* Astronomy

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RE: Red Supergiants

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Title: An Emerging Coherent Picture of Red Supergiant Explosions
Authors: Dovi Poznanski (Tel Aviv University)

A large fraction of supernovae (SNe) arise from the core collapse of red supergiant stars. By comparing the ejecta velocities of a large sample of such SNe with initial stellar masses derived from pre-explosion images of the progenitor star, I find that there is a significant and approximately linear relation between initial mass and ejecta velocity, which in turn implies that the energy released during core collapse and captured by the ejecta depends strongly on this mass as E is proportional to M³. This correlation naturally explains why most such SNe have almost identical "plateau-phase" durations in their light curves, and places an important constraint on the elusive physics of core collapse.

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Title: NLTE water lines in Betelgeuse-like atmospheres
Authors: Julien Lambert, Eric Josselin, Nils Ryde, Alexandre Faure

The interpretation of water lines in red supergiant stellar atmospheres has been much debated over the past decade. The introduction of the so-called MOLspheres to account for near-infrared "extra" absorption has been controversial. We propose that non-LTE effects should be taken into account before considering any extra-photospheric contribution. After a brief introduction on the radiative transfer treatment and the inadequacy of classical treatments in the case of large-scale systems such as molecules, we present a new code, based on preconditioned Krylov subspace methods. Preliminary results suggest that NLTE effects lead to deeper water bands, as well as extra cooling.

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Title: Red supergiants and stellar evolution
Authors: Sylvia Ekström, Cyril Georgy, Georges Meynet, Jose Groh, Anahí Granada

We review the significant role played by red supergiants (RSGs) in stellar populations, and some challenges and questions they raise for theoretical stellar evolution. We present how metallicity and rotation modify the way stars go to the red part of the Hertzsprung-Russell diagram or come back from it, and how RSGs might keep a trace of their main-sequence evolution. We compare theoretical population ratios with observed ones.

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Title: The temperatures of Red Supergiants
Authors: Ben Davies (Liverpool JMU), Rolf-Peter Kudritzki (Hawaii), Bertrand Plez (Montpellier), Scott Trager (Groningen), Ariane Lancon (Strasbourg), Zach Gazak (Hawaii), Maria Bergemann (MPA), Chris Evans (UKATC), Andrea Chiavassa (Nice)

We present a re-appraisal of the temperatures of Red Supergiants (RSGs) using their optical and near-infrared spectral energy distributions (SEDs). We have obtained data of a sample of RSGs in the Magellanic Clouds using VLT+XSHOOTER, and we fit MARCS model atmospheres to different regions of the spectra, deriving effective temperatures for each star from (a) the TiO bands, (b) line-free continuum regions of the spectral energy distributions (SEDs), and (c) the integrated fluxes. We show that the temperatures derived from fits to the TiO bands are systematically {\it lower} than the other two methods by several hundred Kelvin. The TiO fits also dramatically over-predict the flux in the near-IR, and imply extinctions which are anomalously low compared to neighbouring stars. In contrast, the SED temperatures provide good fits to the fluxes at all wavelengths other than the TiO bands, are in agreement with the temperatures from the flux integration method, and imply extinctions consistent with nearby stars. After considering a number of ways to reconcile this discrepancy, we conclude that 3-D effects (i.e.\ granulation) are the most likely cause, as they affect the temperature structure in the upper layers where the TiO lines form. The continuum, however, which forms at much deeper layers, is apparently more robust to such effects. We therefore conclude that RSG temperatures are much warmer than previously thought. We discuss the implications of this result for stellar evolution and supernova progenitors, and provide relations to determine the bolometric luminosities of RSGs from single-band photometry.

