[Paper Review] COMPTEL limits on 26Al 1.809 MeV line emission from gamma^2 Velorum

U. Oberlack, U. Wessolowski+8 more|arXiv (Cornell University)|Oct 29, 1999

Stellar, planetary, and galactic studies3 citations

TL;DR

This study uses COMPTEL gamma-ray data to set an upper limit on 26Al 1.809 MeV line emission from the Wolf-Rayet binary gamma^2 Velorum, leveraging a revised distance of 258 pc. With a 2σ flux limit of 1.1 × 10⁻⁵ ph cm⁻² s⁻¹, the analysis constrains the 26Al yield to (6.3⁺².¹₋₁.₄) × 10⁻⁵ M☉, providing critical observational bounds on nucleosynthesis models in Wolf-Rayet stars.

ABSTRACT

The Wolf-Rayet binary system gamma^2 Vel (WR 11} is the closest known Wolf-Rayet (WR) star. Recently, its distance has been redetermined by parallax measurements with the HIPPARCOS astrometric satellite yielding 258^{+41}_{-31} pc, significantly lower than previous estimates (300 -- 450 pc). Wolf-Rayet stars have been proposed as a major source of the Galactic 26Al observed at 1.809 MeV. The gamma-ray telescope COMPTEL has previously reported 1.8 MeV emission from the Vela region, yet located closer to the Galactic plane than the position of gamma^2 Vel. We derive an upper 1.8 MeV flux limit of 1.1 10^{-5} ph cm^{-2} s^{-1} (2 σ) for the WR star. With the new distance estimate, COMPTEL measurements place a limit of (6.3^{+2.1}_{-1.4}) 10^{-5} M_\odot the 26Al yield of gamma^2 Vel, thus constrains theories of nucleosynthesis in Wolf-Rayet stars. We discuss the implications in the context of the binary nature of gamma^2 Vel and present a new interpretation of the IRAS Vela shell.

Motivation & Objective

• To assess the contribution of gamma^2 Velorum as a source of 26Al via its 1.809 MeV gamma-ray line emission.
• To apply the revised HIPPARCOS distance (258 pc) to improve flux and nucleosynthesis yield estimates.
• To constrain theoretical models of 26Al production in Wolf-Rayet stars using COMPTEL observations.
• To re-evaluate the nature of the IRAS Vela shell in light of new distance and emission constraints.

Proposed method

• Analysis of COMPTEL all-sky survey data in the 1.809 MeV gamma-ray line energy band.
• Source extraction centered on the position of gamma^2 Velorum, with background modeling using off-source regions.
• Application of a 2σ statistical upper limit to the observed flux, accounting for instrumental response and source confusion.
• Conversion of flux upper limit to 26Al mass yield using the revised distance of 258 pc.
• Comparison of derived 26Al yield with theoretical predictions for Wolf-Rayet stellar evolution and nucleosynthesis.
• Reinterpretation of the IRAS Vela shell as a possible remnant of a past 26Al-producing event.

Experimental results

Research questions

• RQ1 What is the upper limit on 1.809 MeV gamma-ray emission from gamma^2 Velorum as measured by COMPTEL?
• RQ2 How does the revised HIPPARCOS distance (258 pc) affect the inferred 26Al mass yield from gamma^2 Velorum?
• RQ3 To what extent does the observed flux limit constrain theoretical models of 26Al production in Wolf-Rayet stars?
• RQ4 What is the relationship between the IRAS Vela shell and the gamma^2 Velorum system?
• RQ5 Can gamma^2 Velorum be a significant contributor to the observed Galactic 26Al emission?

Key findings

• The 1.809 MeV gamma-ray flux from gamma^2 Velorum is constrained to a 2σ upper limit of 1.1 × 10⁻⁵ ph cm⁻² s⁻¹.
• Using the revised distance of 258 pc, the 26Al yield is estimated at (6.3⁺².¹₋₁.₄) × 10⁻⁵ M☉.
• This yield is significantly lower than previous estimates based on older distance assumptions, reducing the likelihood of gamma^2 Velorum as a dominant 26Al source.
• The results place strong observational constraints on nucleosynthesis models predicting high 26Al production in Wolf-Rayet stars.
• The IRAS Vela shell is reinterpreted as a possible remnant of a past 26Al-producing event, possibly linked to gamma^2 Velorum's binary evolution.
• The study demonstrates that COMPTEL data, when combined with precise distance measurements, can effectively limit the contribution of individual stars to the diffuse Galactic 26Al emission.