Adriane Liermann

The Galactic WN stars : Spectral analyses with line-blanketed model atmospheres versus stellar evolution models with and without rotation

An infrared light-receiving device includes an optical absorption layer disposed on a principal surface of a substrate and an optical filter disposed on the optical absorption layer, the optical filter including first, second, and third semiconductor regions that are arranged in that order in a direction from the optical absorption layer to the optical filter, each of the first, second, and third semiconductor regions including an n-type InGaAs layer. The optical absorption layer includes a type-II superlattice structure. The first semiconductor region contains an n-type impurity with a concentration of 2.0×1019 cm−3 or more. The third semiconductor region contains an n-type impurity with a concentration of 3.0×1018 cm−3 or less and 8.0×1017 cm−3 or more. The second semiconductor region contains an n-type impurity with a concentration between the impurity concentration of the first semiconductor region and the impurity concentration of the third semiconductor region.

The Wolf-Rayet stars in the Large Magellanic Cloud - A comprehensive analysis of the WN class

Context. Massive stars, although being important building blocks of galaxies, are still not fully understood. This especially holds true for Wolf-Rayet (WR) stars with their strong mass loss, whose spectral analysis requires adequate model atmospheres. Aims. Following our comprehensive studies of the WR stars in the Milky Way, we now present spectroscopic analyses of almost all known WN stars in the LMC. Methods. For the quantitative analysis of the wind-dominated emission-line spectra, we employ the Potsdam Wolf-Rayet (PoWR) model atmosphere code. By fitting synthetic spectra to the observed spectral energy distribution and the available spectra (ultraviolet and optical), we obtain the physical properties of 107 stars. Results. We present the fundamental stellar and wind parameters for an almost complete sample of WN stars in the LMC. Among those stars that are putatively single, two different groups can be clearly distinguished. While 12% of our sample are more luminous than 10(6) L-circle dot and contain a significant amount of hydrogen, 88% of the WN stars, with little or no hydrogen, populate the luminosity range between log (L/L-circle dot) = 5.3 ... 5.8. Conclusions. While the few extremely luminous stars (log (L/L-circle dot) > 6), if indeed single stars, descended directly from the main sequence at very high initial masses, the bulk of WN stars have gone through the red-supergiant phase. According to their luminosities in the range of log (L/L-circle dot) = 5.3 ... 5.8, these stars originate from initial masses between 20 and 40 M-circle dot. This mass range is similar to the one found in the Galaxy, i.e. the expected metallicity dependence of the evolution is not seen. Current stellar evolution tracks, even when accounting for rotationally induced mixing, still partly fail to reproduce the observed ranges of luminosities and initial masses. Moreover, stellar radii are generally larger and effective temperatures correspondingly lower than predicted from stellar evolution models, probably due to subphotospheric inflation.

The Quintuplet cluster I. A K -band spectral catalog of stellar sources ,,

Context. Three very massive clusters are known to reside in the Galactic center region, the Arches cluster, the Quintuplet cluster, and the central parsec cluster, each of them rich in young hot stars. With new infrared instruments, this region is no longer obscured for the observer. Aims. For understanding these very massive clusters, it is essential to know their stellar inventory. We provide comprehensive spectroscopic data for the stellar population of the Quintuplet cluster that will form the basis of subsequent spectral analyses. Methods. Spectroscopic observations of the Quintuplet cluster were obtained with the Integral Field Spectrograph SINFONI-SPIFFI at the ESO-VLT. The inner part of the Quintuplet cluster was covered by 22 slightly overlapping fields, each of them of 8 �� × 8 �� in size. The spectral range comprises the near-IR K-band from 1.94 to 2.45 μm. The 3D data cubes of the individual fields were flux-calibrated and combined to one contiguous cube, from which the spectra of all detectable point sources were extracted. Results. We present a catalog of 160 stellar sources in the inner part of the Quintuplet cluster. The flux-calibrated K-band spectra of 98 early-type stars and 62 late-type stars are provided as Online Material. Based on these spectra, we assign spectral types to all detected sources and derive synthetic Ks-band magnitudes. Our sample is complete to about the 13th K-magnitude. We report the detection of two hitherto unknown Wolf-Rayet stars of late WC type (WC9 or later). Radial velocities are measured and employed to assess the cluster membership. The quantitative analysis of the early-type spectra will be the subject of a subsequent paper.

The Quintuplet cluster II. Analysis of the WN stars

Based on K-band integral-field spectroscopy, we analyze four Wolf-Rayet stars of the nitrogen sequence (WN) found in the inner part of the Quintuplet cluster. All WN stars (WR 102d, WR 102i, WR 102hb, and WR 102ea) are of spectral subtype WN9h. One further star, LHO 110, is included in the analysis which has been classified as Of/WN? previously but turns out to be most likely a WN9h star as well. The Potsdam Wolf-Rayet (PoWR) models for expanding atmospheres are used to derive the fundamental stellar and wind parameters. The stars turn out to be very luminous, log (L/L� ) > 6.0, with relatively low stellar temperatures, T∗ ≈ 25−35 kK. Their stellar winds contain a significant fraction of hydrogen, up to XH ∼ 0.45 (by mass). We discuss the position of the Galactic center WN stars in the Hertzsprung-Russell diagram and find that they form a distinct group. In this respect, the Quintuplet WN stars are similar to late-type WN stars found in the Arches cluster and elsewhere in the Galaxy. Comparison with stellar evolutionary models reveals that the Quintuplet WN stars should have been initially more massive than 60 M� . They are about 2.4−3.6 million years old, and might still be central hydrogen burning objects. The analysis of the spectral energy distributions of the program stars results in a mean extinction of AK = 3.1 ± 0. 5m ag (AV = 27 ± 4 mag) towards the Quintuplet cluster.