Some runaway stars are known to display IR arclike structures around them, resulting from their interaction with surrounding interstellar material. The properties of these features as well as the ...processes involved in their formation are still poorly understood. We aim to understand the physical mechanisms that shape the dust arc observed near the runaway Ostar AE Aur (HD 34078). The presence of molecular globulettes at such a close distance from an O star is unexpected, and probably related to the high proper motion of HD 34078. Further observational and theoretical work is needed to fully elucidate the processes shaping the gas and dust in bow shocks around runaway O stars.
H 2 , H 3 + and the age of molecular clouds and prestellar cores Pagani, L.; Lesaffre, P.; Roueff, E. ...
Philosophical transactions - Royal Society. Mathematical, Physical and engineering sciences/Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences,
11/2012, Letnik:
370, Številka:
1978
Journal Article
Recenzirano
Odprti dostop
Measuring the age of molecular clouds and prestellar cores is a difficult task that has not yet been successfully accomplished although the information is of paramount importance to help in ...understanding and discriminating between different formation scenarios. Most chemical clocks suffer from unknown initial conditions and are therefore difficult to use. We propose a new approach based on a subset of deuterium chemistry that takes place in the gas phase and for which initial conditions are relatively well known. It relies primarily on the conversion of H
3
+
into H
2
D
+
to initiate deuterium enrichment of the molecular gas. This conversion is controlled by the ortho/para ratio of H
2
that is thought to be produced with the statistical ratio of 3 and subsequently slowly decays to an almost pure para-H
2
phase. This slow decay takes approximately 1 Myr and allows us to set an upper limit on the age of molecular clouds. The deuterium enrichment of the core takes longer to reach equilibrium and allows us to estimate the time necessary to form a dense prestellar core, i.e. the last step before the collapse of the core into a protostar. We find that the observed abundance and distribution of DCO
+
and N
2
D
+
argue against quasi-static core formation and favour dynamical formation on time scales of less than 1 Myr. Another consequence is that ortho-H
2
remains comparable to para-H
2
in abundance outside the dense cores.
Context. Observations have long demonstrated the molecular diversity of the diffuse interstellar medium (ISM). Only now, with the advent of high-performance computing, does it become possible for ...numerical simulations of astrophysical fluids to include a treatment of chemistry, to faithfully reproduce the abundances of the many observed species, and especially that of CO, which is used as a proxy for molecular hydrogen. When applying photon-dominated region (PDR) codes to describe the UV-driven chemistry of uniform density cloud models, it is found that the observed abundances of CO are not well reproduced. Aims. Our main purpose is to estimate the effect of assuming uniform density on the line-of-sight in PDR chemistry models, compared to a more realistic distribution for which total gas densities may well vary by several orders of magnitude. A secondary goal of this paper is to estimate the amount of molecular hydrogen that is not properly traced by the CO (J = 1 → 0) line, the so-called “dark molecular gas”. Methods. We used results from a magnetohydrodynamical (MHD) simulation as a model for the density structures found in a turbulent diffuse ISM with no star-formation activity. The Meudon PDR code was then applied to several lines of sight through this model, to derive their chemical structures. Results. We found that compared to the uniform density assumption, maximal chemical abundances for H2, CO, CH and CN are increased by a factor ~2–4 when taking into account density fluctuations on the line of sight. The correlations between column densities of CO, CH and CN with respect to those of H2 are also found to be in better overall agreement with observations. For instance, at N(H2) ≳ 2 × 1020 cm-2, while observations suggest that d log N(CO) /d log N(H2) ≃ 3.07 ± 0.73, we find d log N(CO) /d log N(H2) ≃ 14 when assuming uniform density, and d log N(CO) /d log N(H2) ≃ 5.2 when including density fluctuations.
We performed high-resolution, 3D MHD simulations and we compared to observations of translucent molecular clouds. We show that the observed populations of rotational levels of H2 can arise as a ...consequence of the multi-phase structure of the ISM.
