spectroscopy overview the science case: high resolution spectroscopy of (cool) stars basic tools for...

SPECTROSCOPY

OVERVIEW

The Science Case: High Resolution Spectroscopy of (cool) stars

Basic tools for specifications and verification One example of similar design and different implementations: UVES, FEROS, HARPS Successful Program: VLT instrumentation

Aspects of Optical design (B. Delabre)

L. Pasquini July 2002

This cycle is directed to ASTRONOMERSDevelops through a logical path which goes from conception to construction and operations of the instrument

SPECTROSCOPY

Science Cases


Science cases: Why do we want to have a new instrument ? Which science to address?

Which Outstanding problems to solve ? How ? Which Telescope ? Which fraction of the time is available ?

Related questions: Which community will serve ? What is already available to them ?

What is the state of art ? The competition ? The alternatives?

Some Practical Aspects: Feasibility, Financial, Timescale

SPECTROSCOPY

WHY H-R Spectroscopy ?


Spectroscopy brings the largest amount of information. The best (and in some case unique) way of making physics.

From Zoccali et al. 2002, search for CN - CH variations in Globular Clusters

SPECTROSCOPY

Cool Stars: Some Topics


Detailed Chemical Abundance: Distance Scale , Stellar Populations, Star Formation History, Chemical Evolution, Age of the Galaxy

Primordial Nucleosynthesis: Li, Be abundance

Stellar Interior: Diffusion, Mixing, oscillations ..

Accurate Radial velocities : Dynamics of complex systems, Binaries, Exo-Planets, short and long term variations

Rotation & magnetic fields: Activity, Dynamo, Solar-stellar connection, Angular Momentum evolution...

The chemical evolution of Globular Clusters:some unexpected (?) results with VLT

ESO Large Programme, PI: Raffaele Gratton (PD)

Content

- Distances to globular clusters and impact on ages: short and long distance scales

- Star-to-star chemical inhomogeneities: the Na-O anticorrelation

- Lithium abundances

- ESO Large Program 165-L0263

Distances to globular clusters and impact on ages:

short and long distance scales

Comparison between confidence range for globular clusters ages and values allowed by Universe geometry

True distance modulus to the LMC according various methods

Globular cluster distances from Main-Sequence fitting to local subdwarfs

Systematic effects and total error budget associatedwith the MS fitting distances to Globular Clusters

Effect (m-M)

Malmquist bias negligibleLutz-Kelker correction 0.02Binaries (in the field) 0.02Binaries (in clusters) 0.03

Photometric calibrations (0.01 mag) 0.04 Reddening scale (0.015 mag) 0.07 Metallicity scale (0.1 dex) 0.08

Total uncertainty (1 ) 0.12

Reddening freeTeff calibration

Star-to-star chemical inhomogeneities:

the Na-O anticorrelation

Variations among MS starsin 47 Tuc (Briley et al. 1994)

Variations in the strength of CH and CN bands

Noticed since early seventies (Osborn 1971) from DDO photometry and spectroscopy

Bimodal distribution along the RGB (Norris & Smith 1980s)

NGC6752

Kraft, Sneden and coworkers:The O-Na anticorrelation for giants in globular clusters

Presence of elements processed through the complete CNO-cycle.At these temperatures 22Ne+p 23Na(Denissenkov & Denissenkova1990; Langer & Hoffman 1995;Cavallo et al. 1996).

At higher temperatures, also26Mg+p 27Al

From Langer et al. 1993

A first mixing episode occurs at the base of the RGB, due to theinward penetration of the outer convective envelope in regions wheresome H-burning (through uncomplete CN-cycle) occurred duringthe latest phases of MS evolution (first dredge-up: Iben 1964).

First dredge up causes only minor effect in metal-poor stars

At the same phases, dilution (by a factor of ~20) of the surface Liabundance occurs

Mixing episodes along the RGB evolution of small mass stars

The maximum inward penetration of the outer convective envelopeat the base of the RGB creates a discontinuity in molecular weight(-barrier) that prevents further mixing, until is canceled by theoutward expansion of the H-burning shell (RGB clump) (Sweigart& Mengel 1979; Charbonnel 1994).

