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197 5Apj. . .195. .6313 the Astrophysical Journal, 195:631-642 .6313 .195. The Astrophysical Journal, 195:631-642, 1975 February 1 . © 1975. The American Astronomical Society. All rights reserved. Printed in U.S.A. 5ApJ. 197 T TAURI NEBULAE AND HERBIG-HARO NEBULAE: EVIDENCE FOR EXCITATION BY A STRONG STELLAR WIND* Richard D. Schwartz*)* Lick Observatory, Board of Studies in Astronomy and Astrophysics, University of California, Santa Cruz Received 1974 July 9 ABSTRACT Radial velocities of the emission nebulae and NGC 1555 (Hind’s Reflection Nebula) which are associated with T Tau suggest the presence of extended mass outflow which is supersonic with respect to the medium in which the star is embedded. It is suggested that radiating shocks may form where the flow interfaces with the intracloud medium, producing emission spectra similar to that predicted by the shock-wave models of Cox. An Nei Te analysis based upon the relative intensities of the near-infrared and blue lines of [O n] in Burnham’s Emission Nebula supports the shock wave hypothesis. The character of the emission-line spectrum, the small filling factor, and the energy balance requirements of the system are also compatible with the hypothesis. The Herbig-Haro Nebulae, if produced by the same mechanism, must involve central objects with a considerably higher rate of mass outflow than is indicated for T Tau. Subject headings: circumstellar shells — nebulae — pre-main-sequence stars — stellar winds I. INTRODUCTION emission does occur, but is so heavily obscured by circumstellar dust that it is not visible, it is difficult to The spectral and morphological similarities of the understand how ionizing radiation could penetrate emission nebulae associated with certain T Tauri stars such a shell to produce the nebula. and the Herbig-Haro (H-H) objects indicate that In this study, new evidence on the emission nebu- identical physical processes are probably involved in losity surrounding T Tau is presented in support of a their production. Among the ionization mechanisms hypothesis that the nebula may be excited by a strong that have been proposed for the nebulae, none can stellar wind which forms shock waves upon inter- account convincingly for the unique spectra displayed action with the intracloud medium. The implications by these objects. The fast-particle (100 keV protons) of this hypothesis for the H-H objects are discussed. ionization mechanism suggested by Magnan and Schatzman (1965) did not consider the Coulomb scattering efficiency of proton-electron encounters. A II. OBSERVATIONS consequence of this scattering is that the thermalizing The Carnegie image tube (CIT) spectrograph at- distance of 100 keV protons at the electron densities 4 -3 tached to the 2.1-meter telescope at Kitt Peak National of ~ 10 cm derived for these objects (Böhm, Perry, Observatory was employed to obtain untrailed spectra and Schwartz 1973; Schwartz 1974) is too small by a of the T Tauri nebula on 1973 December 6. An east- factor of 100 to explain the observed extent of about 16 west oriented slit with a projected width of 2" and a 10 cm for the nebulae. Although seemingly plausible length of y.l was used for two exposures centered 2" for the T Tau emission nebula (Burnham’s Nebula), and 4", respectively, south of the star. This allowed the indirect radiative ionization process suggested by simultaneous detection of Burnham’s Nebula, which Schwartz (1974) is of questionable relevance for the extends about lO" southward from the star, and NGC H-H objects. In that study, it was assumed that the 15551 (Hind’s Nebula), which is situated in the region chromospheric-like circumstellar envelope of T Tau 35" to 60" west of the star. Figure 1 is a photograph of was maintained in a state of essentially complete T Tau and NGC 1555 kindly furnished by George ionization by the deposition of energy from shock Herbig. The location of the spectrograph slit relative waves originating from violent convection in the star. to the star and NGC 1555 is indicated to scale. The Recombination Lyman-continuum photons then es- spectra were recorded on nitrogen-baked IIIa-J plates cape the envelope to ionize the extended, lower- which were calibrated with a spot sensitometer. Figure density gas. Although recent evidence suggests that 2 is a reproduction of the spectrum taken 4" south of T Tauri stars or T Tauri-like stars are associated with the star, covering the range 4800-7600 Â at a disper- some H-H objects (Strom, Strom, and Grasdalen sion of 125 Â mm-1. Figure 3 is an enlargement of the 1974è), there is no