The Mg/Fe characterization
of the MILES spectral library
for stellar population studies
Dr. André Milone (INPE, Brazil)
Dr. Anne Sansom (UCLan, UK)
Dr. Patricia Sánchez-Blázquez (UAM, Spain)
Headlines
Why is it relevant to do this work?
MILES spectral database? What is it used for?
Collecting and homogenizing [Mg/Fe]
Reading Mg/Fe between blended atomic lines
[Mg/Fe] around the new MILES parameter space
Semi-empirical SSP models with variable α/Fe
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Motivation
Current semi-empirical stellar population models:
• based on spectral properties of Galaxy's stars
• biased by the Milky Way's chemical evolution

different behaviour of [E/Fe] vs. [Fe/H]
for each Galactic sub-system
that are imprinted on the stellar spectra
If the elemental abundances were well known
• the stellar library will be more useful
to build up accurate stellar population (SSP) models
→ [Mg/Fe]: indicator of the star formation time scale!
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What are α-elements?
What can their abundances be used for?
Alpha-elements
• 8O16 , 10 Ne20 , 12 Mg24 , 14 Si28 , 16 S32 , 18 Ar40 , 20 Ca40 and 22 Ti48
• based on the capture of α-particles
C12 + 2He4 → 8O16 + 2He4 → 10 Ne20 ...
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
mainly produced in SN-II explosive events (massive stars)
→ Mg is a proxy for the α-elements!
Iron-peak elements
Fe-peak elements are produced in both SN-II and SN-Ia

SN-Ia (low mass stars in binary systems) occur at few 108 years
and continue to occur over Gyrs

→ α/Fe measures the relative contribution by SN-II and SN-Ia
→ The higher the ratio α/Fe, the shorter the star-burst is!
→ Gaussian SFR (Thomas et al. 2005)
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Chemical pattern of α-elements
in the Galactic halo and disc stars
(Pagel & Tautvaisiene 1995)
[O/Fe]
[Mg/Fe]
[Si/Fe]
[Ca/Fe]
[Ti/Fe]
[Fe/H]
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The MILES spectral database
Medium-resolution Isaac Newton Telescope Library of Empirical Spectra
(Sánchez-Blázquez et al. 2006)
985 flux calibrated medium-resolution stellar spectra
λλ3525-7500 Å, FWHM=2.3±0.1 Å
excellent coverage in the 3d HR diagram
2800 ≤ Teff ≤ 50400 °K (±100 °K)
0.0 ≤ log g ≤ 5.0 (±0.2)
-2.7 ≤ [Fe/H] ≤ +1.0 (±0.1 dex)

scales well defined by Cenarro et al. (2007)

errors acceptable for evolutionary stellar population synthesis
However, the MILES' stars …
… can be better chemically characterized ...
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The MILES spectral library
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Collecting and homogenizing [Mg/Fe]
 Abundances from high spectral resolution analyses
• values show an artificial spread among different sources
• several reasons
 Borkova & Marsakov (2005)'s compilation
• scale based on weighted averages for dwarfs and sub-giants
• 218 MILES' stars
 Additional data for 97 MILES stars from 15 other works
• calibration needed
 Calibration to the BM2005's scale
[Mg/Fe]MILES ≡ [Mg/Fe]BM05 = -A/B + (1/B)[Mg/Fe]work
as A≠0 and/or B≠1 applying a 95% confidence level t-test for common samples
(xy linear lsq fits [Mg/Fe]work= A + B [Mg/Fe]BM05)

Total of 261 dwarfs and 54 giants → ~32% of MILES !

accurate precision of ±0.08 dex on average
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Collecting and homogenizing [Mg/Fe]
[Mg/Fe]_work
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[Mg/Fe]_BM05
A control sample for calibrating [Mg/Fe]
255 dwarfs and 51 giants
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A control sample for [Mg/Fe]
306 stars
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Reading [Mg/Fe] between atomic lines

Spectral synthesis by MOOG-LTE code (Sneden 2002)

MARCS's model atmospheres (Gustafsson et al. 2008)
• linearly interpolated for each star (Masseron 2008 software)


Vienna Atomic Line Database

molecular lines from Kurucz (2002)

automatic process plus careful visual inspection!
Two strong magnesium features (e.g. Kirby et al. 2008)



(Kuptal et al. 1992 & others)
MgbIII (λ5183.604 Å), Mg b1
Mg I λ5528.405 Å
Two methods applied
pseudo EW
(based on the growth curve)

line profile fitting

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Imprinted atomic and molecular features
over the stellar spectral types
Elemental abundances in Sun's photosphere (by mass)
H 71.5%, He 27.1%, O 0.6%, C 0.25%, Fe 0.15%,
Ne 0.13%, Si 0.08%, N 0.075%, , Mg 0.07%, ...
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Reading Mg/Fe between atomic lines

