Dissecting Galaxies with Quantitative Spectroscopy of the Brightest Stars in the Universe

Measuring distances to galaxies, determining their chemical composition, investigating the nature of their stellar populations and the absorbing properties of their interstellar medium are fundamental activities in modern extragalactic astronomy helping to understand the evolution of galaxies and the expanding universe. The optically brightest stars in the universe, blue supergiants of spectral A and B, are unique tools for these purposes. With absolute visual magnitudes up to M_V = -9.5 they are the ideal to obtain accurate quantitative information about galaxies through the powerful modern methods of quantitative stellar spectroscopy. The spectral analyis of individual blue supergiant targets provides invaluable information about chemical abundances and abundance gradients, which is more comprehensive than the one obtained from HII regions, as it includes additional atomic species, and which is also more accurate, since it avoids the systematic uncertainties inherent in the strong line studies usually applied to the HII regions of spiral galaxies beyond the Local Group. Simultaneously, the spectral analysis yields stellar parameters and interstellar extinction for each individual supergiant target, which provides an alternative very accurate way to determine extragalactic distances through a newly developed method, called the Flux-weighted Gravity - Luminosity Relationship (FGLR). With the present generation of 10m-class telescopes these spectroscopic studies can reach out to distances of 10 Mpc. The new generation of 30m-class will allow to extend this work out to 30 Mpc, a substantial volume of the local universe.Comment: Karl Schwarzschild Lecture 2009. To appear in Astronomische Nachrichte

Quantitative analysis of WC stars: Constraints on neon abundances from ISO/SWS spectroscopy

Neon abundances are derived in four Galactic WC stars -- gamma Vel (WR11, WC8+O7.5III), HD156385 (WR90, WC7), HD192103 (WR135, WC8), and WR146 (WC5+O8) - using mid-infrared fine structure lines obtained with ISO/SWS. Stellar parameters for each star are derived using a non-LTE model atmospheric code (Hillier & Miller 1998) together with ultraviolet (IUE), optical (INT, AAT) and infrared (UKIRT, ISO) spectroscopy. In the case of gamma Vel, we adopt results from De Marco et al. (2000), who followed an identical approach. ISO/SWS datasets reveal the [NeIII] 15.5um line in each of our targets, while [NeII] 12.8um, [SIV] 10.5um and [SIII] 18.7um are observed solely in gamma Vel. Using a method updated from Barlow et al. (1988) to account for clumped winds, we derive Ne/He=3-4x10^-3 by number, plus S/He=6x10^-5 for gamma Vel. Neon is highly enriched, such that Ne/S in gamma Vel is eight times higher than cosmic values. However, observed Ne/He ratios are a factor of two times lower than predictions of current evolutionary models of massive stars. An imprecise mass-loss and distance were responsible for the much greater discrepancy in neon content identified by Barlow et al. Our sample of WC5--8 stars span a narrow range in T* (=55--71kK), with no trend towards higher temperature at earlier spectral type, supporting earlier results for a larger sample by Koesterke & Hamann (1995). Stellar luminosities range from 100,000 to 500,000 Lo, while 10^-5.1 < Mdot/(Mo/yr) < 10^-4.5, adopting clumped winds, in which volume filling factors are 10%. In all cases, wind performance numbers are less than 10, significantly lower than recent estimates. Carbon abundances span 0.08 < C/He < 0.25 by number, while oxygen abundances remain poorly constrained.Comment: 16 pages,7 figures accepted for MNRA

