DUST EMISSION FROM STAR FORMING REGIONS .2. THE NGC-2024 CLOUD CORE -REVISITED

A previous Paper (Mezger et al., 1988, Paper I) presented first observ ational evidence for the existence of six compact dust condensations i n the dense NGC2024 cloud core. It was suggested that these condensati ons are isothermal protostars and hence would represent the earliest s tages of protostellar evolution. Since then a number of papers have be en published which partly support and partly dispute this interpretati on. Here we present new dust emission maps observed at lambda-1300-mu- m with the Swedish ESO 15-m-Submm Telescope SEST (FWHP THETA(A) = 24") and at lambda-870-mu-m with the IRAM 30-m-Mm-Telescope MRT, providing the yet highest angular resolution of THETA(A) = 8". Combining the ne w results with previously published results (Paper I) and with new hig h resolution (CS)-S-34, (CO)-O-17 and (CO)-O-18 spectroscopy data (Mau ersberger et al., 1991) we address the following questions: i) What is the morphology and physical state of the NGC2024 cloud core? ii) Are the dust condensations isothermal protostars? iii) Do molecular transi tions cease to be reliable tracers of hydrogen column densities at hig h gas densities and low dust temperatures? We arrive at the following conclusions: 1) For an adopted distance of 415 pc, the cloud core has a total hydrogen mass of M(H) almost-equal-to 510m. It consists of an envelope of size almost-equal-to 0.3 x 0.5 pc, total mass M(H) almost- equal-to 300m. and average density <n(H)> almost-equal-to 1 10(5) cm-3 At the center of the envelope and extended roughly in NS direction is a ridge or bar of size 0.04 x 0.3pc, M(H) almost-equal-to 140m., <n(H )> 5 10(6) cm-3 and <N(H)> almost-equal-to 9 10(23) cm-2. Embedded in and close to the center of this bar are seven condensations with avera ge characteristics < M(H) > almost-equal-to 10m., < n(H) > almost-equa l-to 2.5 10(8) cm-3, <N(H)> 1 10(25) cm-2 and <2r> almost-equal-to 4.5 10(16) cm (sect.4). Submm colour temperatures for these core componen ts range from almost-equal-to 10 - 22 K. Only a small amount of dust i n and surrounding the HII region (representing almost-equal-to 30m. of mainly ionized gas) has temperatures of 45 K. The total luminosity is L(IR) almost-equal-to 2.8 10(4) L.(sect.3.1). 2) The compact dust con densations are best represented by isothermal protostars in free-fall contraction, where liberation of gravitational energy provides interna l heating. (sect.6.vi.). 3) The dust condensations are not or only bar ely visible in isotopic CS and CO transitions, probably since molecule s have frozen out on dust grains. It appears that molecule depletion b ecomes important in the density range 10(6) <n(H)/cm-3 < 10(8), at lea st at low dust temperatures T(d) less-than-or-equal-to 20 K (sect.5.2 and 3).