IRON: A KEY ELEMENT FOR UNDERSTANDING THE ORIGIN AND EVOLUTION OF INTERSTELLAR DUST.

Area of Science:

• Astronomy and Astrophysics
• Cosmic Dust Studies
• Stellar Evolution and Supernovae

Background:

• Iron's origin and depletion patterns in interstellar dust are unique among abundant refractory elements.
• Iron is synthesized in Type Ia supernovae (SNe Ia) and core collapse supernovae (CCSN), and outflows from AGB stars, but only CCSN and AGB outflows are confirmed dust sources.
• Significant dust mass is absent in SN Ia ejecta, with over 65% of iron injected into the interstellar medium (ISM) in gaseous form.

Purpose of the Study:

• To investigate the discrepancy between gaseous iron injection and its severe depletion in the ISM.
• To understand the formation pathway of iron within interstellar dust.
• To reconcile the properties of composite grains with observational data.

Main Methods:

• Analysis of ultraviolet and X-ray observations across various ISM lines of sight.
• Comparison of observed iron depletions with solar abundances.
• Theoretical consideration of iron accretion onto existing dust grains.

Main Results:

• Iron is severely depleted in the gas phase of the ISM, with approximately 90% missing compared to solar abundances.
• The majority of missing iron is presumed to be incorporated into interstellar dust.
• Cold accretion onto pre-existing silicate, carbon, or composite grains is the likely mechanism for iron incorporation into dust.

Conclusions:

• Iron's primary growth into dust occurs in the ISM, not within stellar condensation sources like supernovae or AGB stars.
• This finding is independent of dust destruction efficiency models in the ISM.
• Understanding the physical, optical, and chemical properties of these iron-bearing composite grains is crucial for characterizing interstellar dust.