EW method: curve of growth for absorption lines
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Measuring [Mg/Fe] on the MILES spectra
MgbIII spectral synthesis
for a giant
using five α/Fe enhancements
and two methods
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Measuring [Mg/Fe] on the MILES spectra
Mg5528 spectral synthesis
for the previous giant
using five α/Fe enhancements
and two methods
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Reading [Mg/Fe] between atomic lines

Calibration to the BM2005's scale:
separately for each Mg feature (e.g. MgbIII)
[Mg/Fe]_M-R
[Mg/Fe]_H-R

Acceptable internal (based-EW) and systematic errors
MgbIII:
Mg5528:
±0.10 dex & ±0.09 dex
±0.15 dex & ±0.15 dex
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Δ[Mg/Fe]_MR-HR
without dependence on parameters
[Mg/Fe]_
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Reading [Mg/Fe] between atomic lines

Averaging [Mg/Fe]_calibrated from both features
[Mg/Fe]_MgbIII
[Mg/Fe]_Mg5528

Acceptable systematic errors
both features:
±0.12 dex
→ Total coverage of ~46% of MILES from M-R spectra:
152 dwarfs and 298 giants
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The new 4-d HR diagram of MILES
How wide must [Mg/Fe] be in the new MILES parameter space
to build up state-of-the-art SSP models?
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The new 4-d HR diagram of MILES
The [Mg/Fe] coverage in the new MILES parameter space
is enough to build up state-of-the-art SSP models!
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The distribution of [Mg/Fe] in MILES
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[Mg/Fe] versus [Fe/H]
It was possible to statistically recover [Mg/Fe] as a function of the
metallicity for different components of the Galaxy → ~78% of MILES!
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An example of accurate isochronal-based fit
restricted solar chemical composition
around solar age
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SSP modelling with MILES:
4 Gyr, [Fe/H]=0.0, [α/Fe]=+0.2
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A new era for the semi-empirical SSP modelling
From now on,
accurate semi-empirical SSP models can be built up
for several sets of Age, [Fe/H] and [α/Fe].
A g e (G y r)
1 4± 1
6±2
6±2
6±2
6±2
4±1
4±1
4±1
2±1
2±1
2±1
[F e /H ]
-2 .0 0 ± 0 .2 0
-0 .4 0 ± 0 .1 0
-0 .4 0 ± 0 .1 0
-0 .4 0 ± 0 .1 0
0 .0 0 ± 0 .0 5
0 .0 0 ± 0 .1 0
0 .0 0 ± 0 .0 5
0 .0 0 ± 0 .1 0
0 .0 0 ± 0 .1 0
0 .0 0 ± 0 .1 0
+ 0 .2 0 ± 0 .1 0
(Salaris et al. 1993)
[α/ F e ]
+ 0 .4 0 ± 0 .2 0
0 .0 0 ± 0 .1 0
+ 0 .2 0 ± 0 .1 0
+ 0 .4 0 ± 0 .1 0
0 .0 0 ± 0 .1 0
-0 .2 0 ± 0 .1 0
0 .0 0 ± 0 .0 5
+ 0 .2 0 ± 0 .1 0
0 .0 0 ± 0 .1 0
+ 0 .2 0 ± 0 .1 0
-0 .2 0 ± 0 .1
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Single age-[Fe/H] SSP modelling with variable [α/Fe]:
6 Gyr, [Fe/H]= -0.4, [α/Fe]= -0.2, 0.0 & +0.2
Cross-matching theoretical isochrones for non-solar ratios
to real stars and empirical spectrum libraries
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Single age-[Fe/H] SSP modelling with variable [α/Fe]:
14 Gyr, [Fe/H]= -2.0 (±0.2), [α/Fe]= +0.4
Cross-matching theoretical isochrones for non-solar ratios
to real stars and empirical spectrum libraries
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Summary
[Mg/Fe] from high-resolution works



32% of the MILES' stars
excellent accuracy: ±0.08 dex
uniform scale & reference sample for our measurements
[Mg/Fe] measured here
●

robust spectral synthesis with 2 Mg features & 2 methods
accurate ratios for 46% of the MILES' stars: ±0.12 dex
Catalogue of [Mg/Fe] for about 78% of MILES


74% for dwarfs and 82% for giants
excellent coverage in the parameter space
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Further applications
• Semi-empirical modelling of SSPs with variable [α/Fe]
- interpolation code in the parameter space
- comparison with previous models and cluster data
- behaviour of integrated line strengths and colours
→ precise models for several Ages, [Fe/H] & [α/Fe]
(Vazdekis' group, IAC, Spain)
• Dependence of Lick indices on [Mg/Fe]
- in comparison with theoretical predictions of popular star
models (Korn et al. 2005)
→ reliable empirical response functions of indices
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Further applications
Kinematics characterization of MILES' stars
- thin disk, thick disk and/or halo field
Extension of MILES with more stars with [Mg/Fe]
- runs with same instrumental (IDS at 2.50m INT, ENO)
Same approach for calcium in MILES and CaT libraries
(e.g. Kayser et al. 2006)
[Ca/Fe]=[Mg/Fe] in many systems?
[email protected]
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