1. Chapter 3
OB TYPE STARS

2. Not “O and B” type Spectroscopic natural group
At low res – weak H Balmer lines
At high res – detection of He lines
B2V through B Ia supergiants on the HRD
Corresponds to
8M☉
30 MYr
Lower mass limit – 8
Upper age limit – 30 Myr 2

3. outline Optical, UV, IR, X-ray Peculiar ones Modelling
Spectral-type effects
Luminosity-class effects
Peculiar ones
Spectral variation
Rotation
Magnetism
Modelling

4. In the optical O classification (MK system) B classification
He II absorption at blue-violet region
He II 4541/ He I 4471 (1 at O7)
He II 4200/ He I 4026 (1 at O6)
B classification
C, N, O, Si
He I spectra – max intensity at B2
OB classification reexamined (twice the photographic dispersion)
Anomalies in CNO
Si IV – III for Early B type
Si III – II for late type
O9.7, B0.2, B0.7
O9.7 – He II 4541/ Si III 4552 = 1
Si IV 4089/ Si III 4552 ~ 1
O3 in Carina Nebula cluster (no He I lines)
But N IV 4058 & N III 4634, 4640, 4642 emission
O2, O3, O3.5
Initially only O5 – O9 classification was present.
And then O4. Then
OB classification CNO anamolies - > SiIV – III for temperature classification 4

5. Fig 3.2 – OB classification O9.7, B0.7
Si IV 4089/ Si III 4552 ~ 1 for O9.7, B0.7 AND He II 4541/ Si III 4552 = 1 5

6. f+ -- selective emission S IV lines
O3 in Carina Nebula cluster (no He I lines)
But N IV 4058 & N III 4634, 4640, 4642 emission
Fig 3.3 – selective emission lines are when some lines come into emission when others from the same ions remain in absorption.
Ex: Si IV 4089, 4116
f+ -- selective emission S IV lines
f* -- N IV em > N III em + Si IV 6

7. Luminosity class criteria for optical
O & B types
Positive luminosity effects in metallic lines
B types –
Metal/He I are sensitive due to Stark effect
He I has negative luminosity effect
Si IV 4089/ He I 4026
Si IV 4116/ He I 4121
Si III 4552/ He I 4387 (B )
In the MK classification
No luminosity classification below O9/O8
Negative luminosity effect below O9
He II 4686
NIII 4634, 4640, 4642 Of
((f)) – strong He II ab & weak NIII em
(f) - weak He II ab & strong NIII em
f - both strong in em
Late O & B0-0.7
He I has neg lum effect because of diminishing Stark Effect because of decreasing pressure with in extended gaints & supergiants
Of stars 7

8. Fig 3.4 - positive luminosity effect in metals in O stars
Si IV 4089/ He I 4026
Si IV 4116/ He I 4121 is shown in the blue box 8

9. Figs 3.4 & 3.5 half & half
In Si III 4552/ He I 4387 (B ) + ve lum effect 9

10. Fig 3.6 selective emission lines… He II on the left is in absorption and He II on the right is in emission
((f)) – strong He II ab & weak NIII em
(f) - weak He II ab & strong NIII em
f - both strong in em 10

11. The ultraviolet OB stars peak in UV FUV (2000 – 1200Å) NUV
1000Å for T = 30,000 K
P Cygni profiles with few 1000 km/sec – wind
Significant mass loss rate
FUV (2000 – 1200Å)
Forests of Fe III through Fe VI absorption
Ionization – positive luminosity effect
Wavelength of highest density – negative luminosity effect
Stellar wind prominent features
Resonance doublets – N V 1239, 1243; Si IV 1394, 1403; C IV 1548, 1551
Metastable subordinate lines – O V1371, He II 1640, N IV 1718
N V & C IV saturated until O 6.5, fades after
NUV
Fe II, Fe I
Ionization effect same as for FUV 11

12. Si IV not seen in main sequence
Fig C IV and N V wind features are saturated until O6.5 after which they decline
Si IV – not een in MS 12

13. O V 1371 – O2-O3 giant & supergiant
Fig 3.10 Si IV - increases in Si IV & reaches saturation at late O types
O IV – is a unique feature for O2.
O V 1371 – O2-O3 giant & supergiant
Si IV – moderate in O4 (+ve effect) 13

14. Wind features in early-B dwarfs are weak
Fig 3.12 Wind profiles are strong in supergiants through out the B type sequence 14

15. B Supergiant
Fig 3.13 Supergints wind profiles are strong. Between types B 0.5 & B 0.7 the wind profiles undergo abrupt transition to lower terminal velocities & deeper absoprtions best seen in Si IV – this may be due to bistability jump.
Left most is N V – lasts till B0.7.
C II starts at B 0.7 and maxes at B3
C IV stops a B3.
Between 0.5 & 0.7 terminal velociyt decreases due to bistability jump. 15

16. Luminosity class effects
OB winds have positive luminosity effect
Wind density increases with luminosity
Same for ionization potentials of certain features
N V & C IV wind profiles are saturated in early O (up to O 6.5)
Later O & early B – N V, C IV strengths reduce
Si IV – normal
N V & C IV high ionization potential AND high abundance – hence saturation
High ionization but low abundance ex. P V – no saturation
N V, C IV - have high ionization pot.
Si IV – lower so doesn’t saturate but shows nice varition 16

