Of the over 150 different molecular species detected in the interstellar and circumstellar media, approximately 50 contain 6 or more atoms. These molecules, labeled complex by astronomers if not by chemists, all contain the element carbon and so can be called organic. In the interstellar medium, complex molecules are detected in the denser sources only. Although, with one exception, complex molecules have only been detected in the gas phase, there is strong evidence that they can be formed in ice mantles on interstellar grains. The nature of the gaseous complex species depends dramatically on the source where they are found: in cold, dense regions they tend to be unsaturated (hydrogen-poor) and exotic, whereas in young stellar objects, they tend to be quite saturated (hydrogen-rich) and terrestrial in nature. Based on both their spectra and chemistry, complex molecules are excellent probes of the physical conditions and history of the sources where they reside. Because they are detected in young stellar objects, complex molecules are expected to be common ingredients for new planetary systems. In this review, we discuss both the observation and chemistry of complex molecules in assorted interstellar regions in the Milky Way.

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Complex Organic Interstellar Molecules

Aims. Magnetic fields play a primordial role in the star formation process. The Zeeman effect on the CN radical lines is one of the few methods of measuring magnetic fields in the dense gas of star formation regions. Methods. We report new observations of the Zeeman effect on seven hyperfine CN N = 1−0 lines in the direction of 14 regions of star formation. Results. We have improved the sensitivity of previous detections, and obtained five new detections. Good upper limits are also achieved. The probability distribution of the line-of-sight field intensity, including non-detections, provides a median value of the total field Btot = 0.56 mG while the average density of the medium sampled is n(H2) = 4.5 × 10 5 cm −3 . We show that the CN line probably samples regions similar to those traced by CS and that the magnetic field observed mostly pervades the dense cores. The dense cores are found to be critical to slightly supercritical with a mean mass-to-flux ratio M/Φ ∼ 1 to 4 with respect to critical. Their turbulent and magnetic energies are in approximate equipartition.

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CN Zeeman measurements in star formation regions

Titan is the only moon with a substantial atmosphere, the only other thick N2 atmosphere besides Earth's, the site of extraordinarily complex atmospheric chemistry that far surpasses any other solar system atmosphere, and the only other solar system body with stable liquid currently on its surface. The connection between Titan's surface and atmosphere is also unique in our solar system; atmospheric chemistry produces materials that are deposited on the surface and subsequently altered by surface-atmosphere interactions such as aeolian and fluvial processes resulting in the formation of extensive dune fields and expansive lakes and seas. Titan's atmosphere is favorable for organic haze formation, which combined with the presence of some oxygen-bearing molecules indicates that Titan's atmosphere may produce molecules of prebiotic interest. The combination of organics and liquid, in the form of water in a subsurface ocean and methane/ethane in the surface lakes and seas, means that Titan may be the ideal place in the solar system to test ideas about habitability, prebiotic chemistry, and the ubiquity and diversity of life in the universe. The Cassini-Huygens mission to the Saturn system has provided a wealth of new information allowing for study of Titan as a complex system. Here I review our current understanding of Titan's atmosphere and climate forged from the powerful combination of Earth-based observations, remote sensing and in situ spacecraft measurements, laboratory experiments, and models. I conclude with some of our remaining unanswered questions as the incredible era of exploration with Cassini-Huygens comes to an end.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2016JE005240

Titan's atmosphere and climate

We have observed 37 Infrared Dark Clouds (IRDCs), containing a total of 159 clumps, in high-density molecular tracers at 3 mm using the 22 m ATNF Mopra Telescope located in Australia. After determining kinematic distances, we eliminated clumps that are not located in IRDCs and clumps with a separation between them of less than one Mopra beam. Our final sample consists of 92 IRDC clumps. The most commonly detected molecular lines are (detection rates higher than 8%) N2H+, HNC, HN13C, HCO+, H13CO+, HCN, C2H, HC3N, HNCO, and SiO. We investigate the behavior of the different molecular tracers and look for chemical variations as a function of an evolutionary sequence based on Spitzer IRAC and MIPS emission. We find that the molecular tracers behave differently through the evolutionary sequence and some of them can be used to yield useful relative age information. The presence of HNC and N2H+ lines does not depend on the star formation activity. On the other hand, HC3N, HNCO, and SiO are predominantly detected in later stages of evolution. Optical depth calculations show that in IRDC clumps the N2H+ line is optically thin, the C2H line is moderately optically thick, and HNC and HCO+ are optically thick. The HCN hyperfine transitions are blended, and, in addition, show self-absorbed line profiles and extended wing emission. These factors combined prevent the use of HCN hyperfine transitions for the calculation of physical parameters. Total column densities of the different molecules, except C2H, increase with the evolutionary stage of the clumps. Molecular abundances increase with the evolutionary stage for N2H+ and HCO+. The N2H+/HCO+ and N2H+/HNC abundance ratios act as chemical clocks, increasing with the evolution of the clumps.

