Quantitative Spectroscopy of OB Stars Hydrogen, Helium and Carbon

1
Quantitative Spectroscopy of OB StarsHydrogen,
Helium and Carbon

• María Fernanda Nieva(1,2)
• (1) University of Erlangen-Nuremberg (Germany)
• (2) Observatório Nacional (Brazil)

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Outline

• Introduction
• Motivation
• Aims
• Analysis Methodology
• Results
• Conclusions
• Perspectives

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1. Introduction
4
What are OB stars?
OB-type V-III dwarfs (Main Sequence) giants
young/massive
hot/luminous stars
1. introduction
5
What do we observe?
Observer
In,m cos q
q
Stellar atmosphere
Emergent stellar spectrum

• Superposition of
• continuous spectrum
• line spectrum

Flux
l
1. introduction
6
Continuous spectrum
bound-free transitions
Flux
l
free-free transitions
Line spectrum
Normalized Flux
deeper layers
bound-bound transitions
outer layers
l
1. introduction
7
Physical information inferred from stellar line
spectrum

• Atmospheric plasma conditions (through effective
  temperature, surface gravity)
• Chemical composition
• Projected rotational velocity
• Velocitiy fields (micro/macroturbulence)

Others not considered here, e.g. magnetic fields,
stellar winds
1. introduction
8
How? Quantitative Spectroscopy
Theory synthetic lines
Atomic Physics
Model Atoms
Stellar Atmospheres
Norm. Flux
l
Line fits
Observations
Spectra
Physical parameters
- Accuracy depends on all steps of
analysis - Difficult to control
systematic effects
1. introduction
9
Classical model atmospheres

• Plane-parallel geometry
• Homogeneity
• Stationarity
• Hydrostatic equilibrium (momentum conservation)
• Radiative equilibrium (energy conservation)

1. introduction
10
Thermodynamic State
Volume elements independent from each other
Local TE
(T,ne)
Excitation/Ionization of the gas described by
Saha/Boltzmann equations
T,r,ne,t
Volume elements coupled by radiation field
Non-Local TE
Excitation/Ionization of the gas from coupling of
statistical equilibrium radiative transfer
equations
(Tcin,ne,Jn)
g
g
g
T,r,ne,t
1. introduction
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2. Motivation
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Hydrogen, Helium Carbon
Why is carbon important?

• one of most abundant metals in universe
• created in triple alpha reaction (evolved stars)
  
• starting point for nucleosynthesis of heavier
  elements
• CNO cycle (H ? He massive stars)
• basis of organic chemistry

Why are HHe important?

• most abundant elements
• main absorbers in OB dwarfs and giant
  atmospheres
• spectral lines indicators for atmospheric
  parameters

2. motivation
13
Why are chemical abundances of OB stars important?
Observational constraints

• stellar evolution
• basic stellar parameters chemical abundances
  (N/C) empirical evaluation of evolutionary
  models
• galactochemical evolution
• - present-day abundances (young stars)
• - spatial distribution (abundance gradients)

OB stars are luminous one can sample large
distances (Galactic and extragalactic)
Sun
2. motivation
14
Example C abundance Gradient in the Galaxy
H II regions1
Sun
B-type stars2
Chiappini et al. 2003
1 Esteban et al. (1999) 2 Gummersbach et al.
(1998)
2. motivation
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Carbon abundances of OB dwarfs giants in the
solar vicinity
Previous results

• sub-solar chemical evolution of the
  Galaxy ?
• why M/H young stars lt M/H older stars ?
• highly inhomogeneous stellar/galactochemica
  l evolution ?
• why DM/H young stars gt1 order of magnitude ?

(Sun standard?)
Sun
2. motivation
16
Carbon abundances of OB dwarfs giants in the
solar vicinity
Some sources from the literature
carbon
1.6 dex !
2. motivation
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Carbon abundances of OB dwarfs giants in the
solar vicinity
Previous work indicate

• Carbon
• Large non-LTE effects
• e(C II) strong lines ? e(C II) weak lines
• e(C II) ? e(C III) no ionization equilibrium

Temperatures (discrepancies underestimated) Teff
photometric ? Teff spectroscopic up to 5000 K
in OB dwarfs and giants (gt20)

• H,He LTE/NLTE?
• consistent? ALL measurable lines?

