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© 2008. The American Astronomical Society. All rights reserved. We would like to thank an anonymous referee for his/ her thoughtful review of our paper. We acknowledge discussions with Patrick Morrissey related to the possibility of red leaks in the GALEX FUV and NUV bands. R. S.' s contribution to the research described in this publication was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. R. S. thanks NASA for financial support via a GALEX award and an LTSA award. K. F. was partially funded by a SURF scholarship and through the Cornell Presidential Research Scholars (CPRS) program. A. G. d. P. is partially financed by the Spanish Ramón y Cajal program and the Programa Nacional de Astronomía y Astrofísica under grant AYA 2006-02358. C. S. C. is partially funded for this work by the Spanish MCyT under project AYA 2006-14876 and the Spanish MEC under project PIE 200750I028., The search for binarity in AGB stars is of critical importance for our understanding of how planetary nebulae acquire the dazzling variety of aspherical shapes which characterizes this class. However, detecting binary companions in such stars has been severely hampered due to their extreme luminosities and pulsations. We have carried out a small imaging survey of AGB stars in ultraviolet light (using GALEX), where these cool objects are very faint, in order to search for hotter companions. We report the discovery of significant far-ultraviolet excesses toward nine of these stars. The far-ultraviolet excess most likely results either directly from the presence of a hot binary companion or indirectly from a hot accretion disk around the companion., Programa Ramón y Cajal, Programa Nacional de Astronomía y Astrofísica (PNAyA), España, (National Aeronautics and Space Administration (NASA), Ministerio de Ciencia y Tecnología (MCYT), España, Ministerio de Educación y Ciencia (MEC), España, Cornell Presidential Research Scholars (CPRS) program, Depto. de Física de la Tierra y Astrofísica, Fac. de Ciencias Físicas, TRUE, pub

The effect of the choice of carbon support (activated carbon, carbon nanofiber, graphite, and multiwalled carbon nanotube) on the formation of rhenium carbide was evaluated in the hydrodeoxygenation of guaiacol, a pyrolysis oil model compound. Catalysts were prepared by incipient wetness impregnation, carburized at 650ºC under a 25/75 mixture of ethylene/hydrogen, and characterized by XRD, N2-physisorption, TPD, TPR, NH3-TPD, and XPS. The characterization results gave evidence that the choice of support affected the carburization of rhenium, a likely consequence of the nature of oxygen function groups on the supports. Indeed, poor carburization of Re occurred over the carbon nanofiber support, which was rationalized by a lack of carboxylic groups on the support. The study showed that the sites mainly responsible for converting guaiacol could be identified by quantification of CH4 mass signals during TPR measurements. This is an important finding to unravel the critical properties responsible for the HDO catalysis of carburized Re. Most of the catalysts were effective at converting guaiacol to phenol and a few, especially Re/MWCNT-O+ and Re/HSAG, were active for the hydrogenolysis of phenol to benzene (with benzene selectivity reaching 50% at guaiacol conversion of 98%). This indicates that this class of catalysts show great promise in converting guaiacol to desirable aromatic hydrocarbons. © 2023 Elsevier B.V.

Context. This is the second paper of a series making use of Herschel/PACS spectroscopy of evolved stars in the THROES catalogue to study the inner warm regions of their circumstellar envelopes (CSEs). Aims. We analyse the CO emission spectra, including a large number of high-J CO lines (from J = 14-13 to J = 45-44, nu = 0), as a proxy for the warm molecular gas in the CSEs of a sample of bright carbon-rich stars spanning different evolutionary stages from the asymptotic giant branch to the young planetary nebulae phase. Methods. We used the rotational diagram (RD) technique to derive rotational temperatures (T-rot) and masses (M-H2) of the envelope layers where the CO transitions observed with PACS arise. Additionally, we obtained a first order estimate of the mass-loss rates and assessed the impact of the opacity correction for a range of envelope characteristic radii. We used multi-epoch spectra for the well-studied C-rich envelope IRC+10216 to investigate the impact of CO flux variability on the values of T-rot and M-H2. Results. The sensitivity of PACS allowed for the study of higher rotational numbers than before indicating the presence of a significant amount of warmer gas (similar to 200-900 K) that is not traceable with lower J CO observations at submillimetre/millimetre wavelengths. The masses are in the range M-H2 similar to 10(-2)-10(-5)M(Theta), anticorrelated with temperature. For some strong CO emitters we infer a double temperature (warm T-rot similar to 400 K and hot T-rot similar to 820 K) component. From the analysis of IRC+10216, we corroborate that the effect of line variability is perceptible on the T-rot of the hot component only, and certainly insignificant on M-H2 and, hence, the mass-loss rate. The agreement between our mass-loss rates and the literature across the sample is good. Therefore, the parameters derived from the RD are robust even when strong line flux variability occurs, and the major source of uncertainty in the estimate of the

