CO Rotational Lines Research Articles

Far-Infrared CO Rotational Lines in the Orion Molecular Cloud

We present a high signal-to-noise grating spectrum between 43-196.9 μm of the Orion molecular cloud towards the massive star-forming region IRc2, obtained with the Long Wavelength Spectrometer (LWS) on board the Infrared Space Observatory (ISO).

Molecular Lines as Diagnostics of High-Redshift Objects

Models are presented for CO rotational line emission by high-redshift starburst galaxies. The influence of the cosmic microwave background on the thermal balance and the level populations of atomic and molecular species is explicitly included. Predictions are made for the observability of starburst galaxies through line and continuum emission between z=5 and z=30. It is found that the Millimeter Array could detect a starburst galaxy with ~105 Orion regions, corresponding to a star formation rate of about 30 M☉ yr-1, equally well at z=5 or z=30 because of the increasing cosmic microwave background temperature with redshift. Line emission is a potentially more powerful probe than dust continuum emission of very high redshift objects.

High resolution `VUV laser' measurements of the band oscillator strengths of the CO transition

Band-integrated oscillator strengths of the transition for have been measured in absorption using a high-resolution tunable VUV coherent light source (`VUV laser') as a background source. The rotationally resolved absorption spectra are least-squares fitted by precise synthetic spectra in order to take into account the instrumental saturation effects. The fitting parameters are the oscillator strength, the rotational line width and the instrumental width. Saturation effects are reduced by pressure broadening the absorption lines using up to 1.5 bar of argon as a buffer gas. The new data set extends the range covered by recent measurements to . Cross sections for are measured here for the first time. The present results are in general agreement with the latest measurements and with ab initio calculations. The average broadening coefficient of the CO rotational lines by argon is determined in this study for the first time: . The variation of with and J is small and could not be measured in the present experiments.

The Wardle Instability in Interstellar Shocks. II. Gas Temperature and Line Emission

We have modeled the gas temperature structure in unstable C-type shocks and obtained predictions for the resultant CO and H2 rotational line emissions, using numerical simulations of the Wardle instability presented in Paper I. Our model for the thermal balance of the gas includes ion-neutral frictional heating; compressional heating; radiative cooling due to rotational and ro-vibrational transitions of the molecules CO, H2O, and H2; and gas-grain collisional cooling. We obtained results for the gas temperature distribution in—and H2 and CO line emission from—shocks of neutral Alfvénic Mach number 10 and velocity 20 or 40 km s-1 in which the Wardle instability has saturated. Both two- and three-dimensional simulations were carried out for shocks in which the preshock magnetic field is perpendicular to the shock propagation direction, and a two-dimensional simulation was carried out for the case in which the magnetic field is obliquely oriented with respect to the shock propagation direction. Although the Wardle instability profoundly affects the density structure behind C-type shocks, most of the shock-excited molecular line emission is generated upstream of the region where the strongest effects of the instability are felt. Thus the Wardle instability has a relatively small effect on the overall gas temperature distribution in—and the emission-line spectrum from—C-type shocks, at least for the cases that we have considered. In none of the cases that we have considered thus far did any of the predicted emission-line luminosities change by more than a factor of 2.5, and in most cases the effects of instability were significantly smaller than that. Slightly larger changes in the line luminosities seem likely for three-dimensional simulations of oblique shocks, although such simulations have yet to be carried out and lie beyond the scope of this study. Given the typical uncertainties that are always present when model predictions are compared with real astronomical data, we conclude that Wardle instability does not imprint any clear observational signature on the shock-excited CO and H2 line strengths. This result justifies the use of one-dimensional steady shock models in the interpretation of observations of shock-excited line emission in regions of star formation. Our three-dimensional simulations of perpendicular shocks revealed the presence of warm filamentary structures that are aligned along the magnetic field, a result that is of possible relevance to models of water maser emission from C-type shocks.

A Self‐consistent Photoionization–Dust Continuum–Molecular Line Transfer Model of NGC 7027

A model to simulate the entire spectrum (1000 Å to 1 cm) of the high-excitation young planetary nebula NGC 7027 is presented. The ionized, dust, and molecular components of the object are modeled using geometric parameters obtained from visible, radio, infrared, and CO data. The physical processes considered include recombination lines of H and He, collisional excited lines of metals, bf and ff continuum radiations, two-photon radiation, dust continuum radiation, and molecular rotational and vibrational transitions. The dust component is assumed to be heated by a combination of direct starlight and the line and continuum radiation from the ionized nebula. The molecular component of the nebula is coupled to the dust component through the stimulated absorption of the dust continuum radiation. Specifically, we compare the predicted fluxes of the CO rotational lines and the 179.5 μm water rotational line to those observed by the Infrared Space Observatory satellite. From the observed ratio of these lines, we derived a gas phase water-to-H2 abundance ratio of 3.2 × 10-7 assuming that the water is uniformly mixed with the CO.

