CO Abundance Ratio Research Articles

Cosmic-ray-driven enhancement of the C0/CO abundance ratio in W 51 C

Abstract We examine spatial variations of the C0$/$CO abundance ratio (XC/CO) in the vicinity of the γ-ray supernova remnant W 51 C, based on [C i] (3P1–3P0), 12CO(1–0), and 13CO(1–0) observations with the ASTE and Nobeyama 45 m telescopes. We find that XC/CO varies in a range of 0.02–0.16 (0.05 in median) inside the molecular clouds of AV > 100 mag, where photodissociation of CO by the interstellar UV is negligible. Furthermore, XC/CO is locally enhanced by a factor of up to four near the W 51 C center, depending on the projected distance from the W 51 C center. In high-AV molecular clouds, XC/CO is determined by the ratio of the cosmic-ray (CR) ionization rate to the H2 density, and we find no clear spatial variation of the H2 density against the projected distance. Hence, the high CR ionization rate may locally enhance XC/CO near the W 51 C center. We also find that the observed spatial extent of the enhanced XC/CO (∼17 pc) is consistent with the diffusion distance of CRs with an energy of 100 MeV. This fact suggests that the low-energy CRs accelerated in W 51 C enhance XC/CO. The CR ionization rate in the XC/CO-enhanced cloud is estimated to be 3 × 10−16 s−1 on the basis of time-dependent photodissociation region simulations of XC/CO, the value of which is 30 times higher than that in the standard Galactic environment. These results demonstrate that [C i] is a powerful probe to investigate the interaction between CRs and the interstellar medium for a wide area in the vicinity of supernova remnants.

Spatially-resolved relation between [C i] 3P1–3P0 and 12CO (1–0) in Arp 220

Abstract We present $\sim {0.^{\prime \prime }3}$ (114 pc) resolution maps of [C i] 3P1–3P0 (hereafter [C i] (1–0)) and 12CO (1–0) obtained toward Arp 220 with the Atacama Large Millimeter/submillimeter Array. The overall distribution of the [C i] (1–0) emission is consistent with the CO (1–0). While the [C i] (1–0) and CO (1–0) luminosities of the system follow the empirical linear relation for the unresolved ULIRG sample, we find a sublinear relation between [C i] (1–0) and CO (1–0) using the spatially-resolved data. We measure the [C i] (1–0)$/$CO (1–0) luminosity ratio per pixel in star-forming environments of Arp 220 and investigate its dependence on the CO (3–2)$/$CO (1–0) ratio (RCO). On average, the [C i] (1–0)$/$CO (1–0) luminosity ratio is almost constant up to RCO ≃ 1 and then increases with RCO. According to the radiative transfer analysis, a high C i$/$CO abundance ratio is required in regions with high [C i] (1–0)$/$CO (1–0) luminosity ratios and RCO > 1, suggesting that the C i$/$CO abundance ratio varies at ∼100 pc scale in Arp 220. The [C i] (1–0)$/$CO (1–0) luminosity ratio depends on multiple factors and may not be straightforward to interpret. We also find the high-velocity components traced by [C i] (1–0) in the western nucleus, likely associated with the molecular outflow. The [C i] (1–0)$/$CO (1–0) luminosity ratio in the putative outflow is 0.87 ± 0.28, which is four times higher than the average ratio of Arp 220. While there is a possibility that the [C i] (1–0) and CO (1–0) emission traces different components, we suggest that the high line ratios are likely to be because of elevated C i$/$CO abundance ratios based on our radiative transfer analysis. A C i-rich and CO-poor gas phase in outflows could be caused by the irradiation of the cosmic rays, the shock heating, and the intense radiation field.

