Transitions Of HCO Research Articles

The Cygnus Allscale Survey of Chemistry and Dynamical Environments: CASCADE

Context.Molecular outflows are believed to be a key ingredient in the process of star formation. The molecular outflow associated with DR21 Main in Cygnus-X is one of the most extreme molecular outflows in the Milky Way in terms of mass and size. The outflow is suggested to belong to a rare class of explosive outflows formed by the disintegration of protostellar systems.Aims.We aim to explore the morphology, kinematics, and energetics of the DR21 Main outflow, and to compare those properties to confirmed explosive outflows in order to unravel the underlying driving mechanism behind DR21.Methods.We studied line and continuum emission at a wavelength of 3.6 mm with IRAM 30 m and NOEMA telescopes as part of the Cygnus Allscale Survey of Chemistry and Dynamical Environments (CASCADE) program. The spectra include (J= 1−0) transitions of HCO+, HCN, HNC, N2H+, H2CO, and CCH, which trace different temperature and density regimes of the outflowing gas at high velocity resolution (~0.8 km s−1). The map encompasses the entire DR21 Main outflow and covers all spatial scales down to a resolution of 3″ (~0.02 pc).Results.Integrated intensity maps of the HCO+emission reveal a strongly collimated bipolar outflow with significant overlap of the blueshifted and redshifted emission. The opening angles of both outflow lobes decrease with velocity, from ~80 to 20° for the velocity range from 5 to 45 km s−1relative to the source velocity. No evidence is found for the presence of elongated, “filament-like” structures expected in explosive outflows. N2H+emission near the western outflow lobe reveals the presence of a dense molecular structure, which appears to be interacting with the DR21 Main outflow.Conclusions.The overall morphology as well as the detailed kinematics of the DR21 Main outflow are more consistent with a typical bipolar outflow than with an explosive counterpart.

CMZoom III: Spectral line data release

ABSTRACT We present an overview and data release of the spectral line component of the SMA Large Program, CMZoom. CMZoom observed 12CO (2–1), 13CO (2–1), and C18O (2–1), three transitions of H2CO, several transitions of CH3OH, two transitions of OCS, and single transitions of SiO and SO within gas above a column density of N(H2) ≥ 1023 cm−2 in the Central Molecular Zone (CMZ; inner few hundred pc of the Galaxy). We extract spectra from all compact 1.3 mm CMZoom continuum sources and fit line profiles to the spectra. We use the fit results from the H2CO 3(0, 3)–2(0, 2) transition to determine the source kinematic properties. We find ∼90 per cent of the total mass of CMZoom sources have reliable kinematics. Only four compact continuum sources are formally self-gravitating. The remainder are consistent with being in hydrostatic equilibrium assuming that they are confined by the high external pressure in the CMZ. We find only two convincing proto-stellar outflows, ruling out a previously undetected population of very massive, actively accreting YSOs with strong outflows. Finally, despite having sufficient sensitivity and resolution to detect high-velocity compact clouds (HVCCs), which have been claimed as evidence for intermediate mass black holes interacting with molecular gas clouds, we find no such objects across the large survey area.

Nobeyama Survey of Inward Motions toward Cores in Orion Identified by SCUBA-2

In this study, 36 cores (30 starless and six protostellar) identified in Orion were surveyed to search for inward motions. We used the Nobeyama 45 m radio telescope, and mapped the cores in the J = 1 → 0 transitions of HCO+, H13CO+, N2H+, HNC, and HN13C. The asymmetry parameter δV, which was the ratio of the difference between the HCO+ and H13CO+ peak velocities to the H13CO+ line width, was biased toward negative values, suggesting that inward motions were more dominant than outward motions. Three starless cores (10% of all starless cores surveyed) were identified as cores with blue-skewed line profiles (asymmetric profiles with more intense blueshifted emission), and another two starless cores (7%) were identified as candidate blue-skewed line profiles. The peak velocity difference between HCO+ and H13CO+ of them was up to 0.9 km s−1, suggesting that some inward motions exceeded the speed of sound for the quiescent gas (∼10–17 K). The mean of δV of the five aforementioned starless cores was derived to be −0.5 ± 0.3. One core, G211.16−19.33North3, observed using the Atacama Compact Array of the Atacama Large Millimeter/submillimeter Array in DCO+ J = 3 → 2 exhibited blue-skewed features. Velocity offset in the blue-skewed line profile with a dip in the DCO+ J = 3 → 2 line was larger (∼0.5 km s−1) than that in HCO+ J = 1 → 0 (∼0.2 km s−1), which may represent gravitational acceleration of inward motions. It seems that this core is at the last stage in the starless phase, judging from the chemical evolution factor version 2.0 (CEF2.0).

