Astron. Astrophys. 363, 970-983 (2000)

Gamma-ray line emission from OB associations and young open clusters

I. Evolutionary synthesis models

M. Cerviño ^1,2, J. Knödlseder ^2,3, D. Schaerer ¹, P. von Ballmoos ² and G. Meynet <sup>4

1</sup> Observatoire Midi-Pyrénées, 14, avenue Edouard Belin, 31400 Toulouse, France
² Centre d'Etude Spatiale des Rayonnements, CNRS/UPS, B.P. 4346, 31028 Toulouse Cedex 4, France
³ INTEGRAL Science Data Centre, Chemin d'Ecogia 16, 1290 Versoix, Switzerland
⁴ Observatoire de Genève, 1290 Sauverny, Switzerland

Received 16 August 2000 / Accepted 12 October 2000

Abstract

We have developed a new diagnostic tool for the study of gamma-ray emission lines from radioactive isotopes (such as ²⁶Al and ⁶⁰Fe) in conjunction with other multi-wavelength observables of Galactic clusters, associations, and alike objects. Our evolutionary synthesis models are based on the code of Cerviño & Mas-Hesse (1994), which has been updated to include recent stellar evolution tracks, new stellar atmospheres for OB and WR stars, and nucleosynthetic yields from massive stars during hydrostatic burning phases and explosive SN II and SN Ib events.

The temporal evolution of ²⁶Al and ⁶⁰Fe production, the equivalent yield of ²⁶Al per ionising O7 V star (), and other observables are predicted for a coeval population. The main results are:

• The emission of the ²⁶Al 1.809 MeV line is characterised by four phases: stellar wind dominated phase ( 3 Myr), SN Ib dominated phase ( 3-7 Myr), SN II dominated phase ( 7-37 Myr), and exponential decay phase ( 37 Myr).

• The equivalent yield is an extremely sensitive age indicator for the stellar population which can be used to discriminate between Wolf-Rayet star and SN II ²⁶Al nucleosynthesis in the association.

• The ratio of the ⁶⁰Fe/²⁶Al emissivity is also an age indicator that constrains the contribution of explosive nucleosynthesis to the total ²⁶Al production.

We also employed our model to estimate the steady state nucleosynthesis of a population of solar metallicity. In agreement with other works, we predict the following relative contributions to the ²⁶Al production: from stars before the WR phase, from WR stars, from SN Ib, and from SN II. For ⁶⁰Fe we estimate that are produced by SN Ib while come from SN II. Normalising on the total ionising flux of the Galaxy, we predict total production rates of 1.5 Myr^-1 and 0.8 Myr^-1 for ²⁶Al and ⁶⁰Fe, respectively. This corresponds to 1.5 of ²⁶Al and 1.7 of ⁶⁰Fe in the present interstellar medium.

To allow for a fully quantitative analysis of existing and future multi-wavelength observations, we propose a Bayesian approach that allows the inclusion of IMF richness effects and observational uncertainties in the analysis. In particular, a Monte Carlo technique is adopted to estimate probability distributions for all observables of interest. We outline the procedure of exploiting these distributions by applying our model to a fictive massive star association. Applications to existing observations of the Cygnus and Vela regions will be discussed in companion papers.

Key words: stars: abundances stars: early-type stars: supernovae: general Galaxy: open clusters and associations: general gamma rays: observations

Send offprint requests to: Miguel Cerviño

Correspondence to: mcervino@obs-mip.fr

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000

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