Abstract by Michael F. Corcoran
Extremely massive stars (50 solar masses and above) are exceedingly rare in the local Universe but are believed to compose the entire first generation of stars, which lived fast, died young and left behind the first generation of black holes and set the stage for the formation of lower mass stars suitable to support life. For most of their lives, extremely massive stars give mass back to the ISM out of which they formed via strong radiatively-driven winds, though sporadic eruptions may play an important role too. This mass loss plays an important role in the chemical and dynamical evolution of the local interstellar medium prior to the supernova explosion. I'll discuss how high energy thermal (and, in some cases, non-thermal) emission, along with modern simulations in 2 and 3 dimensions, can be used to help determine a physically realistic picture of mass loss in some well-studied systems.
Relevant Papers
cv_corcoran.pdf
Talk Slides
pdf
Friday, April 29, 2011
Friday, April 15, 2011
Dust in the extremely metal poor galaxy I Zw 18
Abstract by Rodrigo Herrera Camus
The blue compact dwarf galaxy I Zw18 is one of the most metal poor systems in the local Universe, with a nebular metallicity of 12 + log(O/H) = 7.2. Because of this, I Zw18 can provide crucial information about the relation between dust-to-gas mass ratio and metallicity in galaxies. In this work we use Spitzer MIPS observations at 70 and 160 micrometer combined with empirical fits from Muñoz-Mateos et al. (2009) to Draine et al. (2007) dust emission models to derive a dust mass upper limit for I Zw 18 of M_Dust = 3 * 10^4 solar masses and a upper limit to the dust-to-gas mass ratio of M_Dust / M_gas = 9.1 * 10^{-4} (3 sigma limits). This upper limit for the dust-to-gas mass ratio is similar to the Milky Way value after scaling by their respective metallicities, and suggests that a linear scaling law could be compatible with metal-poor systems. The upper limit to the dust mass is driven by the confusion limit of Spitzer at 160 micrometer, and we expect it to be significantly improved by future Herschel observations.
Relevant papers
"Dust in I Zw 18 from Hubble Space Telescope Narrowband Imaging" Cannon et al. 2002
"Dust in the Extremely Metal-Poor Blue Compact Dwarf Galaxy I Zw 18: The Spitzer Mid-infrared View" Wu et al. 2007
Talk slides
pdf
The blue compact dwarf galaxy I Zw18 is one of the most metal poor systems in the local Universe, with a nebular metallicity of 12 + log(O/H) = 7.2. Because of this, I Zw18 can provide crucial information about the relation between dust-to-gas mass ratio and metallicity in galaxies. In this work we use Spitzer MIPS observations at 70 and 160 micrometer combined with empirical fits from Muñoz-Mateos et al. (2009) to Draine et al. (2007) dust emission models to derive a dust mass upper limit for I Zw 18 of M_Dust = 3 * 10^4 solar masses and a upper limit to the dust-to-gas mass ratio of M_Dust / M_gas = 9.1 * 10^{-4} (3 sigma limits). This upper limit for the dust-to-gas mass ratio is similar to the Milky Way value after scaling by their respective metallicities, and suggests that a linear scaling law could be compatible with metal-poor systems. The upper limit to the dust mass is driven by the confusion limit of Spitzer at 160 micrometer, and we expect it to be significantly improved by future Herschel observations.
Relevant papers
"Dust in I Zw 18 from Hubble Space Telescope Narrowband Imaging" Cannon et al. 2002
"Dust in the Extremely Metal-Poor Blue Compact Dwarf Galaxy I Zw 18: The Spitzer Mid-infrared View" Wu et al. 2007
Talk slides
Friday, April 1, 2011
The VLT-FLAMES Tarantula survey of Massive Stars
Abstract
This survey is an ESO Large Programme that has obtained multi-epoch optical spectroscopy of over 800 massive stars in the 30 Doradus region of the Large Magellanic Cloud. It is by far the largest homogeneous spectroscopic study of extragalactic early-type stars undertaken to date. The 30 Dor region is the only ‘super star-cluster’ at a well-known distance in which individual objects can be resolved spatially in optical light. This makes it the perfect target for the comprehensive studies required to address some of the fundamental questions that remain in our understanding of massive-star evolution, relying on the analysis of a statistically-significant and unbiased sample. Authors Danny Lennon, Selma E. de Mink, and Nolan Walborn will lead the discussion.
