Abstract by Jean-Claude Bouret
Using near-infrared integral field spectroscopy obtained with SINFONI on the VLT,
we have studied three Galactic HII regions - RCW 79, RCW 82, and RCW 120.
We made a spectral classification of two stellar populations: the ionizing stars of each HII region and the young stellar objects on their borders.
We derived the stellar and wind properties of the ionizing stars using atmosphere models computed with the code CMFGEN. The young stellar objects were classified according to their K-band spectra. Linemaps were also constructed to study the geometry of their close environment.
I will summarize our results about the nature of stars and their environments, the ages of the cluster and show that the YSOs observed on the borders of the HII regions show distinct spectroscopic signatures compared to the ionizing sources, confirming the presence of two stellar populations. This is expected from successive star-forming events, which gives further credit to the triggered star formation scenario.
Talk slides
pdf
Monday, October 25, 2010
Monday, September 27, 2010
MESA-Modules for Experiments in Stellar Astrophysics
Abstract by Aaron Dotter
MESA is an open source project to produce a modern, sophisticated set of modules to be used for computer simulations in stellar astrophysics, including stellar evolution. MESA is capable of modelling massive stars with arbitrary wind models up to core collapse. It does not include a 1-D treatment of rotation at present but that is an active area of development. The current status of the project is described in detail here (http://arxiv.org/abs/1009.1622). I will give a brief overview of MESA massive star evolution.
MESA is an open source project to produce a modern, sophisticated set of modules to be used for computer simulations in stellar astrophysics, including stellar evolution. MESA is capable of modelling massive stars with arbitrary wind models up to core collapse. It does not include a 1-D treatment of rotation at present but that is an active area of development. The current status of the project is described in detail here (http://arxiv.org/abs/1009.1622). I will give a brief overview of MESA massive star evolution.
The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150Msolar stellar mass limit
Abstract by Paul A. Crowther et al.
Spectroscopic analyses of hydrogen-rich WN5-6 stars within the young star clusters NGC3603 and R136 are presented, using archival Hubble Space Telescope and Very Large Telescope spectroscopy, and high spatial resolution near-IR photometry, including Multi-Conjugate Adaptive Optics Demonstrator (MAD) imaging of R136. We derive high stellar temperatures for the WN stars in NGC3603 (T* ~ 42 +/- 2kK) and R136 (T* ~ 53 +/- 3kK) plus clumping-corrected mass-loss rates of 2-5 × 10-5Msolaryr-1 which closely agree with theoretical predictions from Vink et al. These stars make a disproportionate contribution to the global ionizing and mechanical wind power budget of their host clusters. Indeed, R136a1 alone supplies ~7 per cent of the ionizing flux of the entire 30Doradus region. Comparisons with stellar models calculated for the main-sequence evolution of 85-500Msolar accounting for rotation suggest ages of ~1.5Myr and initial masses in the range 105-170Msolar for three systems in NGC3603, plus 165-320Msolar for four stars in R136. Our high stellar masses are supported by consistent spectroscopic and dynamical mass determinations for the components of NGC3603A1. We consider the predicted X-ray luminosity of the R136 stars if they were close, colliding wind binaries. R136c is consistent with a colliding wind binary system. However, short period, colliding wind systems are excluded for R136a WN stars if mass ratios are of order unity. Widely separated systems would have been expected to harden owing to early dynamical encounters with other massive stars within such a high-density environment. From simulated star clusters, whose constituents are randomly sampled from the Kroupa initial mass function, both NGC3603 and R136 are consistent with an tentative upper mass limit of ~300Msolar. The Arches cluster is either too old to be used to diagnose the upper mass limit, exhibits a deficiency of very massive stars, or more likely stellar masses have been underestimated - initial masses for the most luminous stars in the Arches cluster approach 200Msolar according to contemporary stellar and photometric results. The potential for stars greatly exceeding 150Msolar within metal-poor galaxies suggests that such pair-instability supernovae could occur within the local universe, as has been claimed for SN2007bi.
