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 disk object T Cha 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). The mass accretion rate 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 M acc([OI]) and 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 an 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.