Transcript Document

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).
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 2MASS
extinction data of the Perseus molecular
cloud as part of a multi-wavelength
survey of the region through the COordinated
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
B1 region of Perseus.
Abstract
Figure 1: NGC1333, 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
low-mass 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 extinctionthreshold 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 ~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
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
deg2. 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 (2MASS) 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 cm2 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, BaltA, 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, ApJ, 559, 307
Johnstone, D., Di Francesco, J., & Kirk, H. 2004,
ApJ, 611L, 45
Johnstone et al 2005, ApJ, submitted
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
Walawender, J., Bally, J., Kirk, H., Johnstone, D.
2005, AJ, 130, 1795
Williams, J.P., de Geus, E.J., & Blitz, L. 1994,
ApJ, 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, DiFrancesco & 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 (McKee 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 2MASS 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)