Abstract by M. Leutenegger
The radiatively driven winds of O stars are influential in the stars' environments, and are also of great importance in their evolution. The most important parameter of these winds is the stellar mass loss rate. Traditional diagnostics of the mass loss rate, such as H alpha recombination emission and UV P Cygni absorption lines, all have limitations and potential systematic errors. The broad X-ray emission line profiles of O stars offer an alternative mass loss rate diagnostic with different limitations and errors. I will argue that for some stars, X-ray mass loss rate measurements may be the most accurate available. Our group has pursued a modeling program aimed both at a thorough understanding of the underlying physics of X-ray profile formation, as well as robust measurements of mass loss rates. In this talk, I will review our recent work on mass loss rate measurements, and then go into greater detail on the physics of profile formation, including UV photoexcitation of metastable states, resonance scattering, and porosity.
Background Papers
Owocki & Cohen 2001
Feldmeier, Oskinova & Hamann 2003
Owocki & Cohen 2006
Leutenegger et al. 2006
Leutenegger et al. 2007
Cohen et al. 2010
Leutenegger et al. 2010
Monday, November 29, 2010
Friday, November 19, 2010
What Drives the Dynamics of Giant HII Regions? A Study of Stellar Feedback in 30 Doradus
Abstract by L. Lopez
Observations show that star formation is an inefficient and slow process. This result can be attributed to the injection of energy and momentum by stars that prevents free-fall collapse of molecular clouds. The dominant mechanism of this stellar feedback is debated theoretically: possible sources of pressure include the classical warm HII gas, the hot gas shock-heated by supernovae and stellar winds, the direct radiation from stars, and the dust-processed radiation field trapped inside the HII shell. In this talk, I will discuss how one can measure the pressures associated with these feedback processes using multiwavelength imaging (radio, infrared, optical, and X-ray), and I will present the results from applying these techniques to the giant HII region 30 Doradus, the largest star-forming region in the Local Group. For that source, I find that radiation pressure dominates close to the central star cluster, and I will consider the implications regarding the dynamics of 30 Dor and the regulation of star formation in the region.
Background papers
Lopez et al. 2010
Talk slides
pdf
Observations show that star formation is an inefficient and slow process. This result can be attributed to the injection of energy and momentum by stars that prevents free-fall collapse of molecular clouds. The dominant mechanism of this stellar feedback is debated theoretically: possible sources of pressure include the classical warm HII gas, the hot gas shock-heated by supernovae and stellar winds, the direct radiation from stars, and the dust-processed radiation field trapped inside the HII shell. In this talk, I will discuss how one can measure the pressures associated with these feedback processes using multiwavelength imaging (radio, infrared, optical, and X-ray), and I will present the results from applying these techniques to the giant HII region 30 Doradus, the largest star-forming region in the Local Group. For that source, I find that radiation pressure dominates close to the central star cluster, and I will consider the implications regarding the dynamics of 30 Dor and the regulation of star formation in the region.
Background papers
Lopez et al. 2010
Talk slides
Monday, November 15, 2010
Herschel PACS Spectroscopic Signatures of Evolution in Local Ultraluminous Galaxies
Abstract by J. Fischer
Coupling between radiation, gas, and dust is strong in the compact central regions of gas-rich mergers in their ultraluminous infrared galaxy (ULIRG) stage. I discuss the telltale signs of the effects of high ionization parameters, high far-infrared optical depths, and outflows that may be driven by this coupling, as well as the implications of new Herschel far-infrared observations for the evolutionary stage traced by these systems. The massive outflows detected via P-Cygni profile signatures of excited transitions of molecules such as OH and H2O are seen in early PACS observations of both warm and cold ULIRGs and provide evidence of feedback that may be capable of halting the star-formation process.
