Ultraviolet Spectroscopy of HD 44179

TL;DR

This study re-analyzed the ultraviolet spectrum of HD 44179, refining molecular temperature estimates, identifying excitation mechanisms, and examining molecular features to understand the nebula's composition and outflow dynamics.

Contribution

The paper provides improved measurements of CO and other molecular features in HD 44179's spectrum, clarifies excitation processes, and discusses the origin of observed molecules.

Findings

Confirmed non-photoexcitation origin of CO Cameron bands.

Detected high-velocity blue-shifted hot CO bands.

Estimated rovibrational temperatures of OH system.

Abstract

We have re-analyzed the ultraviolet spectrum of HD 44179, the central star(s) of the Red Rectangle nebula, providing improved estimates of the column density, rotational, and vibrational temperatures of the 4th Positive A-X system of CO in absorption. The flux shortward of 2200 A is a complex blend of CO features with no discernible stellar photosphere, making the identification of other molecular species difficult, and the direct derivation of the dust extinction curve impossible. We confirm that the spin-forbidden CO (a-X) Cameron bands are likely produced by either collisional excitation or a chemical reaction, not photoexcitation, but with a higher internal vibrational excitation than previously determined. We also detect the spin-forbidden CO a'-X, d-X, and e-X absorption features. The hot CO (A-X) bands exhibit a blue-shift of ~300 km/s, likely occurring close to the white dwarf star(s) suspected as the original source of the ultraviolet flux in the system, and forming the base of the outflow of material in the Red Rectangle. The OH "comet-band" system near 3000 A is also analyzed, and estimates of its rovibrational temperatures determined. The source of the molecules studied in this system is still unknown, but may be a combination of gaseous material associated with the star(s), or processed material from the surrounding dust torus.