High-resolution Herschel spectroscopy reveals the proto-planetary disk around the young star T Tau N

Proto-planetary disks are the birthplaces of planets. Studying their physical and chemical structure is fundamental to comprehend the formation of our own solar system as well as of extra-solar planetary systems. Such disks are composed of gas and dust. The dust emits continuum excess above the stellar photosphere, while the gas is traced by atomic and molecular lines over a wide range of wavelengths. Observations of these lines allow one to build an inventory of molecular and atomic species in disks and to derive the disks properties, such as their mass and radius. These are fundamental to understand the disk structure and evolution, thus the conditions for planet formation.

From an observational point of view young proto-platenary disks are difficult to observe because they are still surrounded by a residual of the gaseous envelope from which the star has formed. Moreover powerful outflows in these young systems can transport mass away from the star-disk system. The emission from the disk may thus be completely hidden behind the much more extended emission from the original stellar envelope and the outflow. It is however possible to disentangle different gas components in the circumstellar environment through the emission lines from species that probe different physical and chemical conditions. In particular, the cyanogen molecule CN is a precious diagnostic to trace protoplanetary disks around stars which are still embedded in their envelope, because less contaminated by envelope/outflow emission.

A study led by Linda Podio (AstroFIt fellow at the Arcetri Observatory) with Inga Kamp (Kapteyn Institute, the Netherlands), Claudio Codella (Observatory of Arcetri) and other collaborators from Grenoble, UK and USA, makes use of high-resolution spectroscopic observations with HIFI on board the Herschel space observatory to characterize the disk around the young star T Tau N. This few-million-year-old star, located at a distance of 140 pc in the Taurus star forming region, is part of a complicated triple system composed of two more sources, still deeply embedded in their envelope and with two perpendicular outflows.

Sketch of the triple system composed of T Tau N and other two nearby young stars (T Tau Sa, Sb). Two perpendicular outflows are driven by T Tau N and T Tau S (a). The profile of the H2O line (b) reveals the two outflows in the blue and red wings. The two narrow peaks of the CN line (c) originate from the receding and approaching sides of the T Tau N disk. Courtesy: Linda Podio

By comparing the line profiles of H₂O and of CN, the authors are able to distinguish the disk emission from that of the envelope and the outflows. The H₂O line at 556.9 GHz is dominated by the emission from the stellar envelope, but has two broad wings originating from the two high-velocity outflow lobes. The CN line at 566.7 GHz shows a very different profile, characterised by two narrow peaks, one associated with the approaching (blue) side of the disk and the other with the receding (red) side.

From the intensity of the CN line and the velocity separation between the two peaks, the authors can estimate the disk radius (110 AU) and inclination (25 degrees). Notably, the mass of the disk of T Tau N (0.01 solar masses) is larger than the minimum mass needed to form our solar system. The proto-planetary disk around T Tau N could thus give rise to a planetary system like our own Solar System.

The results of this study are published in Probing the gaseous disk of T Tau N with CN 5-4 lines, Podio L. et al, ApJ Letters, 783, 26

Edited by Anna Gallazzi and Linda Podio