The structure of hot molecular cores over 1000 AU

Context. Hot molecular cores (HMCs) are believed to be the cradles of stars of mass above similar to 6 M(circle dot). It is hence important to determine their structure and kinematics and thus study phenomena directly related to the star-formation process, such as outflow, infall, and rotation. Establishing the presence of embedded early-type (proto) stars is also crucial for understanding the nature of HMCs. Aims. To achieve the highest available angular resolution to date, we performed observations of the molecular gas in two well-known HMCs (G10.47+0.03 and G31.41+0.31) with an angular resolution of similar to 0.1. Continuum observations were also made at different wavelengths to detect HII regions associated with early-type stars embedded in the cores. Methods. We used the Very Large Array in its most extended configuration to image the NH(3)(4,4) inversion transition. Continuum measurements were made at 7 mm, 1.3 cm, and 3.6 cm using the A-array configuration. Results. We detected two new continuum sources in G31.41+0.31, which are possibly thermal jets, and confirmed the presence of one ultracompact and two hypercompact HII regions in G10.47+0.03. Evidence that the gas is infalling towards the embedded (proto) stars is provided for both G10.47+0.03 and G31.41+0.31, while in G10.47+0.03 part of the ammonia gas also appears to be expanding in two collimated bipolar outflows. From the temperature profile in the cores, we establish an approximate bolometric luminosity for both sources in the range 1 x 10(5)-7 x 10(5) L(circle dot). Finally, a clear velocity gradient across the core is detected in G31.41+0.31. The nature of this gradient is discussed and two alternative explanations are proposed: outflow and rotation. Conclusions. We propose a scenario where G10.47+0.03 is in a more advanced evolutionary stage than G31.41+0.31. In this scenario, thermal jets develop until the accretion rate is sufficiently high to trap or even quench any HII region. When the jets have pierced the core and the stellar mass has grown sufficiently, hypercompact HII regions appear and the destruction of the HMC begins.