Relevant Papers
"The VLT-FLAMES Tarantula Survey I: Introduction and observational overview" Evans et al. 2011
"The VLT-FLAMES Tarantula Survey II: R139 revealed as a massive binary system" Taylor et al. 2011
This survey is an ESO Large Programme that has obtained multi-epoch optical spectroscopy of over 800 massive stars in the 30 Doradus region of the Large Magellanic Cloud. It is by far the largest homogeneous spectroscopic study of extragalactic early-type stars undertaken to date. The 30 Dor region is the only ‘super star-cluster’ at a well-known distance in which individual objects can be resolved spatially in optical light. This makes it the perfect target for the comprehensive studies required to address some of the fundamental questions that remain in our understanding of massive-star evolution, relying on the analysis of a statistically-significant and unbiased sample. Authors Danny Lennon, Selma E. de Mink, and Nolan Walborn will lead the discussion.
Relevant Papers
"The VLT-FLAMES Tarantula Survey I: Introduction and observational overview" Evans et al. 2011
"The VLT-FLAMES Tarantula Survey II: R139 revealed as a massive binary system" Taylor et al. 2011
Friday, March 18, 2011
Investigating the Chemical Homogeneity of Low-Metallicity Blue Compact Dwarf Galaxies Using Integral Field Spectroscopy
Abstract by By Bethan James
The study of the chemical and physical evolution of Blue Compact Dwarf (BCDs) galaxies is fundamental in our understanding of galaxy formation in the early Universe. Being nearby, BCDs serve as excellent laboratories to study chemical enrichment processes in often unblemished environments, in comparison to primordial galaxies at high-redshift. It has been claimed in the past that in low-metallicity BCDs (12+log(O/H)<8.3), the N/O value is independent of metallicity (O/H ratio), implying the need to invoke a primary production of nitrogen in intermediate-mass stars, in addition to the secondary nitrogen produced from the CNO cycle in high-mass stars. In order to better understand this controversial issue, more extensive spatial information than that offered by traditional longslit spectroscopic methods, is required.
We undertook an integral field spectroscopic study of the nebular gas within a sample of BCDs previously thought to have anomalously high N/O values. Here we present the results of this study for 3 BCDs: two with anomalous N/O values (Mrk996 and UM420) and one with more normal N/O values (UM462). We describe in detail how we derived the physical conditions (Te, Ne) as a function of position within the galaxy, and as a consequence, how this revealed both revised metallicities and normal N/O ratios. Abundances for other elements will also be discussed. In the case of Mrk996, which displays both narrow and broad component emission lines, we associated an increased N/O ratio with the broad component in the nebular gas clearly correlated with WR emission features. This is one of the first clear evidence of nitrogen self-enrichment of an HII region from WR stars.
All the above mentioned results suggest that the nitrogen enrichment seen in the low-metallicity gas of these BCDs is not due to primary nitrogen production, as suggested by current theories. This investigation is also a direct proof that performing spatially resolved spectroscopy, and conducting a separate analysis of broad and narrow emission line components, are vital in deriving the 'true' chemical abundances of BCD galaxies.
The study of the chemical and physical evolution of Blue Compact Dwarf (BCDs) galaxies is fundamental in our understanding of galaxy formation in the early Universe. Being nearby, BCDs serve as excellent laboratories to study chemical enrichment processes in often unblemished environments, in comparison to primordial galaxies at high-redshift. It has been claimed in the past that in low-metallicity BCDs (12+log(O/H)<8.3), the N/O value is independent of metallicity (O/H ratio), implying the need to invoke a primary production of nitrogen in intermediate-mass stars, in addition to the secondary nitrogen produced from the CNO cycle in high-mass stars. In order to better understand this controversial issue, more extensive spatial information than that offered by traditional longslit spectroscopic methods, is required.
We undertook an integral field spectroscopic study of the nebular gas within a sample of BCDs previously thought to have anomalously high N/O values. Here we present the results of this study for 3 BCDs: two with anomalous N/O values (Mrk996 and UM420) and one with more normal N/O values (UM462). We describe in detail how we derived the physical conditions (Te, Ne) as a function of position within the galaxy, and as a consequence, how this revealed both revised metallicities and normal N/O ratios. Abundances for other elements will also be discussed. In the case of Mrk996, which displays both narrow and broad component emission lines, we associated an increased N/O ratio with the broad component in the nebular gas clearly correlated with WR emission features. This is one of the first clear evidence of nitrogen self-enrichment of an HII region from WR stars.
All the above mentioned results suggest that the nitrogen enrichment seen in the low-metallicity gas of these BCDs is not due to primary nitrogen production, as suggested by current theories. This investigation is also a direct proof that performing spatially resolved spectroscopy, and conducting a separate analysis of broad and narrow emission line components, are vital in deriving the 'true' chemical abundances of BCD galaxies.
