Formaldehyde emission from low mass protostars
observed with the JCMT

Formaldehyde is, after water and carbon monoxyde, one of the main component of ices in grains mantles. Recently, Ceccarelli et al. (2000a,b), Maret et al. (2002) and Schöier et al (2002) have shown that in the inner parts of protostellar envelopes, grains mantles evaporate, releasing the ices components into the gas phase, and, among them, formaldehyde. Observations of formaldehyde transitions can be therefore used to determine the physical and chemical conditions -- namely density, temperature and chemical abundances -- in the inner part of protostellar envelopes (Ceccarelli et al. 2003, Maret et al. 2003).

The most accepted scenarios predict that formaldehyde is formed on grain surfaces, by successive hydrogenation of CO. The measure of the formaldehyde abundance in the gaseous phases of the inner part of the envelopes gives some hints on the composition of the grain mantles, and in turn on the grain surface chemistry. Moreover, chemistry models predict that, once in the gas phase, formaldehyde can rapidly form complex molecules, by the so called hot core chemistry (Charnley et al. 1992). This chemistry was thought to exist only in high mass protostars, where the gas temperature and density are high enough to trigger endothermic reactions between species. The very recent detection of O and N bearing molecules, typical of massive hot cores, towards IRAS16293-2422 (Cazaux et al. 2003), emphasizes the chemical similarity that may exist between low and high mass protostars.In order to determine if IRAS16293-2422 is representative of low mass protostars, or rather a peculiar case, one needs to measure the formaldehyde abundance in a larger sample of protostars. We have recently observed a sample of eight Class 0 protostars, located in the Perseus, ρ-Ophiuchus and Taurus complexes, using the James Clerk Maxwell Telescope and the Institut de Radio Astronomie 30 meter telescope. Eight formaldehyde transitions were selected, three ortho and five para, ranging from 140 to 364 GHz. The transitions between 140 GHz and 280 GHz were observed with the IRAM-30m telescope, while transitions at higher frequencies were observed using the JCMT. The choice of these two instruments allow a nearly constant beam size over frequencies. The transitions were chosen to cover a large range of upper level energies, from 20 to 100 cm^-1, in order to probe the physical conditions in the different part of the envelope. Fig. 1 shows a typical example of lines observed towards NGC1333-IRAS4A.

Figure 1: Typical example of formaldehyde lines observed towards NGC1333-IRAS4A. The lines are relatively narrow, with a contribution of the wings extending at larger velocities. Some of the lines show self-absorption and / or absorption by the foreground material.

The formaldehyde emission was modeled using a 1D spherical radiative code. The density and dust temperature profiles determined by Jörgensen (2002), from the simultaneous modeling of the continuum emission at 450 and 850 µm observed by SCUBA and the spectral energy distribution, were adopted. The gas temperature was computed by solving the thermal balance in the envelope (Ceccarelli et al. 1996). Finally, because of the importance of evaporation in the inner parts of the envelope, the formaldehyde abundance has been approximated by a step function: X_out in the outer part of the envelope where the dust temperature T_dust is lower than 100 K, and X_in in the inner parts of the envelope where T_dust > 100 K. These abundances have been determined by a χ² analysis. Fig. 2 show the χ² contours has a function of X_in and X_out for two sources of the sample.

Figure 2: χ² as a function of X_in and X_out for NGC1333-IRAS4A and NGC1333-IRAS2. The contour levels show the 1, 2 and 3 σ confidence level respectively. The observations are only reproduced if there is a jump in the formaldehyde abundance, between 2 and 3 orders of magnitude.

This modelling shows that, in all the protostars but one, the observations can only be reproduced if there is a jump in the formaldehyde abundance, between two and three orders of magnitude. The position of this jump correspond to the radius where the temperature reaches 100 K, the sublimation temperature of grain mantle. In this region, the grain mantle evaporates, releasing the ices components, and among them, formaldehyde. From this point of view, IRAS16293-2422 is not a particuliar case, and all observed protostars but one harbors a hot core, where the chemistry is influenced, if not dominated, by grain mantle evaporation.

References André, P., Ward-Thompson, D., & Barsony, M. 2000, Protostars and Planets IV, 59
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