Iron: A Key Element for Understanding the Origin and Evolution of Interstellar Dust

TL;DR

This paper discusses the unique role of iron in interstellar dust, emphasizing its mostly gaseous origin, severe depletion in the ISM, and its growth through accretion onto existing grains, which challenges traditional dust formation models.

Contribution

It highlights that most interstellar iron is accreted in the ISM rather than formed in stellar sources, redefining understanding of dust growth processes.

Findings

Over 65% of iron is injected into the ISM in gaseous form.

Iron depletion in the gas phase indicates most iron is locked in dust.

Iron's growth occurs mainly through accretion in the ISM, not stellar condensation.

Abstract

The origin and depletion of iron differ from all other abundant refractory elements that make up the composition of the interstellar dust. Iron is primarily synthesized in Type Ia supernovae (SNe Ia) and in core collapse supernovae (CCSN), and is present in the outflows from AGB stars. Only the latter two are observed to be sources of interstellar dust, since searches for dust in SN~Ia have provided strong evidence for the absence of any significant mass of dust in their ejecta. Consequently, more than 65% of the iron is injected into the ISM in gaseous form. Yet, ultraviolet and X-ray observations along many lines of sight in the ISM show that iron is severely depleted in the gas phase compared to expected solar abundances. The missing iron, comprising about 90% of the total, is believed to be locked up in interstellar dust. This suggests that most of the missing iron must have precipitated from the ISM gas by cold accretion onto preexisting silicate, carbon, or composite grains. Iron is thus the only element that requires most of its growth to occur outside the traditional stellar condensation sources. This is a robust statement that does not depend on our evolving understanding of the dust destruction efficiency in the ISM. Reconciling the physical, optical, and chemical properties of such composite grains with their many observational manifestations is a major challenge for understanding the nature and origin of interstellar dust.