Link to paper
j.1365-2966.2010.17167.x
Spectroscopic analyses of hydrogen-rich WN5-6 stars within the young star clusters NGC3603 and R136 are presented, using archival Hubble Space Telescope and Very Large Telescope spectroscopy, and high spatial resolution near-IR photometry, including Multi-Conjugate Adaptive Optics Demonstrator (MAD) imaging of R136. We derive high stellar temperatures for the WN stars in NGC3603 (T* ~ 42 +/- 2kK) and R136 (T* ~ 53 +/- 3kK) plus clumping-corrected mass-loss rates of 2-5 × 10-5Msolaryr-1 which closely agree with theoretical predictions from Vink et al. These stars make a disproportionate contribution to the global ionizing and mechanical wind power budget of their host clusters. Indeed, R136a1 alone supplies ~7 per cent of the ionizing flux of the entire 30Doradus region. Comparisons with stellar models calculated for the main-sequence evolution of 85-500Msolar accounting for rotation suggest ages of ~1.5Myr and initial masses in the range 105-170Msolar for three systems in NGC3603, plus 165-320Msolar for four stars in R136. Our high stellar masses are supported by consistent spectroscopic and dynamical mass determinations for the components of NGC3603A1. We consider the predicted X-ray luminosity of the R136 stars if they were close, colliding wind binaries. R136c is consistent with a colliding wind binary system. However, short period, colliding wind systems are excluded for R136a WN stars if mass ratios are of order unity. Widely separated systems would have been expected to harden owing to early dynamical encounters with other massive stars within such a high-density environment. From simulated star clusters, whose constituents are randomly sampled from the Kroupa initial mass function, both NGC3603 and R136 are consistent with an tentative upper mass limit of ~300Msolar. The Arches cluster is either too old to be used to diagnose the upper mass limit, exhibits a deficiency of very massive stars, or more likely stellar masses have been underestimated - initial masses for the most luminous stars in the Arches cluster approach 200Msolar according to contemporary stellar and photometric results. The potential for stars greatly exceeding 150Msolar within metal-poor galaxies suggests that such pair-instability supernovae could occur within the local universe, as has been claimed for SN2007bi.
Link to paper
j.1365-2966.2010.17167.x
Predicted UV properties of very metal-poor starburst galaxies
Abstract by Anna Raiter et al.
We study the expected properties of starbursts in order to provide the point of reference for interpretation of high-z galaxy surveys and of very metal-poor galaxies. We concentrate mainly on the UV characteristics such as the ionizing spectra, the UV continuum, the Ly alpha and HeII 1640 A line and two-photon continuum emission. We use evolutionary synthesis models covering metallicities from Pop III to solar and a wide range of IMFs. We also combine the synthetic SEDs with the CLOUDY photoionization code for more accurate predictions of nebular emission, and to study possible departures from case B assumed in the synthesis models. The ionizing fluxes, UV continuum properties, and predicted Ly alpha and HeII 1640 A line strengths are presented for synthesis models covering a wider range of parameter space than our earlier calculations. Strong departures from case B predictions are obtained for Ly alpha and two-photon continuum at low metallicities. At low nebular densities both are shown to be enhanced proportionally to the mean energy carried by the Lyman continuum photons emitted by the ionizing source. Larger Ly alpha equivalent widths are therefore predicted at low metallicity. The HeII 1640 A line can be weaker than case B predicts (in terms of flux as well as the equivalent width) due to its ionization parameter dependence and to the enhanced underlying two-photon continuum. Our results have implications for the interpretation of star-forming metal-poor and/or high redshift galaxies, for galaxies among the Ly alpha emitters (LAE) and Lyman Break galaxy (LBG) populations, and for searches of Population III stars in the distant Universe.
Link to paper
2010arXiv1008.2114R
We study the expected properties of starbursts in order to provide the point of reference for interpretation of high-z galaxy surveys and of very metal-poor galaxies. We concentrate mainly on the UV characteristics such as the ionizing spectra, the UV continuum, the Ly alpha and HeII 1640 A line and two-photon continuum emission. We use evolutionary synthesis models covering metallicities from Pop III to solar and a wide range of IMFs. We also combine the synthetic SEDs with the CLOUDY photoionization code for more accurate predictions of nebular emission, and to study possible departures from case B assumed in the synthesis models. The ionizing fluxes, UV continuum properties, and predicted Ly alpha and HeII 1640 A line strengths are presented for synthesis models covering a wider range of parameter space than our earlier calculations. Strong departures from case B predictions are obtained for Ly alpha and two-photon continuum at low metallicities. At low nebular densities both are shown to be enhanced proportionally to the mean energy carried by the Lyman continuum photons emitted by the ionizing source. Larger Ly alpha equivalent widths are therefore predicted at low metallicity. The HeII 1640 A line can be weaker than case B predicts (in terms of flux as well as the equivalent width) due to its ionization parameter dependence and to the enhanced underlying two-photon continuum. Our results have implications for the interpretation of star-forming metal-poor and/or high redshift galaxies, for galaxies among the Ly alpha emitters (LAE) and Lyman Break galaxy (LBG) populations, and for searches of Population III stars in the distant Universe.
