6 Summary and conclusions

In an effort to constrain protostellar evolution and test the predictions of collapse models, we have mapped the column density structure of the circumstellar environment of 49 low-mass embedded YSOs in the dust continuum at 1.3 mm. Our sample includes all 27 Class I (and Class 0) sources presently known in Taurus, along with 9 isolated Bok globules and 9 protostars in Perseus. Our main findings may be summarized as follows:

1.

In good agreement with the predictions of the standard protostellar model of Shu et al. (1987), 16 of the Taurus Class I/0 sources are observed to be surrounded by relatively massive ( $\overline{\mbox{$M_{\mbox{\tiny env}}^{\mbox{\tiny 4200~AU}}$ }}\sim 0.3\ \mbox{$M_\odot$ }$), spatially extended ( $\mbox{$R_{\mbox{\tiny out}}$ }\lower.5ex\hbox{$\; \buildrel > \over \sim \;$ }10\,000$ AU) circumstellar envelopes with a power-law density gradient such as $\rho (r) \propto r^{-2}$ or $\rho(r) \propto r^{-1.5}$. These sources are consistent with being "bona-fide'' protostars, currently in their main accretion phase;

2.

Contrary to expectations, however, the 1.3 mm continuum emission mapped toward 11 Class I sources in Taurus and 1 Class I Bok globule is weak and essentially unresolved. These "peculiar'' Class I sources have very small circumstellar envelope masses, $\mbox{$M_{\mbox{\tiny env}}^{\mbox{\tiny 4200~AU}}$ }< 0.002$- $0.04\ \mbox{$M_\odot$ }$, implying that they cannot be genuine protostars and must be at the very end of their accretion phase. Despite their low bolometric temperatures ( $\overline{\mbox{$T_{\mbox{\tiny bol}}$ }} \sim 350$ K), they are most likely highly reddened PMS stars and/or transition objects between protostars and T Tauri stars;

3.

Like the "bona-fide'' protostars of Taurus, 8 of the 9 embedded YSOs mapped in Bok globules have circumstellar envelope properties that are consistent to first order with the standard model. The envelopes of these objects tend nevertheless to be a factor of $\sim$2 denser than is predicted by a purely thermal model with $\mbox{$T_{\mbox{\tiny cloud}}$ }=10\$ K. Magnetic fields may have contributed a significant fraction of the support in the corresponding pre-collapse cloud cores;

4.

By contrast, the envelopes of 8 Class 0 protostars in Perseus are found to be much denser, by a factor of 3-12, than is expected in the standard collapse model. They are also relatively compact with outer radii of only a few thousand AUs ( $\mbox{$R_{\mbox{\tiny out}}$ }\sim 10\,000$ AU). This is suggestive of a more dynamical picture in which external disturbances initiate the collapse of finite-sized cloud fragments with flat inner density profiles, and supersonic infall velocities develop during a runaway isothermal phase prior to the main accretion stage. Such a picture appears to be more appropriate than the standard model for describing individual collapse in cluster-forming cores. Direct spectroscopic measurements of the velocity field are however needed to confirm this view;

5.

In the close environment of 6 of the 24 Class I YSOs mapped in Taurus, our maps revealed the presence of previously unknown 1.3 mm continuum condensations. All are undetected in the infrared and are good candidates for being new pre-stellar cores and/or young accreting protostars. One of them coincides with the Class 0 object IRAM 04191 already discussed in AMB99. The discovery of these cold condensations shows that infrared observations can only give a partial census of star formation activity in molecular clouds (see also point 2 above), and emphasizes the need for deep, unbiased mapping surveys in the submillimeter band.

Acknowledgements

We would like to thank Roberto Neri for providing the observing parameters necessary to develop our simulation program within the NIC software. We also acknowledge the participation of Sylvie Cabrit in the early phases of this work and the contribution of Sylvain Bontemps during the last observing run. We are also grateful to Anne Dutrey, Frédéric Gueth, Stéphane Guilloteau, and Roberto Neri for their assistance in deriving disk fluxes from our Plateau de Bure interferometric observations.

Figure 7: a) Effect of the dual-beam observing technique on the observed radial intensity profiles. The dash-dotted curve represents an input model with $I(\theta )\propto \theta ^{-1}$ (cf. dotted line) after convolution with the beam of the telescope (dashed curve). The solid curve shows the output model profile that results from our simulation of the dual-beam observation and the various data reduction steps. b) Grid of simulated profiles for a range of input power-law models, $I(\theta )\propto \theta ^{-m}$, with m=0.1 to 2.5