2010A&A...519A.110P

The disc around the Herbig Ae/Be star HD 100546 is one of the most extensively studied discs in the southern sky. Although there is a wealth of information about its dust content and composition, not much is known about its gas and large-scale kinematics. Many recent results have stressed the importance of studying both the gas and dust in discs. ¹²CO is an excellent gas tracer in the submillimetre, and the intensity ratio between lines originating from low and high rotational levels probes the gas temperature. Emerging submillimetre facilities in the Southern hemisphere allow us to characterise the gas and dust content in objects like HD 100546 better. Our aim is to establish whether the disc is gas-rich and to study the disc temperature and kinematics. We detected and studied the molecular gas in the disc at spatial resolution from 7.7'' to 18.9'' using the Atacama Pathfinder Experiment telescope. We observed the lines ¹²CO J=7-6, J=6-5, J=3-2, ¹³CO J=3-2 and [CI] ³P₂-³P₁, as diagnostics of disc temperature, size, chemistry, and kinematics. We use parametric disc models that reproduce the low-J ¹²CO emission from Herbig Ae stars and we vary the basic disc parameters - temperature, mass, and size. With the help of a molecular excitation and radiative transfer code we fit the observed spectral line profiles. Our observations are consistent with more than 10^–3M_☉ of molecular gas in a disc of ≃400AU radius in Keplerian rotation around a 2.5M_☉ star, seen at an inclination of 50°. The detected ¹²CO lines are dominated by gas at 30-70K. Not detecting the [CI] line indicates excess ultraviolet emission above that of a B9 type model stellar atmosphere. Asymmetry in the ¹²CO line emission suggests that one side of the outer disc is colder by 10-20K than the other. A plausible scenario is a warped geometry in the inner disc casting a partial shaddow on the outer disc. We exclude pointing offsets, foreground cloud absorption, and asymmetry in the disc extent as possible causes of the observed line asymmetry. Efficient heating of the outer disc by the star HD 100546 ensures that low- and high-J ¹²CO lines are dominated by the outermost disc regions, indicating a 400AU radius. The ¹²CO J=6-5 line arises from a disc layer higher than disc midplane, and warmer by 15-20K than the layer emitting the J=3-2 line. The existing models of discs around Herbig Ae stars, asuming a B9.5 type model stellar atmosphere, overproduce the [CI] ³P₂-³P₁ line intensity from HD 100546 by an order of magnitude.