The extended molecular envelope of the asymptotic giant branch star π1 Gruis as seen by ALMA

The S-type asymptotic giant branch (AGB) star $\pi^{1}$ Gruis has a known companion at a separation of $\approx$400 AU. The envelope structure, including an equatorial torus and a fast bipolar outflow, is rarely seen in the AGB phase and is particularly unexpected in such a wide binary system. Therefore a second, closer companion has been suggested, but the evidence is not conclusive. The new ALMA $^{12}$CO and $^{13}$CO $J$=3-2 data, together with previously published $^{12}$CO $J$=2-1 data from the Submillimeter Array (SMA), and the $^{12}$CO $J$=5-4 and $J$=9-8 lines observed with Herschel/Heterodyne Instrument for the Far-Infrared (HIFI), is modeled with the 3D non-LTE radiative transfer code SHAPEMOL. The data analysis clearly confirms the torus-bipolar structure. The 3D model of the CSE that satisfactorily reproduces the data consists of three kinematic components: a radially expanding torus with velocity slowly increasing from 8 to 13 km s$^{-1}$ along the equator plane; a radially expanding component at the center with a constant velocity of 14 km s$^{-1}$; and a fast, bipolar outflow with velocity proportionally increasing from 14 km s$^{-1}$ at the base up to 100 km s$^{-1}$ at the tip, following a linear radial dependence. The results are used to estimate an average mass-loss rate during the creation of the torus of 7.7$\times$10$^{-7}$ M$_{\odot}$ yr$^{-1}$. The total mass and linear momentum of the fast outflow are estimated at 7.3$\times$10$^{-4}$ M$_{\odot}$ and 9.6$\times$10$^{37}$ g cm s$^{-1}$, respectively. The momentum of the outflow is in excess (by a factor of about 20) of what could be generated by radiation pressure alone, in agreement with recent findings for more evolved sources. The best-fit model also suggests a $^{12}$CO/$^{13}$CO abundance ratio of 50. Possible shaping scenarios for the gas envelope are discussed

ALMA observations of the nearby AGB star L2Puppis

The circumstellar environment of the AGB star L$_{\rm 2}$ Puppis was observed with ALMA in cycle 3, with a resolution of $15 \times 18 \rm\ mas$. The molecular emission shows a differentially rotating disk, inclined to a nearly edge-on position. In the first paper in this series (paper I) the molecular emission was analysed to accurately deduce the motion of the gas in the equatorial regions of the disk. In this work we model the optically thick $^{\rm 12}$CO $J=$3$-$2 and the optically thin $^{\rm 13}$CO $J=$3$-$2 rotational transition to constrain the physical conditions in the disk. To realise this effort we make use of the 3D NLTE radiative transfer code {\tt LIME}. The temperature structure and velocity structure show a high degree of complexity, both radially and vertically. The radial H$_{\rm 2}$ density profile in the disk plane is characterised by a power law with a slope of $-3.1$. We find a $^{\rm 12}$CO over $^{\rm 13}$CO abundance ratio of 10 inside the disk. Finally, estimations of the angular momentum in the disk surpass the expected available angular momentum of the star, strongly supporting the indirect detection of a compact binary companion reported in paper I. We estimate the mass of the companion to be around 1 Jupiter mass.

HIGH-RESOLUTION CO OBSERVATION OF THE CARBON STAR CIT 6 REVEALING THE SPIRAL STRUCTURE AND A NASCENT BIPOLAR OUTFLOW

CIT 6 is a carbon star in the transitional phase from the asymptotic giant branch (AGB) to the protoplanetary nebulae (pPN). Observational evidences of two point sources in the optical, circumstellar arc segments in an HC$_3$N line emission, and a bipolar nebula in near-infrared provide strong support for the presence of a binary companion. Hence, CIT 6 is very attractive for studying the role of companions in the AGB-pPN transition. We have carried out high resolution $^{12}$CO $J=2-1$ and $^{13}$CO $J=2-1$ observations of CIT 6 with the Submillimeter Array combined with the Submillimeter Telescope (single-dish) data. The $^{12}$CO channel maps reveal a spiral-shell pattern connecting the HC$_3$N segments in a continuous form, and an asymmetric outflow corresponding to the near-infrared bipolar nebula. Rotation of the $^{12}$CO channel peak position may be related to the inner spiral winding and/or the bipolar outflow. An eccentric orbit binary is suggested for the presences of an anisotropic mass loss to the west and a double spiral pattern. The lack of interarm emission to the west may indicate a feature corresponding to the periastron passage of a highly eccentric orbit of the binary. Spatially-averaged radial and spectral profiles of $^{12}$CO $J=2-1$ and $^{13}$CO $J=2-1$ are compared with simple spherical radiative transfer models, suggesting a change of $^{12}$CO/$^{13}$CO abundance ratio from $\sim30$ to $\sim50$ inward in the CSE of CIT 6. The millimeter continuum emission is decomposed into extended dust thermal emission (spectral index $\sim-2.4$) and compact emission from radio photosphere (spectral index $\sim-2.0$).

