Highly Excited H2 in Herbig-Haro 7: Formation Pumping in Shocked Molecular Gas?

TL;DR

This study presents detailed near-infrared spectra of the HH7 bow shock, revealing highly excited molecular hydrogen populations that suggest formation pumping on dust grains as a key process in shock environments.

Contribution

It provides the first detailed spectral analysis showing a hot H2 component consistent with formation pumping, challenging standard shock models.

Findings

Detection of H2 lines from levels up to 50,000 K.

Two-temperature model fit with 98.5% at 1800 K and 1.5% at 5200 K.

Hot component likely due to H2 formation on dust grains after shock dissociation.

Abstract

We have obtained K-band spectra at R~5,000 and angular resolution 0.3" of a section of the Herbig-Haro 7 (HH7) bow shock, using the Near-Infrared Integral Field Spectrograph at Gemini North. Present in the portion of the data cube corresponding to the brightest part of the bow shock are emission lines of H2 with upper state energies ranging from ~6,000 K up to the dissociation energy of H2, ~50,000 K. Because of low signal-to-noise ratios, the highest excitation lines cannot be easily seen elsewhere in the observed region. However, excitation temperatures, measured throughout much of the observed region using lines from levels as high as 25,000 K, are a strong function of upper level energy, indicating that the very highest levels are populated throughout. The level populations in the brightest region are well fit by a two-temperature model, with 98.5% of the emitting gas at T=1800 K and 1.5% at T=5200 K. The bulk of the H2 line emission in HH7, from the 1,800 K gas, has previously been well modeled by a continuous shock, but the 5,200 K component is inconsistent with standalone standard continuous shock models. We discuss various possible origins for the hot component and suggest that this component is H2 newly reformed on dust grains and then ejected from them, presumably following dissociation of some of the H2 by the shock.