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

1. Introduction

OB associations and young open clusters are the most active nucleosynthetic sites in our Galaxy. The combined activity of stellar winds and core-collapse supernovae ejects significant amounts of freshly synthesised nuclei into the interstellar medium. Among those, radioactive isotopes, such as ²⁶Al ( yr) or ⁶⁰Fe ( yr), may eventually be observed by gamma-ray instruments through their characteristic decay-line signatures. Their observation presents direct evidence of recent nucleosynthesis activity, which can be used as a powerful diagnostics tool for studies of present galactic activity.

Galactic 1.809 MeV gamma-ray line emission attributed to the radioactive decay of ²⁶Al has been observed by numerous gamma-ray telescopes, and the detailed mapping of the emission distribution by the COMPTEL telescope has clearly identified massive stars as the source of this radio-isotope (see Prantzos & Diehl 1996 for a review). The most convincing evidence for a massive star origin comes from the close resemblance between the 1.809 MeV and galactic free-free emission, which links ²⁶Al nucleosynthesis to the O star population (Knödlseder et al. 1999a; Knödlseder 1999). Since in general a variety of distinct massive star populations of different ages, sizes, and metallicities contribute to the observed intensities along a line of sight, this indicates that the average properties of the populations are related. However, the correlation between 1.809 MeV and free-free emission also holds for regions far from the Galactic centre where only few massive star associations contribute to the observed emissions. Examples are the Cygnus and the Vela regions where localised 1.809 MeV emission enhancements coincide spatially with maxima of free-free radiation, showing the same relative intensities as the Galaxy as a whole (Knödlseder et al. 1999a).

To fully exploit in a quantitative manner such existing data and in preparation of the upcoming INTEGRAL gamma-ray satellite mission the development of new interpretation tools is necessary. We here present the first results from our modified time-dependent multi-wavelength evolutionary synthesis models. A similar model was recently presented by Plüschke et al. (2000). To model the gamma-ray luminosities the nucleosynthetic production of the long-lived radio-isotopes ²⁶Al and ⁶⁰Fe, has been included in our multi-wavelength code (Cerviño & Mas-Hesse 1994; Cerviño et al. 2000a). These isotopes give rise to the 1.809 MeV (for ²⁶Al) and 1.173 MeV and 1.333 MeV (for ⁶⁰Fe) gamma-ray lines respectively. The model properly accounts for the accumulation of radioactive elements and their respective decay times. Results from state-of-the-art stellar atmospheres of massive stars are included to accurately predict the ionising fluxes from these stars, which are at the origin of thermal free-free emission. Together with the other synthesised observables this provides a versatile tool for gamma-ray to radio analysis of massive star forming regions.

For comparisons with individual Galactic star forming regions (e.g. OB associations, clusters, H II regions) or ensembles of such objects it is not only imperative to model the temporal evolution of their properties. The effects of small number statistics of the massive star population must also be taken into account (Cerviño et al. 2000b). Various studies, including the present one, treat such effects by means of Monte Carlo simulations (e.g. Cerviño & Mas-Hesse 1994, McKee & Williams 1997, Oey & Clarke 1998). Finally for a fully quantitative and objective confrontation with observables an additional step is performed here, to our knowledge for the first time in this context. The observational constraints (e.g. a known number of stars of given spectral types, derived age, distance etc.) and their uncertainties are included in a Bayesian approach providing probability distributions for all derived properties.

Sect. 2 describes the ingredients of our synthesis code. The main predictions from our models are presented in Sect. 3. Uncertainties are briefly discussed in Sect. 4. Our Bayesian approach to model realistic stellar populations is presented in Sect. 5. The main conclusions are summarised in Sect. 6.

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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