Ionized and Photodissociation Regions toward a New Sample of Massive Star-forming Clumps

Abstract

HII regions are an excellent indicator of high-mass star-forming regions and ongoing star formation in galaxies. Powerful outflows from O/B stars in their early stages and, during their whole life, intense UV radiation and stellar winds appreciably alter the chemical composition of the ISM in their vicinity and turn the surfaces of molecular clouds into photodissociation regions. To investigate the feedback of young high-mass stars, a comprehensive sample of regions is necessarily to cover different evolutionary stages and molecular environments. <br /> This thesis presents a study of HII regions and PDRs in a sample of 967 compact dust clumps selected from catalogs of ~10,000 sources identified by the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL). This sample includes star-forming and quiescent clumps to ensure a variety of different evolutionary stages. The primary goal of this thesis is to understand the physical properties and kinematics of HII regions and their influence on the natal dense dust clumps, especially in form of PDRs. The analysis of molecular line surveys conducted toward ATLASGAL dust clumps focuses, first, on radio recombination lines in HII regions and continues with an investigation of PDR tracers. <br /> In particular, millimeter radio recombination lines (mm-RRLs) observed with the IRAM 30m and Mopra 22m telescopes are analyzed. Several RRLs at 3 mm wavelength were covered with principal quantum number n ranging from 39 to 65 and n = 1, 2, 3, and 4 (Hn-alpha, Hn-beta, Hn-gamma, and Hn-delta). This is the largest published sample of mm-RRLs detections: Hn-alpha, Hn-beta , Hn-gamma, and Hn-delta mm-RRLs were detected toward 178, 65, 23, and 22 clumps, respectively. Compact radio continuum counterparts were identified for 134 of the clumps with RRL detections showing that the mm-RRLs are as- sociated with known compact and ultra-compact HII regions. Besides, eight detections represent potentially new HII regions found exclusively from this study. Based on the good agreement of the measured mm-RRL and molecular line velocities, associations of the young HII regions with dense molecular clumps were established. Furthermore, H¹³CO⁺linewidths from clumps with mm-RRLs are broader than in those without them. This indicates a physical connection between the embedded HII region and their natal environments. Nine clumps show broad RRL linewidths (> 40km/s^-1) that re- veal pronounced turbulent motions within the ionized gas. By using the mm-RRLs and radio continuum emission, the estimated electron densities and diameters of the HII regions are n_e <10⁵ cm³ and D > 0.03 pc, respectively. <br /> A dedicated RRL survey was observed with the APEX 12m telescope, covering a variety of (sub)millimeter radio recombination lines (submm-RRLs) with principal quantum numbers ranging from 25 to 30. RRLs observed at different wavelengths allow us to investigate the excitation conditions of ionized gas within HII regions. Comparing mm- and submm-RRLs shows that these RRLs are emitted in local thermodynamic equilibrium (LTE). The average ratio (0.31) of the measured H35 /H28α fluxes is close to the LTE value of 0.28. Furthermore, the submm-RRL luminosities show excellent correlations with the Lyman photon flux and bolometric luminosities of the embedded O/B stars. The HII regions are associated with the most massive and luminous clumps of the sample. The production rate of ionizing photons, Q, was estimated using the H28α line. The measured Q is consistent with the Lyman continuum photon flux determined by 5GHz radio continuum emission. This allows RRLs to be used to measure star formation rates in galaxies. Six RRL sources show a combination of a narrow and a broad Gaussian line feature that suggest a link to high-velocity ionized flows. <br /> In a study of the ISM surrounding HII regions, in 409 ATLASGAL clumps eight molecules (HCO, HOC⁺, C₂H, c-C₃H₂, CN, H¹³CN, HC¹⁵N, HN¹³C) as PDR tracers are investigated plus C¹⁸O and H¹³CO⁺ molecular lines as reference molecules tracing dense molecular gas. Most molecules show detection rates of above 94% except for HCO, H¹³CN, and HC¹⁵N. These three molecules are mostly detected in HII regions. Interestingly, 40 clumps also show CN self-absorption profiles, indicating potential in- falling or expanding motions within the clumps. These lines are broader and brighter toward more evolved clumps. Also, abundances of HCO as well as HCO/H¹³CO⁺ column density ratios significantly drop with increasing H₂ column density. This shows that the HCO molecule is more abundant in PDRs illuminated by FUV radiation than inside molecular clouds. On the other hand, small hydrocarbons (c-C₃H₂ and C₂H) known as typical PDR tracers do not show significant abundance changes over a range of H₂ column density, while toward HII regions the column density ratio of the two molecules shows a moderate increase with dust temperature.