Star Formation Triggered by the Expanding Bubble S111

Abstract This paper investigates the impact of radiative and mechanical feedback from O-type stars on their parent molecular clouds and the triggering of formation of a future generation of stars. We study the infrared bubble S111 created by the embedded massive stellar cluster G316.80–0.05. A significant fraction of gas in shells created due to the compression of the ambient medium by expanding bubbles is photodissociated by the stellar radiation. The kinematics of the shells are thus best studied using spectroscopic observations of singly ionized carbon, the most dominant species. We have used velocity-resolved maps of the 2P3/2 → 2P1/2 transition of [C ii] at 158 μm, the J = 2–1 transition of 13CO and C18O, and the J = 1–0 transition of HCO+ to study the rim of the bubble S111 that partly coincides with the southern part of the infrared dark ridge G316.75. The [C ii] spectra conclusively show evidence of a shell expanding with a moderate velocity of ∼7 km s−1, which amounts to a kinetic energy that is ∼0.5–40 times the thermal energy of the H ii region. The pressure causing the expansion of the H ii region arises mainly from hydrogen ionization and dust-processed radiation. Among the far-infrared sources located in compressed shells, we find the core G316.7799–0.0942 to show broad spectral features consistent with outflow activity and conclude that it is a site of active star formation. Based on the age of the H ii region we conclude that this expanding H ii region is responsible for triggering the current star formation activity in the region.

Chemically tracing the water snowline in protoplanetary disks with HCO+

Context. The formation of planets is expected to be enhanced around snowlines in protoplanetary disks, in particular around the water snowline. Moreover, freeze-out of abundant volatile species in disks alters the chemical composition of the planet-forming material. However, the close proximity of the water snowline to the host star combined with the difficulty of observing water from Earth makes a direct detection of the water snowline in protoplanetary disks challenging. HCO+ is a promising alternative tracer of the water snowline. The destruction of HCO+ is dominated by gas-phase water, leading to an enhancement in the HCO+ abundance once water is frozen out. Aims. Following earlier observed correlations between water and H13CO+ emission in a protostellar envelope, the aim of this research is to investigate the validity of HCO+ and the optically thin isotopologue H13CO+ as tracers of the water snowline in protoplanetary disks and the required sensitivity and resolution to observationally confirm this. Methods. A typical Herbig Ae disk structure is assumed, and its temperature structure is modelled with the thermochemical code DALI. Two small chemical networks are then used and compared to predict the HCO+ abundance in the disk: one without water and one including water. Subsequently, the corresponding emission profiles are modelled for the J = 2−1 transition of H13CO+ and HCO+, which provides the best balance between brightness and the optical depth effects of the continuum emission and is less affected by blending with complex molecules. Models are then compared with archival ALMA data. Results. The HCO+ abundance jumps by two orders of magnitude over a radial range of 2 AU outside the water snowline, which in our model is located at 4.5 AU. We find that the emission of H13CO+ and HCO+ is ring-shaped due to three effects: destruction of HCO+ by gas-phase water, continuum optical depth, and molecular excitation effects. Comparing the radial emission profiles for J = 2−1 convolved with a 0′′.05 beam reveals that the presence of gas-phase water causes an additional drop of only ~13 and 24% in the centre of the disk for H13CO+ and HCO+, respectively. For the much more luminous outbursting source V883 Ori, our models predict that the effects of dust and molecular excitation do not limit HCO+ as a snowline tracer if the snowline is located at radii larger than ~40 AU. Our analysis of recent archival ALMA band 6 observations of the J = 3−2 transition of HCO+ is consistent with the water snowline being located around 100 AU, further out than was previously estimated from an intensity break in the continuum emission. Conclusions. The HCO+ abundance drops steeply around the water snowline, when water desorbs in the inner disk, but continuum optical depth and molecular excitation effects conceal the drop in HCO+ emission due to the water snowline. Therefore, locating the water snowline with HCO+ observations in disks around Herbig Ae stars is very difficult, but it is possible for disks around outbursting stars such as V883 Ori, where the snowline has moved outwards.

