... envelopes

Appendices A and B are only available in electronic form at http://www.edpsciences.org

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... telescope

IRAM is an international venture supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).

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... JCMT

The JCMT is operated by the Joint Astronomy Center in Hilo, Hawaii on behalf of the present organizations: The Particle Physics and Astronomy Research Council in the UK, the National Research Council of Canada and the Netherlands Organization for Scientific Research.

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... 1999

IRAM November 1999 data have also been presented in Loinard et al. (2002a).

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... using

JCMT does not follow the same convention for antenna temperature than IRAM, the JCMT antenna temperature $T_{\rm A}^{*}$ being already corrected for the forward efficiency of the antenna.

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... code

Details on the used LVG code can be found in Ceccarelli et al. (2002).

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... assumed

Since the value of 2 obtained with the rotational diagrams is highly uncertain, we prefer to adopt the canonical value of 3 for the standard model. We will show in Sect. 5.3.3 that the results are not substantially affected by this value.

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... MEPEW

http://www-laog.obs.ujf-grenoble.fr/~ceccarel/mepew/mepew.html

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... densities

L1527 is an exception, with a density of $3\times 10^6$ cm^-3, but, as commented by Jørgensen et al. (2002) this may be due to the contribution of the disk, that may dominate the continuum emission in the inner parts of this source (Loinard et al. 2002b).

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... DUSTY

DUSTY is publicly available at http://www.pa.uky.edu/~moshe/dusty/

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