ALMA observations of the Th 28 protostellar disk. A new example of counter-rotation between disk and optical jet

Abstract

Aims: Recently, differences in Doppler shifts across the base of four close classical T Tauri star jets have been detected with the HST in optical and near-ultraviolet (NUV) emission lines, and these Doppler shifts were interpreted as rotation signatures under the assumption of steady state flow. To support this interpretation, it is necessary that the underlying disks rotate in the same sense. Agreement between disk rotation and jet rotation determined from optical lines has been verified in two cases and rejected in one case. Meanwhile, the near-ultraviolet lines, which may trace faster and more collimated inner spines of the jet than optical lines, either agree or show no clear indication. We propose to perform this test on the fourth system, Th 28. Methods: We present ALMA high angular resolution Band 7 continuum, <SUP>12</SUP>CO(3-2) and <SUP>13</SUP>CO(2-1) observations of the circumstellar disk around the T Tauri star <ASTROBJ>Th 28</ASTROBJ>. <BR /> Results: The sub-arcsecond angular resolution (0.46''× 0.37'') and high sensitivity reached enable us to detect, in CO and continuum, clear signatures of a disk in Keplerian rotation around <ASTROBJ>Th 28</ASTROBJ>. The <SUP>12</SUP>CO emission is clearly resolved, allowing us to derive estimates of disk position angle and inclination. The large velocity separation of the peaks in <SUP>12</SUP>CO, combined with the resolved extent of the emission, indicate a central stellar mass in the range 1-2 M<SUB>☉</SUB>. The rotation sense of the disk is well detected in both <SUP>13</SUP>CO and <SUP>12</SUP>CO emission lines, and this direction is opposite to that implied by the transverse Doppler shifts measured in the optical lines of the jet. <BR /> Conclusions: The <ASTROBJ>Th 28</ASTROBJ> system is now the second system, among the four investigated so far, where counter-rotation between the disk and the optical jet is detected. These findings imply either that optical transverse velocity gradients detected with HST do not trace jet rotation or that modeling the flow with the steady assumption is not valid. In both cases jet rotation studies that rely solely on optical lines are not suitable to derive the launching radius of the jet.