ATLASGAL-selected massive clumps in the inner Galaxy. VI. Kinetic temperature and spatial density measured with formaldehyde

Abstract

Context. Formaldehyde (H<SUB>2</SUB>CO) is a reliable tracer to accurately measure the physical parameters of dense gas in star-forming regions. Aim. We aim to determine directly the kinetic temperature and spatial density with formaldehyde for the 100 brightest ATLASGAL-selected clumps (the TOP100 sample) at 870 μm representing various evolutionary stages of high-mass star formation. <BR /> Methods: Ten transitions (J = 3-2 and 4-3) of ortho- and para-H<SUB>2</SUB>CO near 211, 218, 225, and 291 GHz were observed with the Atacama Pathfinder EXperiment (APEX) 12 m telescope. <BR /> Results: Using non-LTE models with RADEX, we derived the gas kinetic temperature and spatial density with the measured para-H<SUB>2</SUB>CO 3<SUB>21</SUB>-2<SUB>20</SUB>/3<SUB>03</SUB>-2<SUB>02</SUB>, 4<SUB>22</SUB>-3<SUB>21</SUB>/4<SUB>04</SUB>-3<SUB>03</SUB>, and 4<SUB>04</SUB>-3<SUB>03</SUB>/3<SUB>03</SUB>-2<SUB>02</SUB> ratios. The gas kinetic temperatures derived from the para-H<SUB>2</SUB>CO 3<SUB>21</SUB>-2<SUB>20</SUB>/3<SUB>03</SUB>-2<SUB>02</SUB> and 4<SUB>22</SUB>-3<SUB>21</SUB>/4<SUB>04</SUB>-3<SUB>03</SUB> line ratios are high, ranging from 43 to >300 K with an unweighted average of 91 ± 4 K. Deduced T<SUB>kin</SUB> values from the J = 3-2 and 4-3 transitions are similar. Spatial densities of the gas derived from the para-H<SUB>2</SUB>CO 4<SUB>04</SUB>-3<SUB>03</SUB>/3<SUB>03</SUB>-2<SUB>02</SUB> line ratios yield 0.6-8.3 × 10<SUP>6</SUP> cm<SUP>-3</SUP> with an unweighted average of 1.5 (±0.1) × 10<SUP>6</SUP> cm<SUP>-3</SUP>. A comparison of kinetic temperatures derived from para-H<SUB>2</SUB>CO, NH<SUB>3</SUB>, and dust emission indicates that para-H<SUB>2</SUB>CO traces a distinctly higher temperature than the NH<SUB>3</SUB> (2, 2)/(1, 1) transitions and the dust, tracing heated gas more directly associated with the star formation process. The H<SUB>2</SUB>CO line widths are found to be correlated with bolometric luminosity and increase with the evolutionary stage of the clumps, which suggests that higher luminosities tend to be associated with a more turbulent molecular medium. It seems that the spatial densities measured with H<SUB>2</SUB>CO do not vary significantly with the evolutionary stage of the clumps. However, averaged gas kinetic temperatures derived from H<SUB>2</SUB>CO increase with time through the evolution of the clumps. The high temperature of the gas traced by H<SUB>2</SUB>CO may be mainly caused by radiation from embedded young massive stars and the interaction of outflows with the ambient medium. For L<SUB>bol</SUB>/M<SUB>clump</SUB> ≳ 10 L<SUB>☉</SUB>/M<SUB>☉</SUB>, we find a rough correlation between gas kinetic temperature and this ratio, which is indicative of the evolutionary stage of the individual clumps. The strong relationship between H<SUB>2</SUB>CO line luminosities and clump masses is apparently linear during the late evolutionary stages of the clumps, indicating that L<SUB>H_2CO</SUB> does reliably trace the mass of warm dense molecular gas. In our massive clumps H<SUB>2</SUB>CO line luminosities are approximately linearly correlated with bolometric luminosities over about four orders of magnitude in L<SUB>bol</SUB>, which suggests that the mass of dense molecular gas traced by the H<SUB>2</SUB>CO line luminosity is well correlated with star formation. <P />Source and H<SUB>2</SUB>CO parameters (Tables A.1-A.7) are only available at the CDS via anonymous ftp to <A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A> (<A href="http://cdsarc.u-strasbg.fr">http://130.79.128.5</A>) or via <A href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/611/A6">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/611/A6