Silicon carbide together with amorphous carbon are the main components of dust grains in the atmospheres of C-rich AGB stars. Small gaseous Si-C bearing molecules (such as SiC, SiCSi, and SiC2) are efficiently formed close to the stellar photosphere. They likely condense onto dust seeds owing to their highly refractory nature at the lower temperatures (i.e., below about 2500 K) in the dust growth zone which extends a few stellar radii from the photosphere. Beyond this region, the abundances of Si-C bearing molecules are expected to decrease until they are eventually reformed in the outer shells of the circumstellar envelope, owing to the interaction between the gas and the interstellar UV radiation field. Our goal is to understand the time-dependent chemical evolution of Si-C bond carriers probed by molecular spectral line emission in the circumstellar envelope of IRC+10216 at millimeter wavelengths.

AGB stars are important contributors of processed matter to the ISM. However, the physical and chemical mechanisms involved in its ejection are still poorly known. This process is expected to have remarkable effects in the innermost envelope, where the dust grains are formed, the gas is accelerated, the chemistry is active, and the radiative excitation becomes important. A good tracer of this region in C-rich stars is SiS, an abundant refractory molecule that can display maser lines, very sensitive to changes in the physical conditions. We present high angular resolution interferometer observations (HPBW â‰30.″.25) of the v = 0 J = 14-13 and 15-14 SiS maser lines towards the archetypal AGB star IRC+10216, carried out with CARMA and ALMA to explore the inner 1> region around the central star. We also present an ambitious monitoring of these lines along one single pulsation period carried out with the IRAM 30 m telescope.

In this work (Paper I), we analyse Herschel-PACS spectroscopy for a subsample of 23 O-rich and 3 S-type evolved stars, in different evolutionary stages from the asymptotic giant branch (AGB) to the planetary nebula (PN) phase, from the THROES catalogue. (C-rich targets are separately studied in Paper II). The broad spectral range covered by PACS (similar to 55-210 mu m) includes a large number of high-J CO lines, from J = 14-13 to J = 45-44 (v = 0), that allow us to study the warm inner layers of the circumstellar envelopes (CSEs) of these objects, at typical distances from the star of approximate to 10(14)-10(15) cm and approximate to 10(16) cm for AGBs and post-AGB-PNe, respectively. We have generated CO rotational diagrams for each object to derive the rotational temperature, total mass within the CO-emitting region and average mass-loss rate during the ejection of these layers. We present first order estimations of these basic physical parameters using a large number of high-J CO rotational lines, with upper-level energies from E-up similar to 580 to 5000 K, for a relatively big set of evolved low-to-intermediate mass stars in different AGB-to-PN evolutionary stages. We derive rotational temperatures ranging from T-rot similar to 200 to 700 K, with typical values around 500K for AGBs and systematically lower, similar to 200 K, for objects in more advanced evolutionary stages (post-AGBs and PNe). Our values of Trot are one order or magnitude higher than the temperatures of the outer CSE layers derived from low-J CO line studies. The total mass of the inner CSE regions where the PACS CO lines arise is found to range from M-H2 similar to 10(-6) to approximate to 10(-2) M-circle dot, which is expected to represent a small fraction of the total CSE mass. The mass-loss rates estimated are in the range M similar to 10(-7)-10(-4) M-circle dot yr(-1), in agreement (within uncertainties) with values found in the literature. We find a clear anticorrelation between M-H2 and

MEGARA is the new generation IFU and MOS optical spectrograph built for the 10.4m Gran Telescopio CANARIAS (GTC). The project was developed by a consortium led by UCM (Spain) that also includes INAOE (Mexico), IAA-CSIC (Spain) and UPM (Spain). The instrument arrived to GTC on March 28th 2017 and was successfully integrated and commissioned at the telescope from May to August 2017. During the on-sky commissioning we demonstrated that MEGARA is a powerful and robust instrument that provides on-sky intermediate-to-high spectral resolutions RFWHM ~ 6,000, 12,000 and 20,000 at an unprecedented efficiency for these resolving powers in both its IFU and MOS modes. The IFU covers 12.5 x 11.3 arcsec 2 while the MOS mode allows observing up to 92 objects in a region of 3.5 x 3.5 arcmin 2 . In this paper we describe the instrument main subsystems, including the Folded-Cassegrain unit, the fiber link, the spectrograph, the cryostat, the detector and the control subsystems, and its performance numbers obtained during commissioning where the fulfillment of the instrument requirements is demonstrated. © 2018 SPIE.