Submillimeter line emission from LMC N159W: a dense, clumpy PDR in a low metallicity environment

Context. Star formation at earlier cosmological times takes place in an interstellar medium with low metallicity. The Large Magellanic Cloud (LMC) is ideally suited to study star formation in such an environment. Aims. The physical and chemical state of the ISM in a star forming environment can be constrained by observations of submm and FIR spectral lines of the main carbon carrying species, CO, Ci and Cii, which originate in the surface layers of molecular clouds i lluminated by the UV radiation of the newly formed, young stars. Methods. We present high-angular resolution sub-millimeter observations in the N159W region in the LMC obtained with the NANTEN2 telescope of the 12 CO J = 4→ 3, J = 7→ 6, and 13 CO J = 4→ 3 rotational and [Ci] 3 P1− 3 P0 and 3 P2− 3 P1 fine-structure transitions. The 13 CO J = 4→ 3 and [Ci] 3 P 2− 3 P 1 transitions are detected for the first time in the LMC. We deri ve the physical and chemical properties of the low-metallicity molecular gas using an escape probability code and a self-consistent solution of the chemistry and thermal balance of the gas in the framework of a clumpy cloud PDR model. Results. The separate excitation analysis of the submm CO lines and the carbon fine structure lines shows that the emitting gas in th e N159W region has temperatures of about 80 K and densities of about 10 4 cm −3 . The estimated C to CO abundance ratio close to unity is substantially higher than in dense massive star-forming regions in the Milky Way. The analysis of all observed lines together, including the [Cii] line intensity reported in the literature, in the context of a clu mpy cloud PDR model constrains the UV intensity to about �≈ 220 and an average density of the clump ensemble of about 10 5 cm −3 , thus confirming the presence of high density material in the LMC N159W region.

Atomic Oxygen Abundance in Molecular Clouds: Absorption toward Sagittarius B2

We have obtained high-resolution (approximately 35 km/s) spectra toward the molecular cloud Sgr B2 at 63 micrometers, the wavelength of the ground-state fine-structure line of atomic oxygen (O(I)), using the ISO-LWS instrument. Four separate velocity components are seen in the deconvolved spectrum, in absorption against the dust continuum emission of Sgr B2. Three of these components, corresponding to foreground clouds, are used to study the O(I) content of the cool molecular gas along the line of sight. In principle, the atomic oxygen that produces a particular velocity component could exist in any, or all, of three physically distinct regions: inside a dense molecular cloud, in the UV illuminated surface layer (PDR) of a cloud, and in an atomic (H(I)) gas halo. For each of the three foreground clouds, we estimate, and subtract from the observed O(I) column density, the oxygen content of the H(I) halo gas, by scaling from a published high-resolution 21 cm spectrum. We find that the remaining O(I) column density is correlated with the observed (13)CO column density. From the slope of this correlation, an average [O(I)]/[(13)CO] ratio of 270 +/- 120 (3-sigma) is derived, which corresponds to [O(I)]/[(13)CO] = 9 for a CO to (13)CO abundance ratio of 30. Assuming a (13)CO abundance of 1x10(exp -6) with respect to H nuclei, we derive an atomic oxygen abundance of 2.7x10(exp -4) in the dense gas phase, corresponding to a 15% oxygen depletion compared to the diffuse ISM in our Galactic neighborhood. The presence of multiple, spectrally resolved velocity components in the Sgr B2 absorption spectrum allows, for the first time, a direct determination of the PDR contribution to the O(I) column density. The PDR regions should contain O(I) but not (13)CO, and would thus be expected to produce an offset in the O(I)-(13)CO correlation. Our data do not show such an offset, suggesting that within our beam O(I) is spatially coexistent with the molecular gas, as traced by (13)CO. This may be a result of the inhomogeneous nature of the clouds.

