THE UNITED KINGDOM INFRARED TELESCOPE
ANNUAL REPORT
1995 AND 1996

2. Scientific Results during 1995 and 1996

2.2. Selected Scientific Results

2.2.5. MAX, CGS4 and IRCAM and the Environment of T Tauri

T.M. Herbst, M. Robberto and S.V.W. Beckwith (MPIA Heidelberg)

T Tauri objects are young stars with masses similar to those of our Sun which have cleared away most of the material out of which they formed but which are still active. In other words, they are still ejecting material from their vicinities and thus have not yet completely settled into the rather bland existence of our Sun.

As the prototype of its class, T Tau has been the subject of intense ground based study for decades. Our observations of the near environmentof this YSO took place a century after Burnham's initial discovery of a diffuse nebula surrounding the star. About a decade ago T Tau was discovered to be binary, with separation 0.7 arcseconds. The southern object is a so called ``infrared companion.'' These objects, which have now been found toward a number of young stars, are usually too faint to detect at visible wavelengths, yet they are frequently more luminous than their primaries, and dominate the system in the near to mid infrared.

During 1996, we used the United Kingdom Infrared Telescope with its new tip/tilt system built by the MPIA to investigate the near environment of T Tauri. Of interest to us were the interactions of the intense winds from T Tauri with the nearby molecular cloud which had given birth to the object and its companion. In addition to studying the gas surrounding T Tauri we were also eager to get a clear view of the object and its companion.

We imaged the central arcminute of the T Tauri system in the 2.12 m v=1-0 S(1) line of H, using IRCAM3 in combination with the 300 . Fabry Perot interferometer (FP). Figure 7 shows a continuum subtracted version of the S(1) image. More than fifteen distinct arcs and loops appear in the extended molecular hydrogen emission in Figure 7. This region, within 2000 AU of the stars, is unexpectedly rich in phenomena.

  
Figure 7: (left) Fabry Perot images of T Tau in the v=1-0 S(1) transition of molecular hydrogen. North is up and east to the left. Imperfect subtraction of the stellar continuum produces artifacts at the location of the two stars, so we have superimposed the innermost H emission from Herbst et al. (1996 AJ 111, p 2403) in inverse grayscale. The + symbols locate the stars. Note that the bright knot to the northwest of the stars is a line emission object, T Tau NW, and not one of the binary components. (right) Possible outflow geometry (discussed in the text).

Extended molecular hydrogen emission in a star forming region is frequently the signpost of shock waves excited as an outflow from a star encounters the ambient cloud. To test for this possibility, we obtained moderate resolution spectra in the K and H bands, using the UKIRT facility spectrometer, CGS4. These spectra revealed that the molecular hydrogen arcs and loops we had imaged indeed contained shock excited gas.

We propose that the complex morphology of the molecular hydrogen image can be explained by two almost perpendicular outflows, one arising from each of the two stars. The right hand panel of Figure 7 illustrates the proposed geometry. The brighter outflow is oriented NW SE, and probably arises from the primary star. This flow energizes Burnham's Nebula to the south of the stars, a region containing several molecular hydrogen arcs resembling bow shocks opening back toward T Tau. The overall curvature of the NW SE system can arise due to orbital motion of objects and thus the changing location of the origin of the outflow. The second outflow, oriented E W, is distinct in having the H emission oriented parallel to the flow direction. The infrared companion probably produces these structures.

Circumstellar disks may play a central role in the collimation of such outflows. We searched for evidence of such disks by imaging the T Tauri binary from 1 to 20 m in August 1996. We acquired 1 3.4 m images using IRCAM3 with the new tip/tilt secondary and 4.7 20 m images using the MPIA 128128 pixel Si:As mid infrared camera MAX with shift and add techniques. The remarkable sequence of images we obtained is shown in Figure 8. Note that at 1 3.4 m the infrared companion is cleanly separated from the primary star for all bands in which it is detected.

  
Figure 8: Images of the T Tauri binary system between 1.2 and 20 m. The upper four panels contain IRCAM3 images taken with the new tip/tilt secondary, while the lower eight images are shift and add MAX frames. We detected the southern infrared companion between 1.5 m, where seeing and dynamic range are a problem, and 20.6 m, where the diffraction limit of the telescope (1.4'') makes separation of the 0.69 arcsecond binary difficult.

The excellent image quality at UKIRT allowed us to obtain spatially resolved photometry of the binary. This has revealed evidence for circumstellar disks in both stars. The spectral energy distribution of the infrared companion has a strong silicate absorption near 10 m, while the primary shows the silicate feature in emission. This result resolves earlier contradictory observations, some of which showed little or no silicate emission in the primary. After subtraction of a model photosphere plus disk, the companion's absorption feature appears somewhat wider than the primary's emission feature. Our findings are consistent with increased optical depth or particle size, both of which are expected in an infrared companion.

The evidence presented here and elsewhere is beginning to produce a coherent picture of the wind disk ambient cloud interactions in the near environment of T Tauri. Here we see two stars, each with circumstellar disks, producing perpendicular outflows which impact the ambient medium. While this scenario explains most of the phenomena seen on sub arcsecond to arcminute scales, significant mysteries remain. For example, we still are not certain which star produced which outflow, or indeed if one of them produced both. If the winds are collimated by circumstellar disks and/or magnetic fields, the assignment of one outflow to each star raises the question of how the two stars in this close binary can have almost perpendicular angular momenta.

We gratefully acknowledge the contributions of Peter Bizenberger and Christoph Birk in bringing the MAX camera to first light. Tom Geballe and Thor Wold made the UKIRT FP available on short notice, and the entire UKIRT team deserve thanks for successful first runs with MAX. Finally, we are grateful to the MPIA UKIRT collaboration for the tip/tilt system. A complete description of these observations appears in The Astronomical Journal 114, 744 (1997).