The Perseus Molecular Cloud Towards a COMPLETEr Understanding

The Perseus Molecular Cloud: Towards a COMPLETEr Understanding Helen Kirk (UVic, NRC-HIA), Doug Johnstone (NRC-HIA, UVic), James Di. Francesco (NRC-HIA) Introduction The Perseus Molecular Cloud is a region rich with star formation activity (see Fig 1 showing NGC 1333, an active star forming region in Perseus). Multiwavelength observations over large regions of molecular clouds have only recently become possible, presenting the opportunity to learn about large scale processes – for example, the relative importance of turbulence and magnetic fields in the formation and support of structure. Here, we present a combination of 850 m continuum and 2 MASS extinction data of the Perseus molecular cloud as part of a multi-wavelength survey of the region through the CO-ordinated Molecular Probe Line Extinction and Thermal Emission (COMPLETE) Survey. See the poster by J. Walawender for a multi-wavelength analysis including shock structures of the B 1 region of Perseus. Abstract Figure 1: NGC 1333, one of the sites of active star formation in the Perseus Molecular Cloud (image courtesy of J. Walawender) We present results of an analysis of the Perseus molecular cloud using a combination of 850 m continuum data to trace small-scale structure and near -IR extinction data to trace the large-scale structure, including fitting ‘clumps’ found in the sub-millimetre to Bonnor Ebert spheres. The cumulative mass distribution of the sub-millimetre clumps is shown to be dominated by lowmass clumps, with a slope similar to that of the stellar Initial Mass Function. We also demonstrate that the sub-millimetre clumping is only found toward the higher column density regions. This is similar to the extinction-threshold recently discovered for star-forming regions in the Ophiuchus molecular cloud. We conclude by presenting a preliminary analysis that indicates the submillimetre clumps are located offset from peaks in the larger extinction structure. This offset suggests clump formation may have been triggered, consistent with previous results. Figure 3: Cumulative mass distribution of submillimetre clumps. The green line indicates masses calculated assuming a temperature of 15 K, while the blue indicates temperatures derived from a BE sphere analysis Submillimetre Structure Analysis Previous work (e. g. Johnstone et al. 2005) has shown that submillimetre clumps tend to have mass distributions well fit by a broken power law of the form where the number N, varies with mass M as N(M) M- , where ~ 1. 35, similar to the stellar Initial Mass Function (e. g. Salpeter 1955). Fig. 3 (above) shows that our clumps are well fit by a similar law. The turn-over at ~0. 3 M occurs roughly where we expect incompleteness to play a role. We model the clumps as spherically symmetric structures bounded by an external pressure where gravity balances equal levels of thermal and non-thermal support (Bonnor-Ebert spheres; Bonnor 1956, Ebert 1955) which allows us to estimate the internal temperature, bounding pressure and central density of these objects. Each clump is parameterized by their degree of central concentration to find the best fit BE sphere (see Johnstone et al. 2005). Figs 4 and 5 below show the derived clump properties. Triggered Star Formation Fig. 7 shows the distribution of submillimetre clumps (diamonds) with respect to the large scale structure of the cloud, as measured in extinction (contours). The submillimetre clumps all lie off-centre of the peak column density, contrary to that which would be expected by a simple magnetic or turbulent support model. The correlation of the off-axis locations across extinction cores is suggestive of the involvement of a larger process (e. g. the ‘globule-squeezing’ scale of triggered formation; Elmegreen 1998). The arrows plotted indicate the direction to 40 Per, a nearby B star previously suggested as a trigger for star formation in the region (Walawender et al. 2004) Figure 2: Colour – Perseus submillimetre data. The beamsize is 19. 9” and the mean standard deviation is ~8 m. Jy/bm. The contours indicate 2 MASS extinction levels (3, 5, 7, & 9 magnitudes), while the red circles identify the submillimetre clumps. Figure 4 & 5: Model clump properties. We parameterized clumps by their central concentration to fit to BE spheres. Stable BE spheres do not exist for concentrations below 1/3 or above 0. 