An infrared study of the L1551 star formation region

Spectroscopic observations using the Infrared Space Observatory are reporte d towards the two well known infrared sources and young stellar objects L15 51 IRS 5 and L1551 NE, and at a number of locations in the molecular outflo w. The ISO spectrum contains several weak gas-phase lines of OI, CII, [Fe I I] and [Si II], along with solid state absorption lines of CO, CO2, H2O, CH 4 and CH3OH. Hubble Space Telescope (HST) images with the NICMOS infrared c amera reveal a diffuse conical shaped nebulosity, due to scattered light fr om the central object, with a jet emanating from L1551 IRS 5. The continuum spectral energy distribution has been modelled using a 2D radiative transf er model, and fitted for a central source luminosity of 45 L., surrounding a dense torus extending to a distance of similar to 3 x 10(4) AU, which has a total mass of similar to 13 M.. The visual extinction along the outflow is estimated to be approximate to 10 and the mid-plane optical depth to L15 51 IRS 5 to be approximate to 120. This model provides a good fit to the ISO spectral data, as well as to the spatial structures visible on archival HST/NICMOS data, mid-IR maps and sub -millimetre radio interferometry, and to ground-based photometry obtained w ith a range of different aperture sizes. On the basis of the above model, t he extinction curve shows that emission at wavelengths shorter than similar to 2 mum is due to scattered light from close to L1551 IRS 5, while at wav elengths greater than or similar to 4 mum, is seen through the full extingu ishing column towards the central source. Several [FeII] lines were detecte d in the SWS spectrum towards L1551 IRS 5. Although it would seem at first sight that shocks would be the most likely source of excitation for the [Fe II] in a known shocked region such as this, the line intensities do not fit the predictions of simple shock models. An alternative explanation has bee n examined where the [Fe II] gas is excited in hot (similar to 4000 K) and dense (greater than or similar to 10(9) cm(-3)) material located close to t he root of the outflow. The SWS observations did not detect any emission fr om rotationally excited Hz. Observations with United Kingdom Infrared Teles cope (UKIRT) of the vibrationally excited S- and Q-branch lines were howeve r consistent with the gas having an excitation temperature of similar to 25 00 K. There was no evidence of lower temperature (similar to 500 K) H-2 gas which might be Visible in the rotational lines. Observations with UKIRT of the CO absorption bands close to 2.4 mum are best lit with gas temperature s similar to 2500 K, and a column density similar to 6 x 10(20) cm(-2) Ther e is strong circumstantial evidence for the presence of dense (coronal and higher densities) and hot gas (at least 2500 K up to perhaps 5000 K) close to the protostar. However there is no obvious physical interpretation fitti ng all the data which can explain this.