Prestellar cores form from the contraction of cold gas and dust material in dark clouds before they collapse to form protostars. Several concurrent theories exist to describe this contraction but ...they are currently difficult to distinguish. To tell the difference between these theories, measuring the age of prestellar cores could greatly help. However, no reliable clock currently exists. We present a simple chemical clock based on the regulation of the deuteration by the abundance of ortho-Hsub 2 that slowly decays away from the ortho-para statistical ratio of 3 down to or less than 0.001. The chemical dock therefore rules out slow contraction in L183 and steady-state chemical models, since steady state is clearly not reached here. This clock should be applied to other cores to help distinguish slow and fast contraction theories over a large sample of cases.
Observations have long demonstrated the molecular diversity of the diffuse interstellar medium (ISM). Only now, with the advent of high-performance computing, does it become possible for numerical ...simulations of astrophysical fluids to include a treatment of chemistry, to faithfully reproduce the abundances of the many observed species, and especially that of CO, which is used as a proxy for molecular hydrogen. The authors main purpose is to estimate the effect of assuming uniform density on the line-of-sight in photon-dominated region chemistry models, compared to a more realistic distribution for which total gas densities may well vary by several orders of magnitude. They found that compared to the uniform density assumption, maximal chemical abundances for H2, CO, CH and CN are increased by a factor similar to 2-4 when taking into account density fluctuations on the line of sight. The correlations between column densities of CO, CH and CN with respect to those of H2 are also found to be in better overall agreement with observations.
H 2 , H 3 + and the age of molecular clouds and prestellar cores Pagani, L.; Lesaffre, P.; Roueff, E. ...
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
11/2012, Letnik:
370, Številka:
1978
Journal Article
Recenzirano
Measuring the age of molecular clouds and prestellar cores is a difficult task that has not yet been successfully accomplished although the information is of paramount importance to help in ...understanding and discriminating between different formation scenarios. Most chemical clocks suffer from unknown initial conditions and are therefore difficult to use. We propose a new approach based on a subset of deuterium chemistry that takes place in the gas phase and for which initial conditions are relatively well known. It relies primarily on the conversion of H 3 + into H 2 D + to initiate deuterium enrichment of the molecular gas. This conversion is controlled by the ortho/para ratio of H 2 that is thought to be produced with the statistical ratio of 3 and subsequently slowly decays to an almost pure para-H 2 phase. This slow decay takes approximately 1 Myr and allows us to set an upper limit on the age of molecular clouds. The deuterium enrichment of the core takes longer to reach equilibrium and allows us to estimate the time necessary to form a dense prestellar core, i.e. the last step before the collapse of the core into a protostar. We find that the observed abundance and distribution of DCO + and N 2 D + argue against quasi-static core formation and favour dynamical formation on time scales of less than 1 Myr. Another consequence is that ortho-H 2 remains comparable to para-H 2 in abundance outside the dense cores.
H2, H[Formula] and the age of molecular clouds and prestellar cores Pagani, L; Lesaffre, P; Roueff, E ...
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
11/2012, Letnik:
370, Številka:
1978
Journal Article
Recenzirano
Measuring the age of molecular clouds and prestellar cores is a difficult task that has not yet been successfully accomplished although the information is of paramount importance to help in ...understanding and discriminating between different formation scenarios. Most chemical clocks suffer from unknown initial conditions and are therefore difficult to use. We propose a new approach based on a subset of deuterium chemistry that takes place in the gas phase and for which initial conditions are relatively well known. It relies primarily on the conversion of HFormula into H2D+ to initiate deuterium enrichment of the molecular gas. This conversion is controlled by the ortho/para ratio of H2 that is thought to be produced with the statistical ratio of 3 and subsequently slowly decays to an almost pure para-H2 phase. This slow decay takes approximately 1 Myr and allows us to set an upper limit on the age of molecular clouds. The deuterium enrichment of the core takes longer to reach equilibrium and allows us to estimate the time necessary to form a dense prestellar core, i.e. the last step before the collapse of the core into a protostar. We find that the observed abundance and distribution of DCO+ and N2D+ argue against quasi-static core formation and favour dynamical formation on time scales of less than 1 Myr. Another consequence is that ortho-H2 remains comparable to para-H2 in abundance outside the dense cores.
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