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1 Introduction

The lanthanum monoxide molecule has long been recognized to play an important role in cool stellar atmosphere chemistry. Indeed, spectra of S-type red variables show LaO absorption bands of the red (A-X) and yellow-green (B-X) systems, particularly prominent at the light minimum. The strengths of the bands are very sensitive to the temperature [8, (Keenan 1948]; [3, Bidelman 1953)]. In these stars, the abundance of La as an s-process element may be enhanced relatively to light elements, thus favoring the formation of LaO molecules [14, (Wyckoff & Clegg 1978]; [9, Lloyd Evans & Catchpole 1989)]. The C/O abundance ratio also appears as a determining parameter.

We have undertaken to reinvestigate some of the transitions of the LaO radical in the aim to obtain realistic molecular spectroscopic constants which are needed for the modeling of stellar opacities.

Seven electronic states of LaO, namely $X^2\Sigma^+$, $A'^2\Delta$, $A^2\Pi$, $B^2\Sigma^+$, $C^2\Pi$, $D^2\Sigma^+$, and $F^2\Sigma^+$, are known from the observation of six band systems (F-X, D-X, C-X, B-X, C-A', and A-X), extending from ultraviolet to photographic infrared spectral regions [7, (Huber & Herzberg 1979]; see also [4, Carette 1990] and references therein). The B-X system, which is the subject of the present work, was first analysed in rotation by [1, Åkerlind (1962)], considering the ground state as a quartet ($^4\Sigma^+$) state. Only the 0-0, 1-0, and 0-1 bands were concerned. This state was later definitely ascribed as $^2\Sigma^+$ [6, (Green 1971)]. [10, Schoonveld & Sundaram (1974)] estimated the vibrational intervals in both states, up to v=11, from the "band-head corrected" frequencies, and derived approximate values of the vibrational constants. Spin-rotation and hyperfine interactions were studied for $X^2\Sigma^+(v=0, 1)$ and $B^2\Sigma^+(v=0)$ levels only [2, (Bacis et al. 1973]; [5, Childs et al. 1986]; [13, Törring et al. 1988)].

We report the observation and rotational analysis of a number of bands of the $B{\rightarrow}X$ system, involving higher vibrational levels in both states, up to v'=4 and v''=5. Laser excitation technique has been used to obtain LaO fluorescence, and the spectra have been recorded at high resolution with a Fourier transform spectrometer. A consistent set of improved effective spectroscopic constants at equilibrium is determined.

Table 1: Direct fluorescence lines observed upon laser excitation within the $B^2\Sigma^+{\rightarrow}X^2\Sigma^+$ system of LaO

{c c c}
Laser line & Excitation & Fluorescence \\ \hline ...
 ...&(4$\rightarrow$3, 4, 5)$P_{24,23}$(45),$R_{24,23}$(43) \\  

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