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Register a new userOrtho-H2 and the Age of Interstellar Dark Clouds
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Authors
Laurent Pagani
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Interstellar dark clouds are the sites of star formation. Their main component, dihydrogen, exists under two states, ortho and para. H2 is supposed to form in the ortho:para ratio (OPR) of 3:1 and to subsequently decay to almost pure para-H2 (OPR <= 0.001). Only if the H2 OPR is low enough, will deuteration enrichment, as observed in the cores of these clouds, be efficient. The second condition for strong deuteration enrichment is the local disappearance of CO, which freezes out onto grains in the core formation process. We show that this latter condition does not apply to DCO+, which, therefore, should be present all over the cloud. We find that an OPR >= 0.1 is necessary to prevent DCO+ large-scale apparition. We conclude that the inevitable decay of ortho-H2 sets an upper limit of ~6 million years to the age of starless molecular clouds under usual conditions.
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The chemistry in the diffuse interstellar medium initiates the gradual increase of molecular complexity during the life cycle of matter. A key molecule that enables build-up of new molecular bonds and new molecules via proton-donation is H3+. Its evolution is tightly related to molecular hydrogen and thought to be well understood. However, recent observations of ortho and para lines of H2 and H3+ in the diffuse ISM showed a puzzling discrepancy in nuclear spin excitation temperatures and populations between these two key species. H3+, unlike H2, seems to be out of thermal equilibrium, contrary to the predictions of modern astrochemical models. We conduct the first time-dependent modeling of the para-fractions of H2 and H3+ in the diffuse ISM and compare our results to a set of line-of-sight observations, including new measurements presented in this study. We isolate a set of key reactions for H3+ and find that the destruction of the lowest rotational states of H3+ by dissociative recombination largely control its ortho/para ratio. A plausible agreement with observations cannot be achieved unless a ratio larger than 1:5 for the destruction of (1,1)- and (1,0)-states of H3+ is assumed. Additionally, an increased CR ionization rate to 10(-15) 1/s further improves the fit whereas variations of other individual physical parameters, such as density and chemical age, have only a minor effect on the predicted ortho/para ratios. Thus our study calls for new laboratory measurements of the dissociative recombination rate and branching ratio of the key ion H3+ under interstellar conditions.
We investigate the water deuteration ratio and ortho-to-para nuclear spin ratio of H2 (OPR(H2)) during the formation and early evolution of a molecular cloud, following the scenario that accretion flows sweep and accumulate HI gas to form molecular clouds. We follow the physical evolution of post-shock materials using a one-dimensional shock model, with post-processing gas-ice chemistry simulations. This approach allows us to study the evolution of the OPR(H2) and water deuteration ratio without an arbitrary assumption concerning the initial molecular abundances, including the initial OPR(H2). When the conversion of hydrogen into H2 is almost complete, the OPR(H2) is already much smaller than the statistical value of three due to the spin conversion in the gas phase. As the gas accumulates, the OPR(H2) decreases in a non-equilibrium manner. We find that water ice can be deuterium-poor at the end of its main formation stage in the cloud, compared to water vapor observed in the vicinity of low-mass protostars where water ice is likely sublimated. If this is the case, the enrichment of deuterium in water should mostly occur at somewhat later evolutionary stages of star formation, i.e., cold prestellar/protostellar cores. The main mechanism to suppress water ice deuteration in the cloud is the cycle of photodissociation and reformation of water ice, which efficiently removes deuterium from water ice chemistry. The removal efficiency depends on the main formation pathway of water ice. The OPR(H2) plays a minor role in water ice deuteration at the main formation stage of water ice.
The formation of stars and planetary systems is a complex phenomenon, which relies on the interplay of multiple physical processes. Nonetheless, it represents a crucial stage for our understanding of the Universe, and in particular of the conditions leading to the formation of key molecules (e.g. water) on comets and planets. {it Herschel} observations demonstrated that stars form out of gaseous filamentary structures in which the main constituent is molecular hydrogen (H$_2$). Depending on its nuclear spin H$_2$ can be found in two forms: `ortho with parallel spins and `para where the spins are anti-parallel. The relative ratio among these isomers, i.e.,the ortho-to-para ratio (OPR), plays a crucial role in a variety of processes related to the thermodynamics of star-forming gas and to the fundamental chemistry affecting the formation of water in molecular clouds and its subsequent deuteration, commonly used to determine the origin of water in Solar Systems bodies. Here, for the first time, we assess the evolution of the OPR starting from the warm neutral medium, by means of state-of-the-art three-dimensional magneto-hydrodynamic simulations of turbulent molecular clouds. Our results show that star-forming clouds exhibit a low OPR ($ll 0.1$) already at moderate densities ($sim$1000 cm$^{-3}$). We also constrain the cosmic rays ionisation rate, finding that $10^{-16},rm s^{-1}$ is the lower limit required to explain the observations of diffuse clouds. Our results represent a step forward in the understanding of the star and planet formation process providing a robust determination of the chemical initial conditions for both theoretical and observational studies.
C$^+$ is a critical constituent of many regions of the interstellar medium, as it can be a major reservoir of carbon and, under a wide range of conditions, the dominant gas coolant. Emission from its 158$mu$m fine structure line is used to trace the structure of photon dominated regions in the Milky Way and is often employed as a measure of the star formation rate in external galaxies. Under most conditions, the emission from the single [CII] line is proportional to the collisional excitation rate coefficient. We here used improved calculations of the deexcitation rate of [CII] by collisions with H$_2$ to calculate more accurate expressions for interstellar C$^+$ fine structure emission, its critical density, and its cooling rate. The collision rates in the new quantum calculation are $sim$ 25% larger than those previously available, and narrow the difference between rates for excitation by atomic and molecular hydrogen. This results in [CII] excitation being quasi-independent of the molecular fraction and thus dependent only on the total hydrogen particle density. A convenient expression for the cooling rate at temperatures between 20 K and 400 K, assuming an LTE H$_2$ ortho to para ration is $Lambda ({rm LTE~OPR}) = left(11.5 + 4.0,e^{-100,mathrm K/T^{rm kin}}right);e^{-91.25,mathrm K/T^{rm kin}},n ({rm C}^{+}),n({rm H}_2)times 10^{-24};{rm ergs}~{rm cm}^{-3}~{rm s}^{-1}$. The present work should allow more accurate and convenient analysis of the [CII] line emission and its cooling.
We present analytic theory for the total column density of singly ionized carbon (C+) in the optically thick photon dominated regions (PDRs) of far-UV irradiated (star-forming) molecular clouds. We derive a simple formula for the C+ column as a function of the cloud (hydrogen) density, the far-UV field intensity, and metallicity, encompassing the wide range of galaxy conditions. When assuming the typical relation between UV and density in the cold neutral medium, the C+ column becomes a function of the metallicity alone. We verify our analysis with detailed numerical PDR models. For optically thick gas, most of the C+ column is mixed with hydrogen that is primarily molecular (H2), and this C+/H2 gas layer accounts for almost all of the `CO-dark molecular gas in PDRs. The C+/H2 column density is limited by dust shielding and is inversely proportional to the metallicity down to ~0.1 solar. At lower metallicities, H2 line blocking dominates and the C+/H2 column saturates. Applying our theory to CO surveys in low redshift spirals we estimate the fraction of C+/H2 gas out of the total molecular gas to be typically ~0.4. At redshifts 1<z<3 in massive disc galaxies the C+/H2 gas represents a very small fraction of the total molecular gas (<0.16). This small fraction at high redshifts is due to the high gas surface densities when compared to local galaxies.
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