Very early stages of massive stars

Es ist das Ziel dieser Doktorarbeit, die Anfangsbedingungen sowie die frühen Phasen der Entstehung massereicher Sterne mittels geeigneter Beobachtungsstudien zu untersuchen. Hierfür haben wir zwei Objektklassen ausgewählt: Infrarot{Dunkelwolken (IRDCs) sowie ein nachfolgendes Stadium, welches durch die Existenz eines noch jungen, eingebetteten (Proto)- Sternhaufens gekennzeichnet ist. Als ersten Schritt haben wir die Massen und Säulendichten für eine neue Stichprobe von südlichen IRDCs bestimmt unter Verwendung von eigenen Kontinuumsemissionskarten im Millimeter{Bereich sowie von Extinktionsdaten, basierend auf archivierten Infrarot{Karten des Spitzer{Satelliten. Die abgeleiteten Parameter zeigen, daß diese IRDCs potentielle Kandidaten für die Geburtsplätze massereicher Sterne sind und nicht unbedingt nur Sterne niedriger und mittlerer Masse produzieren werden. Insbesondere der Vergleich unserer Resultate mit schon bestehenden Daten für molekulare Kerne niedriger Masse ohne Sternentstehungsaktivität zeigt, daß IRDCs einen Trend zu systematisch höheren Säulendichten zeigen. Die interpolierten zentralen Höchstwerte für die Säulendichten überschreiten auch den kritischen Schwellenwert, der von einigen theoretischen Studien als entscheidend fuer die Entstehung massereicher Sterne angesehen wird. Desweiteren setzen wir uns kritisch mit den Limitierungen der angewandten Emissions- und Extinktionsmethoden auseinander. Zweitens haben wir die physikalischen Bedingungen sowie die chemische Zusammensetzung des mit den IRDCs assoziierten dichten Gases unter Verwendung von Millimeter{Spektroskopie für einer Reihe von Moleküllinien analysiert. Alle Wolken zeigen komplexe HCO+ Linienpro- le, und in einigen IRDCs nden wir SiO{Emission. Diese Befunde weisen auf Einfall- und Ausflus-{Bewegungen zumindest in einigen Substrukturen der IRDCs hin und somit auf schon einsetzende Sternentstehungsaktivität. Im Vergleich mit Kernen niedriger Masse besitzen die IRDCs stärkere und vor allem deutlich breitere Linien, was auf einen erhöhten Grad an Turbulenz hinweisen kann. Trotzdem zeigt unsere Analyse, daß die chemischen Häu gkeiten diverser Moleküle in den IRDCs den entsprechenden Häu gkeiten in sehr jungen massearmen Molekülwolkenkernen sehr ähnlich sind. Dies deutet auf ähnliche chemische Anfangsbedingungen für massearme und massereiche Sternentstehungsregionen hin. Schließlich haben wir die stellare Zusammensetzung und die Umgebungsbedingungen in dem jungen eingebetteten Sternhaufen IRAS 06058+2138 untersucht. Grundlage ist die Analyse aller verfügbaren Beobachtungsdaten, welche einen großen Wellenlngenbereich, vom nahen und mittleren Infrarot bis hin zum Millimeter- und Zentimeterbereich, abdecken. Neue 3D{Infrarotspektroskopie{Daten mit dem VLT{Instrument SINFONI nahmen hierbei eine Schlüsselstellung ein. Wir können den Sternhaufen in drei Teilregionen unterteilen, die sich in unterschiedlichen Stadien der Sternentstehung be nden. Aufgrund des Vergleiches der Alter sowie der relativen Lage dieser Untergebiete folgern wir, daß die Bescha enheit dieser Region nicht durch einen Akt der sogannten getriggerten" Sternentstehung gesteuert worden ist. Deshalb nehmen wir an, daß es sich um einen Fall von rein sequentieller Sternentstehung handelt. Mit dieser Arbeit untermauern wir auch die Wichtigkeit eines multispektralen Ansatzes für solche Beobachtungsstudien. Hiermit können sowohl die stellare Population als auch noch sehr tief eingebettete Objekte simultan untersucht werden, was eine zuverlässigere Rekonstruktion der Sternentstehungsabfolge für solche komplexen Sternhaufen ermöglicht