Observations of High Rotational CO Lines in Post–Asymptotic Giant Branch Stars and Planetary Nebulae

We have observed bright emission in high rotational CO lines (J = 13-12, 17-16, and 22-21) in the post-AGB object GL 2688 and the planetary nebulae, GL 618 and NGC 7027. These stars represent different evolutionary stages, as reflected by their effective temperatures. The observations reveal copious amounts of warm, dense molecular gas (n ~ 107 cm-3; T ~ 103 K), implying that the AGB is terminated by a short-lived phase of high mass-loss rate. We investigate heating mechanisms for these objects. For NGC 7027, UV photons from the central star produce a photodissociation region and the gas cools via CO rotational transitions, as well as via fine-structure atomic lines. A shock is thought to heat the gas in GL 2688, which cools mainly via the [O I] 63 μm line and molecular rotational lines. For GL 618, both mechanisms may coexist.

First detection of thermal H2O Emission from a Herbig-Haro Object

The first detection of thermal H2O emission from an Herbig-Haro flow was made with the LWS (Long Wavelength Spectrometer) aboard ISO (Infrared Space Observatory). In addition to H2O, rotational lines of CO and OH as well as lines from [O I] and [C II] were also recorded from HH 54. These observations are consistent with the concept of interstellar shock waves, but a quantitative unifying shock model, capable of explaining all observations, has yet to be developed.

Disks and Outflows as seen from the IRAM Interferometer

The IRAM interferometer has been used to map 3 high velocity outflows from Class 0 sources in SiO and CO rotational lines. The images show that such outflows are driven by highly collimated, dense, but optically invisible jets, through large travelling bow shocks. Precession is important in explaining some morphological and kinematic aspects. A significant part of the SiO emission is directly associated with the jet. Images of the molecular core surrounding L 1157 in CO isotopic lines show direct evidence for infall in a large, flattened disk.For more evolved objects, “surveys” of mm emission from T Tauri stars provide evidence for circumstellar dust disks with relatively flat surface density distribution and typical radii 100 to 200 AU. Much larger gas disks (radii 800 to 1000 AU) have been detected and imaged in a few objects (2 singles and 2 binaries) through CO isotopomers, but in most sources 13CO is difficult to detect. The spectral line images provide unambiguous evidence for Keplerian rotation. In two cases (GG Tau and DM Tau), direct measurements of the gas mass (independently of the dust) has been made thanks to the detection of 7 molecular species. The molecules are found to be depleted by factors between 5 (for CO) to 100 (for H2CO). These results suggest that circumstellar disks consist in a small, dense, inner disk of outer radius ≃ 150AU (traced by the dust emission), surrounded by a much larger, less massive, outer disk (traced by the gas emission).

ISO LWS Observations of Evolved Stars and Nebulae

This review reports, on behalf of the LWS Consortium1, some of the spectroscopic results on evolved stars and nebulae that have been obtained at far-infrared wavelengths by the Infrared Space Observatory (ISO) since its launch in November 1995. The results reported have been obtained mainly using ISO’s Long Wavelength Spectrometer (LWS; Clegg et al. 1996), but for some objects complementary spectra have also been obtained with the Short Wavelength Spectrometer (SWS; de Graauw et al. 1996). In this review I present some of the results that have been obtained (a) for oxygen-rich cool stars and (b) for carbon stars and carbon-rich post-AGB objects and planetary nebulae. Highlights from the first area include the first detection of the far-infrared rotational lines of water vapour, the dominant coolant in their outflows, and the detailed confirmation of the proposed radiative pump mechanism for circumstellar OH masers. Highlights from the second area include the detection of a rich HCN and CO far-infrared line spectrum from the carbon star IRC+10°216, with HCN found to be the dominant coolant, the detection of very strong shock-excited CO rotational lines from the post-AGB object AFGL 2688, and the first detection of the far-infared rotational lines of CH+ (from the the PDR of the carbon-rich planetary nebula NGC 7027), as well as the first detection in emission of the far-IR rotational lines of CH (also from NGC 7027).

