Cepheids and the Extragalactic Distance Scale

Vicky Scowcroft Carnegie Observatories

DAO Seminar 8th June 2010 Overview

DAO Seminar 8th June 2010 Overview

• A brief history of Cepheids.

DAO Seminar 8th June 2010 Overview

• A brief history of Cepheids.

• Cepheids as part of the distance scale.

DAO Seminar 8th June 2010 Overview

• A brief history of Cepheids.

• Cepheids as part of the distance scale.

• Current problems ☹

DAO Seminar 8th June 2010 Overview

• A brief history of Cepheids.

• Cepheids as part of the distance scale.

• Current problems ☹

• Cepheid populations:

• Optical - M33

• Infra-red - LMC

DAO Seminar 8th June 2010 Overview

• A brief history of Cepheids.

• Cepheids as part of the distance scale.

• Current problems ☹

• Cepheid populations:

• Optical - M33

• Infra-red - LMC

• Interesting finds ☺

DAO Seminar 8th June 2010 The discovery of Cepheid variables

John Goodricke • Variability of discovered by Edward Pigott in 1784.

• and EP later discovered variability of Delta Cephei.

• Calculated period as 5.14 days - exceptionally close to today’s value (5.37 days) - time is easy to measure accurately☺

DAO Seminar 8th June 2010 The discovery of Cepheid variables

John Goodricke • Variability of Eta Aquilae discovered by Edward Pigott in 1784.

• John Goodricke and EP later discovered variability of Delta Cephei.

• Calculated period as 5.14 days - exceptionally close to Goodricke’s gruelling today’s value (5.37 days) - observing schedule for time is easy to measure Delta Cep ended in accurately☺ pneumonia and death!

DAO Seminar 8th June 2010 What is a Cepheid?

• Young

• M = few M⊙

3 4 • L = 10 to 10 L⊙

• Live on the instability strip:

• MW, LMC, SMC

• Unstable to radial pulsations

DAO Seminar 8th June 2010 Why do Cepheids pulsate?

DAO Seminar 8th June 2010 Why do Cepheids pulsate?

L

He+ → He2+

DAO Seminar 8th June 2010 Why do Cepheids pulsate?

κ↑, T↑, P↑, R↑ L L+

He+ → He2+ He2+ → He+

DAO Seminar 8th June 2010 Why do Cepheids pulsate?

κ↑, T↑, P↑, R↑ T↓, κ↓, P↓, R ↓ L L+ L-

He+ → He2+ He+ → He2+ He2+ → He+

DAO Seminar 8th June 2010 Why do Cepheids pulsate?

κ↑, T↑, P↑, R↑ T↓, κ↓, P↓, R ↓ L L+ L-

He+ → He2+ He+ → He2+ He2+ → He+ Single mode pulsation: 1 2 2R P assume HSE 4πR3 / Π= ∝ Π= v vs s ! ρ !3GM "

DAO Seminar 8th June 2010 Why do Cepheids pulsate?

κ↑, T↑, P↑, R↑ T↓, κ↓, P↓, R ↓ L L+ L-

He+ → He2+ He+ → He2+ He2+ → He+ Single mode pulsation: 1 2 2R P assume HSE 4πR3 / Π= ∝ Π= v vs s ! ρ !3GM " 1 2 2 4 L3/2 / L =4πR σT Π ∝ ! T 6 "

DAO Seminar 8th June 2010

Discovery of the PL relation Leavitt & Pickering, 1912 Pickering, & Leavitt Henrietta Leavitt

“... the brighter variables have the longer periods...” Strong relationship between period and magnitude at maximum M = a log P + b and minimum

DAO Seminar 8th June 2010 Cepheids as standard candles

DAO Seminar 8th June 2010 Cepheids as standard candles Take a single population of Cepheids...

Observed PL: m = a log P + b b Absolute PL: M = a log P + b0

DAO Seminar 8th June 2010 Cepheids as standard candles Take a single population of Cepheids...

Observed PL: m = a log P + b b Absolute PL: M = a log P + b0

Distance can be found easily: μ = m - M = a log P + b - a log P - b0 = b - b0

DAO Seminar 8th June 2010 Cepheids as standard candles Take a single population of Cepheids...

Observed PL: m = a log P + b b Absolute PL: M = a log P + b0

Distance can be found easily: Simples? μ = m - M = a log P + b - a log P - b0 = b - b0

DAO Seminar 8th June 2010 Not so simple...