The Infall Motion in the Low-mass Protostellar Binary NGC 1333 IRAS 4A1/4A2

Abstract We report ALMA observations of NGC 1333 IRAS 4A, a young low-mass protostellar binary, whose components are referred to as 4A1 and 4A2. With multiple H2CO transitions and HNC (4−3) observed at a resolution of 0.″25 (∼70 au), we investigate the gas kinematics of 4A1 and 4A2. Our results show that on the large angular scale (∼10″), 4A1 and 4A2 each display a well-collimated outflow along the N–S direction, and an S-shaped morphology is discerned in the outflow powered by 4A2. On the small scale (∼0.″3), 4A1 and 4A2 exhibit distinct spectral features toward the continuum centroid, with 4A1 showing simple symmetric profiles predominantly in absorption and 4A2 demonstrating rather complicated profiles in emission as well as in absorption. Based on radiative transfer modeling exercises, we find that the physical parameters inferred from earlier low-resolution observations cannot be directly extrapolated down to the inner region of 4A1. Possible reasons for the discrepancies between the observed and modeled profiles are discussed. We constrain the mass infall rate in 4A1 to be at most around 3 × 10−5 M ⊙ yr−1 at the layer of 75 au. For the kinematics of the inner envelope of 4A2, the absorbing dips in the H2CO spectra are skewed toward the redshifted side and likely signatures of inward motion. These absorbing dips are relatively narrow. This is, like the case for 4A1, significantly slower than the anticipated inflow speed. We estimate a mass infall rate of (3.1–6.2) × 10−5 M ☉ yr−1 at the layer of 100 au in 4A2.

Revealing the chemical structure of the Class I disc Oph-IRS 67

Context. Recent results suggest that the first steps towards planet formation may be already taking place in protoplanetary discs during the first 100 000 yr after stars form. It is therefore crucial to unravel the physical and chemical structures of such discs in their earliest stages while they are still embedded in their natal envelopes and compare them with more evolved systems. Aims. The purpose of this paper is to explore the structure of a line-rich Class I protobinary source, Oph-IRS 67, and analyse the differences and similarities with Class 0 and Class II sources. Methods. We present a systematic molecular line study of IRS 67 with the Submillimeter Array (SMA) on 1–2′′ (150–300 AU) scales. The wide instantaneous band-width of the SMA observations (~30 GHz) provide detections of a range of molecular transitions that trace different physics, such as CO isotopologues, sulphur-bearing species, deuterated species, and carbon-chain molecules. Results. We see significant differences between different groups of species. For example, the CO isotopologues and sulphur-bearing species show a rotational profile and are tracing the larger-scale circumbinary disc structure, while CN, DCN, and carbon-chain molecules peak at the southern edge of the disc at blue-shifted velocities. In addition, the cold gas tracer DCO+ is seen beyond the extent of the circumbinary disc. Conclusions. The detected molecular transitions can be grouped into three main components: cold regions far from the system, the circumbinary disc, and a UV-irradiated region likely associated with the surface layers of the disc that are reached by the UV radiation from the sources. The different components are consistent with the temperature structure derived from the ratio of two H2CO transitions, that is, warm temperatures are seen towards the outflow direction, lukewarm temperatures are associated with the UV-radiated region, and cold temperatures are related with the circumbinary disc structure. The chemistry towards IRS 67 shares similarities with both Class 0 and Class II sources, possibly due to the high gas column density and the strong UV radiation arising from the binary system. IRS 67 is, therefore, highlighting the intermediate chemistry between deeply embedded sources and T-Tauri discs.