Link to paper
2010arXiv1008.2114R
Monday, September 13, 2010
An investigation of high metallicity LGRB hosts
Abstract by John Graham
One of the most powerful means to study the formation and evolution of gamma-ray bursts is by observing there environments. GRBs can be subdivided into two largely distinct classes based on duration and energy distribution: short-hard and long-soft. While short bursts have been detected in nearby galaxies of all types, long burst hosts are dominated by blue irregulars leading to speculation of metal poor host galaxies, a result which has now been confirmed via emission line metallicity diagnostics. However three exceptions to this trend have been found, LGRBs 051022, 020819B & 050826. Here I will discuss our observations of the former 2, the unique properties of these bursts and their hosts, and our ongoing efforts to probe these systems at greater spatial resolution (in concert with X-ray and ratio afterglow positions and astrometry) to narrow our observations from the galaxies as a whole to the burst's progenitor regions. Finally I intend to open discussion to the audience on potential mechanisms for generating LGRB exceptions from high metallicity progenitor stars while still on average favoring low metallicity.
Talk slides
pdf
key
One of the most powerful means to study the formation and evolution of gamma-ray bursts is by observing there environments. GRBs can be subdivided into two largely distinct classes based on duration and energy distribution: short-hard and long-soft. While short bursts have been detected in nearby galaxies of all types, long burst hosts are dominated by blue irregulars leading to speculation of metal poor host galaxies, a result which has now been confirmed via emission line metallicity diagnostics. However three exceptions to this trend have been found, LGRBs 051022, 020819B & 050826. Here I will discuss our observations of the former 2, the unique properties of these bursts and their hosts, and our ongoing efforts to probe these systems at greater spatial resolution (in concert with X-ray and ratio afterglow positions and astrometry) to narrow our observations from the galaxies as a whole to the burst's progenitor regions. Finally I intend to open discussion to the audience on potential mechanisms for generating LGRB exceptions from high metallicity progenitor stars while still on average favoring low metallicity.
Talk slides
key
Monday, August 30, 2010
Mass Buildup and Rotational Spindown of Massive Star Magnetospheres
Abstract by Stan Owocki
The generally slow rotation of the sun and other evolved cool stars has long been inferred to stem from the gradual angular momentum loss from their magnetized winds over their multi-Gyr lifetimes. By contrast the relatively short-lived, hot, massive stars generally have rapid rotation; but recent advances in spectropolarimetry have revealed examples of magnetic OB-stars that also have very slow rotation periods. This talk reviews the theory of magnetic spindown, with emphasis on how the seminal Weber-Davis analysis that was developed for the sun is adapted for massive stars, showing for example that their stronger winds and fields can make possible spindown times that are comparable to their relatively short (several Myr) lifetimes. For the magnetic B2V star sigma Ori E, the predicted spindown time of 1.3 Myr has turned out to be in remarkably good agreement with recent direct measurement of spindown from photometric monitoring. I also discuss whether the expected scalings of spindown time with magnetic and wind properties can explain the exceedingly slow rotation periods(> 500 days!) inferred for some magnetic O-type stars.
Background papers
Ud-Doula et al. 2009
Townsend et al. 2010
Talk slides
pdf
key
The generally slow rotation of the sun and other evolved cool stars has long been inferred to stem from the gradual angular momentum loss from their magnetized winds over their multi-Gyr lifetimes. By contrast the relatively short-lived, hot, massive stars generally have rapid rotation; but recent advances in spectropolarimetry have revealed examples of magnetic OB-stars that also have very slow rotation periods. This talk reviews the theory of magnetic spindown, with emphasis on how the seminal Weber-Davis analysis that was developed for the sun is adapted for massive stars, showing for example that their stronger winds and fields can make possible spindown times that are comparable to their relatively short (several Myr) lifetimes. For the magnetic B2V star sigma Ori E, the predicted spindown time of 1.3 Myr has turned out to be in remarkably good agreement with recent direct measurement of spindown from photometric monitoring. I also discuss whether the expected scalings of spindown time with magnetic and wind properties can explain the exceedingly slow rotation periods(> 500 days!) inferred for some magnetic O-type stars.
Background papers
Ud-Doula et al. 2009
Townsend et al. 2010
Talk slides
key
Friday, August 20, 2010
How accurately can star cluster properties be derived from integrated photometry?
Abstract by Soeren Larsen
For star clusters in galaxies beyond the Local Group, determination ofphysical properties such as age, mass, metallicity, etc, generally
relies on comparison of observed colors with Simple Stellar Population (SSP)
models. However, the integrated colors of star clusters may not always
be well approximated by such models. Resolved color-magnitude diagrams are
available for several massive clusters, and are not reproduced in detail by currently
available stellar isochrones. This may due to a variety of effects,
including stellar rotation and/or binary stars, but also possibly a significant
age spread within the clusters. The consequences for integrated colors
have yet to be quantified. For lower-mass clusters, integrated colors
become strongly affected by the stochastic sampling of the stellar IMF, and
a given combination of physical cluster properties no longer corresponds to
unique observables. This seriously hampers our ability to reliably derive
physical properties for clusters below ~10000 Msun, and may in some cases
lead to age- and mass estimates being wrong by several orders of magnitudes.
Talk slides
Subscribe to:
Posts (Atom)