On June 25th 2017, the new intermediate-resolution optical IFU and MOS of the 10.4-m GTC had its first light. As part of the tests carried out to verify the performance of the instrument in its two modes (IFU and MOS) and 18 spectral setups (identical number of VPHs with resolutions R=6000-20000 from 0.36 to 1 micron) a number of astronomical objects were observed. These observations show that MEGARA@GTC is called to fill a niche of high-throughput, intermediateresolution IFU and MOS observations of extremely-faint narrow-lined objects. Lyman-¿ absorbers, star-forming dwarfs or even weak absorptions in stellar spectra in our Galaxy or in the Local Group can now be explored to a new level. Thus, the versatility of MEGARA in terms of observing modes and spectral resolution and coverage will allow GTC to go beyond current observational limits in either depth or precision for all these objects. The results to be presented in this talk clearly demonstrate the potential of MEGARA in this regard. © 2018 SPIE.

We present continuum and molecular line emission ALMA observations of OH 231.8+4.2, a well studied bipolar nebula around an asymptotic giant branch (AGB) star. The high-angular resolution (~0.¿¿2¿0.¿¿3) and sensitivity of our ALMA maps provide the most detailed and accurate description of the overall nebular structure and kinematics of this object to date. We have identified a number of outflow components previously unknown. Species studied in this work include 12CO, 13CO, CS, SO, SO2, OCS, SiO, SiS, H3O+, Na37Cl, and CH3OH. The molecules Na37Cl and CH3OH are first detections in OH 231.8+4.2, with CH3OH being also a first detection in an AGB star. Our ALMA maps bring to light the totally unexpected position of the mass-losing AGB star (QX Pup) relative to the large-scale outflow. QX Pup is enshrouded within a compact (¿60 AU) parcel of dust and gas (clump S) in expansion (Vexp ~ 5¿7 km s¿1) that is displaced by ~ 0.¿¿6 to the south of the dense equatorial region (or waist) where the bipolar lobes join. Our SiO maps disclose a compact bipolar outflow that emerges from QX Pup¿s vicinity. This outflow is oriented similarly to the large-scale nebula but the expansion velocities are about ten times lower (Vexp ¿ 35 km s¿1). We deduce short kinematical ages for the SiO outflow, ranging from ~50¿80 yr, in regions within ~150 AU, to ~400¿500 yr at the lobe tips (~3500 AU). Adjacent to the SiO outflow, we identify a small-scale hourglass-shaped structure (mini-hourglass) that is probably made of compressed ambient material formed as the SiO outflow penetrates the dense, central regions of the nebula. The lobes and the equatorial waist of the mini-hourglass are both radially expanding with a constant velocity gradient (Vexp ¿ r). The mini-waist is characterized by extremely low velocities, down to ~1 km s¿1 at ~150 AU, which tentatively suggest the presence of a stable structure. The spatio-kinematics of the large-scale, high-velocity lobes (HV lobes), and the dense equatoria

We present new high angular resolution interferometer observations of the v = 0 J = 14-13 and 15-14 SiS lines toward IRC+10216, carried out with the Combined Array for Research in Millimeter-wave Astronomy and the Atacama Large Millimeter Array. The maps, with angular resolutions of ≃0.″25 and 0.″55, reveal (1) an extended, roughly uniform, and weak emission with a size of ≃0.″5; (2) a component elongated approximately along the east-west direction peaking at ≃0.″13 and 0.″17 at both sides of the central star; and (3) two blue- and redshifted compact components peaking around 0.″07 to the NW of the star. We have modeled the emission with a 3D radiation transfer code, finding that the observations cannot be explained only by thermal emission. Several maser clumps and one arc-shaped maser feature arranged from 5 to 20 from the central star, in addition to a thin shell-like maser structure at ≃13 , are required to explain the observations. This maser-emitting set of structures accounts for 75% of the total emission, while the other 25% is produced by thermally excited molecules. About 60% of the maser emission comes from the extended emission, and the rest comes from the set of clumps and the arc. The analysis of a time monitoring of these and other SiS and SiS lines carried out with the IRAM 30 m telescope from 2015 to present suggests that the intensity of some spectral components of the maser emission strongly depends on the stellar pulsation, while other components show a mild variability. This monitoring evidences a significant phase lag of ≃0.2 between the maser and near-IR light curves.

