Smoke in the Pipe Nebula: dust emission and grain growth in the starless core FeSt 1-457

Forbrich, Jan; Lada, Charles J.; Lombardi, Marco; Roman-Zuñiga, Carlos; Alves, João
Astronomy & Astrophysics, Volume 580, id.A114, 10 pp. (2015).


Context. The availability of submillimeter dust emission data in an unprecedented number of bands provides us with new opportunities to investigate the properties of interstellar dust in nearby clouds. 

Aims: The nearby Pipe Nebula is an ideal laboratory to study starless cores. We here aim to characterize the dust properties of the FeSt 1-457 core, as well as the relation between the dust and the dense gas, using Herschel, Planck, 2MASS, ESO Very Large Telescope, APEX-Laboca, and IRAM 30 m data. 
Methods: We derive maps of submillimeter dust optical depth and effective dust temperature from Herschel data that were calibrated against Planck. After calibration, we then fit a modified blackbody to the long-wavelength Herschel data, using the Planck-derived dust opacity spectral index β, derived on scales of 30' (or ~1 pc). We use this model to make predictions of the submillimeter flux density at 850 μm, and we compare these in turn with APEX-Laboca observations. Our method takes into account any additive zeropoint offsets between the Herschel/Planck and Laboca datasets. Additionally, we compare the dust emission with near-infrared extinction data, and we study the correlation of high-density-tracing N2H+ emission with the coldest and densest dust in FeSt 1-457. 
Results: A comparison of the submillimeter dust optical depth and near-infrared extinction data reveals evidence for an increased submillimeter dust opacity at high column densities, interpreted as an indication of grain growth in the inner parts of the core. Additionally, a comparison of the Herschel dust model and the Laboca data reveals that the frequency dependence of the submillimeter opacity, described by the spectral index β, does not change. A single β that is only slightly different from the Planck-derived value is sufficient to describe the data, β = 1.53 ± 0.07. We apply a similar analysis to Barnard 68, a core with significantly lower column densities than FeSt 1-457, and we do not find evidence for grain growth but also a single β. Finally, our previously reported finding of a correlation of N2H+ emission with lower effective dust temperatures is confirmed for FeSt 1-457 in mapping observations. 
Conclusions: While we find evidence for grain growth from the dust opacity in FeSt 1-457, we find no evidence for significant variations in the dust opacity spectral index β on scales 0.02 <x< 1 pc (or 36''<x< 30'). The correction to the Planck-derived dust β that we find in both cases is on the order of the measurement error, not including any systematic errors, and it would thus be reasonable to directly apply the dust β from the Planck all-sky dust model. As a corollary, reliable effective temperature maps can be derived which would be otherwise affected by β variations. Finally, we note that the angular resolution of extinction maps for the study of nearby starless cores remains unsurpassed.