STELLAR SPECTRAL CLASSIFICATION THE FIRST STEP IN QUANTITATIVE SPECTRAL ANALYSIS PART I AND II Ewa Niemczura Astronomical Institute, University of Wrocław

References Book: „Stellar spectral classification”, R.O. Gray & C.J. Corbally Lecture and paper: „Stellar spectral classification”. R.O. Gray, Spring School of Spectroscopic Data Analyses, Wrocław

Classification – essential activity of Science

How to do spectral classification? Via direct comparison with spectra of standard stars; Spectral region/resolution are of secondary importance; Spectral type and luminosity class are fundamental data only if no theory or other data is used; Standard stars: Anchor points, Primary standards, Secondary standards; Method: visual (or authomatical); we need comparison stars: when observing your stars, observe also few standards; Complicated and iterative process.

Why spectral classification is important? Independent source of information and the beginning point for further spectral analysis: locate star in the H-R diagram; first estimate of T eff, logg, [m/H], rotation velocity; Identification of chemically peculiar stars and astrophysicaly interesting objects; useful “reality check” to an analysis based on stellar atmosphere theory.

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Stellar spectral classification

O and B-type stars O-type stars: T eff from ~28000 to ~50000 K M from 16 to ~100 M sun B-type stars: T eff from ~10000 to ~28000 K M from 2 to ~16 M sun

Spectra of O- and B-type stars Optical part H He C N O Ne Mg Al Si S Fe

Spectral classification Optical part – spectral type Balmer lines of hydrogen Helium lines: He I (B-type stars) Helium lines: He II (O-type stars)

Spectral classification O-type stars Helium lines: He I Helium lines: He II Balmer lines of hydrogen

Spectral classification (B-type stars) Optical part – spectral type

Spectral classification Optical part – spectral type Helium lines: He I (from O-type stars, maximum at a spectral type of B2; disappearing at a spectral type of A0) Helium lines: He II (O-type stars) Balmer lines of hydrogen (maximum at a spectral type of A2) Spectral classification: He I,II and Balmer lines In case of B-type stars: helium abundance anomalies Solution: silicon lines ratios: Si III/Si II and Si IV/Si III

Spectral classification (B-type stars) Optical part – spectral type

Spectral classification Optical part – spectral type Helium lines: He I Helium lines: He II Balmer lines of hydrogen Spectral classification: He I and Balmer lines Helium abundance anomalies Silicon lines ratios: Si III/Si II and Si IV/Si III Balmer lines and Si lines are luminosity sensitive Mg II (4481 Å): ratio He I (4471 Å)/Mg II (4481 Å)

Spectral classification (B-type stars) Optical part – spectral type

Spectral classification Optical part – spectral type Helium lines: He I Helium lines: He II Balmer lines of hydrogen Spectral classification: He I and Balmer lines Helium abundance anomalies Silicon lines ratios: Si III/Si II and Si IV/Si III Balmer lines and Si lines are luminosity sensitive Mg II (4481 Å): ratio He I (4471 Å)/Mg II (4481 Å)

Spectral classification Optical part – luminosity class Balmer lines of hydrogen (luminosity sensitivity is greatest in the late B-type stars)

Spectral classification Optical part – luminosity class Balmer lines of hydrogen (luminosity sensitivity is greatest in the late B-type stars) B-type stars: O II lines (increase in strength with increasing luminosity) Ratios: O II with Balmer lines and the He I lines

Spectral classification (B-type stars) Optical part – luminosity class

Spectral classification Optical part – luminosity class Balmer lines of hydrogen (luminosity sensitivity is greatest in the late B-type stars) O II lines (increase with strength with increasing luminosity) Rations: O II with Balmer lines and the He I lines But: CNO peculiarities Solution: ratios of silicon lines to He I