Further mixing (due e.g. to meridional circulations activated by corerotation is possible only after the RGB clump

Role of the molecular weight barrier

Molecular weight-barrier along the RGB(from Charbonnel et al. 1998)

Field stars conform this theoretical paradigma (Gratton et al. 2000)

However abundances ofO and Na are not affected:

mixing is not deep enough to reach regions where complete CNO cycle occurs

There is a systematic difference between field and cluster stars.

Important: this might be correlated with the 2nd parameter effect- Systematic different core-rotation core and total mass at He-flash- Mixing of He

It may also affect HB magnitudes (and then distance scales)

Possible hints for a correlationbetween the 2nd parameter andthe Na-O anticorrelation may besuggested by these graphs byCarretta et al. (1996)

What is going on in cluster stars?

There are mainly two scenarios:

- Deep mixing episodes: may only occur along the RGB, after the clump (temperature is not large enough in TO-stars)

- Accretion: should be present independent of the evolutionary phase (the material comes from now extincted TP AGB stars, undergoing hot bottom burning). Accretion might occur: . on protostars (Cottrell & Da Costa) . on already formed stars (D’Antona, Gratton & Chieffi)

Not distinguishable from observations of bright giants

Observations of stars fainter than the clump

Lithium abundances

Lithium abundancesand primordial nucleosynthesis

(figure from Suzuki et al. 2000)

Lithium abundancesfrom halo stars onthe Spite’s paletau

(data from Suzuki et al. 2000)

Main concern:Surface Li depletiondue to sedimentationdue to some mixing

(figure from Vauclair & Charbonnel 1998)

Role of globular clusters

We may compare abundances in TO and subgiants looking forcostraints about sedimentation:

- comparing abundances in TO-stars and subgiants effects of sedimentation should be canceled when the outer convective envelope penetrates inward (dilution is independent of diffusion)

- elements other than Li provide costraints on effects of sedimentation

Comparison between abundances for TO-stars and subgiants

Previous observations of Li in globular clusters(figure from Charbonnel et al. 2000)

Field stars GC stars:filled symbols: Deliyannis et al.open symbols: Pasquini & Molaro Castilho et al.

Previous observations of Li in globular clusters

Li abundances in NGC6397 stars from Castilho et al. (2000)

PI: R. Grattonco-authors: P. Bonifacio, A. Bragaglia, E. Carretta, V. Castellani,M. Centurion, A. Chieffi, R. Claudi, G. Clementini, F. D’Antona,S. Desidera, P. Francois, F. Grundhal, S. Lucatello, P. Molaro, L. Pasquini, C. Sneden, M. Spite, F. Spite, O. Straniero

VLT2 (Kueyen)+UVES12 nights in June and September 200012 nights in August and October 2001

ESO Large Program 165-L0263:

Distances, Ages and Metal Abundancesin Globular Cluster Dwarfs

OBSERVATIONS

Clusters selected for observations

The closest globular clusters (but M4 for which differential reddening is important)

cluster V(TO) [Fe/H]

NGC6397 16.4 -1.82NGC6752 17.2 -1.4247 Tuc 17.6 -0.70

NGC 6397 and NGC 6752 - Stars selected for observations:14 TO stars and 12 subgiants (below the RGB clump) in NGC6397 and NGC6752

47 Tucanae - Stars selected for observations:3 TO stars and 8 subgiants (below the RGB clump)

Field star sample:34 metal-poor stars with good parallaxesfrom the Hipparcossatellite

Green points:single stars

Red squares:binaries

ANALYSIS

Our spectra have R~40,000, andS/N~80-100 for stars in NGC6397,S/N~20-60 for stars in NGC 6752and 47 Tucanae..The spectral range is 3500-9000 Å.

