The Mass Distribution of Starless and Protostellar Cores in Gould Belt Clouds

Abstract

Using data from the SCUBA Legacy Catalogue (850 μm) and Spitzer Space Telescope (3.6-70 μm), we explore dense cores in the Ophiuchus, Taurus, Perseus, Serpens, and Orion molecular clouds. We develop a new method to discriminate submillimeter cores found by Submillimeter Common-User Bolometer Array (SCUBA) as starless or protostellar, using point source photometry from Spitzer wide field surveys. First, we identify infrared sources with red colors associated with embedded young stellar objects (YSOs). Second, we compare the positions of these YSO candidates to our submillimeter cores. With these identifications, we construct new, self-consistent starless and protostellar core mass functions (CMFs) for the five clouds. We find best-fit slopes to the high-mass end of the CMFs of –1.26 ± 0.20, –1.22 ± 0.06, –0.95 ± 0.20, and –1.67 ± 0.72 for Ophiuchus, Taurus, Perseus, and Orion, respectively. Broadly, these slopes are each consistent with the –1.35 power-law slope of the Salpeter initial mass function at higher masses, but suggest some differences. We examine a variety of trends between these CMF shapes and their parent cloud properties, potentially finding a correlation between the high-mass slope and core temperature. We also find a trend between core mass and effective size, but we are very limited by sensitivity. We make similar comparisons between core mass and size with visual extinction (for AV ≥ 3) and find no obvious trends. We also predict the numbers and mass distributions of cores that future surveys with SCUBA-2 may detect in each of these clouds.

Full text

Page 1

arXiv:1001.0978v3 [astro-ph.GA] 18 Feb 2010
The Mass Distributions of Starless and Protostellar Cores in Gould Belt Clouds
Sarah I. Sadavoy1,2, James Di Francesco2,1, Sylvain Bontemps3, S. Thomas Megeath4, Luisa M.
Rebull5, Erin Allgaier4, Sean Carey5, Robert Gutermuth6, Joe Hora7, Tracy Huard8, Caer-Eve
McCabe5, James Muzerolle9, Alberto Noriega-Crespo5, Deborah Padgett5, Susan Terebey10
ssadavoy@uvic.ca
ABSTRACT
Using data from the SCUBA Legacy Catalogue (850 µm) and Spitzer Space Telescope
(3.6 - 70 µm), we explore dense cores in the Ophiuchus, Taurus, Perseus, Serpens, and
Orion molecular clouds. We develop a new method to discriminate submillimeter cores
found by SCUBA as starless or protostellar, using point source photometry from Spitzer
wide field surveys. First, we identify infrared sources with red colors associated with
embedded young stellar objects (YSOs). Second, we compare the positions of these
YSO-candidates to our submillimeter cores. With these identifications, we construct
new, self-consistent starless and protostellar core mass functions (CMFs) for the five
clouds. We find best fit slopes to the high-mass end of the CMFs of −1.26 ± 0.20,
−1.22 ± 0.06, −0.95 ± 0.20, and −1.67 ± 0.72 for Ophiuchus, Taurus, Perseus, and
Orion, respectively. Broadly, these slopes are each consistent with the −1.35 power-law
slope of the Salpeter initial mass function (IMF) at higher masses, but suggest some
differences. We examine a variety of trends between these CMF shapes and their parent
cloud properties, potentially finding a correlation between the high-mass slope and core
1Department of Physics and Astronomy, University of Victoria, PO Box 355, STN CSC, Victoria BC CANADA,
V8W 3P6
2National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria BC
CANADA, V9E 2E7
3Observatoire de Bordeaux, Site de Floirac, 2 rue de l‘Observatoire, 33270 Floirac, FRANCE
4Ritter Observatory, MS-113, University of Toledo, 2801 W. Bancroft St., Toledo, OH 43606, USA
5Spitzer Science Center, Mail Stop 220-6, California Institute of Technology, 1200 East California Boulevard,
Pasadena, CA 91125
6University of Massachusetts, Smith College, Northampton, MA 01063
7Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-42, Cambridge, MA 02138, USA
8Department of Astronomy, University of Maryland, College Park, MD 20742, USA
9Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA
10Department of Physics and Astronomy PS315, 5151 State University Drive, California State University at Los
Angeles, Los Angeles, CA 90032, USA