ALMA Resolves 30 Doradus: Sub-parsec Molecular Cloud Structure Near the Closest Super-Star Cluster

We present ALMA observations of 30 Doradus -- the highest resolution view of molecular gas in an extragalactic star formation region to date (~0.4pc x 0.6pc). The 30Dor-10 cloud north of R136 was mapped in 12CO 2-1, 13CO 2-1, C18O 2-1, 1.3mm continuum, the H30alpha recombination line, and two H2CO 3-2 transitions. Most 12CO emission is associated with small filaments and clumps (<1pc, ~1000 Msun at the current resolution). Some clumps are associated with protostars, including "pillars of creation" photoablated by intense radiation from R136. Emission from molecular clouds is often analyzed by decomposition into approximately beam-sized clumps. Such clumps in 30 Doradus follow similar trends in size, linewidth, and surface density to Milky Way clumps. The 30 Doradus clumps have somewhat larger linewidths for a given size than predicted by Larson's scaling relation, consistent with pressure confinement. They extend to higher surface density at a given size and linewidth compared to clouds studied at 10pc resolution. These trends are also true of clumps in Galactic infrared-dark clouds; higher resolution observations of both environments are required. Consistency of clump masses calculated from dust continuum, CO, and the virial theorem reveals that the CO abundance in 30 Doradus clumps is not significantly different from the LMC mean, but the dust abundance may be reduced by ~2. There are no strong trends in clump properties with distance from R136; dense clumps are not strongly affected by the external radiation field, but there is a modest trend towards lower dense clump filling fraction deeper in the cloud.Comment: accepted to Ap

G048.66-0.29: physical state of an isolated site of massive star formation

We present continuum observations of the infrared dark cloud (IRDC) G48.66-0.22 (G48) obtained with Herschel, Spitzer, and APEX, in addition to several molecular line observations. The Herschel maps are used to derive temperature and column density maps of G48 using a model based on a modified blackbody. We find that G48 has a relatively simple structure and is relatively isolated; thus, this IRDC provides an excellent target to study the collapse and fragmentation of a filamentary structure in the absence of complicating factors such as strong external feedback. The derived temperature structure of G48 is clearly non-isothermal from cloud to core scale. The column density peaks are spatially coincident with the lowest temperatures (~17.5 K) in G48. A total cloud mass of ~390 M ⊙ is derived from the column density maps. By comparing the luminosity-to-mass ratio of 13 point sources detected in the Herschel/PACS bands to evolutionary models, we find that two cores are likely to evolve into high-mass stars (M sstarf >= 8 M ⊙). The derived mean projected separation of point sources is smaller than in other IRDCs but in good agreement with theoretical predications for cylindrical collapse. We detect several molecular species such as CO, HCO+, HCN, HNC, and N2H+. CO is depleted by a factor of ~3.5 compared to the expected interstellar abundance, from which we conclude that CO freezes out in the central region. Furthermore, the molecular clumps, associated with the submillimeter peaks in G48, appear to be gravitationally unbound or just pressure confined. The analysis of critical line masses in G48 shows that the entire filament is collapsing, overcoming any internal support

Herschel observations of EXtraordinary Sources: Analysis of the full Herschel/HIFI molecular line survey of Sagittarius B2(N)

A sensitive broadband molecular line survey of the Sagittarius B2(N) star-forming region has been obtained with the HIFI instrument on the Herschel Space Observatory, offering the first high-spectral resolution look at this well-studied source in a wavelength region largely inaccessible from the ground (625-157 um). From the roughly 8,000 spectral features in the survey, a total of 72 isotopologues arising from 44 different molecules have been identified, ranging from light hydrides to complex organics, and arising from a variety of environments from cold and diffuse to hot and dense gas. We present an LTE model to the spectral signatures of each molecule, constraining the source sizes for hot core species with complementary SMA interferometric observations, and assuming that molecules with related functional group composition are cospatial. For each molecule, a single model is given to fit all of the emission and absorption features of that species across the entire 480-1910 GHz spectral range, accounting for multiple temperature and velocity components when needed to describe the spectrum. As with other HIFI surveys toward massive star forming regions, methanol is found to contribute more integrated line intensity to the spectrum than any other species. We discuss the molecular abundances derived for the hot core, where the local thermodynamic equilibrium approximation is generally found to describe the spectrum well, in comparison to abundances derived for the same molecules in the Orion KL region from a similar HIFI survey.Comment: Accepted to ApJ. 64 pages, 14 figures. Truncated abstrac

