F-type Stars “Stars in transition”

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 • 6100-7400 K • 1.0-1.4 Mʘ • Radius= 1.0-1.7 Rʘ • Luminosity= 1-20 Lʘ • Lifetime= 1000-10000 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 (10-100 km/s)  narrow lines • Cooler temperatures allow for molecules to form (ex. CH G line)

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, λλ4395-4400, λ4417, and λ4444. Used in ratio with luminosity independent lines • SrII 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

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 (λλ8498-8662) triplet • 83375- 8770Å 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’smethod 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 λλ4172-8 blend), above the main sequence • Some are δ Scutipulsators. But lack of Am star helium convection for δ Scutipulsator

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 SrII 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

23. 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 (Kaler237) • 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)

26. High-Latitude F Supergiants • Most supergiant F stars: Population I (galactic disk) • Some at high latitudes: high velocity, low metallicity, photometricallyand 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, 2009. Web. • Kaler, James B. Stars and Their Spectra: An Introduction to the Spectral Sequence. 2nd ed. Cambridge: Cambridge University Press, 2011. Print. • Jascheck, Carlos and Mercedes Jascheck. The Classification of Stars. Cambridge: Cambridge University Press, 1987. Print.

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