We discuss the interstellar absorption from many atomic and molecular species seen in high-resolution Hubble Space Telescope/STIS UV and high signal-to-noise ratio optical spectra of the moderately reddened B3–5 V star HD 62542. This remarkable sight line exhibits both very steep far-UV extinction and a high fraction of hydrogen in molecular form, with strong absorption from CH, C2, CN, and CO, but weak absorption from CH+ and most of the commonly observed diffuse interstellar bands. Most of the material resides in a single narrow velocity component, offering a rare opportunity to probe the primarily molecular core of a single interstellar cloud with little associated diffuse atomic gas. Detailed analyses of the spectra indicate that (1) the molecular fraction in the main cloud is high (f(H2) ≳ 0.8); (2) the gas is fairly cold (Tk = 40–43 K; from the rotational excitation of H2 and C2); (3) the local hydrogen density nH ∼ 1500 cm−3 (from C2 excitation, fine-structure excitation of C0, and simple chemical models); (4) the unusually high excitation temperatures for 12CO and 13CO may be largely due to radiative excitation; (5) N(C+):N(CO):N(C) ∼ 100:10:1; (6) the depletions of many elements are more severe than those seen in any other sight line and the detailed pattern of depletions differs from those derived from larger samples of Galactic sight lines; and (7) the various neutral/first ion ratios do not yield consistent estimates for electron density, even when the effects of grain-assisted recombination and low-temperature dielectronic recombination are considered.

Aiming for a new and more comprehensive DIB catalog between 4000 and 9000 Å, we revisited the Atlas Catalog based on the observations of HD 183143 and HD 204827. Twenty-five medium to highly reddened sight lines were selected, sampling a variety of spectral types of the background star and the interstellar environments. The median signal-to-noise ratio (S/N) of these spectra is ∼1300 around 6400 Å. Compared to the Atlas Catalog, 22 new DIBs were found, and the boundaries of 27 (sets of) DIBs were adjusted, resulting in an updated catalog containing 559 DIBs that we refer to as the Apache Point Observatory Catalog of Optical Diffuse Interstellar Bands. Measurements were then made based on this catalog. We found our survey most sensitive between 5500 and 7000 Å, due largely to the local S/N of the spectra, the relative absence of interfering stellar lines, and the weakness of telluric residuals. For our data sample, the number of DIBs detected in a given sight line is mostly dependent on EB−V and less on the spectral type of the background star. Some dependence on the molecular fraction fH2 is observed, but it is less well determined owing to the limited size of the data sample. The variations of the wavelengths of each DIB in different sight lines are generally larger than those of the known interstellar lines CH+, CH, and K i. Those variations could be due to the inherent error in the measurement, or to differences in the velocity components among sight lines.

We present the results of a ground-based observing campaign designed to produce the first homogeneous census of the population of the broadest (FWHM ≥ 6 Å) diffuse interstellar bands (DIBs) in the Galaxy. New low-resolution optical spectra for 21 sightlines spanning a reddening range of three magnitudes, using stars of spectral types O7 through A3, were examined together with corresponding higher-resolution echelle spectra. A new pipeline developed for the reduction and analysis of the low-resolution spectra is presented. Of the 34 broad features reported in the literature, 22 are covered in this survey. Of those 22, 9 are confirmed as broad DIBs, 2 are rejected, and the remaining 11 are uncertain, due to severe blending with stellar and/or telluric lines. No new broad DIBs are identified. New homogeneous measurements of the strength of the broad 6177 Å DIB are compared with the corresponding strengths of five narrower DIBs known to trace different phases of the diffuse ISM: the 4963 Å C2-DIB (which can reside in diffuse molecular gas) and the 5780, 5797, 6284, and 6613 Å DIBs (which primarily trace atomic gas). The 6177 Å DIB correlates quite well with the 5780 and 6284 Å DIBs but shows no correlation with the 4963 Å C2-DIB—suggesting that its carrier is found primarily in the atomic gas.

Context. Medium-resolution echelle spectra of the Red Square Nebula surrounding the star MWC 922 are presented. The spectra have been obtained in 2010 and 2012 using the X-shooter spectrograph mounted on the Very Large Telescope (VLT) in Paranal, Chile. The spectrum covers a wavelength range between 300 nm-2.5 mu m and shows that the nebula is rich in emission lines.Aims. We aim to identify the emission lines and use them as a tool to determine the physical and chemical characteristics of the nebula. The emission lines are also used to put constraints on the structure of the nebula and on the nature of the central stars.Methods. We analyzed and identified emission lines that indicated that the Red Square Nebula consists of a low density bipolar outflow, eminent in the broad emission component seen in [Fe II], as well as in P Cygni line profiles indicative of fast outflowing material. The narrow component in the [Fe II] lines is most likely formed in the photosphere of a surrounding disk. Some of the emission lines show a pronounced double peaked profile, such as Ca II, indicating an accretion disk in Keplerian rotation around the central star. [O I] emission lines are formed in the neutral atomic zone separating the ionized disk photosphere from the molecular gas in the interior of the disk, which is prominent in molecular CO emission in the near-IR. [NII] and [S II] emission clearly originates in a low density but fairly hot (7 000-10 000 K) nebular environment. Hi recombination lines trace the extended nebula as well as the photosphere of the disk.Results. These findings put constraints on the evolution of the central objects in MWC 922. The Red Square shows strong similarities to the Red Rectangle Nebula, both in morphology and in its mid-IR spectroscopic characteristics. As for the Red Rectangle, the observed morphology of the nebula reflects mass-loss in a binary system. Specifically, we attribute the biconical morphology and the associated rung-like structure to the action of intermittent jets blown by the accreting companion in a dense shell, which has been created by the primary. We stress, though, that despite the morphological similarities, these two objects represent very different classes of stellar objects.

