Molecular Line Intensities Research Articles

Pulsation-induced Spectroscopic Variability of IRAS Z02229+6208

Monitoring of high-resolution spectra in the optical and near-infrared wavelength region was carried out for the early post-AGB stage star IRAS Z02229+6208 with the CARMENES instrument to reveal spectroscopic variability over the pulsation cycle of 154 days. Significant changes are seen in both the intensity and position of carbon-bearing molecular lines. Strong and broad CN red system lines, blueshifted up to 9.4 km s−1 relative to the stellar photosphere, are seen near the light minimum. Near the light maxima, the molecular features are weak and most of them are in emission, e.g., broad emissions in the CN red system (0,0) lines are seen. A site of formation of molecular absorptions and emissions appears to be an extended atmosphere of a cool post-AGB star affected by stellar pulsation. In addition, we observed narrow absorption lines of the C2 Phillips (2, 0), (3, 0) system and the CN red system (2,0), most likely related to the circumstellar envelope produced during the evolution on the asymptotic giant branch (AGB). An expansion velocity of the circumstellar shell was found to be V exp = −11.1 km s−1 relative to the stellar mass center, in agreement with that estimated before from CO lines. The observed spectroscopic variability seems to be typical for pulsating early post-AGB stage stars.

Do simulated molecular clouds look like real ones?

ABSTRACT Simulations of molecular clouds often begin from highly idealized initial conditions, such as a uniform-density sphere with an artificially imposed turbulent velocity field. While the resulting structures may appear qualitatively similar to those detected in continuum and line observations, it is unclear whether they are genuinely representative of real molecular clouds. Recent observational work has discovered a tight, often close-to-linear relationship between the integrated intensity of molecular lines and the total column density of the cloud material. We combine magnetohydrodynamical simulations, time-dependent chemistry, and radiative transfer to produce synthetic molecular line observations of model clouds. We find similarly tight correlations between line intensity and column density to those observed, although the linear behaviour is only seen in isolated (as opposed to colliding) model clouds. This linear relationship is not due to optically thin emission; all lines investigated have high optical depths, and the increase in integrated intensity with column density is due to higher velocity dispersion along the line of sight. Overall, the idealized models commonly used in the literature appear to be reasonably accurate representations of real molecular clouds.

AGN-driven Cold Gas Outflow of NGC 1068 Characterized by Dissociation-sensitive Molecules

Recent developments in (sub)millimeter facilities have drastically changed the amount of information obtained from extragalactic spectral scans. In this paper, we present a feature extraction technique using principal component analysis (PCA) applied to arcsecond-resolution (1.″0–2.″0 = 72–144 pc) spectral scan data sets for the nearby type-2 Seyfert galaxy NGC 1068, using Band 3 of the Atacama Large Millimeter/submillimeter Array. We apply PCA to 16 well-detected molecular line intensity maps convolved to a common 150 pc resolution. In addition, we include the [S iii]/[S ii] line ratio and [C i] 3 P 1–3 P 0 maps in the literature, both of whose distributions show a remarkable resemblance to that of a kiloparsec-scale biconical outflow from the central active galactic nucleus. We identify two prominent features: (1) central concentration at the circumnuclear disk (CND) and (2) two peaks across the center that coincide with the biconical outflow peaks. The concentrated molecular lines in the CND are mostly high-dipole molecules (e.g., H13CN, HC3N, and HCN). Line emissions from molecules known to be enhanced in an irradiated interstellar medium, CN, C2H, and HNC, show similar concentrations and extended components along the bicone, suggesting that molecule dissociation is a dominant chemical effect of the cold molecular outflow of this galaxy. Although further investigation should be made, this scenario is consistent with the faintness or absence of emission lines from CO isotopologues, CH3OH, and N2H+ in the outflow, which are easily destroyed by dissociating photons and electrons.

Chemistry of nebulae around binary post-AGB stars: A molecular survey of mm-wave lines

