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Blue large-amplitude pulsators as a new class of variable stars

Abstract

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 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.

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Figure 1: Phased I-band light curves of blue large-amplitude pulsators (BLAPs) detected by the OGLE survey.
Figure 2: Colour–magnitude diagrams for stars in the fields with detected BLAPs.
Figure 3: Magellan-Baade moderate-resolution spectra at opposite phases of the cycle of the prototype object OGLE-BLAP-001.
Figure 4: Gemini-South low-resolution spectra for three BLAPs.
Figure 5: Location of the BLAPs in the Hertzsprung–Russell diagram.

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Acknowledgements

We thank M. Kubiak and G. Pietrzyński, former members of the OGLE team, for their contribution to the collection of the OGLE photometric data over the past years. The OGLE project has received funding from the National Science Centre, Poland (grant number MAESTRO 2014/14/A/ST9/00121 to A.U.). M.L. acknowledges support from the Alexander von Humboldt Foundation. The Las Campanas Observatory, which hosts the Warsaw Telescope, Swope Telescope and Magellan Telescopes, is operated by the Carnegie Institution for Science. The Gemini Observatory is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the National Science Foundation (NSF) on behalf of the Gemini partnership: the NSF (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), and Ministério da Ciência, Tecnologia, Inovações e Comunicações (Brazil).

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Contributions

P.P. coordinated the research, obtained and analysed part of the observations and prepared the manuscript. W.A.D. proposed the envelope model and calculated its characteristics. M.L. fitted model atmospheres to the spectroscopic data. R.A. obtained and reduced Gemini spectra. R.P. and F.diM. obtained part of the follow-up photometric observations. The remaining authors, including also P.P. and R.P., collected the OGLE observations. All authors commented on the manuscript and were involved in the scientific discussion.

Corresponding author

Correspondence to Paweł Pietrukowicz.

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The authors declare no competing financial interests.

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Pietrukowicz, P., Dziembowski, W., Latour, M. et al. Blue large-amplitude pulsators as a new class of variable stars. Nat Astron 1, 0166 (2017). https://doi.org/10.1038/s41550-017-0166

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