Opening the Treasure Chest in Carina

Pillars and globules are the best examples of the impact of the radiation and wind from massive stars on the surrounding interstellar medium. We mapped the G287.84-0.82 cometary globule (with the Treasure Chest cluster embedded in it) in the South Pillars region of Carina (i) in [C II], 63 μm [O I], and CO(11–10) using the heterodyne receiver array upGREAT on SOFIA and (ii) in J = 2–1 transitions of CO, 13CO, C18O, and J = 3–2 transitions of H2CO using the APEX telescope in Chile. We used these data to probe the morphology, kinematics, and physical conditions of the molecular gas and the photon-dominated regions (PDRs) in G287.84-0.82. The velocity-resolved observations of [C II] and [O I] suggest that the overall structure of the pillar (with red-shifted photoevaporating tails) is consistent with the effect of FUV radiation and winds from η Car and O stars in Trumpler 16. The gas in the head of the pillar is strongly influenced by the embedded cluster, whose brightest member is an O9.5 V star, CPD −59°2661. The emission of the [C II] and [O I] lines peak at a position close to the embedded star, while all the other tracers peak at another position lying to the northeast consistent with gas being compressed by the expanding PDR created by the embedded cluster. The molecular gas inside the globule was probed with the J = 2–1 transitions of CO and isotopologs as well as H2CO, and analyzed using a non-local thermodynamic equilibrium model (escape-probability approach), while we used PDR models to derive the physical conditions of the PDR. We identify at least two PDR gas components; the diffuse part (~ 104 cm−3) is traced by [C II], while the dense (n ~ 2–8 × 105 cm−3) part is traced by [C II], [O I], and CO(11–10). Using the F = 2–1 transition of [13C II] detected at 50 positions in the region, we derived optical depths (0.9–5), excitation temperatures (80–255 K) of [C II], and N(C+) of 0.3–1 × 1019 cm−2. The total mass of the globule is ~1000 M⊙, about half of which is traced by [C II]. The dense PDR gas has a thermal pressure of 107–108 K cm−3, which is similar to the values observed in other regions.

Herschel/HIFI spectral line survey of the Orion Bar

Photon Dominated Regions (PDRs) are interfaces between the mainly ionized and mainly molecular material around young massive stars. Analysis of the physical and chemical structure of such regions traces the impact of far-ultraviolet radiation of young massive stars on their environment. We present results on the physical and chemical structure of the prototypical high UV-illumination edge-on Orion Bar PDR from an unbiased spectral line survey with a wide spectral coverage. A spectral scan from 480-1250 GHz and 1410-1910 GHz at 1.1 MHz resolution was obtained by the HIFI instrument onboard the Herschel Space Observatory. For molecules with multiple transitions we used rotational diagrams to obtain excitation temperatures and column densities. For species with a single detected transition we used an optically thin LTE approximation. In case of species with available collisional rates, we also performed a non-LTE analysis to obtain kinetic temperatures, H2 volume densities, and column densities. About 120 lines corresponding to 29 molecules (including isotopologues) have been detected in the Herschel/HIFI line survey, including 11 transitions of CO, 7 transitions of 13CO, 6 transitions of C18O, 10 transitions of H2CO, and 6 transitions of H2O. Most species trace kinetic temperatures in the range between 100 and 150 K and H2 volume densities in the range between 10^5 and 10^6 cm^-3. The species with temperatures and / or densities outside of this range include the H2CO transitions tracing a very high temperature (315 K) and density (1.4x10^6 cm^-3) component and SO corresponding to the lowest temperature (56 K) measured as a part of this line survey. The observed lines/species reveal a range of physical conditions (gas density /temperature) involving structures at high density / high pressure, obsoleting the traditional 'clump / interclump' picture of the Orion Bar.

