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Register a new userOn the Properties of Blue Large-Amplitude Pulsators. No BLAPs in the Magellanic Clouds
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Pawel Pietrukowicz
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We present the properties of the recently discovered class of variable stars, Blue Large-Amplitude Pulsators (BLAPs). These extremely rare, short-period pulsating objects were detected thanks to regular, high-cadence observations of hundreds of millions of Milky Way stars by the OGLE variability survey. The new variables closely resemble classical pulsators, Cepheids, and RR Lyrae-type stars, but at effective temperatures at which pulsations are due to the presence of iron-group elements. Theory shows that BLAPs are evolved low-mass stars with a giant-like structure, but their origin remains a mystery. In this contribution, we report the negative result of a search for BLAPs in the whole Magellanic System.
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Blue Large-Amplitude Pulsators (BLAPs) are a recently discovered class of pulsating star, believed to be proto-white dwarfs, produced by mass stripping of a red giant when it has a small helium core. An outstanding question is why the stars in this class of pulsator seem to form two distinct groups by surface gravity, despite predictions that stars in the gap between them should also pulsate. We use a binary population synthesis model to identify potential evolutionary pathways that a star can take to become a BLAP. We find that BLAPs can be produced either through common envelope evolution or Roche lobe overflow, with a Main Sequence star or an evolved compact object being responsible for the envelope stripping. The mass distribution of the inferred population indicates that fewer stars would be expected in the range of masses intermediate to the two known groups of pulsators, suggesting that the lack of observational discoveries in this region may be a result of the underlying population of pre-white dwarf stars. We also consider metallicity variation and find evidence that BLAPs at $Z = 0.010$ (half-Solar) would be pulsationally unstable and may also be more common. Based on this analysis, we expect the Milky Way to host around 12000 BLAPs and we predict the number density of sources expected in future observations such as the Legacy Survey of Space and Time at the Vera Rubin Observatory.
The single degenerate (SD) model, one of the leading models for the progenitors of Type Ia supernovae (SNe Ia), predicts that there should be binary companions that survive the supernova explosion which, in principle, should be detectable in the Galaxy. The discovery of such surviving companions could therefore provide conclusive support for the SD model. Several years ago, a new type of mysterious variables was discovered, the so-called blue large-amplitude pulsators (BLAPs). Here we show that all the properties of BLAPs can be reasonably well reproduced if they are indeed such surviving companions, in contrast to other proposed channels. This suggests that BLAPs could potentially be the long-sought surviving companions of SNe Ia. Our model also predicts a new channel for forming single hot subdwarf stars, consistent with a small group in the present hot-subdwarf-star sample.
Regular intrinsic brightness variations observed in many stars are caused by pulsations. These pulsations provide information on the global and structural parameters of the star. The pulsation periods range from seconds to years, depending on the compactness of the star and properties of the matter that forms its outer layers. Here, we report the discovery of more than a dozen of previously unknown short-period variable stars: blue large-amplitude pulsators. These objects show very regular brightness variations with periods in the range of 20-40 min and amplitudes of 0.2-0.4 mag in the optical passbands. The phased light curves have a characteristic sawtooth shape, similar to the shape of classical Cepheids and RR Lyrae-type stars pulsating in the fundamental mode. The objects are significantly bluer than main sequence stars observed in the same fields, which indicates that all of them are hot stars. Follow-up spectroscopy confirms a high surface temperature of about 30,000 K. Temperature and colour changes over the cycle prove the pulsational nature of the variables. However, large-amplitude pulsations at such short periods are not observed in any known type of stars, including hot objects. Long-term photometric observations show that the variable stars are very stable over time. Derived rates of period change are of the order of 10^-7 per year and, in most cases, they are positive. According to pulsation theory, such large-amplitude oscillations may occur in evolved low-mass stars that have inflated helium-enriched envelopes. The evolutionary path that could lead to such stellar configurations remains unknown.
Blue Large Amplitude Pulsators (BLAPs) are blue stars showing high amplitude (>0.2 mag) pulsations on a timescale of a few tens of mins. They form a new class of variable star recently discovered using OGLE data. It has lead to a number of investigations searching for the origin of these pulsations. This short study presents the Gaia DR2 data of ten BLAPs which have parallax measurements. We have dereddened their colours using Gaia DR2 data from the stars in their immediate field and find that six show absolute magnitude and intrinsic colour consistent with expectations, whilst four stars have a less certain classification. This work highlights the extra information which Gaia DR2 data can provide to help classify those variable stars which do not currently have moderate resolution optical spectra and make searches for BLAPs in wide field high cadence surveys more systematic and robust.
Following the discovery of blue large-amplitude pulsators (BLAPs), single star evolu- tion models of post red giant branch stars that have undergone a common envelope (CE) ejection in the form of a high mass loss rate have been constructed and analysed for pulsation stability. The effects of atomic diffusion, particularly radiative levitation, have been examined. Two principal models were considered, being post-CE stars of 0.31 and 0.46 M$_{odot}$. Such stars are likely, in turn, to become either low-mass helium white dwarfs or core helium-burning extreme horizontal-branch stars. The inclusion of radiative levitation leads to opacity driven pulsations in both types of post-CE object when their effective temperatures are comparable to those of BLAPs, with similar periods. The extent of the instability region for models in these simulations, which are not in thermal balance, is larger than that found for static models, in agreement with previous theory. By comparing to observations, and making some simple evolutionary assumptions, we conclude the 0.31 M$_{odot}$ star is the more likely candidate for BLAPs. The rate of period change is negative for both cases, so the origin of BLAPs with positive rates of period change remain uncertain.
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