CP stars – Introduction I
CP subgroups
Classical
name
Prestons’s
group
Discovery criteria Spectral
types
Magnetic
Field
Am – Fm CP1 weak Ca II and/or Sc II;
enhanced metals
A0 – F4 N
Bp – Ap CP2 enhanced Sr, Cr, Eu, and/or Si B6 – F4 Y
HgMn CP3 enhanced Hg II and/or Mn II B6 – A0 N
He – weak CP4 weak He I compared with
colours
B2 – B8 Y
He – strong enhanced He I compared with
colours
B0 – B2 Y
Preston (1974, ARA&A, 12, 257), Pedersen & Thomsen (1977, A&AS, 30, 11)
Metallicity - Basics
• Metallicity as [X:Y:Z]
• X = Hydrogen
• Y = Helium
• Z = „the rest“
𝑋 ≡
𝑚 𝐻
𝑀
Y ≡
𝑚 𝐻𝑒
𝑀
Z = σ𝑖>𝐻𝑒
𝑚𝑖
𝑀
= 1 – X – Y
Metallicity - designations
• In the literature you will find
– [Z]
– [Fe/H]
– [M/H]
– [Element 1 / Element 2]
• Relations for the transformation are necessary
Metallicity – designations
Metallicity - designations
• [dex], e.g. [Fe/H] = -0,5 dex
dex factor dex factor
-2 0,01 0,1 1,26
-1,5 0,03 0,2 1,58
-1 0,10 0,3 2,00
-0,9 0,13 0,4 2,51
-0,8 0,16 0,5 3,16
-0,7 0,20 0,6 3,98
-0,6 0,25 0,7 5,01
-0,5 0,32 0,8 6,31
-0,4 0,40 0,9 7,94
-0,3 0,50 1 10,00
-0,2 0,63 1,5 31,62
-0,1 0,79 2 100,00
The Sun as standard star
• „Our“ standard star for the normalisation of
the metallicity is the Sun
• We define:
– Mass
– Luminosity = absolute (bolometric) magnitude
– Temperature = spectral type = colour
– Age
– Chemical composition
– Internal structure (rotation, magnetic field,
convection, diffusion, pulsation, …)
Abundance analysis - Sun
• Review article: Asplund et al., 2009, Annual
Review of Astronomy & Astrophysics, 47, 481
• Ingredients:
– Stellar atmosphere
– Atomic line data
– High resolution spectra
– Analysis method
– Starting parameter
• Gray, 2005, The Observation and Analysis of
Stellar Photospheres, Cambridge University Press
Abundance - Sun
• Problems with
–Hydrogen
–Helium
–Elements with only a few lines
–Elements with only weak lines
• LTE versus NLTE (Non-Local
Thermodynamic Equilibrium)
Abundance - Sun
Asplund et al.
Abundance - Sun
Asplund et al.
Abundance - Sun
Asplund et al.
Metallicity => different opacity
Isochrones for 10 Myr
Metallicity - isochrones
Schaller et al., 1993, A&AS, 101, 415
Different He abundances – [Z]
constant
What means „normal“?
• Is the abundance of the Sun representative
for all stars in our neighbourhood?
• If not, what is the spread?
• Is the metallicity in the solar
neighbourhood representative for the
whole Milky Way?
• Is the metallicity depending on the age?
What means „normal“?
• Allende Prieto et al., 2004, A&A, 420, 183:
analysis of 118 FGK stars within 15 pc
around the Sun
• Based on
Hipparcos
parallaxes
What means „normal“?
• Wide spread of [Fe/H] abundances
• Sun -0.1 dex underabundant
What means „normal“?
What means „normal“?
• Netopil et al., 2016, A&A, 585, A150
• Metallicities of open clusters
Abundances CP1 stars
Ghazaryan et al., 2018, MNRAS, 480, 2953
Abundances CP2/4 stars
Ghazaryan et al., 2018, MNRAS, 480, 2953
Abundances CP3 stars
Ghazaryan et al., 2018, MNRAS, 480, 2953
Abundance - Sun
Asplund et al.
Basic Classification
⚫ Parameters to estimate:
1. Spectral type (temperature)
2. Luminosity class (log g, age)
3. v sin i (rotation)
4. [metallicity]
⚫ How?
1. Line strengths
2. Line ratios
3. Equivalent widths
⚫ Why?
1. Very efficient
2. Well tested
Used Notation I
⚫ O-B-A-F-G-K-M-(-R-N), Yerkes = MK Classification
⚫ „Official subclasses“
⚫ O: 4, 5, 6, 7, 8, 9, 9.5
⚫ B: 0, 0.5, 1, 2, 3, 5, 7, 8, 9.5
⚫ A: 0, 2, 3, 5, 7
⚫ F: 0, 2, 3, 5, 7, 8, 9
⚫ G: 0, 2, 5, 8
⚫ K: 0, 2, 3, 4, 5
⚫ M: 0, 1, 2, 3, 4, 7, 8
⚫ For these subclasses you will find also “official”
standard stars
Used Notation II
⚫ These subclasses were extended during time,
see for example Gray (1989, AJ 89, 1049)
⚫ Additional suffices: n, nn, e, weak, st…
⚫ Especially for B/A/F stars: i.e. hA0kA5mA3 V
this means that the hydrogen lines (h) have the
characteristics as in a A0 star, CaH&K (k) A5
and the metallic lines (m) A3
Freedman & Kaufmann, 2002, Universe, Sixth Edition, Freeman Company
Classification spectroscopy I
Chemically peculiar stars
Hümmerich et al., 2021, A&A, 640, A40
Chemically peculiar stars
Hümmerich et al., 2021, A&A, 640, A40
Chemically peculiar stars
Hümmerich: An Atlas of LAMOST Low-Resolution Spectra of Chemically Peculiar Stars