W Virginis stars
W Virginis stars are type II Cepheids that exhibit pulsations with periods between about 5 days (in the Magellanic Clouds we adopted 4 days) and 20 days. The periods and amplitudes of W Virginis stars often vary irregularly from cycle to cycle. The light curves exhibit systematic changes of shapes and mean amplitudes with periods. The shortest-period W Virginis stars show faster rise to maximum light and a slower fall. This asymmetry decreases with period and for a period of about 8 days the light curves of W Virginis stars are roughly symmetric. The amplitudes of pulsation have their minimum also for periods around 8 days. For typical longer-period W Virginis stars the descending branch of light curves is steeper than the ascending branch. For periods of 13 days and longer a secondary bump prior to maximum is common.
Below we present nine typical I-band light curves of W Virginis stars identified in the Galactic bulge. The photometry was collected in 2010-2012 during the fourth phase of the OGLE project. The light curves are arranged in order of increasing period. To show the amplitude variations we plot both - unfolded (left panels) and folded (right panels) - light curves for each star. Move the mouse pointer over the image to see the designation and J2000 equatorial coordinates of a given star.
There are also W Virginis stars that do not fit this pattern, for example:
Peculiar W Virginis stars
In the Large and Small Magellanic Clouds we isolated a subclass of type II Cepheids stars, which we called peculiar W Virginis stars (Soszyński et al. 2008, 2010). These stars are on average bluer and brighter than regular W Virginis stars, so peculiar W Virginis stars do not follow the period-luminosity relation of regular type II Cepheids. Besides, both subclasses of W Virginis stars are characterized by a different light curve morphology. Compare two pairs of regular and peculiar W Virginis stars with similar periods:
The latter light curve shows significant scatter of observing points due to variations of periods and additional ellipsoidal modulation superimposed on the pulsation light curve. After removing the effect of period changes and separating pulsation and ellipsoidal variations the light curves look like this:
The ellipsoidal variability is an indicator of binarity. It turns out that a large fraction (at least 33%) of peculiar W Virginis stars have additional ellipsoidal or eclipsing modulation of their light curves, so they are members of binary systems. Here are two other examples from the Large and Small Magellanic Clouds:
Note that even after subtraction of the eclipsing variability, the pulsation light curves show significant scatter, in particular in the latter case - OGLE-SMC-T2CEP-28. Why? Because the observed pulsation amplitude changes with the orbital phase. Below we present one season of observations of OGLE-SMC-T2CEP-28. From this light curve we subtracted the eclipsing modulation, so we can see here just pure pulsations. The amplitudes of pulsations change with half the orbital period, which reflects complex oscillations of a star which is distorted by tidal interactions from its companion. The observed pulsation amplitudes change depending on the angle at which we observe the star. Move the mouse pointer over the image to see additional information.
Taking into account that due to random distribution of orbital inclinations only a fraction of binary systems can be observed as ellipsoidal or eclipsing variables, it is justified to conclude that all peculiar W Virginis stars are members of binary systems. It means that peculiar W Virginis stars pulsate because they have close companions. The evolutionary channel that leads a star to the Cepheid instability strip may be similar to that occured in the Binary Evolution Pulsators described by Pietrzyński et al. (2012).