The initial conditions of isolated star formation — VI. SCUBA mapping of pre-stellar cores

By J. Kirk, D. Ward-Thompson, P. André. 2005

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Observations have been carried out with the Submillimetre Common-User Bolometer Array (SCUBA) at the James Clerk Maxwell Telescope (JCMT) of regions of comparatively isolated star formation in molecular cloud cores. Some 52 starless cores were observed, which are molecular cloud cores that do not contain any sign of protostellar activity such as infrared sources or bipolar outflows. These are all therefore candidate pre-stellar cores, which are believed to represent the stage of star formation that precedes the formation of a protostar. Of the 52 cores, 29 were detected at 850 ?m at varying levels of signal-to-noise ratio greater than 3? at peak, while 23 of the cores were observed but not detected. The mean detection lower limit of the data corresponds roughly to an AV? 15 under typical assumptions. The detected cores were split into ‘bright’ cores and ‘intermediate’ cores, depending on their peak flux density at 850 ?m. Those with peak 850-?m flux densities greater than 170 mJy beam?1 were designated ‘bright’ cores. Those with peak 850-?m flux densities less than this value were designated ‘intermediate’ cores. This dividing line corresponds to a mean detection limit of 10? at peak, and an approximate AV? 50 under typical assumptions. Of the 29 detected cores, 13 are found to be bright and 16 are intermediate. The data are combined with our previously published ISO data, and the physical parameters of the cores, such as density and temperature, are calculated. The bright cores are detected with sufficiently high signal-to-noise ratio to allow their structure to be mapped. Radial flux density profiles of these show flattened inner regions and sharp boundaries, consistent with previous observations of pre-stellar cores. Detailed fitting of the bright core radial profiles shows that they are not critical Bonnor–Ebert spheres, in agreement with previous findings. However, we find that intermediate cores, such as B68 (which has previously been claimed to be a Bonnor–Ebert sphere), may in fact be consistent with the Bonnor–Ebert criterion, suggesting perhaps that cores pass through such a phase during their evolution. We also find that the masses of the bright cores have a mean value of approximately the same order as their virial masses. We make rough estimates of core lifetimes based on the statistics of detections and find that the lifetime of a pre-stellar core is roughly ?3 × 105 yr, while that of a bright core is ?1.5 × 105 yr. Comparisons with some models that regulate collapse using either magnetic fields or turbulence show that no model can match all of the data. Models that are tuned to fit the total pre-stellar core lifetime do not predict the relative numbers of cores seen at each stage.