Spectral classification Optical part – luminosity class

Balmer lines of hydrogen (luminosity sensitivity is greatest in the late B-type stars) O II lines (increase with strength with increasing luminosity) Rations: O II with Balmer lines and the He I lines But: CNO peculiarities Solution: ratios of silicon lines to He I N II (3995 Å)

Spectral classification Optical part – luminosity class

Spectral classification (O-type stars) Optical part – luminosity class N III Si IV S IV

Spectral classification (O-type stars) Optical part – luminosity class Balmer lines Si IV / H  Si IV/ He I

Spectral classification Optical part Spectral classification: iterative process Spectral type, B-type stars: Balmer lines Helium lines Other lines if necessary Spectral type, O-type stars: He II lines Luminosity class: Balmer lines + metal lines Warning: chemically peculiar stars!

Chemically peculiar stars B-type stars Helium-strong stars, He-s Spectral types: B3 or earlier Strong lines of He I

Chemically peculiar B stars: He-strong Spectral types: B3 or earlier Strong lines of He I Strong magnetic fields Photometric and spectroscopic variability – oblique rotator model (magnetic axis is inclined with respect to the rotational axis).

Oblique rotator model: magnetic axis is inclined with respect to the rotational axis.

Chemically peculiar B-type stars Helium-weak stars, He-w Spectral types: B3 or later Weak lines of He I Variability Magnetic field

Chemically peculiar B-type stars Helium-weak star subclasses:  Si stars ( enhanced Si II lines)  PGa stars  SrTi stars

Chemically peculiar B-type stars Helium-weak stars, He-w Spectral types: B3 or later Weak lines of He I Variability Magnetic field Helium-weak stars subclasses:  Si stars (enhanced Si II lines, hotter than classical Ap Si stars)  SrTi stars (enhanced Sr and Ti, lines, hotter than classical Ap Sr stars) Magnetic He-weak B-type stars – hot end of the magnetic Ap stars

Chemically peculiar B-type stars Helium-weak stars, He-w Spectral types: B3 or later Weak lines of He I Variability Magnetic field Helium-weak stars subclasses:  Si stars (enhanced Si II lines, hotter than classical Ap Si stars)  SrTi stars (enhanced Sr and Ti lines, hotter than classical Ap Sr stars)  PGa stars (enhanced P and Ga lines, hot end of HgMn stars, no detection of magnetic field)

Chemically peculiar B-type stars HgMn (mercury-manganese) stars: Spectral types: B7-B9, luminosity class III-V Strong lines of Hg II and Mn II; mild helium deficiencies and other peculiarities Magnetic field Periodic spectral variations (non-uniform distribution of abundances on the stellar surface, so the presence of a magnetic field) Connection with hot-Am stars?

Chemically peculiar B-type stars Helium-strong Helium-weak Si SrTi PGa HgMn Bp stars Si II Cr II Sr II Eu II

Chemically peculiar B-type stars Summary TypeSpTMagnetic fieldVariability He-sB1-B3Yes He-w SiB3-B8Yes He-w SrTiB3-B8Yes He-w PGaB3-B7?? HgMnB7-B9Yes Bp (Si,Cr,Sr,Eu) late B, A, F0 Yes

Chemically peculiar B-type stars: 3 He – 4 He Peculiar profiles of helium lines: 3 He – 4 He stars 4 He 3 He

Chemically peculiar B-type stars: 3 He – 4 He Bohlender (2005)

CP stars on H-R diagram Bohlender (2005)

A and F-type stars A-type stars: T eff from ~7500 to ~10000K; M from ~1.4 to ~2.1 M sun F-type stars: T eff from ~6000 to ~7500K; M from ~1.04 to ~1.4 M sun

10000 K 9000 K 8000 K 7000 K 6000 K

Spectral classification (A-type stars) Optical part – spectral type

Hydrogen Balmer lines – maximum at ~A2 Calcium Ca II K line; Lines of metals, Fe I (4271 Å, 4046 Å, 4383 Å), Ca I (4226 Å), Mn I (4030 Å) Problem: hydrogen lines and metal lines (e.g. Fe II and Ti II) – sensitive to luminosity (logg). Solution: Ca II K, and ratio of Ca II K to H  or H  – prime spectral type criterion. For „normal” stars – the same spectral type from all three criteria. CP stars: Ca II K lines may be weak; metal lines: abnormal!

