0×10−12 and f(CH3PH2/H2) < 8.4×10−12. However, this result does not mean that more complex organophosphorus compounds do not exist in clouds such as Sgr B2(N). Accurate laboratory spectra for many possible species such as PH2CHO or CH3CH2PH2 are simply not available, limiting our ability to probe the extent of phosphorus synthesis in the ISM.

Following Molecular Material into the Solar System Solar systems form in collapsing dense clouds. From studies of meteorites, it is clear that some material of interstellar origin has been delivered to Earth. For example, there are isotopic excesses in deuterium (D) and 15 N in insoluble organic matter, or IOM, which are believed to reflect the partial preservation of interstellar molecules originally present in cold dense clouds or at the edge of the protoplanetary disk (e.g., Busemann et al. 2006; Alexander et al. 2007). However, making a direct connection between material delivered to Earth, which is in the solid state, and interstellar molecular matter, which is principally studied in the gas-phase, is difficult. Pre-solar grains, which come from outside the Solar System, offer a possible avenue by which a clearer connection can be established. These grains, which are extracted from meteorites, can be classified according to their 12C/13C and 14 N/15 N ratios. Silicon carbide grains of this type fall into three major populations 1) “mainstream” plus Z and Y groups, 2) A + B grains, and 3) X type (Lodders and Amari 2005). Based primarily on models of nucleosynthesis, mainstream SiC grains are thought to originate in AGB stars, along with Z and Y types, while X grains come from supernovae. The A + B group are attributed to “J-type” stars, anomalous carbon stars that have high lithium abundances and no significant s-element enhancement (Abia and Isern 2000). These three populations are shown in Fig. 8, adapted from Lodders and Amari. Millimeter observations of interstellar molecules and their isotopically-substituted forms offer a direct way to measure 12C/13C and 14 N/15 N ratios in astronomical sources. The high spectral resolution of such measurements neatly separates the isotopologues. For example, the J = 3→2 line of HCN lies at 265,886 MHz, while those of H13CN and HC15N are at 259,012 MHz and 258,157 MHz, respectively. The instrumental spectral resolution is better than 1 MHz, and typical spectral linewidths are a few MHz. Comparison of observed line intensities then enables a direct determination of a given isotope ratio, provided the lines are not saturated.

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Fig. 8 Carbon (12C/13C) and nitrogen (14 N/15 N) isotope ratios (in black and white) determined for a large sample of pre-solar SiC grains (from Lodders and Amari 2005). Superimposed on the diagram are the ratios (in red) measured in envelopes of AGB and J-type stars, as well as towards supernovae remnants (SNR), using millimeter-wave spectra of HCN and its 13C and 15 N isotopologues (Adande, Woolf, and Ziurys, in prep.) The SiC grains fall into three general categories, Bmainstream^ plus Y and Z, A + B, and X, thought to arise from AGB stars, J-type stars, and supernovae, respectively. The astronomical observations confirm these assignments

Observations of HCN and its 13C and 15 N-substituted species have been conducted towards envelopes of AGB and J-type stars and towards supernovae remnants (SNR) by Adande et al. (2015, Carbon and nitrogen isotope ratios in circumstellar gas and supernovae remnants: Linking the ISM to pre-solar grains. in preparation). Preliminary results are also shown in Fig. 8. The red circles show the ratios measured towards the J-type stars VY Cn and VY Dra, which fall in the A + B grain population, while the red squares indicate those found in AGB envelopes, which lie in the “mainstream” grain region. The ratios obtained toward the SNRs (red triangles) fall among those of the X-type population. Although preliminary, these results appear to confirm the identity of the SiC grain groups and theoretical predictions of nucleosynthesis. Moreover, these measurements make a clear link between molecular material in stellar envelopes and that found in meteorites. These results suggest that further direct connections can be made between interstellar and solar system material.

Conclusions Millimeter wavelength astronomical observations are clearly demonstrating that gas-phase molecular material is far more common in the Galaxy than previously thought. Interstellar molecules are increasingly found in significant abundances under harsh environmental conditions. For example, polyatomic species such as H2CO, HCN, and C3H2 are present in planetary nebulae, contained in dense, self-shielding clumps that then seed the diffuse ISM. Molecular material is therefore “recycled” in the ISM, suggesting that chemical timescales could be as


long as the lifetime of the Galaxy. Therefore, greater chemical complexity can be achieved than previously thought, perhaps leading to even more complex prebiotic or even biological compounds that could significantly influence the beginning of life on a planet such as the Earth. The limit of molecular complexity in interstellar space is not currently known. There are substantial challenges in evaluating this complexity, including the problem of spectral-line confusion, as well as the difficulty in laboratory spectroscopy measurements. However, even at this juncture, it is clear that simple prebiotic building blocks such as H2CO and NH2CHO are ubiquitous constituents of the Galaxy. Less is known about interstellar phosphorus chemistry, partly because of the dearth of laboratory spectroscopic studies. A definite connection between interstellar and solar system material, as found in meteorites, appears to exist. There are many pieces to this puzzle, however, and only a few such pieces have been assembled. Overall, there is a vast amount of interstellar molecular material in the Galaxy, and its association with life’s origins is highly plausible. Acknowledgments This research has been supported by NASA Exobiology grant NNX10AR83G and NSF grants AST-1140030 and AST-1211502. The 12 m and SMT are operated by the Arizona Radio Observatory (ARO), Steward Observatory, University of Arizona, with support through the NSF University Radio Observatories program (URO: AST-1140030).

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