We show the correlation between EWs measured with an authomaticprocedure on spectra of two TO stars in NGC6752 (upper panel) and NGC6397 (lower panel)

Typical errors are 3 mÅ for stars inNGC 6397, and 5 mÅ for stars inNGC 6752 and 47 Tucanae

Accurate EWs can be derived fromour spectra

Teff’s from spectra:

- Balmer line profiles

Analysis procedure strictly identical for field and cluster stars

Reddening free

Comparison between Teff’s from H and from colours(calibration by Kurucz, model without overshooting)

Green points:single starsRed squares:binaries

Mean offset:-6827 Kr.m.s.=159 K

Reddeningzero pointerror:E(B-V)=0.008

Our Teff scale agrees quitewell with that of Alonso et al. based on the IRFM

Average difference is

T(Us)-T(A)=2912 K

(r.m.s.= 78 K, 42 stars)

Eliminating five outliers:

T(Us)-T(A)=0.906T(A) +564 K

(r.m.s.= 35 K, 37 stars)

Results• Impact of microscopic diffusion on models of

low mass stars

• The O-Na anticorrelation among globular cluster TO-stars

• Lithium abundances in TO-stars and subgiants of globular clusters

• Distances and Ages of Globular Clusters

• Comparison between abundances in GC and field stars

• Rotation of TO-stars in globular clusters

Impact of microscopic diffusion on models of low mass stars

Impact of microscopic diffusion on models of low mass stars

Microscopic diffusion is a basic physical mechanism, thatshould be included in stellar models

It causes sedimentation of heavy elements, mainly He; in lowmass (M~0.8 M0), metal-poor ([Fe/H]-2) stars near the TO,also O and Fe are expected to be depleted significantly

The net effects of sedimentation are:- ages are reduced by about 10%- Li abundances may be significantly reduced with respect to the original value

Our observations of TO and subgiants in NGC6397 (M~0.8 M0,[Fe/H]=-2.0) allow to costrain sedimentation effects

Abundances in stars of NGC6397

Star S/N [Fe/H] [O/Fe]

TO-stars 1543 91 -2.02 0.16 1622 82 -2.02 0.11 1905 92 -2.06 0.11201432 97 -2.00 0.08202765 59 -2.02 0.21 <> -2.020.01

Subgiants 669 91 -2.01 0.26 793 105 -2.04 <0.26206810 85 -2.10 0.48

<> -2.050.03

Prediction of models with microscopic diffusion (0.8 Mo)

Model [Fe/H] TO-subgiants

Castellani et al. 2001 -0.25 for [Fe/H]= -2.0Salasnich et al. 2000 -0.29 for [Fe/H]= -1.3 -0.78 for [Fe/H]= -2.3Chieffi & Straniero 1997 -0.38 for [Fe/H]= -2.3

NGC6397 +0.030.04 for [Fe/H]= -2.0

Conclusion: Models predict much larger sedimentation due to microscopicdiffusion than actually observed. There should be somemechanism that prevents sedimentation

The O-Na anticorrelation among globular cluster stars

The O-Na anticorrelation among globular cluster stars

There are mainly two scenarios:

- Deep mixing episodes: may only occur along the RGB (temperature is not large enough in TO-stars)

- Pollution: should be present independent of the evolutionary phase (the material comes from now extincted TP AGB stars, undergoing hot bottom burning). Pollution might occur: . on protostars (Cottrell & Da Costa) . on already formed stars (D’Antona, Gratton & Chieffi)

Our observations of TO-stars in NGC6752 (a cluster which exhibits a clear O-Na among giants) allows to make a definitive test on the deep mixing scenarios

Na doublet at 8183-94 Åin TO-stars of NGC6752(these stars have virtuallyidentical atmospheric parameters)

There is a clear star-to-starvariation in Na abundances

OI triplet at 7771-75 Åin TO-stars of NGC6752.

These stars have virtuallyidentical atmosphericparameters.

There is a clear star-to-starvariation in O-abundances,anticorrelated withvariations in Na abundances

The O-Na anticorrelationamong stars in NGC6752.Filled squares: TO starsEmpty squares: subgiants.

The observed anticorrelationis very similar to thatobserved in giants

The correlation between the Strömgren c1 index and the Na abundance among stars in NGC6752.Filled squares: TO starsEmpty squares: subgiants

The c1 index is correlatedwith Na abundances amongsubgiants.

The Mg-Al anticorrelationamong stars in NGC6752.Upper panel: TO starsLower panel: subgiants.

Na rich stars are Al-richand Mg-poor.

This is most clear amongsubgiants.

