The Massive Star Population of Cygnus OB2 Nicholas J. Wright, Janet E. Drew, Michael Mohr-Smith Centre for Astrophysics Research, University of Hertfordshire We have compiled a significantly updated and comprehensive census of massive stars in the nearby Cygnus OB2 association by gathering and homogenising data from across the literature. The census contains 169 primary OB stars, including 52 O-type stars and 3 Wolf-Rayet stars. Spectral types and photometry are used to place the stars in a Hertzprung-Russell diagram, which is compared to both non-rotating and rotating stellar evolution models, from which stellar masses and ages are calculated. The star formation history and mass function of the association are assessed, and both are found to be heavily influenced by the evolution of the most massive stars to their end states. We find that the mass function of the most massive stars is consistent with a `universal' power-law slope of $Gamma = 1.3$. The age distribution inferred from stellar evolutionary models with rotation and the mass function suggest the majority of star formation occurred more or less continuously between 1 and 7~Myr ago, in agreement with studies of low- and intermediate mass stars in the association. We identify a nearby young pulsar and runaway O-type star that may have originated in Cyg~OB2 and suggest that the association has already seen its first supernova. Finally we use the census and mass function to calculate the total mass of the association of $16500^{+3800}_{-2800}$~M$_odot$, at the low end, but consistent with, previous estimates of the total mass of Cyg~OB2. Despite this Cyg~OB2 is still one of the most massive groups of young stars known in our Galaxy making it a prime target for studies of star formation on the largest scales. Reference: MNRAS, accepted Status: Manuscript has been accepted Weblink: http://arxiv.org/abs/1502.05718 Comments: Email: nick.nwright@gmail.com --- Submitted on Tue Mar 3 6:31:19 CST 2015------------------------- 2.5D global-disk oscillation models of the Be shell star ζ Tauri I. Spectroscopic and polarimetric analysis C. Escolano (1), A. C. Carciofi (1), A. T. Okazaki (2), T. Rivinius (3), D. Baade (4), and S. Štefl (5) (1) - Instituto de Astronomia, Geofísica e Ciencias Atmosféricas (São Paulo, Brazil); (2) - Hokkai-Gakuen University (Sapporo, Japan); (3) - ESO (Santiago, Chile); (4) - ESO (Garching, Germany); (5) - ALMA (Santiago, Chile) Context. A large number of Be stars exhibit intensity variations of their violet and red emission peaks in their HI lines observed in emission. This is the so-called V/R phenomenon, usually explained by the precession of a one-armed spiral density perturbation in the circumstellar disk. That global-disk oscillation scenario was confirmed, both observationally and theoretically, in the previous series of two papers analyzing the Be shell star ζ Tauri. The vertically averaged (2D) global-disk oscillation model used at the time was able to reproduce the V/R variations observed in Hα, as well as the spatially resolved interferometric data from AMBER/VLTI. Unfortunately, that model failed to reproduce the V/R phase of Br15 and the amplitude of the polarization variation, suggesting that the inner disk structure predicted by the model was incorrect. Aims. The first aim of the present paper is to quantify the temporal variations of the shell-line characteristics of ζ Tauri. The second aim is to better understand the physics underlying the V/R phenomenon by modeling the shell-line variations together with the V/R and polarimetric variations. The third aim is to test a new 2.5D disk oscillation model, which solves the set of equations that describe the 3D perturbed disk structure but keeps only the equatorial (i.e., 2D) component of the solution. This approximation was adopted to allow comparisons with the previous 2D model, and as a first step toward a future 3D model. Methods. We carried out an extensive analysis of ζ Tauri’s spectroscopic variations by measuring various quantities characterizing its Balmer line profiles: red and violet emission peak intensities (for Hα, Hβ, and Br15), depth and asymmetry of the shell absorption (for Hβ, Hγ, and Hδ), and the respective position (i.e., radial velocity) of each component. We attempted to model the observed variations by implementing in the radiative transfer code HDUST the perturbed disk structure computed with a recently developed 2.5D global-disk oscillation model. Results. The observational analysis indicates that the peak separation and the position of the shell absorption both exhibit variations following the V/R variations and, thus, may provide good diagnostic tools of the global-disk oscillation phenomenon. The shell absorption seems to become slightly shallower close to the V/R maximum, but the scarcity of the data does not allow the exact pattern to be identified. The asymmetry of the shell absorption does not seem to correlate with the V/R cycle; no significant variations of this parameter are observed, except during certain periods where Hα and Hβ exhibit perturbed emission profiles. The origin of these so-called triple-peak phases remains unknown. On the theoretical side, the new 2.5D formalism appears to improve the agreement with the observed V/R variations of