Using an internal coordinate Gaussian basis and a space-fixed Cartesian coordinate kinetic energy operator to compute a vibrational spectrum with rectangular collocation.

We demonstrate that it is possible to use basis functions that depend on curvilinear internal coordinates to compute vibrational energy levels without deriving a kinetic energy operator (KEO) and without numerically computing coefficients of a KEO. This is done by using a space-fixed KEO and computing KEO matrix elements numerically. Whenever one has an excellent basis, more accurate solutions to the Schrödinger equation can be obtained by computing the KEO, potential, and overlap matrix elements numerically. Using a Gaussian basis and bond coordinates, we compute vibrational energy levels of formaldehyde. We show, for the first time, that it is possible with a Gaussian basis to solve a six-dimensional vibrational Schrödinger equation. For the zero-point energy (ZPE) and the lowest 50 vibrational transitions of H2CO, we obtain a mean absolute error of less than 1 cm-1; with 200 000 collocation points and 40 000 basis functions, most errors are less than 0.4 cm-1.

DENSE GAS IN MOLECULAR CORES ASSOCIATED WITH PLANCK GALACTIC COLD CLUMPS

ABSTRACT We present the first survey of dense gas toward Planck Galactic Cold Clumps (PGCCs). Observations in the J = 1–0 transitions of HCO+ and HCN toward 621 molecular cores associated with PGCCs were performed using the Purple Mountain Observatory’s 13.7 m telescope. Among them, 250 sources were detected, including 230 cores detected in HCO+ and 158 in HCN. Spectra of the J = 1–0 transitions from 12CO, 13CO, and C18O at the centers of the 250 cores were extracted from previous mapping observations to construct a multi-line data set. The significantly low detection rate of asymmetric double-peaked profiles, together with the good consistency among central velocities of CO, HCO+, and HCN spectra, suggests that the CO-selected Planck cores are more quiescent than classical star-forming regions. The small difference between line widths of C18O and HCN indicates that the inner regions of CO-selected Planck cores are no more turbulent than the exterior. The velocity-integrated intensities and abundances of HCO+ are positively correlated with those of HCN, suggesting that these two species are well coupled and chemically connected. The detected abundances of both HCO+ and HCN are significantly lower than values in other low- to high-mass star-forming regions. The low abundances may be due to beam dilution. On the basis of an inspection of the parameters given in the PGCC catalog, we suggest that there may be about 1000 PGCC objects that have a sufficient reservoir of dense gas to form stars.

Molecular Line Emission Towards High-Mass Clumps: The MALT90 Catalogue

AbstractThe Millimetre Astronomy Legacy Team 90 GHz survey aims to characterise the physical and chemical evolution of high-mass clumps. Recently completed, it mapped 90 GHz line emission towards 3 246 high-mass clumps identified from the ATLASGAL 870 μm Galactic plane survey. By utilising the broad frequency coverage of the Mopra telescope’s spectrometer, maps in 16 different emission lines were simultaneously obtained. Here, we describe the first catalogue of the detected line emission, generated by Gaussian profile fitting to spectra extracted towards each clumps’ 870 μm dust continuum peak. Synthetic spectra show that the catalogue has a completeness of > 95%, a probability of a false-positive detection of < 0.3%, and a relative uncertainty in the measured quantities of < 20% over the range of detection criteria. The detection rates are highest for the (1–0) transitions of HCO+, HNC, N2H+, and HCN (~77–89%). Almost all clumps (~95%) are detected in at least one of the molecular transitions, just over half of the clumps (~53%) are detected in four or more of the transitions, while only one clump is detected in 13 transitions. We find several striking trends in the ensemble of properties for the different molecular transitions when plotted as a function of the clumps’ evolutionary state as estimated from Spitzer mid-IR images, including (1) HNC is relatively brighter in colder, less evolved clumps than those that show active star formation, (2) N2H+ is relatively brighter in the earlier stages, (3) that the observed optical depth decreases as the clumps evolve, and (4) the optically thickest HCO+ emission shows a ‘blue-red asymmetry’ indicating overall collapse that monotonically decreases as the clumps evolve. This catalogue represents the largest compiled database of line emission towards high-mass clumps and is a valuable data set for detailed studies of these objects.

