*Astron. Astrophys. Suppl. Ser.* **142**, 325-338

**M.S. Burgin ^{1,2} -
A. Chalabaev^{2}**

**Send offprint request: **A. Chalabaev

1 - Astro-Space Center FIAN, Profsoyuznaya 84/32, 117810 Moscow,
Russia

e-mail: mburgin@dpc.asc.rssi.ru

2 -
CNRS, Laboratoire d'Astrophysique, Observatoire de
Grenoble, UMR 5571, BP. 53X, F-38041 Grenoble
Cedex, France

e-mail: Almas.Chalabaev@obs.ujf-grenoble.fr

Received June 14; accepted September 21, 1999

When planning new facilities, one is interested to know whether and by
how much the new technique is superior to already existing ones. We
describe a general approach permitting us to evaluate the relative merits of various
techniques used in astrophysical observations, following the theory of
model parameter estimation. It is applied to compare
two-aperture optical long baseline interferometry to classical
spectroscopy, both used to define the model parameters of the
P Cyg wind. The wind modeling was done using an efficient
approximation for computation of the line source function; it allowed us
to analyse about 10^{5} points in the parametric space of P Cyg
envelope models. It is shown that interferometry offers no real
advantage if the object can be described by stationary spherically
symmetric models with a priori fixed thermal structure. However, if
the object must be described by a model with a large number of free
parameters, e.g. when the thermal structure of the envelope is not
fixed a priori, then the interferometric measurements can reduce the
error in model parameters determination by an order of magnitude. In
the case of P Cyg, the reduction of errors provided by interferometry
is highest for the baseline lengths in the range 45-90 m. This
illustrates the capacity of the proposed method to be used for
optimization of interferometric configurations. The
simplifications adopted for this first trial application are
critically analyzed, and future improvements are indicated.

**Key words: **techniques: interferometric -- techniques: spectroscopic
-- P Cyg -- stars: mass-loss

- 1 Introduction
- 2 Relative figure of merit of observational methods
- 3 The relative figure of merit of OLBI vs. spectroscopy
- 4 Models of the P Cyg wind. Computed observables
- 5 Results
- 6 Conclusions
- 7 Final remarks
- Appendix A: Computation of and
- Appendix B: Simple approximation for the escape probability
- References

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