L1448

   
7 Interpretation: L1448

Modelling in L1448 is less conclusive since only a few rotational transitions of H₂ per position were detected. We supplement the data with the vibrational excitation as measured in the K-band, and indicated by the ratio R=I(1-0S(1))/I(2-1S(1)) for the northern regions by Davis & Smith (1995). The ratio is constant over the flow from L1448-mm, with $R=10.4\pm2.3$, and constant over a counterflow which appears to stem from L1448-IRS3 with $R=\,\sim$20.

7.1 L1448N2

Figure 15: Models CBOW (full) and JBOW (dotted) plotted against the L1448N2 data. (1) a C-type bow with speed 100kms-1 and pre-shock density of 106cm-3, fully molecular, ion fraction $2\times 10^{-7}$ and bow geometry s=1.4, and (2) a J-type bow with pre-shock density of 105cm-3, fully molecular fraction upstream, speed 80kms-1 and bow shape s=1.4, are displayed.

The H₂ and CO in the L1448 N2 location can be interpreted by both JBOW and CBOW models with long-flanked shapes (Fig.15). The predicted CO excitation and CO flux levels are consistent with the observed data.

An upper limit on the extinction can be set from the H₂ 0-0 S(3) line, on assuming the flux is reduced by extrinsic silicate absorption but cannot lie above a linear fit to the rest of the data on the CDR diagram. This yields a very low 2 $\mu$m extinction of less than 0.4 magnitudes and, indeed, bow shock fits consistent with no extinction.

The H₂ ratio R=I(1-0S(1))/I(2-1S(1)) observed by Davis & Smith (1995) for the N2 region is 13.5 (taking the flux-weighted average of the knots). This is consistent with the model value of 11.5 (CBOW) but compares less well with 17.6 (JBOW). A low density bow with pre-shock density of 10⁵cm^-3 also fits the H₂ and CO diagrams with bow shape s = 1.2, but the predicted ratio R = 36.4 is then too large.

Extraordinary low levels of [OI]63$\mu$m emission are found in L1448. This is inconsistent with the cool H₂ spectra. The J-bow model, as for CepA, predicts at least an order of magnitude too much [OI]63$\mu$m emission, generated in the long cool bow tails. The C-bow model yields a [OI](63$\mu$m)/CO(J=17-16) flux ratio of 73, even with the low oxygen abundance of 10^-4, still a factor of 17 larger than observed.

One solution would be that the pre-shock chemistry in our model is incorrect: the oxygen is, instead, tied up in H₂O or other species. This may be feasible if the pre-shock gas is warm and the atomic hydrogen fraction is very low, both factors which reduce the atomic oxygen abundance when the oxygen chemistry is in equilibrium. This argument is supported by the global structure of the outflow as a parsec-scale molecular flow (Eislöffel 2000), in which the gas entering shock N2 has been preprocessed through previous shocks converting the atomic oxygen into H₂O.

A more likely solution is however that the oxygen fine-structure lines are optically thick. The modelled [OI]63$\mu$m to [OI] 145$\mu$m flux ratio is 112, an order of magnitude larger than observed, evidence that the 63 $\mu$m line is optically thick. Hollenbach & McKee (1989) state a column density of $4.9\times 10^{20}$cm^-2 would provide an optical depth of unity at the line centre for this transition. We find that the column of cool gas in the wings of the C-type bows is typically $2{-}4\times 10^{21}$cm^-2.

The predicted surface filling factor of the H₂ emission is low. This problem arises from the low observed fluxes in comparison to CepheusA. For the L1448N2 location, the 0-0S(5) surface brightness in the SWS aperture is 10^-6Wm^-2. The CBOW model predicts a bow-average surface brightness of $5.8\times 10^{-5}$Wm^-2, which implies an aperture filling factor of 0.02. This is rather low although the 1-0S(1) emission certainly does not fill the SWS aperture. A solution investigated here is that the hydrogen in the outflow is predominantly atomic. Then, the powerful shocks would generate less H₂ emission. We also find that a lower density is then permissible in the excitation modelling. We thus arrive at a model with just ten per cent of the atoms tied up in molecular hydrogen, and a pre-shock density of $3\times 10^5$cm^-3. The 0-0S(5) SWS filling factor is then 0.4 and the CO boost factor is 18. The atoms would reform on grains in a time of order $3\times 10^{16}$/n = 3000yr, assuming standard grain properties. This model, however, predicts high intrinsic oxygen 63 $\mu$m emission since the high atomic fraction would ensure that H₂O and OH are dissociated by collisions with H, producing abundant atomic oxygen.

There is quite uniform [CII] emission across L1448. This is not expected to arise directly from the outflows but is consistent with excitation by the local radiation field (see Nisini et al. 1999 for further discussion).

7.2 L1448N1

Figure 16: The L1448N1 data with the CBOW model displayed for N2 but with s = 1.35.

The H₂ rotational excitation is even lower in the N1 location. We find a CBOW model with s = 1.35 and pre-shock density 10⁶cm^-3 fits the data (Fig.16). This predicts R = 13.4 which is somewhat high. We also find that a higher density reproduces the observed 1-0/2-1 ratio but that this density then overestimates the CO flux level. A lower density has the opposite result. The [OI](63$\mu$m)/CO (J=17-16) flux ratio is 140 for O and CO abundances both of $2\times 10^{-4}$, again requiring the oxygen to be tied up in molecules or dust or that the line is optically thick. The modelled [OI]63$\mu$m to [OI] 145$\mu$m flux ratio is 112, an order of magnitude larger than observed. This suggests that the 63$\mu$m line is optically thick.

7.3 L1448C

Figure 17: Model CBOW plotted against the L1448C data. The C-type bow applied to N2 is taken here, with a lower CO boost factor providing a consistent fit.

The central region cannot be reliably analysed since the H₂ data we have are unsatisfactory. Nisini et al. (1999), however, have published upper limits for a few other lines which we include in our modelling. This clearly corroborates that an excitation of the H₂ rotational spectra in this region is very similar to N2 and N1 (Fig.17).

The water lines are difficult to use as shock diagnostics (e.g. Nisini et al. 1999). We have used the Tables and Figures provided by Kaufman & Neufeld (1996) to predict fluxes from the CBOW models in some detail. We find that the observed ratio of 4₁₄-3₀₃/0-0S(2) is consistent although the data are inadequate for a detailed test.

7.4 L1448 South

We do not have comprehensive data sets for the areas south of the source. For the S1 location, we take the same models as for the central location, and check their consistency with the available data. This then predicts the 0-0S(5) flux of $8\times 10^{-16}$Wm^-2. We take the analysis of S1 no further here.

Heading south, in Fig.18 we show that shock configurations are again necessary to explain the H₂ data for location S2 (the S3 region is only slightly warmer). We find: (1) the H₂ excitation appears very similar to the Northern locations but (2) the CO emission is considerably less. The obvious implication of lower CO fluxes is that the density is lower. However, the H₂ line strengths are comparable to those in the North of the outflow. Moreover, a lower density would alter the predicted shock ensemble: we find that a C-type bow with s = 1.2 (dot-dashed lines) would be necessary with n = 10⁵ cm^-3. This would make the fact that the excitation appears similar as in the North as pure coincidence. An alternative answer would be that extinction has decreased the apparent excitation. We find, however, that the 0-0 S(3) flux is not sufficiently depressed to allow for much extinction.

The answer we favour is that the CO abundance is low, with CO frozen out onto grains. Then, all other parameters employed for the N2 CBOW model remain unchanged, as shown by the solid-line fit displayed in Fig.18.

L1448