Abstract
After studying how line width depends on spatial scale in low-mass star-forming regions, we propose that dense cores (Myers & Benson 1983) represent an inner scale of a self-similar process that characterizes larger scale molecular clouds. In the process of coming to this conclusion, we define four distinct types of line width-size relation (ΔvRai), which have power-law slopes a1, a2, a3, and a4, as follows: Type 1—multitracer, multicloud intercomparison; Type 2—single-tracer, multicloud intercomparison; Type 3—multitracer study of a single cloud; and Type 4—single-tracer study of a single cloud. Type 1 studies (of which Larson 1981 is the seminal example) are compendia of Type 3 studies which illustrate the range of variation in the line width-size relation from one region to another. Using new measurements of the OH and C18O emission emanating from the environs of several of the dense cores studied in NH3 by Barranco & Goodman (1998; Paper I), we show that line width increases with size outside the cores with a4 ~ 0.2. On scales larger than those traced by C18O or OH,12CO and 13CO observations indicate that a4 increases to ~0.5 (Heyer & Schloerb 1997). By contrast, within the half-power contour of the NH3 emission from the cores, line width is virtually constant, with a4 ~ 0. We interpret the correlation between increasing density and decreasing Type 4 power-law slope as a transition to coherence. Our data indicate that the radius Rcoh at which the gas becomes coherent (i.e., a4 → 0) is of order 0.1 pc in regions forming primarily low-mass stars. The value of the nonthermal line width at which coherence is established is always less than but still of order of the thermal line width of H2. Thus coherent cores are similar to, but not exactly the same as, isothermal balls of gas. Two other results bolster our proposal that a transition to coherence takes place at ~0.1 pc. First, the OH, C18O, and NH3 maps show that the dependence of column density on size is much steeper (N R-0.9) inside Rcoh than outside of it (N R-0.2), which implies that the volume filling factor of coherent cores is much larger than in their surroundings. Second, Larson (1995) has recently found a break in the power law characterizing the clustering of stars in Taurus at 0.04 pc, just inside of Rcoh. Larson and we interpret this break in slope as the point at which stellar clustering properties change from being determined by the (fractal) gas distribution (on scales greater than 0.04 pc) to being determined by fragmentation processes within coherent cores (on scales less than 0.04 pc). We speculate that the transition to coherence takes place when a dissipation threshold for the MHD turbulence that characterizes the larger scale medium is crossed at the critical inner scale Rcoh. We suggest that the most likely explanation for this threshold is the marked decline in the coupling of the magnetic field to gas motions due to a decreased ion/neutral ratio in dense, high filling factor gas.
Highlights
With C18O and OH observations Taken together, the two studies show that a line widthÈsize power-law scaling relation similar to the one discovered more than a decade ago by Larson (1981) appears to have a break in slope at a size scale roughly comparable to the full width at half maximum (FWHM) contour of an
In ° 4, we show that the qualitative tendency for line width to decrease with increasing antenna temperature apparent in the Ðgures can be transformed into a quantitative line widthÈsize relation
What is often called a ““ cloud ÏÏ consists of gas we label ““ cha† ÏÏ
Summary
IslaLnodws-omf acsaslm(Di1nÈa10mMor_e)tduerbnuselecnotrseesaa. pTpheeasretcoobreesr, ewlahtiicvhe are the birthplace of many of the stars in our Galaxy, have average density D104 cm~3 and size of approximately a few tenths of a parsec, and are characterized by very small internal velocity dispersions of approximately several tenths of a kilometer per second (see Myers & Benson 1983 ; Benson & Myers 1989). Barranco & Goodman (1998 ; hereafter, Paper Ith) ipsrpesaepnetrhpiguhts-stehnossietivoibtyseNrvHat3iomnaspipnitnogcoofndteexntsebycocroems,paanrd-. Larson (1981) examined the variations in line width within individual clouds To do this, he compared concentric spectral line maps of various density tracers for a number of regions. He compared concentric spectral line maps of various density tracers for a number of regions In so doing, he found a correlation similar to equation (1). Myers (1983) reformulated LarsonÏs Laws to take this e†ect into account He denoted the line width as the quadrature sum of a nonthermal and a thermal component,. This slope is always positive (so that line width increases with size), but in the thermal region, the observed line width is approximately constant (i.e., independent of scale). We suggest that the regions of nearly constant line width, while not purely thermal, are still physically distinguished from their surroundings in that they are ““ coherent,ÏÏ and we suggest ways in which these coherent cores might arise (° 5)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have