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The disc around the Herbig Ae/Be star HD 100546 is one of the mostextensively studied discs in the southern sky. Although there is a wealth ofinformation about its dust content and composition, not much is known about itsgas and large scale kinematics. We detect and study the molecular gas in thedisc at spatial resolution from 7.7" to 18.9" using the APEX telescope. Thelines 12CO J=7-6, J=6-5, J=3-2, 13CO J=3-2 and [C I] 3P2-3P1 are observed,diagnostic of disc temperature, size, chemistry, and kinematics. We useparametric disc models that reproduce the low-J 12CO emission from Herbig~Aestars and vary the basic disc parameters - temperature, mass and size. Usingthe molecular excitation and radiative transfer code RATRAN we fit the observedspectral line profiles. Our observations are consistent with more than 0.001Msun of molecular gas in a disc of approximately 400 AU radius in Keplerianrotation around a 2.5 Msun star, seen at an inclination of 50 degrees. Thedetected 12CO lines are dominated by gas at 30-70~K. The non-detection of the[C I] line indicates excess ultraviolet emission above that of a B9 type modelstellar atmosphere. Asymmetry in the 12CO line emission suggests that one sideof the outer disc is colder by 10-20~K than the other, possibly due to a shadowby a warped geometry of the inner disc. Pointing offsets, foreground cloudabsorption and asymmetry in the disc extent are excluded scenarios. Efficientheating of the outer disc ensures that low- and high-J 12CO lines are dominatedby the outermost disc regions, indicating a 400 AU radius. The 12CO J=6--5 linearises from a disc layer higher above disc midplane, and warmer by 15-20~K thanthe layer emitting the J=3--2 line. The existing models of discs around HerbigAe stars, assuming a B9.5 type model stellar atmosphere overproduce the [CI]3P2--3P1 line intensity from HD 100546 by an order of magnitude.

Warm molecular gas and kinematics in the disc around HD 100546