TW Hya from the Submillimeter to X-rays: Chemistry of a Nearby Protoplanetary Disk

In contrast, a handful of T Tauri stars are notable because of the absence of dark clouds in their general vicinity. TW Hya is the prototype of this group. It is fully 13 degrees from the nearest cloud (Rucinski and

Krautter 1983). Yet there is little doubt as to its T Tauri status: like classical T Tauri stars, it exhibits strong HI emission (as well as other optical emission lines), Li absorption, and excess continuum flux at near- infrared and ultraviolet wavelengths (Rucinski & Krautter). It is a strong submillimeter continuum source (Weintraub et al. 1989) and displays relatively strong CO emission (Zuckerman et al. 1995).

Figure 1: Molecular spectra observed toward TW Hya with the JCMT. Abscissa is heliocentric radial velocity and ordinate is relative main-beam brightness temperature. The dashed curve overlaying the CO (3-2) line profile represents the best-fit Gaussian.

Thus, it appears TW Hya possesses a compact, dusty molecular envelope that persists despite the general lack of interstellar molecular gas in its vicinity. Most likely the molecular gas orbits the star; specifically, given its narrow emission lines, the CO probably lies in a nearly face-on disk (Zuckerman et al.). Such a (relatively gas-rich) disk may closely resemble the early solar nebula. If TW Hya is as close as some of the other young stars in its vicinity (e.g. HD 98800, which may be as close as 20 pc; Zuckerman & Becklin 1993), and if the material orbiting TW Hya has a nearly face-on orientation, then it is likely one of the best young systems to examine for indications of the formation of planets.

To better understand the chemistry and physical conditions of the circumstellar molecular gas around TW Hya, we are conducting a molecular line survey of this enigmatic star with the JCMT (Kastner et al. 1996). To date, we have detected the J=2-1, 3-2, and 4-3 transitions of CO, J=2-1 and 3-2 transitions of 13CO, J=3-2 and 4-3 transitions of HCN, and the J=4-3 transition of HCO+. Spectra of most of these molecular lines are presented in Figure 1. Note the narrow, "wing-less"' line profiles (even for the CO lines), which are unlike those typically observed toward T Tauri stars embedded in molecular clouds. The absence of clear evidence of mass outflow suggests that the molecular gas is bound, and supports the interpretation that the emission arises in a face-on disk.

The large abundances of HCN and HCO+ (inferred from the strengths of these lines relative to 13CO emission) suggest a chemistry that more closely resembles that of molecule-rich planetary nebulae and massive star formation regions than low-mass (T Tauri) star-forming cloud cores. Thus the circumstellar chemistry of TW Hya likely has deviated significantly from that of its parent cloud (although no evidence exists for the existence, past or present, of said cloud!). Since T Tauri stars are often X-ray sources, we investigated one possible mechanism for this chemical evolution, namely, X-irradiation of the circumstellar gas. Indeed, from analysis of archival ROSAT data, we find that TW Hya is a very strong X-ray source, with an X-ray flux of 3 x 10(-12) ergs cm(-2) s(-1) - about an order of magnitude larger than the X-ray fluxes typical of T Tauri stars in nearby clouds such as Taurus. Since TW Hya is probably not very young (an age of at least 107 yr is suggested by the apparent dispersal of its parent molecular cloud), and hence likely is not as intrinsically luminous in X-rays as deeply cloud-embedded T Tauri stars, its unusually large X-ray flux is likely due to its proximity to Earth (see below). Even at a distance of only 30 pc, however, our calculations indicate that the flux of X-ray photons incident on its circumstellar molecular envelope is sufficient to explain the inferred overabundance of HCO+, via reactions triggered by ionization of molecular hydrogen.

The relative intensities of the various transitions of CO and HCN suggest that the molecular emission region is rather warm (about 50 K or warmer) and dense (about 10(7) cm(-3)). If gas-dust collisions are the dominant heating mechanism for the molecular gas - which is far from certain, given the possibility that the strong UV and/or X-ray emission from TW Hya are important for gas heating - then this relatively high gas temperature would suggest the emitting region is approximately the size of our solar system, i.e., about 35 AU in radius. The intensities of the CO lines combined with beam-filling considerations then indicate that TW Hya is not likely to be much further from Earth than about 30 pc. This distance is consistent with the notion that TW Hya is at a rather advanced age for a T Tauri star; its luminosity at 30 pc is a mere 0.1 L(sun), i.e., the main-sequence luminosity of a star of its spectral type (K7).

If this distance determination is confirmed (an accurate distance to TW Hya will soon be available from Hipparcus observations) then, at 30 pc, TW Hya

would be the closest T Tauri star to Earth that is orbited by a prominent molecular disk. At this distance (and assuming that the gas is collisionally heated), the angular radius of the disk (about 1") would correspond to the radius of the orbit of Neptune. Such a disk should be marginally resolveable with the next generation of submillimeter interferometers at Mauna Kea Observatory, raising the potential for mapping out the gas distribution in a region that corresponds closely with the region occupied by the massive planets in our solar system. From these and other observations at very high resolution, TW Hya may have much more to tell us about how such planets came to be.

Joel Kastner (MIT),

Ben Zuckerman (UCLA)

& Thierry Forveille (Grenoble)

References

Kastner, J., Zuckerman, B., & Forveille, T. 1996, submitted to AJ.

Rucinski, S.M., & Krautter, J. 1983, A&A 121, 217

Weintraub D.A., Sandell, G., & Duncan 1989, ApJ, 340, L69

Zuckerman, B., & Becklin, E.E. 1993, ApJ 406, L25

Zuckerman, B., Forveille, T., & Kastner, J.H. 1995, Nature 373, 494