The Earliest Phases of Star Formation (EPoS): A Herschel Key Program - The precursors to high-mass stars and clusters

(Abridged) We present an overview of the sample of high-mass star and cluster forming regions observed as part of the Earliest Phases of Star Formation (EPoS) Herschel Guaranteed Time Key Program. A sample of 45 infrared-dark clouds (IRDCs) were mapped at PACS 70, 100, and 160 micron and SPIRE 250, 350, and 500 micron. In this paper, we characterize a population of cores which appear in the PACS bands and place them into context with their host cloud and investigate their evolutionary stage. We construct spectral energy distributions (SEDs) of 496 cores which appear in all PACS bands, 34% of which lack counterparts at 24 micron. From single-temperature modified blackbody fits of the SEDs, we derive the temperature, luminosity, and mass of each core. These properties predominantly reflect the conditions in the cold, outer regions. Taking into account optical depth effects and performing simple radiative transfer models, we explore the origin of emission at PACS wavelengths. The core population has a median temperature of 20K and has masses and luminosities that span four to five orders of magnitude. Cores with a counterpart at 24 micron are warmer and bluer on average than cores without a 24 micron counterpart. We conclude that cores bright at 24 micron are on average more advanced in their evolution, where a central protostar(s) have heated the outer bulk of the core, than 24 micron-dark cores. The 24 micron emission itself can arise in instances where our line of sight aligns with an exposed part of the warm inner core. About 10% of the total cloud mass is found in a given cloud's core population. We uncover over 300 further candidate cores which are dark until 100 micron. These are candidate starless objects, and further observations will help us determine the nature of these very cold cores.Comment: Accepted for publication in A&A, 81 pages, 68 figures. For full resolution image gallery (Appendix B), see http://www.mpia.de/~ragan/epos.htm

CHEMICAL EVOLUTION IN HIGH-MASS STAR-FORMING REGIONS: RESULTS FROM THE MALT90 SURVEY

The chemical changes of high-mass star-forming regions provide a potential method for classifying their evolutionary stages and, ultimately, ages. In this study, we search for correlations between molecular abundances and the evolutionary stages of dense molecular clumps associated with high-mass star formation. We use the molecular line maps from Year 1 of the Millimetre Astronomy Legacy Team 90 GHz (MALT90) Survey. The survey mapped several hundred individual star-forming clumps chosen from the ATLASGAL survey to span the complete range of evolution, from prestellar to protostellar to H II regions. The evolutionary stage of each clump is classified using the Spitzer GLIMPSE/MIPSGAL mid-IR surveys. Where possible, we determine the dust temperatures and H2 column densities for each clump from Herschel Hi-GAL continuum data. From MALT90 data, we measure the integrated intensities of the N2H+, HCO+, HCN and HNC (1-0) lines, and derive the column densities and abundances of N2H+ and HCO+. The Herschel dust temperatures increase as a function of the IR-based Spitzer evolutionary classification scheme, with the youngest clumps being the coldest, which gives confidence that this classification method provides a reliable way to assign evolutionary stages to clumps. Both N2H+ and HCO+ abundances increase as a function of evolutionary stage, whereas the N2H+ (1-0) to HCO+ (1-0) integrated intensity ratios show no discernable trend. The HCN (1-0) to HNC(1-0) integrated intensity ratios show marginal evidence of an increase as the clumps evolve

Radiative transfer simulations of infrared dark clouds

The determination of prestellar core structure is often based on observations of (sub)millimeter dust continuum. However, recently the Spitzer Space Telescope provided us with IR images of many objects not only in emission but also in absorption. We developed a technique to reconstruct the density and temperature distributions of protostellar objects based on radiation transfer (RT) simulations both in mm and IR wavelengths. Best-fit model parameters are obtained with the genetic algorithm. We apply the method to two cores of Infrared Dark Clouds and show that their observations are better reproduced by a model with an embedded heating source despite the lack of 70 μm emission in one of these cores. Thus, the starless nature of massive cores can only be established with the careful case-by-case RT modeling