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Imaging the Small-Scale Circumstellar Gas Around T~ Tauri Stars

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Imaging the Small-Scale Circumstellar Gas Around T~ Tauri Stars References Andr´e, P., & Montmerle, T. 1994, ApJ, 420, 837 Bastien, P. 1982, A&AS, 48, 153 Beckwith, S. V. W., & Sargent, A. I. 1991, ApJ, 381, 250 Beckwith, S. V. W., & Sargent, A. I. 1993a, in Protostars and Planets III, edited by E. H. Levy and J. L. Lunine (University of Arizona Press, Tucson) p. 521 Beckwith, S. V. W., & Sargent, A. I. 1993b, ApJ, 402, 280 Beckwith, S. V. W., Sargent, A. I., Scoville, N. Z., Masson, C. R., Zuckerman, B. & Phillips, T. G. 1986, ApJ, 309, 755 Beckwith, S. V. W., Sargent, A. I., Chini, R. &G¨usten, R. 1990, AJ, 99, 924 Cohen, M., & Kuhi, L. V. 1979, ApJS, 41, 743 Cohen, M., Emerson, J. P. & Beichman, C. A. 1989, ApJ, 339, 455 d’Antona, F., & Mazzitelli, I. 1993, ApJS, 90, 467 de Geus, E. J., Bronfman, L., & Thaddeus, P. 1990, A&A, 231, 137 Dutrey A., Guilloteau, S. & Simon, M. 1994, A&A, 286, 149 Elias, J. 1978, ApJ, 224, 857 Elsasser, H., & Staude, H. J. 1978, A& A, 70, L3 Galli, D., & Shu, F. H. 1993, ApJ, 417, 220 arXiv:astro-ph/9503034v1 8 Mar 1995 Ghez, A., Neugebauer, G. & Mathews, K. 1993, AJ, 106, 2005 Hartmann, L., Hewett, R., Stahler, S., & Mathieu, R. D. 1986, ApJ, 309,275 Hayashi, M., Ohashi, N. & Miyama, S. M. 1993, ApJ, 418, L71 Henning, T., Pfau, W., Zinnecker, H., & Prusti, T. 1994, A&A, 276, 129 Herbig, G. H., & Bell, K. R. 1988, in Lick Observatory Bulletin No. 1111 (University of California) Herbig, G. H. 1977, ApJ, 214, 747 Hirth, G. A., Mundt, R., Solf, J., & Ray, T. P. 1994, ApJ, 427, L99 – 2 – Leinert, Ch., Hass, M., Richi, A., Zinnecker, H., & Mundt, R. 1991, A&A, 250, 407 Kawabe, R., Ishiguro, M., Omodaka, T., Kitamura, Y., & Miyama, S. M. 1993, ApJ, L63 Kenyon. S. J., Calvet, N., & Hartmann, L. 1993, ApJ, 414, 676 Koerner, D. W., Sargent, A. I., & Beckwith, S. V. W. 1993a, Icarus. 106, 2 Koerner, D. W., Sargent, A. I., & Beckwith, S. V. W. 1993b, ApJ, 408, L93 Nakajima, T., & Golimowski, D. A. 1995, AJ, in press Sargent, A. I., & Beckwith, S. 1987, ApJ, 323, 294 Sargent, A. I., & Beckwith, S. V. W. 1989, in Structure and Dynamics of the Interstellar Medium, IAU Colloquium No. 120, 215 Sargent, A. I., & Beckwith, S. 1991, ApJ, 382, L31 Sargent, A. I., & Beckwith, S. 1993, in Millimeter and Submillimeter Wave Interferometry, edited by M. Ishiguro and R. Kawabe, (Bookcrafters, San Francisco) p. 232 Scoville, N. Z., Sargent, A. I., Sanders, D. B., Claussen, M. J., Masson, C. R., Lo, K., & Phillips, T. G. 1986, ApJ, 303, 416 Stahler, S. W., Korycansky, D. G., Brothers, M. J., & Touma, J. 1994, ApJ, 431, 341 Stapelfeldt, K. R., Burrows, C. J., Krist, J., Hester, J. J., Trauger, J., Ballester, G., Casertano, S., Clarke, J., Crisp, D., Evans, R. W., Gallagher, J. S., Griffiths, R. E., Hoessel, J. G., Holtzman, J. A., Mould, J. R., Scowen, P., Watson, A. M., & Westphal, J. A. 1994, ApJ, submitted Strom, K. M., Strom, S. E., Edwards, S., Cabrit, S., & Skrutskie, M. 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Telephone: (818)356-6622; FAX: (818)568-9352; E-mail: [email protected] – 4 – No. pages: 32 No. Figures: 5 – 5 – Abstract We have detected circumstellar molecular gas around a small sample of T Tauri stars through aperture synthesis imaging of CO(2 1) emission at 2-3′′ resolution. RY Tauri, → ∼ DL Tauri, DO Tauri, and AS 209 show resolved and elongated gaseous emission. For RY Tau, the deconvolved, half-maximum radius along the direction of elongation, PA 48◦, ∼ is 110 AU. Corresponding radii and orientations for the other sources are: DL Tau – 250 AU at PA 84◦; DO Tau – 350 AU at PA 160◦; AS 209 – 290 AU at PA 138◦. RY Tau, ∼ ∼ ∼ DL Tau, and AS 209 show velocity gradients parallel to the elongation, suggesting that the circumstellar material is rotating. RY Tau and AS 209 also exhibit double-peaked spectra characteristic of a rotating disk. Line emission from DO Tau is dominated by high-velocity blue-shifted gas which complicates the interpretation. Nevertheless, there is in each case sufficient evidence to speculate that the circumstellar emission may arise from a protoplanetary disk similar to that from which our solar system formed. – 6 – 1. Introduction There is increasing evidence that the nebular environment assumed to have given rise to Solar System bodies is a common feature of the