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ISO far-infrared observations of the high-latitude cloud L1642 . II. Correlated variations of far-infrared emissivity and temperature of "classical large" dust particles
Aims.Our aim is to compare the infrared properties of big, "classical"dust grains with visual extinction in the cloudL1642. In particular, we study the differences ingrain emissivity between diffuse and dense regions in the cloud. Methods: The far-infrared properties of dust are based on large-scale100 μm and 200 μm maps. Extinction through the cloud was derivedby using the star count method in the B- and I-bands, and colour excessmethod in the J, H, and Ks bands. Radiative transfercalculations were used to study the effects of increasing absorptioncross-section on the far-infrared emission and dust temperature. Results: Dust emissivity, measured by the ratio of far-infrared opticaldepth to visual extinction, τ(far-IR)/A_V, increases with decreasingdust temperature in L1642. There is about a two-foldincrease in emissivity over the dust temperature range of 19 K-14 K.Radiative transfer calculations show that, in order to explain theobserved decrease of dust temperature towards the centre ofL1642, an increase of absorption cross-section ofdust at far-IR is necessary. This temperature decrease cannot beexplained solely by the attenuation of interstellar radiation field.Increased absorption cross-section also manifests itself as an increasedemissivity. We find that, due to temperature effects, the apparent valueof optical depth τ_app(far-IR), derived from 100 μm and 200 μmintensities, is always lower than the true optical depth.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands, and the UK) and with the participation of ISAS and NASA.Appendices A-D are only available in electronic form athttp://www.aanda.org
| ISO far infrared observations of the high latitude cloud L 1642. I. The density and temperature structure
We have performed a large-scale mapping of the high-latitude, moderateextinction dark cloud L 1642 with ISO (Infrared Space Observatory) at200 μm to study the properties of dust, the virial equilibriumcondition of the cloud, and their relation to star formation. The cloudconsists of three denser regions which are connected by diffusematerial. Only one of the regions, the one most massive and with thegreatest optical depth, is related to a temperature mimimum. Thepre-main sequence binary stars IRAS 04325-1419 and IRAS 04327-1419,probably born within the cloud, are located close to this temperaturemimimum coinciding with the column density maximum. The minimum dusttemperature is ˜13.8 K. The ratio2Ekin/|Epot| for the star-forming region is about1.2, and thus it is close to being gravitationally bound. However, theother regions are not gravitationally bound. On a 1 arcmin-10 arcmin spatial scale the radial density distributions, derived from far-infaredoptical depth maps, are similar between the star-forming and nonstar-forming regions. The differences that distinguish the star-formingcore appear to be its higher density and lower dust temperature.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the United Kingdom) and with the participation of ISASand NASA.
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