evidence for strong chromospheric- like emission (i.e., strong H and K lines of Ca n and other permitted metallic lines) in H-H objects. If such 1 In the early literature, NGC 1555 is described as a round condensation lying about 45" southwest of T Tau. (See * Lick Observatory Bulletin, No. 673. Barnard 1895, 1899 for an historical account of the observa- t Visiting astronomer at Kitt Peak National Observatory, tions from the time of Hind’s discovery of the nebula in which is operated by the Association for Research in As- 1852.) Direct plates taken with the Crossley reflector by tronomy, Inc., under contract with the National Science Curtis in 1914-1915 confirm Barnard’s description of the Foundation. nebula. 631 © American Astronomical Society • Provided by the NASA Astrophysics Data System .6313 .195. SCHWARTZ 632 . region 6200-6800 Â from the plate taken 2" south of TABLE 1 5ApJ. the star. The strong continuum in figure 3 resulted Relative Line Intensities in Nebulae from scattering of stellar continua during the exposure. 197 Lampland (1936) observed the nebula for over 20 Burnham’s years, and reported a dramatic increase in brightness A(Â) Identification Nebula* HH It N- in 1935. Baade’s 100-inch (2.5-m) photograph taken in 1940 (see Joy 1945) shows that NGC 1555 had become 3726 [O ii] 108 73 (648) 3729 ....... [O ii] 63 56 greatly extended to the west and north from its earlier 3868 [Ne in] 6 8 (45) position, a change which may have been associated with 3889 H8 10 9 (27) Lampland’s observation. Crossley plates and 120-inch 3934 ....... Ca ii 9 16 (16) (3-m) plates taken in the period 1950-1962 reveal that 3968 Ca ii, He, [Ne m] 38 24 (41) 4071 [S ii] 67 69 36 little morphological change has occurred in the 4102 HS 30 23 36 nebula since 1940. 4340 Hy 41 36 51 Several features emerge from these spectra which 4571 Mg I] 7 5 were undetected in the earlier study (Schwartz 1974). 4861 H]8 Í6Ó 100 100 4959 [O hi] 8 18 40 First, the greater red sensitivity of the CIT system 5007 [O in] 22 51 96 allowed detection of the near-infrared lines of He i 5159 [Fe ii] 15 29 18 À7065, [Fe n] A7155, [Ca il] A7291, [O n] A7330, and a 5200 [Ni] 22 30 8 blend of [O n] A7319 and [Ca h] A7324. In addition, 5754 [Nil] (5)§ 4 5876 He i (10)§ 12 lines of He i A5876 and [N n] A5754 have been de- 6300 [O i] 126 A 124 tected. Second, a new emission nebula has been dis- 6363 ...... [O I] 45 70 44 covered near the inner edge of NGC 1555. The shorter 6548 [N ii] 55 87 31 slit length used in the earlier study did not allow the 6563 Ha 340 408 296 6584 [Nil] 134 221 77 detection of features in the vicinity of NGC 1555. 6717 [S ii] 106 114 (172) Figures 2 and 3 show that the bulk of NGC 1555 is a 6731 [S ii] 171 198 reflection of the spectrum of T Tau, dominated by a 7065 He i 7 strong Ha feature superposed upon a stellar con- 7155 [Fe ii] 15 7291 [Ca ii] 13 tinuum. The spectrum correlates well with photo- 7319 + 7324 [O ii], [Ca ii] 24 graphs of the region (cf. fig. 1), indicating that the 7330 [O ii] 16 I reflection nebula extends from about 35" to 60" west of the star. The discrete emission nebula, especially * Schwartz 1973. t Böhm et al. 1973. strong in the lines of [S n], [N n], and Ha, but posses- t Oster brock and Dufour 1973. sing no apparent continuum, appears in the region § Intensities poorly calibrated. from 20" to 35" west of the star, perhaps merging II Lines appear in H-H 1 with about same relative inten- slightly with the reflection nebula. Third, a close sities, but are not calibrated relative to Hß. examination reveals significant changes in the radial # Wavelength region not covered by Osterbrock and velocity structure of the emission lines as one moves Dufour spectrum. westward from Burnham’s Nebula, through the new emission nebula, and into NGC 1555 (see especially fig. 3). earlier study using the Lick automated Gaertner microphotometer. In addition, because the brighter III. DATA ANALYSIS lines on the present deep-exposure plates are too a) N , T of Burnham's Nebula from the [O n] Lines dense to retrieve any intensity information, the infra- e e red lines have been normalized to the Hß intensity In the previous study (Schwartz 1974), the physical (= 100) by using the intensity of the intermediate line conditions in Burnham’s Nebula were studied by [N ii] A6584 for which an intensity could be obtained means of a diagnostic Ae, Te plot which relied largely on the present plates. The relative intensity of the upon the relative line strengths of different atomic and [Ca ii] A7291 line obtained in this way agrees to within ionic species.
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