arXiv subjects

J. M. Kirk

Publications and source records attributed to J. M. Kirk.

40 records · Page 3Linked to original sources

arXiv2003

SCUBA Polarization Measurements of the Magnetic Field Strengths in the L183, L1544, and L43 Prestellar Cores

We have mapped linearly polarized dust emission from L183 with the JCMT SCUBA polarimeter and have analyzed these and our previously published data for the prestellar cores L183, L1544, and L43 in order to estimate magnetic field strengths in the plane of the sky, $B_{pos}$. The analysis used the Chandrasekhar-Fermi technique, which relates the dispersion in polarization position angles to $B_{pos}$. We have used these estimates of the field strengths (neglecting the unmeasured line-of-sight component) to find the mass-to-magnetic flux ratios $λ$ (in units of the critical ratio for magnetic support). Results are $B_{pos} \approx 80$ $μ$G and $λ\approx 2.6$ for L183, $B_{pos} \approx 140$ $μ$G and $λ\approx 2.3$ for L1544, and $B_{pos} \approx 160$ $μ$G and $λ\approx 1.9$ for L43. Hence, without correction for geometrical biases, for all three cores the mass-to-flux ratios are supercritical by a factor of $\sim 2$, and magnetic support cannot prevent collapse. However, a statistical mean correction for geometrical bias may be up to a factor of three; this correction would reduce the individual $λ$'s to $λ_{cor} \approx 0.9$, 0.8, and 0.6, respectively; these values are approximately critical or slightly subcritical. These data are consistent with models of star formation driven by ambipolar diffusion in a weakly turbulent medium, but cannot rule out models of star formation driven by turbulence.

astro-ph↗

arXiv2003

The Structure of Cold Molecular Cloud Cores

A brief summary is presented of our current knowledge of the structure of cold molecular cloud cores that do not contain protostars, sometimes known as starless cores. The most centrally condensed starless cores are known as pre-stellar cores. These cores probably represent observationally the initial conditions for protostellar collapse that must be input into all models of star formation. The current debate over the nature of core density profiles is summarised. A cautionary note is sounded over the use of such profiles to ascertain the equilibrium status of cores. The magnetic field structure of pre-stellar cores is also discussed.

astro-ph↗

arXiv2001

The initial conditions of isolated star formation. V: ISOPHOT imaging and the temperature and energy balance of pre-stellar cores

ISO data taken with the long-wavelength imaging photo-polarimeter ISOPHOT are presented of 18 pre-stellar cores at three far-infrared wavelengths - 90, 170 and 200 microns. Most of the cores are detected clearly at 170 and 200 but only one is detected strongly at 90 microns, indicating that mostly they are very cold, with typical temperatures of only 10-20K. Colour temperature images are constructed for each of the cores. Most of the cores are seen to be either isothermal, or to have associated temperature gradients from the core centres to their edges, with all except one being cooler at the centre. We compare the data with previous ISOCAM absorption data and calculate the energy balance for those cores in common between the two samples. We find that the energy radiated by each core in the far-IR is similar to that absorbed at shorter wavelengths. Hence there is no evidence for a central heating source in any of the cores - even those for which previous evidence for core contraction exists. This is all consistent with external heating of the cores by the local interstellar radiation field, confirming their pre-stellar nature.

astro-ph↗

arXiv2000

First Observations of the Magnetic Field Geometry in Pre-stellar Cores

We present the first published maps of magnetic fields in pre-stellar cores, to test theoretical ideas about the way in which the magnetic field geometry affects the star formation process. The observations are JCMT-SCUBA maps of 850 micron thermal emission from dust. Linear polarizations at typically ten or more independent positions in each of three objects, L1544, L183 and L43 were measured, and the geometries of the magnetic fields in the plane of the sky were mapped from the polarization directions. The observed polarizations in all three objects appear smooth and fairly uniform. In L1544 and L183 the mean magnetic fields are at an angle of around 30 degrees to the minor axes of the cores. The L43 B-field appears to have been influenced in its southern half, such that it is parallel to the wall of a cavity produced by a CO outflow from a nearby T Tauri star, whilst in the northern half the field appears less disturbed and has an angle of 44 degrees to the core minor axis. We briefly compare our results with published models of magnetized cloud cores and conclude that no current model can explain these observations simultaneously with previous ISOCAM data.

astro-ph↗