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Register a new userChemical Abundances in a Turbulent Medium -- H$_2$, OH$^+$, H$_2$O$^+$, ArH$^+$
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Supersonic turbulence results in strong density fluctuations in the interstellar medium (ISM), which have a profound effect on the chemical structure. Particularly useful probes of the diffuse ISM are the ArH$^+$, OH$^+$, H$_2$O$^+$ molecular ions, which are highly sensitive to fluctuations in the density and the H$_2$ abundance. We use isothermal magnetohydrodynamic (MHD) simulations of various sonic Mach numbers, $mathcal{M}_s$, and density decorrelation scales, $y_{rm dec}$, to model the turbulent density field. We post-process the simulations with chemical models and obtain the probability density functions (PDFs) for the H$_2$, ArH$^+$, OH$^+$ and H$_2$O$^+$ abundances. We find that the PDF dispersions increases with increasing $mathcal{M}_s$ and $y_{rm dec}$, as the magnitude of the density fluctuations increases, and as they become more coherent. Turbulence also affects the median abundances: when $mathcal{M}_s$ and $y_{rm dec}$ are high, low-density regions with low H$_2$ abundance become prevalent, resulting in an enhancement of ArH$^+$ compared to OH$^+$ and H$_2$O$^+$. We compare our models with Herschel observations. The large scatter in the observed abundances, as well as the high observed ArH$^+$/OH$^+$ and ArH$^+$/H$_2$O$^+$ ratios are naturally reproduced by our supersonic $(mathcal{M}_s=4.5)$, large decorrelation scale $(y_{rm dec}=0.8)$ model, supporting a scenario of a large-scale turbulence driving. The abundances also depend on the UV intensity, CR ionization rate, and the cloud column density, and the observed scatter may be influenced by fluctuations in these parameters.
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We report observations of the reactive molecular ions OH$^+$, H$_2$O$^+$, and H$_3$O$^+$ towards Orion KL with Herschel/HIFI. All three $N=1-0$ fine-structure transitions of OH$^+$ at 909, 971, and 1033GHz and both fine-structure components of the doublet {it ortho}-H$_2$O$^+$ $1_{11}-0_{00}$ transition at 1115 and 1139GHz were detected; an upper limit was obtained for H$_3$O$^+$. OH$^+$ and H$_2$O$^+$ are observed purely in absorption, showing a narrow component at the source velocity of 9 kms$^{-1}$, and a broad blueshifted absorption similar to that reported recently for HF and {it para}-H$_{2}^{18}$O, and attributed to the low velocity outflow of Orion KL. We estimate column densities of OH$^+$ and H$_2$O$^+$ for the 9 km s$^{-1}$ component of $9 pm 3 times 10^{12}$cm$^{-2}$ and $7 pm 2 times 10^{12}$cm$^{-2}$, and those in the outflow of $1.9 pm 0.7 times 10^{13}$cm$^{-2}$ and $1.0 pm 0.3 times 10^{13}$cm$^{-2}$. Upper limits of $2.4times 10^{12}$cm$^{-2}$ and $8.7times 10^{12}$cm$^{-2}$ were derived for the column densities of {it ortho} and {it para}-H$_3$O$^+$ from transitions near 985 and 1657GHz. The column densities of the three ions are up to an order of magnitude lower than those obtained from recent observations of W31C and W49N. The comparatively low column densities may be explained by a higher gas density despite the assumption of a very high ionization rate.