Spectral classification (A-type stars) Optical part – luminosity class

Primary criterium: wings of hydrogen lines (< A6-A7); Hydrogen lines are sensitive to luminosity class and spectral type: iterative process; ~F2 – hydrogen lines are not sensitive to luminosity > A7 – luminosity class from ionised lines of Fe and Ti Problem: CP stars (e.g. metal-weak Boo)

Spectral classification (A-type stars) Optical part – luminosity class

Primary criterium: wings of hydrogen lines (< A6-A7); Hydrogen lines are sensitive luminosity class and spectral type: iterative process; ~F2 – hydrogen lines are not sensitive to luminosity > A7 – luminosity class from ionised lines of Fe and Ti; ratios of Fe II, Ti II lines to Fe I lines Around A7 – difficult to determine luminosity class (especially to separate dwarf and giants). Problem: CP stars (e.g. metal-weak Boo)

Chemically peculiar Am stars Am, metallic-line A-type stars: A- and early F-type stars with Ca II K-line spectral type earlier than the metallic-line spectral type (at least 5 spectral subclacces). Proto-Am stars: the difference is less than 5 spectral subclasses. e.g. spectral type of 63 Tau: kA1.5hA9mF3

Chemically peculiar Am stars

Am, metallic-line A-type stars: A- and early F-type stars with Ca II K-line spectral type earlier than the metallic-line spectral type (at least 5 spectral subclacces). Proto-Am stars: the difference is less than 5 spectral subclasses. e.g. spectral type of 63 Tau: kA1.5hA9mF3 (III) Anomalous luminosity effect (ALE): lines of 4395– 4444Å, 4395–4400Å and 4417Å – dwarfs; Fe II/Ti II 4172–9Å blend – giants.

Chemically peculiar Am stars Am, metallic-line A-type stars: A- and early F-type stars with Ca II K-line spectral type earlier than the metallic-line spectral type (at least 5 spectral subclacces). Proto-Am stars: the difference is less than 5 spectral subclasses. Anomalous luminosity effect (ALE): lines of 4395– 4444Å, 4395–4400Å and 4417Å – dwarfs; Fe II/Ti II 4172–9Å blend – giants. Peculiar abundance pattern: calcium and scandium are underabundant; iron-peak elements and heavier elements are overabundant.

Chemically peculiar Am stars Lines: Sc, Sr, Y, Zr, Ca, Fe Gebran et al. (2010)

Chemically peculiar Am stars

Am stars mechnism: chemical separation driven by radiative and gravitational acceleration. In normal A-type stars: chemical separation < effects of rotation (meridional circulation). Am stars are slow rotators: chemical separation > mixing by meridional circulation. Later spectral types: strong convection.

Chemically peculiar Am stars Mixing–length theoryTurbulent convectionConvective overshooting B. Smalley, Spring School of Spectroscopic Data Analyses, Wrocław 2013

Chemically peculiar Am stars Summary Complicated spectral types Chemical peculiarities ALE Slowly rotators Most in binarys systems Pulsating stars (SuperWASP, Kepler observations) Magnetic fields?

Chemically peculiar Ap stars Ap, peculiar A-type stars: only selected elements have greatly enhanced abundances. Most of the Ap stars are B-type stars in terms of effective temperature; but the coolest are early F-type stars.

