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

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2. The observations

In this study, the Infrared Space Observatory (ISO) spectrometers (ISO - Long wavelength Spectrometer - LWS - Clegg et al. 1996, and Short Wavelength Spectrometer - SWS - de Graauw et al. 1996) were used to obtain spectra towards IRS 5 and at 10 positions on the outflow, as summarised in Table 1. Some of these pointing positions lay along the centre of the outflow axis, and some at the edge of the outflow cavity (see Fig. 1).

[FIGURE] Fig. 1. Palomar POSS II Digital Sky Survey picture of the L1551 region, with the position and names of the LWS pointing positions indicated, along with the beamsize, which was [FORMULA] 80" (Clegg et al. 1996; Swinyard et al. 1996).


Table 1. ISO observation log. The fluxes of the O I and C II lines are in Wcm-2. All errors and upper limits are 1 [FORMULA]

An ISO SWS spectrum was obtained towards IRS 5 using the S01 mode (2.4-45 µm, scan speed 4, resolution [FORMULA] 1000-2000, integration time 6590 seconds). The SWS aperture varied from 14" [FORMULA]20" to 17" [FORMULA]40" for the 2-40 µm regions respectively (de Graauw et al. 1996). The SWS and LWS data were reduced using the standard ISO analysis software ISAP v1.6 and LIA 6. Small corrections were made for fringing, and to align adjacent detector scans, but overall the standard pipeline data was of high quality. The SWS observations were made with the long dimension of the slit oriented at position angle 171o (measured anticlockwise from north) - this lies almost orthogonal to the direction of the outflow jet (see Fig. 2) and limb-brightened outflow lobes (Rainey et al. 1987).

[FIGURE] Fig. 2. HST image of the central part of the reflection nebula close to L1551 IRS 5 centred at 8900 Å. The SWS apertures at the shortest and longest wavelengths are shown as boxes centred at the co-ordinates listed in Table 1. The two knots visible to the right of the larger SWS aperture are features known from previous optical studies to be associated with working surface or shock activity (Fridlund & Liseau 1988).

Images were taken from the HST archive to examine the near-infrared distribution closer to IRS 5. These data, using the HST/NICMOS camera were cleaned of residual cosmic rays which remained after the standard pipeline processing and mosaicked together. Narrow band images of the [FORMULA]2.12 H2, [FORMULA]1.87 Paschen lines were examined, as well as broad filters which included the [FORMULA]2.12 H2 + CO band-head region, the [FORMULA]1.64 [Fe II] line, and a 1.12 µm continuum filter (in this paper we will adopt a naming convention for lines of the form [FORMULA]2.12 H[FORMULA] to mean the H2 line at 2.12 µm).

Near-infrared long-slit grating spectroscopy was available from the UKIRT CGS4 archive. The CGS4 camera contained a 256 [FORMULA] 256 InSb array, and was operated as a long-slit spectrograph covering the 1-5 µm spectral region. A 75 l/mm grating provided a resolving powers of [FORMULA] 500 along an 80" slit, which was 1[FORMULA] @ wide. Spectra covering the interval 1.86 to 2.52 µm were reduced using the standard STARLINK package CGS4DR, and calibrated against the standard stars BS 1001 and BS 1665.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001