1 Introduction

The nearby carbon-star CW Leo/IRC+10216 has experienced a phase of high mass loss for the last $\sim 10^5$ years and is at the verge of evolving toward a Planetary Nebula. As attested by its non-solar isotopic composition, it has already expelled most of its convective shell in the form of a thick, slowly expanding circumstellar envelope (CSE), formed of molecules and dust grains. This CSE is one of the brightest 10 $\mu$m objects and one of the richest molecular sources in the sky: to date, over 53 molecular species have been detected in this object (Table 1).

 

Table 1: Molecules in this 2 mm line survey of IRC+10216

Molecule	Isotopomer or v-state	Number of Observed Transitions	Table	Previous IRAM data1
SiS	SiS	2	4	this work
	29SiS	2	4	Kahane et al. 1988, 1992
	30SiS	2	4	Kahane et al. 1988, 1992
	Si34S	2	4	Kahane et al. 1988, 1992
	Si33S	2	4	Kahane et al. 1988, 1992
	29Si34S	2	4	this work
	30Si34S	1	4	this work
	SiS v=1	2	4	this work
	SiS v=2	2	4	this work
	SiS v=3	1	4	this work
SiO	SiO	1	4	this work
	29SiO	1	4	Kahane et al. 1988, 1992
	30SiO	1	4	Kahane et al. 1988, 1992
SiC	SiC	3	4	Cernicharo et al. 1989, 1992
SiN	SiN	5	4	this work
SiC2	SiC2	12	5	this work
	SiC2 $\nu_3=1$	2	5	this work
	29SiC2	11	5	Cernicharo et al. 1986c
	30SiC2	8	5	Cernicharo et al. 1986c
	Si13CC	16	5	Cernicharo et al. 1991a
SiC3	SiC3	5	5	this work
ClNa	ClNa	4	6	Cernicharo & Guélin 1987
	37ClNa	2	6	Cernicharo & Guélin 1987
ClK	ClK	4	6	Cernicharo & Guélin 1987
	37ClK	2	6	this work
ClAl	ClAl	2	6	Cernicharo & Guélin 1987
	37ClAl	3	6	this work
AlF	AlF	2	6	Cernicharo & Guélin 1987
MgNC	MgNC	6	6	Guélin et al. 1986
	26MgNC	2	6	Guélin et al. 1995
	25MgNC	1	6	Guélin et al. 1995
NaCN	NaCN	26	6	this work
C3H	C3H	8	7	this work
	C3H $\nu_4=1$	4	7	this work
c-C3H	c-C3H	8	7	this work
C3H2	C3H2	4	7	this work
	C13CCH2	1	7	this work
H2C4	H2C4	12	7	Cernicharo et al. 1991b
C4H	C4H	10	7	this work
	C4H $\nu_7=1$	20	7	Guélin et al. 1987
	C4H $\nu_7=2$	17	7	Guélin et al. 1987
	13CCCCH	1	7	this work
	C13CCCH	3	7	this work
	CC13CCH	2	7	this work
	CCC13CH	6	7	this work
C5H	C5H	4	7	Cernicharo et al. 1986a,b, 1987a
C6H	C6H	18	7	Guélin et al. 1987a
				& Cernicharo et al. 1987b
HCCN	HCCN	4	8	Guélin & Cernicharo 1991
CH3CN	CH3CN	5	8	this work
HC3N	HC3N	4	8	this work
	HC3N $\nu_7=1$	5	8	this work
	H13CCCN	4	8	this work
	HC13CCN	4	8	this work
	HCC13CN	4	8	this work
HC5N	HC5N	5	8	this work
C3N	C3N	8	8	this work
	13CCCN	4	8	this work
	C13CCN	4	8	this work
	CC13CN	2	8	this work

 

Table 1: continued

Molecule	Isotopomer or v-state	Number of Observed Transitions	Table	Previous IRAM data1
CP	CP	4	9	Guélin et al. 1990
PN	PN	1	9	this work
CS	CS	1	9	this work
	CS v=1	1	9	this work
	13CS	1	9	Kahane et al. 1988
	C34S	1	9	Kahane et al. 1988
	C33S	1	9	Kahane et al. 1988
	13C34S	1	9	Kahane et al. 1988
C2S	C2S	8	9	Cernicharo et al. 1987c
C3S	C3S	5	9	Cernicharo et al. 1987c
	C334S	1	9	this work
H2S	H2S	1	9	this work

(1) Reference to previous publications of partial data from this line survey.


Other species in IRC+10216 not in this 2 mm survey:

NH3	SiH4
CN	CO	HCN	HNC	HCO+	CH4	MgCN
C2	C2H	HCCH	H2CCH2
C3	H2C3
C4Si
C5	C5N
H2C6
C7H	HC7N
C8H
HC9N

Figure 1: The $\lambda$ 2 mm IRAM 30-m telescope spectral survey of IRC+10216. The data has been smoothed to a resolution of 3 MHz ($\simeq 10$ km s-1). The spectrum is dominated by the lines of seven species: CS, SiS, SiO, SiC2, C4H, C3N, and HC3N The ordinate scale is $T_{\rm A}^*$, the antenna temperature, corrected for spillover losses and atmosphere attenuation. The inset zooms the spectra between 132.8 and 139 GHz

In view of the wide range of physical conditions present in the stellar atmosphere and in the envelope, the molecules form through a variety of chemical processes: 3-body processes in the hot and dense atmosphere of the star (R= R_*), surface reactions in the dust condensation region ( R= 3-10  R_*), gas phase neutral-neutral or ion-molecule reactions in the CSE ( $R\geq 100~ R_*$; see e.g. Glassgold 1996). The molecules present in the atmosphere and in the dust condensation region are best detected through their ro-vibrational lines in the infrared; those present in the cold CSE are best observed through their rotational lines at millimeter wavelengths. Because the molecular rotational temperatures in the CSE are between 10 and 50 K, most molecules, including the small carbon chains and the small metal compounds, have their strongest lines around 2 mm.

Several spectral line surveys of IRC+10216 have been reported in the literature. They include the far Infrared survey with the Infrared Space Obseravtory (ISO) (Cernicharo et al. [1996]), the 0.8 mm James Clerk Maxwell Telescope (JCMT) and Caltech Submillimeter Observatory (CSO) surveys (Avery et al. [1992]; Groesbeck et al. [1994]), the 3-4 mm Onsala survey (Johansson et al. [1985]) and the 6-10 mm Nobeyama survey (Kawaguchi et al. [1995]). In this article, we present the results of a systematic spectral survey of the 2.3-1.7 mm window. Our survey was made with the IRAM 30-m telescope, the telescope with the largest effective area throughout this window.