DAO Seminar 8th June 2010 Not so simple... M = a log P + b

B V I

DAO Seminar 8th June 2010 Not so simple... M = a log P + b

B V I

PL changes with bandpass

DAO Seminar 8th June 2010 Not so simple... M = a log P + b

B V I

Extinction?

DAO Seminar 8th June 2010 Not so simple... M = a log P + b Linearity? B V I

DAO Seminar 8th June 2010 Not so simple... M = a log P + b ?

B V I

DAO Seminar 8th June 2010 Not so simple... M(P, λ,A,Z) = a(P,λ,Z) log P + b(A, λ,Z)

B V I

DAO Seminar 8th June 2010 Calibration of the PL relation

• Reddening is easy(ish) to fix.

• Some evidence that the PL changes slope around 10

Ngeow et al. (2009) days, but not conclusive.

0.0 0.5 log P 1.0 1.5 • Limiting samples to either Fouqué et al. (2007) above or below 10 days removes the effect.

• Metallicity is a more complicated problem...

DAO Seminar 8th June 2010 Why would metallicity matter?

DAO Seminar 8th June 2010 Why would metallicity matter?

• Compare two Cepheids with equal M, different

Z... low Z

high Z

const. P

IS

DAO Seminar 8th June 2010 Why would metallicity matter?

• Compare two Cepheids with equal M, different

Z... low Z • Tracks show that for constant M and P, low Z high Z will be brighter than high Z star. const. P

IS

DAO Seminar 8th June 2010 Why would metallicity matter?

• Compare two Cepheids with equal M, different

Z... low Z • Tracks show that for constant M and P, low Z high Z star will be brighter than high Z star. const. P

• But PLC projection IS widens the IS, so does it really matter?

DAO Seminar 8th June 2010 What are we measuring?

Metallicity effect can be added into the PL relation:

M = a log P + b - γΔZ

change in measured μ δ (m − M) γ = 0 δlogZ change in metallicity

DAO Seminar 8th June 2010 Observational tests

Theory says that metallicity does make a difference, but is it observable?

DAO Seminar 8th June 2010 Observational tests

Freedman & Madore (1990): No (statistically significant) change in μ with Z

DAO Seminar 8th June 2010 Observational tests

Sasselov et al. (1997): SMC Cepheids too faint when compared with LMC Cepheids. dμ = 0.44 log (Z / ZLMC)

DAO Seminar 8th June 2010 Observational tests

HST Key Project result: γ = − 0.24 ± 0.16 mag dex-1 Kennicutt et al. (1998)

DAO Seminar 8th June 2010 Observational tests

Romaniello et al. (2008) DAO Seminar 8th June 2010 Observational tests

This is the opposite direction to the theoretical suggestions!

DAO Seminar 8th June 2010 M33 to the rescue!

Many previous studies have used selections of • ★ Single reddening law for the whole sample . (?) ★ Range of distances, reddenings and M33 is ideal! can introduce uncertainties. • ★ Well studied - distance is well known ★ Difficult to ensure you’re always on the independently. same zero-point. ★ Well defined metallicity gradient. • Using a single gets rid of these issues. ★ Small inclination angle - we can assume all ★ All Cepheids can be assumed to have the Cepheids have the same true value of μ. same μ.

DAO Seminar 8th June 2010 M33 to the rescue!

Many previous studies have used selections of • ★ Single reddening law for the whole sample galaxies. (?) ★ Range of distances, reddenings and M33 is ideal! metallicities can introduce uncertainties. • ★ Well studied - distance is well known ★ Difficult to ensure you’re always on the independently. same zero-point. ★ Well defined metallicity gradient. • Using a single galaxy gets rid of these issues. ★ Small inclination angle - we can assume all ★ All Cepheids can be assumed to have the Cepheids have the same true value of μ. same μ.

