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X-Shooter Spectroscopy of Massive Stars in the Local Group and Beyond

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X-Shooter Spectroscopy of Massive Stars in the Local Group and Beyond Astronomical Science X-shooter Spectroscopy of Massive Stars in the Local Group and Beyond Hugues Sana1 general relativity. Massive stars affect the portant role in explaining the observed Alex de Koter1, 2 dynamical evolution of the star clusters in frequencies of different evolutionary Miriam Garcia 3, 4 which they reside and are thought to play phases and of supernova types. Olga Hartoog1 a crucial role in the formation and evo- Lex Kaper1, 5 lution of galaxies. They are also the can- With the latest generation of 8–10-metre- Frank Tramper1 didate first stars, anticipated to have re- class telescopes, massive stars in more Artemio Herrero 3, 4 ionised the Universe some few hundred distant galaxies can now be individually Norberto Castro 6 million years after the Big Bang. Given resolved, allowing us to probe a wider their importance for various fields of astro- span in environmental properties, albeit physics, a clear understanding of the for- so far mostly at low spectral resolution 1 Astronomical Institute Anton Pannekoek, mation and evolution of massive stars is (R ~ 2000) and within the Local Group Amsterdam University, the Netherlands essential. (e.g., Bresolin et al., 2007). Although quite 2 Utrecht University, the Netherlands a number of exciting objects have been 3 Instituto de Astrofísica de Canarias, At optical and near-infrared wavelengths identified, detailed quantitative spectro- La Laguna, Tenerife, Spain massive stars emerge from their natal scopic analyses of the most massive 4 Departamento de Astrofísica, Universi- clouds typically within the first million stars have remained cumbersome for dad de La Laguna, Tenerife, Spain years after formation, such that a census obvious reasons: low signal-to-noise ratio 5 Vrije Universiteit Amsterdam, of their masses, rotation rates and multi- and/or modest spectral resolution com- the Netherlands plicity characteristics may provide impor- plicate (or prevent) the removal of nebular 6 Institute of Astronomy & Astrophysics, tant insight into the end product of the emission, among other corrections. National Observatory of Athens, Greece star formation process. A detailed under- Besides enabling a better nebular sub- standing of their evolution further requires traction, the higher spectral resolution the identification and analysis of stars offered by X-shooter allows a more accu- Combined with the collecting power of in various evolutionary phases up to the rate surface temperature, gravity and the Very Large Telescope, X-shooter pre-supernova stage, thus deriving evo- mass-loss determination. It also allows is the most sensitive medium-resolution lutionary connections between these for the analysis of weak metallic lines, spectrograph currently in operation, phases. Comparisons between obser- crucial to derive abundances and accu- allowing us to perform quantitative vations and stellar evolution models must rate projected rotational velocities. These spectroscopy of objects that, up to be done for stars in different chemical quantities are the key to establish and now, were deemed too faint. In addition, environments as the ratios of stars in characterise the evolutionary stages of with its unique wavelength coverage, different phases (e.g., that of O to Wolf– massive stars and their feedback. Here X-shooter provides access to an un- Rayet stars), as well as the ratio of we report on the first analyses of massive precedentedly large number of diag- Type Ib/c to Type II supernovae, are found star mass loss at sub-SMC metallicity, nostic lines. We review our recent work to be metallicity dependent. With most which appears to contradict our expecta- on massive stars in Local Group dwarf long-duration gamma-ray bursts occur- tions. galaxies and in NGC 55, an irregular ring at low metallicity and the anticipated galaxy in the foreground of the Sculptor role played by massive stars in the early Group. The observations were obtained Universe, including its re-ionisation and Mass loss at low metallicity as part of the ESO Science Verifica- galaxy formation, it is particularly interest- tion Programme and the NOVA–Dutch ing to study massive stars in low metallic- The gas outflow of hot massive stars is Guaranteed Time Observation Pro- ity environments. driven by radiation pressure on metallic gramme. The aim is to investigate the ions in the star’s atmosphere, and conse- evolutionary status of high-mass stars These considerations have motivated the quently its strength. is predicted to scale in various environments and to test study of massive stars in the Large and with metallicity (M Z 0.69 ± 0.10; Vink et al., the theory of radiation-driven stellar Small Magellanic Clouds (LMC and SMC 2001). This prediction has been verified winds at metallicities below that of the respectively). Massive OB, Luminous for massive stars in the Galaxy and in the Small Magellanic Cloud. Blue Variable (LBV) and Wolf–Rayet (WR) Magellanic Clouds observed in the VLT– stars in the Magellanic Clouds have Flames Survey of Massive Stars (Evans et been studied intensively in the past dec- al.,. 