Stellar Spectral classification The First Step in Quantitative Spectral Analysis PART I AND II

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STELLAR SPECTRAL CLASSIFICATIONTHE FIRST STEP IN QUANTITATIVE SPECTRAL ANALYSISPART I AND II

Ewa NiemczuraAstronomical Institute, University of Wrocław

eniem@astro.uni.wroc.pl

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 2013

• http://stellar.phys.appstate.edu/Standards/

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 Teff, 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.

O Be A Fine

Girl Kiss Me

Stellar spectral classification

O and B-type stars

• O-type stars: • Teff from ~28000 to ~50000 K

• M from 16 to ~100 Msun

• B-type stars: • Teff from ~10000 to ~28000 K

• M from 2 to ~16 Msun

𝑇 eff=40kK

𝑇 eff=35kK

𝑇 eff=30 kK

𝑇 eff=25 kK𝑇 eff=20 kK

𝑇 eff=15kK

𝑇 eff=40kK𝑇 eff=35kK𝑇 eff=30 kK

𝑇 eff=25 kK𝑇 eff=20 kK

𝑇 eff=15kK

𝑇 eff=10kK

Spectra of O- and B-type starsOptical part

HHeCNONeMgAlSiSFe

Spectral classificationOptical 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

• 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

• 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)

• 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 classificationOptical 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

• 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

• 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 (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 classificationOptical 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

Helium-weak star subclasses: Si stars ( enhanced Si II lines) PGa stars SrTi stars

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

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)

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?

• Helium-strong• Helium-weak

• Si • SrTi • PGa

• HgMn • Bp stars

• Si II• Cr II• Sr II• Eu II

Chemically peculiar B-type starsSummary

Type SpT Magnetic field Variability

He-s B1-B3 Yes Yes

He-w Si B3-B8 Yes Yes

He-w SrTi B3-B8 Yes Yes

He-w PGa B3-B7 ? ?

HgMn B7-B9 Yes Yes

Bp (Si,Cr,Sr,Eu)

late B, A, F0

Yes Yes

Chemically peculiar B-type stars: 3He – 4He

• Peculiar profiles of helium lines: 3He – 4He stars

4He3He

Chemically peculiar B-type stars: 3He – 4He

Bohlender (2005)

CP stars on H-R diagram

Bohlender (2005)

A and F-type stars

• A-type stars: • Teff from ~7500 to ~10000K; M from ~1.4 to ~2.1 Msun

• F-type stars: • Teff from ~6000 to ~7500K; M from ~1.04 to ~1.4 Msun

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

• 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.

• Peculiar abundance pattern: calcium and scandium are underabundant; iron-peak elements and heavier elements are overabundant.

Chemically peculiar Am starsLines: Sc, Sr, Y, Zr, Ca, Fe

Gebran et al. (2010)

Chemically peculiar Am starsAm 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.

Mixing–length theory Turbulent convection Convective overshooting

B. Smalley, Spring School of Spectroscopic Data Analyses, Wrocław 2013

Chemically peculiar Am starsSummary

• 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.

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 starsPredominant chemical peculiarities

Chemically peculiar Ap starsComplex 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 starsSome interesting cases

ClCoAuHg…

Chemically peculiar Ap starsSome interesting cases

Chemically peculiar Ap starsPrzybylski star

Strongest lines: Singly ionised lanthanides;

Fe: deficient

Lines of Pm, Tc

Half-life:Tc: 4.2x106yrPm: 17.7 yr

Chemically peculiar Ap starsSummary

• 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: 10-14 Msunyr-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 103877 – 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!

Stellar Spectral classification The First Step in Quantitative Spectral Analysis PART I AND II

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