Page 2

– 2 –
temperature. We also find a trend between core mass and effective size, but we are very
limited by sensitivity. We make similar comparisons between core mass and size with
visual extinction (for AV ≥ 3) and find no obvious trends. We also predict the numbers
and mass distributions of cores that future surveys with SCUBA-2 may detect in each
of these clouds.
1.Introduction
The origin of stellar mass is not well understood. We know stars form in very cold and dense
regions of gas and dust, deeply embedded within molecular clouds, but the dominant processes
that guide the formation of small scale structure and the collapse of those structures into stars
is unclear. We define zones of relatively high density on the small scales in molecular clouds as
“cores,” which have masses ∼ M⊙within a radius of ∼ 0.07 pc and would form a single star or a
stellar system with a few stars (Di Francesco et al. 2007). Thus, molecular cloud cores represent the
cumulative result of all physical processes that organize cloud mass into relatively high densities
on small scales.
The masses of molecular cloud cores are important probes of the initial conditions of star
formation, and the relationships between these cores and any stellar products may be key to
understanding the origin of stellar mass. Previous studies of these populations (for examples,
see Motte et al. 1998; Nutter & Ward-Thompson 2007; Ward-Thompson et al. 2007a; Enoch
et al. 2008) have revealed that core mass functions (CMFs) roughly follow lognormal shapes that
closely resemble the observed stellar initial mass function (IMF). In particular, these studies have
shown great similarity between the high-mass slopes of CMFs and the high mass slope of the IMF.
The CMF, however, may still depend on physical conditions within the parent cloud. Indeed,
the relationship between the CMF and IMF is likely complex and a variety of factors, such as
fragmentation (e.g., Dobbs et al. 2005), competitive accretion (e.g., Bonnell et al. 2004), turbulence
(e.g., Elmegreen 2002), magnetic fields (e.g., Shu et al. 2004), and radiative feedback (e.g., Offner
et al. 2009) may be relevant.
The continuity between CMFs and the stellar IMF suggests that stellar mass is related to
how material in molecular clouds is first collected into stellar precursors. Unfortunately, previous
studies generally examined small regions within larger clouds (e.g., the L1688 region of Ophiuchus
in Motte et al. 1998) and over different wavelengths (e.g., 850 µm, 1.1 mm). As well, each group
had developed a different set of conditions to identify small scale structure in molecular clouds,
thereby these studies follow different numbers of cores, even for the same region (e.g., for Perseus,
Hatchell et al. 2005; Kirk et al. 2006). In spite of the inconsistent analyses, these previous studies
seem to produce CMFs that resemble the IMF shape. Nevertheless, it is difficult to compare CMFs
from different studies and test for differences between clouds. It would be advantageous to look at
core populations in various clouds with different physical conditions using a self-consistent analysis.

Page 3

– 3 –
In addition, observations of dense cores over the last decade have revealed populations of
cores with and without embedded young stars (Di Francesco et al. 2007). Cores that contain
a young stellar object (YSO) will have lost some of the surrounding material to outflows or to
accretion onto the central body (Myers 2008). Also, their intrinsic temperatures may differ, biasing
estimates of their masses. Thus, to obtain an accurate CMF, starless cores must be differentiated
from protostellar cores, ie., those containing YSOs.
Distinguishing between starless and protostellar cores depends on detecting an infrared source
embedded in the dense material. Deeply embedded sources can be very faint, however, making
such detections difficult. As well, many observed infrared sources are background galaxies or bright
giant stars in our Galaxy, which must be identified as external to the cloud. Therefore, multi-band
mid- to far-infrared photometry has emerged as an effective means for identifying and classifying
embedded YSOs from external sources of infrared emission (e.g., Allen et al. 2004; Megeath et al.
2004; Harvey et al. 2006; Gutermuth et al. 2008; Megeath et al. 2009; Evans et al. 2009; Rebull
et al. 2009).
Already, numerous studies have developed techniques to identify protostellar core populations
from starless core populations in molecular clouds (e.g., Jørgensen et al. 2007; Enoch et al. 2009).
These techniques, however, employ very different constraints in infrared colors, magnitudes, or
locations of YSO candidates with respect to the cores. As such, the CMFs produced by each
technique will contain different biases, making comparisons between these studies unclear. Ideally,
we would want to use the same constraints to compare CMFs over a large sample of molecular
clouds to explore differences and similarities in core populations with environment.
Here, we use 850 µm data from the Submillimeter Common-User Bolometer Array (SCUBA)
and just-released Spitzer data with a new self-consistent method of classifying cores to identify
starless and protostellar cores in five clouds (Ophiuchus, Taurus, Perseus, Serpens, and Orion).
With this discrimination, we produce consistent CMFs across the five clouds and thus, we can
directly compare these distributions. In §2, we discuss our sample choices, including the target
clouds, the infrared and submillimeter data used in this study, and the individual core populations
and selection criteria. In §3, we discuss our new core classification technique and compare our
results with two other previously developed methods. In §4, we examine the CMFs produced from
our own classification method, and we compare these to standard formulations of the IMF. Also, we
examine trends in the CMFs between the clouds, compare core properties with their surrounding
environments, and make predictions as to what forthcoming instruments will detect. In §5, we
summarize our results.

