Infrared giants vs. supergiants

II. CO observations

¹ Institut d'Astrophysique de Paris, CNRS, 98bis boulevard Arago, F-75014 Paris, France
² European Southern Observatory, Casilla 19001, Santiago 19, Chile
³ STScI, 3700 San Martin Dr., Baltimore, MD 21218, U.S.A.

Send offprint request to: E. Josselin

Received: 21 March 1997
Accepted: 17 September 1997

Abstract

We report systematic observations of millimeter CO emission from a sample of 109 oxygen–rich evolved stars (AGB and supergiants), colour–selected from the IRAS Point Source Catalog . CO has been searched with good sensitivity in 81 sources (74% of the sample). CO is detected in 54 sources and a significant upper limit has been achieved in 27 sources. 
In our previous paper we reported on the statistical results of these observations. We showed that in almost 50% of the sources, the ratio of the IRAS flux to CO intensity, , is larger by a factor of 3 to more than 10 than what is expected according to the correlation found by Nyman et al. (1992). Supergiants only exhibit very high values (≳ 200). In most cases, the observed spread in the values of this ratio can be explained by a large range of luminosities. This leads to a new criterion to identify AGB stars: an object with must have a low mass progenitor. 
Here we study the correlations between and various physical properties of the sources. Most sources with high values of also have low galactic latitudes, small IRAS variability indices, and early spectral types (typically M1–M5). Conversely, there is no dependence on the IRAS colours, nor on the intensity of silicate emission. However, a few AGB stars exhibit large ; other factors than luminosity are required to explain these values. Different hypotheses, such as the possible presence of a chromosphere, a low ¹²C abundance or a variable mass–loss rate, are examined. Considering the global high OH detection rate , we studied the correlations with CO and OH emission. The detection of OH seems to be a useful discriminator of mechanisms that enhance .