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Title: Spectral Types of Red Supergiants in NGC 6822 and the Wolf-Lundmark-Melotte Galaxy
Authors: Emily M. Levesque, Philip Massey

We present moderate-resolution spectroscopic observations of red supergiants (RSGs) in the low-metallicity Local Group galaxies NGC 6822 (Z = 0.4Zsun) and Wolf-Lundmark-Melotte (WLM; Z = 0.1Zsun). By combining these observations with reduction techniques for multislit data reduction and flux calibration, we are able to analyze spectroscopic data of 16 RSGs in NGC 6822 and spectrophotometric data of 11 RSGs in WLM. Using these observations we determine spectral types for these massive stars, comparing them to Milky Way and Magellanic Clouds RSGs and thus extending observational evidence of the abundance-dependent shift of RSG spectral types to lower metallicities. In addition, we have uncovered two RSGs with unusually late spectral types (J000158.14-152332.2 in WLM, with a spectral type of M3 I, and J194453.46-144552.6 in NGC 6822, with a spectral type of M4.5 I) and a third RSG (J194449.96-144333.5 in NGC 6822) whose spectral type has varied from a M2.5 in 1997 to a K5 in 2008. All three of these stars could potentially be members of a recently-discovered class of extreme RSG variables.

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Title: Physical Properties of Red Supergiants
Authors: Emily Levesque
(Version v2)

Red supergiants (RSGs) are a He-burning phase in the evolution of moderately massive stars (10-25 solar masses). For many years, the assumed physical properties of these stars placed them at odds with the predictions of evolutionary theory. We have recently determined new effective temperatures and luminosities for the RSG populations of galaxies with a factor of ~8 range in metallicity, including the Milky Way, the Magellanic Clouds, and M31. We find that these new physical properties greatly improve the agreement between the RSGs and the evolutionary tracks, although there are still notable difficulties with modelling the physical properties of RSGs at low metallicity. We have also examined several unusual RSGs, including VY CMa in the Milky Way, WOH G64 in the LMC, and a sample of four RSGs in the Magellanic Clouds that show considerable variations in their physical parameters, most notably their effective temperatures. For all of these stars we reexamine their placement on the H-R diagram, where they have appeared to occupy the "forbidden" region to the right of the Hayashi track. We have updated current understanding of the physical properties of VY CMa and WOH G64; in the case of the unusual Magellanic Cloud variables, we conclude that these stars are undergoing an unstable evolutionary phase not previously associated with RSGs.

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The Scutum-Crux Red Supergiant Clusters

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2MASS K-band image of RSGC2 (upper), and finding chart (lower).
Coordinates are centred on 18 39 20.4, -6 01 41 (2000), following Stephenson (1990).
The sizes of the plotting symbols scale linearly with K-band magnitude.

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Title: A massive cluster of Red Supergiants at the base of the Scutum-Crux arm
Authors: Ben Davies (RIT), Don F. Figer (RIT), Rolf-Peter Kudritzki (IfA, Hawaii), John MacKenty (STScI), Francisco Najarro (CSIC, Madrid), Artemio Herrero (IAC, Spain)
(Version v3)

We report on the unprecedented Red Supergiant (RSG) population of a massive young cluster, located at the base of the Scutum-Crux Galactic arm. We identify candidate cluster RSGs based on {\it 2MASS} photometry and medium resolution spectroscopy. With follow-up high-resolution spectroscopy, we use CO-bandhead equivalent width and high-precision radial velocity measurements to identify a core grouping of 26 physically-associated RSGs -- the largest such cluster known to-date. Using the stars' velocity dispersion, and their inferred luminosities in conjunction with evolutionary models, we argue that the cluster has an initial mass of ~40,000\msun, and is therefore among the most massive in the galaxy. Further, the cluster is only a few hundred parsecs away from the cluster of 14 RSGs recently reported by Figer et al (2006). These two RSG clusters represent 20% of all known RSGs in the Galaxy, and now offer the unique opportunity to study the pre-supernova evolution of massive stars, and the Blue- to Red-Supergiant ratio at uniform metallicity. We use GLIMPSE, MIPSGAL and MAGPIS survey data to identify several objects in the field of the larger cluster which seem to be indicative of recent region-wide starburst activity at the point where the Scutum-Crux arm intercepts the Galactic bulge. Future abundance studies of these clusters will therefore permit the study of the chemical evolution and metallicity gradient of the Galaxy in the region where the disk meets the bulge.

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The Scutum Red Supergiant Clusters summary of physical properties

RSGC2 - 26 RSGs
Mcl= (4 ± 1) x 10^4 M
Age = 17 ± 4 Myr

RSGC1 - 14 RSGs
M cl = (3 ± 1) x 10^4 M
Age = 12 ± 2 Myr

GLIMPSE, l=24..27°, b=0.0