A self-consistent photoionization-dust continuum-molecular line transfer model of NGC7027

A model to simulate the entire spectrum (1000 Å to 1 cm) of the high-excitation young planetary nebula NGC 7027 is presented. The ionized, dust, and molecular components of the object are modeled using geometric parameters obtained from visible, radio, infrared, and CO data. The physical processes considered include recombination lines of H and He, collisional excited lines of metals, bf and ff continuum radiations, two-photon radiation, dust continuum radiation, and molecular rotational and vibrational transitions. The dust component is assumed to be heated by a combination of direct starlight and the line and continuum radiation from the ionized nebula. The molecular component of the nebula is coupled to the dust component through the stimulated absorption of the dust continuum radiation. Specifically, we compare the predicted fluxes of the CO rotational lines and the 179.5 μm water rotational line to those observed by the Infrared Space Observatory satellite (Liu et al. 1996, A&A in press).

The CO Abundance on Neptune from HST Observations

We have marginally detected two CO absorption bands in the UV reflection spectrum of Neptune measured by the HST Faint Object Spectrograph (FOS). They are the (1, 0) and (0,0) Cameron bands at 1992 and 2063 Å. The interpretation of these features with a radiative transfer model—including Rayleigh, Raman, and particulate scattering processes—implies a CO mole fraction of (2.7 ± 1.8) × 10−6in the 30–800 mbar pressure range. The total uncertainty takes into account the statistical error bars as well as systematic effects (correction for straylight in the FOS data, temperature dependence of the CO absorption). Our CO mole fractions are in agreement with previous determinations made from millimeter measurements of CO rotational lines both in the stratosphere and in the troposphere. Carbon monoxide thus appears to be well-mixed in Neptune's atmosphere, which would confirm its internal origin. We have also examined the possible influence of NH3and HC3N on Neptune's spectrum in the 1950–2150 Å range, but could only estimate upper limits for these species (respectively 3 × 10−4and 6 × 10−3cm-am) that are much larger than the abundances predicted by current photochemical models.

On the Vertical Thermal Structure of Pluto's Atmosphere

A radiative–conductive model for the vertical thermal structure of Pluto's atmosphere is developed with a non-LTE treatment of solar heating in the CH43.3 μm and 2.3 μm bands, non-LTE radiative exchange and cooling in the CH47.6 μm band, and LTE cooling by CO rotational line emission. The model includes the effects of opacity and vibrational energy transfer in the CH4molecule. Partial thermalization of absorbed solar radiation in the CH43.3 and 2.3 μm bands by rapid vibrational energy transfer from the stretch modes to the bending modes generates high altitude heating at sub-microbar pressures. Heating in the 2.3 μm bands exceeds heating in 3.3 μm bands by approximately a factor of 6 and occurs predominantly at microbar pressures to generate steep temperature gradients ∼10–20 K km−1forp> 2 μbar when the surface or tropopause pressure is ∼3 μbar and the CH4mixing ratio is a constant 3%. This calculated structure may account for the “knee” in the stellar occultation lightcurve. The vertical temperature structure in the first 100 km above the surface is similar for atmospheres with Ar, CO, and N2individually as the major constituent. If a steep temperature gradient ∼20 K km−1is required near the surface or above the tropopause, then the preferred major constituent is Ar with 3% CH4mixing ratio to attain a calculated ratio ofT/M(= 3.5 K amu−1) in agreement with inferred values from stellar occultation data. However, pure Ar and N2ices at the same temperature yield an Ar vapor pressure of only ∼0.04 times the N2vapor pressure. Alternative scenarios are discussed that may yield acceptable fits with N2as the dominant constituent. One possibility is a 3 μbar N2atmosphere with 0.3% CH4that has 106 K isothermal region (T/M= 3.8 K amu−1) and ∼8 K km−1surface/tropopause temperature gradient. Another possibility would be a higher surface pressure ∼10 μbar with a scattering haze forp> 2 μbar. Our model with appropriate adjustments in the CH4density profile to Triton's inferred profile yields a temperature profile consistent with the UVS solar occultation data (Krasnopolsky, V. A., B. R. Sandel, and F. Herbert 1992.J. Geophys. Res.98, 3065–3078.) and ground-based stellar occultation data (Elliot, J. L., E. W. Dunham, and C. B. Olkin 1993.Bull. Am. Astron. Soc.25, 1106.).