Interaction between the W51C Supernova Remnant and a Molecular Cloud. II. Discovery of Shocked CO and HCO+

We carried out 12CO J = 1-0, J = 2-1,13CO J = 1-0, and HCO+ J = 1-0 line observations of the supernova remnant W51C in order to search for shocked molecules associated with the shocked H I reported in a companion paper. We discovered shocked CO distributed along an ~8' (or ~14 pc at a distance of 6 kpc) long arc structure. The position and morphology of the CO distribution coincide with the shocked H I. The shocked CO has line-of-sight velocities between +20 and +50 km s-1 with respect to the ambient cloud and velocity widths of 8-21 km s-1. The CO emission is optically thin, and Tex 10 K. The shocked CO is clumpy, and we have identified five clumps. Their column densities are in the range N(CO) = 1.2-4.0 × 1016 cm-2. Total mass of the shocked CO is 0.16 M☉. The HCO+/CO abundance ratio is 1.2-1.3 × 10-4, typical of quiescent molecular clouds. The fast-moving CO and HCO+ are probably the molecules that had been destroyed by a fast, dissociative J-type shock and re-formed behind the shock front. We find that the CO/H I abundance ratio is generally less than 4 × 10-5, except for some weak features. The upper limit implies that almost all the hydrogen nuclei in the shocked gas is atomic, not molecular. We conclude that the shock is old enough to re-form CO and HCO+ molecules but not old enough to completely re-form H2 molecules. We discuss the variation of the CO content of the shocked gas.

Discovery of Far-Infrared Pure Rotational Transitions of CH[TSUP]+[/TSUP] in NGC 7027

We report the discovery of the rotational spectrum of CH+ in the Infrared Space Observatory Long Wavelength Spectrometer (LWS) spectrum of the planetary nebula NGC 7027. The identification relies on a 1996 reanalysis of the LWS spectrum by Liu et al. and on new LWS data. The strong line at 179.62 μm (coinciding with the 212-101 transition of water vapor) and the lines at 119.90 and 90.03 μm (reported as unidentified by Liu et al.), whose frequencies are in the harmonic relation 2 : 3:4, are shown to arise from the J = 2-1, 3-2, and 4-3 rotational transitions of CH+. This identification is strengthened by the new LWS spectra of NGC 7027, which clearly show the next two rotational lines of CH+ at 72.140 and 60.247 μm. This is the first time that the pure rotational spectrum of CH+ has been observed. This discovery opens the possibility of probing the densest and warmest zones of photodissociation regions. We derive a rotational temperature for the CH+ lines of 150 ± 20 K and a CH+/CO abundance ratio of 2-6 × 10-4.

CO and OH absorption toward the low-latitude radio source 2023+336

We report interferometric observations of (12)CO (0 to 1) 115 GHz and OH 1667 MHz absorption toward the extragalactic radio continuum source 2023+336. Our data show two partially blended absorption features near +2 km/s LSR which probably originate in local molecular clouds in the Cygnus rift, approximately 0.7 kpc away. The CO lines are optically thin, with tau(sub CO) = 0.5 +/- 0.1, while tau(sub OH) = 0.030 +/- 0.004. The detected absorption features have a combined velocity width of 3.0 km/s FWHM, greater than the width of the corresponding (12)CO emission features, Full Width of Half Maximum (FWHM) = 2.3 km/s, indicating that the absorbing gas in a approximately 1 AU diameter column shows a greater velocity range than the gas in the 0.2 pc diameter emission beam. CO excitation temperatures T(sub ex), range from 2.7 to 7.0 K across the line profile, implying that the gas along this line of sight is rotationally quite cold. By measuring the (12)CO excitation temperature and optical depth, we derive an abundance ratio, (CO)/(H2), of 6 x 10(exp -6), which is more than an order of magnitude lower than is seen elsewhere. Our low observed CO optical depth is inconsistent with the ratio of (12)CO to (13)CO emission line integrals, 16:1, which implies that the (12)CO is optically thick. As a solution to these anomalies, we propose that the CO absorption and emission arise from preferentially different parts of the molecular medium. Other solutions would require an anomalous (12)CO/(13)CO abundance ratio and an anomalous CO/H2 abundance ratio, or much lower excitation temperatures. Since we find no absorption at negative velocities with tau greater than 0.23 (2.5 sigma), there is no compelling evidence for the existence of an extensive population of cold molecular gas in the outer Galaxy along this line of sight.

