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Astron. Astrophys. 364, 741-762 (2000)

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5. Conclusions

Observations have been made towards the two well known infrared sources L1551 IRS 5 and L1551 NE, and at a number of locations in the molecular outflow, using the LWS and SWS spectrometers on the ISO satellite, and several other near-IR telescopes. The present work possibly adds to the complexity by unveiling a plethora of unexpected phenomena, such as e.g. the occurence of significant extinction at mid and far infrared wavelengths and the existence of dense and hot regions (thermally stable?) over extended scales. The main results of this study are:

  1. The ISO LWS spectrum consists of a relatively strong continuum, superposed with a few weak lines of O I, C II and possibly OH. Emission from other species such as CO or H2O was not detected. This might indicate that either the molecules have been destroyed, perhaps in a shock, or that the environment is unable to excite them to emit in the far and near infrared.

  2. The ISO SWS spectrum of L1551 IRS 5 contains solid state absorption lines of CO, CO2, H2O, CH4 and CH3OH, which correspond to column densities of [FORMULA] 7.7[FORMULA]1018 cm-2, 5.4[FORMULA]1017 cm-2, 7.6[FORMULA]1016 cm-2 and 2.6[FORMULA]1018 cm-2 respectively.

  3. Examination of archival HST NICMOS images reveals a diffuse conical shaped nebulosity, due to scattered light from the central object, with a jet emanating from L1551 IRS 5. It is likely that the emission in this jet-like feature is dominated by [Fe II] lines.

  4. The continuum spectral energy distribution has been modelled using a 2D radiative transfer model. The continuum is well fitted for a central source luminosity of 45 [FORMULA], surrounded by a flared disc with an opening angle of 90o. The outer parts of the torus extend to a distance of [FORMULA] 3[FORMULA]104 AU, and has a total (gas + dust) mass of [FORMULA] 13 [FORMULA]. The extinction towards the outflow is estimated to be [FORMULA] 10 and the mid-plane optical depth to L1551 IRS 5 to be [FORMULA] 120. This model provides a good fit to the ISO data, as well as the available HST/NICMOS data, mid-IR maps, submm interferometry, and ground-based photometry with a range of different aperture sizes.

  5. On the basis of the above model, an extinction curve has been estimated, which shows that the emission at wavelengths shorter than [FORMULA] 2 µm is due to scattered light from close to L1551 IRS 5, while at wavelengths [FORMULA] 4 µm, is seen through the full extinguishing column towards the central source. This need to be taken careful account of when comparing line intensities at different wavelengths.

  6. Three [Fe II] lines were detected in the SWS spectrum towards L1551 IRS 5, with a fourth line at [FORMULA] 1.5 [FORMULA], and upper limits on several others. Although it would seem at first sight that shocks would be the most likely source of excitation in a known shocked region such as this, the line intensities do not fit the predictions of simple shock models. The problem with such shock interpretation of the [Fe II] lines is that the line ratios and strengths imply densities and temperatures which are outside the range considered by Hollenbach & McKee. However, as explained in 4.1.1 4.1.3, the overall energy budget and observed densities place stringent limits on possible shock models, leading us to explore other models. An alternative explanation has been examined where the [Fe II] gas is hot ([FORMULA] 4000 K) and dense ([FORMULA] cm-3). Although this provides an acceptable fit to the relative line intensities, it provides no constraints as to the precise heating mechanism of the gas - although it seems likely that it would have to occur very close to the root of the outflow. The lack of detection on Br [FORMULA] [FORMULA] 4.052 or Br [FORMULA] [FORMULA] 2.626, and the known low surface temperature ([FORMULA] 5500 K) of the central protostellar object argue against efficient excitation in a high UV field environment, making shocks the most likely way to explain the [Fe II] and [Si II] emission intensities.

  7. The SWS observations did not detect any emission from rotationally excited H2. Observations with UKIRT of the vibrationally excited S and Q-branch lines were consistent with the gas having an excitation temperature of [FORMULA] 2500 K. Given the likely opacity to the central source which was predicted in our modelling, it is unlikely that we would have detected emission due to this hot gas component. Similarly, there was no evidence of lower temperature ([FORMULA] 500 K) gas, as has been inferred towards many other sources.

  8. Observations with UKIRT of the CO absorption bands close to 2.4 µm are best fit with gas temperatures [FORMULA] 2500 K, and a column density [FORMULA] 6[FORMULA]1020 cm-2.

  9. Evidence for dense (coronal and higher densities) and hot (at least 2500 K up to perhaps 5000 K) gas is manifest in a multitude of observables; a) the overall SED, b) the CO bands, c) for a `normal' [CO]/[H2] = 8 10-5, the implied H2 colum density N(H2) = 6 1020/8 10-5 = 8 1024 [FORMULA] 1025 cm-2 for gas at T [FORMULA] 2500 K, d) the H2 emission (presence of rovibrational emission and absence of pure rotational lines) e) the [Fe II] spectrum, and f) that the SiII 35 micron is also consistent with this gas.

    Although each of these individual pieces of evidence may have several interpretations, we argue that the combination of them all makes it highly likely that such a hot and dense gas phase is present. The problem as how to produce and to maintain this region (which has not previously been presented in the literature elsewhere) is beyond the scope of the present paper. Our simple modelling is based on steady state assumptions about processes which are most probably highly dynamic in nature. Any attempt to provide an explanation(s) would at the moment be based on pure speculation, and so we instead indicate the need for extensive theoretical work, which is beyond the scope of the present observational paper.

  10. Observations at a number of other locations along the molecular outflow, and towards the Herbig-Haro object HH29, revealed no emission apart from weak C II and O I lines.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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