ISO far-infrared observations of the high-latitude cloud L1642

Abstract

Aims. Our aim is to compare the infrared properties of big, "classical" dust grains with visual extinction in the cloud L1642. In particular, we study the differences in grain emissivity between diffuse and dense regions in the cloud. Methods. The far-infrared properties of dust are based on large-scale 100 µm and 200 µm maps. Extinction through the cloud was derived by using the star count method in the B-and I-bands, and colour excess method in the J, H, and K s bands. Radiative transfer calculations were used to study the effects of increasing absorption cross-section on the far-infrared emission and dust temperature. Results. Dust emissivity, measured by the ratio of far-infrared optical depth to visual extinction, τ(far-IR)/A V , increases with decreasing dust temperature in L1642. There is about a two-fold increase in emissivity over the dust temperature range of 19 K−14 K. Radiative transfer calculations show that, in order to explain the observed decrease of dust temperature towards the centre of L1642, an increase of absorption cross-section of dust at far-IR is necessary. This temperature decrease cannot be explained solely by the attenuation of interstellar radiation field. Increased absorption cross-section also manifests itself as an increased emissivity. We find that, due to temperature effects, the apparent value of optical depth τ app (far-IR), derived from 100 µm and 200 µm intensities, is always lower than the true optical depth.

Multiwavelength study of the high-latitude cloud L1642: chain of star formation

“…To compare with the earlier results of Lehtinen et al (2007, Fig. 3), we derived total least-squares fits of τ 250 vs. τ J also separately for different regions.…”

“…L1642 has also been widely studied using other methods such as dust emission in mid-and far-infrared (Laureijs et al 1987;Reach et al 1998;Verter & Rickard 1998;Verter et al 2000;Lehtinen et al 2004Lehtinen et al , 2007 and optical (360-600 nm) surface brightness (Laureijs et al 1987;Mattila et al 2012). Mattila et al (2007) studied scattered Hα light in L1642.…”

Dust coagulation processes as constrained by far-infrared observations

“…Lagache et al 1998;Bernard et al 1999;Stepnik et al 2003;del Burgo et al 2003;Planck Collaboration 2011b). Additionally, an increase in the dust opacity by a factor between 2 and 4 is observed in these denser regions at long wavelengths, which is inferred from the low dust temperature, and not always by comparison to the gas or A V (Bernard et al 1999;Hotzel et al 2001;Cambrésy et al 2001;Stepnik et al 2003;Bianchi et al 2003;del Burgo et al 2003;del Burgo & Laureijs 2005;Kiss et al 2006;Lehtinen et al 2007;Ridderstad et al 2006;Bot et al 2009;Paradis et al 2009;Planck Collaboration 2011b). Moreover, the abundance of the very small grains (VSGs, 1−15 nm in radius), that are stochastically heated and responsible for the 60 μm emission, decreases by ∼80-100% in the transition from diffuse to dense molecular clouds (Laureijs et al 1991;Abergel et al 1994Abergel et al , 1996Lagache et al 1998;Stepnik et al 2003;Kramer et al 2003).…”

Planckearly results. XXV. Thermal dust in nearby molecular clouds

“…An increase of τ 250 /N H in the FIR by a factor ≥1.5−4 (always with N H derived from the NIR extinction) has been detected from ISOPHOT observations of several high-latitude translucent clouds (Cambrésy et al 2001;Burgo et al 2003;Ridderstad et al 2006;Kiss et al 2006;Lehtinen et al 2007) and TMC-2 (Burgo & Laureijs 2005), by Spitzer observations of the Perseus molecular cloud (Schnee et al 2008) and of the Taurus molecular cloud (Flagey et al 2009), and more rencentely by Herschel observations of Galactic dense cores (Juvela et al 2011). Because the decrease in temperature is generally observed to be associated with a decrease of the 60 μm over 100 μm intensity ratio, which traces the abundance of small grains relative to that of big grains, coagulation has often been invoked to explain this emissivity increase (Bernard et al 1999;Stepnik et al 2003;Cambrésy et al 2005).…”

“…Because there is no embedded heating star, the distribution of dust temperatures along a "cold" line of sight, with a low value of the measured temperature, should be generally broader than along lines of sight with higher measured temperatures. Moreover, the measured temperatures for cold lines of sight are always warmer than the average temperature along that line of sight, so the values of τ 250 for cold lines of sight may underestimate the average values for dust particles 2 (see also Cambrésy et al 2001;Lehtinen et al 2007;Schnee et al 2008). 2.…”