TY - JOUR A1 - Sun, Ning-Chen A1 - de Grijs, Richard A1 - Subramanian, Smitha A1 - Cioni, Maria-Rosa L. A1 - Rubele, Stefano A1 - Bekki, Kenji A1 - Ivanov, Valentin D. A1 - Piatti, Andrés E. A1 - Ripepi, Vincenzo T1 - The VMC Survey. XXII. Hierarchical star formation in the 30 Doradus-N158-N159-N160 star-forming complex JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We study the hierarchical stellar structures in a similar to 1.5 deg(2) area covering the 30. Doradus-N158-N159-N160 starforming complex with the VISTA Survey of. Magellanic Clouds. Based on the young upper main-sequence stars, we find that the surface densities cover a wide range of values, from log(Sigma.pc(2))less than or similar to -2.0 to log(Sigma. pc(2)) greater than or similar to 0.0. Their distributions are highly non-uniform, showing groups that frequently have subgroups inside. The sizes of the stellar groups do not exhibit characteristic values, and range continuously from several parsecs to more than 100. pc; the cumulative size distribution can be well described by a single power law, with the power-law index indicating a projected fractal dimension D-2 = 1.6 +/- 0.3. We suggest that the phenomena revealed here support a scenario of hierarchical star formation. Comparisons with other star-forming regions and galaxies are also discussed. KW - galaxies: clusters: general KW - infrared: stars KW - Magellanic Clouds KW - stars: formation Y1 - 2017 U6 - https://doi.org/10.3847/1538-4357/835/2/171 SN - 0004-637X SN - 1538-4357 VL - 835 IS - 2 PB - Institute of Physics Publ. CY - Bristol ER - TY - JOUR A1 - Sun, Ning-Chen A1 - de Grijs, Richard A1 - Subramanian, Smitha A1 - Bekki, Kenji A1 - Bell, Cameron P. M. A1 - Cioni, Maria-Rosa L. A1 - Ivanov, Valentin D. A1 - Marconi, Marcella A1 - Oliveira, Joana M. A1 - Piatti, Andres E. A1 - Ripepi, Vincenzo A1 - Rubele, Stefano A1 - Tatton, Ben L. A1 - van Loon, Jacco Th. T1 - The VMC Survey. XXII. Hierarchical Star Formation in the 30 Doradus-N158–N159–N160 Star-forming Complex JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - Star formation is a hierarchical process, forming young stellar structures of star clusters, associations, and complexes over a wide range of scales. The star-forming complex in the bar region of the Large Magellanic Cloud is investigated with upper main-sequence stars observed by the VISTA Survey of the Magellanic Clouds. The upper main-sequence stars exhibit highly nonuniform distributions. Young stellar structures inside the complex are identified from the stellar density map as density enhancements of different significance levels. We find that these structures are hierarchically organized such that larger, lower-density structures contain one or several smaller, higher-density ones. They follow power-law size and mass distributions, as well as a lognormal surface density distribution. All these results support a scenario of hierarchical star formation regulated by turbulence. The temporal evolution of young stellar structures is explored by using subsamples of upper main-sequence stars with different magnitude and age ranges. While the youngest subsample, with a median age of log(tau/yr) = 7.2, contains the most substructure, progressively older ones are less and less substructured. The oldest subsample, with a median age of log(tau/yr) = 8.0, is almost indistinguishable from a uniform distribution on spatial scales of 30-300. pc, suggesting that the young stellar structures are completely dispersed on a timescale of similar to 100. Myr. These results are consistent with the characteristics of the 30. Doradus complex and the entire Large Magellanic Cloud, suggesting no significant environmental effects. We further point out that the fractal dimension may be method dependent for stellar samples with significant age spreads. KW - infrared: stars KW - Magellanic Clouds KW - stars: formation Y1 - 2017 U6 - https://doi.org/10.3847/1538-4357/aa911e SN - 0004-637X SN - 1538-4357 VL - 849 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Steinke, Martin A1 - Oskinova, Lida A1 - Hamann, Wolf-Rainer A1 - Sander, Andreas Alexander Christoph A1 - Liermann, A. A1 - Todt, Helge Tobias T1 - Analysis of the WN star WR102c, its WR nebula, and the associated cluster of massive stars in the Sickle Nebula JF - Tectonophysics : international journal of geotectonics and the geology and physics of the