early stages of stellar evolution. Circumstellar dust disks are detected around a large fraction of T Tauri stars (e.g., Cohen, Emerson, & Beichman 1989; Strom et al. 1989; Beckwith et al. 1990; Andr´e& Montmerle 1994; Henning et al. 1994) and in at least one case, the velocity field of the associated circumstellar gas is consistent with that of a model of a disk in Keplerian rotation (Koerner, Sargent, & Beckwith 1993a, henceforth KSB). Disk properties, such as mass, size, and kinematics, are sufficiently similar to those expected for the early solar nebula (cf. Beckwith & Sargent 1993a) that continued study is likely to afford us the opportunity to constrain theories of planetary system formation. A better understanding of the detailed properties of these circumstellar environments may also narrow down the field of truly proto-planetary disks. The small fraction of T Tauri stars for which circumstellar gas has been observed (cf. Sargent & Beckwith 1993) contrasts markedly with the 50% detection rate achieved by continuum surveys. This is largely because single telescope observations of the circumstel- lar dust continuum emission are readily obtainable. In general, unambiguous detection of circumstellar gas on small spatial scales requires molecular line millimeter-wave interfer- ometer measurements; historically, these have been time consuming. Flattened structures of circumstellar gas with radii of order 1000 AU have been observed around the T Tauri stars, HL Tauri (Beckwith et al. 1986; Sargent & Beckwith 1987), T Tauri (Weintraub et al. 1987; 1989), DG Tauri (Sargent & Beckwith 1989; Ohashi et al. 1991), GG Tauri (Kawabe et al. 1993; Koerner, Sargent, & Beckwith 1993b; Dutrey, Simon, & Guilloteau – 7 – 1994) and GM Aurigae (KSB). The earliest interpretations suggested that circumstellar material was centrifugally supported out to radial distances of 1000 AU (Sargent & Beck- with 1987; Weintraub et al. 1989). Recent observations indicate that gas at these large radii is still in a state of collapse (e.g., Hayashi et al. 1993; van Langevelde, van Dishoeck, & Blake 1994). However, it also appears that some of these objects may not represent most T Tauri stars in general. For example, Hubble Space Telescope observations imply that HL Tau is heavily embedded in its parent cloud and is therefore at a very early stage of pre-main sequence evolution (Stapelfeldt et al. 1994); the spectral energy distribution (SED) of T Tau, a binary object, can be understood only if there is a large component of non-planar dust (Kenyon et al. 1993), and GG Tau is part of an unusual quadruple star system and surrounded by a large torus (Leinert et al. 1991; Ghez et al. 1993; Koerner et al. 1993b; Dutrey et al. 1994). To date, GM Aur seems the best example of a “typical” T Tauri star. It is relatively unobscured with AV = 0.1 (Strom et al. 1989) and is significantly older than the other stars which show circumstellar gas. It is also not a binary and affords a closer example of the early Solar System environment than the large fraction of T Tauri stars with a stellar companion (Ghez et al. 1993). Here, there is unambiguous evidence that the surrounding molecular gas is centrifugally supported out to 1000 AU, but emission beyond a 150 AU radius is tenous compared to the other circumstellar structures observed. This star/disk system may well represent a planet-forming environment (KSB). Obviously, we would like to observe a range of circumstellar gaseous environments for a larger sample of T Tauri stars to determine detailed properties of the disks and generalize these to compare with the conditions in the early solar nebula. We are continuing a program of aperture synthesis imaging of the environments of T Tauri stars with a view to building up a statistical base – 8 – of information on the circumstellar gas. High resolution molecular line images of four more systems, RY Tauri, DL tauri, DO Tauri, and AS 209 are presented and discussed here. 2. Observations RY Tau, DL Tau, DO Tau, and AS 209 are among the strongest sources of millimeter continuum emission in the nearest star-forming regions (Beckwith et al. 1990; Andr´e& Montmerle 1994). Their coordinates (Herbig & Bell 1988) and relevant properties are listed in Table 1. The spectral index, α2.2−12µ, was derived from K-band magnitudes (Cohen & Kuhi 1979; Ghez 1993) and IRAS 12 µm fluxes (Weaver & Jones 1992).
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