[Paper Review] The Molecular Condensations Ahead of Herbig-Haro Objects. III. Radiative and dynamical perturbations of the HH 2 condensation

J. M. Girart, S. Viti+2 more|ArXiv.org|Apr 15, 2005

Astrophysics and Star Formation Studies47 references11 citations

TL;DR

This study investigates radiative and dynamical perturbations in the molecular gas ahead of Herbig-Haro object HH 2 using high-resolution BIMA observations and chemical modeling. It reveals that despite the cloud's apparent quiescence, UV radiation and outflow winds from the VLA 1 protostar induce complex, structured chemistry and kinematics, indicating active disruption of molecular material through non-dissociative shocks and photochemistry, with up to 10–15% of the region’s mass potentially being driven out.

ABSTRACT

We have carried out an extensive observational study (from BIMA data) and made a preliminary theoretical investigation of the molecular gas around HH2. The molecular maps show a very complex morphological, kinematical and chemical structure. The overall main conclusion of this work confirms the findings of Paper I and II, by demonstrating that in addition to the strong photochemical effects caused by penetration of the UV photons from HH2 into molecular cloud, a range of complex radiative and dynamical interactions occur. Thus, despite the apparent `quiescent' nature of the molecular cloud ahead of HH2, the kinematical properties observed within the field of view suggest that it is possibly being driven out by powerful winds from the VLA 1 protostar.

Motivation & Objective

• To understand the radiative and dynamical effects of HH 2’s UV radiation and VLA 1 outflow on surrounding molecular gas.
• To determine whether the apparent quiescence of the molecular cloud ahead of HH 2 is genuine or masked by hidden dynamical processes.
• To characterize the chemical and kinematical structure of distinct subregions (Ahead Core, SO2 Clump, West Core, High Velocity Region) using multi-line molecular emission.
• To assess the role of photochemistry and shock processes in shaping molecular abundances and temperature structures.
• To evaluate whether the observed chemistry and kinematics can be explained by UV irradiation, shocks, or a combination of both.

Proposed method

• Acquired high-angular and spectral resolution molecular line data using the BIMA interferometer across multiple frequency setups (72–113 GHz), covering key transitions of molecules like HCO+, SO2, HCN, CH3OH, and CO isotopologues.
• Mapped molecular gas in four subregions: the Ahead Core, SO2 Clump, West Core, and High Velocity Region, based on morphological, kinematical, and chemical distinctions.
• Conducted a four-point chemical analysis of the SO2 Clump to test for substructure, using relative molecular abundances and density gradients as constraints.
• Applied chemical models of irradiated clumps and PDRs to interpret observed abundances, particularly for species like HCO+, CS, CH3OH, and SO2.
• Modeled shock chemistry using non-dissociative shock models to explain enhanced HCO+ without strong SiO or CH3OH emission.
• Used multi-wavelength data (e.g., [SII], Hα, mid-IR) to identify shocked gas and PDR-like structures, and to test geometric interpretations (e.g., ring structures as projection effects).

Experimental results

Research questions

• RQ1 What is the origin of the complex chemical structure observed in the Ahead Core and SO2 Clump, and does it reflect substructure or external irradiation?
• RQ2 How do UV photons from HH 2 and outflow shocks alter molecular abundances and excitation conditions in the pre-existing dense gas?
• RQ3 Why is HCO+ enhanced in the High Velocity Region while other shock tracers (SiO, CS, CH3OH) are not detected?
• RQ4 Can the observed morphology of the West Core and its surrounding ring structure be explained by projection effects or by a physical PDR/warm interface?
• RQ5 To what extent is the molecular gas ahead of HH 2 being dynamically disrupted by the VLA 1 outflow, and what fraction of the mass is involved?

Key findings

• The Ahead Core, located ahead of HH 2, exhibits a non-homogeneous structure with varying molecular abundances, indicating it is composed of small, chemically distinct clumps exposed to a weak UV field.
• The SO2 Clump, more exposed to HH 2’s UV radiation, shows increasing density and molecular abundances toward HH 2, suggesting compression by VLA 1 winds and substructure consistent with photochemical processing.
• The West Core, surrounded by a ring of shocked gas and mid-IR emission, is best explained by a combination of an old, photo-processed dense clump and a PDR or warm interface created by outflow interaction, with HCO+ emission likely originating in the PDR or interface.
• The High Velocity Region is traced exclusively by HCO+ and not by other shock tracers, indicating non-dissociative shocks and strong UV fields, though models overpredict HCN and CN abundances.
• Despite its apparent quiescence, the molecular cloud is dynamically perturbed: kinematical evidence suggests it is being driven out by VLA 1 winds, with up to 10–15% of the region’s mass potentially disrupted.
• The study confirms that both radiative (UV-driven photochemistry) and dynamical (shock, wind compression) processes leave distinct chemical signatures, demonstrating complex interactions beyond simple irradiation.