Hα and Br15, under the proviso that a large value of the viscosity parameter, α = 0.8, be adopted. It remains challenging for the models to reproduce consistently the amplitude and the average level of the polarization data. The 2D formalism provides a better match to the peak separation, although the variation amplitude predicted by both the 2D and 2.5D models is smaller than the observed value. Shell-line variations are difficult for the models to reproduce, whatever formalism is adopted. Reference: A&A Status: Manuscript has been accepted Weblink: http://arxiv.org/abs/1503.00590 Comments: Email: cyril.escolano@gmail.com --- Submitted on Tue Mar 3 9:11:17 CST 2015------------------------- On the consistent treatment of the quasi-hydrostatic layers in hot star atmospheres Andreas Sander (1), Tomer Shenar (1), Rainer Hainich (1), Angel Gímenez-García (2), Helge Todt (1), Wolf-Rainer Hamann (1) (1) - Institut for Physics and Astronomy, University of Potsdam, Germany; (2) - Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, Spain CONTEXT: Spectroscopic analysis remains the most common method to derive masses of massive stars, the most fundamental stellar parameter. While binary orbits and stellar pulsations can provide much sharper constraints on the stellar mass, these methods are only rarely applicable to massive stars. Unfortunately, spectroscopic masses of massive stars heavily depend on the detailed physics of model atmospheres. AIMS: We demonstrate the impact of a consistent treatment of the radiative pressure on inferred gravities and spectroscopic masses of massive stars. Specifically, we investigate the contribution of line and continuum transitions to the photospheric radiative pressure. We further explore the effect of model parameters, e.g., abundances, on the deduced spectroscopic mass. Lastly, we compare our results with the plane-parallel TLUSTY code, commonly used for the analysis of massive stars with photospheric spectra. METHODS: We calculate a small set of O-star models with the Potsdam Wolf-Rayet (PoWR) code using different approaches for the quasi-hydrostatic part. These models allow us to quantify the effect of accounting for the radiative pressure consistently. We further use PoWR models to show how the Doppler widths of line profiles and abundances of elements such as iron affect the radiative pressure, and, as a consequence, the derived spectroscopic masses. RESULTS: Our study implies that errors on the order of a factor of two in the inferred spectroscopic mass are to be expected when neglecting the contribution of line and continuum transitions to the radiative acceleration in the photosphere. Usage of implausible microturbulent velocities, or the neglect of important opacity sources such as Fe, may result in errors of approximately 50% in the spectroscopic mass. A comparison with TLUSTY model atmospheres reveals a very good agreement with PoWR at the limit of low mass-loss rates. Reference: A&A, in press Status: Manuscript has been accepted Weblink: http://arxiv.org/abs/1503.01338 Comments: Email: ansander@astro.physik.uni-potsdam.de --- Submitted on Thu Mar 5 2:34:24 CST 2015------------------------- 3D Printing Meets Computational Astrophysics: Deciphering the Structure of Eta Carinae's Inner Colliding Winds Thomas I. Madura(1), Nicola Clementel(2), Theodore R. Gull(1), Chael J.H. Kruip(2), and Jan-Pieter Paardekooper(3,4) 1 - NASA Goddard Space Flight Center; 2 - Leiden Observatory; 3 - Universitat Heidelberg; 4 - Max Planck Institute for Extraterrestrial Physics We present the first 3D prints of output from a supercomputer simulation of a complex astrophysical system, the colliding stellar winds in the massive (>120 M_Sun), highly eccentric (e~0.9) binary star system Eta Carinae. We demonstrate the methodology used to incorporate 3D interactive figures into a PDF journal publication and the benefits of using 3D visualization and 3D printing as tools to analyze data from multidimensional numerical simulations. Using a consumer-grade 3D printer (MakerBot Replicator 2X), we successfully printed 3D smoothed particle hydrodynamics (SPH) simulations of Eta Carinae's inner (r ~ 110 au) wind-wind collision interface at multiple orbital phases. The 3D prints and visualizations reveal important, previously unknown 'finger-like' structures at orbital phases shortly after periastron (phi~1.045) that protrude radially outward from the spiral wind-wind collision region. We speculate that these fingers are related to instabilities (e.g. thin-shell, Rayleigh-Taylor) that arise at the interface between the radiatively-cooled layer of dense post-shock primary-star wind and the fast (3000 km/s), adiabatic post-shock companion-star wind. The success of our work and easy identification of previously unrecognized physical features highlight the important role 3D printing and interactive graphics can play in the visualization and understanding of complex 3D time-dependent numerical simulations of astrophysical phenomena. Reference: For publication in MNRAS Status: Manuscript has been accepted Weblink: http://arxiv.org/abs/1503.00716 Comments: To view 3D interactive figures and movie, use Adobe PDF viewer. Email: thomas.i.madura@nasa.gov --- Submitted on Thu Mar 5 15:13:36 CST 2015-------------------------