2 Observations and data reduction

In this Section the observational constraints used in the radiative transfer modelling of the circumstellar envelope around IRAS 16293-2422 are presented. The SED and submillimetre continuum brightness maps constrain the physical structure of the envelope. Millimetre molecular line observations provide further information on the physical structure, in particular the large scale velocity field, and allow for studies of the chemistry in the envelope.

 

 

Table 1: The spectral energy distribution of IRAS 16293-2422.

$\lambda$	${F_{\rm obs}} ^{\rm a}$	${\Delta F_{\rm obs}} ^{\rm b}$	${\theta_{{\rm mb}}} ^{\rm c}$		${F_{\rm mod}} ^{\rm d}$
$[\mu{\rm m}]$	$[{\rm Jy}]$	$[{\rm Jy}]$	$[\arcsec]$	${\rm Ref.}$	$[{\rm Jy}]$
2900	0.60	0.13	60.0	3	0.58
1300	6.97	2.24	30.0	3	7.5
850	30.8	6.2	15.0	1	31.3
450	220.4	44.5	8.8	1	220.3
100	1032.0	226.0	237.0	2	852.4
60	254.9	59.5	160.0	2	162.4

$^{\rm a}$ Observed flux integrated over emitting region.
$^{\rm b}$ A calibration uncertainty of 20% has been added.
$^{\rm c}$ Size of the main beam.
$^{\rm d}$ Flux predicted by best fit model using a density structure described by
    a single power-law and OH5 dust opacities. Note that the 60 $\mu$m and 2.9 mm
    fluxes are not included in the modelling (see text for details).

Refs. - (1) This paper; (2) IRAS Point Source Catalogue; (3) Walker et al. (1990).

2.1 Spectral energy distribution

The SED of IRAS 16293-2422, as presented in Table 1 is used to constrain the total amount of material present in the circumstellar envelope. Care has been taken that only total fluxes integrated over the whole emitting region for wavelengths $\leq$1.3 millimetre (mm) are used in the SED modelling to ensure that emission from the envelope itself is dominating the observed flux. The contribution from the disk(s) to the observed emission starts to become important at wavelengths longer than $\sim$1 mm. However, any disk emission would be significantly diluted in the relatively large beams used here (e.g., Motte & André 2001). That thermal emission from an extended dusty envelope is responsible for the majority of the observed millimetre flux is confirmed by the radiative transfer analysis performed in Sect. 4. The model fluxes predicted by the best fit envelope model are compared with observations in Table 1. Additionally, the observed IRAS flux at 60 $\mu$m is not used in the analysis since the emission at that wavelength emanates from the inner hot parts of the envelopes where the dust grain properties are probably significantly different from those in the cooler outer parts. Dust grains in the outer parts will be coated by a layer of ice which, as the temperature increases towards the star, starts to evaporate thereby changing the optical properties of the dust grains. In Sect. 4, the effect of varying the dust opacities will be investigated. A relative calibration uncertainty of 20% is added to all flux measurements, in addition to any statistical errors, which dominates the error budget and gives all points on the SED more or less equal weights.

Figure 1: SCUBA images at 450 and 850 $\mu$m of IRAS 16293-2422. The contours start at the 3$\sigma$ level (0.9 and 0.24 Jy beam-1 for 450 and 850 $\mu$m, respectively) and increase by multiples of 2.

2.2 Submillimetre continuum observations

Submillimetre continuum observations were retrieved from the James Clerk Maxwell Telescope (JCMT) public archive. The data were obtained during an observational run in April 1998, using the Submillimetre Common-User Bolometer Array (SCUBA), and consist of pairs of images at 450 $\mu$m and 850 $\mu$m. The dual SCUBA array contains 91 pixels in the short-wavelength array and 37 pixels in the long-wavelength array, each covering a hexagonal 2$\farcm$3 field. The SCUBA bolometer array camera is described in some detail in Holland et al. (1999).

The imaging was made using the jiggle-mapping mode to produce fully sampled maps. In this mode the SCUBA bolometers instantaneously under-sample the sky and a 64 point jiggle pattern is carried out by the telescope to fully sample both the long and short wavelength arrays. In practice, the 64 point pattern is broken down into four 16-point sub-patterns that spend 1 s integrating on the source at each gridpoint. After a sub-pattern has been completed the telescope is nodded and the pattern is repeated again so that sky subtraction can be made. In all, it takes 128 s to complete one full on/off source jiggle map. The jiggle mapping mode is the preferred observational mode for SCUBA when imaging sources smaller than the chop throw. Usually, a chop throw of 2$\arcmin$ is used to ensure chopping off the arrays. A larger chop throw would result in poor sky subtraction and loss in image quality.

The data were reduced in a standard way, as described in Sandell (1997), using the SCUBA reduction package SURF (Jenness & Lightfoot 1997). The images were calibrated using simultaneous observations of Uranus retrieved from the JCMT archive. The sky opacities at 450 $\mu$m and 850 $\mu$m were estimated using the 1.3 mm opacity, monitored by the Caltech Submillimeter Observatory and listed for each individual observation, using the relations in Archibald et al. (2000). The validity of these relations have been checked and confirmed using SCUBA sky dips. The total calibration uncertainty is estimated to be approximately $\pm$20% at 450 $\mu$m and about $\pm$10% at 850 $\mu$m. Care was taken to select data taken during good to excellent submillimetre conditions. The beam is determined from the Uranus observations and is dominated by an approximately Gaussian main beam with a deconvolved FWHM of $14\farcs 2 \times 16 \farcs0$ and 8$\farcs$5$\times$9$\farcs$1 at 850 $\mu$m and 450 $\mu$m, respectively. A substantial error beam is, however, present at these wavelengths (see below) picking up significant amounts of flux. The error lobe pick up is estimated to be approximately 15% and 45% at 850 $\mu$m and 450 $\mu$m, respectively, and is taken explicitly into account in the analysis.

