Distance Determination to M31
Using Cepheids and Eclipsing
Binaries: The Catalog
Vilardell, F.a; Ribas, I.b; Jordi, C.a,b; Guinan, E.F.c; Giménez, Á.d,e
aDepartament d’Astronomia i Meteorologia, Universitat de Barcelona, Spain
bInstitut d’Estudis Espacials de Catalunya (CSIC), Spain
cDepartment of Astronomy & Astrophysics, Villanova University, USA
dResearch and Scientific Support Department, ESA, the Netherlands
eLaboratorio de Astrofísica Espacial y Física Fundamental, Spain
Index
1. Introduction
2. Our project
3. Data reduction
4. The Catalog
5. Distance determination targets
6. Preliminary results on Cepheids
7. Conclusions & future work
1
Introduction: The Cosmic Distance Ladder
• The Cosmic Distance Ladder extends from our Solar System to the
most distant galaxies B H0!!
• This implies scales ranging from ~1011 m (AU) to ~1026 m (a few
Gpc) B 15 orders of magnitude!
• No single distance calibrator can cover such huge interval B The
Cosmic Distance Ladder is built rung by rung
Observable universe
Distant galaxies
Local Group/nearby galaxies
100 Mpc
1 Mpc
Milky Way
10 kpc
Nearby stars
100 pc
Solar System
0.0001 pc
Introduction: Local Group Galaxies
• An accurate distance to Local Group galaxies is therefore a critical
stepping stone for the entire distance scale
• Once a Local Group galaxy's distance is known, all of its various
stellar populations are available as potential “standard candles”
Galaxy
Type
mv-Mv (mag)
LMC
Sm...
18.4
SMC
Im
18.8
M31
Sb
24.3
IC1613
dG
24.4
M33
Sc
24.5
Sext A
I...
26.1
NGC 300
Sd
26.1
DDO 216
dG
26.2
NGC 2403
Sc
27.6
6 magnitudes!!
2
Introduction: LMC vs M31
LMC:
• Closest galaxy B Traditionally
used for this purpose
• Possible irregular geometry
• Low metallicity (~50% solar)
• Limited stellar population
M31:
• Simple geometry
• Chemical composition similar to
Milky Way
• Large and diverse stellar
population
Image: http:/seds.lpl.arizona.edu/ messier/Jpg/m31.jpg
Introduction: Distances to M31
Great number of distance determinations:
Method
Still large
Differences
(10-20%)
Distance
(kpc)
Distance
(m-M)
References
Cepheids
690±40
24.20±0.14
Baade & Swope (1963)
Cepheids
710±30
24.26±0.08
Welch (1986)
RR Lyrae
740±50
24.34±0.15
Pritchet & van den Berg (1987)
Novae
710±70
24.27±0.20
Capaccioli et al. (1989)
Cepheids
740±40
760±30
820±50
24.33±0.12
24.41±0.09
24.58±0.12
Freedman & Madore (1990)
Carbon stars
745±10
24.36±0.03
Brewer et al. (1995)
GC dynamical models
640±60
24.03±0.23
Ostriker & Gnedin (1997)
RGB
780±30
24.47±0.07
Holland (1998)
Cepheids
752±17
24.38±0.05
Freedmant et al. (2001)
Cepheids
790±40
24.29±0.11
Joshi et al. (2003)
3
Introduction: Eclipsing Binaries
Eclipsing binaries can be used as distance tracers because the
components’ physical properties are determined to great
accuracy…
LMC example:
The method
HV 982 (V=14.6)
Physical
properties
rA=RA/a
rB=RB/a
TeffB/TeffA
LB/LA
Orbital
properties
P
i
e
ω
Light curve:
Radial velocity
curve:
Physical
properties
MAsin3i
MBsin3i
Orbital
properties
a sin i
vγ
(P,e,ω)
Introduction: Eclipsing Binaries
…and also the distance.
The method
R 
f⊕ =  A 
 d 
λ
2
2

 RB  λ 
λ
 FA +   FB  × 10 −0.4 E ( B −V )[ k ( λ −V ) + R (V )]


 RA 
LMC example:
HV 982 (V=14.6)
UV+optical+IR photometry
UV/optical
spectrophotometry
FOS+STIS
TeffA
[m/H]
Aλ
RA/d
4
Index
1. Introduction
2. Our project
3. Data reduction
4. The Catalog
5. Distance determination targets
6. Preliminary results on Cepheids
7. Conclusions & future work
Our Project: Observational system
Main goal
Determine accurate (direct) distances to
M31 using Eclipsing Binaries
• Precise photometric data:
2.5m Isaac Newton Telescope (La Palma, Spain)
WFC: 6000×6000 pixels CCD mosaic (34’×34’)
• Multi-object spectroscopy
Image: www.ing.iac.es/ PR/int_sun.gif
8m Gemini Telescope (Hawaii)
GMOS: multi-object spectrograph
Image: www.brouhaha.net/ bytemarks/gemini.html
• Temperatures/absorption determination
GALEX: UV spectrophotometry/photometry
IR/optical photometry
Image: http://spaceflightnow.com/pegasus/galex/images/galexart.jpg
Accurate distances using Cepheids too!
