Since the discovery of the first molecules in space in the 1940's (e.g. Adams 1941), the number of molecules detected and identified has increased to more than one hundred. With the increase in detections came an increase in the complexity of the detected molecules (e.g. Guelin et al. 1998). It is now clear that molecular clouds, and in particular clouds harboring newly formed stars, are extremely efficient factories of rather complex molecules.
The identification of more complex molecules fueled the search for amino acids - the pre-biotic molecules - in space. Glycine (NH2CH2COOH) is one of the simplest amino acids and therefore a privileged object of several detection attempts. The first attempts were carried out on high mass star forming regions like SgrB2, Orion and W3OH (Brown et al. 1979; Hollis et al. 1980; Snyder et al. 1983; Guelin & Cernicharo 1989) for the reason that they are the most luminous FIR sources in the sky. The searches were, however, unsuccessful: Combes et al. (1996) eventually argued that the upper limit which they obtained (N(Glycine) cm-2) cannot be further lowered, as in their search they had reached the line confusion limit.
Although high mass protostars were the first regions in the Galaxy in which complex molecules were detected, it is now clear that low mass protostars are also intense factories of complex molecules, and that their neighborhood may provide an even more favorable environment for the formation and survival of complex molecular species (van Dishoeck & Blake 1998). For example, the doubly deuterated formaldehyde D2CO is about 20 times more abundant in the solar type protostar IRAS16293-2422 (hereinafter IRAS16293) than in Orion (Ceccarelli et al. 1998; Loinard et al. 2000). A search of two dozen protostars confirmed that low luminosity protostars have similar high abundances of D2CO with respect to H2CO (), whereas high luminosity protostars have much lower abundances () (Loinard et al. in preparation).
We have recently argued that solar type protostars, like their more massive counterparts, also possess hot cores where dust and gas temperatures exceed 100 K (Ceccarelli et al. 2000a,b). Our claim is based on the simultaneous modeling of H2O, O, SiO and H2CO line emissions from IRAS16293 by means of an accurate model of the structure of the protostellar envelopes (Ceccarelli et al. 1996). Additional evidence suggesting the presence of such hot cores inside low mass protostars is provided by NIR observations of the H2O and CO absorption lines in El 29 (Boogert et al. 2000), a relatively evolved solar type protostar. In such hot cores the grain mantles evaporate, injecting plenty of complex molecules in the gas phase, as is the case for massive hot cores. In these cases, the so called "high temperature-driven chemistry" produces even more complex molecules (Charnley et al. 1997). A recent search suggests that formaldehyde has an abundance in the hot cores of massive protostars (van der Tak, van Dishoeck & Caselli 2000). We have found 100 times more H2CO in the hot core of the solar type protostar IRAS16293 (Ceccarelli et al. 2000b). Whether hot cores in low mass protostars are chemically richer than in massive protostars is not clear, but it is likely that they have different chemical compositions. In our opinion, solar-type protostars with signs of hot cores may have a better chance of having detectable amounts of glycine than do massive protostars.
The conditions that may make low mass protostars more favorable for searching for glycine than massive protostars are:
a) their longer evolutionary timescale, which allow more time for the accretion and/or synthesis of molecules in the mantle
b) the less harsh environment to which molecules are exposed in the gas phase, which increases the probability of their survival.
Finally, the only place in the Universe where the presence of amino acids has definitively been shown is our planetary system, i.e. around a solar type star. Even though this does not imply at all that amino acids are formed in the protostellar phase it is a further reason to search for glycine around solar type protostars.
In this article we describe the first search for glycine carried out on a solar type protostar. After presenting the results of our observations (Sect. 2), we discuss in Sect. 3 the upper limit to the glycine column density and abundance that we derive from these observations and compare them with model predictions.
© European Southern Observatory (ESO) 2000
Online publication: October 30, 2000