Astron. Astrophys. 324, 203-210 (1997)

1. Introduction

The Taurus molecular cloud (TMC) is one of the best targets to study low-mass star formation, since it is located at a distance of only 140 pc. The region is associated with T Tauri stars, identified by infrared and optical observations (e.g. Strom et al. 1989 ; Kenyon et al. 1990 ; Weintraub 1990 ), low-luminosity objects extracted from the IRAS Point Source Catalogue (PSC, IRAS 1985 ), molecular outflows (e.g. Fukui 1989 ; Fukui et al. 1993 ) and water maser sources (Wouterloot et al. 1993 ), indicating ongoing star formation. The TMC contains several dark molecular clouds of size 1 pc and densities of 10³ cm^-3 (e.g. Walmsley et al. 1980 ; Wouterloot & Habing 1985 ; Ungerechts & Thaddeus 1987 ; Mizuno et al. 1995 ). These in turn contain dense molecular cores, high density ( 10⁴ cm^-3) substructures which have been classified by Myers et al. (1983 ) and Myers & Benson (1983 ). For the sky distribution of the molecular cores in the TMC, see Kenyon et al. (1990 ). Subsequent multimolecular observations (e.g. Benson & Myers 1984 ; Fuller 1989 ; Fuller & Myers 1993 ) have confirmed that dense cores are smaller than 0.1 pc with masses ranging from 0.5 to 10 solar masses and kinetic temperatures of the order of 10 K. Beichman et al. (1984 ; 1986 ) have shown that about 50% of the molecular cores are associated with IRAS sources, indicating that these objects may contain, in their interiors, young stellar objects (YSOs). Therefore, it is now clear that there exists a close association between low-mass YSOs and molecular clumps. Since the TMC is not strongly affected by the nearby Cas-Tau OB association (Blaauw 1991 ), it is clear that it provides unique opportunities to study spontaneous low-mass star formation and, in particular, to investigate the nature of the earliest stages of low-mass star formation, strictly connected with dense molecular clumps.

Following the definition given by Myers & Benson (1983 ; i.e. sources with 0.4 K at the Haystack and Green Bank antennas), 16 dense molecular cores have so far been identified in the TMC (Myers & Benson 1983 ; Benson & Myers 1989 ). In order to obtain a more complete sample of TMC cores, to gain information about the nature of the molecular gas before or at the onset of YSO formation and to clarify the evolutionary stages connected with the collapse process, we report observations of the NH₃ () = (1,1) and (2,2) inversion lines and the cyanodiacetylene (HC₅ N) J=9-8 rotational line towards 4 newly discovered dense cores.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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