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

An infrared study of the L1551 star formation region

G.J. White ^1,2,11, R. Liseau ¹, A.B. Men'shchikov ¹, K. Justtanont ¹, B. Nisini ⁶, M. Benedettini ³, E. Caux ¹², C. Ceccarelli ⁴, J.C. Correia ², T. Giannini ^6,3,10, M. Kaufman ⁵, D. Lorenzetti ⁶, S. Molinari ⁷, P. Saraceno ³, H.A. Smith ⁸, L. Spinoglio ³ and E. Tommasi <sup>9

1</sup> Stockholm Observatory, 133 36 Saltsjöbaden, Sweden
² University of London, Queen Mary & Westfield College, Department of Physics, Mile End Road, London E1 4NS, England, UK
³ Istituto di Fisica Spazio Interplanetario, CNR Area Ricerca Tor Vergata, Via Fosso del Cavaliere, 00133 Roma, Italy
⁴ Laboratoire d'Astrophysique de l'Observatoire de Grenoble, 414, Rue de la Piscine, Domaine Universitaire de Grenoble, B.P. 53, 38041 Grenoble Cedex 9, France
⁵ San Jose State University, Department of Physics, San Jose, CA 95192-0106, USA
⁶ Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio, Italy
⁷ California Institute of Technology, IPAC, MS 100-22, Pasadena, California, USA
⁸ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁹ Italian Space Agency, Via di Villa Patrizi 13, 00161 Roma, Italy
¹⁰ Universita La Sapienza, Istituto Astronomico, Via Lancisi 29, 00161 Roma, Italy
¹¹ Mullard Radio Astronomy Observatory, The Cavendish Laboratory, Cambridge CB3 0HE, England, UK
¹² CESR, B.P. 4346, 31028 Toulouse Cedex 4, France

Received 29 February 2000 / Accepted 14 August 2000

Abstract

Spectroscopic observations using the Infrared Space Observatory are reported towards the two well known infrared sources and young stellar objects L1551 IRS 5 and L1551 NE, and at a number of locations in the molecular outflow. The ISO spectrum contains several weak gas-phase lines of O I, C II, [Fe II] and [Si II], along with solid state absorption lines of CO, CO₂, H₂O, CH₄ and CH₃OH. Hubble Space Telescope (HST) images with the NICMOS infrared camera reveal a diffuse conical shaped nebulosity, due to scattered light from the central object, with a jet emanating from L1551 IRS 5. The continuum spectral energy distribution has been modelled using a 2D radiative transfer model, and fitted for a central source luminosity of 45 , surrounding a dense torus extending to a distance of 310⁴ AU, which has a total mass of 13 . The visual extinction along the outflow is estimated to be 10 and the mid-plane optical depth to L1551 IRS 5 to be 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 with a range of different aperture sizes. On the basis of the above model, the extinction curve shows that emission at wavelengths shorter than 2 µm is due to scattered light from close to L1551 IRS 5, while at wavelengths 4 µm, is seen through the full extinguishing column towards the central source. Several [Fe II] lines were detected 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 been examined where the [Fe II] gas is excited in hot ( 4000 K) and dense ( 10⁹ cm^-3) material located close to the root of the outflow. The SWS observations did not detect any emission from rotationally excited H₂. Observations with United Kingdom Infrared Telescope (UKIRT) of the vibrationally excited S- and Q-branch lines were however consistent with the gas having an excitation temperature of 2500 K. There was no evidence of lower temperature ( 500 K) H₂ gas which might be visible in the rotational lines. Observations with UKIRT of the CO absorption bands close to 2.4 µm are best fit with gas temperatures 2500 K, and a column density 610²⁰ cm^-2. There 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 fitting all the data which can explain this.

Key words: ISM: dust, extinction ISM: individual objects: L1551 ISM: jets and outflows stars: pre-main sequence infrared: ISM: lines and bands infrared: stars

Send offprint requests to: g.j.white@ukc.ac.uk

SIMBAD Objects

Contents

• 1. Introduction
• 2. The observations
• 3. Infrared continuum
  • 3.1. ISO observations
    • 3.1.1. SWS and LWS continuum
    • 3.1.2. Solid state features
    • 3.1.3. Spectral shape
  • 3.2. HST NICMOS images
  • 3.3. Two-dimensional radiative transfer model
    • 3.3.1. Geometry
    • 3.3.2. Grain properties
    • 3.3.3. Parameter space
    • 3.3.4. Spectral energy distribution
    • 3.3.5. Densities and temperatures
    • 3.3.6. Submillimetre and millimetre visibilities
    • 3.3.7. Far-IR and millimetre intensity profiles
    • 3.3.8. HST image at 2.12 µm
    • 3.3.9. Average density estimates
• 4. Emission lines
  • 4.1. [Fe II]
    • 4.1.1. Observed transitions
    • 4.1.2. [Fe II] excitation and radiative transfer
    • 4.1.3. [Fe II] model calculations and results
  • 4.2. Si II
  • 4.3. OH
  • 4.4. H₂O
  • 4.5. H₂
  • 4.6. CO vibrational bands
  • 4.7. CO rotational lines
  • 4.8. O I
  • 4.9. HH 29
  • 4.10. L1551 NE
  • 4.11. Other locations along the outflow
• 5. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
helpdesk.link@springer.de