Densitometry and Thermometry of Starburst Galaxies

With a goal toward deriving the physical conditions in external galaxies, we have conducted a survey and subsequent high spatial resolution imaging of formaldehyde (H2CO) and ammonia (NH3) emission and absorption in a sample of starburst galaxies. In this article we present the results from a subset of this survey which focuses on high spatial resolution measurements of volume density-and kinetic temperature-sensitive transitions of the H2CO molecule. The volume density structure toward the nuclear region of NGC 253 has been derived from ? ≠4 arcsec NRAO Very Large Array (VLA) measurements of the 110-111 and 211-212 K-doublet transitions of H2CO. The kinetic temperature structure toward NGC 253 and NGC 4945 has been derived from ? ≠0.5-1.0 arcsec measurements of the H2CO 3K-1K+1-2K-1K+1 (near 218 GHz) and 5K-1K+1-4K-1K+1 (near 365 GHz) transitions acquired using the Atacama Large Millimeter/submillimeter Array (ALMA). These measurements have allowed us to characterize the dense gas and kinetic temperature structure within these star forming galaxies, which is a first step toward associating dense star-forming gas and the heating processes at work within galaxies.

The comparison of H2CO (110–111), C18O (1–0) and the continuum towards molecular clouds

We present large scale observations of C18O (1–0) towards four massive star forming regions: MON R2, S156, DR17/L906 and M17/M18. The transitions of H2CO (110–111), C18O (1–0) and the 6 cm continuum are compared in these four regions. Our analysis of the observations and the results of the Non—LTE model shows that the brightness temperature of the formaldehyde absorption line is strongest in a background continuum temperature range of about 3 – 8 K. The excitation of the H2CO absorption line is affected by strong background continuum emission. From a comparison of H2CO and C18O maps, we found that the extent of H2CO absorption is broader than that of C18O emission in the four regions. Except for the DR17 region, the maximum in H2CO absorption is located at the same position as the C18O peak. A good correlation between intensities and widths of H2CO absorption and C18O emission lines indicates that the H2CO absorption line can trace the dense, warm regions of a molecular cloud. We find that N(H2CO) is well correlated with N(C18O) in the four regions and that the average ratio of column densities is 〈N(H2CO)/N(C18O)〉 ∼ 0.03.

Complex organic molecules in protoplanetary disks

(Abridged) Protoplanetary disks are vital objects in star and planet formation, possessing all the material which may form a planetary system orbiting the new star. We investigate the synthesis of complex organic molecules (COMs) in disks to constrain the achievable chemical complexity and predict species and transitions which may be observable with ALMA. We have coupled a 2D model of a protoplanetary disk around a T Tauri star with a gas-grain chemical network including COMs. We compare compare synthesised line intensities and calculated column densities with observations and determine those COMs which may be observable in future. COMs are efficiently formed in the disk midplane via grain-surface chemical reactions, reaching peak grain-surface fractional abundances 1e-6 - 1e-4 that of the H nuclei number density. COMs formed on grain surfaces are returned to the gas phase via non-thermal desorption; however, gas-phase species reach lower fractional abundances than their grain-surface equivalents, 1e-12 - 1e-7. Including the irradiation of grain mantle material helps build further complexity in the ice through the replenishment of grain-surface radicals which take part in further grain-surface reactions. There is reasonable agreement with several line transitions of H2CO observed towards several T Tauri star-disk systems. The synthesised line intensities for CH3OH are consistent with upper limits determined towards all sources. Our models suggest CH3OH should be readily observable in nearby protoplanetary disks with ALMA; however, detection of more complex species may prove challenging. Our grain-surface abundances are consistent with those derived from cometary comae observations providing additional evidence for the hypothesis that comets (and other planetesimals) formed via the coagulation of icy grains in the Sun's natal disk.