5
Our Project: and DIRECT
Project to locate Eclipsing binaries and Cepheids
Fields of study: M31 and M33
Instrumental characteristics:
• Photometric data: two telescopes
(1.2 and 1.3 meter)
• Spectroscopic data: Keck II
Achievements:
• 89 eclipsing binaries
• 332 Cepheids
Our Project: Current Status
Photometric data:
• Five years campaign (1999-2003): 21 nights
• 260 images in both B and V filters
• Data reduction: done!
Multi-object spectroscopy:
• Target stars selected
• 14 hours granted (queue mode, 2004B)
6
Index
1. Introduction
2. Our project
3. Data reduction
4. The Catalog
5. Distance determination targets
6. Preliminary results on Cepheids
7. Conclusions & future work
Data reduction: DIA
Difference Image Analysis
technique (Wozniak, 2000)
Template construction
Image subtraction
Differential fluxes
M i = −2.5 × log( Ftemp + ∆Fi )
7
Data reduction: DAOPHOT
DAOPHOT: PSF photometry
(Stetson, 1987)
Instrumental magnitudes
Standard stars
(Landolt, 1992)
Standard magnitudes
Index
1. Introduction
2. Our project
3. Data reduction
4. The Catalog
5. Distance determination targets
6. Preliminary results on Cepheids
7. Conclusions & future work
8
The Catalog: Some Numbers
• 236649 stellar objects identified
• 37265 with σ<0.1 magnitudes
• 2480 variable stars (~50% periodic)
• 317 eclipsing binaries
• 250 Cepheids
The Catalog: Complete Sample
Complete to V~22.3 & B~23.2
9
The Catalog: Complete Sample
Error distribution of standard magnitudes
σ ~ 0.02 mag
σ ~ 0.03 mag
The Catalog: Complete Sample
DIRECT comparison
<∆V> = 0.02 mag
σV = 0.05 mag
<∆B> = -0.02 mag
σB = 0.05 mag
Consistent with DIRECT magnitudes
10
The Catalog: Complete Sample
Diverse stellar population
The Catalog: Complete Sample
Upper main sequence
11
The Catalog: Complete Sample
Upper Red Giant Branch
The Catalog: Complete Sample
B – V < 0.1 mag
GALEX image
12
The Catalog: Eclipsing binaries & Cepheids
317 Eclipsing binaries
250 Cepheids
The Catalog: Eclipsing binaries & Cepheids
317 Eclipsing binaries
250 Cepheids
13
The Catalog: Eclipsing binaries & Cepheids
P = 2.30482 d
P = 6.72115 d
P = 2.71215 d
P = 8.45520 d
Index
1. Introduction
2. Our project
3. Data reduction
4. The Catalog
5. Distance determination targets
6. Preliminary results on Cepheids
7. Conclusions & future work
14
Distance Determination Targets
Generic constraints:
• Precise relative radius B Deep eclipses, detached (eccentric)
• Double lined spectra B Similar eclipse deepness
• Accurate Teff determination B Late O/early B stars (V > 18.5 – 19)
Specific constraints (Gemini + GMOS):
• Radial velocity measures (±10 km/s) B S/N > 20 B
18.5 – 19.0 > V > 19.5 – 20.0
• Multiobject spectroscopy B maximum number of EB’s in 5’×5’
Distance Determination Target Stars
6 EB’s + 4 Cepheids
P = 5.7526 d
P = 3.5497 d
15
Index
1. Introduction
2. Our project
3. Data reduction
4. The Catalog
5. Distance determination targets
6. Preliminary results on Cepheids
7. Conclusions & future work
Preliminary Results on Cepheids
Bimodal frequency distribution
• Two maxima:
Ö ~5 days (log P ~ 0.7)
Ö ~13 days (log P ~ 1.1)
• Dip 8-10 days (log P ~ 0.9−1.0)
• Reason not certainly known:
Present study
• Deficiency of F mode pulsators
(Buchler et al., 1997)
• Increases with the metallicity of the
galaxy
Antonello et al. (2002)
16
Preliminary Results on Cepheids
Period – Luminosity relationship
Fitted values (Sandage, 2004):
V = (-2.70±0.09)×log(P) + a0
B = (-2.34±0.08)×log(P) + b0
a0 = 23.5 σV = 0.4 mag
b0 = 24.1 σB = 0.5 mag
µV ~ 25.0 mag
µB ~ 25.3 mag
E(B-V)~0.3 mag B AV~0.9 mag
µ0 ~ 24.1 mag
Index
1. Introduction
2. Our project
3. Data reduction
4. The Catalog
5. Distance determination targets
6. Preliminary results on Cepheids
7. Conclusions & future work
17
Conclusions
• Accurate photometric data: 32000 stars
• Light curves of 2480 variables have been obtained
• Detected 317 eclipsing binaries + 250 Cepheids
• Selection of eclipsing binaries for distance determination B GMOS
Future work
• Radial velocities determination
• UV + IR photometry B Teff
- Distance to M31 (direct)
- First determinations of fundamental properties of stars in
M31 (EB’s & Cepheids)
• Test stellar evolution models
18