http://iopscience.iop.org/article/10.1088/0004-637X/756/1/60/pdf

CHEMISTRY IN INFRARED DARK CLOUD CLUMPS: A MOLECULAR LINE SURVEY AT 3 mm

While unbiased surveys observable from ground-based telescopes have previously been obtained towards several high mass protostars, very little exists on low mass protostars. To fill up this gap, we carried out a complete spectral survey of the bands at 3, 2, 1 and 0.8 mm towards the solar type protostar IRAS16293-2422. The observations covered about 200\,GHz and were obtained with the IRAM-30m and JCMT-15m telescopes. Particular attention was devoted to the inter-calibration of the obtained spectra with previous observations. All the lines detected with more than 3 sigma and free from obvious blending effects were fitted with Gaussians to estimate their basic kinematic properties. More than 4000 lines were detected (with sigma \geq 3) and identified, yielding a line density of approximatively 20 lines per GHz, comparable to previous surveys in massive hot cores. The vast majority (~2/3) of the lines are weak and due to complex organic molecules. The analysis of the profiles of more than 1000 lines belonging 70 species firmly establishes the presence of two distinct velocity components, associated with the two objects, A and B, forming the IRAS16293-2422 binary system. In the source A, the line widths of several species increase with the upper level energy of the transition, a behavior compatible with gas infalling towards a ~1 Mo object. The source B, which does not show this effect, might have a much lower central mass of ~0.1 Mo. The difference in the rest velocities of both objects is consistent with the hypothesis that the source B rotates around the source A. This spectral survey, although obtained with single-dish telescope with a low spatial resolution, allows to separate the emission from 2 different components, thanks to the large number of lines detected. The data of the survey are public and can be retrieved on the web site http://www-laog.obs.ujf-grenoble.fr/heberges/timasss.

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TIMASSS: the IRAS 16293-2422 millimeter and submillimeter spectral survey. I. Observations, calibration, and analysis of the line kinematics

We have carried out observations toward the W3 complex and G34.3+0.15 using the TRAO 14 m radio telescope to examine in detail the chemical variations occurring while molecular clouds evolve from the prestellar to the H II region phase. Observations include spectral surveys of these objects between 84.7 and 115.6 GHz; mapping observations toward W3(OH) with the emissions of CS (2-1), HCN (1-0), HNC (1-0), and HCO+ (1-0); and mapping of CS (2-1) emission toward W3 IRS 5. Chemical model calculations are used to estimate the age of W3(OH) by comparing with the fractional abundances of detected molecules. We found that G34.3+0.15 and W3(OH) are at a similar evolutionary stage, although large differences in the fractional abundances are found in CH3CN and HC3N. Overall, the properties of the detected species and abundances in three regions support the view that chemistry varies as molecular clouds evolve from a cold, collapsing phase to a high-temperature phase, such as the hot core and H II phase. Chemical model calculations for W3(OH) indicate that the evolutionary age of the cloud is 104-105 yr with temperature in the range 10-60 K.

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A Molecular Line Survey of W3(OH) and W3 IRS 5 from 84.7 to 115.6 GHz: Observational Data and Analyses

Based on the simultaneous observational results of SiO and H{sub 2}O masers toward 401 evolved stars, we have performed statistical analyses. We find that the peak and integrated intensities of SiO masers are stronger than those of H{sub 2}O masers in both Mira variables and OH/IR stars at most optical phases. However, the relative intensity ratios of H{sub 2}O to SiO masers in OH/IR stars are larger than those in Mira variables. Moreover, the intensity ratios of H{sub 2}O to SiO masers are found to be increased around the optical phases from 0-0.4. The H{sub 2}O photon luminosities also tend to be more dependent on the optical phase compared with those of SiO masers. These facts suggest that H{sub 2}O masers are more sensitive to expanding the motion of circumstellar envelopes and also shock waves arising from the pulsations of the central star compared with SiO masers. This result may also be related to the differences in the maser location and pumping mechanism between H{sub 2}O and SiO masers. The full width at zero power of SiO masers in Mira variables shows similar values to those of OH/IR stars, while those of H{sub 2}O masers in OH/IR stars show largermore » values than those of Mira variables. These differences may originate from the different mass-loss rates and the different location of the two masers. The mean velocity shift of SiO and H{sub 2}O masers with respect to the stellar velocity was investigated as a function of optical phase. The velocity shift of the H{sub 2}O masers shows that the redshifted emission dominates during the phases from 0.3-0.6, while the blueshifted emission appears at phase 0.6 and coexists with the redshifted emission during other phases. These features show an associated pattern with the CO ΔV = 3 radial velocity curve, which exhibits a typical pulsation motion. On the other hand, the velocity shift of the SiO v = 2 maser shows slightly similar features to the H{sub 2}O maser, while that of SiO v = 1 does not show these similar features. Finally, the distributions of all 401 observed sources are investigated in the IRAS two-color diagram in relation to the late evolutionary processes of asymptotic giant branch stars.« less