We present recent Atacama Large Millimeter/submillimeter Array (ALMA)-based studies of circumstellar envelopes (CSEs) around Asymptotic Giant Branch (AGB) stars and pre-Planetary Nebulae (pPNe). In only a few years of operation, ALMA is revolutionising the field of AGB-to-PN research by providing unprecedentedly detailed information on the complex nebular architecture (at large but also on small scales down to a few ∼10 AU from the centre), dynamics and chemistry of the outflows/envelopes of low-to-intermediate mass stars in their late stages of the evolution. Here, we focus on continuum and molecular line mapping studies with high angular resolution and sensitivity of some objects that are key to understanding the complex PN-shaping process. In particular, we offer (i) a brief summary of ALMA observations of rotating disks in post-AGB objects and (ii) report on ALMA observations of OH 231.8+4.2 providing the most detailed and accurate description of the global nebular structure and kinematics of this iconic object to date.

[Context] The mechanism behind the shaping of bipolar planetary nebulae is still poorly understood. It is becoming increasingly clear that the main agents must operate at their innermost regions, where a significant equatorial density enhancement should be present and related to the collimation of light and jet launching from the central star preferentially towards the polar directions. Most of the material in this equatorial condensation must be lost during the asymptotic giant branch as stellar wind and later released from the surface of dust grains to the gas phase in molecular form. Accurately tracing the molecule-rich regions of these objects can give valuable insight into the ejection mechanisms themselves., [Aims] We investigate the physical conditions, structure and velocity field of the dense molecular region of the planetary nebula NGC 6302 by means of ALMA band 7 interferometric maps., [Methods] The high spatial resolution of the CO and CO J = 3-2 ALMA data allows for an analysis of the geometry of the ejecta in unprecedented detail. We built a spatio-kinematical model of the molecular region with the software SHAPE and performed detailed non-LTE calculations of excitation and radiative transfer with the shapemol plug-in., [Results] We find that the molecular region consists of a massive ring out of which a system of fragments of lobe walls emerge and enclose the base of the lobes visible in the optical. The general properties of this region are in agreement with previous works, although the much greater spatial resolution of the data allows for a very detailed description. We confirm that the mass of the molecular region is 0.1 M. Additionally, we report a previously undetected component at the nebular equator, an inner, younger ring inclined ~60° with respect to the main ring, showing a characteristic radius of 7.5 × 10 cm, a mass of 2.7 × 10M, and a counterpart in optical images of the nebula. This inner ring has the same kinematical age as the northwest optical lobes, implying it was ejected approximately at the same time, hundreds of years after the ejection of the bulk of the molecular ring-like region. We discuss a sequence of events leading to the formation of the molecular and optical nebulae, and briefly speculate on the origin of this intriguing inner ring.

[Aims] We aim to study the presence of both rotation and expansion in post-AGB nebulae, in particular around IW Car, a binary post-AGB star that was suspected to be surrounded by a Keplerian disk., [Methods] We obtained high-quality ALMA observations of 12CO and 13CO J = 3−2 lines in IW Car. The maps were analyzed by means of a simplified model of CO emission, based on those used for similar objects., [Results] Our observations clearly show the presence of gas components in rotation, in an equatorial disk, and expansion, which shows an hourglass-like structure with a symmetry axis perpendicular to the rotation plane and is probably formed of material extracted from the disk. Our modeling can reproduce the observations and shows moderate uncertainties. The rotation velocity corresponds to a central stellar mass of approximately 1 M⊙. We also derive the total mass of the molecule-rich nebula, found to be of ~4 × 10-3M⊙; the outflow is approximately eight times less massive than the disk. From the kinematical age of the outflow and the mass values derived for both components, we infer a (future) lifetime of the disk of approximately 5000–10 000 yr.