17. Fig 3.14 – same stars as 3.6 shown earlier in optical.17

18. The Infrared
Classification of O stars in NIR is based on ionization of He
He I ( µm), He I ( ) & He II ( )
He II disappears after O9 ( no He II in B)
He I disappears before O4
He I in IR lasts only till B3 (unlike optical)
He I ( ) in K-band
Not suitable for classification (atmospheric lines)
More sensitive to winds rather than photospheric changes
H band lines (1.4 – 2 µm) (weaker than K band lines)
He I , He II
Sensitive to gravity & temperature changes
Photospheric lines
Few more in K-band – good temperature correlation
C IV doublet – 2.07, , µm
Strong broad feature at µm
Only seen in O stars
Since H band is weaker than K band, it is more preferred in cases of extinction. But H band is better behaved than K band because they vary according to photospheric changes.
The He I line goes into emission in supergiants and dwarfs cooler than O7. 18

19. C IV – seen in hottest O stars seen in O4 - O7 – depends on S/N
Fig 3.17 Smooth varation of He I & He II with temperature
C IV – seen in hottest O stars
seen in O4 - O7 – depends on S/N
– unidentified – broad feature (N III or C III)
Seen in hotter than O8 & dominates over nearby He I line 19

20. Luminosity class criteria
O5, O7, O9.5, B2
Decreasing gravity, lines become narrower & deeper
Except He I (no significant Stark broadening)
O9 & early B supergiants
He I at 2.149, 2.160, 2.181, 2.184
Luminous Blue Variables & P Cygni stars
Mg II at &
Early & mid O stars
Hɣ emission in Ia supergiants
Luminosity can be predicted if temperature is known
Equivalent width increases with decreasing gravity in IR
In optical, opposite behavior
Stark broadening as a function of electron density
Strong He lines with decreasing gravity = non-LTE effects 20

21. Peculiar categories OBN & OBC Rapid Rotators Magnetic Rotators
ZAMS OB Stars
Metal Deficient
OB/LBV Stars 21

22. OBN & OBC Stars OBC – normal main sequence abundances
Supergiant OB – CNO mixed in the atmospheres & winds
OBN – higher degree of mixing
Anomalies in late supergiant ON/OC
C III 4650 weakening wrt N III 4640 in ONs
N III 4097 weakening in OCs
N II 3995 very weak in BCs
CNO is important for modelling massive stars with rotation
Have small absolute ages & lie near the MS
More rapid mixing process
Fig 22

23. Fig 3.18 – ON/ OC supergiant spectra23

24. Rapid rotators km/s Line widths dominated by turbulence
km/s
Line widths dominated by turbulence
Lower luminosity classes – higher velocity
Peculiar rapid rotators include
Oe ~ Be like – show rotating disks in Balmer line profiles
Onfp/ Oef – absorption reversal in He II 4686 broad line
ONn – rotationally enhanced mixing (significant to study evolution)
Onfp! – very luminous
Currently growing rapidly in Magellanic Clouds
Candidates for stellar merger, gamma ray burst progenitors! 24

25. Question - self absorption in broadedned He II in Onfp – is this the one on the top most right25

26. Magnetic rotators Of?p Compared to Of supergiants Are not supergiants
C III 4647, 4650, 4651 ≈ N III 4634, 4640, 4642
Usually C III is weaker in Of I
Are not supergiants
wrt Si IV resonance lines
H, He & C III/ N III emission line rations – variation in decades
Ex: HD
Large spectral variations (Oblique rotator, magnetically confined wind disk)
C III disappearance
Spectral type change from O 6.5 – 8 (He I filling)
He II 4686, Hα changes entirely
Hα from P Cygni to absorption
Long rotation period (magnetic braking)
They were thought to be supergiants so in comparison to supergiants, C III = N III 26

27. 27

28. ZAMS OB Stars Inverse Of effect (compared to V) Compared to Class-V
He II 4686 absorption stronger relative to other He lines
Emission filling occurs in Class-V spectra
Weak or absent in Vz
Seen in very young regions
Spectra classified as “Vz”
Compared to Class-V
Extreme youth
Subluminous
Broad lines
Weak UV stellar wind profiles
Early O – He II 4686 absorption > He II 4541
O7 – He II 4541 = He I 4471
⇒ He II 4686 > He II 4541 & He I 4471 in Vz
Later than O7 – 4541 << 4686 in V
So, late O Vz has 4686 >> He I 4471 28

29. 29

30. X-ray spectra Strongest lines migrate towards longer wavelengths
With advancing spectral type
Ionization effect
Rapid decline of close pairs of He & H like ionic lines
Mg XII/Mg XI in early O supergiants
Si XIV/Si XIII in early O main sequence
Reversal of Ne X/Ne IX in both
Probably colliding wind binaries 30

31. O Supergiant/ (Giant) 31

32. Astrophysical modeling
CNO abundance important
Because degree of mixing depends on rotation
Single tracks with different initial rotation cross in HRD
After sometime they no longer have the same mass
For ex: OC may be slow rotators on MS
& ON were fast rotators & OB low speed rotators
OC 7.5 ((f)) and O7 Iaf+
Normal abundance in first
Enhanced N in second
Initial mass 40 M☉ & OC slow rotator – loses 1M☉
Of fast rotator – loses 10 M☉ 32

33. All non-rotating O – burn H in the core
Rotating giants – H almost exhausted
Rotating supergiants – post-H burning
Humphreys-Davidson limit – peculiar categories 33

34. 34

35. 35