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An 18.9 min blue large-amplitude pulsator crossing the ‘Hertzsprung gap’ of hot subdwarfs

Abstract

Blue large-amplitude pulsators (BLAPs) represent a new and rare class of hot pulsating stars with unusually large amplitudes and short periods. The evolutionary path that could give rise to such kinds of stellar configurations is unclear. Here we report a comprehensive study of the peculiar BLAP discovered by the Tsinghua University–Ma Huateng Telescopes for Survey (TMTS), namely, TMTS J035143.63+584504.2 (TMTS-BLAP-1). This new BLAP has an 18.9 min pulsation period and is similar to the BLAPs with a low surface gravity and extended helium-enriched envelope, suggesting that it is a low-gravity BLAP at the shortest-period end. In particular, the long-term monitoring data reveal that this pulsating star has an unusually large rate of period change, namely, \(\dot{P}/P\) = 2.2 × 10–6 yr–1. Such a significant and positive value challenges its origins from both helium-core pre-white-dwarfs and core helium-burning subdwarfs, but is consistent with that derived from shell helium-burning subdwarfs. The particular pulsation period and unusual rate of period change indicate that TMTS-BLAP-1 is at a short-lived (~106 yr) phase of shell helium ignition before the stable shell helium burning; in other words, TMTS-BLAP-1 is going through a ‘Hertzsprung gap’ of hot subdwarfs.

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Fig. 1: Distribution of all the known BLAPs and candidates in the period−amplitude diagram.
Fig. 2: Keck LRIS spectra and best-fitting parameters for TMTS-BLAP-1 at four non-overlapping pulsation phases.
Fig. 3: Normalized WWZ plots for TMTS-BLAP-1.
Fig. 4: O − C diagram for the pulsation period of TMTS-BLAP-1.
Fig. 5: HR diagram and period versus rate of period change diagram for three candidate BLAP models.

Data availability

The ZTF r- and g-band photometry can be obtained from the NASA/IPAC Infrared Science Archive (https://irsa.ipac.caltech.edu). The ATLAS o- and c-band magnitudes can be obtained from the ATLAS forced photometry server (https://fallingstar-data.com/forcedphot). All the reduced light curves and spectra used for this work, as well as some evolutionary tracks, are available via Zenodo at https://doi.org/10.5281/zenodo.6425425. Source data are provided with this paper.

Code availability

The codes of Tlusty (v. 207) and Synspec (v. 53) that are used for generating (non-local thermodynamic equilibrium) model atmospheres and producing synthetic spectra are available at https://www.as.arizona.edu/h̃ubeny, and the services of online spectral analyses (XTgrid) are provided from Astroserver (www.astroserver.org). The Python package libwwz (v. 1.2.0) for WWZ analysis can be obtained from https://pypi.org/project/libwwz. The general tools for timing analysis are provided from Python package gatspy (v. 0.3) (http://www.astroml.org/gatspy or https://zenodo.org/record/47887). The software MESA (v. 12115) used for stellar evolutionary calculations is available at http://mesastar.org.

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Acknowledgements

We are grateful to P. Pietrukowicz for sharing very useful information about OGLE BLAPs. The work of X.W. is supported by the National Science Foundation of China (NSFC grants 12033003 and 11633002), the Ma Huateng Foundation, the Scholar Program of Beijing Academy of Science and Technology (DZ:BS202002) and the Tencent Xplorer Prize. C.W. is supported by the National Natural Science Foundation of China (NSFC grant 12003013). P.N. acknowledges support from the Grant Agency of the Czech Republic (GAČR 22-34467S) and from the Polish National Science Centre under projects UMO-2017/26/E/ST9/00703 and UMO-2017/25/B/ST9/02218. The Astronomical Institute in Ondřejov is supported by project RVO:67985815. T.W. is grateful for support from the B-type Strategic Priority Program of the Chinese Academy of Sciences (grant XDB41000000), the National Key R&D Program of China (grant 2021YFA1600402), the NSFC of China (grants 11873084 and 12133011), the Youth Innovation Promotion Association of the Chinese Academy of Sciences and the Ten Thousand Talents Program of Yunnan for Top-notch Young Talents. T.W. also acknowledges with gratitude the computing time granted by the Yunnan Observatories and those for the facilities at the Yunnan Observatories Supercomputing Platform and the ‘PHOENIX Supercomputing Platform’ jointly operated by the Binary Population Synthesis Group and The Stellar Astrophysics Group at Yunnan Observatories, Chinese Academy of Sciences. A.V.F.’s group at University of California Berkeley received financial support from the Miller Institute for Basic Research in Science (where A.V.F. was a Miller Senior Fellow), the Christopher R. Redlich Fund and many individual donors. Y.C. is funded by the China Postdoctoral Science Foundation (grant 2021M691821). Some of the data presented here were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA; the observatory was made possible by the generous financial support of the W. M. Keck Foundation. This work has made use of data from the Asteroid Terrestrial-Impact Last Alert System (ATLAS) project. The ATLAS project is primarily funded to search for near-Earth objects (NEOs) through NASA grants NN12AR55G, 80NSSC18K0284 and 80NSSC18K1575; byproducts of the NEO search include images and catalogues from the survey area. This work was partially funded by Kepler/K2 grant J1944/80NSSC19K0112 and HST GO-15889, and STFC grants ST/T000198/1 and ST/S006109/1. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy; the Queen’s University Belfast; the Space Telescope Science Institute; the South African Astronomical Observatory; and The Millennium Institute of Astrophysics (MAS), Chile. Based in part on observations obtained with the Samuel Oschin 48 inch telescope and the 60 inch telescope at Palomar Observatory as part of the Zwicky Transient Facility (ZTF) project. ZTF is supported by the US National Science Foundation under grants AST-1440341 and AST-2034437, as well as a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin, Lawrence Livermore National Laboratories, Lawrence Berkeley National Laboratory and IN2P3 (France). The operations are conducted by COO, IPAC and UW. This research has used the services of www.astroserver.org under reference UMVVTX. It has also used the VizieR catalogue access tool, CDS (https://doi.org/10.26093/cds/vizier). We also used data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC; https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for DPAC has been provided by national institutions, particularly institutions participating in the Gaia Multilateral Agreement. This work used the International Variable Star Index (VSX)71 database, operated at the American Association of Variable Star Observers (Cambridge).