[Context] M 2-9 is a young planetary nebula (PN) that shows the characteristics of its last ejections in unprecedented detail. These last ejections are thought to trigger the post-asymptotic giant branch evolution., [Aims] To assemble an overall picture of how M 2-9 was shaped, we analyzed the characteristics of the different molecular gas components and their relation with the warmer parts of the nebula that are visible in the optical domain., [Methods] CO and CO J = 3-2 line emission maps were obtained with the Atacama Large Millimeter/submillimeter Array with high angular-resolution and sensitivity., [Results] Two equatorial rings are found to host most of the cold molecular gas in M 2-9, as has been described for previous CO J = 2-1 emission observations. In addition, we have detected a double crown-shaped structure that is symmetric with respect to the main nebular axis, which is located 1.5′′ away from both sides of the equatorial plane. Their distribution and kinematics show a very close relationship with the inner molecular ring: Both are part of the same small hourglass structure formed ∼900 yr ago. Two clearly distinct ejections with a remarkable axial symmetry are found to have shaped the molecular gas distribution in M 2-9, in agreement with the ejection processes that were probably responsible for the optical lobes. For the first time, the physical conditions of the different molecular components in M 2-9 are comprehensively analyzed with a radiative transfer model. They are found to follow standard laws, like those obtained in other young PN, with densities and temperatures decreasing with radius and ballistic expansion. A total mass of ∼5 × 10M was derived for the detected molecular component, the larger and older equatorial ring hosting most (∼90%) of this gas.

We present interferometric and single-dish molecular line observations of the interstellar bullet-outflow source IRAS 05506+2414, whose wide-angle bullet spray is similar to the Orion BN/KL explosive outflow and likely arises from an entirely different mechanism than the classical accretion-disk-driven bipolar flows in young stellar objects. The bullet-outflow source is associated with a large pseudo-disk and three molecular outflows - a high-velocity outflow (HVO), a medium-velocity outflow (MVO), and a slow, extended outflow (SEO). The size (mass) of the pseudo-disk is 10,350 au ×6400 au (0.64-0.17 M ); from a model-fit assuming infall and rotation, we derive a central stellar mass of 8-19 M . The HVO (MVO) has an angular size ∼5180 (∼3330) au and a projected outflow velocity of ∼140 km s (∼30 km s). The SEO size (outflow speed) is ∼0.9 pc (∼6 km s). The HVO's axis is aligned with (orthogonal to) that of the SEO (pseudo-disk). The velocity structure of the MVO is unresolved. The scalar momenta in the HVO and SEO are very similar, suggesting that the SEO has resulted from the HVO interacting with ambient-cloud material. The bullet spray shares a common axis with the pseudo-disk and has an age comparable to that of MVO (few hundred years), suggesting that these three structures are intimately linked. We discuss several models for the outflows in IRAS 05506+2414 (including dynamical decay of a stellar cluster, chance encounter of a runaway star with a dense cloud, and close passage of two protostars), and conclude that second-epoch imaging to derive proper motions of the bullets and nearby stars can help to discriminate between them.

[Aims] We aim to study the rotating and expanding gas in the Red Rectangle, which is a well known bipolar nebula surrounding a double stellar system whose primary is a post-asymptotic giant branch (post-AGB) star. We analyze the properties of both components and the relation between them. Rotating disks have been very elusive in post-AGB nebulae, in which gas is almost always found to be in expansion., [Methods] We present new high-quality ALMA observations of this source in C17O J = 6−5 and H13CN J = 4−3 line emission and results from a new reduction of already published 13CO J = 3−2 data. A detailed model fitting of all the molecular line data, also discussing previous maps and single-dish observations of lines of CO, CII, and CI, was performed using a sophisticated code that includes an accurate nonlocal treatment of radiative transfer in 2D (assuming axial symmetry). These observations (of low- and high-opacity lines requiring various degrees of excitation) and the corresponding modeling allowed us to deepen the analysis of the nebular properties. We also stress the uncertainties, particularly in the determination of the boundaries of the CO-rich gas and some properties of the outflow., [Results] We confirm the presence of a rotating equatorial disk and an outflow, which is mainly formed of gas leaving the disk. The mass of the disk is ~0.01 M⊙, and that of the CO-rich outflow is around ten times smaller. High temperatures of ≳100 K are derived for most components. From comparison of the mass values, we roughly estimate the lifetime of the rotating disk, which is found to be of about 10 000 yr. Taking data of a few other post-AGB composite nebulae into account, we find that the lifetimes of disks around post-AGB stars typically range between about 5000 and more than 20 000 yr. The angular momentum of the disk is found to be high, ~9 M⊙ AU km s-1, which is comparable to that of the stellar system at present. Our observations of H13CN show a particularly wide velocity dispersion and indicate that this molecule is only abundant in the inner Keplerian disk, at ≲60 AU from the stellar system. We suggest that HCN is formed in a dense photodissociation region (PDR) due to the UV excess known to be produced by the stellar system, following chemical mechanisms that are well established for interstellar medium PDRs and disks orbiting young stars. We further suggest that this UV excess could lead to an efficient formation and excitation of PAHs and other C-bearing macromolecules, whose emission is very intense in the optical counterpart.

