TY - JOUR
T1 - H I-to-H2 Transition Layers in the Star-forming Region W43
AU - Bialy, Shmuel
AU - Bihr, Simon
AU - Beuther, Henrik
AU - Henning, Thomas
AU - Sternberg, Amiel
N1 - Publisher Copyright:
© 2017. The American Astronomical Society. All rights reserved..
PY - 2017/2/1
Y1 - 2017/2/1
N2 - The process of atomic-to-molecular (H i-to-H2) gas conversion is fundamental for molecular-cloud formation and star formation. 21 cm observations of the star-forming region W43 revealed extremely high H i column densities, of 120-180 , a factor of 10-20 larger than predicted by H i-to-H2 transition theories. We analyze the observed H i with a theoretical model of the H i-to-H2 transition, and show that the discrepancy between theory and observation cannot be explained by the intense radiation in W43, nor be explained by variations of the assumed volume density or H2 formation rate coefficient. We show that the large observed H i columns are naturally explained by several (9-22) H i-to-H2 transition layers, superimposed along the sightlines of W43. We discuss other possible interpretations such as a non-steady-state scenario and inefficient dust absorption. The case of W43 suggests that H i thresholds reported in extragalactic observations are probably not associated with a single H i-to-H2 transition, but are rather a result of several transition layers (clouds) along the sightlines, beam-diluted with diffuse intercloud gas.
AB - The process of atomic-to-molecular (H i-to-H2) gas conversion is fundamental for molecular-cloud formation and star formation. 21 cm observations of the star-forming region W43 revealed extremely high H i column densities, of 120-180 , a factor of 10-20 larger than predicted by H i-to-H2 transition theories. We analyze the observed H i with a theoretical model of the H i-to-H2 transition, and show that the discrepancy between theory and observation cannot be explained by the intense radiation in W43, nor be explained by variations of the assumed volume density or H2 formation rate coefficient. We show that the large observed H i columns are naturally explained by several (9-22) H i-to-H2 transition layers, superimposed along the sightlines of W43. We discuss other possible interpretations such as a non-steady-state scenario and inefficient dust absorption. The case of W43 suggests that H i thresholds reported in extragalactic observations are probably not associated with a single H i-to-H2 transition, but are rather a result of several transition layers (clouds) along the sightlines, beam-diluted with diffuse intercloud gas.
KW - ISM: clouds
KW - ISM: individual objects (W43)
KW - ISM: structure
KW - galaxies: star formation
KW - photondominated region (PDR)
UR - http://www.scopus.com/inward/record.url?scp=85012024874&partnerID=8YFLogxK
U2 - 10.3847/1538-4357/835/2/126
DO - 10.3847/1538-4357/835/2/126
M3 - מאמר
AN - SCOPUS:85012024874
VL - 835
JO - Astrophysical Journal
JF - Astrophysical Journal
SN - 0004-637X
IS - 2
M1 - 126
ER -