Detecting CO at High Redshift

Searches for molecular line emission from high redshift galaxies have become one of the recent highlights in millimeter astronomy, largely because detection of this emission enables one to study the potential for star formation in galaxies at epochs close to galaxy formation. Such information is crucial to models of galaxy evolution. Thus far, most of the searches have been to try to detect any of the rotational lines of CO, although many authors have also inferred the presence of molecular gas through detections of cold dust in the submillimeter region of the spectrum. In addition to providing information about the physical properties of the molecular gas in distant galaxies (when more than one transition or isotope is detected), the CO lines can be used to place stringent constrints on the dynamical masses of these systems. Moreover, since millimeter data has spectral resolutions of typically a few tens of km/s, one can pin down the redshift of the host galaxy with extremely high precision. One of the driving forces in most of the searches for CO emission at high redshift is the fact that molecular gas is known to be an important constituent in the low redshift counterparts to the types of objects that one expects to find at high redshifts, the Ultraluminous Infrared Galaxies (ULIRGs), (e.g. Mirabel and Sanders 1985; Sanders et al. 1986), powerful radio galaxies (e.g. Mazzarella et al. 1993), and nearby quasars (e.g. Barvainis et al. 1989), for example.

Modeling of the Dust and Gas Outflows from OH 26.5+0.6: The Superwind

We have observed the extreme OH/IR star, OH 26.5+0.6, in the infrared dust continuum and in the sub- millimeter rotational lines of CO. Mid-infrared images reveal the compact nature of the circumstellar shell (less than 0.5 sec). A deep 9.7 microns absorption feature and an absorption at 18 microns show that the dust mass-loss rate is very high. However, the low antenna temperatures of CO J = 1-0 and 2-1 lines suggest that the outer part of the circumstellar shell is much more tenuous. In order to resolve this discrepancy, we have observed the J = 3-2 and 4-3 CO rotational transitions. We have developed a model for the circumstellar shell for OH 26.5 + 0.6 which is consistent with the infrared and submillimeter observations. The dust and gas data are well fitted by a two-shell model, consisting of a dense shell surrounded by a more tenuous shell. The former we identify with the superwind (M = 5.5 x 10(exp -4) solar mass/ yr), and the latter we identify with mass loss on the asymptotic giant branch (AGB) (M = 10(exp -6) solar mass/ yr). The transition between the two mass-loss phases is shown to be rather abrupt ((Delta)t less than 150 yr). Depending on the mass of the progenitor, this superwind phase may be the last thermal pulse (for M(sub *) less than 1.5 solar mass), or the first of a series of the superwind phases (for up to 8 solar mass), punctuated by a period of low mass-loss rates, before the star evolves off the AGB.

A Multiwavelength Study of 30 Doradus: The Interstellar Medium in a Low-Metallicity Galaxy

We report maps of the 158 microns [C II] fine-structure line, the 63 microns and 146 microns [O I] fine-structure lines, the 2.2 microns H I Brγ line, the 2.1 microns H<SUB>2</SUB> 1-0 S(1) ro-vibrational line, and the 2.6 mm CO (1-0) rotational line toward the 30 Doradus complex in the Large Magellanic Cloud. <P />Comparing our Brγ map with Hα and Hβ measurements, we find that visual to near-infrared extinction and reddening follow the standard dust extinction law and that the Brγ extinction is small, which allows for a reliable determination of the Lyman-continuum intensity. The Lyman continuum as derived from the Brγ emission and the far-UV derived from the far-infrared continuum match the average spectrum of the exciting stars in the 30 Doradus cluster. The observed H_{2 }line intensity may be produced in dense clumps exposed to the stellar radiation fields. <P />The maps of all tracers emphasize a shell-like structure of the 30 Doradus region, which is seen approximately edge-on. The warm molecular gas traced by the H<SUB>2</SUB> line and the ionized gas traced by the Brγ line are intermixed, while the cold molecular gas as traced by CO (1-0) and the photodissociated gas as traced by [C ii] are coextensive over tens of parsecs. This distribution can be explained only by a highly fragmented structure of the interstellar medium that allows UV radiation to penetrate deep into the molecular cloud. Clumpiness is also the key to understanding the extremely high [C II]/CO line intensity ratio. Depending on cloud geometry and physical conditions, the relative beam-filling factors of the partly atomic, partly molecular photodissociated gas as seen in the FIR tracers, and of the purely molecular gas traced by CO, can differ substantially in a clumpy, low-metallicity environment. This effect also leads to a greatly increased H<SUB>2</SUB>/CO conversion factor because a major part of the H<SUB>2</SUB> molecular gas may be contained in the photodissociation region where CO has been destroyed.

Observations of high co rotational lines in post-AGBs and PN

We present a preliminary interpretation of high CO rotational line data obtained from KAO. The possibility of either a PDR or a shock model is considered in order to explain the observations.

