Dense cores in the Seahorse infrared dark cloud: physical properties from modified blackbody fits to the far-infrared-submillimetre spectral energy distributions

We used data from WISE, IRAS, and Herschel in conjuction with our previous observations with SABOCA and LABOCA, and constructed the far-IR to submillimetre spectral energy distributions (SEDs) of dense cores in the filamentary Seahorse infrared dark cloud (IRDC) G304.74+01.32. For the 12 analysed cores, which include two IR dark cores (no WISE counterpart), nine IR bright cores, and one HII region, the mean dust temperature of the cold (warm) component, the mass, luminosity, H$_2$ number density, and surface density were derived to be $13.3\pm1.4$ K ($47.0\pm5.0$ K), $113\pm29$ M$_{\odot}$, $192\pm94$ L$_{\odot}$, $(4.3\pm1.2)\times10^5$ cm$^{-3}$, and $0.77\pm0.19$ g cm$^{-3}$, respectively. The HII region IRAS 13039-6108a was found to be the most luminous source in our sample ($(1.1\pm0.4)\times10^3$ L$_{\odot}$). All the cores were found to be gravitationally bound (i.e. the virial parameter $\alpha_{\rm vir}<2$). Seven out of 12 of the analysed cores (58%) were found to lie above the mass-radius thresholds of high-mass star formation proposed in the literature. The surface densities of $\Sigma>0.4$ g cm$^{-3}$ derived for these seven cores also exceed the corresponding threshold for high-mass star formation. Five of the analysed cores (42%) show evidence of fragmentation into two components in the SABOCA 350 $\mu$m image. In addition to the HII region source IRAS 13039-6108a, some of the other cores in Seahorse also appear to be capable of giving birth to high-mass stars. The dense core population in the Seahorse IRDC has comparable average properties to the cores in the well-studied Snake IRDC G11.11-0.12. The Seahorse core fragmentation mechanisms appear to be heterogenous, including cases of both thermal and non-thermal Jeans instability. High-resolution follow-up studies are required to address the fragmented cores' genuine potential of forming high-mass stars.