Chemically peculiar Ap stars

Ap or peculiar A-type stars: only selected elements have greatly enhanced abundances. Most of the Ap stars are B-type stars; the coolest are early F-type stars. For spectral classification: Ca II K-line; but line is often peculiarly weak or strong, or has an unusual profile, the correlation with effective temperature is quite poor. Hydrogen lines; but in extreme Ap stars, the structure of the stellar atmosphere is distorted – unusual hydrogen line profiles (e.g. roAp stars).

Chemically peculiar Ap stars Predominant chemical peculiarities

Chemically peculiar Ap stars

Complex blend is in Ap stars: the most important contributer is Eu II, but lines of Fe I and Fe II and rare earths Ce II and Gd II are involved.

Chemically peculiar Ap stars Some interesting cases Cl Co Au Hg …

Chemically peculiar Ap stars Some interesting cases

Chemically peculiar Ap stars Przybylski star Strongest lines: Singly ionised lanthanides; Fe: deficient Lines of Pm, Tc Half-life: Tc: 4.2x10 6 yr Pm: 17.7 yr

Chemically peculiar Ap stars Summary Chemically peculiar stars Difficult to do spectral classification Most are slow rotators Spots on the surface (as Bp stars) Magnetic stars (oblique rotation model) roAp – rapidly oscillating Ap stars (e.g. Przybylski star)  UMa

Chemically peculiar stars: Boo λ Bootis stars: metal-weak, population I A-type stars

Chemically peculiar stars: Boo λ Bootis stars: spectral type (from the hydrogen lines): from B9.5 to F0 with possible members as late as F3; weak Mg II λ4481 lines; general metal-weak character; broad hydrogen lines (stars on or near the main-sequence); rotation velocities typical for A stars; circumstellar disc (not all Boo stars); explanation of CP: selective accretion/diffusion theory (metal- depleted gas from IS is accreted by the star, required accretion rate: M sun yr -1 ; gas can be associated from IS, circumstellar disc or cometary bodies). no magnetic fields? rare objects.

Spectral classification (F stars) Spectral type

Hydrogen lines – prime spectral type criteria; least affected by differences in metallicity; Metal lines: Ca I 4226Å, Fe I 4046Å, 4384Å; ratios of metal lines with hydrogen lines; G-band due to CH diatomic molecule (from F3-F4); But: CP metal weak stars.

Spectral classification (F stars) Spectral type

Spectral classification (F-type stars) Luminosity class

F0 – F6: lines of ionized iron and titanium; blends at λλ4172–8, λλ4395–4400, λ4417, λ4444, and the entire Ti II – Fe II “forest” near 4500 Å; Ratios with lines that do not show a strong luminosity sensitivity, such as Fe I λ4046, λ4271, and λ4383, and Ca I λ4226; F6 and later: ratio Sr II λ4077/Fe I λ4046, or λ4077/Hδ; F8 and later: Ca II H and K.

Spectral classification (F-type stars) Luminosity class

Chemically peculiar stars:  Pup ρ Puppis: group of unusually late, probably evolved Am- type stars.

Chemically peculiar stars:  Pup ρ Puppis stars: group of unusually late, probably evolved Am-type stars. ρ Pup, θ Gru, and HD – prototypes of the ρ Puppis class of stars: (1) late Am stars (show ALE); (2) hydrogen-line spectral types are F5, late for Am stars; (3) luminosity types, determined from the Fe II, Ti II λλ4172–9 blend, and Sr II λλ4077 and 4216 lines are from II–III to Ib.

Chemically peculiar stars: F-type Sr λ4077 Strong Stars and Barium Dwarfs

Chemically peculiar stars: F-type λ4077 Strong Stars and Barium Dwarfs “λ4077 strong” – Sr II λ4077 line appears abnormally strong (F5 – G-type stars); some of them are late Am or ρ Pup stars some of them are late-F, early-G dwarfs with Sr overabundance and overabundances of other s-process elements including Ba – barium dwarfs.

Conclusions Spectral classification yields good starting estimates for the physical parameters of your star. Spectral Classification is an essential first step in stellar spectral analysis!