Long Secondary Periods
Long secondary period (LSP) is a somewhat unfortunate name for a phenomenon exhibited by at least a third of long period variables. LSPs range from several months to several years - an order of magnitude longer than the pulsation ("primary") periods in the same red giant stars. The amplitudes the LSP modulation reach one magnitude at visual wavelengths and in many cases are larger than the pulsation amplitudes. LSPs occupy sequence D in the period-luminosity diagram, therefore LSP variables are sometimes called sequence D stars.
Many researchers have explored various scenarios for the origin of LSPs in red giant and supergiant stars, but were unable to give a satisfactory solution to this problem. The LSP variations have been attributed to radial or non-radial stellar pulsation, turnover of giant convective cells, episodic dust ejection, magnetic activity, or binarity. In the latter scenario, the red giant has a close-orbiting substellar or stellar companion submerged in a dusty cloud with a comet-like tail which obscures the giant star once per orbit.
Below we present I-band light curves of several LSP variables from the Galactic bulge. Left panels show unfolded time-series photometry collected during 10 years of the OGLE-IV project. Right panels present the same light curves folded with the LSPs (given in the panels).
In some sequence D stars, the amplitudes of the light curves significantly vary from cycle to cycle and sometimes the LSP variations disappear completely for a period of time. In the images above, we present light curves with relatively stable amplitudes of the LSP changes and small amplitudes of stellar pulsations (visible as short-period oscillations in the light curves) to highlight the characteristic properties of the LSP modulations. Note that a typical folded light curve can be divided into two parts, each lasting approximately half of the LSP cycle. During the first part, the brightness does not vary at all (except pulsations) or it increases slowly with time. The second part of the light curve has a triangular shape - the luminosity decreases, reaches a minimum, and then increases. In the largest-amplitude LSP variables, the duration of this triangle minimum may extend and exceed half the cycle, while the maxima often take a rounded shape. Move the mouse pointer over the LSP light curves below to see this effect.
Interestingly, the flat maxima characterize only the optical light curves of LSP variables. In the infrared wavelengths, LSP light curves often exhibit double minima which can be interpreted as the primary and secondary eclipses (Soszyński et al. 2021). Below we present optical (blue points) and infrared (red points) light curves of six LSP variables from the Large Magellanic Cloud. The infrared observations were obtained in the 4.6 μm bandpass by the NEOWISE project. Note that the secondary minima are only visible in the infrared range.
The secondary minima in the infrared light curves are one of the predictions of the binary model of the LSPs in red giant stars. In this scenario, the red giant has a close-orbiting substellar or stellar companion surrounded by a comet-like dusty cloud. The cloud absorbs the stellar radiation over a wide range of the electromagnetic spectrum and re-emits this energy at infrared wavelengths. When the red giant star is obscured by the cloud, we can observe the primary eclipse in all spectral ranges. When the dusty cloud is hidden behind the red giant, we can detect the secondary minimum only in the infrared domain.
Radial velocity curves are currently available for several dozen LSP variables (e.g. Nicholls et al. 2009). The velocity semi-amplitudes are of the order of 1-3 km/s, which is indicative of a brown dwarf or a very low-mass main-sequence star companion. However, the high prevalence of LSPs among long-period variables (at least 30%) is at odds with the so-called "brown dwarf desert" - a paucity of brown-dwarf companions relative to planets around main-sequence stars. To explain this inconsistency, it should be assumed that the low-mass companion was initially a planet that increased its mass accreting material from the envelope of its host star. If this is true, the LSP variables could serve as tracers of extrasolar planetary systems in the Milky Way and other galaxies.
Artistic impression of a red giant star obscured by a dusty cloud surrounding a low-mass companion.
Author: Matylda Soszyńska.