Observational studies of Cepheid amplitudes

Abstract

Context. The dependence of amplitude on the pulsation period differs from other Cepheid-related relationships. Aims. We attempt to revise the period-amplitude (P-A) relationship of Galactic Cepheids based on multi-colour photometric and radial velocity data. Reliable P-A graphs for Galactic Cepheids constructed for the U, B, V, R C , and I C photometric bands and pulsational radial velocity variations facilitate investigations of previously poorly studied interrelations between observable amplitudes. The effects of both binarity and metallicity on the observed amplitude, and the dichotomy between short-and long-period Cepheids can both be studied. Methods. A homogeneous data set was created that contains basic physical and phenomenological properties of 369 Galactic Cepheids. Pulsation periods were revised and amplitudes were determined by the Fourier method. P-A graphs were constructed and an upper envelope to the data points was determined in each graph. Correlations between various amplitudes and amplitude-related parameters were searched for, using Cepheids without known companions. Results. Large amplitude Cepheids with companions exhibit smaller photometric amplitudes on average than solitary ones, as expected, while s-Cepheids pulsate with an arbitrary (although small) amplitude. The ratio of the observed radial velocity to blue photometric amplitudes, A V RAD /A B , is not as good an indicator of the pulsation mode as predicted theoretically. This may be caused by an incorrect mode assignment to a number of small amplitude Cepheids, which are not necessarily first overtone pulsators. The dependence of the pulsation amplitudes on wavelength is used to identify duplicity of Cepheids. More than twenty stars previously classified as solitary Cepheids are now suspected to have a companion. The ratio of photometric amplitudes observed in various bands confirms the existence of a dichotomy among normal amplitude Cepheids. The limiting period separating short-and long-period Cepheids is 10.47 days. Conclusions. Interdependences of pulsational amplitudes, the period dependence of the amplitude parameters, and the dichotomy have to be taken into account as constraints in modelling the structure and pulsation of Cepheids. Studies of the P-L relationship must comply with the break at 10. d 47 instead of the currently used "convenient" value of 10 days.

On the fine structure of the Cepheid metallicity gradient in the Galactic thin disk

“…For the FO pulsators we adopted the ratio between optical and NIR amplitudes for FU Cepheids with log P ≤ 1.2 (see Klagyivik & Szabados 2009). This assumption relies on the theoretical and empirical evidence that FOs, once their period is fundamentalized, display pulsation properties very similar to FU Cepheids with periods shorter than log P ≤ 1.2.…”

“…During the last few years the dependence of the luminosity amplitude on metallicity has been investigated both from the theoretical point of view (Bono et al 2000b) and the observational (Klagyivik & Szabados 2009;Pedicelli et al 2010;Klagyivik et al 2013). In particular, Szabados & Klagyivik (2012), by using a large sample (327) of Galactic Cepheids with accurate pulsation parameters and spectroscopic metal abundances, found evidence that the luminosity and the radial velocity amplitudes slightly decrease with increasing iron abundance.…”

Anchors for the cosmic distance scale: the Cepheid QZ Normae in the open cluster NGC 6067

“…Visual estimates from those plates will be needed unless surveyed by DASCH, which is a project aimed at digitizing the Harvard collection (Grindlay et al 2012). However, QZ Nor and V340 Nor are small-amplitude Cepheids (Klagyivik & Szabados 2009), which will exacerbate the uncertainties. Lastly, despite recent gains in bolstering the short and intermediate period regimes of the Galactic Cepheid calibration, considerable effort remains to constrain the long-period domain.…”

Observational studies of Cepheid amplitudes

“…Motivated by the fact that the dependence of the pulsation amplitude on log P is neither linear nor unique, we studied the P − A relationship of Galactic Cepheids in Paper I (Klagyivik & Szabados 2009). We revised the P − A graphs for the U, B, V, R C , and I C photometric bands.…”