Docstoc

Center for Astrophysics

Document Sample
Center for Astrophysics Powered By Docstoc
					                                                                        The Perseus Molecular Cloud:
                                                                     Towards a COMPLETEr Understanding
           Helen Kirk (UVic, NRC-HIA), Doug Johnstone (NRC-HIA, UVic), James DiFrancesco (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).           Multi-                                                                                           Abstract
wavelength observations over large                                                                 We present results of an analysis of the Perseus molecular cloud using a
regions of molecular clouds have only                                                              combination of 850 m continuum data to trace small-scale structure and
recently become possible, presenting the                                                           near-IR extinction data to trace the large-scale structure, including fitting
opportunity to learn about large scale                                                             ‘clumps’ found in the sub-millimetre to Bonnor Ebert spheres. The cumulative
processes – for example, the relative                                                              mass distribution of the sub-millimetre clumps is shown to be dominated by
importance of turbulence and magnetic                                                              low-mass clumps, with a slope similar to that of the stellar Initial Mass
fields in the formation and support of                                                             Function. We also demonstrate that the sub-millimetre clumping is only found
structure. Here, we present a combination                                                          toward the higher column density regions. This is similar to the extinction-
of 850m continuum and 2MASS                                                                       threshold recently discovered for star-forming regions in the Ophiuchus                                               Figure 3: Cumulative mass distribution of submillimetre
extinction data of the Perseus molecular                                                           molecular cloud. We conclude by presenting a preliminary analysis that                                                   clumps. The green line indicates masses calculated
cloud as part of a multi-wavelength                                                                indicates the submillimetre clumps are located offset from peaks in the larger                                        assuming a temperature of 15 K, while the blue indicates
survey of the region through the CO-                                                               extinction structure. This offset suggests clump formation may have been                                                   temperatures derived from a BE sphere analysis
ordinated     Molecular      Probe      Line                                                       triggered, consistent with previous results.
Extinction and Thermal Emission                         Figure 1: NGC1333, one of the sites of
                                                          active star formation in the Perseus
(COMPLETE) Survey. See the poster by
                                                        Molecular Cloud (image courtesy of J.                                                                                                                     Submillimetre Structure Analysis
J. Walawender for a multi-wavelength                                  Walawender)                                                                                                                        Previous work (e.g. Johnstone et al. 2005) has shown that submillimetre
analysis including shock structures of the                                                                                                                                                        clumps tend to have mass distributions well fit by a broken power law of the form
B1 region of Perseus.                                                                                                                                                                             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
                                                                                                                       Triggered Star Formation                                                   our clumps are well fit by a similar law. The turn-over at ~0.3 M occurs roughly
                                                                                                              Fig. 7 shows the distribution of submillimetre clumps                               where we expect incompleteness to play a role.
                                                                                                              (diamonds) with respect to the large scale structure of the                                We model the clumps as spherically symmetric structures bounded by an
                                                                                                              cloud, as measured in extinction (contours).             The                        external pressure where gravity balances equal levels of thermal and non-thermal
                                                                                                              submillimetre clumps all lie off-centre of the peak column                          support (Bonnor-Ebert spheres; Bonnor 1956, Ebert 1955) which allows us to
                                                                                                              density, contrary to that which would be expected by a                              estimate the internal temperature, bounding pressure and central density of these
                                                                                                              simple magnetic or turbulent support model.              The                        objects. Each clump is parameterized by their degree of central concentration to find
                                                                                                              correlation of the off-axis locations across extinction cores                       the best fit BE sphere (see Johnstone et al. 2005). Figs 4 and 5 below show the
                                                                                                              is suggestive of the involvement of a larger process (e.g.                          derived clump properties.
