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SPETTROSCOPIA DI STELLE CALDE
DI RAMO ORIZZONTALE IN
AMMASSI GLOBULARI
COFIN 2001 – Bologna, 12 giugno 2003
Theoretical and observational framework
Same core mass
(0.5 M)
Different total
mass.
HB
morphology
Horizontal Branch
Theoretical and observational framework
•Stellar evolution:
(internal structure)
Same core mass
(0.5 M)
Different total
mass.
HB
morphology
Blue tail
• Possibly the
prime contributors
to the UV emission
in elliptical
galaxies.
• Population
synthesis
of extragalactic non
resolved systems.
• Star formation
history modeling
in dwarf galaxies
of the Local Group.
Theoretical and observational framework
Blue Tails
The most extreme
espresion of the
second parameter
problem
Why hot HB stars
can loose so
much mass?
Menv < 0.2 M
Temperatures up
to ~ 35 000 K
Theoretical and observational framework
Blue Tails
• Gaps: regions
underpopulated in
stars, which appear
in the blue HB
sequences of many
globular clusters.
Theoretical and observational framework
Blue Tails
• Gaps: regions
underpopulated in
stars, which appear
in the blue HB
sequences of many
globular clusters.
Piotto et al. (1999)
Same mass
Theoretical and observational framework
Blue Tails
Ferraro et al. (1998)
Differences in:
• Gaps: regions
underpopulated in
stars, which appear
in the blue HB
sequences of many
globular clusters.
Evolution
Mass loss
[CNO/Fe]
He mixing
Rotation
Origin (binaries)
Abundances
Same mass or same temperature
Theoretical and observational framework
Blue Tails
• Gaps: regions
underpopulated in
stars, which appear
in the blue HB
sequences of many
globular clusters.
Sweigart (2001)
• Diffusive processes:
Abundance anomalies
Michaud, Vauclair & Vauclair (1983):
•Radiative levitation of metals and
gravitational settling of helium.
• Atmosphere must be stable (non-convective
and slowly rotating) to avoid re-mixing).
Theoretical and observational framework
Blue Tails
• Gaps: regions
underpopulated in
stars, which appear
in the blue HB
sequences of many
globular clusters.
• Diffusive processes:
Sweigart (2001)
He
Fe
Mg
Ti
Si
Ca
P
Cr
CNO
Abundance anomalies
Behr et al. (2000)
Theoretical and observational framework
Blue Tails
• Gaps: regions
underpopulated in
stars, which appear
in the blue HB
sequences of many
globular clusters.
• Diffusive processes:
Abundance anomalies
Low gravities
•Moehler et al. (1995, 1997, 2000)
•de Boer et al. (1995)
•Crocker et al. (1998)
Theoretical and observational framework
Blue Tails
• Gaps: regions
underpopulated in
stars, which appear
in the blue HB
sequences of many
globular clusters.
• Diffusive processes:
Abundance anomalies
Low gravities
Luminosity jump
Grundahl et al. (1999)
Theoretical and observational framework
Blue Tails
• Gaps: regions
underpopulated in
stars, which appear
in the blue HB
sequences of many
globular clusters.
• Diffusive processes:
Abundance anomalies
Low gravities
Luminosity jump
• Fast rotation
• Peterson et al. (1983-1995) : M3, M4, M5, M13, NGC 288, halo.
• Cohen & McCarthy (1997) : M92
• Behr et al. (1999-2000) : M3, M13, M15, M68, M92, NGC 288.
• Kinman et al. (2000) : metal-poor halo
Theoretical and observational framework
Blue Tails
• Gaps: regions
underpopulated in
stars, which appear
in the blue HB
sequences of many
globular clusters.
• Diffusive processes:
Abundance anomalies
Low gravities
Luminosity jump
• Fast rotation
Many open questions on HB
morphology and hot HB stars nature
The origine of blue tails: why hot HB
stars loose so much mass?
Is there any relation between fast rotation
and HB morphology?
How is the distribution of stellar rotation
along the HB?
Which is the origine of fast stellar rotation
on HB stars?
The spectroscopic approach
Ultraviolet Visual Echelle Spectrograph (UVES) + VLT
R ~ 40 000 => 0.1 Å (7.5 km/s)
3730 – 4990 Å
The spectroscopic approach
Ultraviolet Visual Echelle Spectrograph (UVES) + VLT
Exposure times: 800s – 2.5 h/star
61 hot HB stars observed
The spectroscopic approach
Ultraviolet Visual Echelle Spectrograph (UVES) + VLT
Exposure times: 800s – 2.5 h/star
61 hot HB stars observed
The spectroscopic approach
Ultraviolet Visual Echelle Spectrograph (UVES) + VLT
Exposure times: 800s – 2.5 h/star
61 hot HB stars observed
The spectroscopic approach
ROTATIONAL VELOCITY
Analysis procedure: Cross-correlation technique
Projected rotational velocity (v sin i)
determined via the CCF (Tonry &
Davis, 1979) using rotation
standard stars of similar spectral
type (Peterson et al. 1987).
