Post on 30-Apr-2022
Antonino P. Milone
Spectroscopy of stellar populations
A. P. Milone Stellar Populations University of Padova, 2019
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
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.
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)
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
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
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
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
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
Dominant features in the spectrum
A. P. Milone Stellar Populations University of Padova, 2019
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.
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)
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)
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).
Most-relevant stellar parameters
Multi-band photometry
A. P. Milone Stellar Populations University of Padova, 2019
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
Photometry of stars with well-known stellar parameters provide empirical Teff-color relations:
Multi-band photometry and stellar parameters
Alonso et al. 1999
– 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
– 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
In solar type stars the Balmer jump also depends on gravity.
Balmer jump
Stroemgren photometry
Source M. Asplund
Infrared flux method
Casagrande et al. 2010
It is based on the comparison between the mochromatic flux in the IR with the bolometric flux.