Page 4

– 4 –
2.Clouds
2.1.Cloud Properties
Our analysis focused on the Ophiuchus, Taurus, Perseus, Serpens, and Orion molecular clouds.
These clouds were chosen due to a wealth of available data (e.g., millimeter, infrared) and their
close proximity (all < 500 pc), resulting in maps with good linear resolution. Such small scale
observations are necessary to resolve cores as well as to probe the physical properties and structure
inside cores (Ward-Thompson et al. 2007a). The five clouds studied here represent a variety of
physical environments. For example, the Taurus cloud is undergoing only low-mass star formation
(Hartmann 2000) whereas the Orion region contains active regions of massive star formation and
is found within a large OB association (Peterson & Megeath 2008). We found total cloud masses
using our visual extinction maps (see §2.3.3 and Figure 1). Assuming that 1 magnitude of extinction
corresponds to 1021molecules of hydrogen gas (1 AV = 1021H2cm−2) and a mean molecular weight
of 2.33, we measured the mass of the cloud via,
M(M⊙) = 56.4
?
θ
deg
?2?
d
100 pc
?2?
AV,i
(1)
where θ is the angular size of the extinction map pixels, d is the cloud distance (listed in Table
1), and AV,iis the extinction in a given pixel element. Table 1 gives the mass measurements we
obtained for each cloud and the area over which we measured the mass. We constrained our cloud
mass and size measurements to the regions with AV > 1. For Serpens, the cloud is located close
to the Galactic Plane, making its boundary less certain. Thus, we are likely including extinction
from the nearby Auriga cloud and over-estimating its mass.
2.2.Core Properties
For each cloud, we assumed the core dust temperatures, Td, and the 850 µm opacities, κ850,
were constant. Temperatures and opacities of core dust probably deviate within a given region due
to different circumstances, such as proximity to an embedded cluster. Table 2 lists the assumed
values for Tdand Table 1 gives the assumed values of distance for each cloud.
We note that the temperatures listed in Table 2 were obtained via different techniques. Friesen
et al. (2009) and Rosolowsky et al. (2008) derived kinetic temperatures, TK, of dense gas in Ophi-
uchus and Perseus, respectively, using ammonia hyperfine structure lines, and we assume these
temperatures are similar to Tdsince the densities in the cores are expected to be high (Goldsmith
2001). For the L1544 region in Taurus, Andr´ e et al. (2000) used spectral energy distribution (SED)
fitting from ISO, SCUBA, and IRAM observations to obtain Td= 13 K. As part of the COM-
PLETE survey, Schnee et al. (2005) used the 60µm/100µm flux density ratio for Serpens (see their
Figure 5) to estimate its dust temperature. For Orion, Wilson et al. (1999) observed ammonia
and derived rotational temperatures of ∼ 20 K. As with Ophiuchus and Perseus, we assume the

Page 5

– 5 –
rotational temperature is similar to the dust temperature since the gas traced by ammonia is dense
(∼ 104cm−3).
Clearly a common origin of Tdwould be preferable, but note that the masses of all cores in the
CMFs will scale with Tdand to first order the CMF shape will not depend on Td(see §4 for further
discussion; we assume a 30% error in Tdto derive uncertainties in our masses and CMFs). Self-
consistent determinations of Tdfor cores in these clouds will soon be possible through SED fitting of
75−500 µm data from the Herschel Gould Belt Survey (Andr´ e & Saraceno 2005; Ward-Thompson
et al. 2007b).
2.3.Data
We obtained our data from large-scale surveys and included wavelengths from the submillimeter
(850 µm) to the infrared (∼ 2 µm to 70 µm). We used the submillimeter data to probe the densest
regions of each cloud and the infrared data to identify embedded protostars through emission and
characterize the extended cloud structure through extinction. We discuss each of these data sets
in turn below.
2.3.1.SCUBA Maps
We obtained our submillimeter data from the SCUBA Legacy Catalogue (SLC; see Di Francesco
et al. 2008 for more details), which is an archive of all 850 µm and 450 µm mapping observations
from SCUBA on the James Clerk Maxwell Telescope (JCMT). These observations generally focused
on the high extinction regions within each cloud, thus the SLC data are patchy. The SLC includes
two sub-catalogues: the Fundamental Catalogue, which contains only objects identified from data
with high quality atmospheric corrections (consisting of ∼ 78 % of map data with an areal coverage
of ∼ 19.6 deg2), and the Extended Catalogue, which includes all the data regardless of quality (areal
coverage of ∼ 29.3 deg2). For consistency, the SLC used the algorithms developed by Johnstone
et al. (2000a) to similarly reduce these data. As well, the SLC used the 2D Clumpfind algorithm
(Williams et al. 1994) to identify structures in the continuum emission. Clumpfind identifies flux
peaks over a certain noise level and then uses flux contours (down to a minimum threshold level)
to assign boundaries to the cores and measure total fluxes (see Di Francesco et al. 2008). For a
discussion on the biases associated with different clump-finding algorithms, see Curtis & Richer
(2009).
To identify our submillimeter cores, we only used the 850 µm data, since the 450 µm obser-
vations have a greater absolute flux uncertainty than the 850 µm data by over a factor of two (Di
Francesco et al. 2008). Since we are more interested in accurate core fluxes than wide areal coverage
of the clouds, we drew our samples from the Fundamental Catalogue. Table 3 lists the cloud areal
coverage observed by each of the surveys, where the second column indicates the area mapped in