interior of the earth N2 - Context. The massive Wolf-Rayet type star WR102c is located near the Quintuplet Cluster, one of the three massive star clusters in the Galactic centre region. Previous studies indicated that WR102c may have a dusty circumstellar nebula and is among the main ionising sources of the Sickle Nebula associated with the Quintuplet Cluster. Aims. The goals of our study are to derive the stellar parameters of WR102c from the analysis of its spectrum and to investigate its stellar and nebular environment. Methods. We obtained observations with the ESO VLT integral field spectrograph SINFONI in the K-band, extracted the stellar spectra, and analysed them by means of stellar atmosphere models. Results. Our new analysis supersedes the results previously reported for WR102c. We significantly decrease its bolometric luminosity and hydrogen content. We detect four early OB type stars close to WR102c. These stars have radial velocities similar to that of WR102c. We suggest that together with WR102c these stars belong to a distinct star cluster with a total mass of similar to 1000 M-circle dot. We identify a new WR nebula around WR102c in the SINFONI map of the di ff use Br gamma emission and in the HST Pa ff images. The Br gamma line at di ff erent locations is not significantly broadened and similar to the width of nebular emission elsewhere in the H i i region around WR102c. Conclusions. The massive star WR102c located in the Galactic centre region resides in a star cluster containing additional early-type stars. The stellar parameters of WR102c are typical for hydrogen-free WN6 stars. We identify a nebula surrounding WR102c that has a morphology similar to other nebulae around hydrogen-free WR stars, and propose that the formation of this nebula is linked to interaction of the fast stellar wind with the matter ejected at a previous evolutionary stage of WR102c. KW - stars: early-type KW - stars: individual: WR 102c KW - stars: Wolf KW - Rayet KW - Galaxy: center KW - HII regions KW - infrared: stars Y1 - 2016 U6 - https://doi.org/10.1051/0004-6361/201527692 SN - 1432-0746 VL - 588 PB - EDP Sciences CY - Les Ulis ER - TY - GEN A1 - Martins, Fabrice A1 - Bergemann, Maria A1 - Bestenlehner, Joachim M. A1 - Crowther, Paul A. A1 - Hamann, Wolf-Rainer A1 - Najarro, Francisco A1 - Nieva, Maria Fernanda A1 - Przybilla, Norbert A1 - Freimanis, Juris A1 - Hou, Weizhen A1 - Kaper, Lex T1 - SpS5 - II. Stellar and wind parameters T2 - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - The development of infrared observational facilities has revealed a number of massive stars in obscured environments throughout the Milky Way and beyond. The determination of their stellar and wind properties from infrared diagnostics is thus required to take full advantage of the wealth of observations available in the near and mid infrared. However, the task is challenging. This session addressed some of the problems encountered and showed the limitations and successes of infrared studies of massive stars. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 688 KW - infrared: stars KW - stars: early-type KW - stars: late-type KW - stars: fundamental parameters KW - stars: mass loss KW - stars: abundances Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-414956 SN - 1866-8372 IS - 688 ER - TY - GEN A1 - Liermann, Angelika A1 - Hamann, Wolf-Rainer A1 - Oskinova, Lida T1 - The quintuplet cluster III. Hertzsprung-Russell diagram and cluster age (vol 540, pg A14, 2012) T2 - Astronomy and astrophysics : an international weekly journal KW - open clusters and associations: individual: Quintuplet KW - infrared: stars KW - stars: early-type KW - stars: late-type KW - Hertzsprung-Russell and C-M diagrams KW - errata, addenda Y1 - 2014 U6 - https://doi.org/10.1051/0004-6361/201117534e SN - 0004-6361 SN - 1432-0746 VL - 563 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Liermann, A. A1 - Hamann, Wolf-Rainer A1 - Oskinova, Lida T1 - The Quintuplet cluster III. Hertzsprung-Russell diagram and cluster age JF - Astronomy and astrophysics : an international weekly journal N2 - The Quintuplet, one of three massive stellar clusters in the Galactic center (GC), is located about 30 pc in projection from Sagittarius A*. We aim at the construction of the Hertzsprung-Russell diagram (HRD) of the cluster to study its evolution and to constrain its star-formation history. For