Mid-IR excess of evolved and
MS stars in 47 Tuc
Pier Giorgio Prada Moroni
Dipartimento di Fisica “E. Fermi”
Università di Pisa
Collaborators
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Giuseppe Bono (INAF-OAR)
Pauline Barmby (Harvard-Smithsonian CfA)
Ivan Ferraro (INAF-OAR)
Massimo Marengo(Harvard-Smithsonian CfA)
Roberto Buonanno (Univ. Roma Tor Vergata)
Carlo Corsi (INAF-OAR)
DATA SET
• IRAC-Spitzer: 3.6, 4.5, 5.8, 8 μm
(GTO Prog. 623 Nov. 14 2004; Cycle 2 GOP 20298
Sept. 21 2005)
• MIPS-Spitzer: 24 μm
(Cycle 3 GTO 30031 June 12 2006)
• U,B,V,I: ground based telescopes
DATA SET
• Mid-IR Catalogue: 42000 stars
• Optical Catalogue: 173000 stars
Theretical isochrones
• FRANEC (Cariulo et al. 2004)
• Z=0.004 Y=0.24
• BCs and color index for the optical bands
by Bessell et al. 1998
IRAC/MIPS color transformation
• Kurucz’s atmospheric models: ATLAS 9
(Castelli et al. 2003)
IRAC/MIPS transmission curves
IRAC/MIPS: extinction coefficients
• IRAC bands:
• MIPS 24 μm: table 1 by Mathis 1990
47 Tuc
• DM0= 13.35±0.15
• E(B-V)=0.03±0.02
(Zoccali et al. 2001, Percival
et al. 2002, Gratton et al. 2003,
Salaris et al. 2008)
• Age= 11 Gyr
Spitzer-IRAC 3.6 - 4.5
Spitzer-IRAC 5.4 - 8
Spitzer-IRAC 3.6
• Large spread in
optical-MIR colors
Δ (B-3.6) ~ 2 mag
• Observed color
excess » photometric
error
IR-excess sources
• Red extragalactic sources
• Binaries or photometric blends
• Circumstellar dusty envelopes
• Circumstellar dusty disk
Working hypothesis:debris disk
• Chiang & Goldreich 1997
• Jura 2003
Spitzer-IRAC 3.6 - 4.5
Spitzer-IRAC 5.4 - 8
Spitzer-IRAC
SED
CONCLUSIONS
• Mid-IR excess for evolved and MS stars
• Dusty disks
FIR/Ftot
• MS star: ~0.016
• RGB star: ~0.02-0.03
• HB star: ~0.0036
Mass estimate
• Silicate grains: χ= 1000 – 0.05 cm2/g
• M≥ Aring/ χ
• MS: 1021-1025 g
• RGB: 1022-1026 g
• HB: 1023-1027 g
Circumstellar envelope
AGB Stars
AGB Stars
AGB Stars
Binary/photometric blend: BD
Burrows et al. 1997
Binary/photometric blend: VLM
VLM MS (Brocato et al. 1998)
Binary/photometric blend: RGB
HB M=0.65 Mo
RGB M=0.9 Mo
MS M=0.86 Mo
Photometric error MIR-bands
RGB Bump Stars
RGB Stars
Metallicity
• [Fe/H]=-0.67±0.05 (Carretta et al. 2004)
• [α /Fe]=0.3 (Gratton et al. 2003)
Salaris et al. 1993; Degl’Innocenti, Prada Moroni & Ricci 2006
• Z=0.04
DATA SET
• IRAC-Spitzer: 3.6, 4.5, 5.8, 8 μm
1. GTO Prog. 623 Nov. 14 2004: tot. int. time 135 sec, 22’x26’
2. Cycle 2 GOP 20298 Sept. 21 2005: tot. int. time 940 sec, 5’x15’
• MIPS-Spitzer: 24 μm
Cycle 3 GTO 30031 June 12 2006: tot. int. Time 100 sec, 5.4’x10.6’
• U,B,V,I: ground based telescopes
20’x18’