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Authors

Contributions

J.L., C.W., X.W. and P.N. drafted the manuscript. A.V.F., T.W. and Y.C. also helped with the manuscript, and A.V.F. edited it in detail. X.W. is the PI of TMTS and SNOVA. J.L. discovered this source by analysing the large-volume data from TMTS observations and performed the timing analysis to determine its rate of period change. C.W. computed the stellar evolution models for helium-burning stars and helium-core pre-WDs, and H.X. provided some key ideas for these models. T.W. contributed to the asteroseismic theory and analysis. P.N. determined the atmospheric parameters from Keck I LRIS spectra. Y.C., S.Y., Y.L. and D.X. assisted in the spectral analysis. The Keck I LRIS spectra were provided by A.V.F.’s group (including A.V.F, T.G.B., W.-K.Z. and Y.Y.). A.I., A.E. and Jujia Zhang contributed to the observations with SNOVA and the Lijiang 2.4 m telescope, and X. Zeng reduced these data. X.W., J.M., G.X., J.Z. and J.L. contributed to the building, pipeline and database of TMTS. G.X., J.M., X.J., H.S., Z.W., L.C., F.G., Z.C., W. Li, W. Lin, H.L. and X. Zhang contributed to the operations of TMTS.

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Correspondence to Xiaofeng Wang.

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Extended data

Extended Data Fig. 1 TMTS L-band (close to white-light) light curves of TMTS-BLAP-1 taken on December 24 and 25, 2020 (UT).

The red solid lines represent the best-fitting models of Fourier series truncated at fourth harmonic.

Extended Data Fig. 2 Phase-folded light curves for every subset of ATLAS, ZTF, TMTS, and SNOVA data.

Every subset of ATLAS and ZTF data covers up to 20 ~ days, while each subset of TMTS and SNOVA data covers only one night. The observed time of maximum light (\({{{{\rm{T}}}}}_{\max }^{{{{\rm{O}}}}}-2,450,000\)) for every subset is shown above the plots. Since the phases here were all calculated using the ephemeris of Eq. (1), the pulsation phases ϕ = 0 (the vertical dot-dashed lines) here correspond to the calculated times of maximum light, namely \({{{{\rm{T}}}}}_{\max }^{{{{\rm{C}}}}}\).

Extended Data Fig. 3 Folded light curve and surface parameters against pulsation phase.

a, corrected ZTF r-band folded light curve with a best-fitting 3-harmonic Fourier model overplotted (red solid line); b,c,d, radial velocity (RV), effective temperature (Teff) and surface gravity (log g) against pulsation phase. The red solid curves are the best-fitting sinusoidal curves, and the purple dashed line in panel d represents the prediction from the time-derivative of the best-fitting model of radial velocity3.

Extended Data Fig. 4 O-C diagram for the pulsation period of ZGP-BLAP-09.

The observed time of maximum light (\({{{{\rm{T}}}}}_{\max }^{{{{\rm{O}}}}}\)) was obtained from the 20 ~ day subsets of ATLAS and ZTF. The O-C values were calculated following the the ephemeris \({{{{\rm{T}}}}}_{\max }^{{{{\rm{C}}}}}={{{{\rm{BJD}}}}}_{{{{\rm{TDB}}}}}\,2,458,218.5012+0.0161558353\times {{{\rm{E}}}}\). Because ZGP-BLAP-09 lacks similar cyclic behavior in the diagram, the O-C variability is modeled only by assuming the linear period change.

Extended Data Fig. 5 Phase-folded light curves of TMTS-BLAP-1.

The folded light curves are derived from ZTF r-band (panels a,b) and ATLAS o-band (panels c,d) observations. a,c, The light curves are folded using a constant period inferred from the Lomb–Scargle periodogram. b,d, The light curves are folded using the new ephemeris derived from the O-C diagram. The red solid lines represent the best-fitting 3-harmonic Fourier models.

Supplementary information

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Supplementary Figs. 1 and 2 and Tables 1–3.

Source data

Source Data Fig. 1

Statistical source data.

Source Data Fig. 5

Statistical source data.

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Lin, J., Wu, C., Wang, X. et al. An 18.9 min blue large-amplitude pulsator crossing the ‘Hertzsprung gap’ of hot subdwarfs. Nat Astron (2022). https://doi.org/10.1038/s41550-022-01783-z

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