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Astron. Astrophys. 325, 685-692 (1997)

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Appendix A

A.1. Observed emission excess around 1.3 mm

As mentioned in Table 1, S22 and VLA1 show a 1.3 mm excess compared to the obtained fit (see Fig. 1). Extended emission is excluded as the 1.3 mm beam size is not significantly larger than the others. Possible calibration error has been searched for and excluded. Some other sources in our sample are also studied by Reipurth et al. (1993) and measurements are in good agreement, within the error bars. Radio continuum emission has been measured for 67 objects of the L1641 molecular cloud (Morgan et al. 1990). Fluxes at 6 and 20 cm were used to evaluate the expected free-free emission at 1.3 mm. In two cases, this contribution is negligible (less than 0.2% of the 1.3 mm flux; see also Walker et al. 1990). Therefore, free-free emission cannot be responsible for this excess. New measurements are needed to confirm the observed excesses.

A.2. Discussion of individual sources

Some sources in our sample have a complicated morphology not resolved by IRAS (see Pravdo & Chester 1987) and this can affect the determination of the parameters. To estimate the fit reliability, we have investigated evidence for IRAS confusion.

  • S06 - L1641-N
    In the near IR, this source is a cluster of objects but one is dominant (see Chen et al. 1993a) and coincides with the the 2.7 mm peak (Wilking et al. 1990).
    The parameters obtained for this source do not agree with those obtained for the rest of the sample. The fit tends to pass through the upper limits of the submillimetre measurements (see Fig. 1). This is also revealed by the difference in [FORMULA] values between S59 and S06, two sources which have similar ([FORMULA] / [FORMULA]) values (see Fig. 2) and which should be in a similar evolutionary stage, if one considers that their initial envelope masses are not too different. S06 is also the only source associated with a relatively low dust temperature (see Table 2) considering its 100 µm flux. Finally, the fit obtained using its associated IRAS-PSC fluxes (lower than those determined by SNS) gives T =34.7 K, [FORMULA] =1.25 and M =2.1  [FORMULA], similar to those obtained for S59. Moreover, if we fix the same temperature for S06 and S59 (see also Reipurth et al. 1993), an agreement is found between the two sets of obtained parameters. Finally, we found no evolutionary track (with an accretion rate of 10-5   [FORMULA] /year) that reproduces the observed quantities for S06 (see Fig. 3).
    We think that extension and clustering in this source lead to an overestimate of the fluxes. For consistency, we have kept the parameters obtained with the least-squares fit, but point out possible problems.
  • S11 - MSSB-8
    The multiple components of this source were not resolved by IRAS (Chen & Tokunaga 1994; SNS). However, we consider the fit to be reliable (see Sect. 3.2).
  • S22 - MSSB-18
    This source exhibits no clear near IR counterpart (see Fig. 2 in Chen & Tokunaga 1994). The source is seen at longer wavelengths to the south of the IRAS position (SNS), in agreement with the observed displacement of the submillimetre peak. We consider the fit to be reliable. Due to the observed excess, we excluded the measured 1.3 mm flux from the fit procedure and took the one which agrees with the fit for all the quantities linked to it. The great similarity of the measured submillimetre continuum fluxes of this source with those of S32 shows the reliability to this procedure.
  • S31
    This source, together with S32 and VLA1, is found in a confused region (Pravdo & Chester 1987) which renders the association between IRAS and submillimetre fluxes highly doubtful. Therefore, we excluded this source from the analysis.
    An upper limit for the bolometric luminosity obtained using available IRAS-PSC fluxes is given in Table 1. As tested by our fit procedure, these fluxes are also too high to be associated with the submillimetre measurements. However, the use of an upper limit for the bolometric luminosity allow us to discuss some properties of this source.
    The submillimetre spectral slope of S31 is very similar to that of other sources of our sample and, apart from an overestimate of its IR fluxes, we think that its nature as a Class I source is not in question.
  • S32 - MSSB-21
    At 100 µm, this source is the dominant component of a blend (SNS), so the flux may be overestimated. As for the previous source, the fluxes obtained by Chen et al. (1993b) lead to unrealistic [FORMULA] values. We consider the fit to be reliable (see Fig. 1).
  • S55 - L1641-S
    This source is associated with a strong nebulosity in the near IR (see Chen & Tokunaga 1994) and lies on a broad pedestal in the four IRAS bands (SNS), giving the unusual shape of its IR spectrum for a Class I source. Cohen (1990) determined IR fluxes and we run our fit procedure, with consistent results (high values for T and [FORMULA]). Moreover, fixing the same dust temperature (in the range 25 - 35 K) for all the sources (see also Reipurth et al. 1993), we performed a least-squares fit using only the submillimetre points and found that S55 remains the most evolved source of our sample. Therefore we consider the fit to be reliable. Our 1.3 mm measurement agrees with that of Reipurth et al. (1993).
  • S59 - L1641-S3
    This strong submillimetre source is relatively isolated and has well determined IR fluxes. We consider the fit to be reliable.
  • S72
    Even though there are large errors in the parameters values due to large errors in the 100 µm flux, we consider the fit to be reliable.
  • S85 - L1641-S4
    This source lies in a confused region. The 60 and 100 µm fluxes measured by Chen et al. (1993b) are associated with 100% uncertainty. Fluxes determined by SNS and the PSC agree. We consider the fit to be reliable.
  • VLA1
    This strong millimetre source is located near S31 and S32 and has no near IR counterpart (see Chen & Tokunaga 1994). Due to confusion, no IRAS fluxes have been determined. We took the 50 and 100 µm fluxes from Harvey et al. (1986). Our 1.3 mm measurement agrees with that of Reipurth et al. (1993), within the errors. Their 870 µm and 1.3 mm flux measurements are shown in Fig. 1.
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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998