Higher Rotational Transitions Research Articles

Rotational excitation of highly excited H2O by H2

ABSTRACT Water is a key molecule for interstellar chemistry. Observations with Herschel telescope show significant population of very high rotational transitions (j ≳ 8) in young stellar objects, indicating significant amounts of water in hot (T ≳ 1500 K) and dense (n ≳ 106 cm−3) gas. Non-local thermodynamic equilibrium (LTE) modelling of these observations requires the knowledge of the collisional and radiative properties of highly excited water at high temperature. The aim of this work is to calculate a new set of excitation rate coefficients for both para- and ortho-H2O induced by collisions with H2 for energy levels up to j = 17. Quantum scattering calculations were performed using a reduced dimensional approach and the coupled states approximation. Rate coefficients were obtained for 97 pure rotational energy levels of both para- and ortho-H2O and for temperatures up to 2000 K. With the forthcoming launch of the James Webb Space Telescope, these new collisional data will allow us to gain more insight into the physical conditions in star- and planet-forming regions.

Double Resonance Rotational Spectroscopy of Weakly Bound Ionic Complexes: The Case of Floppy CH_{3}^{+}-He.

A novel rotational spectroscopy method applicable to ions stored in cold traps is presented. In a double resonance scheme, rotational excitation is followed by vibrational excitation into a dissociative resonance. Its general applicability is demonstrated for the CH_{3}^{+}-He complex, which undergoes predissociation through its C-H stretching modes ν_{1} and ν_{3}. High resolution rotational transitions are recorded for this symmetric top, and small unexpected splittings are resolved for K=1. Advantages and potential future applications of this new approach are discussed.

C2H observations toward the Orion Bar

C$_2$H is one of the first radicals to be detected in the interstellar medium. Its higher rotational transitions have recently become available with the Herschel Space Observatory. We aim to constrain the physical parameters of the C$_2$H emitting gas toward the Orion Bar. We analyse the C$_2$H line intensities measured toward the Orion Bar CO$^+$ Peak and Herschel/HIFI maps of C$_2$H, CH, and HCO$^+$, and a NANTEN map of [CI]. We interpret the observed C$_2$H emission using radiative transfer and PDR models. Five rotational transitions of C$_2$H have been detected in the HIFI frequency range toward the CO$^+$ peak. A single component rotational diagram gives a rotation temperature of ~64 K and a beam-averaged C$_2$H column density of 4$\times$10$^{13}$ cm$^{-2}$. The measured transitions cannot be explained by any single parameter model. According to a non-LTE model, most of the C$_2$H column density produces the lower-$N$ C$_2$H transitions and traces a warm ($T_{\rm{kin}}$ ~ 100-150 K) and dense ($n$(H$_2$)~10$^5$-10$^6$ cm$^{-3}$) gas. A small fraction of the C$_2$H column density is required to reproduce the intensity of the highest-$N$ transitions ($N$=9-8 and N=10-9) originating from a high density ($n$(H$_2$)~5$\times$10$^6$ cm$^{-3}$) hot ($T_{\rm{kin}}$ ~ 400 K) gas. The total beam-averaged C$_2$H column density in the model is 10$^{14}$ cm$^{-2}$. Both the non-LTE radiative transfer model and a simple PDR model representing the Orion Bar with a plane-parallel slab of gas and dust suggest, that C$_2$H cannot be described by a single pressure component, unlike the reactive ion CH$^+$, which was previously analysed toward the Orion Bar CO$^+$ peak. The physical parameters traced by the higher rotational transitions ($N$=6-5,...,10-9) of C$_2$H may be consistent with the edges of dense clumps exposed to UV radiation near the ionization front of the Orion Bar.

MOLECULAR TRACERS OF TURBULENT SHOCKS IN GIANT MOLECULAR CLOUDS

Giant molecular clouds contain supersonic turbulence and simulations of magnetohydrodynamic turbulence show that these supersonic motions decay in roughly a crossing time, which is less than the estimated lifetimes of molecular clouds. Such a situation requires a significant release of energy. We run models of C-type shocks propagating into gas with densities around 10^3 cm^(-3) at velocities of a few km / s, appropriate for the ambient conditions inside of a molecular cloud, to determine which species and transitions dominate the cooling and radiative energy release associated with shock cooling of turbulent molecular clouds. We find that these shocks dissipate their energy primarily through CO rotational transitions and by compressing pre-existing magnetic fields. We present model spectra for these shocks and by combining these models with estimates for the rate of turbulent energy dissipation, we show that shock emission should dominate over emission from unshocked gas for mid to high rotational transitions (J >5) of CO. We also find that the turbulent energy dissipation rate is roughly equivalent to the cosmic-ray heating rate and that the ambipolar diffusion heating rate may be significant, especially in shocked gas.

