1. F-type Stars “Stars in transition”
Caroline Roberts
Polaris A and B, F-type stars; credit: NASA

2. Basic Information 3% of solar neighborhood
Examples: Polaris, Dubhe, Canopus, Procyon K Mʘ
Radius= Rʘ
Luminosity= 1-20 Lʘ
Lifetime= million yrs

3. Basic Information
Through the F-type classification, stars develop a convective layer to their atmosphere
Surface chemical peculiarities washed out in early F-types
Present in later F-types; brings up material from the core
Convection  magnetic field  stellar wind  rapid rotation breaking ( km/s)  narrow lines
Cooler temperatures allow for molecules to form (ex. CH G line)
CH is carbyne?

4. Optical Spectra Main identifier is strength of hydrogen lines
Ca II K-line, but plateaus past F3
Fe I λλ4046 and 4383 and Ca I λ4226 lines: strengthening through class
F3 or F4: G-band from the CH diatomic molecule

7. Optical Spectra- Luminosity
Fe II lines and Ti II lines: “forest” around 4200Å but in particular λλ4172-9, λλ , λ4417, and λ4444. Used in ratio with luminosity independent lines
Sr II λ4077, 4216, often in ratio with Fe I (mostly used in late F)

12. Optical Spectra- Luminosity
In Fe II and Ti II lines, metastable levels play a role as well as electron density
Microturbulence broadens lines. Turbulence in the star’s atm > than mean free path
Bolton 1971: microturbulence becomes important factor as well as pressure/gravity
You must up the turbulence (broaden) when you up the pressure (narrow) or it won’t work out

13. Ultraviolet Spectra Classification limited in the UV
Prominent shape, highly metallicity dependent
Mg II H & K lines are the main identifiers. Also: Fe II and Fe I blend at λ2745 and Mg I at λ2852
Blends of hundreds of lines. Line labeled by main contributor

15. Infrared Spectra
Near Infrared: three main features: Hα, O I λ8446 triplet, and the Ca II (λλ ) triplet
Å lacks telluric (atmospheric) lines, good to study: higher Paschen lines, O I λ7774, and the Ca II triplet
“Dead zone” in late classifications where spectra look similar

17. J, H, and K Bands J Band has two main lines: Paschen β and γ
H Band: early stars: high Brackett lines in early stars, intermediate classes: there are neutral metal lines (Mg I, Si I, and Al I), latest classes: spectra dominated by metals
K Band: main line is Brackett γ

18. Population II F-type Stars
Metal-weak stars
Population I stars are in the disk of galaxy while Population II stars come from the halos and are older
Higher velocity

19. Population II F-type Stars
Turn off around FV, most metal weak Population II stars are F5 or later, though a few as early as F0
Low electron densities  more ionization
Houk’s method for classification: Identify the temperature type + “w” for metal weakness + metallicity type (Ex. G0wF2)
Imprecise: dependence on T
Many variations of classification
Gray’s method for classification: Line pattern ratios (hydrogen lines, metallic lines both metallicity dependent (Cr I/Fe I) and independent, given “m±#” designation (Ex. F9 V m-2.25)

21. ρ Puppis Stars
No longer a δ Del: group re-studied in 1989 and divided into smaller groups, Am, δ Del, ρ Puppis, etc.
Late Am stars, F5 and later hydrogen line spectra, luminosity class of Ib-III (Sr II λ4077, λ4216, Fe II/Ti II λλ blend), above the main sequence
Some are δ Scuti pulsators. But lack of Am star helium convection for δ Scuti pulsator

22. F-type λ4077 Strong Stars and Barium Dwarfs
F5 or later, Sr II strong
Many Am or δ Del/ρ Puppis stars, some also barium dwarfs (not just Sr II lines; other s-process lines too)
Cores, convective envelopes
Possible binaries: overflow or wind accretion brings s-process elements onto the star via a companion. Possible but not certain
Dwarfs do not have the cores necessary for these processes, and even if they did, their convective envelopes should have washed out these spectral lines.

23. credit: Swinburne University of Technology
Variable Stars
Instability Strip
Changing spectral type
Repeating pulsation caused by a changing balance between radiation pressure and gravity
credit: Swinburne University of Technology

24. RR Lyrae Stars
H/He ionization inside the star. Normal T increase would decrease Χ, but here it increases it. A “squeeze” of pressure gives a resulting outward push of radiation pressure. The star expands, decreases density, repeats (Kaler 237)
Not as spectrally variable as their A-type companions in the blue- violet
Metallicity classified by ΔS = hydrogen type – Ca II K type (Ex. ΔS = F0 – A2 = 8)

25. Middle RR Lyrae is metal weaker

26. High-Latitude F Supergiants
Most supergiant F stars: Population I (galactic disk)
Some at high latitudes: high velocity, low metallicity, photometrically and spectroscopically variable
Weaker H lines, Sr II λ4216 Ca I λ4226 and Fe II λ4233 are not equal like they should be
Possibly ejected, or born above the plane

27. References
Gray, Richard O. and Christopher J. Corbally. Stellar Spectral Classification. Princeton, NJ: Princeton University Press, Web.
Kaler, James B. Stars and Their Spectra: An Introduction to the Spectral Sequence. 2nd ed. Cambridge: Cambridge University Press, Print.
Jascheck, Carlos and Mercedes Jascheck. The Classification of Stars. Cambridge: Cambridge University Press, Print.

28. Canopus, an F-type Star; credit: NASA
Thank You
Canopus, an F-type Star; credit: NASA