Context. There is a class of binary post-asymptotic giant branch (post-AGB) stars that exhibit remarkable near-infrared excess. Such stars are surrounded by Keplerian or quasi-Keplerian disks, as well as extended outflows composed of gas escaping from the disk. This class can be subdivided into disk- and outflow-dominated sources, depending on whether it is the disk or the outflow that represents most of the nebular mass, respectively. The chemistry of this type of source has been practically unknown thus far. Aims. Our objective is to study the molecular content of nebulae around binary post-AGB stars that show disks with Keplerian dynamics, including molecular line intensities, chemistry, and abundances. Methods. We focused our observations on the 1.3, 2, 3 mm bands of the 30mIRAM telescope and on the 7 and 13 mm bands of the 40 m Yebes telescope. Our observations add up ~600 h of telescope time. We investigated the integrated intensities of pairs of molecular transitions for CO, other molecular species, and IRAS fluxes at 12, 25, and 60 μm. Additionally, we studied isotopic ratios, in particular 17O/18O, to analyze the initial stellar mass, as well as 12CO/13CO, to study the line and abundance ratios. Results. We present the first single-dish molecular survey of mm-wave lines in nebulae around binary post-AGB stars. We conclude that the molecular content is relatively low in nebulae around binary post-AGB stars, as their molecular lines and abundances are especially weaker compared with AGB stars. This fact is very significant in those sources where the Keplerian disk is the dominant component of the nebula. The study of their chemistry allows us to classify nebulae around AC Her, the Red Rectangle, AI CMi, R Sct, and IRAS 20056+1834 as O-rich, while that of 89 Her is probably C-rich. The calculated abundances of the detected species other than CO are particularly low compared with AGB stars. The initial stellar mass derived from the 17O/18O ratio for the Red Rectangle and 89 Her is compatible with the central total stellar mass derived from previous mm-wave interferometric maps. The very low 12CO/13CO ratios found in binary post-AGB stars reveal a high 13CO abundance compared to AGB and other post-AGB stars.

An Analysis of the $B^{3}\Pi _{2} - X^{3}\Delta _{2}$ (0, 0) Band System of the TiO Molecule in Laboratory and Sunspot Spectra

The $B^{3}\Pi _{2} - X^{3}\Delta _{2}$ (0, 0) band system of the titanium monoxide (TiO) molecule was excited in a DC copper arc with a constant deviation spectrometer. The resulting spectrum has been analyzed using image J software. Relative intensity measurements of the P- and R-branch molecular lines of the (0, 0) band with rotational quantum ( $J$ ) numbering have been obtained. The measured intensity of rotational molecular lines and the $J$ numbering were used to estimate the excitation rotational temperature of the source emitting the spectrum of TiO molecules. Also the presence of TiO spectral lines of the $B^{3}\Pi _{2} - X^{3}\Delta _{2}$ (0, 0) band in the wavenumber region of 14 500 to 16 000 cm−1 has been confirmed in the umbral spectrum, from the atlases recorded at the National Solar Observatory using the Fourier transform spectrometer (FTS) of the McMath-Pierce Solar Telescope on Kitt Peak. The combined laboratory and sunspot spectral line measurements have been used to obtain the improved molecular structure parameters for the electronic states $B^{3}\Pi _{2}$ and $X^{3}\Delta _{2}$ of the TiO molecule. Using equivalent width measurements of well resolved and identified lines in the sunspot spectrum with known rotational quantum number, the effective rotational temperature was found to be $2555 \pm 780$ K. This proves the presence of TiO molecules in sunspot and other higher temperature astrophysical sources.

Twenty-Five-Fold Reduction in Measurement Uncertainty for a Molecular Line Intensity.

To accurately attribute sources and sinks of molecules like CO_{2}, remote sensing missions require line intensities (S) with relative uncertainties u_{r}(S)<0.1%. However, discrepancies in S of ≈1% are common when comparing different experiments, thus limiting their potential impact. Here we report a cavity ring-down spectroscopy multi-instrument comparison which revealed that the hardware used to digitize analog ring-down signals caused variability in spectral integrals which yield S. Our refined approach improved measurement accuracy 25-fold, resulting in u_{r}(S)=0.06%.

Molecular-cloud-scale Chemical Composition. III. Constraints of Average Physical Properties through Chemical Models

Abstract It is important to understand the origin of molecular line intensities and chemical composition at the molecular-cloud scale in Galactic sources because it serves as a benchmark to compare with the chemical compositions of extragalactic sources. Recent observations of 3-mm spectra averaged over a 10 pc scale show similar spectral patterns among sources for molecular lines HCN, HCO+, CCH, HNC, HNCO, c-C3H2, CS, SO, N2H+, and CN. To constrain the average physical properties emitting such spectral patterns, we model molecular spectra using a time-dependent gas-grain chemical model followed by a radiative transfer calculation. We use a grid of physical parameters such as the density n = 3 × 102–3 × 104 cm−3, temperature, T = 10–30 K, visual extinction A V = 2, 4, 10 mag, cosmic-ray ionization rate ζ = 10−17–10−16 s−1, and sulfur elemental abundance S/H = 8 ×10−8–8 × 10−7. A comparison with observations indicates that spectra are well reproduced with a relatively low density of n = (1–3) × 103 cm−3, T = 10 K, ζ = 10−17 s−1, and a short chemistry timescale of 105 yr. This short chemistry timescale may indicate that molecular clouds are constantly affected by turbulence, and exposed to low-density, low-A V regions that “refresh” the chemical clock by UV radiation. The relatively low density obtained is orders of magnitude lower than the commonly quoted critical density in the optically thin case. This range of density is consistent with results from recent observational analysis of molecular-cloud-scale mapping.