We calculated reaction rate constants including atom tunneling of the reaction of dihydrogen with the hydroxy radical down to a temperature of 50 K. Instanton theory and canonical variational theory with microcanonical optimized multidimensional tunneling (CVT/$mu$OMT) were applied using a fitted potential energy surface [J. Chem. Phys. 138, 154301 (2013)]. All possible protium/deuterium isotopologues were considered. Atom tunneling increases at about 250 K (200 K for deuterium transfer). Even at 50 K the rate constants of all isotopologues remain in the interval $ 4 cdot 10^{-20}$ to $4 cdot 10^{-17}$ cm$^3$ s$^{-1}$ , demonstrating that even deuterat
Context: Herschel observations suggest that the H$_2$O distribution in outflows from low-mass stars resembles the H$_2$ emission. It is still unclear which of the different excitation components that characterise the mid- and near-IR H$_2$ distribution is associated with H$_2$O. Aim: The aim is to spectrally resolve the different excitation components observed in the H$_2$ emission. This will allow us to identify the H$_2$ counterpart associated with H$_2$O and finally derive directly an H$_2$O abundance estimate with respect to H$_2$. Methods: We present new high spectral resolution observations of H$_2$ 0-0 S(4), 0-0 S(9), and 1-0 S(1) towards HH 54, a bright nearby shock region in the southern sky. In addition, new Herschel-HIFI H$_2$O (2$_{12}$$-$1$_{01}$) observations at 1670~GHz are presented. Results: Our observations show for the first time a clear separation in velocity of the different H$_2$ lines: the 0-0 S(4) line at the lowest excitation peaks at $-$7~km~s$^{-1}$, while the more excited 0-0 S(9) and 1-0 S(1) lines peak at $-$15~km~s$^{-1}$. H$_2$O and high-$J$ CO appear to be associated with the H$_2$ 0-0 S(4) emission, which traces a gas component with a temperature of 700$-$1000 K. The H$_2$O abundance with respect to H$_2$ 0-0 S(4) is estimated to be $X$(H$_2$O)$<$1.4$times$10$^{-5}$ in the shocked gas over an area of 13$^{primeprime}$. Conclusions: We resolve two distinct gas components associated with the HH 54 shock region at different velocities and excitations. This allows us to constrain the temperature of the H$_2$O emitting gas ($leq$1000 K) and to derive correct estimates of H$_2$O abundance in the shocked gas, which is lower than what is expected from shock model predictions.
In the cold neutral medium, high out-of-equilibrium temperatures are created by intermittent dissipation processes, including shocks, viscous heating, and ambipolar diffusion. The high-temperature excursions are thought to explain the enhanced abundance of CH$^{+}$ observed along diffuse molecular sight-lines. Intermittent high temperatures should also have an impact on H$_2$ line luminosities. We carry out simulations of MHD turbulence in molecular clouds including heating and cooling, and post-process them to study H$_2$ line emission and hot-gas chemistry, particularly the formation of CH$^+$. We explore multiple magnetic field strengths and equations of state. We use a new H$_2$ cooling function for $n_{rm H} leq 10^5,{rm cm}^{-3}$, $Tleq 5000,{rm K}$, and variable H$_2$ fraction. We make two important simplifying assumptions: (i) the ${rm H}_2/{rm H}$ fraction is fixed everywhere, and (ii) we exclude from our analysis regions where the ion-neutral drift velocity is calculated to be greater than 5 km/s. Our models produce H$_2$ emission lines in accord with many observations, although extra excitation mechanisms are required in some clouds. For realistic r.m.s. magnetic field strengths ($approx 10$ $mu$G) and velocity dispersions, we reproduce observed CH$^+$ abundances. These findings contrast with those of Valdivia et al. (2017). Comparison of predicted dust polarization with observations by {it Planck} suggests that the mean field $gtrsim 5 mu$G, so that the turbulence is sub-Alfvenic. We recommend future work treating ions and neutrals as separate fluids to more accurately capture the effects of ambipolar diffusion on CH$^+$ abundance.
Transmission spectroscopy of exoplanets has the potential to provide precise measurements of atmospheric chemical abundances, in particular of hot Jupiters whose large sizes and high temperatures make them conducive to such observations. To date, several transmission spectra of hot Jupiters have revealed low amplitude features of water vapour compared to expectations from cloud-free atmospheres of solar metallicity. The low spectral amplitudes in such atmospheres could either be due to the presence of aerosols that obscure part of the atmosphere or due to inherently low abundances of H$_2$O in the atmospheres. A recent survey of transmission spectra of ten hot Jupiters used empirical metrics to suggest atmospheres with a range of cloud/haze properties but with no evidence for H$_2$O depletion. Here, we conduct a detailed and homogeneous atmospheric retrieval analysis of the entire sample and report the H$_2$O abundances, cloud properties, terminator temperature profiles, and detection significances of the chemical species. Our present study finds that the majority of hot Jupiters have atmospheres consistent with sub-solar H$_2$O abundances at their day-night terminators. The best constrained abundances range from $mathrm{log(H_2O)}$ of $-5.04^{+0.46}_{-0.30}$ to $-3.16^{+0.66}_{-0.69}$, which compared to expectations from solar-abundance equilibrium chemistry correspond to $0.018^{+0.035}_{-0.009}times$ solar to $1.40^{+4.97}_{-1.11}times$ solar. Besides H$_2$O we report statistical constraints on other chemical species and cloud/haze properties, including cloud/haze coverage fractions which range from $0.18^{+0.26}_{-0.12}$ to $0.76^{+0.13}_{-0.15}$. The retrieved H$_2$O abundances suggest sub-solar oxygen and/or super-solar C/O ratios, and can provide important constraints on the formation and migration pathways of hot giant exoplanets.
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