2008), where the empirical relation Massive stars are linked to a wide vari- ade (e.g., Mokiem et al., 2007; Evans et M Z 0.78 ± 0.17 was found (Mokiem et al., ety of astrophysical processes and phe- al., 2008). One important result that 2007). The theory has, however, never nomena. Their intense radiation fields, has emerged from these studies is that been tested at sub-SMC metallicity. Be- strong stellar winds and violent super- the rate of gas outflow driven by radia- cause the evolution of massive stars is nova explosions stir the ambient interstel- tion pressure on spectral lines is both greatly influenced by the amount of mass lar medium, which is typically a site of predicted and found to be metallicity, Z, and angular momentum lost through star formation. Their explosions produce dependent in a range from ZA to 0.2 ZA. their strong stellar winds, determining the the neutron stars and black holes that This mass-loss rate versus metallicity wind strength was one of the main goals are used to test extreme physics and dependence is expected to play an im- of our quantitative spectroscopic analysis. The Messenger 148 – June 2012 33 Astronomical Science Sana H. et al., X-shooter Spectroscopy of Massive Stars TW . 5( " ((( Ck !l6 '''' '' ''D (( ' 'D(( '¡ 'D((( 'b 'D(( 'a 'D(( 'D(( '` -NQL@KHRD l5 6@UDKDMFSGÄ 45!5K2 " (5 TW k . 5( 'D(( . 5 " (5 'D(( !l6 'D(( '_ -NQL@KHRDC l5 6@UDKDMFSGÄ Figure 1 (above). Part of the X-shooter normalised than predicted mass-loss rates for an- Figure 2 (below). The wind momentum–luminosity spectra of two sources in IC 1613 are shown: A13 other two massive stars in IC 1613 diagram is shown. Dashed lines indicate theoretical (O3V, lower spectrum in the two panels) and B17 predictions from Vink et al. (2001) for different metal- (WO3, upper spectrum in the two panels). The WO (objects 7 and 8 in Figure 2). Although licities. Solid lines and shaded areas indicate the spectrum has been reduced in flux by a factor of five more stars need to be observed and observed wind momentum–luminosity relations for and shifted to fit the panel. The main spectral lines analysed to draw firm conclusions, these the Galaxy, LMC and SMC samples (Mokiem et are labelled. The vertical dashed line marks the tran- unexpected results may have interesting al., 2007). The shift between the predictions and the sition between the UVB and VIS arms of X-shooter. empirical measurements can be explained by implications. For example, the single- wind inhomogeneities, which are not included in the star channel to produce long-duration analysis. Our observations in metal-poor galaxies gamma-ray bursts depends on a star are overplotted, objects 1–6 are from Tramper et al. The Local Group dwarf galaxies IC 1613, retaining a rapidly spinning core at the (2011). Objects 7 and 8 are from Herrero et al. (2012) and Herrero et al. (2011), respectively. Although the the Wolf–Lundmark–Melotte (WLM) gal- end of its life, and the star therefore can- error bars are large, the positions of objects 5, 6 and axy and NGC 3109 are ideally suited to not lose too much angular momentum 8 are incompatible with theoretical predictions. Fig- probe the properties of massive stars in through its wind. This result would thus ure adapted from Tramper et al. (2011). low-metallicity environments. These galaxies all have a metallicity of approxi- ("l 5E &@K@WX mately ZA /7 (i.e. lower than that of the ("l KKK +," SMC), a very low foreground reddening, ("l! K and a young stellar population. Their dis- ("l"KKKE 2," tances are such that one can still resolve 6+,l K@ and individually study the brightest ob- -&"lK ("l KKK jects. We have thus obtained X-shooter ("lKE spectra of several massive stars in these galaxies (four in IC 1613, and one each L LN in WLM and NGC 3109). The targets were # chosen to probe the whole range of NF + O-type stars, with spectral types ranging from O3 V to O9.5 I. An example spec- C trum is displayed in Figure 1. 99 We find that the stars in our sample appear to exhibit surprisingly strong 9 9 winds, with a mass-loss rate expected for 9 9 ,NJHDLDS@K 9 9 5HMJDS@K LMC metallicity (objects 2–6 in Figure 2; Tramper et al., 2011). In a similar study, Herrero et al. (2011) also report stronger +NF++ 34 The Messenger 148 – June 2012 Figure 3. The Q vs. U–B imply that fewer progenitors are pro- duced through this channel. Also the sin- diagram of IC 1613 is shown with the locus of gle-star population at low metallicity O and B stars.
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