The nature of the embedded intermediate-mass T Tauri star DK Chamaeleontis^⋆

¹ Dublin Institute for Advanced Studies, School of Cosmic Physics, 31 Fitzwilliam Place, Dublin 2, Ireland
e-mail: rgarcia@mpifr-bonn.mpg.de
² INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monteporzio Catone, Italy
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany

Received: 3 May 2011
Accepted: 29 August 2011

Abstract

Context. Most of our knowledge about star formation is based on studies of low-mass stars, whereas very little is known about the properties of the circumstellar material around young and embedded intermediate-mass T Tauri stars (IMTTSs) mostly because they are rare, typically more distant than their lower mass counterparts, and their nearby circumstellar surroundings are usually hidden from us.

Aims. We present an analysis of the excitation and accretion properties of the young IMTTS DK Cha. The nearly face-on configuration of this source allows us to have direct access to the star-disk system through the excavated envelope and outflow cavity.

Methods. Based on low-resolution optical and infrared spectroscopy obtained with SofI and EFOSC2 on the NTT we derive the spectrum of DK Cha from  ~ 0.6 μm to  ~2.5 μm. From the detected lines we probe the conditions of the gas that emits the H i IR emission lines and obtain insights into the origin of the other permitted emission lines. In addition, we derive the mass accretion rate (Ṁ_acc) from the relationships that connect the luminosity of the Br γ and Pa β lines with the accretion luminosity (L_acc).

Results. The observed optical/IR spectrum is extremely rich in forbidden and permitted atomic and molecular emission lines, which makes this source similar to very active low-mass T Tauri stars. Some of the permitted emission lines are identified as being excited by fluorescence. We derive Brackett decrements and compare them with different excitation mechanisms. The Paβ/Brγ ratio is consistent with optically thick emission in LTE at a temperature of  ~3500 K, originated from a compact region of  ~5 R_⊙ in size: but the line opacity decreases in the Br lines for high quantum numbers n_up. A good fit to the data is obtained assuming an expanding gas in LTE, with an electron density at the wind base of  ~10¹³ cm^-3. In addition, we find that the observed Brackett ratios are very similar to those reported in previous studies of low-mass CTTSs and Class I sources, indicating that these ratios are not dependent on masses and ages. Finally, L_acc  ~  9 L_⊙ and Ṁ_acc  ~  3  ×  10^-7 M_⊙ yr^-1 values were found. When comparing the derived Ṁ_acc value with that found in Class I and IMTTSs of roughly the same mass, we found that Ṁ_acc in DK Cha is lower than that found in Class I sources but higher than that found in IMTTSs. This agrees with DK Cha being in an evolutionary transition phase between a Class I and II source.