Subfields: Astronomy and Astrophysics, Atmospheric Science, Spectroscopy

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Citations in the corpus

SDC13 infrared dark clouds: longitudinally collapsing filaments? (2013, background): “Their widths3 as measured on the MAMBO map are ∼0.3 pc. Taking a 3D velocity dispersion σ3D = √ 3σtot = 1.0(±0.3) km s−1, we obtain a crossing time tcross = 0.3(±0.1) Myr.”
Star Formation in the Milky Way and Nearby Galaxies (2012, background): “Prestellar cores are centrally condensed, and can be modeled as Bonnor-Ebert spheres (Ward-Thompson et al. 1999; Evans et al. 2001; Kirk et al. 2005), which have nearly power-law (n(r) ∝ r−2) envelopes around a core with nearly constant density.”
On the internal structure of starless cores II. A molecular survey of L1498 and L1517B (2006, methodology): “(Recent density determinations for these two cores by Kirk et al. 2005 using SCUBA data seem to confirm our estimates, although Shirley et al. 2005 have derived also from SCUBA data a central density for L1498 that is more than 3 times lower than ours.)”
The initial conditions of star formation in the Ophiuchus main cloud: Kinematics of the protocluster condensations , (2007, background): “We may conservatively estimate their evolutionary timescale to be∼ 3 t f f , which is appropriate for magnetically supercritical dense cores (e.g. Ciolek & Mouschovias 1994) and typical of isolated prestellar cores detected in the submillimeter continuum (Kirk et al. 2005).”
The stellar and sub-stellar IMF of simple and composite populations (2011, background): “38 2 SOME ESSENTIALS 2.6 Binary systems (II) The deduced maximum binary period in the BBP of 108.43 d corresponds to a spatial scale of ≈ 104AU which is the typical dimension of a pre-stellar cloud core (Kirk et al. 2005).”
Dense core formation by fragmentation of velocity-coherent filaments in L1517 (2011, background): “Cores A and C have received less attention due to their weaker emission in both high-density tracers and mm-continuum (Benson & Myers 1989; Ladd & Myers 1991; Kirk et al. 2005), so their structure is less well known.”
The mass function of dense molecular cores and the origin of the IMF (2006, background): “This range is lower than that (3×104−5 cm−3) usually associated with typical star forming cores and even the starless cores typically traced by dust continuum observations (Motte et al. 1998; Johnstone et al. 2001; Kirk et al. 2005).”
THE SPITZER c2d LEGACY RESULTS: STAR-FORMATION RATES AND EFFICIENCIES; EVOLUTION AND LIFETIMES (2008, result): “The inferred lifetimes of 0.32 Myr and 0.11 Myr are quite con- 15 sistent with values found by Kirk et al. (2005) and with the overall relation found by Ward-Thompson et al. (2007) indicating lifetimes decreasing toward, but not yet reaching, the free-fall time (tff ) as the density increases, as…”
Magnetic Fields in Dark Cloud Cores: Arecibo OH Zeeman Observations (2008, background): “Later workers (Kirk et al. 2005; Kandori et al. 2005) inferred similarly short core lifetimes.”
MAMBO Mapping Of Spitzer c2d Small Clouds And Cores (2008, background): “Several surveys of isolated cores with and without active star formation have been conducted in the past ≈15 years (e.g., Motte & André 2001; Visser et al. 2002; Kirk et al. 2005; Young C. H. et al. 2006; Wu et al. 2007).”
Comparing Star Formation on Large Scales in the c2d Legacy Clouds: Bolocam 1.1 mm Dust Continuum Surveys of Serpens, Perseus, and Ophiuchus (2007, background): “Recent advances in millimeter and submillimeter wavelength continuum detectors have enabled a number of large-scale surveys of nearby molecular clouds (e.g. Johnstone et al. 2004; Kirk et al. 2005; Hatchell et al. 2005; Enoch et al. 2006; Stanke et al. 2006; Young et al. 2006).”
Observing the gas temperature drop in the high-density nucleus of L 1544 (2007, background): “On large scales >0.05 pc, there is already evidence for higher dust temperatures on core edges than in core centers (Ward-Thompson et al. 2002; Pagani et al. 2004; Kirk et al. 2005) but on the scale of 0.01 pc (15′′ at 140 pc), the situation is less clear.”
Initial highlights of the HOBYS key program, the Herschel imaging survey of OB young stellar objects (2010, background): “Such a result suggests that the lifetime of prestellar cores in Rosette is a few times shorter than found in nearby lowmass star-forming regions (Kirk et al. 2005, 1−4×105 yr), and a few times longer than measured in high-mass star-forming complexes (Motte et al. 2007; Russeil et al. 2010, 3−9 ×…”
Identifying the Low-Luminosity Population of Embedded Protostars in the c2d Observations of Clouds and Cores (2007, background): “…(4) 2MASS; (5) IRAS ; (6) Young et al. (2006a); (7) Motte & André (2001); (8) Bourke et al. (2006); (9) M. Dunham et al. (2008), in preparation; (10) Kirk et al. (2005); (11) Kauffmann et al. (2008); (12) Young et al. (2003); (13) Lehtinen et al. (2005); (14) K. Brede et al. (2008), in…”
The earliest phases of high-mass star formation: a 3 square degree millimeter continuum mapping of Cygnus X (2007, background): “11 of Kirk et al. 2005 and references therein).”