DAO Seminar 8th June 2010 First test: WIYN survey of M33

DAO Seminar 8th June 2010 First test: WIYN survey of M33

DAO Seminar 8th June 2010 Observations

• Four fields in two regions: ★ 1, 2: = 0.52 kpc, <12 + log O/H> = 8.852

★ 3, 4: = 5.11 kpc, <12 + log O/H> = 8.296 • Assuming double-component Z gradient from Magrini et al. (2007)

• 25 epochs of BVIC observations using the WIYN 3.5 m Imager (first 20 epochs) and Mini-Mosaic (last 5 epochs) cameras. • Overlapping fields of view mean some stars have up to 38 observations. Original copyright NOAO

DAO Seminar 8th June 2010 Photometry

DAO Seminar 8th June 2010 Photometry

• Photometry process automated using DAOPHOT and ALLFRAME. • ALLFRAME used to get consistent object list and deeper detections. • Imager photometry calibrated using Landolt standards, Mini-Mosaic calibrated using secondary standards. • used USNO-B for outer field, Massey et al. (2006) catalogues for inner field. • Massey et al. also used for photometry comparison.

DAO Seminar 8th June 2010 Photometry comparisons Scowcroft et al., 2009 Inner field Outer field

Excellent agreement between this work and Massey et al. (2006). Around 5% of our stars appeared in both catalogues. Average ΔV = −0.009 (inner), −0.014 (outer)

DAO Seminar 8th June 2010 Cepheid selection

• Cepheids identified using Fourier Parameters (J.B. Marquette for the CFHT M33 survey). • Magnitudes determined using 3rd order Fourier fit to light curves. • Bad Cepheids: weird amplitude ratios, bad data points, incorrect periods. FINAL SAMPLE: 91 inner, 28 outer DAO Seminar 8th June 2010 BVI PL relations

PL relations for BVI fit using slope of LMC B V IC • relations in Fouqué et al. (2007), zero-point fit using an iterative weighted-least-squares method. • Differences found in zero-point between inner and outer field for all bands. • Several possibilities: 1. Different reddening/. B V IC 2. Blending affecting inner field.

3. Different P distribution - nonlinearity of PL.

4. Metallicity effects.

Scowcroft et al., 2009

DAO Seminar 8th June 2010 Getting rid of reddening Reddening can be removed from the problem using the reddening-free Wesenheit index:

Wvi = V - RVI (V - I) RVI = AV / E(V - I)

Wvi=[V0 + RVI (V−I)0]+[AV−RVIE(V−I)]

DAO Seminar 8th June 2010 Getting rid of reddening Reddening can be removed from the problem using the reddening-free Wesenheit index:

Wvi = V - RVI (V - I) RVI = AV / E(V - I)

Wvi=[V0 + RVI (V−I)0]+[AV−RVIE(V−I)]

DAO Seminar 8th June 2010 Getting rid of reddening Reddening can be removed from the problem using the reddening-free Wesenheit index:

Wvi = V - RVI (V - I) RVI = AV / E(V - I)

Wvi=[V0 + RVI (V−I)0]+[AV−RVIE(V−I)]

All reddening terms cancel, apparent index equal to absolute index ➔ reddening free!

DAO Seminar 8th June 2010 Reddening-free PL relations

Scowcroft et al., 2009

DAO Seminar 8th June 2010 Reddening-free PL relations

• Reddening-free PL relations still show an offset ➔ different zero- points can’t be caused be differences in the reddening.

Scowcroft et al., 2009

DAO Seminar 8th June 2010 Reddening-free PL relations

• Reddening-free PL relations still show an offset ➔ different zero- points can’t be caused be differences in the reddening. • Could be the different period distributions... but cutting the sample to P < 10 days and refitting slope and ZP shows it’s not. • Blending ruled out using amplitude ratios, inspection of light curves etc.

Scowcroft et al., 2009

DAO Seminar 8th June 2010 Reddening-free PL relations

• Reddening-free PL relations still show an offset ➔ different zero- points can’t be caused be differences in the reddening. • Could be the different period distributions... but cutting the sample to P < 10 days and refitting slope and ZP shows it’s not. • Blending ruled out using amplitude ratios, inspection of light curves etc. • Only one explanation left... Different [O/H] in the two regions is causing the offset ? Scowcroft et al., 2009

DAO Seminar 8th June 2010 Quantifying the metallicity effect

Magrini et al. (2007) • Using 2 component [O/H] gradient from Inner Magrini et al. (2007)

★ Clear difference between inner and outer fields. • Using the average radii of the fields to get Outer [O/H] gives γ = -0.29 ± 0.09 mag dex-1. • Consistent with other previous work, such as the Hubble Key Project and the calculation from NGC 4258 by Macri et al. (2006)

New metallicity corrected PL relation: Wvi = -3.32 log P + 22.01 + 0.29 ([O/H]LMC - [O/H]) Assuming [O/H]LMC = 8.34

DAO Seminar 8th June 2010 Second test:

DAO Seminar CFHT Survey 8th June 2010 The data

• Covered 1 square degree -- the whole of the galaxy. • 34 g’, 33 r’, 36 i’ observations taken between 2003 and 2005 in queue service observing mode. • Initial data reduction and image subtraction performed by Hartman et al. (2006). • Variables detected by image subtraction

★ Cepheids found by position in IS.