Analysis of Hydrogen Cyanide Hyperfine Spectral Components towards Star Forming Cores

Abstract Although hydrogen cyanide has become quite a common molecular tracing species for a variety of astrophysical sources, it, however, exhibits dramatic non-LTE behaviour in its hyperfine line structure. Individual hyperfine components can be strongly boosted or suppressed. If these so-called hyperfine line anomalies are present in the HCN rotational spectra towards low or high mass cores, this will affect the interpretation of various physical properties such as the line opacity and excitation temperature in the case of low mass objects and infall velocities in the case of their higher mass counterparts. Anomalous line ratios are present either through the relative strengths of neighboring hyperfine lines or through the varying widths of hyperfine lines belonging to a particular rotational line. This work involves the first observational investigation of these anomalies in two HCN rotational transitions, J=1→0 and J=3→2, towards both low mass starless cores and high mass protostellar objects. The degree of anomaly in these two rotational transitions is considered by computing the ratios of neighboring hyperfine lines in individual spectra. Results indicate some degree of anomaly is present in all cores considered in our survey, the most likely cause being line overlap effects among hyperfine components in higher rotational transitions.

A Multitransition HCO+Study in NGC 2264G: Anomalous Emission of theJ = 1 → 0 Line

We present multitransition observations of the HCO+ molecule toward the very young star-forming region associated with the NGC 2264G molecular outflow. Anomalous emission is observed in the lowest rotational transition; the J = 4 → 3 and J = 3 → 2 transitions clearly trace the dense core encompassing the exciting source of the molecular outflow, whereas the HCO+ J = 1 → 0 is barely detected at a much lower intensity and has a much broader line shape. Analysis of the data strongly suggests that the HCO+ J = 1 → 0 emission arising from the core is being absorbed efficiently by a cold low-density envelope around the core or a foreground cloud. This result seems exceptional, yet the J = 1 → 0 HCO+ and HCN emission from other dense cores (especially those in giant molecular clouds) may be affected. In these cases, the rare isotopes of these molecules and higher rotational transitions of the main isotopes should be used to study these regions. Two quiescent clumps, JMG 99 G1 and G2, are detected in the blue lobe of the NGC 2264G molecular outflow, close to shock-excited near-IR H2 knots. These clumps belong to the class of radiatively excited clumps, i.e., the radiation from the shock evaporates the dust mantles and initiates a photochemical process, enhancing the emission of the HCO+.

Theoretical modelling of SiO maser lineshapes

Various factors affecting the lineshapes of SiO masers have been considered: saturation, competitive gain, velocity redistribution and properties of the circumstellar envelope supporting the masers. The lineshape of a single maser spot is likely to be dominated by conditions of complete velocity redistribution (CVR) for low to moderate amplification, changing to NVR (no velocity redistribution) only for strongly saturating masers. Both CVR and NVR regimes can produce peculiar lineshapes. When many maser ‘cells’ were considered, distributed at random in the stellar envelope, the individual lineshapes were combined to produce synthetic spectra. Maser action is predicted in higher rotational transitions than those observed to date, for example the J=7-6 transition of v=2.

Fast molecular shocks. II - Emission from fast dissociative shocks

The line radiations emitted in the cooling gas behind a fast dissociative shock are studied. The intensities emitted in high rotational transitions of the neutral molecules CO, SiO, HCN, CN, NO, and SO are estimated, as well as in rovibrational transitions of the molecular ions HeH(+) and OH(+) in radio recombination lines of atomic hydrogen and in fine-structure transitions of C, C(+), O, and Si(+). The predictions are compared with the observed intensities of line emission from the Orion-KL region. For Orion-KL the observations do not exclude, but probably do not require, the presence of a fast dissociative shock. Emission from SiO in high-J rotational states and from vibrationally excited OH(+), HeH(+), HeH(+), and SO(+) may be detectable from dissociative shocks under suitable conditions of preshock density and shock velocity; such emission may prove to be a useful diagnostic probe of fast shock activity.

Laser magnetic resonance of sulphur radicals

High rotational transitions of the radical NS have been observed using an FIR optically-pumped laser magnetic resonance (LMR) spectrometer. The spectra were recorded at wavelengths of 680 and 514 μm (from C 2H 3Br and DCOF lasers, respectively) and have been assigned to individual Zeeman components of the rotational transitions J = 8.5→ 9.5 and J = 11.5→ 12.5 in the lowest vibrational level of the 2Π 3 2 ground electronic state. New FIR LMR spectra of a sulphur radical have been detected with a water-vapour discharge laser oscillating on the 108 μm D 2O line. The spectra are tentatively attributed to HCS, a radical not previously detected, and the assignment is based mainly on chemical evidence at this stage.

Application of the PHD approximation to the high rotational transitions in the Ar + N 2 system

The p-helicity-decoupling (PHD) scheme is applied to high rotational transitions in the Ar + N 2 system and found to give more accurate cross sections and opacity functions than other known approximations.

Tunable laser measurements of water vapor transitions in the vicinity of 5 μm

The present results confirm earlier tunable laser observations that the atmospheric widths of high rotational energy transitions are considerably narrower than previously predicted. Intensities, self-broadening coefficients, nitrogen and air shifts, and relative positions have also been measured.