UVES spectroscopy of T Chamaeleontis: line variability, mass accretion rate, and spectro-astrometric analysis

Abstract

Although advances in exoplanet detection techniques have seen an increase in discoveries, observing a planet in the earliest stages of formation still remains a difficult task. Here four epochs of spectra of the transitional disc object T Chamaeleontis are analysed to determine whether spectro-astrometry can be used to detect a signal from its proposed protoplanet, T Cha b. The unique properties of T Cha are also further constrained. Hα and [O I] λ 6300, the most prominent lines, were analysed using spectro-astrometry. Hα being a direct accretion tracer is the target for the T Cha b detection, while [O I] λ 6300 is considered to be an indirect tracer of accretion. [O I] λ 6300 is classified as a broad low-velocity component (BC LVC). \dot{M}_acc was derived for all epochs using new [O I] λ 6300 LVC relationships and the Hα line luminosity. It is shown that a wind is the likely origin of the [O I] λ 6300 line and that the [O I] λ 6300 line serves as a better accretion tracer than Hα in this case. From the comparison between \dot{M}_acc({[O I]}) and \dot{M}_acc(Hα) it is concluded that T Cha is not an intrinsically weak accretor but rather that a significant proportion of the Hα emission tracing accretion is obscured. T Cha b is not detected in the spectro-astrometric analysis yet a detection limit of 0.5 mas is derived. The analysis in this case was hampered by spectro-astrometric artefacts and by the unique properties of T Cha. While it seems that spectro-astrometry as a means of detecting exoplanets in TDs can be challenging, it can be used to put a limit on the strength of the Hα emission from accreting planetary companions, and thus can have an important input into the planning of high-angular resolution observations.