★ Classified by Fourier parameters. • Photometry calibrated using INT observations from October 2008.

DAO Seminar 8th June 2010 g’r’i’ PL relations

• Unlike BVI, the g’r’i’ PL relations have never been found observationally before

★ Semi-empirical versions exist (Ngeow & Kanbur, 2008), but are transformations of the LMC BVI relations. • This time we need to fit the slope and the zero-point, therefore need to eliminate possible systematics. • Period distribution changes with Z, so use Z = ZLMC sample to fit PL initially.

• Use Cepheids with R = Router to correct zero-point for reddening

DAO Seminar 8th June 2010 PL fitting

Router ZLMC

• ZLMC sample (243 Cepheids) used to fit the slopes.

• Router sample (56 Cepheids) used to find reddening corrected zero-points.

★ Assume CCM reddening law holds, use AV from WIYN survey to get Ag,r,is. g’ = -2.41 log P - 1.58 First ever r’ = -2.72 log P - 1.72 observational i’ = -2.84 log P - 1.71 measurement!!

DAO Seminar 8th June 2010 Wesenheit PL fitting

• Wesenheit PL relations have the same issue ➔ never found observationally before.

• Used ZLMC sample to find slope and zero-point this time.

• Scatter is larger for Wri as colour coefficient is larger.

Wgi = -3.38 log P + 22.66 Wri = -3.32 log P + 22.67

First observational Scowcroft, 2010 determination!!

DAO Seminar 8th June 2010 Do these apply to the whole sample?

• Now use the whole sample.

★ PL fit using ZLMC sample - green and yellow

★ Z > ZLMC - red and pink

★ Z < ZLMC - dark and light blue • In both cases, low Z tend to lie below the line, high Z tend to lie above it. • Doesn’t make any any assumptions about metallicity gradient here, colours represent different radii. • Looks like metallicity is having an effect (unless you can think of something else Scowcroft, 2010 that would go with radius?)

DAO Seminar 8th June 2010 Is metallicity affecting μ?

• Clearly an increase in measured μ with increasing radial distance ➔ can be interpreted as a metallicity effect. • Instead of taking the difference from two fields, find the slope ∆μ / ∆R. • Tested 3 [O/H] gradients: steep, shallow and 2 component (for Wgi):

★ γsteep = -0.33 ± 0.05

★ γshallow = -3.50 ± 3.33

★ γ2-comp = -0.489 ± 0.208 • If different [O/H] is the cause, the shallow gradient gives a very strange result. Scowcroft, 2010

DAO Seminar 8th June 2010 The metallicity problem Result for γ is only as good as the metallicity gradient...

Rosolowsky & Simon (2008)

We can’t agree on the gradient, so can we say anything about γ?

DAO Seminar 8th June 2010 Cepheids to the rescue! Can use both periods of double-mode Cepheids to derive metallicity from pulsation models. Clear trends for galaxies with no Z gradient

M33 crosses over the SMC other trends, implying a significant Z gradient. LMC M33

MW

Beaulieu et al. 2006

Result from first 5 BCs consistent with steep measurements, eventually will use ~40 BCs.

DAO Seminar 8th June 2010 Is the PL linear?

Slope change dominated by doesn’t appear high R Cepheids to be physical - caused by superposition of different populations? looks like a slope change

Scowcroft, 2010

DAO Seminar 8th June 2010 Third test: The Carnegie Hubble project

DAO Seminar 8th June 2010 Third test: The Carnegie Hubble project

DAO Seminar 8th June 2010 What is the CHP?

• Aim: To measure H0 to 3% using data from the Warm Spitzer mission.

• Collaborators:

• OCIW - Wendy Freedman, Barry Madore, Jane Rigby, Andy Monson, Eric Persson, Mark Seibert.

• Peter Stetson (DAO), Laura Sturch (BU).

