Identification of strontium in the merger of two neutron stars

Abstract: Half of all the elements in the universe heavier than iron were created by rapid neutron capture. The theory for this astrophysical $r$-process' was worked out six decades ago and requires an enormous neutron flux to make the bulk of these elements. Where this happens is still debated. A key piece of missing evidence is the identification of freshly-synthesised $r$-process elements in an astrophysical site. Current models and circumstantial evidence point to neutron star mergers as a probable $r$-process site, with the optical/infraredkilonova' emerging in the days after the merger a likely place to detect the spectral signatures of newly-created neutron-capture elements. The kilonova, AT2017gfo, emerging from the gravitational-wave--discovered neutron star merger, GW170817, was the first kilonova where detailed spectra were recorded. When these spectra were first reported it was argued that they were broadly consonant with an outflow of radioactive heavy elements, however, there was no robust identification of any element. Here we report the identification of the neutron-capture element strontium in a re-analysis of these spectra. The detection of a neutron-capture element associated with the collision of two extreme-density stars establishes the origin of $r$-process elements in neutron star mergers, and demonstrates that neutron stars contain neutron-rich matter.

• The paper identifies strontium in the kilonova AT2017gfo from the neutron star merger GW170817 using detailed spectral analysis.
• It employs LTE spectral synthesis, MOOG, and TARDIS modeling to match broad absorption features and analyze rapid expansion velocities.
• The findings support r-process nucleosynthesis theories and establish neutron star mergers as significant contributors to heavy element production.

Identification of Strontium in the Merger of Two Neutron Stars

This paper presents a significant finding in the field of astrophysics, specifically regarding the synthesis of heavy elements through rapid neutron capture, or the 'r-process', in astrophysical events. The research focuses on the identification of strontium in the kilonova associated with the neutron star merger event identified as GW170817. This detection is pivotal as it provides direct spectroscopic evidence that neutron star mergers are a site for r-process element production, supporting theories that have been debated for decades.

Summary of Findings

The spectroscopic analysis of the kilonova AT2017gfo, emerging from the neutron star merger GW170817, is the primary focus of the study. This event was previously identified through gravitational waves, providing a unique opportunity to study the aftermath of such high-density astronomical interactions. The identification of strontium was accomplished through detailed re-analysis of the spectra obtained with ESO's VLT/X-shooter instrument. The spectra covered a range from ultraviolet to near-infrared, enabling comprehensive observation of the kilonova over several days post-merger.

The initial analysis indicated deviations from a single-temperature blackbody spectrum, suggesting the presence of broad absorption features at specific wavelengths (350 nm and 810 nm). Using LTE spectral synthesis and radiative transfer modeling, the researchers identified these absorption lines as being attributable to strontium, providing robust evidence for its presence.

Methodological Approach

The research employed multiple approaches to confirm the findings:

• LTE Spectral Synthesis: Utilized to simulate potential absorption features, yielding strong strontium lines that matched the observed spectra.
• MOOG and TARDIS: These codes were employed for spectrum synthesis and radiative transfer modeling, ensuring results were consistent across varying methods and spectral line lists.
• Doppler Analysis: To account for the high expansion velocities resultant from the merger, the spectral lines were analyzed for blueshifts indicative of rapid radial expansion.

Implications and Discussion

This detection holds significant implications for astrophysics, notably affirming that neutron star mergers are indeed sites for r-process nucleosynthesis. This lends strong observational support to theoretical models predicting that such mergers could account for a substantial fraction of the universe’s heavy, neutron-rich elements. The presence of strontium underscores the need to consider lighter r-process elements, alongside heavier lanthanides, in future kilonova emission modeling efforts.

Furthermore, the identification of r-process elements in neutron star mergers corroborates the composition of neutron stars as neutron-rich environments, which was previously hypothesized but not directly observed through spectroscopic means in kilonovae. This provides a compelling case for the role of mergers in galaxy chemical evolution.

Future Directions

The study opens several avenues for future research, particularly in enhancing models of kilonova spectra to incorporate a broader range of r-process elements. Continued efforts will be required to refine our understanding of the diversity in r-process yields among different neutron star mergers. Additionally, subsequent gravitational wave detections paired with detailed spectroscopic follow-ups could provide further tests for the universality of these findings, informing both stellar nucleosynthesis models and the lifecycle of matter in the universe.

In conclusion, this research marks a critical step in substantiating neutron star mergers as key contributors to the cosmic distribution of heavy elements, with strontium identification acting as a vital observational milestone in the study of r-process element formation. Further investigations and technological advancements in spectral analysis are required to extend these findings to broader astrophysical contexts.