The C(0)/CO ratio problem - A possible new precision test of the decaying neutrino theory

view Abstract Citations (7) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The C 0/CO Ratio Problem: A Possible New Precision Test of the Decaying Neutrino Theory Sciama, D. W. Abstract We reconsider Tarafdar's (1991) suggestion that decay photons from neutrinos in molecular clouds could explain the anomalously large C(0)/CO abundance ratio found in the clouds. In particular, we follow our recent proposal that the decay photons can ionize nitrogen, so that their energy E(gamma) must exceed 14.53 eV. Tarafdar found that the clouds would be too opaque unless the energy of the decay photons lies below the photodissociation continuum of H2, which requires that E(gamma) is less than 14.68 eV. This upper limit is similar to, but firmer than, a limit previously derived from consideration of the intergalactic ionizing flux of decay photons. The domain of validity of the neutrino decay theory would then be nonzero but extremely small, with E(gamma) = 14.605 +/- 0.075 eV. We also find for the mass of the decaying neutrinos is 29.21 +/- 0.15 eV. This mass, when combined with estimates of the age of the universe, implies that Omega is approximately equal to 1. If Omega = 1 exactly, then our theory could be valid only if the Hubble constant is 56.3 +/- 0.5 km/s Mpc. Publication: The Astrophysical Journal Pub Date: September 1993 DOI: 10.1086/187025 Bibcode: 1993ApJ...415L..31S Keywords: Carbon; Carbon Monoxide; Elementary Particles; Interstellar Matter; Neutrinos; Particle Theory; Gas Dissociation; Molecular Clouds; Astrophysics; ELEMENTARY PARTICLES; ISM: MOLECULES; RADIATION MECHANISMS: MISCELLANEOUS full text sources ADS | data products SIMBAD (2)

Ground-based constraints on the H2O(+)/CO(+) abundance ratio and dust impact rate in comet P/Giacobini-Zinner - Comparison with the ICE spacecraft results

The results of ground-based spectrophotometry of the comet P/Giacobini-Zinner (1984e) near perihelion and the time of the International Cometary Explorer (ICE) encounter are presented. The authors find that the abundance ratio [H<SUB>2</SUB>O<SUP>+</SUP>/CO<SUP>+</SUP>] is 1.2±0.6 compared with the ICE result of approximately 5. Analysis of the dust production as measured by the optical continuum predicts a dust impact rate on the S antenna of the ICE of ≤0.01 s<SUP>-1</SUP> at closest approach. This upper limits is consistent with the ICE results.

