Comparing the asteroseismic properties of pulsating pre-extremely low mass white dwarf and delta Scuti stars

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    Julieta P. Sanchez Arias, UNLP, National University of La Plata, Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET), CONICET, Inst Astrofis La Plata, Alejandra D. Romero, Univ Fed Rio Grande do Sul, Universidade Federal do Rio Grande do Sul, Inst Fis, Alejandro H. Corsico, UNLP, National University of La Plata, Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET), CONICET, Inst Astrofis La Plata, Ingrid Pelisoli, Univ Fed Rio Grande do Sul, Universidade Federal do Rio Grande do Sul, Inst Fis,
  • Victoria Antoci
  • S. O. Kepler, Univ Fed Rio Grande do Sul, Universidade Federal do Rio Grande do Sul, Inst Fis, Leandro G. Althaus, UNLP, National University of La Plata, Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET), CONICET, Inst Astrofis La Plata, Mariela A. Corti, Consejo Nacl Invest Cient & Tecn, Instituto Argentino de Radioastronomia, Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET), National University of La Plata, CCT La Plata, Inst Argentino Radioastron

Context. Pulsating extremely low-mass pre-white dwarf stars (pre-ELMV), with masses between similar to 0.15 M-circle dot and similar to 0.30 M-circle dot, constitute a new class of variable stars showing g- and possibly p-mode pulsations with periods between 320 and 6000 s (frequencies between 14.4 and 270 c/d), driven by the kappa mechanism operating in the second He ionization zone. On the other hand, main sequence delta Scuti stars, with masses between 1.2 and 2.5 M-circle dot, pulsate in low-order g and p modes with periods in the range [700-28 800] s (frequencies in the range [3-123] c/d), driven by the kappa mechanism operating in the He II ionization zone and the turbulent pressure acting in the HI ionization layer. Interestingly enough, the instability strips of pre-ELM white dwarf and delta Scuti stars nearly overlap in the T-eff vs. log g diagram, leading to a degeneracy when spectroscopy is the only tool to classify the stars and pulsation periods only are considered.

Aims. Pre-ELM white dwarf and delta Scuti stars are in very different stages of evolution and therefore their internal structure is very distinct. This is mirrored in their pulsational behavior, thus employing asteroseismology should allow us to distinguish between these groups of stars despite their similar atmospheric parameters.

Methods. We have employed adiabatic and non-adiabatic pulsation spectra for models of pre-ELM white dwarfs and delta Scuti stars, and compare their pulsation periods, period spacings, and rates of period change.

Results. Unsurprisingly, we found substantial differences in the period spacing of delta Scuti and pre-ELM white dwarf models. Even when the same period range is observed in both classes of pulsating stars, the modes have distinctive signature in the period spacing and period difference values. For instance, the mean period difference of p-modes of consecutive radial orders for delta Scuti model are at least four times longer than the mean period spacing for the pre-ELM white dwarf model in the period range [2000-4600] s (frequency range [18.78-43.6] c/d). In addition, the rate of period change is two orders of magnitudes larger for the pre-ELM white dwarfs compared to delta Scuti stars. In addition, we also report the discovery of a new variable star, SDSSJ075738.94 + 144827.50, located in the region of the T-eff versus log g diagram where these two kind of stars coexist.

Conclusions. The characteristic spacing between modes of consecutive radial orders (p as well as g modes) and the large differences found in the rates of period change for delta Scuti and pre-ELM white dwarf stars suggest that asteroseismology can be employed to discriminate between these two groups of variable stars. Furthermore, we found that SDSSJ075738.94 + 144827.50 exhibits a period difference between p modes characteristic of a delta Sct star, assuming consecutive radial order for the observed periods.

Original languageEnglish
Article number80
JournalAstronomy & Astrophysics
Volume616
Number of pages14
ISSN1432-0746
DOIs
Publication statusPublished - 24 Aug 2018

    Research areas

  • asteroseismology, stars: evolution, stars: interiors, stars: oscillations, stars: variables: delta Scuti, white dwarfs, GAMMA DORADUS STARS, PG-1159 EVOLUTIONARY MODELS, RED-GIANT STAR, ORDER G-MODES, SX PHE STARS, STELLAR EVOLUTION, AGB STARS, OVERSHOOT, OPACITIES, ROTATION

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