2. motivation
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3. Aims
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Aim of this work

• Solve classical problem of carbon abundance
  determination in OB stars
• Construction of robust C II/III/IV model atom
• Derivation of reliable atmospheric parameters

3. aims
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4. Analysis Methodology
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Spectral modelling

• Hybrid non-LTE approach
• - LTE model atmosphere ATLAS9 (Kurucz 1993)
• - non-LTE line formation DETAILSURFACE
  (Giddings, 1981 Butler Giddings 1985 updated
  by K. Butler)
• Model atoms
• H (Przybilla Butler 2004)
• He I/II (Przybilla 2005)
• C II/III/IV this work

radiative transfer statistical equilibrium
4. method
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Observations

• 6 early-B III-V apparently slow-rotators
• randomly distributed in solar vicinity (RgRSunlt
  500 pc)
• from associations and field
• Spectra high S/N resolution, wide wavelength
  coverage (FEROS, ESO/La Silla)
• Spectra near-IR (FOCES, SUBARU) for 2 stars

21500 lt Teff lt 32000 K 3.1 lt log g lt 4.3 dex
4. method
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Analysis

• Based on H, He I/II, C II-IV
• C highly sensitive to
• Self-consistent iteration on all variables
• Teff, log g, x, z, v sin i, e(He), e(C)
  atomic data
• reduction (where possible) of systematic errors

- atmospheric parameters - input atomic data
Results

• Accurate stellar parameters
• Empirically calibrated C model
• Accurate C abundances

• 6 early-B III-V stars
• 21500 lt Teff lt 32000 K
• 3.1 lt log g lt 4.3 dex

4. method
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Step 2 Carbon
Analysis
Step 1 H, He I/II
I
Initial Teff , log g
I
Initial Teff , log g, x, z, e(He), v sin i
Detailed analysis for each star
NLTE C populations
Stellar atmosphere
Synthetic H/He/C profiles
NLTE H/He populations
Variables
Comparison with observed spectra
Synthetic H/He profiles
Atmospheric
Teff , log g, x, z, e(C), v sin i
Comparison with observed spectra
Parameter verification
Empirical calibration of C model atom
Atomic data gt1300 radiat. transitions gt5300
collis. transitions
Parameter verification
M
Modified Teff , log g, x, z, e(He), v sin i
New set of atomic data
M
Modified Teff , log g
MI ?
no
MI ? se(C) min?
yes
no
Final Teff , log g, x, z, e(He), v sin i

• Accurate Stellar Parameters
• Calibrated C model for 1 Star
• Accurate Carbon Abundance

