SIMBAD references

2006MNRAS.368.1463D - Mon. Not. R. Astron. Soc., 368, 1463-1478 (2006/May-3)

Properties of dust and detection of Hα emission in LDN 1780.


Abstract (from CDS):

We present ISOPHOT observations between 60 and 200µm and a near-infrared extinction map of the small intermediate-density cloud Lynds Dark Nebula (LDN) 1780 (Galactic coordinates l = 359° and b = 36{bsl00064}8). For an angular resolution of 4arcmin, the visual extinction maximum is AV= 4.4mag. We have used the ISOPHOT data together with the 25-, 60- and 100-µm IRIS maps to disentangle the warm and cold components of large dust grains that are observed in translucent clouds and dense clouds. The warm and cold components in LDN 1780 have different properties (temperature, emissivity) and spatial distributions, with the warm component surrounding the cold component. The warm component is mainly in the illuminated side of the cloud facing the Galactic plane and the Scorpius-Centaurus (Sco-Cen) OB association, as in the case of the Hi excess emission. The cold component is associated with the 13CO(J = 1-0) line integrated (W13), which trace molecular gas at densities of ∼103/cm3. The warm component has a uniform colour temperature of 25±1K (assuming β = 2), and the colour temperature of the cold component slightly varies between 15.8 and 17.3K (β = 2, ΔT = 0.5K). The ratio between the emission at 200µm of the cold component [Icν(200)] and AVis Icν(200)/AV= 12.1±0.7MJysr–1mag–1 and the average ratio τ200/AV= (2.0±0.2)x10–4 mag–1. The far-infrared emissivity of the warm component is significantly lower than that of the cold component. The Hα emission [Iν(Hα)] and AVcorrelate very well; a ratio Iν(Hα)/AV= 2.2±0.1Rayleighmag–1 is observed. This correlation is observed for a relatively large range of column densities and indicates the presence of a source of ionization that can penetrate deep into the cloud (reaching zones with optical extinctions AVof 2mag). Based on modelling predictions, we reject out a shock front as precursor of the observed Hα surface brightness although that process could be responsible of the formation of LDN 1780. Using the ratio Iν(Hα)/AV, we have estimated an ionization rate for LDN 1780 that results to be ∼10–16 γs–1. We interpret this relatively high value as due to an enhanced cosmic ray radiation rate of ∼10 times the standard value. This is the first time such an enhancement is observed in a moderately dense molecular cloud. The enhancement in the ionization rate could be explained as the result of a confinement of low-energy (∼100MeV) cosmic rays by self-generated magnetohydrodynamics waves in agreement with the recent modelling results of Padoan & Scalo. The origin of the cosmic rays could be from supernovae in the Sco-Cen OB association and/or the runaway ζ Ophiuchus. The observed low- 13CO abundance and relatively high temperatures of the dust in LDN 1780 support the existence of a heating source that can come in through the denser regions of the cloud.

Abstract Copyright: 2006 The Authors. Journal compilation © 2006 RAS

Journal keyword(s): ISM: clouds - dust, extinction - infrared: ISM

Simbad objects: 13

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