A FEROS spectroscopic study of the extreme O supergiant He 3-759

We present a study of the extreme O-type supergiant He 3-759 using new high-resolution FEROS data, revealing that it is a near spectroscopic twin of HD 151804 (O8 Iaf). We investigate the extinction towards He 3-759 using a variety of methods, revealing A_V ~ 4.7 mag. If we assume He 3-759 has an identical absolute K-band magnitude to HD 151804 we find that it lies in the Sagittarius-Carina spiral arm at a distance of ~6.5 kpc. We derive the physical and wind properties for He 3-759, revealing T* = 30.5 kK, log L/L(sun) = 5.9 and dM/dt = 10^-5.17 M(sun)/yr for a clumped wind whose terminal velocity is estimated at 1000 km/s. The atmosphere of He 3-759 is enriched in helium (X_He = 49%) and nitrogen (X_N = 0.3%). A reanalysis of HD 151804 and HD 152408 (WN9ha) reveals similar parameters except that the WN9ha star possesses a stronger wind and reduced surface hydrogen content. HD 151804 and HD 152408 lie within the Sco OB1 association, with initial masses of ~60 M(sun) and ages ~2.7 Myr, consistent with NGC 6231 cluster members using standard Geneva isochrones. Improved agreement with observed surface abundances are obtained for similar initial masses with more recent Geneva group predictions from which higher ages of ~3.75 Myr are obtained. No young, massive star cluster is known to be associated with He 3-759.Comment: 6 pages, 4 figures, accepted for A&

A Disk Around the Planetary-Mass Companion GSC 06214-00210 b: Clues About the Formation of Gas Giants on Wide Orbits

We present Keck/OSIRIS 1.1-1.8 um adaptive optics integral field spectroscopy of the planetary-mass companion to GSC 06214-00210, a member of the ~5 Myr Upper Scorpius OB association. We infer a spectral type of L0+/-1, and our spectrum exhibits multiple signs of youth. The most notable feature is exceptionally strong PaBeta emission (EW=-11.4 +/- 0.3 A) which signals the presence of a circumplanetary accretion disk. The luminosity of GSC 06214-00210 b combined with its age yields a model-dependent mass of 14 +/- 2 MJup, making it the lowest-mass companion to show evidence of a disk. With a projected separation of 320 AU, the formation of GSC 06214-00210 b and other very low-mass companions on similarly wide orbits is unclear. One proposed mechanism is formation at close separations followed by planet-planet scattering to much larger orbits. Since that scenario involves a close encounter with another massive body, which is probably destructive to circumplanetary disks, it is unlikely that GSC 06214-00210 b underwent a scattering event in the past. This implies that planet-planet scattering is not solely responsible for the population of gas giants on wide orbits. More generally, the identification of disks around young planetary companions on wide orbits offers a novel method to constrain the formation pathway of these objects, which is otherwise notoriously difficult to do for individual systems. We also refine the spectral type of the primary from M1 to K7 and detect a mild (2-sigma) excess at 22 um using WISE photometry.Comment: 25 pages, 13 figures; Accepted by Ap

HELIOS-Retrieval: An Open-source, Nested Sampling Atmospheric Retrieval Code, Application to the HR 8799 Exoplanets and Inferred Constraints for Planet Formation

We present an open-source retrieval code named HELIOS-Retrieval (hereafter HELIOS-R), designed to obtain chemical abundances and temperature-pressure profiles from inverting the measured spectra of exoplanetary atmospheres. In the current implementation, we use an exact solution of the radiative transfer equation, in the pure absorption limit, in our forward model, which allows us to analytically integrate over all of the outgoing rays (instead of performing Gaussian quadrature). Two chemistry models are considered: unconstrained chemistry (where the mixing ratios are treated as free parameters) and equilibrium chemistry (enforced via analytical formulae, where only the elemental abundances are free parameters). The nested sampling algorithm allows us to formally implement Occam's Razor based on a comparison of the Bayesian evidence between models. We perform a retrieval analysis on the measured spectra of the HR 8799b, c, d and e directly imaged exoplanets. Chemical equilibrium is disfavored by the Bayesian evidence for HR 8799b, c and d. We find supersolar C/O, C/H and O/H values for the outer HR 8799b and c exoplanets, while the inner HR 8799d and e exoplanets have substellar C/O, substellar C/H and superstellar O/H values. If these retrieved properties are representative of the bulk compositions of the exoplanets, then they are inconsistent with formation via gravitational instability (without late-time accretion) and consistent with a core accretion scenario in which late-time accretion of ices occurred differently for the inner and outer exoplanets. For HR 8799e, we find that spectroscopy in the K band is crucial for constraining C/O and C/H. HELIOS-R is publicly available as part of the Exoclimes Simulation Platform (ESP; www.exoclime.org).Comment: 27 pages, 21 figures, 3 tables, published in A