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2 Observations and data analysis

We used the Submillimetre Common User Bolometer Array (SCUBA - Holland et al. 1999) on the James Clerk Maxwell Telescope (JCMT[*]) on 1998 December 15 and 16 to carry out a submillimeter continuum mapping of NGC 2068 and NGC 2071. Five sub-fields were imaged simultaneously at $850~\mu$m and $450~\mu$m in the standard scan-map mode to produce two mosaics of a $\sim $32 $\hbox{$^\prime$ }\times 18\hbox{$^\prime$ }$ field (see Fig. 1). Each sub-field was covered twelve times using three different chop throws in both right ascension and declination. Pointing and calibration checks were made on HL Tau at regular intervals. The zenith atmospheric optical depth was measured to be $\sim $0.25 at $850~\mu$m and $\sim $1.4 at $450~\mu$m. The FWHM beam size as measured on Uranus was $\sim $13 $\hbox{$^{\prime\prime}$ }$ at $850~\mu$m and $8\hbox{$^{\prime\prime}$ }$ at $450~\mu$m. The mosaics were reduced with a SURF script from R. Tilanus using the "Emerson 2'' restoration algorithm (Emerson 1995).

With a spatial extent of 3.7 pc $\times$ 2.1 pc, our mosaics cover $75\%$ of the region mapped in CS(2-1) by Lada et al. (1991a - hereafter LBS) around NGC 2068/2071. In the submm continuum, the CS dense cores of LBS have a filamentary appearance, with typical dimensions $\sim $0.08  ${\rm pc} \times 0.5$ pc (aspect ratio $\sim $0.15), and are highly fragmented. Within these extended filaments, a total of 82 condensations with lengthscales characteristic of YSO circumstellar structures (see Table 1, only available in electronic form at http://www.edpsciences.org and at http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/ 372/L41, and Fig. 2) were identified using a multiresolution wavelet analysis (cf. Starck et al. 1998; MAN98; Motte & André 2001). The outer sizes of the condensations were estimated from their radial intensity profiles at $850~\mu$m. The ambient background observed on larger lengthscales was then subtracted to allow a proper derivation of the integrated flux densities of the condensations (see MAN98). Unlike the filaments, the (resolved) condensations are roughly circular (mean aspect ratio $\sim $0.6) with an average deconvolved FWHM diameter of $\sim $13 $\hbox{$^{\prime\prime}$ }$ (i.e. $\sim $5000 AU).

  \begin{figure}
\par\includegraphics[width=8cm,clip]{Da182_f1.ps}\par\par\end{figure} Figure 1: Dust continuum mosaic of the NGC 2068/2071 region at $450~\mu$m, smoothed to an effective angular resolution of $18\hbox {$^{\prime \prime }$ }$. Contour levels go from 1.2 to 9.6 Jy/beam with steps of 1.2 Jy/beam and from 20 to 50 Jy/beam by 10 Jy/beam. The mean rms noise level is $\sim 0.4~\rm Jy/18\hbox{$^{\prime\prime}$ }$-beam. Several embedded YSOs (italicized names) and CS cores (LBS numbers and triangles) are indicated.

Our $850~\mu$m mosaic (e.g. Fig. 2) mainly traces optically thin thermal dust emission from cold, dense structures in the cloud. The map contours should thus primarily reflect the column density distribution of dust and gas. The condensation masses were estimated from the integrated $850~\mu$m fluxes (cf. MAN98), adopting recommended values for the dust opacity per unit mass column density (of dust and gas): $\kappa_{850} = 0.02~{\rm cm}^{2} \, {\rm
g}^{-1}$ for protostellar envelopes and $\kappa_{850} = 0.01~{\rm
cm}^{2} \, {\rm g}^{-1}$ for starless condensations (cf. Henning et al. 1995). The dust temperature $T_{\rm d} $ was taken to be 20-40 K and 15 K, respectively, in agreement with published temperatures (Harju et al. 1993; Gibb & Little 2000). Under these assumptions, our 5$\sigma$ column density sensitivity is $N_{\rm H2} \sim 1{-}2 \times 10^{22}$ cm-2. The condensations have masses ranging from $\sim $0.3 $M_\odot$ to $\sim $$M_\odot$ ($\sim $$M_\odot$ for NGC 2071-IRS), with a factor $\mathrel{\mathchoice {\vcenter{\offinterlineskip\halign{\hfil
$\displaystyle ...2 absolute uncertainty. The average density $\langle n_{\rm H2} \rangle$ of the condensations is $3{-}40 \times 10^6~{\rm cm}^{-3}$ while that of the filaments is $\sim $ $5 \times 10^5~{\rm cm}^{-3}$.

The $850~\mu$m emission of seven condensations identified in the vicinity of the luminous embedded infrared source NGC 2071-IRS (e.g. Harvey et al. 1979) may not arise only from dust. NGC 2071-IRS (called LBS8-MM18 in Table 1) drives a prominent bipolar flow (e.g. Chernin & Masson 1992), which is responsible for a northeast-southwest ridge of broad-band $850~\mu$m emission. The spectral index observed between $450~\mu$m and $850~\mu$m in this ridge is atypical: $\alpha \mathrel{\mathchoice {\vcenter{\offinterlineskip\halign{\hfil
$\displays...
...r{\offinterlineskip\halign{\hfil$\scriptscriptstyle ... (where $S_\nu \propto \nu^\alpha$) instead of $\alpha \sim 4$ as measured for the extended emission outside the ridge. Based on the CO(3-2) and HCO+(4-3) maps of Chernin & Masson (1992) and Girart et al. (1999), we estimate that line emission may contribute up to $20{-}100\%$ of the SCUBA $850~\mu$m emission in the outflow region. We are, however, confident that the 75 other condensations represent genuine dust continuum sources. Many of them are detected at both $450~\mu$m and $850~\mu$m (cf. Figs. 1 and 2).


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