June 2006Lectures on Stellar Populations Bolometric Corrections and Colors We do not observe...

June 2006 Lectures on Stellar Populations

Bolometric Correctionsand Colors

We do not observe Bolometric, we observe through filters:

oioi

ii M

L

LLogM ,

,

5.2

0

dSLL ii

system throughput

kL

LLogMMBC

iiboli

5.2 k

F

FLog

i

5.2

depends on Teff, gravity and Z

depends on .... stellar radius

ijjiBCBCCol


Average of Observed Stellar Spectra:Dwarfs

O 50000 3.5e+14

A 10000 5.7e+11

G 6000 7.3e+10

M 3500 8.5e+09

TSp T(K) F c.g.s.


Dwarfs SED & Filters

IVB

U

Cool stars detected in Red

Hot stars detected in Blue

BC strongly depends on TSp

COLORS:

kL

LLogMM

2

121 5.2

are Temperature Indicators

Cool stars are Red

Hot stars are Blue


Effect of gravity

Gravity effects are very

Important for very cool stars

A0

B0

B5

K5

M2

M5


COLORS: Empirical

Johnson 1966 ARAA 4 193

B-V colors are good Teff indicators

for late A, F, G and early K stars

For Hot stars TSp is preferred


Bolometric Corrections: Empirical

Hottest and Coolest stars

are 3-4 mags fainter in V

than in Bolometric

Gravity dependence can

amount to 0.5mags


Model Atmospheres:Kurucz Grid revised by Castelli

Models Empirical


Model Atmospheres:dependence on gravity

Models Empirical


Model Atmospheres:dependence on

Metallicity

Blanketing

Molecules


Model Atmospheres:Calibration

• The Models do a good job for the SED of Dwarfs, especially for intermediate Spectral Types

• Not too bad for Giants and Supergiants also• Major problems are met al low Temperatures (Opacity, Molecules)• Anyway, the use of Model Atmospheres becomes a MUST because:

they allow us to compute Colors and BCs for various Metallicities

AND for whatever filters combinations

To do that we:

Take a grid of Models

Perform calibration

Produce Tables of BC, Col function of (Teff ,Log g, [M/H])


oV

obol

V

bolV

oVobolVo

V

bolV

F

FLog

F

FLogBC

MMBC

kF

FLogBC

,

,

,,

5.25.207.0

07.0

5.2

Vega

Vega

Vega

F

FLog

F

FLogCol

Col

kF

FLogCol

,2

,1

2

1

2

1

5.25.2

0

5.2

Balmer Jump

Go Back


Colors from Model Atmospheres

Origlia and Leitherer 1998: Bessel, Castelli and Pletz models through Ground Based Filters


Bolometric Correction from Model Atmospheres

Nice and smooth

BUT

Probably off for

Late K and M stars

Have you noticed that lines of different colors

Span different Temperature Range?

THIS IS NOT A SUPERMONGO FALIURE:


Tracks on the Log Teff – Log g Plane

WE LACK LOW GRAVITY MODELS FOR MASSIVE STARS

WE LACK LOW TEMPERATURE AND LOW GRAVITY MODELS

FOR LOW MASS STARS (AT HIGH METALLICITIES)


M&M: attach empirical calibrations

Montegriffo et al. (1998) traslated

Go back


Bessel, Castelli & Pletz (1998, A&A 333, 231)

Compare Kurucz’s revised models (ATLAS9)+ Gustafsson et al revised (NMARCS) models for red dwarfs and giants to empirical colors and BCs for stars in the Solar Neighbourhood (i.e. about solar metallicity).

They show color-temperature, color-color, and BC-color relations.

Conclude that :

1. There is a general good agreement for most of the parameter space

2. B-V predicted too blue for late type stars, likely due to missing atomic and molecular opacity

3. NMARCS to be preferred to ATLAS9 below 4000 K


Hot Dwarfs

A-K Dwarfs

GKM Giants

The models are shown as curves

The data are shown as points

The ptype encodes the literature source


Dwarfs

Giants

K

NM


GiantsDwarfs

Dwarfs


BaSeL Grid(Lejeune, Cuisinier and Buser 1997 +)

• Collect Model Atmospheres from Kurucz +Bessel + Fluks (for RGs) + Allard (for M dwarfs)•Correct the model spectra so as to match empirical calibration•Put the corrected models on the net


Lejeune Models: Z dependenceCheck with Globulars’ Ridge Lines

BaSeL 2.2 : Corrected Models at solar Z

& Z theoretical dependence

BaSeL 3.1: Corrected models at various Z

based on GCs Ridge Lines

5 GGs with [Fe/H]=-2.2 to -0.7 in UBVRIJHKL

For each get Te from V-K (using BaSel 2.2)

BCs vs (Te,g)

BaSeL 3.1 Padova 2000: Correction at various Z

made to match GCs Ridge Lines with

Padova 2000 isochrones

”It is virtually impossible to establish a unique calibrationIn terms of Z which is consistent with both color –temperatureRelations AND GCs ridge lines (with existing isochrones)”

Westera et al. 2002


Libraries with high Spectral resolution

Recently developed for Population Synthesis Studies, Stellar spectroscopy, Automatic Classification of Stellar and Galaxy Spectra … not so important for Broad Band Colors

