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

SÁNCHEZ ARIAS, JULIETA P.; ROMERO, ALEJANDRA D.; CÓRSICO, ALEJANDRO H.; PELISOLI, INGRID; ANTOCI, VICTORIA; KEPLER, S. O.; ALTHAUS, LEANDRO G.; CORTI, MARIELA A.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Año: 2018 vol. 616

0004-6361

Context. Pulsating extremely low-mass pre-white dwarf stars (pre-ELMV),with masses between 0.15 M⊙ and 0.30 M⊙,constitute a new class of variable stars showing g- and possibly p-modepulsations with periods between 320 and 6000 s (frequencies between 14.4and 270 c/d), driven by the κ mechanism operating in the second Heionization zone. On the other hand, main sequence δ Scuti stars,with masses between 1.2 and 2.5 M⊙, pulsate in low-orderg and p modes with periods in the range [700-28 800] s (frequencies inthe range [3-123] c/d), driven by the κ mechanism operating in theHe II ionization zone and the turbulent pressure acting in the HIionization layer. Interestingly enough, the instability strips ofpre-ELM white dwarf and δ Scuti stars nearly overlap in theTeff vs. log g diagram, leading to a degeneracy whenspectroscopy is the only tool to classify the stars and pulsationperiods only are considered. Aims: Pre-ELM white dwarf andδ Scuti stars are in very different stages of evolution andtherefore their internal structure is very distinct. This is mirrored intheir pulsational behavior, thus employing asteroseismology should allowus to distinguish between these groups of stars despite their similaratmospheric parameters. Methods: We have employed adiabatic andnon-adiabatic pulsation spectra for models of pre-ELM white dwarfs andδ Scuti stars, and compare their pulsation periods, periodspacings, and rates of period change. Results: Unsurprisingly, wefound substantial differences in the period spacing of δ Scuti andpre-ELM white dwarf models. Even when the same period range is observedin both classes of pulsating stars, the modes have distinctive signaturein the period spacing and period difference values. For instance, themean period difference of p-modes of consecutive radial orders forδ Scuti model are at least four times longer than the mean periodspacing 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 rateof period change is two orders of magnitudes larger for the pre-ELMwhite dwarfs compared to δ Scuti stars. In addition, we alsoreport the discovery of a new variable star, SDSSJ075738.94+144827.50,located in the region of the Teff versus log g diagram wherethese two kind of stars coexist. Conclusions.The characteristic spacingbetween modes of consecutive radial orders (p as well as g modes) andthe large differences found in the rates of period change for δScuti and pre-ELM white dwarf stars suggest that asteroseismology can beemployed to discriminate between these two groups of variable stars.Furthermore, we found that SDSSJ075738.94+144827.50 exhibits a perioddifference between p modes characteristic of a δ Sct star,assuming consecutive radial order for the observed periods.