Dust grains play a key role in the physics of star-forming regions , even though they constitute only \sim 1 % of the mass of the interstellar medium .
The derivation of accurate dust parameters such as temperature ( T _ { \mathrm { d } } ) , emissivity spectral index ( \beta ) and column density requires broadband continuum observations at far-infrared wavelengths .
We present Herschel -SPIRE Fourier Transform Spectrometer ( FTS ) measurements of three starless cores : L1521E , L1521F and L1689B , covering wavelengths between 194 and 671 \mu m. This paper is the first to use our recently updated SPIRE-FTS intensity calibration , yielding a direct match with SPIRE photometer measurements of extended sources .
In addition , we carefully assess the validity of calibration schemes depending on source extent and on the strength of background emission .
The broadband far-infrared spectra for all three sources peak near 250 \mu m. Our observations therefore provide much tighter constraints on the spectral energy distribution ( SED ) shape than measurements that do not probe the SED peak .
The spectra are fitted using modified blackbody functions , allowing both T _ { \mathrm { d } } and \beta to vary as free parameters .
This yields T _ { \mathrm { d } } of 9.8 \pm 0.2 K , 15.6 \pm 0.5 K and 10.9 \pm 0.2 K and corresponding \beta of 2.6 \mp 0.9 , 0.8 \mp 0.1 and 2.4 \mp 0.8 for L1521E , L1521F and L1689B respectively .
The derived core masses are 1.0 \pm 0.1 , 0.10 \pm 0.01 and 0.49 \pm 0.05 M _ { \sun } , respectively .
The core mass/Jeans mass ratios for L1521E and L1689B exceed unity indicating that they are unstable to gravitational collapse , and thus pre-stellar cores .
By comparison , the elevated temperature and gravitational stability of L1521F support previous arguments that this source is more evolved and likely a protostar .