Adsorption of formaldehyde molecule on Stone–Wales defected graphene doped with Cr, Mn, and Co: A theoretical study

Adsorption of H2CO molecule on Cr, Mn and Co doped Stone–Wales defected graphene were theoretically studied using density functional theory (DFT) method. It was found that H2CO molecule had no considerable interaction with perfect or SW-defected graphene, but the presence of Stone–Wales defect in graphene enhanced the adsorption of H2CO, which exhibited larger binding energy and smaller bond distance. Chemisorptions were observed on the transition metal (TM) atoms (Cr, Mn and Co) doped structures. Compared with TM-doped perfect graphene, the binding energy of H2CO molecule on TM-doped defective graphene can be enlarged by the introduction of SW-defect. The density of states (DOS) showed that the contribution of hybridization between O atom of H2CO molecule and transition metal atom is mainly from the p or d orbitals. Furthermore, adsorption of H2CO affected the electronic conductance of the Cr and Mn doped defective graphene, which can be seen signal of gas sensor. It is expected that the results could provide useful information for the design of H2CO sensing devices.

THE HELIX NEBULA VIEWED IN HCO+: LARGE-SCALE MAPPING OF THEJ= 1 → 0 TRANSITION

The J = 1 → 0 transition of HCO+ at 89 GHz has been mapped across the Helix Nebula (NGC 7293) with 70'' spatial resolution (1.68 km s–1 velocity resolution) using the Arizona Radio Observatory 12 m telescope. This work is the first large-scale mapping project of a dense gas tracer (n(H2) ~ 105 cm–3) in old planetary nebulae. Observations of over 200 positions encompassing the classical optical image were conducted with a 3σ noise level of ~20 mK. HCO+ was detected at most positions, often exhibiting multiple velocity components indicative of complex kinematic structures in dense gas. The HCO+ spectra suggest that the Helix is composed of a bipolar, barrel-like structure with red- and blue-shifted halves, symmetric with respect to the central star and oriented ~10° east from the line of sight. A second bipolar, higher velocity outflow exists as well, situated along the direction of the Helix plumes. The column density of HCO+ across the Helix is N tot ~ 1.5 × 1010-5.0 × 1011 cm–2, with an average value N ave ~ 1 × 1011 cm–2, corresponding to an abundance, relative to H2, of f ~ 1.4 × 10–8. This value is similar to that observed in young PN, and contradicts chemical models, which predict that the abundance of HCO+ decreases with nebular age. This study indicates that polyatomic molecules readily survive the ultraviolet field of the central white dwarf, and can be useful in tracing nebular morphology in the very late stages of stellar evolution.

Analytic Morse/long-range potential energy surfaces and predicted infrared spectra for CO–H2 dimer and frequency shifts of CO in (para-H2)N N = 1–20 clusters

A five-dimensional ab initio potential energy surface (PES) for CO-H2 that explicitly incorporates dependence on the stretch coordinate of the CO monomer has been calculated. Analytic four-dimensional PESs are obtained by least-squares fitting vibrationally averaged interaction energies for vCO = 0 and 1 to the Morse/long-range potential function form. These fits to 30,206 points have root-mean-square (RMS) deviations of 0.087 and 0.082 cm(-1), and require only 196 parameters. The resulting vibrationally averaged PESs provide good representations of the experimental infrared data: for infrared transitions of para H2-CO and ortho H2-CO, the RMS discrepancies are only 0.007 and 0.023 cm(-1), which are almost in the same accuracy as those values of 0.010 and 0.018 cm(-1) obtained from full six-dimensional ab initio PESs of V12 [P. Jankowski, A. R. W. McKellar, and K. Szalewicz, Science 336, 1147 (2012)]. The calculated infrared band origin shift associated with the fundamental of CO is -0.179 cm(-1) for para H2-CO, which is the same value as that extrapolated experimental value, and slightly better than the value of -0.176 cm(-1) obtained from V12 PESs. With these potentials, the path integral Monte Carlo algorithm and a first order perturbation theory estimate are used to simulate the CO vibrational band origin frequency shifts of CO in (para H2)N-CO clusters for N = 1-20. The predicted vibrational frequency shifts are in excellent agreement with available experimental observations. Comparisons are also made between these model potentials.

