Self-generated ultraviolet radiation in molecular shock waves

Abstract

Context. Shocks are ubiquitous in the interstellar and intergalactic media, where their chemical and radiative signatures reveal the physical conditions in which they arise. Detailed astrochemical models of shocks at all velocities are necessary to understand the physics of many environments including protostellar outflows, supernova remnants, and galactic outflows. Aims. We present an accurate treatment of the self-generated ultraviolet (UV) radiation in models of intermediate velocity (V S = 25-60 km s −1), stationary, weakly magnetised, J-type, molecular shocks. We show how these UV photons modify the structure and chemical properties of shocks and quantify how the initial mechanical energy is reprocessed into line emission. Methods. We develop an iterative scheme to calculate the self-consistent UV radiation field produced by molecular shocks. The shock solutions computed with the Paris-Durham shock code are post-processed using a multi-level accelerated Λ-iteration radiative transfer algorithm to compute Lyman α, Lyman β, and two-photon continuum emission. The subsequent impacts of these photons on the ionisation and dissociation of key atomic and molecular species as well as on the heating by the photoelectric effect are calculated by taking the wavelength dependent interaction cross-sections and the fluid velocity profile into account. This leads to an accurate description of the propagation of photons and the thermochemical properties of the gas in both the postshock region and in the material ahead of the shock called the radiative precursor. With this new treatment, we analyse a grid of shock models with velocities in the range V S = 25-60 km s −1 , propagating in dense (n H 10 4 cm −3) and shielded gas. Results. Self-absorption traps Lyα photons in a small region in the shock, though a large fraction of this emission escapes by scattering into the line wings. We find a critical velocity V S ∼ 30 km s −1 above which shocks generate Lyα emission with a photon flux exceeding the flux of the standard interstellar radiation field. The escaping photons generate a warm slab of gas (T ∼ 100 K) ahead of the shock front as well as pre-ionising C and S. Intermediate velocity molecular shocks are traced by bright emission of many atomic fine structure (e.g. O and S) and metastable (e.g. O and C) lines, substantive molecular emission (e.g. H 2 , OH, and CO), enhanced column densities of several species including CH + and HCO + , as well as a severe destruction of H 2 O. As much as 13-21% of the initial kinetic energy of the shock escapes in Lyα and Lyβ photons if the dust opacity in the radiative precursor allows it. Conclusions. A rich molecular emission is produced by interstellar shocks regardless of the input mechanical energy. Atomic and molecular lines reprocess the quasi totality of the kinetic energy, allowing for the connection of observable emission to the driving source for that emission.

Atomic Shocks in the Outflow of L1551 IRS 5 Identified with SOFIA-upGREAT Observations of [OI]

“…For example, in irradiated C-shock models, the O abundance decreases with shock velocity, while the H 2 O abundance increases, resulting in a decreasing [O i]/H 2 O ratio (Melnick & Kaufman 2015; Godard et al 2019), the opposite to this SOFIA observation shows. A high UV radiation would promotes photodissociation, efficiently destroying H 2 O; however, Lehmann et al (2020) find only J-shocks can produce sufficient UV radiation to generate significant photodissociation. Smooth, steady-state disk winds can reproduce the Herschel H 2 O observations that trace outflows and contain little dust to block the UV radiation due from the accreting protostar (Yvart et al 2016).…”

“…Estimated abundances of O, CO, and H2O The total volatile oxygen abundance to H (X(O total )) is assumed as 2 × 10 −4 .velocities(Lehmann et al 2020), or disk winds. In Figure14, we show the [O i]/H 2 O intensity ratio as a function of velocity.…”

Observations and analysis of CH$^+$ vibrational emissions from the young, carbon-rich planetary nebula NGC 7027: a textbook example of chemical pumping

“…Dense PDRs (unassociated with evolved stars) and UV-and self-irradiated shock waves (e.g. Godard et al 2019, Lehmann et al 2020 are other environments in which CH + rovibrational emissions are potentially detectable; the required conditions are especially prevalent in starburst galaxies (e.g. Falgarone et al 2017).…”

Signatures of UV radiation in low-mass protostars I. Origin of HCN and CN emission in the Serpens Main region

“…The spatial extent of the enhanced CN/HCN ratio and 13 CO 6-5 emission suggest that UV photons in most protostars are likely produced in-situ, in the immediate surrounding of the outflow shocks (see Section 5.1, Yıldız et al 2015). Recent models by Lehmann et al (2020) provide predictions for molecular abundances arising in shocks where UV emission originates from the shock itself, i.e. the self-generated UV radiation.…”

“…Observations of CN and HCN in the Serpens Main are consistent with the Lehmann et al ( 2020) shock models with velocities of 35 km s −1 , but only for the regions with the highest CN/HCN ratios (Figure 13). These shocks produce UV fields G eff ∼ 25, where G eff is the flux of UV photons normalized to the average interstellar UV field (Lehmann et al 2020), and have a relatively short lifetime of ∼ 10 3 yrs. The UV fields are lower than G 0 of ∼ 10 3 predicted by the chemical model with UV (Section 4), which could be due to many different factors and assumptions.…”