The final 450 $\mu$m and 850 $\mu$m images are presented in Fig. 1. All offsets reported are relative to the adopted position of the protobinary star IRAS 16293-2422 ( $\alpha_{2000}=16^{\rm h}32^{\rm m}22\fs 91$, $\delta_{2000}=- 24\degr 28\arcmin 35\farcs 6$). The locations of the two protostars IRAS 16293A (MM1) and IRAS 16293B (MM2) relative to this position are (-3$\arcsec$, -1$\arcsec$) and (-5$\arcsec$, +3$\arcsec$), respectively. The emission appears to have an overall spherical symmetry and is centered on the adopted central position within the pointing accuracy of the telescope. For comparison, the pointing accuracy of the JCMT is estimated to be about $\pm$1.5$\arcsec$ in both elevation and azimuth. Also visible is a second, weak, component apparent near the eastern edge of the SCUBA maps. This second component, or IRAS 16293E (+77$\arcsec$, -22$\arcsec$), which was first identified by its strong ammonia emission, is most probably also a class 0 protostar (Mizuno et al. 1990; Castets et al. 2001).

 

 

Table 2: Observational results from SCUBA images.

$\lambda$	${\tau} ^{\rm a}$	${F_{\rm tot}} ^{\rm b}$	${F_{\rm peak}} ^{\rm c}$	${\rm rms}$	${\theta_{{\rm mb}}} ^{\rm d}$
$[\mu{\rm m}]$		$[{\rm Jy}]$	$[{\rm Jy~beam}^{-1}]$	$[{\rm Jy~beam}^{-1}]$	$[\arcsec]$
450	0.5-0.7	220.4	72.0	0.3	8.8
850	0.12-0.15	30.8	16.8	0.08	15.0

$^{\rm a}$ Zenith opacity of the atmosphere.
$^{\rm b}$ Total flux integrated over emitting region. The calibration uncertainty is estimated to be $\sim$20%.
$^{\rm c}$ Flux at stellar position. The calibration uncertainty is estimated to be $\sim$20%.
$^{\rm d}$ Geometrical mean of the beam size.

  Table 3: Additional molecular line observations of IRAS 16293-2422 using the JCMT.

$^{\rm a}$ Total integrated intensity calculated over full extent of line. The calibration uncertainty in the intensity scale is estimated to be $\sim$15-20%.
$^{\rm b}$ Estimated from a Gaussian fit to the observed spectrum. A colon (:) indicates an uncertain value due to low signal-to-noise or, in the majority of cases,
    a significant departure from a Gaussian line profile.
Refs. - (1) JCMT public archive; (2) This paper.

The FWHM of the emission centered on the stellar position is 20$\farcs$8$\times$19$\farcs$4 and 21$\farcs$9$\times$19$\farcs$3 at 850 $\mu$m and 450 $\mu$m, respectively. The deconvolved envelope sizes assuming both the beam and brightness distribution to be described by Gaussian functions are $\sim$14$\arcsec$ at 850 $\mu$m and $\sim$19$\arcsec$ at 450 $\mu$m. Thus, only the 450 $\mu$m emission appears to be resolved. The observational results are summarized in Table 2. To compare the observed brightness distributions with the predictions from a spherically symmetric model, the SCUBA maps were azimuthally averaged in bins with half the corresponding beam size in width. Moreover, care was taken to block out any contribution from IRAS 16293E. The resulting radial brightness distributions are shown in Fig. 3.

2.3 Millimetre molecular line observations

A survey of the millimetre molecular line emission towards IRAS 16293-2422 was presented in Blake et al. (1994) and van Dishoeck et al. (1995). This large data set forms the base for the molecular excitation analysis performed in this paper. The absolute calibration uncertainty of the intensities is estimated to be $\sim$30%. In addition, we have searched the JCMT public archive for complementary millimetre line observations. This additional set of data, taken at face value, is presented in Table 3. Lines for which multi-epoch observations are available in the JCMT archive typically display intensities that are consistent to $\sim$20%. This was also the conclusion reached by Schöier & Olofsson (2001) for a large survey of carbon stars. When newer data were available for a particular transition, they were usually adopted, due to a higher signal-to-noise and/or greater spectral resolution. In one case [H¹³CO⁺ ( $J = 3\rightarrow 2$)] the old data set was found to have a significantly lower line intensity, possibly due to pointing problems. Additional H₂CO data published recently by Loinard et al. (2000) were further used in order to increase the number of observed transitions for this molecule.

The detected molecular line emission probes the full radial range of the envelope, providing additional constraints on the physical structure of IRAS 16293-2422. Only information on the lowest transitions of the molecules, which occur at millimetre wavelengths and probe the very coldest outer parts, is lacking. The observed line shapes provide valuable information on the velocity structure in the envelope. For example, the single-dish observations of abundant molecules like CO and CS typically show lines with strong self-absorption and some degree of asymmetry. The variation of the line profiles among the CO and CS isotopomers is potentially a sensitive probe of infall models and will be further investigated in Sect. 4.3.