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<section id="classical-cepheids-ceph">
<h1>Classical Cepheids (ceph)<a class="headerlink" href="#classical-cepheids-ceph" title="Link to this heading">¶</a></h1>
<p>Classical Cepheids are young, bright (100-10,000 solar luminosities) supergiant stars that pulsate with periods of 1 to 100 days (typically several days). They are located in the main instability strip in the H-R diagram. Cepheids follow a famous pulsation period-luminosity relation, allowing the absolute magnitude of a Cepheid, and thus its distance, to be inferred from the pulsation period. Thus, Cepheids are used to measure distances to nearby galaxies.</p>
<section id="classification-and-numbers">
<h2>Classification and numbers<a class="headerlink" href="#classification-and-numbers" title="Link to this heading">¶</a></h2>
<ul class="simple">
<li><p>Supertypes</p>
<ul>
<li><p>variable</p></li>
<li><p>periodic</p></li>
<li><p>pulsator</p></li>
</ul>
</li>
<li><p>Occurrence rate: rare, about 10<sup>3</sup> expected in ZTF data, mostly in the Galactic plane and in M31</p></li>
</ul>
</section>
<section id="ztf-light-curves">
<h2>ZTF light curves<a class="headerlink" href="#ztf-light-curves" title="Link to this heading">¶</a></h2>
<p><img alt="ZTF cepheids" src="_images/cepheid_F.png" />
<img alt="ZTF cepheids" src="_images/cepheid_F_1.png" />
<img alt="ZTF cepheids" src="_images/cepheid_1O.png" />
<img alt="ZTF cepheids" src="_images/cepheid_F1O.png" /></p>
<section id="description">
<h3>Description<a class="headerlink" href="#description" title="Link to this heading">¶</a></h3>
<p>Fundamental-mode Cepheids are easy to recognise by their distinctive sawtooth light curve shape (with a rapid rise to maximum light and a slower decline) and period range (from 1 to 100 days). A secondary bump may be seen in the light curves of some Cepheids with periods in the range 6-20 days. Cepheids pulsating in the first overtone have lower amplitudes and more symmetric light curves than fundamental-mode Cepheids. Their periods (in the Milky Way) range from 0.24 to 8 days. The lower period limit is arbitrary since there is no natural boundary between first-overtone Cepheids and delta Scuti stars. Some Cepheids pulsate both in the fundamental mode and first overtone, the period ratio ranges from 0.68 to 0.77.</p>
</section>
<section id="light-curve-characteristics">
<h3>Light curve characteristics<a class="headerlink" href="#light-curve-characteristics" title="Link to this heading">¶</a></h3>
<ul class="simple">
<li><p>periodic variable</p></li>
<li><p>period range: 1-100 days (fundamental-mode pulsators), 0.24-8 days (first-overtone pulsators)</p></li>
<li><p>amplitude: moderate amplitude (~0.3 to 0.8 mag)</p></li>
<li><p>light curve shape:</p>
<ul>
<li><p>smooth, sawtooth variations (blue/green filters show stronger sawtooth pattern); steep rise and slow decay</p></li>
<li><p>Classical Cepheids with pulsation periods 6-20 days may have a secondary bump</p></li>
</ul>
</li>
</ul>
</section>
<section id="other-characteristics-and-selection-methods">
<h3>Other characteristics and selection methods<a class="headerlink" href="#other-characteristics-and-selection-methods" title="Link to this heading">¶</a></h3>
<ul class="simple">
<li><p>intrinsic Cepheids colors: Bp-Rp = 0.8 – 2.0 mag, but as these stars are located near the Galactic plane, the reddening may be significant</p></li>
<li><p>absolute magnitude: -6<G<-2</p></li>
<li><p>located close to the Galactic plane (within ~10 degrees)</p></li>
</ul>
<p><img alt="HR diagram of classical Cepheids" src="_images/hr__cepheid.png" /></p>
<p><img alt="RA/Dec diagram of classical Cepheids" src="_images/radec__cepheid.png" /></p>
<p><img alt="Period histogram of classical Cepheids" src="_images/period__cepheid.png" /></p>
</section>
</section>
<section id="references-and-further-reading">
<h2>References and further reading:<a class="headerlink" href="#references-and-further-reading" title="Link to this heading">¶</a></h2>
<ul class="simple">
<li><p>Soszynski, I. et al. (2008) <a href="https://ui.adsabs.harvard.edu/abs/2008AcA....58..163S/abstract">The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. I. Classical Cepheids in the Large Magellanic Cloud</a></p></li>
<li><p>Udalski, A. et al. (2018) <a href="https://ui.adsabs.harvard.edu/abs/2018AcA....68..315U/abstract">OGLE Collection of Galactic Cepheids</a></p></li>
<li><p>Rimoldini, L. et al. (2019) <a href="https://ui.adsabs.harvard.edu/abs/2019A%26A...625A..97R/abstract">Gaia Data Release 2. All-sky classification of high-amplitude pulsating stars</a></p></li>
<li><p>Skowron, D. et al. (2020) <a href="https://ui.adsabs.harvard.edu/abs/2019Sci...365..478S/abstract">A three-dimensional map of the Milky Way using classical Cepheid variable stars</a></p></li>
</ul>
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