Abstract
We investigate the effect of line of sight temperature variations and noise on two commonly used methods to determine dust properties from dust continuum observations of dense cores. One method employs a direct fit to a modified blackbody SED; the other involves a comparison of flux ratios to an analytical prediction. Fitting fluxes near the SED peak produces inaccurate temperature and dust spectral index estimates due to the line of sight temperature (and density) variations. Longer wavelength fluxes in the Rayleigh-Jeans part of the spectrum (>~ 600 micron for typical cores) may more accurately recover the spectral index, but both methods are very sensitive to noise. The temperature estimate approaches the density weighted temperature, or "column temperature," of the source as short wavelength fluxes are excluded. An inverse temperature - spectral index correlation naturally results from SED fitting, due to the inaccurate isothermal assumption, as well as noise uncertainties. We show that above some "threshold" temperature, the temperatures estimated through the flux ratio method can be highly inaccurate. In general, observations with widely separated wavelengths, and including shorter wavelengths, result in higher threshold temperatures; such observations thus allow for more accurate temperature estimates of sources with temperatures less than the threshold temperature. When only three fluxes are available, a constrained fit, where the spectral index is fixed, produces less scatter in the temperature estimate when compared to the estimate from the flux ratio method.
Highlights
Some of the coolest regions in molecular clouds are dense, starless cores
The systematic exclusion of short wavelength fluxes results in different T and β estimates; for an isothermal source, the fit T would always be the same. If this trend occurs when there are only a few observations, short wavelength observations can still be used to determine whether a source is isothermal or not
Though we cannot exclude the possibility that the spectral index of dust decreases with increasing temperature, we have shown that simple power-lawmodified blackbody fits to observed data can result in misleading T–β relationships which appear like those sometimes claimed to be of physical origin
Summary
Some of the coolest regions in molecular clouds are dense, starless cores. These dust enshrouded objects are often in the process of forming one (or a few) protostar(s) (Benson & Myers 1989). Since the temperatures of the cores are 15 K, the emergent continuum SED peaks in the far-infrared (FIR) or submillimeter wavelength regimes (see Figure 1). Ground- and space-based observations by Submillimeter Common-User Bolometric Array (SCUBA), MAMBO, Bolocam, Two Micron All Sky Survey (2MASS), Infrared Astronomical Satellite (IRAS), ISO, and Spitzer have detected dust emission from many environments, and they have provided much information about starless cores (e.g., Ward-Thompson et al 2002; Schnee et al 2007; Kauffmann et al 2008). A thorough consideration of the nature of dust-continuum emission, and the uncertainty associated with measuring it, is timely
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