FORMALDEHYDE DENSITOMETRY OF STARBURST GALAXIES: DENSITY-INDEPENDENT GLOBAL STAR FORMATION

Accurate techniques which allow for the derivation of the spatial density in star formation regions are rare. A technique which has found application for the derivation of spatial densities in Galactic star formation regions utilizes the density-sensitive properties of the K-doublet transitions of formaldehyde (H2CO). In this paper, we present an extension of our survey of the formaldehyde 1(10)-1(11) (lambda = 6.2 cm) and 2(11)-2(12) (lambda = 2.1 cm) K-doublet transitions of H2CO in a sample of 56 starburst systems (Mangum etal. 2008). We have extended the number of galaxies in which both transitions have been detected from 5 to 13. We have improved our spatial density measurements by incorporating kinetic temperatures based upon NH3 measurements of 11 of the galaxies with a total of 14 velocity components in our sample (Mangum etal. 2013). Our spatial density measurements lie in a relatively narrow range of from 10^(4.5) to 10^(5.5) cm^(-3). This implies that the Schmidt-Kennicutt relation between L_(IR) and M_(dense): (1) Is an indication of the dense gas mass reservoir available to form stars, and (2) Is not directly dependent upon a higher average density driving the star formation process in the most luminous starburst galaxies. We have also used our H2CO measurements to derive two separate measures of the dense gas mass which are generally smaller, in many cases by a factor of 10^2-10^3, than those derived using HCN. This disparity suggests that H2CO traces a denser, more compact, component of the giant molecular clouds in our starburst galaxy sample. We also report measurements of the rotationally-excited lambda = 6.3 cm 2P_(1/2) J=1/2 state of OH and the H111alpha radio recombination line taken concurrently with our H2CO 1(10)-1(11) measurements.

CHEMICAL COMPLEXITY IN THE HELIX NEBULA: MULTI-LINE OBSERVATIONS OF H2CO, HCO+, AND CO

Observations of CO, HCO+, and H2CO have been carried out at nine positions across the Helix Nebula (NGC 7293) using the Submillimeter Telescope and the 12 m antenna of the Arizona Radio Observatory. Measurements of the J = 1 → 0, 2 → 1, and 3 →2 transitions of CO, two transitions of HCO+ (J = 1 → 0 and 3 →2), and five lines of H2CO (J Ka, Kc = 10, 1 → 00, 0, 21, 2 → 11, 1, 20, 2 → 10, 1, 21, 1 → 11, 0, and 30, 3 →20, 2) were conducted in the 0.8, 1, 2, and 3 mm bands toward this highly evolved planetary nebula. HCO+ and H2CO were detected at all positions, along with three transitions of CO. From a radiative transfer analysis, the kinetic temperature was found to be T K ~ 15-40 K across the Helix with a gas density of n(H2) ~ 0.1-5 × 105 cm–3. The warmer gas appears to be closer to the central star, but high density material is distributed throughout the nebula. For CO, the column density was found to be N tot ~ 0.25-4.5 × 1015 cm–2, with a fractional abundance of f (CO/H2) ~ 0.3-6 × 10–4. Column densities for HCO+ and H2CO were determined to be N tot ~ 0.2-5.5 × 1011 cm–2 and 0.2-1.6 × 1012 cm–2, respectively, with fractional abundances of f (HCO+/H2) ~ 0.3-7.3 × 10–8 and f (H2CO/H2) ~ 0.3-2.1 × 10–7—several orders of magnitude higher than predicted by chemical models. Polyatomic molecules in the Helix appear to be well-protected from photodissociation and may actually seed the diffuse interstellar medium.