The physical conditions for massive star formation: Dust continuum maps and modeling

Fifty-one dense cores associated with water masers were mapped at 350 μm. These cores are very luminous, 10³ < L_bol/L _ȯ<10⁶, indicative of the formation of massive stars. Dust continuum contour maps, radial intensity profiles, and photometry are presented for these sources. The submillimeter dust emission peak is, on average, nearly coincident with the water maser position. The spectral energy distributions and normalized radial profiles of dust continuum emission were modeled for 31 sources using a one-dimensional dust radiative transfer code, assuming a power-law density distribution in the envelope, n =n _f(r/r_f)^-p. The best-fit density power-law exponent, p, ranged from 0.75 to 2.5 with (p) = 1.8 ± 0.4, similar to the mean value found recently by Beuther and coworkers in a large sample of massive star-forming regions. The mean value of p is also comparable to that found in regions forming only low-mass stars, but (n_f) is over 2 orders of magnitude greater for the massive cores. The mean p is incompatible with a logatropic sphere (p = 1), but other star formation models cannot be ruled out. Different mass estimates are compared and mean masses of gas and dust are reported within a half-power radius determined from the dust emission, 〈logM(<r_dec)〉 =2.0 ± 0.6, and within a radius where the total density exceeds 10⁴ car^-3, 〈log M(<r_n)〉 = 2.5 ± 0.6. Evolutionary indicators commonly used for low-mass star formation, such as T_bol and L _boi/L_smm may have some utility for regions forming massive stars. Additionally, for comparison with extragalactic star formation studies, the luminosity-to-dust mass ratio is calculated for these sources, (L_bol/M_D) = 1.4 x 10⁴ L_ȯ/M _ȯ, with a method most parallel to that used in studies of distant galaxies. This ratio is similar to that seen in high-redshift starburst galaxies.