yes
To Step 2
Verify Step 1
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Sensitivity of C II to atomic data e.g.
photoionization cross-sections
(several lines are also sensitive to other atomic
data)
4. method
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Sensitivity of C II to atomic data e.g.
photoionization cross-sections
Quantified for the 6 calibration stars
C II l5145 ? not sensitive to non-LTE C II l4267
? very sensitive to non-LTE
-0.8 dex
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4. method
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Sensitivity of C II/III/IV to atmospheric
parameters
Present solution Teff 31200300 K log g
3.950.05 dex x 8 1 km s-1 e(C) lowest 1s
Present solution
Teff -2000 K
log g 0.2 dex
x 5 km s-1
HR 3055
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Quantitatively
Teff -2000 K
HR 3055
log g 0.2 dex
x 5 km s-1
C IV up to 1.10 dex (x10) C III up to 0.35 dex
(x2.5) C II up to -0.40 dex (x2.5)
Present solution Teff 31200300 K log g
3.950.05 dex x 8 1 km s-1
Main effect from Teff
4. method
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Teff scales Present (He I/II, C II/III/IV
ionization equilibria) vs. Literature
(photometric spectroscopic)
4. method
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Teff vs. Non-LTE effects on C abundances relative
to final results
LTE
DTeff-2000 K
HR 3055
Present solution Teff 31200300 K log g
3.950.05 dex x 8 1 km s-1
4. method
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5. Results
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Near-IR
Simultaneous fits to most measurable H/He lines
Visual
H Balmer
H Paschen Data FOCES,
Calar Alto, Spain
He I
He I K-Band Data Subaru, Hawaii
He II
Data FEROS, ESO
HR 3055
5. results
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Fits to C lines
Data FEROS, ESO S/N up to 800
C II
All lines have very similar abundances (low
1s-uncertainties)
C III
C IV
t Sco
5. results
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Non-LTE vs. LTE for individual C lines
5. results
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C abundances in the Solar Vicinity
Present results vs. literature
carbon
OB III-V stars
reduced systematic errors in atmospheric
parameters atomic data
Statistics has to be improved
5. results
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C abundances in the Solar Vicinity
Present results vs. literature
carbon
OB III-V stars
For the same sample stars Kilian (1992) obtain
systematic lower e(C) and a larger spread
Statistics has to be improved
5. results
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C abundances in the Solar Vicinity
Present results vs. literature
carbon
e(C)young e(C)Sun(AGS)
AGS Asplund, Grevesse Sauval (2005) NEW
Sun
AG Grevesse Sauval (1998) OLD
5. results
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6. Conclusions
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Conclusions

• Solution to classical problem of C abundance
  determination in OB stars (dwarfs giants)
• C II/III/IV ionization equilibrium for the
  sample
• For the hotter stars C II/III/IV He I/II
  provide same atmospheric parameters (also SEDs)
• Highly uniform and solar C abundances (limit.
  of sample)
• Accurate data for radiative collisional
  transitions are required for a robust model atom
  for NLTE
• Temperature scale highly important in the
  analysis, sometimes more than non-LTE effects

6. conclusions
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Within context of Galactochemical Evolution

• C abundances derived here are consistent with
• New solar value (older star)
• Orion H II region gas-phase
• Uniformity of C abundance here agrees with
• Uniform gas-phase of interstellar medium (up to
  1.5 kpc from the Sun)
• Predicted C abundance gradient (0.04 dex for
  Rg1 kpc)

6. conclusions
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Within context of Stellar Evolution

• Uniformity of C abundance here agrees with
  predicted depletion of 0.03 dex for OB-type
  dwarfs giants (max. for vinit 300 km/s)

6. conclusions
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7. Perspectives
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Perspectives

• Application of available model atoms for other
  metals to the programme stars (multiple
  ionization equilibria?)
• Application of (simplified) present methodology
  to other stars in the Galaxy solar vicinity
  more distant stars (gradients)
• Extragalactic stars (e.g. Magellanic Clouds) with
  remaining limitation quality of spectra

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Extra
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Grotrian Diagram for C II (observed multiplets)
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Sensitivity to collisional excitation
cross-sections
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Spectral Energy Distributions
parameters H, He, C fits
Data IUE fluxes Johnson 2Mass photometry
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Evolutionary tracks with chemical information
constraining mixing induced by rotation
Evolution of N/C ratios at surface of rotating
models for vini 300 kms-1
Meynet Maeder 2000
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2 C II emission lines (not reproduced by LTE)
LTE only absorption lines radiation through
cooler medium
Teff
C II NLTE
calibration stars
NLTE coupling radiation field atomic
transitions emission upper levels overpopulated
Carbon abundance similar rest of lines
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Chemical Evolution of the Galaxy Temporal end
point (present-day abundances)
Sun ? sources
Chiappini et al. 2003
? Chemical Evolution Models for the Milky Way
2. motivation
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Photoionization cross-sections (C II)
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Alternative temperature indicator photometric
indices
Photometric filters
Photometric temperature calibrations also require
model atmospheres
Sensitivity
l
Flux
l
Photometric index difference of magnitudes
Photometric magnitudes
1. introduction