Astron. Astrophys. 339, 159-164 (1998)

1. Introduction

Pre Main Sequence stars are characterized by the presence of strong, ionized winds that are the main mechanism through which the parent dense envelopes are swept out. Given the relatively high visual extinction which characterizes these early type stars, the analysis of infrared HI recombination lines is one of the main diagnostic tools to derive key parameters of the winds, such as the mass loss rate and the number of ionizing photons, which are directly linked to the star properties and its evolutionary status (see e.g. Simon et al. 1983, Evans et al. 1987, Nisini et al. 1995). However, any model for the emission of HI recombination lines from ionized moving envelopes relies on a number of parameters which cannot all be constrained by using only the limited number of lines accessible from the ground, especially when the star spectral type and distance are poorly known.

The most widely adopted procedure to derive information from the usually observed lines (which are commonly limited to Br, Br and Pf), is to use the line ratios to check for optically thick effects in the line emission due to the presence of dense winds and to derive the wind mass loss rate from the line absolute intensities, on the main assumption that the wind is completely ionized, and adopting a gas velocity law. This procedure can, however, lead to inaccurancies if these assumptions are not correct. For example, it has been found that winds in low luminosity stars are mostly neutral, and therefore the mass loss rates derived assuming full ionization can be heavily underestimated (Natta et al. 1988).

In this contribution we will show how the observation of many HI lines from several different spectral series, now possible with the ISO spectrometers, allows a better definition of the physical model for their emission. In particular, the aim of this study is to show how the analysis of the line decrement in the same spectral series can give complementary information which is fundamental for a correct interpretation of the observed lines. We present the spectra of two Herbig Ae/Be stars: MWC1080 and CoD 11721, which have both an early spectral type relative to the majority of the Herbig Ae/Be stars and a luminosity high enough to produce a detectable ionized region. Moreover, they also have other observed HI recombination lines, both optical and near IR, useful to complement our study.

© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998
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