Cepheids

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• Cepheids
• Another relative rung of the distance ladder
• Adric Riedel

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Five Main Sections

• 1. Cepheids
• 2. Mechanism
• 3. Astrophysical Uses
• 4. Problems
• 5. Polaris

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1.) Delta Cepheids (Pop I)
F, G or K supergiant (6-10 solar masses)
Timescales of 1 to 100 days
Absolute Magnitudes between -1 and -6
Found in galactic disks and arms
Variations of between 0.1 and 2 magnitudes
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1.) The Instability strip

• Runs through the entire H-R diagram
• Characterized by pulsating stars with unstable
  envelopes
• Mostly due to HeII-gt HeIII ionization

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1.) W Virginis Stars (Pop II)
1.5 magnitudes fainter for the same period (1
solar mass AGB)
BL Hercules (CWA) Timescales of 1-4 days
Absolute Magnitudes between 0.5 and -4.5
W Virginis (CWB) Timescales of 10-30 days
Variations of between 0.1 and 2 magnitudes
Found in bulge and halo
Much looser Period-Luminosity Relation than
Classical Cepheids
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The distance of the Andromeda nebula...had been
determined by type I cepheids, whereas
distances... in our own galaxy were based on type
II cepheids (the cluster-type variables). --W.
Baade, 1956, PASP, 68, 5B.
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1.) Other 'Cepheids'

• Dwarf Cepheids Delta Scuti (Pop I) / SX
  Phoenicis
• Main sequence stars (not useful for distance
  scales)
• Cluster Cepheids RR Lyra (Pop II)
• Beta Cepheids Beta Ursa Majoris
• Blue Supergiants with iron instability
• Complicated period dependence

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1.) Cepheid Light Curve

• Characteristic sawtooth light curve (small subset
  is symmetric)
• W Virginis have a hump on the descending side

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2.) How it works

• Helium ionizes in the envelope
• Ionized Helium is more opaque
• The Helium expands outwards, pushed by radiation
• Once the envelope pushes out, the Helium cools,
  recombines, and the star contracts again.

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2.) Pulsation Mechanism

• The star's brightest point (and highest
  temperaturetype) is when its outward radial
  velocity is largest.
• Maximum radius is actually halfway down the
  descending slope.
• Minimum radius occurs just after the sharp rise
  begins

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3.) The Period-Luminosity Relationship
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3.) The Period-Luminosity Relationship

• Latest calibration Fouque et al. 2007
• M a log(P) b

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3.) The Period-Luminosity Relationship
http//www.institute-of-brilliant-failures.com/Ima
ge57.gif
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3.) Major areas of current Cepheid research

• Refining the Cepheid relation (Foque et al. 2007
  AA)
• Calculating the distances to galaxies (Cook et
  al. 2007, NOAO proposal Coma Cluster)
• Calibrating other distance relations (Rizzi et
  al. 2007 ApJ Tip of the Red Giant Branch)
• Calculating H0 (Van Leeuwen et al. 2007 MNRAS
  72 km/sec/Mpc)

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3.) Distance Estimates
http//www.astro.gla.ac.uk/users/kenton/C185/ladde
r.gif
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3.) Distance Estimates
http//seds.org/messier/more/m100_hst2.html
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4.) Problem Relative measurement

• Relies on accuracy of other direct and indirect
  distance measurements for calibration
• Hipparcos and HST have fairly accurate
  trigonometric parallaxes for some Cepheids
• Use many methods
• Baade-Wesselink method for distances can now use
  interferometric radii (CHARA!)
• ZAMS parallaxes (Main Sequence Fitting)
• Statistical Parallax
• Infrared Surface Brightness

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4.) Problem Reddening

• As massive young stars, all Classical Cepheids
  are near the galactic plane (reddening
  corrections needed!)
• Reddening corrections are available and
  improving.
• Reddening for extragalactic sources less
  problematic, but far less well known

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4.) Problem Metallicity

• Must distinguish between (ex) Classical Cepheids
  and W Virginis stars, which look very similar.
• With a known metallicity and good models, ALL
  Cepheids can be treated as Classical
• (See Aloisi et al. 2007 ApJL, who did a study of
  Classical Cepheids in I Zwicky 18)

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4.) Problem The Relation Itself

• Distance measurements can be no more accurate
  than the period-luminosity relation itself.
• The fit is being improved as time goes on (Fouque
  et al. AA 2007)

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5.) Polaris

• Cepheid luminosity and periods change
• Overtone pulsation

Engle Guinan 2008
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5.) Polaris
Engle Guinan 2008
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5.) Polaris
Engle Guinan 2008
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Works Cited

• Aloisi et al. 2007 ApJL 667, 151A
• AAVSO. W Virginis 2003. http//www.aavso.org/vst
  ar/vsots/spring03.shtml
• Baade, W. 1956 PASP 68 5B.
• Binney Merrifield Galactic Astronomy,
  Princeton University Press 1998
• Cook, K. 2007 NOAO proposal 536C
• Engle, S. Guinan, E. 2008 (in preparation)
• Fouque et al. 2007 AA 476, 73F
• Leavitt, H Pickering, E.C. 1912 HarCir 173, 1L
• Shapley, H. 1918 PASP 30, 42
• Rizzi et al. 2007 ApJ 661, 815R
• Van Leeuwen et al. 2007 MNRAS 379, 723