C-N anticorrelation in subgiants of NGC6752

CN-band at 3883 Å G-band

Stars are ordered according to increasing Na abundance

[N/Fe]=1.0

[N/Fe]=1.1

[N/Fe]=1.3

[N/Fe]=0.0

[N/Fe]=1.2

[N/Fe]=1.3

[N/Fe]=1.2

[N/Fe]=1.45

[N/Fe]=1.5

[C/Fe]=-0.05

[C/Fe]=-0.40

[C/Fe]=-0.15

[C/Fe]=-0.35

[C/Fe]=-0.35

[C/Fe]=-0.65

[C/Fe]=-0.60

[C/Fe]=-0.25

[C/Fe]=-0.35

C and N abundances in NGC6752 subgiants

[(C+N)/Fe]=0

All O transformed into N

C and N abundances in subgiants of NGC6397

[N/Fe]=1.4

[N/Fe]=1.3

[N/Fe]=1.5

[C/Fe]=+0.05

[C/Fe]=-0.10

[C/Fe]=0.0

Very high N abundance ![O/Fe]=+0.210.05 but[(C+N+O)/Fe]=+0.580.10

Conclusions:

The O-Na anticorrelation is present among TO-stars andsubgiants in NGC6752. For the same stars, also a Mg-Alanticorrelation is observed

This clearly rules out deep mixing as explanation for theO-Na anticorrelation

The sum of C+N abundances is not constant: a substantialfraction of O is transformed into N in some NGC6752 stars

N is overabundant by a large factor in subgiants of NGC6397:while O is almost solar, the sum of C+N+O is overabundantas in halo field stars

Lithium abundances in TO-stars and subgiants

of globular clusters

NGC 6397

Li doublet inTO-stars of NGC6397Line strength is the same in all stars

Average Liabundance:log n(Li)=2.34r.m.s=0.056 dexMaximumintrinsic scatter0.035 dexThis is to befulfilled by stellarmodels whichpredict Li depletion.

If this is primordialLi then the baryonicdensity is:bh2=0.0160.004orbh2=0.0050.002

Li abundances infield and (Na-poor) cluster stars.

They occupy thesame location

Dilution factor is about 15 for both field (Gratton et al. 2000) andcluster stars, in agreement with theoretical predictions

Spite’splateau

Lithium abundancesand primordial nucleosynthesis

(figure from Suzuki et al. 2000)

NGC 6752

Li doublet in TO-stars of NGC6752There are clear star-to-star variations

EW=18.5 mÅ

EW= 5.9 mÅ

EW=15.7 mÅ

EW=19.5 mÅ

EW=17.0 mÅ

EW=18.5 mÅ

EW=28.2 mÅ

EW=32.9 mÅ

EW=33.2 mÅ

Na-Li anticorrelationfor TO stars in NGC6752

Li is anticorrelated with Na in TO-stars of NGC6752; howeversome Li is observed also in most Na-rich, O-poor stars

Field star value

How it is possible that some Liis observed also when productsof complete CNO-burningare observed?This is possible in accretionscenarios since there are phasesin which massive TP-AGB stars produce Li and other where theydestroy Li (Ventura et al. 2001)

Conclusions:

Stars at the TO of NGC6397, and O-rich TO-stars in NGC6752have Li abundances very close to those of stars on the Spite’s plateau

Na-rich, O-poor TO-stars in NGC6752 have Li abundances lowerthan that of stars on the Spite’s plateau, but some Li is still observed

The observed dilution factor for subgiants is similar to that predictby current models

Distances and Ages of Globular Clusters

Colour of the main sequence at MV=6

Line is not best fit, but the prediction of models by Chieffi & Straniero

Reddenings toward NGC6397, NGC6752 and 47 Tuc

Comparing the Teff-colour relations for field and cluster stars:

Source E(B-V) NGC 6397 E(B-V) NGC6752 E(B-V) 47Tuc

(b-y) 0.1780.007 0.045 0.007 0.0210.005(B-V) 0.1860.006 0.0350.007 0.0130.007average 0.1830.005 0.0400.005 0.0180.004