A study of DCO+ emission regions in interstellar clouds

view Abstract Citations (96) References (45) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A study of DCO+ emission regions in interstellar clouds. Wootten, A. ; Loren, R. B. ; Snell, R. L. Abstract Observations of the J = 1-0 lines of HCO(+), (H-13)CO(+), and DCO(+) and of the J = 2-1 line of DCO(+) taken with similar beam sizes are used in constructing models of the DCO(+) emission regions in eleven molecular clouds. The densities and extents of the regions as determined from the DCO(+) observations are similar to the densities and extents of the same regions modeled on formaldehyde observations. The DCO(+)/(H-13)CO(+) abundance ratio ranges from 0.2 to 6 in this sample. The observations, interpreted in the light of newly measured reaction rates, constrain electron abundances in some regions to be very low, X(e) being less than 1 x 10 to the -7. Independent limits placed on X(e) using (C-13)O and (H-13)CO(+) observations yield more sensitive limits, X(e) being less than 6 x 10 to the -8. It is found that carbon monoxide abundance limits are consistent with those found at the edges of dark clouds. Publication: The Astrophysical Journal Pub Date: April 1982 DOI: 10.1086/159814 Bibcode: 1982ApJ...255..160W Keywords: Astronomical Models; Deuterium Compounds; Emission Spectra; Interstellar Matter; Line Spectra; Molecular Clouds; Abundance; Carbon Monoxide; Formic Acid; Interstellar Chemistry; Positive Ions; Radiative Transfer; Reaction Kinetics; Temperature Dependence; Astrophysics full text sources ADS | data products SIMBAD (27) NED (1)

The CO-12/CO-13 abundance ratio toward Zeta Ophiuchi

view Abstract Citations (75) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The 12 CO/13 CO abundance ratio toward ZET Oph. Wannier, P. G. ; Penzias, A. A. ; Jenkins, E. B. Abstract The CO-12/CO-13 abundance ratio toward Zeta Ophiuchi, and interstellar material physical properties, are determined by means of a system of UV absorption lines gathered by Copernicus and IUE spacecraft observations. The observations imply a low CO isotope ratio, at 55 + or - 11, and a CO-12 column density of 2.4 + or - 0.3 x 10 to the 15th/sq cm. A small velocity dispersion, at 0.46 + or - 0.15 km/sec, is implied by further analysis, along with a kinetic temperature greater than 70 K. Publication: The Astrophysical Journal Pub Date: March 1982 DOI: 10.1086/159710 Bibcode: 1982ApJ...254..100W Keywords: Abundance; Carbon Monoxide; Carbon 12; Carbon 13; Interstellar Gas; Ultraviolet Spectra; Absorption Spectra; Iue; Line Spectra; Oao 3; Spaceborne Astronomy; Astrophysics full text sources ADS | data products MAST (2) SIMBAD (1)

Some new results on shock chemistry in IC 443

view Abstract Citations (62) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Some new results on shock chemistry in IC 443. Denoyer, L. K. ; Frerking, M. A. Abstract New observations have been made of CO, CO-13, SiO, SO, H2CO, HCO(+), N2H(+), CS, OCS, HCN, and OH in the shocked clouds of IC 443. It is found that at position IC 443 B, (1) the shocked CO is optically thin; (2) the HCO(+)/CO abundance ratio is 4-9 x 10 to the -4 th, representing a tenfold enhancement over that of normal interstellar clouds; (3) there is no enhancement of SO or SIO, as occurs in Orion KL; and (4) there is optically thin preshock OH, confirming a hundredfold enhancement of the OH/CO ratio in the shock. Publication: The Astrophysical Journal Pub Date: May 1981 DOI: 10.1086/183548 Bibcode: 1981ApJ...246L..37D Keywords: Abundance; Interstellar Gas; Molecular Clouds; Shock Waves; Supernova Remnants; Carbon Isotopes; Carbon Monoxide; Emission Spectra; Hydrogen Clouds; Hydroxyl Emission; Line Spectra; Molecular Excitation; Molecular Spectra; Optical Thickness; Silicon Oxides; Spectrum Analysis; Astronomy full text sources ADS | data products SIMBAD (2)

On the interstellar CO+/CO abundance ratio.

view Abstract Citations (14) References (8) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the interstellar CO+/CO abundance ratio. Hobbs, L. M. Abstract A search at high sensitivity for interstellar absorption in the A4250 line of CO + yields negative results toward 14 stars, despite the pervasive presence of large amounts of CO in the interstellar gas. This result is in accord with theory, but still lacks by a large factor sufficient sensitivity to test any details of the theory. Subject headings: abundances - molecules, interstellar - interstellar matter Publication: The Astrophysical Journal Pub Date: May 1973 DOI: 10.1086/152089 Bibcode: 1973ApJ...181..795H full text sources ADS | data products SIMBAD (14)