antonino p. milone - università degli studi di padova
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Antonino P. Milone
Spectroscopy of stellar populations
A. P. Milone Stellar Populations University of Padova, 2019
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Homework Homework (by Friday)(by Friday)::– Derive age, distance and reddening of Horologium I*.– Compare your results with those by Brown et al. (2014) based on six UFDs.
– Try to address the following ‘big questions’:
Do true-fossil galaxies exist ?
Is the UFD ‘Horologium I’ a true-fossil galaxy?
Data on more UFDs available on our web pageData on more UFDs available on our web page
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Hydrogen lines
A. P. Milone Stellar Populations University of Padova, 2019
– Lyman series (from n=1). UV. – Balmer transition (from n=2). Optical.– Paschen transition (from n=3). NIR.
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Helium lines
A. P. Milone Stellar Populations University of Padova, 2019
– Helium is the second most-abundant element is stars – Helium lines are detectable only in very hot stars (O-B)
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Metal lines
A. P. Milone Stellar Populations University of Padova, 2019
– Metal lines become stronger as effective temperature decreases– Dominate the spectra of F, G, K stars
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Metal lines
A. P. Milone Stellar Populations University of Padova, 2019
– Example: Optical-NIR ( synthetic) spectrum of a bright main-sequence star of the globular cluster Omega Centauri
Milone et al. 2017
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Metal lines
A. P. Milone Stellar Populations University of Padova, 2019
– Example: Optical-NIR ( synthetic) spectrum of a bright main-sequence star of the globular cluster Omega Centauri
Milone et al. 2017
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Metal lines
A. P. Milone Stellar Populations University of Padova, 2019
– Note that stellar magnitudes are the convolution of the stellar spectrum with the transmission of the filter
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Molecular lines
A. P. Milone Stellar Populations University of Padova, 2019
– Molecular lines form in cool stars (M-, L-, T-types) – The flux is significantly reduced in the bands
Electron transitions Visible and UV linesVibrational transitions Infrared linesRotational transitions Radio-wave lines
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Dominant features in the spectrum
A. P. Milone Stellar Populations University of Padova, 2019
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Application: Ages from the MS knee
A. P. Milone Stellar Populations University of Padova, 2019
The Main-Sequence knee is a feature of the CMD caused by the collision induced absorption of molecular hydrogen
It is well visible in CMDs made with near-infrared photometry.
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Ages from the MS knee
A. P. Milone Stellar Populations University of Padova, 2019
The magnitude difference between the Main-Sequence turn-off and the Main-Sequence knee is used to infer the age of star clusters.
Bono et al. (2010)
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Ages from the MS knee
A. P. Milone Stellar Populations University of Padova, 2019
Assignment: Do this method work? Why? Which are the advantages? Which are the disadvantages?
Bono et al. (2010)
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Ages from the MS knee
A. P. Milone Stellar Populations University of Padova, 2019
– Challenge: the magnitude and color of the MS knee strongly depends on the oxygen abundance!
Synthetic spectra of two faint-MS stars (M-dwarfs) in Omega Centauri. The spectra have the same atmospheric parameters but different oxygen abundance. See Milone et al. (2012, 2017).
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Most-relevant stellar parameters
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Multi-band photometry
A. P. Milone Stellar Populations University of Padova, 2019
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Stellar parameters and in particular effective temperatures can be obtained by comparing stellar colours.
To get the maximum leverage the two wavelength regions should be well separated in wavelength.
Multi-band photometry and stellar parameters
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Photometry of stars with well-known stellar parameters provide empirical Teff-color relations:
Multi-band photometry and stellar parameters
Alonso et al. 1999
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– Opacity, ĸ, indicates the ability of stellar matter to absorb radiation (energy transport equation).
– Need to consider all microscopic processes that can absorb photons at each frequency, ν: Bound-bound absorption (bb) Bound-free absorption (bf) Free-free absorption (ff) Electron scatter (es)
– Opacity in stellar interiors depends on: Level populations of different ion species (dependence on T, –Boltzmann equation– and on the degree of ionisation).Ionization balance (dependence on electron density and T, –Saha equation– )
Electron density (dependence on density, ρ, on T and local composition, Xi).
In summary:
Opacity
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– For wavelength <365nm photoionization from the level n=2 of HI can occur. – The corresponding increase in opacity diminishes the emergent flux shortward of 365nm: this is the Balmer jump.
In hot stars, the Balmer jump depends only on the effective temperature.
Balmer jump
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In solar type stars the Balmer jump also depends on gravity.
Balmer jump
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Stroemgren photometry
Source M. Asplund
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Infrared flux method
Casagrande et al. 2010
It is based on the comparison between the mochromatic flux in the IR with the bolometric flux.