https://iopscience.iop.org/article/10.1088/0004-6256/147/1/22/pdf

STATISTICAL STUDIES BASED ON SIMULTANEOUS SiO AND H2O MASER SURVEYS TOWARD EVOLVED STARS

We present the results of simultaneous observations of SiO v = 1, 2, 29SiO v = 0, J = 1-0, and H2O 616-523 maser lines toward 152 known stellar H2O maser sources using the Yonsei 21 m radio telescope of the Korean VLBI Network from 2009 June to 2011 January. Both SiO and H2O masers were detected from 62 sources with a detection rate of 40.8%. The SiO-only maser emission without H2O maser detection was detected from 27 sources, while the H2O-only maser without SiO maser detection was detected from 22 sources. Therefore, the overall SiO maser emission was detected from 89 sources, resulting in a detection rate of 58.6%. We have identified 70 new detections of the SiO maser emission. For both H2O and SiO maser detected sources, the peak and integrated antenna temperatures of SiO masers are stronger than those of H2O masers in both Mira variables and OH/IR stars and the relative intensity ratios of H2O to SiO masers in OH/IR stars are larger than those in Mira variables. In addition, distributions of 152 observed sources were investigated in the IRAS two-color diagram.

http://iopscience.iop.org/article/10.1088/0004-6256/145/1/22/pdf

Simultaneous Observations of SiO and H2O Masers toward Known Stellar H2O Maser Sources

We present the results of simultaneous time monitoring observations of SiO J = 2-1 and J = 3-2 maser emission for 10 late-type stars (8 Mira variables, 1 OH/IR star, and 1 supergiant) with the 14 m radio telescope at Taeduk Radio Astronomy Observatory from 1999 January to 2001 February. The SiO v = 1, J = 2-1 and J = 3-2 maser emission was detected at almost all observational epochs. The SiO v = 2, J = 2-1 maser was detected from 4 late-type stars (VY CMa, R Cas, χ Cyg, R Leo) and the v = 2, J = 3-2 maser was detected from 7 stars (R Aqr, TX Cam, R Cas, χ Cyg, W Hya, R Leo, IK Tau). The v = 3, J = 2-1 and J = 3-2 masers were also detected from χ Cyg and TX Cam, respectively. Based on these observational data, line profile and peak velocity variations with respect to stellar velocity, antenna temperatures, and their ratio variations as a function of optical phase of central star were investigated. As main results, the line profile and the peak velocity variation of the v = 1, J = 3-2 maser with pulsation phase was found to differ from the v = 1, J = 2-1 transition. Similarly, the J = 2-1 and J = 3-2 transitions also differ between rovibrational transitions at a given pulsation phase. However, it is difficult to find significant correlations between the peak velocity variation relative to the stellar velocity of either the J = 3-2 or J = 2-1 transitions over pulsation phase, due to limited time sampling in these data. The peak and integrated antenna temperature (PT and IT) ratios among rotational ladders and vibrational states are investigated. These ratios between rotational ladders of the v = 1, J = 2-1, and J = 3-2 masers are averaged to be the peak antenna temperature ratio, PT(v = 1, J = 3-2)/PT(v = 1, J = 2-1) ≈ 0.29, and the integrated antenna temperature ratio, IT(v = 1, J = 3-2)/IT(v = 1, J = 2-1) ≈ 0.21, respectively. In the v = 2 state, these ratios are PT(v = 2, J = 3-2)/PT(v = 2, J = 2-1) ≈ 7.94 and IT(v = 2, J = 3-2)/IT(v = 2, J = 2-1) ≈ 8.50, respectively. The peak and integrated antenna temperature ratios between vibrational states are also averaged to be PT(v = 2, J = 3-2)/PT(v = 1, J = 3-2) ≈ 1.29, IT(v = 2, J = 3-2)/IT(v = 1, J = 3-2) ≈ 1.02, PT(v = 2, J = 2-1)/PT(v = 1, J = 2-1) ≈ 0.06, and IT(v = 2, J = 2-1)/IT(v = 1, J = 2-1) ≈ 0.05, respectively. These intensity ratios for the v = 2, J = 2-1 and v = 2, J = 3-2 masers suggest that line overlaps operating in the v = 2, J = 2-1 transition do not similarly affect the v = 2, J = 3-2 transition.

Sang Joon Kim

Papers

A Molecular Line Survey of W3(OH) and W3 IRS 5 from 84.7 to 115.6 GHz: Observational Data and Analyses

STATISTICAL STUDIES BASED ON SIMULTANEOUS SiO AND H2O MASER SURVEYS TOWARD EVOLVED STARS

Simultaneous Observations of SiO and H2O Masers toward Known Stellar H2O Maser Sources

Time Monitoring Observations of SiO J = 2-1 and J = 3-2 Maser Emission toward Late-Type Stars

Diagnostic Simulations of the Lunar Sodium Tail and Bright Spot