Circumstellar envelopes (CSEs) around asymptotic giant branch (AGB) stars are the main sites of molecular formation. OH 231.8+4.2 is a well studied oxygen-rich CSE around an intermediate-mass evolved star that, in dramatic contrast to most AGB CSEs, displays bipolar molecular outflows accelerated up to ~400 km s-1. OH 231.8+4.2 also presents an exceptional molecular richness probably due to shock-induced chemical processes. We report the first detection in this source of four nitrogen-bearing species, HNCO, HNCS, HC3N, and NO, which have been observed with the IRAM-30 m radiotelescope in a sensitive mm-wavelength survey towards this target. HNCO and HNCS are also first detections in CSEs. The observed line profiles show that the emission arises in the massive (~0.6 M⊙) central component of the envelope, expanding with low velocities of Vexp~ 15–30 km s-1, and at the base of the fast lobes. The NO profiles (with FWHM~ 40–50 km s-1) are broader than those of HNCO, HNCS, and HC3N and, most importantly, broader than the line profiles of 13CO, which is a good mass tracer. This indicates that the NO abundance is enhanced in the fast lobes relative to the slow, central parts. From LTE and non-LTE excitation analysis, we estimate beam-average rotational temperatures of Trot~ 15–30 K (and, maybe, up to ~55 K for HC3N) and fractional abundances relative to H2 of X(HNCO) ~ [0.8–1] × 10-7, X(HNCS) ~ [0.9–1] × 10-8, X(HC3N) ~ [5–7] × 10-9, and X(NO) ~ [1–2] × 10-6. NO is, therefore, amongst the most abundant N-bearing species in OH 231.8+4.2. We performed thermodynamical chemical equilibrium and chemical kinetics models to investigate the formation of these N-bearing species in OH 231.8+4.2. The model underestimates the observed abundances for HNCO, HNCS, and HC3N by several orders of magnitude, which indicates that these molecules can hardly be products of standard UV-photon and/or cosmic-ray induced chemistry in OH 231.8+4.2 and that other processes (e.g. shocks) play a major

OH 231.8+4.2, a bipolar outflow around a Mira-type variable star, displays a unique molecular richness amongst circumstellar envelopes (CSEs) around O-rich AGB and post-AGB stars. We report line observations of the HCO+ and H13CO+ molecular ions and the first detection of SO+, N2H+, and (tentatively) H3O+ in this source. SO+ and H3O+ have not been detected before in CSEs around evolved stars. These data have been obtained as part of a full mm-wave and far-IR spectral line survey carried out with the IRAM 30 m radio telescope and with Herschel/HIFI. Except for H3O+, all the molecular ions detected in this work display emission lines with broad profiles (FWHM ~ 50−90 km s-1), which indicates that these ions are abundant in the fast bipolar outflow of OH 231.8. The narrow profile (FWHM ~ 14 km s-1) and high critical densities (>106 cm-3) of the H3O+ transitions observed are consistent with this ion arising from denser, inner (and presumably warmer) layers of the fossil remnant of the slow AGB CSE at the core of the nebula. From rotational diagram analysis, we deduce excitation temperatures of Tex~ 10−20 K for all ions except for H3O+, which is most consistent with Tex≈ 100 K. Although uncertain, the higher excitation temperature suspected for H3O+ is similar to that recently found for H2O and a few other molecules, which selectively trace a previously unidentified, warm nebular component. The column densities of the molecular ions reported here are in the range Ntot≈ [1−8] × 1013 cm-2, leading to beam-averaged fractional abundances relative to H2 of X(HCO+) ≈ 10-8, X(H13CO+) ≈2 × 10-9, X(SO+) ≈4 × 10-9, X(N2H+) ≈2 × 10-9, and X(H3O+) ≈7 × 10-9 cm-2. We have performed chemical kinetics models to investigate the formation of these ions in OH 231.8 as the result of standard gas phase reactions initiated by cosmic-ray and UV-photon ionization. The model predicts that HCO+, SO+, and H3O+ can form with abundances comparable to the observed average values in the external laye