Calculation of molecular line intensity in stellar atmospheres

AbstractMolecular line intensity calculations are not a straightforward task. We present a description of the basics for including molecular lines in synthetic spectra and of the input data needed. We aim both at describing ways in which molecular lines are computed in the context of photospheres of F–G–K stars, and to present a new online available version of our code for spectrum synthesis of cool stars. We apply calculations to molecular bands in the ultraviolet, visible, and near-infrared main features, and comparisons with spectra of reference stars are shown. We provide user-friendly tools for the free use of the code Pfant. The code is available athttp://trevisanj.github.io/PFANT.

Spectral line-shape study by cavity-enhanced complex refractive index spectroscopy

We present a unique experimental setup that enables measurement of high-resolution complex refractive index spectra of gas sample using several cavity-enhanced methods: frequency-stabilized cavity ring-down spectroscopy, cavity mode-width spectroscopy and one-dimensional cavity mode-dispersion spectroscopy. Presented examples of applications cover molecular spectral line-shape investigations, line intensities and unperturbed line center frequency measurements and high-precision detection of systematic instrumental errors of various spectroscopic methods.

Interpreting observations of molecular outflow sources: the MHD shock code mhd_vode

The planar MHD shock code mhd_vode has been developed in order to simulate both continuous (C) type shock waves and jump (J) type shock waves in the interstellar medium. The physical and chemical state of the gas in steady-state may also be computed and used as input to a shock wave model. The code is written principally in FORTRAN 90, although some routines remain in FORTRAN 77. The documented program and its input data are described and provided as supplementary material, and the results of exemplary test runs are presented. Our intention is to enable the interested user to run the code for any sensible parameter set and to comprehend the results. With applications to molecular outflow sources in mind, we have computed, and are making available as supplementary material, integrated atomic and molecular line intensities for grids of C- and J-type models; these computations are summarized in the Appendices.

A Spitzer–IRS spectroscopic atlas of early-type galaxies in the Revised Shapley–Ames Catalog

We produce an atlas of homogeneously reduced and calibrated low resolution IRS spectra of the nuclear regions of nearby early-type galaxies (i.e. Es and S0s, ETGs), in order to build a reference sample in the mid-infrared window. From the Spitzer Heritage Archive we extract ETGs in the "Revised Shapley-Ames Catalog of Bright Galaxies" having an IRS SL and/or LL spectrum. We recover 91 spectra out of 363 galaxies classified as ETGs in the catalog: 56 E (E0-E6), 8 mixed E/S0+S0/E, 27 S0 (both normal and barred - SB0) plus mixed types SB0/Sa+SB0/SBa. For each galaxy, we provide the fully reduced and calibrated spectrum, the intensity of nebular and molecular emission lines as well as of the Polycyclic Aromatic Hydrocarbons (PAHs) after a template spectrum of a passively evolving ETG has been subtracted. Spectra are classified into five mid-infrared classes, ranging from AGN (class-4) and star forming nuclei (class-3), transition class-2 (with PAHs) and class-1 (no-PAHs) to passively evolving nuclei (class-0). A demographic study of mid-infrared spectra shows that Es are significantly more passive than S0s: 46^{+11}_{-10}% of Es and 20^{+11}_{-7}% of S0s have a spectrum of class-0. Emission lines are revealed in 64^{+12}_{-6}% of ETGs. The H_2-S(1) line is found with similar rate in Es (34^{+10}_{-8}%) and in S0s (51^{+15}_{-12}%). PAHs are detected in 47^{+8}_{-7}% of ETGs, but only 9^{+4}_{-3}% have PAHs ratios typical of star forming galaxies. Several indicators, such as peculiar morphologies and kinematics, dust--lane irregular shape, radio and X-ray properties, suggest that mid-infrared spectral classes are connected to phases of accretion/feedback phenomena occurring in the nuclei of ETGs.