Harris 0.18 0.04 0.05Schlegel maps 0.187 0.056 0.032

Main sequence fitting distance to NGC6397 and NGC6752

NGC6397 NGC6752E(B-V) 0.1830.005 0.0400.005[Fe/H] -2.030.04 -1.42 0.04

Main sequence fitting distance to 47 Tucanae

E(B-V) 0.0180.004[Fe/H] -0.660.04

Main parameters for NGC6397, NGC6752 and 47 Tuc

Parameter NGC6397 NGC6752 47 Tuc

[Fe/H] -2.030.04 -1.430.04 -0.66 0.04

(m-M)V 12.55 13.30 13.48 (from B-V)(m-M)V 12.60 13.15 13.55 (from b-y)(m-M)V 12.580.05 13.220.07 13.520.06 (average)

V(TO) 16.560.02 17.390.03 17.680.05 (new measure)V(HB) 13.110.10 13.840.10 14.130.10 (using Rosenberg V)MV(TO) 3.980.06 4.170.08 4.160.07MV(HB) 0.530.11 0.620.12 0.610.11

Age (Gyr) 14.10.9 14.11.1 11.10.8 (Chieffi & Straniero isochrones)

Mean Age = 13.1 1.0 Gyr

Comparison between location in the cmd of stars of NGC6397and field subdwarfs with parallaxes error /<0.12 and -2.43<[Fe/H]<-1.63Green points are bona fide single stars; red are known binaries

Comparison between location in the cmd of stars of NGC6752and field subdwarfs with parallaxes error /<0.12 and -1.72<[Fe/H]<-1.12Green points are bona fide single stars; red are known binaries

Comparison between location in the cmd of stars of 47 Tucanaeand field subdwarfs with parallaxes error /<0.12 and -0.76<[Fe/H]<-0.56Green points are bona fide single stars; red are known binaries

Systematic effects and total error budget associatedwith the MS fitting distances to Globular Clusters

Effect (m-M)

Malmquist bias negligibleLutz-Kelker correction 0.02Binaries (in the field) 0.02Binaries (in clusters) 0.03

Reddening scale (0.008 mag) 0.04 Metallicity scale (0.04 dex) 0.03

Total uncertainty (1 ) 0.07

Reddening freeTeff calibration

Rotation of TO-stars in globular clusters

A consistent fraction of stars on the horizontalbranch of globular clusters rotate at rather highvelocities (Peterson 1983; Behr et al. 2000a, 2000b)

The origin of the angular momentum of these starsis unclear.

Given the large cluster ages, a low rotation velocityis expected for main sequence stars, due to dissipationby the dynamo mechanism (Skumanich law)

However no data have been insofar obtained formain sequence stars in globular clusters

Line FWHM isderived by crosscorrelating withtemplates

This method can bealso used to check forcontaminating stars

Line FWHM can becalibrated in terms ofrotation velocities (or upper limits of) using stars with knownrotational velocities

TO-stars and subgiants in globular clusters arevery slow rotators. Upper limits to rotational velocities are obtained by subtracting broadeningdue to microturbulence from the FWHM

If stars rotate, scatter is expected depending onrandom orientation of the rotational axis

Very stringent upper limits to rotation can beobtained by comparing the observed scatter withthat expected from random axis orientation

The limit is:v sin i 1.7 km/s

The limit on rotation isv 2 km/s

SUMMARY

- NGC 6397 is a very homogeneous cluster ([Fe/H]=-2.030.04)

- Abundances for TO-stars and subgiants agree within a few percent, costraining the impact of diffusion on stellar models

- The O-Na (and Mg-Al) anticorrelation is present among TO stars in NGC6752. This rules out internal mixing as the cause of the O-Na anticorrelation

- Stars at the TO of NGC6397, and O-rich TO-stars in NGC6752 have Li abundances very close to those of stars on the Spite’s plateau

- Li is anticorrelated with Na in TO-stars of NGC6752; however some Li is observed also in most Na-rich, O-poor stars

- Derivation of distances and ages in progress

SPECTROSCOPY

Li and Be as probe of stellar Interior


Li and Be are the two elements most largely depleted in the solar photosphere; Li is destroyed by proton capture reaction at 2.5 million K ; Be 1 million K hotter. (cfr. Steigman)

Li7 is produced by primordial nucleosynthesis, Be by spallation produced by high energy galactic cosmic rays .