Background papers
Fischer et al. 2010
Abel et al. 2009
Gonzalez-Alfonso et al. 2008
Talk slides
pdf
Coupling between radiation, gas, and dust is strong in the compact central regions of gas-rich mergers in their ultraluminous infrared galaxy (ULIRG) stage. I discuss the telltale signs of the effects of high ionization parameters, high far-infrared optical depths, and outflows that may be driven by this coupling, as well as the implications of new Herschel far-infrared observations for the evolutionary stage traced by these systems. The massive outflows detected via P-Cygni profile signatures of excited transitions of molecules such as OH and H2O are seen in early PACS observations of both warm and cold ULIRGs and provide evidence of feedback that may be capable of halting the star-formation process.
Background papers
Fischer et al. 2010
Abel et al. 2009
Gonzalez-Alfonso et al. 2008
Talk slides
Monday, November 8, 2010
Weak Winds of Late-Type O Stars
Abstract by W. Marcolino
The radiatively driven wind theory provides a good agreement to measured
mass-loss rates for several O stars. However, low luminosity stars - of late spectral types - present rates much lower than expected, up to 2 orders of magnitude. Such huge discrepancy have serious consequences to stellar evolution theory and hydrodynamical models. In this work we present and discuss this so-called weak wind problem. We present results regarding five late-type O stars (O8-9V). The expanding atmosphere code CMFGEN was used to determine their stellar and wind properties. We discuss other mass-loss rate diagnostics besides the CIV 1549 line in the UV. We also show that although X-rays seem a nice way to recover the expected mass-loss rates and fit the observed spectra, the X-ray luminosity needed is in contrast with observations. Some perspectives and work in progress on the weak wind problem are discussed.
Background papers
Vink et al. 2000
Martins et al. 2005
Marcolino et al. 2009
Talk Slides
pdf
The radiatively driven wind theory provides a good agreement to measured
mass-loss rates for several O stars. However, low luminosity stars - of late spectral types - present rates much lower than expected, up to 2 orders of magnitude. Such huge discrepancy have serious consequences to stellar evolution theory and hydrodynamical models. In this work we present and discuss this so-called weak wind problem. We present results regarding five late-type O stars (O8-9V). The expanding atmosphere code CMFGEN was used to determine their stellar and wind properties. We discuss other mass-loss rate diagnostics besides the CIV 1549 line in the UV. We also show that although X-rays seem a nice way to recover the expected mass-loss rates and fit the observed spectra, the X-ray luminosity needed is in contrast with observations. Some perspectives and work in progress on the weak wind problem are discussed.
Background papers
Vink et al. 2000
Martins et al. 2005
Marcolino et al. 2009
Talk Slides
Monday, November 1, 2010
Observational Evidence for a Correlation Between Macroturbulent Broadening and Line-profile Variations in OB Supergiants
Abstract by Simón-Díaz et al.
The spectra of O and B supergiants (Sgs) are known to be affected by a significant form of extra line broadening (usually referred to as macroturbulence) in addition to that produced by stellar rotation. Recent analyses of high-resolution spectra have shown that the interpretation of this line broadening as a consequence of large-scale turbulent motions would imply highly supersonic velocity fields in photospheric regions, making this scenario quite improbable. Stellar oscillations have been proposed as a likely alternative explanation. As part of a long-term observational project, we are investigating the macroturbulent broadening in O and B Sgs and its possible connection with spectroscopic variability phenomena and stellar oscillations. In this Letter, we present the first encouraging results of our project, namely, firm observational evidence for a strong correlation between the extra broadening and photospheric line-profile variations in a sample of 13 Sgs with spectral types ranging from O9.5 to B8.rs.
Background papers
Simón-Díaz et al. 2010
Talk slides (by Alex Fullerton)
pdf
The spectra of O and B supergiants (Sgs) are known to be affected by a significant form of extra line broadening (usually referred to as macroturbulence) in addition to that produced by stellar rotation. Recent analyses of high-resolution spectra have shown that the interpretation of this line broadening as a consequence of large-scale turbulent motions would imply highly supersonic velocity fields in photospheric regions, making this scenario quite improbable. Stellar oscillations have been proposed as a likely alternative explanation. As part of a long-term observational project, we are investigating the macroturbulent broadening in O and B Sgs and its possible connection with spectroscopic variability phenomena and stellar oscillations. In this Letter, we present the first encouraging results of our project, namely, firm observational evidence for a strong correlation between the extra broadening and photospheric line-profile variations in a sample of 13 Sgs with spectral types ranging from O9.5 to B8.rs.
