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Astron. Astrophys. 331, 669-696 (1998)

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2. The selected fields

The three fields harbour potential sites of star formation, on the basis of several observational criteria which are described below. They have been selected on purpose in very ordinary regions of molecular clouds, because this material of low average brightness in all sorts of tracers (far-infrared continuum emission, molecular line emission) builds up a significant fraction of the disk emission in spirals comparable to our Galaxy. COBE observations suggest that the bulk of the CO emission in our Galaxy is not dominated by the emission from the vicinity of hot star forming regions but by a very cold widespread component (Wright et al. 1991). All the fields lie in the vicinity of the Sun (d [FORMULA] 150 pc) to obtain as high a linear resolution as possible. They all contain a low-mass dense core identified in [FORMULA], [FORMULA], [FORMULA], HNC and/or [FORMULA] lines at low angular resolution with no signpost of star formation. The three fields lie out of the Galactic Plane (L134A, [FORMULA], [FORMULA] ; L1512, [FORMULA], [FORMULA], Polaris, [FORMULA], [FORMULA]) and are similarly confined by the pressure, [FORMULA] /k [FORMULA] K [FORMULA], of the HI layer estimated to be as thick as 1 or 2 kpc (Cox, 1991).

2.1. The Polaris flare

The Polaris Flare was first known as a large spur of atomic hydrogen rising over more than [FORMULA] above the galactic plane (Heiles 1984, 1989), then detected by IRAS as part of the high latitude cirrus clouds which appear as a net of intertwined low 100µm brightness filaments found over most of the sky (Low et al. 1984). It was first mapped in the [FORMULA] (J=1-0) transition by Heithausen & Thaddeus (1990) at a resolution of 8.7' (FWHM). The average [FORMULA] column density at the position of the field selected for high resolution observations and deduced from the [FORMULA] (J=1-0) intensity is also low, [FORMULA] [FORMULA] using the factor [FORMULA] [FORMULA] / [FORMULA] (Fig. 1a). This factor may be more appropriate here than the lower value determined by de Vries, Heithausen & Thaddeus (1987) in the nearby high latitude cloud in Ursa Major because the field selected for molecular line studies (shown in Fig. 1a) stands out as lying in one of the coldest spots in the entire Flare in terms of infrared color, [FORMULA] (60µm)/ [FORMULA] (100µm) [FORMULA], and its 100µm emissitivity per H nucleus is lower than for the bulk of the cirrus component. The average extinction at the scale of 0.2 [FORMULA] (resoltuion of the CO map) at the location of the IRAM field is [FORMULA].

[FIGURE] Fig. 1a-c. Large scale maps showing the locations of the fields observed at high angular resolution (black boxes): a [FORMULA] (J=1-0) map of the Polaris Flare (from Heithausen & Thaddeus 1990). The 6' [FORMULA] 8' field is located at [FORMULA], [FORMULA]. The first contour level and step are 2 [FORMULA]. b [FORMULA] (J=1-0) map (T. Dame, private communication) of the Taurus-Auriga-Perseus complex from Ungerechts & Thaddeus (1987). The 5' [FORMULA] 10' field of L1512 is located at [FORMULA] (1950)=05h 00m 54.5s, [FORMULA] (here the black box has been enlarged by a factor [FORMULA] 3 for clarity). First level 2 [FORMULA] and step 3 [FORMULA]. c 100µm IRAS map of L134A (from Laureijs et al. 1991). The 3' [FORMULA] 16' field observed is located at [FORMULA] (1950)=15h 50m 58.1s, [FORMULA]. The contours correspond to -1 MJy sr-1 (dashed line) and 2, 5, 8... MJy sr-1 (solid lines).

One square degree around the selected field had also been previously observed with an angular resolution of 3.9' with the KOSMA telescope in [FORMULA] and [FORMULA] (J=1-0) and at lower resolution, at the Effelsberg telescope, in the 18cm lines of OH and the [FORMULA] transition of [FORMULA] (Großman et al. 1990). The structure appears to have chemical and excitation properties similar to those of a dark cloud. According to these authors, its morphology, velocity field and large OH abundance suggest the presence of a shock associated to the expansion of the North Celestial Pole Loop (NCP). Higher resolution observations in the [FORMULA] (J=1-0), HCN, HNC, [FORMULA] (1,1) and (2,2), [FORMULA] (J=1-0) and (J=2-1) lines (Großmann & Heithausen 1992) and SO lines (Heithausen, Corneliussen & Großmann 1995) revealed significant sub-structure down to the highest resolution ([FORMULA] pc). A low mass ([FORMULA]) dense core has been identified with an average density [FORMULA] [FORMULA] over about 0.05 pc and an abundance ratio X(HNC)/X(HCN)= 0.8 [FORMULA] 0.1 representative of low temperature chemistry.

