F-type Stars“Stars in transition”

Caroline Roberts

Polaris A and B, F-type stars; credit: NASA

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

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)

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

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• Sr II λ4077, 4216, often in ratio with Fe I (mostly used in late F)

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

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

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

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

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

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)

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 Scuti pulsators. But lack of Am star helium convection for

δ Scuti pulsator

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

Variable Stars

• Instability Strip • Changing spectral

type• Repeating pulsation

caused by a changing balance between radiation pressure and gravity

credit: Swinburne University of Technology

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)

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

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.

Thank You

Canopus, an F-type Star; credit: NASA