BL Herculis stars
BL Herculis stars are type II Cepheids with the shortest pulsation periods - between 1 and about 5 days. Below we present several typical I-band light curves of BL Herculis stars arranged in order of increasing periods. All of them were collected in 2010-2012 during the OGLE-IV survey of the Galactic bulge. Note the secondary bumps, which show up on the descending branch of the shortest-period light curves, and which progress backward in phase to the ascending branch for longer periods. This phenomenon is analogous to the Hertzsprung progression in classical Cepheids. For periods of about 3 days the secondary bumps disappear. Move the mouse pointer over the image to see the designation and J2000 equatorial coordinates of the star.
Using the photometric data obtained by the OGLE survey, we were able for the first time to confirm observationally a phenomenon theoretically predicted twenty years ago by Buchler & Moskalik (1992). In the Galactic bulge we discovered two BL Herculis stars with period doubling (Smolec et al. 2012) - their light curves display regular alternations of higher and lower maxima and minima. Move the cursor over the image to see this effect.
Double-mode BL Herculis stars
Until 2018, all type II Cepheids were considered pure fundamental-mode pulsating stars. Smolec et al. (2018) analyzed type II Cepheids in the OGLE Collection of Variable Stars and discovered the first two double-mode BL Herculis stars. These objects pulsate simultaneously in the fundamental and first-overtone radial modes. Currently, five double-mode BL Herculis stars are known in the Universe (as for June 2022). Below, we present two examples of such objects. The left panels show the original OGLE photometry folded with the fundamental-mode period, the middle and right panels show disentangled light curves corresponding to both modes.
First-overtone BL Herculis stars
Examining the huge photometric database obtained by the OGLE project, Soszyński et al. (2019) found BL Herculis pulsating exclusively in the first overtone. Currently, five stars of this type are known in our Galaxy and the Large Magellanic Cloud (as for June 2022). Two example light curves of single-mode first-overtone type II Cepheids are shown in the figures below.