2.2. L1512

This field is located in a weakly [FORMULA] (J=1-0) emitting edge of the Taurus-Auriga-Perseus complex ([FORMULA] [FORMULA] from Ungerechts & Thaddeus, 1987) and is shown in Fig. 1b. The average [FORMULA] column density at the scale of 0.5 pc, deduced from the [FORMULA] (J=1-0) line intensity and the above conversion factor is only [FORMULA] a few [FORMULA] [FORMULA] which corresponds to quite a diffuse environment at that scale. The average extinction at this scale is only [FORMULA].

The field contains one opaque core, originally identified as an opaque spot of a few arc minutes on the Palomar Sky Survey prints (Myers, Linke & Benson 1983) and later on mapped in the [FORMULA] (J,K)=(1,1) and (2,2) lines (Myers & Benson 1983; Benson & Myers 1989). This core is one of the few cores in the complex not associated to an IRAS infra-red point source (Beichman et al. 1986) and is therefore considered not to have given birth yet to any star (or group of stars) of bolometric luminosity larger than 0.1 [FORMULA]. The dense core has been subsequently observed in many molecular transitions at higher angular resolution, in [FORMULA] transitions at 18.3 and 85.3 GHz (Cox, Walmsley & Güsten 1989), in the J [FORMULA] transition of [FORMULA] and the [FORMULA] (1,1) line (Fuller & Myers 1993). The diameter of the region of [FORMULA] emission is 0.075 pc, and is slightly smaller for the [FORMULA] emission. The widths of the [FORMULA] lines are the narrowest ever observed in a dense core but cannot be accounted for by purely thermal motions by comparison to the [FORMULA] linewidths, because the linewidths do not scale as the inverse square root of the molecular mass. The gas temperature in the core is estimated to be 11.6 K and the non-thermal contribution to the observed motions [FORMULA] km s-1. This core has also been observed in the [FORMULA] (J=1-0) and CS(J=2-1) transitions (Fuller 1989) and the size of the CS and [FORMULA] emission regions is about twice as large as that of the [FORMULA] and [FORMULA] cores (Myers, Fuller, Goodman & Benson 1991). Its properties at the [FORMULA] 0.1 pc scale can be summarized as follows: it is a low-mass core, with local [FORMULA] density derived from millimeter line excitation ranging from [FORMULA] to [FORMULA] [FORMULA], depending on the set of lines chosen.

2.3. L134A

Lynds 134 is a high latitude complex of molecular clouds of low average column density ([FORMULA] at the parsec scale, except for a central region of size [FORMULA] pc which is extremely opaque ([FORMULA]) also known from Myers & Benson (1983) and Benson & Myers (1989) to contain low-mass dense cores. The dense core in L134A is associated with a sudden drop of the 60 µm emission (Laureijs et al. 1991). This drop is interpreted as a sharp change in the dust properties as the column density increases. The field has also been observed in HI at the VLA (van der Werf et al. 1988). It lies at less than 30 pc of [FORMULA] Oph and is irradiated by the UV field of the nearby Sco OB2 association. Laureijs et al. (1991) have estimated from the 100µm IRAS emission of the dust that the ambient UV field there is 5 times stronger than the average value in the Solar Neighborhood. The location of the field mapped at high angular resolution is indicated on the 100µm map of Laureijs et al. (1991) in Fig. 1c.

All the fields are therefore low average column density clouds at the parsec scale ([FORMULA] at their distance), contain one so-called dense core which is indeed a high column density region ([FORMULA]) of a few arc minutes in size, and the fields mapped with the IRAM-30m telescope cover a large fraction of the dense cores and of their environment. One field (L1512) lies at the edge of a giant complex (the Taurus-Auriga-Perseus complex) while the two others are more isolated. Polaris may be in a region of enhanced cosmic-ray flux given its connexion with the expanding NCP loop. L134A has a higher ambient UV flux. Each of the cores therefore belongs to slightly different radiative, gravitational and cosmic-ray environments.

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© European Southern Observatory (ESO) 1998

Online publication: February 16, 1998