We study the behavior of eight diffuse interstellar bands (DIBs) in different interstellar environments, as characterized by the fraction of hydrogen in molecular form (f(H2)), with comparisons to the corresponding behavior of various known atomic and molecular species. The equivalent widths of the five "normal" DIBs (lambda lambda 5780.5, 5797.1, 6196.0, 6283.8, and 6613.6), normalized to EB-V, show a "lambda-shaped" behavior: they increase at low f(H2), peak at f(H2) similar to 0.3, and then decrease. The similarly normalized column densities of Ca, Ca+, Ti+, and CH+ also decline for f(H2) > 0.3. In contrast, the normalized column densities of Na, K, CH, CN, and CO increase monotonically with f(H2), and the trends exhibited by the three C-2 DIBs (lambda lambda 4726.8, 4963.9, and 4984.8) lie between those two general behaviors. These trends with f(H2) are accompanied by cosmic scatter, the dispersion at any given fH2 being significantly larger than the individual errors of measurement. The lambda-shaped trends suggest the balance between creation and destruction of the DIB carriers differs dramatically between diffuse atomic and diffuse molecular clouds; additional processes aside from ionization and shielding are needed to explain those observed trends. Except for several special cases, the highest W-lambda(5780)/W-lambda(5797) ratios, characterizing the so-called "sigma-zeta effect," occur only at f(H2) < 0.2. We propose a sequence of DIBs based on trends in their pair-wise strength ratios with increasing f(H2). In order of increasing environmental density, we find the lambda 6283.8 and lambda 5780.5 DIBs, the lambda 6196.0 DIB, the lambda 6613.6 DIB, the lambda 5797.1 DIB, and the C-2 DIBs.

A central focus of astrobiology is the determination of abiotic formation routes to important biomolecules. The dissociation mechanisms of these molecules lend valuable insights into their synthesis pathways. Because of the detection of organic anions in the interstellar medium (ISM), it is imperative to study their role in these syntheses. This work aims to experimentally and computationally examine deprotonated adenine and guanine dissociation in an effort to illuminate potential anionic precursors to purine formation. Collision-induced dissociation (CID) products and their branching fractions are experimentally measured using an ion trap mass spectrometer. Deprotonated guanine dissociates primarily by deammoniation (97%) with minor losses of carbodiimide (HNCNH) and/or cyanamide (NH2CN), and isocyanic acid (HNCO). Deprotonated adenine fragments by loss of hydrogen cyanide and/or isocyanide (HCN/HNC; 90%) and carbodiimide (HNCNH) and/or cyanamide (NH2CN; 10%). Tandem mass spectrometry (MS(n)) experiments reveal that deprotonated guanine fragments lose additional HCN and CO, while deprotonated adenine fragments successively lose HNC and HCN. Every neutral fragment observed in this study has been detected in the ISM, highlighting the potential for nucleobases such as these to form in such environments. Lastly, the acidity of abundant fragment ions is experimentally bracketed. Theoretical calculations at the B3LYP/6-311++G(d,p) level of theory are performed to delineate the mechanisms of dissociation and analyze the energies of reactants, intermediates, transition states, and products of these CID processes.

Studies of interstellar chemistry have grown in number and complexity by both observations and laboratory measurements, and nitrogen-containing aromatics have been implicated as important interstellar molecules. In this paper, the gas-phase collision induced dissociation (CID) processes of protonated pyridazine (1,2-diazine), pyrimidine (1,3-diazine), and pyrazine (1,4-diazine) cations (C4H5N2+) are investigated in detail both experimentally and theoretically. The major neutral loss for all three CID processes is HCN, leading to the formation of C3H4N+ isomers; our density functional theory (DFT) calculations support and elucidate our experimental results. The formation of C3H4N+ isomers from the reaction of abundant interstellar acrylonitrile (CH2CHCN) and H+is also studied employing DFT calculations. Our results lead to a novel mechanism for interstellar protonated diazine formation from the consecutive reactions of CH2CHCN+ H+ + HCN. Moreover, our results motivate the continuing search for interstellar C3H4N+ isomers as well as polycyclic aromatic N-containing hydrocarbons (PANHs).

