1 Introduction

Globular Filaments (GF) are nearby (100-700 pc distance) filamentary dark clouds containing small numbers of cloud cores connected by lower density gas and dust. Schneider & Elmegreen (1979) compiled the first catalog of such clouds, containing 23 globular filaments. These clouds look like strings (the low opacity material) with beads (the denser cores) strung along their lengths, often in a periodic fashion. They may represent swept-up material created by blast waves or strong stellar winds (Schneider & Elmegreen 1979; Heiles 1997) or they could be a natural stable state for magnetized galactic molecular clouds.

At an estimated distance of 150 pc, the Lupus complex of molecular dark clouds, which is located near the Scorpio-Centaurus (or Sco OB2) OB association (Humphreys 1978), is one of the most active nearby low-mass star-forming regions (Schwartz 1977; Krautter 1991; Hughes et al. 1994; Wichmann et al. 1997; Krautter et al. 1997). This complex consists of five filamentary dark clouds (Tachihara et al. 1996), and among these are globular filaments GF 17 and GF 20, also known as Lupus 4 and Lupus 2 dark clouds, respectively. GF 17 is a large filament, spanning about $1.5^{\circ}$ in the plane of the sky (Fig. 1), while GF 20 is a smaller filament extending for about $40^{\prime}$. The filamentary structure is particularly well delineated in GF 20 (Fig. 2) where a chain of small globules can be seen extending to the northeast and away from a larger condensation. Apparently there is no evidence for interaction with the surrounding environment in both GF 17 and GF 20 (Schneider & Elmegreen 1979). A total of eight T Tauri stars are known to be associated with GF 17, and four young stellar objects are associated with GF 20.

Figure 1: Digital Sky Survey image of the central region of globular filament GF 17. The two rectangular boxes indicate the regions mapped in 13CO in the present study. The region within the eastern box is referred to as the filamentary region, and the region within the western box as the main core region.

Figure 2: Digital Sky Survey image of the globular filament GF 20.

Despite several studies dedicated to this star forming region, observations of molecular emission in Lupus have been rather limited, probably due to its southernly declination ( ${<}{-}30^{\circ}$). The Lupus complex of dark clouds was surveyed in the CO(1-0) line over an area of ${\sim} 170$ sq deg by Murphy et al. (1986) with an effective resolution of 30'. They estimated the mass of the cloud complex to be $3\times10^{4}~M_{\odot}$, comparable to that of the nearby Ophiuchus complex of dark clouds. More recently, Tachihara et al. (1996) performed large-scale ¹³CO(1-0) observations of the complex (with the exception of GF 17) with an effective resolution of $8^{\prime}$, and Gahm et al. (1993) made CO(1-0) observations of GF 20 with $40^{\prime\prime}$ resolution but covering only the main condensation of the cloud. Using optical star counts, Andreazza & Vilas-Boas (1996) derived masses of $197~M_{\odot}$ and $59~M_{\odot}$ for GF 17 and GF 20, respectively.

The main purpose of these few sudies was to better characterize the star formation activity within the dark clouds in Lupus. However, the Lupus 1 and 3 dark clouds have been much more actively engaged in star formation (see Krautter 1991, for a review) than GF 17 or GF 20. Consequently, the physical properties (such as gas kinematics and temperatures, and density structure) of the molecular material as well as the dust content of the GF 17 and GF 20 globular filaments remain virtually unknown.

As part of a comprehensive study of the structure, physical conditions, and dynamical states of dark globular filaments (Moreira et al. 2000; Moreira & Yun 2002), we have performed large-scale observations of the physical properties of the gas and dust within GF 17 and GF 20. The elements of our study were (1) 12, 25, 60, and 100 $\mu$m IRAS co-added images, and (2) large-scale millimeter molecular line maps in the J=1-0 transition of CO, ¹³CO, and C¹⁸O with unprecedented spatial coverage and resolution. In this paper we present and discuss the correlations between gas and dust in GF 17 and GF 20. Analysis of the gas velocity structure in GF 17 and GF 20 strongly indicates that these clouds have been shaped by the interaction with expanding HI shells originated in the nearby Sco OB2 association.

In Sect. 2 we present the observations and data reduction. Our method of analysis of the IRAS co-added images and millimeter molecular line data is presented in Sect. 3. In Sects. 4 and 5 we present and discuss our results. Section 6 summarizes our findings.