Friday, March 4, 2011
Probing the stellar populations of z ~ 1.5-3.5 galaxies using rest-frame UV and optical morphology
Abstract by Nicholas Bond
I discuss some of my recent research studying the properties of the stellar populations of high-redshift galaxies by correlating their morphological properties with their spectral energy distributions. Using samples of star-forming galaxies at z ~ 1.5 - 3.5, including BX, BzK, Lyman break galaxies, and Lyman Alpha Emitters (LAEs), we implement comparative morphological diagnostics to spatially differentiate the young and old stars. Although the morphological differences between rest-frame UV and rest-frame optical light are generally small, they are statistically significant for the majority of star-forming galaxies at z >~ 2, and larger than for early-type galaxies at similar redshifts.
We then connect this work to LAEs, which are thought be among the most pristine of the high-redshift galaxies. Despite the common belief that a young, dust-free environment is needed to produce an LAE, there is increasing evidence for an underlying population of older stars in many LAEs, especially towards lower redshifts (z ~ 2).
Date: March 4th, 2010 (Friday)
Location: STScI, room N420
http://adsabs.harvard.edu/abs/2011ApJ...729...48B
http://adsabs.harvard.edu/abs/2011arXiv1101.3017G
http://adsabs.harvard.edu/abs/2010ApJ...716L.200B
http://adsabs.harvard.edu/abs/2010arXiv1005.3006G
http://adsabs.harvard.edu/abs/2010ApJ...714..255G
http://adsabs.harvard.edu/abs/2009ApJ...705..639B
http://adsabs.harvard.edu/abs/2003ApJ...598..827P
I discuss some of my recent research studying the properties of the stellar populations of high-redshift galaxies by correlating their morphological properties with their spectral energy distributions. Using samples of star-forming galaxies at z ~ 1.5 - 3.5, including BX, BzK, Lyman break galaxies, and Lyman Alpha Emitters (LAEs), we implement comparative morphological diagnostics to spatially differentiate the young and old stars. Although the morphological differences between rest-frame UV and rest-frame optical light are generally small, they are statistically significant for the majority of star-forming galaxies at z >~ 2, and larger than for early-type galaxies at similar redshifts.
We then connect this work to LAEs, which are thought be among the most pristine of the high-redshift galaxies. Despite the common belief that a young, dust-free environment is needed to produce an LAE, there is increasing evidence for an underlying population of older stars in many LAEs, especially towards lower redshifts (z ~ 2).
Date: March 4th, 2010 (Friday)
Location: STScI, room N420
http://adsabs.harvard.edu/abs/2011ApJ...729...48B
http://adsabs.harvard.edu/abs/2011arXiv1101.3017G
http://adsabs.harvard.edu/abs/2010ApJ...716L.200B
http://adsabs.harvard.edu/abs/2010arXiv1005.3006G
http://adsabs.harvard.edu/abs/2010ApJ...714..255G
http://adsabs.harvard.edu/abs/2009ApJ...705..639B
http://adsabs.harvard.edu/abs/2003ApJ...598..827P
Wednesday, February 2, 2011
Integrated spectroscopy of stellar clusters
Abstract by Andrea Ahumada
We present flux-calibrated integrated spectra in the optical spectral range (3600-6900 Å) of Galactic open clusters and Magellanic Clouds (MCs) stellar clusters mainly obtained with the 2.15 m telescope at "Complejo Astronómico El Leoncito" (CASLEO, San Juan, Argentina). Age and foreground reddening were simultaneously derived by comparing the continuum distribution and line strengths of the cluster spectra with those of template spectra. The present data will let us upgrade the spectral library of solar-metallicity and MCs metallicities which will be useful for several astrophysical applications.
Date: Friday, February 4th, 11:30 - 12.30 pm, room N420
Host: Nolan Walborn
We present flux-calibrated integrated spectra in the optical spectral range (3600-6900 Å) of Galactic open clusters and Magellanic Clouds (MCs) stellar clusters mainly obtained with the 2.15 m telescope at "Complejo Astronómico El Leoncito" (CASLEO, San Juan, Argentina). Age and foreground reddening were simultaneously derived by comparing the continuum distribution and line strengths of the cluster spectra with those of template spectra. The present data will let us upgrade the spectral library of solar-metallicity and MCs metallicities which will be useful for several astrophysical applications.
Date: Friday, February 4th, 11:30 - 12.30 pm, room N420
Host: Nolan Walborn
Monday, November 29, 2010
Modeling of X-ray emission line profiles in O stars
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
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
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