72. Data Reduction and Structure Identification Submillimetre data at 850 m of the Perseus molecular cloud was obtained using SCUBA on the JCMT. The data we present are comprised of our own observations and publicly available archival data for a total of ~3. 5 deg 2. We use the matrix inversion technique to convert the data into an image (Johnstone et al. 2000), flattening the map on large scales to eliminate any artificial structure induced by chopping (see Fig. 2). The extinction data (contours on Fig. 2) were derived from the Two Micron All Sky Survey (2 MASS) stellar reddening data (Ridge et al. 2006) as part of COMPLETE. We identified structure in the 850 m map, using the automated routine Clumpfind (Williams et al. 1994) which has the advantage of not using an assumed shape for structure. We identify 54 submillimetre clumps (see Fig. 2). These clumps match well to those identified in a similar analysis by Hatchell et al. (2005). The total flux of each clump is converted into mass using a dust opacity of 850 = 0. 02 cm 2 g-1, a typical internal temperature of 15 K and distance of 250 pc (Cernis 1993). References Bonnor, W. B. 1956, MNRAS, 112, 195 Cernis, K. 1993, Balt. A, 2, 214 Ebert, R. 1955, Z. Astrophys. , 37, 217 Elmegreen, B. 1998, ASPC, 148, 150 Hatchell et al 2005, A&A, 440, 151 Johnstone et al 2000, Ap. J, 559, 307 Johnstone, D. , Di Francesco, J. , & Kirk, H. 2004, Ap. J, 611 L, 45 Johnstone et al 2005, Ap. J, submitted Kirk et al 2006, in prep Mc. Kee, C. 1989, Ap. J, 345, 782 Ridge et al 2006, in prep Salpeter, E. E. 1955, Ap. J, 121, 161 Walawender et al 2004, AJ, 127, 2809 Walawender, J. , Bally, J. , Kirk, H. , Johnstone, D. 2005, AJ, 130, 1795 Williams, J. P. , de Geus, E. J. , & Blitz, L. 1994, Ap. J, 428, 693 Extinction Threshold Figure 7: Positions of submillimetre clumps in Perseus (diamonds) relative to the underlying (extinction) structure (contours). Arrows indicate direction from each extinction core to 40 Per, a candidate for triggering star formation in the region Discussion Fig. 2 shows a lack of clumps in low extinction regions of the cloud – here we demonstrate this is not an observational bias. Following Johnstone, Di. Francesco & Kirk (2004), we use extinction as an indication of external pressure, and calculate a BE sphere’s observable properties for a given importance of self-gravity. Fig. 6 demonstrates that for a given level of importance of self-gravity, our lack of detections at AV = 5 - 7 is inconsistent with the detections at higher extinctions. A similar result was found by Johnstone et al. (2004) in Ophiuchus but at a higher threshold. The magnetic support scenario can explain an extinction threshold of AV = 4 – 8 (Mc. Kee 1989) – ambipolar diffusion is only efficient in structure formation at high extinction where the ionized fraction is low. More research is needed to determine if the turbulent support scenario is supported by our results. We present analysis of 850 m continuum and 2 MASS extinction data of 3. 5 square degrees of the Perseus molecular cloud. We find the mass distribution of the submillimetre clumps is similar to that found in studies of other star forming regions, with the slope of the cumulative number distribution being similar to that of the IMF. Most of the clumps are well fit by a Bonnor Ebert sphere model with temperatures from 10 to 19 K. Similar to previous work in the Ophiuchus molecular cloud (Johnstone et al. 2004), the extinction data indicate that the submillimetre structure forms only above a certain AV. An extinction threshold would be supported by a model of magnetic support, but it is less clear for the turbulent support scenario. Finally, we present analysis on the locations of submillimetre clumps within the extinction structure, suggesting that they are consistent with a triggered formation scenario, with the B star 40 Per a possible candidate for this triggering. This work is part of the http: //cfa-www. harvard. edu/COMPLETE The work here forms a part of my MSc thesis, which can be viewed at http: //orca. phys. uvic. ca/~hkirk/thesis. pdf project Fig 6: Observed clump properties (total flux, peak flux and radius) versus extinction. The curves indicate the expected relation for a BE sphere of constant importance of self-gravity. The diamonds indicate clumps found in a region we believe to is evolved (see Kirk et al. 2006)