The Thermal Structure of Pluto's Atmosphere: Clear vs Hazy Models

The problem of Pluto's atmosphere thermal structure, as determined from the June 9, 1988 occultation data, is reassessed in the light of the detection of N 2 and CO ices, along with CH 4, on Pluto's surface and of recent surface temperature measurements. Firm inferences from these observations are that N 2 must be the major gas of Pluto's atmosphere and that the temperature at the microbar level is about 105 K. We estimate that CH 4 and CO are present at the 0.1% level of N 2 in the atmosphere and we develop simple 1-D aeronomical models (solving the heat equation) for various atmospheric cases. It is found in particular that (i) if CO cooling is omitted, heating by CH 4 can explain the temperature rise between the surface and the μbar level, but not the drop-off observed in several occultation lightcurves below 1215 km, unless the CH 4 mixing ratio is larger than 10%; (ii) if, as expected, CO is present at the ≈0.1% level, and CH 4 is lower than 10%, then cooling by CO rotational lines is so large that CH 4 heating cannot explain a 105 K temperature at 1-2 μbar. A possible way to explain the temperature rise between the surface and 1 μbar and the lightcurve drop-off is to invoke heating by a Titan-like haze layer with extinction optical depth ≈(0.4-4) × 10 -3 for particle sizes in the range 0.01-2 μm. Such a haze can plausibly result from methane photolysis and subsequent chemistry, provided that the CH 4 mixing ratio is significantly larger than 10 -4. A similar haze is not expected to be present on Triton (where the methane mixing ratio is about (2-20) × 10 -5), which is consistent with the fact that Triton's atmosphere near the μbar level is much colder than Pluto's. Estimating production and fallout time constants suggests that haze particles in Pluto's atmosphere must be small (≤0.1 μm).

The [C ii] 158 Micron Emission from the Horsehead Nebula

We have mapped the [C II] 158 μm line and observed several rotational lines of CO, 13 CO and CS toward selected positions in the Horsehead extinction region in IC 434. The observations show that the region has a gas density of about 10 4 cm −3 and an external UV flux 20-100 times the average interstellar UV field. Although this is a regime where the C + emission varies rapidly with UV intensity, fine-structure line emission from gas with this range of physical conditions has not been investigated previously. Comparisons of our results with models of photodissociation regions show that existing plane-parallel photodissociation region models are in general agreement with the observed intensity

Kiloparsec-scale molecular gas excitation in spiral galaxies

We combine beam-matched ^13^CO, ^12^CO J = 3 - 2 and J = 2 - 1 line data to infer the molecular gas excitation conditions in the central 500 to 1600 pc diameters of a small sample of infrared-bright external galaxies: NGC 253, IC 342, M83, Maffei 2, and NGC 6946. We find that the central 170 to 530 pc diameter regions have typical molecular gas densities ranging from &lt;~ 10^4^ cm^-3^ (in M83) to &gt;~ 10^5^ cm^-3^ (in NGC 253) and that, outside of these regions, the densities are likely to be &lt;~ 10^4^ cm^-3^. The molecular clouds outside the inner 170-530 pc are at least as warm as the molecular clouds in our Galaxy. Column densities derived from integrated ^13^CO line strengths and Hα surface brightnesses suggest that the star formation rate is enhanced in the central 170-530 pc diameters by an order of magnitude over that inferred for the outer star-forming disks in spiral galaxies. The total luminosity of each sample galaxy over all CO rotational lines is ~10^5^-10^6^ L_sun_, which is within an order of magnitude of that of the important cooling line of [C II] at 158 microns. Additional observations of the J = 1 - 0 lines of C^18^O and ^13^CO suggest that emission from the central kiloparsec of spiral galaxies may be optically thick in the ^13^CO J = 1 - 0 line. If so, estimates of molecular gas mass using ^13^CO integrated line strengths could be too low. The sum of the molecular line observations imply the existence of two molecular gas components: a low-density [n(H_2_) &lt;~ 10^3^-10^4^ cm^-3^] component and a warm (T_K_ &gt;~ 50 K), high-density (n(H_2_) &gt;~ 10^4^-10^5^ cm^-3^] component. The warm, dense component is probably associated with star formation.

Mass-loss rates for IRAS-bright red giants - A revised model of circumstellar CO emission

We investigate the effect of self-consistent calculations of gas kinetic temperature on the predicted CO rotational emission line intensities and profiles of expanding circumstellar envelopes. In regimes of large mass-loss rate (M), the line intensities are sharply decreased relative to the standard model owing to the close coupling of the gas excitation temperature to kinetic temperature at high density