Background papers
Simón-Díaz et al. 2010
Talk slides (by Alex Fullerton)
The Great Observatories Origins Deep Survey: Constraints on the Lyman Continuum Escape Fraction Distribution of Lyman--Break Galaxies at z of 3.4--4.5
Abstract by E. Vanzella et al.
We use ultra-deep ultraviolet VLT/VIMOS intermediate-band and VLT/FORS1 narrow-band imaging in the GOODS Southern field to derive limits on the distribution of the escape fraction (f_esc) of ionizing radiation for L >~ L*(z=3) Lyman Break Galaxies (LBGs) at redshift 3.4--4.5. Only one LBG, at redshift z=3.795, is detected in its Lyman continuum (LyC; S/N~5.5), the highest redshift galaxy currently known with a direct detection. Its ultraviolet morphology is quite compact (R_eff=0.8, kpc physical). Three out of seven AGN are also detected in their LyC, including one at redshift z=3.951 and z850 = 26.1. From stacked data (LBGs) we set an upper limit to the average f_esc in the range 5%--20%, depending on the how the data are selected (e.g., by magnitude and/or redshift). We undertake extensive Monte Carlo simulations that take into account intergalactic attenuation, stellar population synthesis models, dust extinction and photometric noise in order to explore the moments of the distribution of the escaping radiation. Various distributions (exponential, log-normal and Gaussian) are explored. We find that the median f_esc is lower than ~6% with an 84% percentile limit not larger than 20%. If this result remains valid for fainter LBGs down to current observational limits, then the LBG population might be not sufficient to account for the entire photoionization budget at the redshifts considered here, with the exact details dependent upon the assumed ionizing background and QSO contribution thereto. It is possible that f_esc depends on the UV luminosity of the galaxies, with fainter galaxies having higher f_esc, and estimates of f_esc from a sample of faint LBG from the HUDF (i775<28.5) are in broad quantitative agreement with such a scenario.
Background papers
Vanzella et al. 2010
We use ultra-deep ultraviolet VLT/VIMOS intermediate-band and VLT/FORS1 narrow-band imaging in the GOODS Southern field to derive limits on the distribution of the escape fraction (f_esc) of ionizing radiation for L >~ L*(z=3) Lyman Break Galaxies (LBGs) at redshift 3.4--4.5. Only one LBG, at redshift z=3.795, is detected in its Lyman continuum (LyC; S/N~5.5), the highest redshift galaxy currently known with a direct detection. Its ultraviolet morphology is quite compact (R_eff=0.8, kpc physical). Three out of seven AGN are also detected in their LyC, including one at redshift z=3.951 and z850 = 26.1. From stacked data (LBGs) we set an upper limit to the average f_esc in the range 5%--20%, depending on the how the data are selected (e.g., by magnitude and/or redshift). We undertake extensive Monte Carlo simulations that take into account intergalactic attenuation, stellar population synthesis models, dust extinction and photometric noise in order to explore the moments of the distribution of the escaping radiation. Various distributions (exponential, log-normal and Gaussian) are explored. We find that the median f_esc is lower than ~6% with an 84% percentile limit not larger than 20%. If this result remains valid for fainter LBGs down to current observational limits, then the LBG population might be not sufficient to account for the entire photoionization budget at the redshifts considered here, with the exact details dependent upon the assumed ionizing background and QSO contribution thereto. It is possible that f_esc depends on the UV luminosity of the galaxies, with fainter galaxies having higher f_esc, and estimates of f_esc from a sample of faint LBG from the HUDF (i775<28.5) are in broad quantitative agreement with such a scenario.
Background papers
Vanzella et al. 2010
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