Azines are important in many extraterrestrial environments, from the atmosphere of Titan to the interstellar medium. They have been implicated as possible carriers of the diffuse interstellar bands in astronomy, indicating their persistence in interstellar space. Most importantly, they constitute the basic building blocks of DNA and RNA, so their chemical reactivity in these environments has significant astrobiological implications. In addition, N and O atoms are widely observed in the ISM and in the ionospheres of planets and moons. However, the chemical reactions of molecular anions with abundant interstellar and atmospheric atomic species are largely unexplored. In this paper, gas-phase reactions of deprotonated anions of benzene, pyridine, pyridazine, pyrimidine, pyrazine, and s-triazine with N and O atoms are studied both experimentally and computationally. In all cases, the major reaction channel is associative electron detachment; these reactions are particularly important since they control the balance between negative ions and free electron densities. The reactions of the azine anions with N atoms exhibit larger rate constants than reactions of corresponding chain anions. The reactions of azine anions with O atoms are even more rapid, with complex product patterns for different reactants. The mechanisms are studied theoretically by employing density functional theory; spin conversion is found to be important in determining some product distributions. The rich gas-phase chemistry observed in this work provides a better understanding of ion-atom reactions and their contributions to ionospheric chemistry as well as the chemical processing that occurs in the boundary layers between diffuse and dense interstellar clouds.

Cyanate (OCN-) is the only ion to date whose presence has been confirmed in the icy mantles that coat interstellar dust grains. Understanding the chemical behavior of cyanate at a fundamental level is therefore integral to the advancement of astrochemistry. We seek to unravel the chemistry of this intriguing anion through a combination of gas-phase experiments and theoretical explorations. Our approach is twofold: first, employing a flowing afterglow-selected ion flow tube apparatus, the reactions between OCN- and three of the most abundant atomic species in the interstellar medium, hydrogen, nitrogen, and oxygen, are examined. Hydrogen atoms readily react by associative detachment, but the remarkable stability of OCN- does not give rise to an observable reaction with either nitrogen or oxygen atoms. To explain these results, the potential energy surfaces of several reactions are investigated at the B3LYP/6-311++ G(d, p) level of theory. Second, collision induced dissociation experiments involving deprotonated uracil, thymine, and cytosine in an ion trap mass spectrometer reveal an interesting connection between these pyrimidine nucleobase anions and OCN-. Theoretical calculations at the B3LYP/6-311++ G(d, p) level of theory are performed to delineate the mechanisms of dissociation and explore the possible role of OCN- as a biomolecule precursor.

We have studied the gas-phase reactions of CF(+) with 24 neutral species. Reaction rate constants and product branching fractions are measured at 298 K using a flowing afterglow-selected ion flow tube. Experimental work is supported by computational chemistry calculations to provide insight into the reactivity of classes of neutral molecules. Reactions of CF(+) with small triatomic species and oxygen-containing organic molecules produce the stable molecule CO. The product branching fractions are discussed, and the potential energy surfaces for a few representative reactions are examined. CF(+) is highly reactive with complex molecules and will likely be destroyed in dense environments in the interstellar medium. However, the lack of reactivity with small diatomic molecules will likely enable its survival in diffuse regions.

We present the first high resolution UV spectra toward Herschel 36, a Trapezium-like system of high-mass stars contained within the Lagoon Nebula (M8, NGC 6523). The spectra reveal extreme rovibrational excitation of molecular hydrogen in material at a single velocity or very small range of velocities, with this component presumably lying near the star system and undergoing fluorescent excitation. The overall H-2 excitation is similar to, but apparently larger than, that seen toward HD 37903 which previously showed the largest vibrationally excited H-2 column densities seen in UV absorption spectra. While the velocities of the highly excited H2 lines are consistent within each observation, it appears that they underwent a similar to 60 km s(-1) redshift during the 3.6 yr between observations. In neither case does the velocity of the highly excited material match the velocity of the bulk of the line-of-sight material which appears to mostly be in the foreground of M8. Recent work shows unusually excited CH and CH+ lines and several unusually broad diffuse interstellar bands toward Herschel 36. Along with the H-2 excitation, all of these findings appear to be related to the extreme environment within similar to 0.1 pc of the massive young stellar system.

We have studied the reactions of polycyclic aromatic hydrocarbon cations and their nitrogen-containing analogs with H atoms. Reaction rate constants are measured at 300 K using a flowing afterglow-selected ion flow tube. We have implemented the laser induced acoustic desorption technique to allow the study of large, non-volatile species in the gas phase. The extension of this work from previous studies shows that the reactivity of polycyclic aromatic hydrocarbon cations with H atoms reaches a constant value for large cations. There is a small difference in reactivity when comparing molecules of different size and geometry; however, no difference in reactivity was found when nitrogen was incorporated into the ring.