• Split into six sections:

• Cepheids: Galactic, LMC, Local Group, Beyond the Local Group.

• Tully-Fisher relation

• Supernovae hosts

DAO Seminar 8th June 2010 Why Spitzer?

DAO Seminar 8th June 2010 Why Spitzer? • Infra-red observations (practically) eliminates reddening and extinction.

• Single telescope eliminates many systematics.

• Metallicity effects should also disappear.

DAO Seminar 8th June 2010 Why Spitzer? • Infra-red observations (practically) eliminates reddening and extinction.

• Single telescope eliminates many systematics.

• Metallicity effects should also disappear.

• Amplitudes of Cepheid light curves drop:

• Need less observations to get a good average magnitude.

• Harder to find - but we already know where they are.

DAO Seminar 8th June 2010 Cepheids in the LMC • Sample of 84 Cepheids taken from Persson et al. 2007

• Majority chosen to have P>10 days, but a few below to calibrate to galactic sample.

• Data reduced using the SSC APEX software - PRF fitting.

• Matched to archival data in optical and NIR to produce 12 band light curves and PL.

DAO Seminar 8th June 2010 Pretty light curves...

DAO Seminar 8th June 2010 IRAC PL relations

• PL relations derived from well sampled light curves.

• Magnitudes from GLOESS fitting.

• Size of points ≈ size of error bars.

• Consistent with results from SAGE data

[3.6] = -3.281 (log P - 1) +11.673 [4.5] = -3.168 (log P - 1) +11.114

DAO Seminar 8th June 2010 Scatter in the PL

Scatter in the IRAC PLs is due to the geometry of the = 0.141

SAGE LMC, which will be = 0.108 σ

σ corrected for.

Using well sampled light curves, 4% distances to individual stars are achievable! σ = 0.100 σSAGE = 0.135 DAO Seminar 8th June 2010 Comparison with Galactic PL Preliminary Galactic PL Relation -10 LogP > 1 Consistent -3.59logP -2.28 rms=0.079 -3.29logP -2.70 with LMC at -3.48logP -2.36 rms=0.083 -3.19logP -2.76 log P > 1 -8

-6 -0.2 0 0.2 Sample contains HST Some overlap with LMC at P Cepheids, as between 6 -4 well as and10 days targets Residual Correlation

-2 0 0.5 1 1.5 2 2.5 Log(Period) Monson et al, in prep.

DAO Seminar 8th June 2010 IRAC colours IRAC bands

IRAC bands should trace radius variations, colour should be around 0 and flat, but it’s not... DAO Seminar 8th June 2010 The CO effect

DAO Seminar 8th June 2010 The CO effect • CO band-head is present in the [4.6] IRAC band.

• As the Cepheid heats up, the CO dissociates, resulting in an increase in [4.6] flux.

• Looks like the effect turns off at low P (high T).

DAO Seminar 8th June 2010 The CO effect • CO band-head is present in the [4.6] IRAC band.

• As the Cepheid heats up, the CO dissociates, resulting in an increase in [4.6] flux.

• Looks like the effect turns off at low P (high T).

• May be useful as an abundance tracer - more investigation needed.

DAO Seminar 8th June 2010 Radial velocities

[3.6] traces radius variations, so should be able to use it for measurements.

DAO Seminar 8th June 2010 What’s next?

• LMC observations will be complete within the next month.

• Will produce definitive calibration of PL in IRAC bands.

• Reddening and metallicity free Cepheid distance to LMC.

• Applied for more Spitzer time to add the SMC.

• Will allow us to compare samples at three metallcities, investigate the possibility of CO as an abundance tracer.

• Combining Galactic and LMC [3.6] data will produce well- calibrated PL covering both sides of the possible period break.

DAO Seminar 8th June 2010 Summary • Optical:

• Wesenheit indices in the optical (Wvi, Wgi, and Wri) are all affected by the composition of the Cepheid, such that the higher metallicity Cepheids appear brighter, and therefore closer.

• Infra-red:

• Distances accurate to 4% for individual stars can be obtained using the Spitzer 3.6μm band.

• The CO band-head in the 4.5 μm band may be useful as an abundance tracer, but should not be used for distances.

• Our Spitzer light curves are good enough that they can be used to study the structural properties, and may be used to obtain radial velocities.

• Cepheids are great ☺

DAO Seminar 8th June 2010