WHY ONE-DIMENSIONAL MODELS FAIL IN THE DIAGNOSIS OF AVERAGE SPECTRA FROM INHOMOGENEOUS STELLAR ATMOSPHERES

We investigate the feasibility of representing a structured multi-dimensional stellar atmosphere with a single one-dimensional average stratification for the purpose of spectral diagnosis of the atmosphere's average spectrum. In particular we construct four different one-dimensional stratifications from a single snapshot of a magneto-hydrodynamic simulation of solar convection: one by averaging its properties over surfaces of constant height, and three different ones by averaging over surfaces of constant optical depth at 500 nm. Using these models we calculate continuum, and atomic and molecular line intensities and their center-to-limb variations. From analysis of the emerging spectra we identify three main reasons why these average representations are inadequate for accurate determination of stellar atmospheric properties through spectroscopic analysis. These reasons are: non-linearity in the Planck function with temperature, which raises the average emergent intensity of an inhomogeneous atmosphere above that of an average-property atmosphere, even if their temperature-optical depth stratification is identical; non-linearities in molecular formation with temperature and density, which raise the abundance of molecules of an inhomogeneous atmosphere over that in a one-dimensional model with the same average properties; the anisotropy of convective motions, which strongly affects the center-to-limb variation of line-core intensities. We argue therefore that a one-dimensional atmospheric model that reproduces the mean spectrum of an inhomogeneous atmosphere necessarily does not reflect the average physical properties of that atmosphere, and are therefore inherently unreliable.

Spectral characteristics of an atmospheric-pressure arc discharge during the synthesis of fullerene derivatives

The electron concentration and temperature are measured when elements having various ionization potentials are introduced into the carbon-helium plasma that forms in a high-frequency atmospheric-pressure arc discharge. The effect of the introduced elements on the relative fullerene content in a carbon condensate is also studied. The plasma parameters are determined by in situ emission spectral analysis during the synthesis of fullerene derivatives with scandium, sodium, and silicon. The variation of the atomic and molecular line intensities of the plasma radiation with the distance from the carbon-arc axis is investigated upon the introduction of various doping substances. The optimal temperature conditions for the synthesis of fullerene derivatives with Sc, Na, and Si are found.

Molecular line intensities as measures of cloud masses – II. Conversion factors for specific galaxy types

We present theoretically-established values of the CO-to-H2 and C-to-H2 conversion factors that may be used to estimate the gas masses of external galaxies. We consider four distinct galaxy types, represented by M51, NGC 6946, M82 and SMC N27. The physical parameters that best represent the conditions within the molecular clouds in each of the galaxy types are estimated using a chi^2 analysis of several observed atomic fine structure and CO rotational lines. This analysis is explored over a wide range of density, radiation field, extinction, and other relevant parameters. Using these estimated physical conditions in methods that we have previously established, CO-to-H2 conversion factors are then computed for CO transitions up to J=9-8. For the conventional CO(1-0) transition, the computed conversion factor varies significantly below and above the canonical value for the Milky Way in the four galaxy types considered. Since atomic carbon emission is now frequently used as a probe of external galaxies, we also present, for the first time, the C-to-H2 conversion factor for this emission in the four galaxy types considered.

Molecular line intensities as measures of cloud masses - I. Sensitivity of CO emissions to physical parameter variations

A reliable estimate of the molecular gas content in galaxies plays a crucial role in determining their dynamical and star-forming properties. However, H-2, the dominant molecular species, is difficult to observe directly, particularly in the regions where most molecular gas is thought to reside. Its mass is therefore commonly inferred by assuming a direct proportionality with the integrated intensity of the (CO)-C-12(J = 1 --> 0) emission line, using a CO-to-H-2 conversion factor, X. Although a canonical value for X is used extensively in such estimates, there is increasing evidence, both theoretical and observational, that the conversion factor may vary by over an order of magnitude under conditions different from those of the local neighbourhood. In an effort to understand the influence of changing environmental conditions on the conversion factor, we derive theoretical estimates of X for a wide range of physical parameters using a photon-dominated region (PDR) time-dependent chemical model, benchmarking key results against those of an independent PDR code to ensure reliability. Based on these results, the sensitivity of the X factor to change in each physical parameter is interpreted in terms of the chemistry and physical processes within the cloud. In addition to confirming previous observationally derived trends, we find that the time-dependence of the chemistry, often neglected in such models, has a considerable influence on the value of the conversion factor.