Current view is that primordial Li is close to the pop II ‘plateau’ , then it has enriched by a factor 10 during the Galactic history, to the levels observed in young pop I stars, where is internally destroyed. Its study in Pop II and Pop I stars tells us about Primordial nucleosynthesis and barion density (PopII)

Internal structure of the stars and mixing mechanism (PopI).

SPECTROSCOPY

Li and Be in Pop I stars


Li7 is depleted 100 times in the solar photosphere with respect to meteorites.The original gas has been transported to high temperature, or diluted with Li poor gas, or Li has diffused from surface..

Key Observations: Open Clusters (Pasquini et al. 1997 A&A 325, 535, Pasquini 2000, IAU 198,Natal); 2001, A&A374,1017, Randich et al. 2000, A&A 356,L25; 2002, A&A 387,222 ) + Homogeneous, Known ages + Chemically Homogeneous (?) sample + Well determined relative parameters + Good (but not excellent!) age and metallicity distribution in the Galaxy - FAINT ! --------> Large Telescopes, State-of-the-art instruments.. - Be lines are in the UV, at 313 nm UVES !!!!

SPECTROSCOPY



Li on Pre- Main Sequence . IC2602, 2391, 4665 are young, a few million years old. G stars are on M-S, K stars not yet. They also have different metallicities.

G stars have Li abundances very close to the solar meteoritic value (LogN(Li)=3.3)) ----> No PMS depletion

Colder stars (K stars, below 5000 K ) show a clear signature of depletion, may be metallicity dependent on PMS

Note that there is almost no difference with much older (100 Myr) Pleiades.

SPECTROSCOPY



Li on Main Sequence

Hyades (700 Myr)IC4651 and NGC3680 (1.6 Gyr)M67 (4 Gyr) . Main sequence depletion is presentamong G stars, most action in the first Gyr(s).

No spread is observed at a given T up to ~1.6 Gyrs.

Dual behaviour for older stars, (M67), factor 10 difference, as observed in the field.

SPECTROSCOPY



The Li behaviour in open clusters does not quantitatively agree with models , either ‘classical’ (e.g. where only convection is acting) or ‘mixing’ ones(e.g. rotationally induced mixing , Pinsonneault et al. 2000, IAU 198, Natal)

SPECTROSCOPY

Be in Pop I stars


Since Be is destroyed at higher temperatures, Li-Be diagrams are powerful tests for stellar interior models (see e.g. Delyiannis et al. 2000 IAU 198, Natal)

Be is in the UV, in a CROWDED region ----> HR UV spectrograph + Spectral synthesis

SPECTROSCOPY



Crucial test: In M67 main sequence stars exist with same effective temperature, but Li abundances difering by a factor 10.

UVES + dychroic simultaneous observations (Li and Be) up to V=14.3 !!!

That is about 2 magnitude fainter than any previous Be observations !

While Li differs ---------> Be is the same

The MIXING MUST BE SHALLOW ! Enough to burn Li, but not Be.

SPECTROSCOPY



Rotationally -induced mixing models have precise predictions : Li depletion must be accompanied by some degree of Be depletion. The line shows predictions from models. The constancy of Be abundance regardless of Li (as well other parameters, such as age) is impressive.

SPECTROSCOPY



Out of possible models, only gravitational waves are compatible with the observed behaviour of Be(point-dashed lines). All other models (diffusion, rotational induced mixing) would predict too much Be depletion.

SPECTROSCOPY



Gravitational waves alone (point-dashed models for 1.7 and 4 Gyrs), however, are primarily functions of fundamental stellar parameters, so they cannot explain the observations of Li star to star variations in M67 and field stars (including the Sun). Dashed lines: rotational models of 1.5 and 4 Gyrs for starting rotational velocity of 30 and 10 Km/sec respectively.

spectroscopy overview the science case: high resolution spectroscopy of (cool) stars basic tools for...

Documents

instrument slide

l0263 slide

timescale slide

calibration slide

globular clusters ages

nao anticorrelation

globular clusters effect

local subdwarfs slide