Detecting scattered light from low-mass molecular cores at 3.6μm

Abstract

Context. Recently discovered scattered light at 3−5 μm from low-mass cores (so-called "coreshine") reveals the presence of grains around 1 μm, which is larger than the grains found in the low-density interstellar medium. But only about half of the 100+ cores investigated so far show the effect. This prompts further studies on the origin of this detection rate. Aims. We aim to supply criteria for detecting scattered light at 3.6 μm from molecular cloud cores. Methods. From the 3D continuum radiative transfer equation, we derive the expected scattered light intensity from a core placed in an arbitrary direction seen from Earth. We use the approximation of single scattering, consider extinction up to 2nd-order Taylor approximation, and neglect spatial gradients in the dust size distribution. We analyze how scattered light can outshine the absorbing effect of extinction in front of background radiation by the core for given grain properties, anisotropic interstellar radiation field and background field. The impact of the directional characteristics of the scattering on the detection of scattered light from cores is calculated for a given grain size distribution, and local effects like additional radiation field components are discussed. The surface brightness profiles of a core with a 1D density profile are calculated for various Galactic locations, and the results are compared to the approximate detection limits. Results. We find that for optically thin radiation and a constant size distribution, a simple limit for detecting scattered light from a low-mass core can be derived that holds for grains with sizes smaller than 0.5 μm. The extinction by the core prohibits detection in bright parts of the Galactic plane, especially near the Galactic center. For scattered light received from low-mass cores with grain sizes beyond 0.5 μm, the directional characteristics of the scattering favors the detection of scattered light above and below the Galactic center, and to some extent near the Galactic anti-center. We identify the local incident radiation field as the major unknown causing deviations from this simple scheme. Conclusions. The detection of coreshine at a wavelength of 3.6 μm is a complex interplay of the incident radiation, the extinction of the background radiation, the grain properties, and the core properties like sky position and mass.

Galactic cold cores

“…The effect was also seen in , where WISE MIR observations were analysed and both LDN 183 and LDN 134 were found to be sources of strong MIR emission. If the signal is interpreted as scattering of the interstellar radiation field, it seems to imply the presence of very large, micrometre-sized dust particles (Steinacker et al , 2014Lefèvre et al 2014). Thus, our detection of increased submillimetre opacity in these clouds agrees with the evidence provided by MIR wavelengths.…”

A common column density threshold for scattering at 3.6μm and water-ice in molecular clouds

“…Following the discussion in Steinacker et al (2014), the location of Lupus IV at (l, b) = (336.7, +07.8) indeed favours the scenario scattering exceeds the extinction of the background for a maximum grain size of 1 µm or more. The spectroscopic sample was not chosen specifically for sight lines where CS was present above the background, and this presents an unbiased selection of sight lines to measure CS.…”

“…In this Letter we investigate the connection between the large grains observed through scattering at 3.6 µm and the water-ice abundance in Lupus IV. The observed scattered light is a function of the dust distribution, but also of the geometry between the molecular cloud and the scattering source, the radiation field being scattered, and the strength of the background radiation, all depending on the location in the Galaxy (Steinacker et al 2014;Lefèvre et al 2014). The large-scale geometric effects, for instance the phase function, and the radiation field, are nearly constant, which reduces the main uncertainties to the relative geometry between the molecular clouds and the radiation field.…”

CORESHINE : a tracer of grain growth in dark clouds

“…Several questions arise to whether a constant amount of large grains is present throughout the cloud or if the large grain fraction increases with density, the importance of the local interstellar radiation field, of the global interstellar radiation field anisotropy and of the cloud background field. The latter can prevent the detection of coreshine if too high, as already advocated in [20] and demonstrated in [30,31]. The precise estimate of the background field is a compulsory requisite to be able to constrain the coreshine intensity in the model.…”

“…Without modelling, the coreshine effect would remain simply the most direct proof of the existence of grown grains in clouds, but coreshine can be used as a tool to explore both the 3D structure of clouds and the grain properties [11,30,31]. Several questions arise to whether a constant amount of large grains is present throughout the cloud or if the large grain fraction increases with density, the importance of the local interstellar radiation field, of the global interstellar radiation field anisotropy and of the cloud background field.…”