Observational Librariestake a sample of well observed stars with known parameters Log Te, Log g, [Fe/H]

and derive their spectra

STELIB – Le Borgne et al. 2003249 spectra between 3200 and 9500 A, sp.res. ~ 3 A

INDO-US – Valdes et al. 2004 885 spectra between 3460 and 9464 A+ 400 with smaller wavelength rangesp. res. ~ 1 A


ADSD: DATA BASE OF DATA BASES

Sordo and Munari 2006:

WEB interface to access to aLarge (294) number of spectroscopic Databases

Total number of stars is 16046

Interrogation tool to search in theDatabase is included


Libraries with high Spectral resolution

THEORETICAL MODELSUsually constructed on top of a model atmosphere (Kurucz) +

Code for synthetic spectrum which solves monochromatic radiative transport with a large list of lines not very important for broad band colors, but could suggest diagnostic tools

Martins et al. 2005: 1654 spectra between 3000 and 7000 A with sp. res. ~0.3 ASpecial care to describe non-LTE and sphericity effects


Martins et al. 2005

30262 4.18 0.02

13622 3.80 0.05

7031 4.04 0.01

4540 0.88 0.02

3700 1.3 0.01

3540 0 0.02

Check versus STELIB stars

Check versus INDO-US stars

3910 1.6 0.01

300004.50.02

140004.50.02

35001.00.01

45000.00.01

70004.00.02

40001.00.02

35000.00.02


Other Models:

Bertone et al. : 2500 spectra with resolution of ~ 0.3 A UV grid Optical gridbetween 850 and 4750 A 3500 and 7000 A Te from 3000 to 50000 K 4000 to 50000 K Log g from 1 to 5 0 to 5 [M/H] from -2.5 to +0.5 -3 to +0.3

Munari et al. : 67800 spectra between 2500 and 10500 A with res of ~1 A cover Te from 3500 to 47500 K, Log g from 0 to 5 [M/H] from -2.5 to +0.5 and [A/Fe]=0,+0.4

Coelho et al. : spectra between 3000 and 1800 A with res of ~0.02 A cover Te from 3500 to 7000 K, Log g from 0 to 5 [M/H] from -2.5 to +0.5 and [A/Fe]=0,+0.4


Converted Tracks: B and V


Converted Tracks: V and I


What have we learnt

When passing from the theoretical HRD to the theoretical CMD we should remember that:

• At Zo the model atmospheres are adequate for most TSp

• There are substantial problems for cool stars, especially at low gravities

• The theoretical trend with Z is not well tested

• The tracks on the CMD reflect these uncertainties

The transformed tracks make it difficult to sample well the upper MS

(large BC); the intermediate MS merges with the blue part

of the loops; the colors (and the luminosities) of the Red giants and Supergiants are particularly uncertain.


Uncertainty of Stellar Models

Gallart, Zoccali and Aparicio 2005 compare various sets of models (isochrones) to

gauge the theoretical uncertainty when computing simulations with one set.


Age-dating from Turn-off Magnitude

In general the turn-off magnitudeat given age agrees

Teramo models fit the turn offMagnitude with older ages(at intermediate ages)

Notice some difference in isochrone shapes , and SGBfor old isochrones


Deriving metallicity from RGB

The RGBs can be very differentespecially at high Z

The difference is already substantialat MI=1.5 where the BCs can stillbe trusted (Te ~ 4500)

The comparison to Saviane’s linesSeem to favour Teramo at high Z,but the models do not bendenough at the bright end.


Deriving distance from RGB Tip

The RGB Tip is an effective distance indicator in the I band and at low ZsThe theoretical location depends on the bolometric magnitude and onThe BC in the I band.

There is a trend of Padova models to yield relatively faint TRGB atall metallicities.

Observations are not decisive,But undersampling at TRGB shouldlead to systematically faint observed TRGB.


Magnitude location of the HB

The HB luminosity can be used as distance indicator as well as to deriveAges of GCs, from the difference between the HB and the TO luminosity(dependence on Z is crucial for this).

The models show substantial discrepancies, again with Padova models fainter thanTeramo.

Observations are very discrepant as well;major difficulties stem from• the correction for luminosity evolution on the Horizontal Branch;• the necessity to trace the ZAHB to the sameTeff point in both observations and models.


Summary

• The TO magnitude at given age of the stellar population seems independent of the set of tracks , except for obvious systematics withinput physics (but Teramo models need further investigation) this feature can be safely used for age-dating;• The TO temperatures, and in general the shape of the isochrones, seems more uncertain, as they differ in different sets;• The colors of RGB stars and their dependence on metallicity are very uncertain; the derivation of Z and Z distribution from RGB stars needs a careful evaluation on systematic error;• The magnitude level of the ZAHB and its trend with Z show a substantial discrepancy in the various sets of models AND in the various observational data sets. This is a major caveat for the distance and age determinations based on the level of HB stars. A theoretical error of about 0.2 is also to be associated to the distance determination from the TRGB.

June 2006Lectures on Stellar Populations Bolometric Corrections and Colors We do not observe...

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