Conor Byrne
Blue Large-Amplitude Pulsators are a newly-discovered category of pulsating variable star, with two different sub-groups being discovered in 2017 and 2019. They show brightness variations over a time periods ranging from 3 minutes to 40 minutes, with an amplitude of a few per cent of their total brightness. The evolutionary status of these stars was uncertain upon their discovery, although one suggestion is that they could be low-mass pre-white dwarfs, stars which have had their outer layers removed by a close binary companion, leaving a nearly naked helium core object which is contracting to become a white dwarf.
In this study I produced a sequence of pre-white dwarf stellar evolution models for a range of masses, to test the hypothesis that BLAPs are low-mass pre-white dwarfs. The models were also used to test whether objects in this phase of evolution are sufficiently unstable for pulsations to occur. These models were produced using the MESA open-source stellar evolution code, and the presence of pulsations was determined using the GYRE stellar oscillation code.
It was found that pre-white dwarf stars with masses between 0.28 and 0.31 times the mass of the Sun can provide a reliable explanation for the evolutionary status of BLAPs. The models also indicate that these stars should pulsate, with periods that match the observations, but only if the process of radiative levitation is taken into account. Radiative levitation causes iron and nickel to accumulate in large amounts providing the opacity bump to drive the pulsations. Preliminary predictions are made for the prospect of further objects being found in the region of parameter space between the two distinct sub-groups of BLAP currently known. The morphology of the pulsation instability region may also provide insight into common envelope evolution, a poorly understood phase of close binary interactions.
Blue large-amplitude pulsators (BLAPs) were discovered by the OGLE survey in 2017. 14 objects were identified with short period, large amplitude brightness variations. Follow-up measurements determined these objects are hot and have high surface gravities placing them below the Main Sequence. Another similar group of stars with higher surface gravities, were discovered by the ZTF survey in 2019.
The evolutionary status of these objects is uncertain. One suggestion is that they are low-mass pre-white dwarfs (WDs). Low-mass WDs are WDs with a mass of less than 0.5 Msun. These can only form through binary interactions, like common envelope ejection. If a companion star removes the hydrogen envelope from a red giant, the inert core will contract and become a white dwarf.
METHODS
The MESA stellar evolution code was used to create models of low-mass WDs. A 1 Msun star was evolved to the Red Giant Branch and then interrupted by sudden mass-loss, replicating the effect of common envelope evolution. This was done for several core masses to produce a sequence of low-mass WDs.
The effects of radiative levitation were considered in these models. Radiative levitation opposes the gravitational force and causes elements with many transition lines like iron and nickel to move upwards and accumulate in a thin layer which creates an opacity bump to drive the pulsations. The same process causes pulsations in hot subdwarf stars.
ANALYSIS
Each model was analysed along its evolution track using the GYRE stellar oscillation code. By carrying out non-adiabatic analyses, it can be determined whether the star is stable or will pulsate. A number of instability regions were identified, including one which encompasses both the OGLE BLAPs and the ZTF BLAPs. Examination of the unstable models in this region identified the iron opacity bump as the source of pulsation driving.
RESULTS
By comparing the mass fraction of iron in the pulsation driving region of each model to the locations of the instability regions, it can be noted that the pulsations arise as soon as radiative levitation takes hold and iron has had time to accumulate in the driving region, highlighting the importance of radiative levitation in these stars. By comparing the models and observations, good agreement is found between the computed and observed pulsation periods.
DISCUSSION
The exact location of the low-temperature edge of the instability strip is sensitive to assumptions about common envelope ejection, such as the remnant envelope mass, so BLAPs may be a useful tool for studying close binary interactions.
The fact that pulsations in both OGLE and ZTF BLAPs are driven by the same mechanism and share an instability region means we should expect to find BLAPs in the ‘gap’ the two groups. Based solely on the amount of time each model spends in the instability region, an expected mass distribution was produced, showing that pre-WDs with masses between 0.26 Msun and 0.32 Msun spend the longest amount of time in the instability region.
To date, only 4 of the 14 BLAPs discovered by OGLE have had their atmospheric parameters determined. Placing more of these objects on the log(g)-log(Teff) lane would provide a better understanding of how the known population is distributed in terms of mass.
CONCLUSIONS
BLAPs are identified to be pulsating low-mass pre-white dwarfs, with masses between around 0.28 and 0.31 Msun.
The pulsations are a result of Z-bump opacity which provides a significant driving force when the effects of radiative levitation are considered.
Further BLAPs may be found in ongoing and future sky surveys, to fill the ‘gap’ between the current groups.
Binary population synthesis is required to obtain more accurate expectations for mass distribution.