this purpose we use the most complete spectral catalog of the Quintuplet stars. Based on the K-band spectra we determine stellar temperatures and luminosities for all stars in the catalog under the assumption of a uniform reddening towards the cluster. We find two groups in the resulting HRD: early-type OB stars and late-type KM stars, well separated from each other. By comparison with Geneva stellar evolution models we derive initial masses exceeding 8 M-circle dot for the OB stars. In the HRD these stars are located along an isochrone corresponding to an age of about 4 Myr. This confirms previous considerations, where a similar age estimate was based on the presence of evolved Wolf-Rayet stars in the cluster. We derive number ratios for the various spectral subtype groups (e.g. N-WR/N-O, N-WC/N-WN) and compare them with predictions of population synthesis models. We find that an instantaneous burst of star formation at about 3.3 to 3.6 Myr ago is the most likely scenario to form the Quintuplet cluster. Furthermore, we apply a mass-luminosity relation to construct the initial mass function (IMF) of the cluster. We find indications for a slightly top-heavy IMF. The late-type stars in the LHO catalog are red giant branch (RGB) stars or red supergiants (RSGs) according to their spectral signatures. Under the assumption that they are located at about the distance of the Galactic center we can derive their luminosities. The comparison with stellar evolution models reveals that the initial masses of these stars are lower than 15 M-circle dot implying that they needed about 15 Myr (RSG) or even more than 30 Myr (RGB) to evolve into their present stage. It might be suspected that these late-type stars do not physically belong to the Quintuplet cluster. Indeed, most of them disqualify as cluster members because their radial velocities differ too much from the cluster average. Nevertheless, five of the brightest RGB/RSG stars from the LHO catalog share the mean radial velocity of the Quintuplet, and thus remain highly suspect for being gravitationally bound members. If so, this would challenge the cluster formation and evolution scenario. KW - stars: late-type KW - Hertzsprung-Russell and C-M diagrams KW - infrared: stars KW - stars: early-type KW - open cluster and associations: individual: Quintuplet Y1 - 2012 U6 - https://doi.org/10.1051/0004-6361/201117534 SN - 0004-6361 VL - 540 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Oskinova, Lida A1 - Steinke, M. A1 - Hamann, Wolf-Rainer A1 - Sander, A. A1 - Todt, Helge Tobias A1 - Liermann, Adriane T1 - One of the most massive stars in the Galaxy may have formed in isolation JF - Monthly notices of the Royal Astronomical Society N2 - Very massive stars, 100 times heavier than the sun, are rare. It is not yet known whether such stars can form in isolation or only in star clusters. The answer to this question is of fundamental importance. The central region of our Galaxy is ideal for investigating very massive stars and clusters located in the same environment. We used archival infrared images to investigate the surroundings of apparently isolated massive stars presently known in the Galactic Centre (GC). We find that two such isolated massive stars display bow shocks and hence may be 'runaways' from their birthplace. Thus, some isolated massive stars in the GC region might have been born in star clusters known in this region. However, no bow shock is detected around the isolated star WR 102ka (Peony nebula star), which is one of the most massive and luminous stars in the Galaxy. This star is located at the centre of an associated circumstellar nebula. To study whether a star cluster may be 'hidden' in the surroundings of WR 102ka, to obtain new and better spectra of this star, and to measure its radial velocity, we obtained observations with the integral-field spectrograph SINFONI at the ESO's Very Large Telescope. Our observations confirm that WR 102ka is one of the most massive stars in the Galaxy and reveal that this star is not associated with a star cluster. We suggest that WR 102ka has been born in relative isolation, outside of any massive star cluster. KW - stars: individual: WR 102ka KW - Galaxy: centre KW - infrared: stars Y1 - 2013 U6 - https://doi.org/10.1093/mnras/stt1817 SN - 0035-8711 SN - 1365-2966 VL - 436 IS - 4 SP - 3357 EP - 3365 PB - Oxford Univ. Press CY - Oxford ER -