Molecular line emission from planetary and protoplanetary nebulae

Molecular line emission in the mm- and submm-wave domains is found to be a very useful tool to study planetary nebulae (PNe). Molecular lines, particularly the low- transitions of carbon monoxide, are known to probe most of the nebular material in young PNe and protoplanetary nebulae (PPNe). Many quantitative results on these objects have been so obtained, including general structure, total mass and density distribution, kinetic temperatures, velocity fields, etc. In more evolved PNe, however, the molecular line intensity (or even detection) is quite uncertain, due to photodissociation. Molecular lines often trace just certain parts of these nebulae, like equatorial tori or axial clumps. Other molecules are in general more affected by photodissociation than CO, but photo- or shock-induced chemistry produces high abundances of radicals and large molecules in some intermediate-evolution objects. The presence of rotating disks in some objects is finally discussed.

A technique for extrapolating absorption coefficient measurements to high temperatures

An extrapolation technique that provides semi-quantitative estimates for the infrared absorption coefficients of gaseous fuels at temperatures beyond those for which measurements are generally practical (>700 K) is presented. The new method is based on a simplified expression for molecular line intensities consisting of three fitting parameters and two variables (temperature and frequency). The accuracy of the extrapolations was tested first by comparing predictions of absorption coefficients for CO, CO 2, and H 2O vapor to the corresponding values obtained directly from the HITEMP molecular database. Finally, to establish the practical utility of the method, the spectrum of propane at 1000 K obtained from the extrapolation technique was compared to actual experimental measurements.

Photoemission resonance and its quenching during destruction of the molecular structure of a C60 fullerite under synchrotron radiation

The photoelectron spectra of a C60 fullerene condensate are investigated. Under conditions where the photoionization (HOMO-ɛ 1) and Auger (KVV*) transitions are at resonance, the intensity of molecular lines in the photoelectron spectra increases by a factor of several tens. It is found that even insignificant destruction of the molecular structure of fullurenes under synchrotron radiation leads to quenching of the observed resonance. The quenching of the resonance manifests itself in a decrease in the intensity of the molecular lines in the photoemission spectra. The revealed effect can be used to determine the degree of radiation-induced modification of fullerenes.

Pressure-broadening coefficients and line strengths of H 2O near 1.39 μm: application to the in situ sensing of the middle atmosphere with balloonborne diode lasers

Since 1998, we have developed two balloonborne diode laser spectrometers, SDLA and micro-SDLA, to yield in situ concentration data of H 2O in the middle atmosphere by absorption spectroscopy in the 1.39-μm spectral range. In this paper, we revisit the molecular parameters, line intensities and pressure-broadening coefficients, for several lines of the ν 1+ν 3 and 2ν 1 bands, which are useful for the monitoring of atmospheric H 2O. The new values are thoroughly compared to existing molecular database and previous determinations. Calculations are performed to yield the pressure-effect coefficients: the theoretical model is described and the ab initio predictions are compared to laboratory measurements achieved with a laser diode spectrometer. The improvement in the H 2O monitoring obtained with the new set of molecular data is evaluated directly from vertical H 2O concentration profiles.

Dust in the Photospheric Environment. II. Effect on the Near‐Infrared Spectra of L and T Dwarfs

We report an attempt to interpret the spectra of L and T dwarfs with the use of the Unified Cloudy Model (UCM). For this purpose, we extend the grid of the UCMs to the cases of log g = 4.5 and 5.5. The dust column density relative to the gas column density in the observable photosphere is larger at the higher gravities, and molecular line intensity is generally smaller at the higher gravities. The overall spectral energy distributions (SEDs) are f_{J} f_{K} in early T dwarfs (L/T transition objects), and finally f_{J} > f_{H} > f_{K} in middle and late T dwarfs, where f_{J}, f_{H}, and f_{K} are the peak fluxes at J, H, and K bands, respectively, in f_{nu} unit. This tendency is the opposite to what is expected for the temperature effect, but can be accounted for as the effect of thin dust clouds formed deep in the photosphere together with the effect of the gaseous opacities including H_2 (CIA), H_2O, CH_4, and K I. Although the UCMs are semi-empirical models based on a simple assumption that thin dust clouds form in the region of T_{cr} < T < T_{cond} (T_{cr} = 1800K is an only empirical parameter while T_{cond} about 2000K is fixed by the thermodynamical data), the major observations including the overall SEDs as well as the strengths of the major spectral features are consistently accounted for throughout L and T dwarfs. In view of the formidable complexities of the cloud formation, we hope that our UCM can be of some use as a guide for future modelings of the ultracool dwarfs as well as for interpretation of observed data of L and T dwarfs.