Upload
archibald-hunter
View
217
Download
1
Tags:
Embed Size (px)
Citation preview
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
L. Pasquini July 2002
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 ?
L. Pasquini July 2002
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
L. Pasquini July 2002
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
L. Pasquini July 2002
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
L. Pasquini July 2002
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 and Be in Pop I stars
L. Pasquini July 2002
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 and Be in Pop I stars
L. Pasquini July 2002
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
Li and Be in Pop I stars
L. Pasquini July 2002
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
L. Pasquini July 2002
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
Li and Be in Pop I stars
L. Pasquini July 2002
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
Li and Be in Pop I stars
L. Pasquini July 2002
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
Li and Be in Pop I stars
L. Pasquini July 2002
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
Li and Be in Pop I stars
L. Pasquini July 2002
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.