                                                                                                              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 ~8mJy/bm. The contours indicate 2MASS 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                                                                                                                                                                                 Extinction Threshold
            Submillimetre data at 850 m of the Perseus molecular cloud was                                                                                                                                 Fig. 2 shows a lack of clumps in low extinction regions of the cloud –
    obtained using SCUBA on the JCMT. The data we present are comprised of                                                                                                                                  here we demonstrate this is not an observational bias. Following
    our own observations and publicly available archival data for a total of ~3.5                                                                                                                           Johnstone, DiFrancesco & Kirk (2004), we use extinction as an indication
    deg2. We use the matrix inversion technique to convert the data into an image                                                                                                                           of external pressure, and calculate a BE sphere’s observable properties for
    (Johnstone et al. 2000), flattening the map on large scales to eliminate any                                                                                                                            a given importance of self-gravity. Fig. 6 demonstrates that for a given
    artificial structure induced by chopping (see Fig. 2).                                                                                                                                                  level of importance of self-gravity, our lack of detections at AV = 5 - 7 is
            The extinction data (contours on Fig. 2) were derived from the Two                                                                                                                              inconsistent with the detections at higher extinctions. A similar result was
    Micron All Sky Survey (2MASS) stellar reddening data (Ridge et al. 2006) as                                                                                                                             found by Johnstone et al. (2004) in Ophiuchus but at a higher threshold.
    part of COMPLETE.                                                                                                                                                                                       The magnetic support scenario can explain an extinction threshold of AV =
            We identified structure in the 850 m map, using the automated routine                          Figure 7: Positions of submillimetre clumps in Perseus (diamonds) relative to the
                                                                                                                                                                                                            4 – 8 (McKee 1989) – ambipolar diffusion is only efficient in structure
    Clumpfind (Williams et al. 1994) which has the advantage of not using an                                underlying (extinction) structure (contours). Arrows indicate direction from each
                                                                                                             extinction core to 40 Per, a candidate for triggering star formation in the region             formation at high extinction where the ionized fraction is low. More
    assumed shape for structure. We identify 54 submillimetre clumps (see Fig. 2).                                                                                                                          research is needed to determine if the turbulent support scenario is
    These clumps match well to those identified in a similar analysis by Hatchell et                                                                                                                        supported by our results.
    al. (2005). The total flux of each clump is converted into mass using a dust
    opacity of 850 = 0.02 cm2 g-1, a typical internal temperature of     15 K and
                                                                                                                                           Discussion
                                                                                                        We present analysis of 850 m continuum and 2MASS extinction data of 3.5
    distance of 250 pc (Cernis 1993).
                                                                                                        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
           References                                                                                   that of the IMF. Most of the clumps are well fit by a Bonnor Ebert sphere
           Bonnor, W.B. 1956, MNRAS, 112, 195                                                           model with temperatures from 10 to 19 K. Similar to previous work in the
           Cernis, K. 1993, BaltA, 2, 214                                                               Ophiuchus molecular cloud (Johnstone et al. 2004), the extinction data indicate
           Ebert, R. 1955, Z. Astrophys., 37, 217
                                                                                                        that the submillimetre structure forms only above a certain AV. An extinction
           Elmegreen, B. 1998, ASPC, 148, 150
           Hatchell et al 2005, A&A, 440, 151                                                           threshold would be supported by a model of magnetic support, but it is less clear
           Johnstone et al 2000, ApJ, 559, 307                                                          for the turbulent support scenario. Finally, we present analysis on the locations
           Johnstone, D., Di Francesco, J., & Kirk, H. 2004,                                            of submillimetre clumps within the extinction structure, suggesting that they are
                   ApJ, 611L, 45                                                                        consistent with a triggered formation scenario, with the B star 40 Per a possible
           Johnstone et al 2005, ApJ, submitted                                                         candidate for this triggering.
           Kirk et al 2006, in prep
           McKee, C. 1989, ApJ, 345, 782
           Ridge et al 2006, in prep
           Salpeter, E.E. 1955, ApJ, 121, 161
           Walawender et al 2004, AJ, 127, 2809
                                                                                 This work is part of the                                                                                    project                     Fig 6: Observed clump properties (total flux, peak flux and radius)
           Walawender, J., Bally, J., Kirk, H., Johnstone, D.                                                                                                                                                            versus extinction. The curves indicate the expected relation for a
                   2005, AJ, 130, 1795                                                                             http://cfa-www.harvard.edu/COMPLETE                                                                   BE sphere of constant importance of self-gravity. The diamonds
           Williams, J.P., de Geus, E.J., & Blitz, L. 1994,                                                                                                                                                              indicate clumps found in a region we believe to is evolved (see
                   ApJ, 428, 693                                                                                                                                                                                         Kirk et al. 2006)
                                                                                                 The work here forms a part of my MSc thesis, which can be viewed at
                                                                                                              http://orca.phys.uvic.ca/~hkirk/thesis.pdf