The origin of the heavy elements

Supernovae (exploding massive stars at the ends of their lives) are the main sites of heavy-element production, leading to chemical enrichment of galaxies and, ultimately, the universe. Depending on the mass of the stellar progenitor, supernova explosions may be thermonuclear or caused by the collapse of the stellar core (for progenitor stars with masses greater than eight times that of the Sun). The quantity of elements synthesized during the final stage of violent nucleosynthesis depends on the type and physics of the explosion. Direct measurements of the elements produced and constraints on the nature of both the stellar progenitors and explosion mechanism are required to understand the origin of the elements, and also how they are spread into the interstellar medium and drive heavy-element enrichment of galaxies.

The interaction of the high-velocity material ejected in the explosion (traveling at a few tens of thousands of km/s) with the surrounding medium leads to shocks that play a crucial role in generating the hot interstellar medium and possibly stimulate star formation. Heated to tens of millions of degrees, both the ejected stellar matter and the surrounding medium are powerful thermal x-ray sources. The composition of the ejecta is accessible in x-rays through observations of young supernova remnants (≤ 1000yr).

Over the last two decades, x-ray CCD spectro-imagers have profoundly changed our appreciation of the x-ray universe, opening up the field of spatially resolved x-ray spectroscopy. The first satellite equipped with x-ray CCDs was the Japanese Advanced Satellite for Cosmology and Astrophysics (ASCA), launched in 1993. With improved spatial resolution and sensitivity, the two large x-ray satellites launched in 1999, NASA's Chandra X-ray Observatory and the European Space Agency's (ESA) X-ray Multi-Mirror Mission (XMM-Newton), considerably advanced our understanding of supernova remnants. X-ray spectroscopy of young supernova remnants (see Figure 1) is important for determining their explosion and progenitor types by measuring their composition and emitting conditions.¹ The ‘Kepler’ supernova remnant (observed by Johannes Kepler in 1604) has been controversial for decades. While its high latitude above the symmetry plane of the Milky Way galaxy argued in favor of an old, metal-poor, and low-mass progenitor, implying a type Ia explosion (thermonuclear combustion of a white dwarf, a low-mass star at the end of its evolution), optical observations revealed the presence of circumstellar material. This evidence requires a young, massive, metal-rich progenitor featuring a strong stellar wind and a core-collapse explosion.² The prominence of iron emission in the ejecta spectra, combined with the absence of oxygen emission, is in agreement with the expected nucleosynthetic products from a type Ia supernova and inconsistent with core-collapse yields that predict high oxygen abundances relative to iron.³ The observed circumstellar medium might indicate the possibility of a younger, more massive progenitor for thermonuclear supernovae.

Figure 1. X-ray spectrum of the Tycho type Ia supernova remnant, obtained with the European Space Agency's (ESA) X-ray Multi-Mirror Mission (XMM-Newton). The emission lines originate from the heavy elements synthesized in the supernova and expanding into the surrounding medium. O: Oxygen. Fe: Iron. Ne: Neon. Mg: Magnesium. Si: Silicon. S: Sulfur. Ar: Argon. Ca: Calcium. (Credit: ESA/French Atomic Energy Commission-Institute of Research into the Fundamental Laws of the Universe/A. Decourchelle et al.)

Spatially resolved x-ray spectroscopy allows mapping of the emission lines from heavy elements in the ejecta, revealing the spatial distribution of the elements synthesized in the explosion and providing information on the degree of asymmetry of the explosion mechanism. In the Tycho type Ia remnant (observed in 1572 by the Danish astronomer Tycho Brahe), we found that the silicon- and iron-line maps indicate overall large-scale mixing of elements that were initially produced in an onion-skin structure,⁴ accompanied by inhomogeneities on small scales. We observed similar results for Kepler's supernova remnant, which supports a type Ia origin.⁵ Indeed, core-collapse supernova remnants seem inherently asymmetric, as illustrated by x-ray observations of the Cassiopeia A remnant, where jets and spatial inversion of a significant part of the supernova core are observed: iron, which is produced in deeper layers, is now located outside the silicon layer (see Figure 2).^6,7

Figure 2. One-million-second Chandra x-ray composite image of the core-collapse supernova remnant Cassiopeia A, showing thermal emission from silicon (red) and iron (blue) in the shocked ejecta. The nonthermal emission from electrons accelerated to tera-electronvolt energies at the shock is shown in green. (Credit: NASA/Chandra X-ray Center/Goddard Spaceflight Center/U. Hwang et al.)

Bulk x-ray Doppler-velocity maps are limited to the Cassiopeia A remnant. They reveal its 3D asymmetry, which is in agreement with theoretical expectations from a core-collapse explosion.^8,9 In addition, we found that the large flux of titanium-44 radioactive lines, determined with the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) satellite, also favors an asymmetric supernova explosion.¹⁰

In summary, supernova ejecta plasma properties vary on small scales. This is caused by asymmetries in the explosion's geometry and mixing of the various layers of matter synthesized during the explosion, which have important repercussions for nucleosynthetic yields. Future x-ray missions will perform high-spectral-resolution, spatially resolved x-ray spectroscopy that will enable measurements of Doppler velocities and determination of the 3D structure of young supernova remnants. These measurements are essential to understand the nature of supernova progenitors, the explosion mechanism, and their impact on the heavy-element enrichment of galaxies. This will be achieved with the International X-ray Observatory, a mission currently under study at ESA, the Japan Aerospace Exploration Agency, and NASA.