NGC 346 in The Small Magellanic Cloud. IV. Triggered Star Formation in the HII Region N66

Stellar feedback, expanding HII regions, wind-blown bubbles, and supernovae are thought to be important triggering mechanisms of star formation. Stellar associations, being hosts of significant numbers of early-type stars, are the loci where these mechanisms act. In this part of our photometric study of the star-forming region NGC346/N66 in the Small Magellanic Cloud, we present evidence based on previous and recent detailed studies, that it hosts at least two different events of triggered star formation and we reveal the complexity of its recent star formation history. In our earlier studies of this region (Papers I, III) we find that besides the central part of N66, where the bright OB stellar content of the association NGC346 is concentrated, an arc-like nebular feature, north of the association, hosts recent star formation. This feature is characterized by a high concentration of emission-line stars and Young Stellar Objects, as well as embedded sources seen as IR-emission peaks that coincide with young compact clusters of low-mass pre-main sequence stars. All these objects indicate that the northern arc of N66 encompasses the most current star formation event in the region. We present evidence that this star formation is the product of a different mechanism than that in the general area of the association, and that it is triggered by a wind-driven expanding HII region (or bubble) blown by a massive supernova progenitor, and possibly other bright stars, a few Myr ago. We propose a scenario according to which this mechanism triggered star formation away from the bar of N66, while in the bar of N66 star formation is introduced by the photo-ionizing OB stars of the association itself.Comment: Astrophysical Journal, In Press. 10 pages, 4 figures, emulateapj LaTeX style. Figures with Scaled-down resolution. Related Press Releases: http://www.nasa.gov/mission_pages/spitzer/news/spitzer-20081008.html and http://www.eso.org/public/outreach/press-rel/pr-2008/pr-34-08.htm

NGC 346 in the Small Magellanic Cloud. III. Recent Star Formation and Stellar Clustering Properties in the Bright HII Region N 66

In the third part of our photometric study of the star-forming region NGC 346/N~66 and its surrounding field in the Small Magellanic Cloud (SMC), we focus on the large number of low-mass pre-main sequence (PMS) stars revealed by the Hubble Space Telescope Observations with the Advanced Camera for Surveys. We investigate the origin of the observed broadening of the pre-main sequence population in the $V-I$, $V$ CMD. The most likely explanations are either the presence of differential reddening or an age spread among the young stars. Assuming the latter, simulations indicate that we cannot exclude the possibility that stars in NGC 346 might have formed in two distinct events occurring about 10 and 5 Myr ago, respectively. We find that the PMS stars are not homogeneously distributed across NGC 346, but instead are grouped in at least five different clusters. On spatial scales from 0.8$''$ to 8$''$ (0.24 to 2.4 pc at the distance of the SMC) the clustering of the PMS stars as computed by a two-point angular correlation function is self-similar with a power law slope $\gamma \approx -0.3$. The clustering properties are quite similar to Milky Way star forming regions like Orion OB or $\rho$ Oph. Thus molecular cloud fragmentation in the SMC seems to proceed on the same spatial scales as in the Milky Way. This is remarkable given the differences in metallicity and hence dust content between SMC and Milky Way star forming regions.Comment: Accepted for publication in ApJ. 16 pages, 13 (low-resolution) figures, emulateapj.cls LaTeX styl

Measurement of charged particle spectra in deep-inelastic ep scattering at HERA

Charged particle production in deep-inelastic ep scattering is measured with the H1 detector at HERA. The kinematic range of the analysis covers low photon virtualities, 5 LT Q(2) LT 100 GeV2, and small values of Bjorken-x, 10(-4) LT x LT 10(-2). The analysis is performed in the hadronic centre-of-mass system. The charged particle densities are measured as a function of pseudorapidity (n(*)) and transverse momentum (p(T)(*)) in the range 0 LT n(*) LT 5 and 0 LT p(T)(*) LT 10 GeV in bins of x and Q(2). The data are compared to predictions from different Monte Carlo generators implementing various options for hadronisation and parton evolutions

Simulation and Calibration of the Specific Energy Loss of the Central Jet Chambers of the H1 Detector and Measurement of the Inclusive D∗±D^{*\pm} Meson Cross Section in Photoproduction at HERA

In dieser Arbeit wird die Photoproduktion von D*pm–Mesonen in ep–Kollisionen bei HERA untersucht. Die D*–Mesonen werden anhand ihres ’goldenen’ Zerfalls D*pm->Kmppipmpi_spm mit dem H1–Detektor nachgewiesen. Gegenüber früheren Analysen konnte der systematische Fehler durch zwei wesentliche Verbesserungen verringert werden. Zum einen ermöglicht die Simulation des Fast Track Trigger, der auf Spuren in den zentralen Spurkammern basiert, eine korrekte Berücksichtigung der Abhängigkeiten der Triggereffizienz von unterschiedlichen kinematischen Grössen. Zum anderen erlaubt die Verwendung des spezifischen Energieverlusts eine bessere Unterdrückung von nicht–resonantem Untergrund. Um die Teilchenidentifikation anhand des spezifischen Energieverlustes in der Analyse verwenden zu können, wurde die Simulation des Energieverlustes in den Spurkammern des H1–Experiments verbessert und die notwendigen Korrekturfunktionen und Kalibrationen bestimmt. Mit dieser verbesserten H1– Detektorsimulation wird der Wirkungsquerschnitt der Photoproduktion von D* –Mesonen in den HERAII Daten bestimmt. Die Daten entsprechen einer integrierten Luminosität von 113 pb-1. Die Messung erfolgt im kinematischen Bereich von Q2 1.8 GeV und im Bereich zentraler Pseudorapiditäten |eta(D*) | < 1.5 werden bestimmt und perturbativen QCD Vorhersagen in führender und nächstführender Ordnung gegenübergestellt

Measurement of D∗±D^*\pm Meson Production and Determination of F2ccˉ\it F_{2}^{c\bar{c}} at low Q2\it Q^{2} in Deep-Inelastic Scattering at HERA

Inclusive production of D* mesons in deep-inelastic $\it ep$ scattering at HERA is studied in the range 5 < $Q^{2}$ < 100 $GeV^{2}$ of the photon virtuality and 0.02 < y < 0.7 of the inelasticity of the scattering process. The observed phase space for the D* meson is $p_{T}$ $(D_{*})$ > 1.25 GeV and $\mid\mu(D_{*})\mid$ < 1.8. The data sample corresponds to an integrated luminosity of 348 $pb^{−1}$ collected with the H1 detector. Single and double differential cross sections are measured and the charm contribution $F_{2}^{c\bar{c}}$ to the proton structure function $\it F_{2}$ is determined. The results are compared to perturbative QCD predictions at next-to-leading order implementing different schemes for the charm mass treatment and with Monte Carlo models based on leading order matrix elements with parton showers