2
The spectroscopic approach
ROTATIONAL VELOCITY
Analysis procedure: Cross-correlation technique
Projected rotational velocity (v sin i)
determined via the CCF (Tonry &
Davis, 1979) using rotation
standard stars of similar spectral
type (Peterson et al. 1987).
v sin i = A  2 - 2o = A   rot
2

The spectroscopic approach
ROTATIONAL VELOCITY
Analysis procedure: Cross-correlation technique
Projected rotational velocity (v sin i)
determined via the CCF (Tonry &
Davis, 1979) using rotation
standard stars of similar spectral
type (Peterson et al. 1987).
v sin i = A  2 - 2o = A   rot
2
The spectroscopic approach
ROTATIONAL VELOCITY
2
The spectroscopic approach
ROTATIONAL VELOCITY
2
The spectroscopic approach
ROTATIONAL VELOCITY
2
The spectroscopic approach
ROTATIONAL VELOCITY RESULTS
Recio-Blanco et al., ApJL 572, 2002
• Fast HB rotation, although maybe not present in all clusters, is
a fairly common feature.
• The discontinuity in the rotation rate seems to coincide with
the luminosity jump
- All the stars with Teff > 11 500 K have vsin i < 12 km/s
- Stars with Teff < 11 500 K show a range of rotational
velocities, with some stars showing vsin i up to 30km/s.
2
• Apparently, the fast rotators are more abundant in
NGC 1904, M13, and NGC 7078 than in NGC 2808 and
NGC 6093 ( statistics? ).
The spectroscopic approach
ABUNDANCE ANALYSIS
10 stars in NGC 1904
Program: WIDTH3 (R. Gratton, addapted by D. Fabbian)
Tested in 2 hot HB stars from the literature
Stellar model atmosphere (Kurucz, 1998)
Line list: Moore et al. 1966, Hambly et al. 1997, Kurucz & Bell (1995)
2
Observed equivalent widths (EW) : ROSA (R. Gratton)
The spectroscopic approach
ABUNDANCE ANALYSIS
• Atmospheric parameters (Teff, log g, )
Photometric Teff determination
2
The spectroscopic approach
ABUNDANCE ANALYSIS
• Atmospheric parameters (Teff, log g, )
Photometric Teff determination
Behr et al. (1999) measurements in M13 :
log g = 4.83

 = -4.7 
2
log (Teff) – 15.74
log (Teff) + 20.9
• Error determinations ( EW, Teff, log g, , Z )
The spectroscopic approach
ABUNDANCE ANALYSIS
2
log Teff (K)
The spectroscopic approach
ABUNDANCE ANALYSIS
2
log Teff (K)
The spectroscopic approach
ABUNDANCE ANALYSIS
2
log Teff (K)
The spectroscopic approach
ABUNDANCE ANALYSIS
2
log Teff (K)
The spectroscopic approach
ABUNDANCE ANALYSIS
2
log Teff (K)
The spectroscopic approach
ABUNDANCE ANALYSIS
2
log Teff (K)
The spectroscopic approach
ABUNDANCE ANALYSIS
2
log Teff (K)
The spectroscopic approach
ABUNDANCE ANALYSIS
2
log Teff (K)
The spectroscopic approach
ABUNDANCE ANALYSIS RESULTS
• Radiative levitation of metals and helium depletion is
detected for HB stars hotter than ~11 000 K in NGC 1904 for
the first time.
Fe, Ti, Cr and other metal species are enhanced to
supersolar values.
He abundance below the solar value.
• Slightly higher abundances in NGC 1904 than in M13 (?)
(Fabbian et al. 2003, in preparation).
2
The spectroscopic approach
POSSIBLE INTERPRETATIONS
• Why some blue HB stars are spinning so fast?
1) Angular momentum transferred from the core to the outer envelope:
Magnetic braking on MS only affects a star’s envelope (Peterson et al. 1983,
Pinsonneault et al. 1991)
Problems : Sun (Corbard et al. 1997, Charbonneau et al. 1999)
Young stars (Queloz et al. 1998).
Core rotation developed during the RGB (Sills & Pinsonneault 2000)
Problems : no correlation between v sin i and the star’s distance to the ZAHB.
2) HB stars re-acquire angular momentum:
2
Swallowing substellar objects (Peterson et al. 1983, Soker & Harpaz 2000.)
Problems : No planets found in globular clusters yet.
Close tidal encounters (Recio-Blanco et al. 2002).
Problems : Only a small subset of impact parameters.
The spectroscopic approach
POSSIBLE INTERPRETATIONS
• Why is there a discontinuity in the rotational velocity rate?
Important : the change in velocity distribution can possibly be associate
to the jump.
1)
Angular momentum transfer prevented by a gradient in molecular weight
(Sills & Pinsonneault 2000).
2) Removal of angular momentum due to the enhanced mass loss expected for
Teff > 11 500 K (Recio-Blanco et al. 2002, Vink & Cassisi 2002 models).
2
HOT HB STARS: BINARITY
Intermediate resolution spectroscopy with FORS2 +VLT
R  8000 (4 nights allocated).
~80 stars in 3 Galactic Globular clusters (NGC6752, NGC5986
and M80).
Analysis procedure: Radial velocity variations with crosscorrelation technique.
2
HOT HB STARS: BINARITY
2
CMD by Momany et al. (2002)
• First results comming soon => Graduate thesis by C. Moni Bidin