Observations of warm molecular gas and kinematics in the disc around HD 100546

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² European Southern Observatory, Karl Schwarzschild str. 2, 85748 Garching, Germany e-mail: opanic@eso.org
³ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
⁴ Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ Nederlandse Onderzoeksschool Voor Astronomie (NOVA), PO Box 9513, 2300 RA Leiden, The Netherlands
⁶ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands

Received: 15 January 2010
Accepted: 3 May 2010

Abstract

Context. 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.

Aims. Our aim is to establish whether the disc is gas-rich and to study the disc temperature and kinematics.

Methods. We detected and studied the molecular gas in the disc at spatial resolution from 77 to 189 using the Atacama Pathfinder Experiment telescope (This publication is based on data acquired with the Atacama Pathfinder Experiment (APEX). APEX is a collaboration between the Max-Planck-Institut für Radioastronomie, the European Southern Observatory, and the Onsala Space Observatory.). We observed the lines ¹²CO J = 7–6, J = 6–5, J = 3–2, ¹³CO J = 3–2 and [C I] 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.

Results. Our observations are consistent with more than 10^-3  of molecular gas in a disc of ≈400 AU radius in Keplerian rotation around a 2.5  star, seen at an inclination of 50°. The detected ¹²CO lines are dominated by gas at 30–70 K. Not detecting the [C I] 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 10–20 K 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.

Conclusions. 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 400 AU radius. The ¹²CO J = 6–5 line arises from a disc layer higher than disc midplane, and warmer by 15–20 K 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.