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Register a new userDiscovery of Three Pulsating, Mixed-atmosphere, Extremely Low-mass White Dwarf Precursors
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Added by
Alexandros Gianninas
Publication date
2016
fields
Physics
and research's language is
English
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We report the discovery of pulsations in three mixed-atmosphere, extremely low-mass white dwarf (ELM WD, M $leqslant$ 0.3 M$_{odot}$) precursors. Following the recent discoveries of pulsations in both ELM and pre-ELM WDs, we targeted pre-ELM WDs with mixed H/He atmospheres with high-speed photometry. We find significant optical variability in all three observed targets with periods in the range 320--590 s, consistent in timescale with theoretical predictions of $p$-mode pulsations in mixed-atmosphere $approx$ 0.18 M$_{odot}$ He-core pre-ELM WDs. This represents the first empirical evidence that pulsations in pre-ELM WDs can only occur if a significant amount of He is present in the atmosphere. Future, more extensive, time-series photometry of the brightest of the three new pulsators offers an excellent opportunity to constrain the thickness of the surface H layer, which regulates the cooling timescales for ELM WDs.
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We present the discovery of an unusual, tidally-distorted extremely low mass white dwarf (WD) with nearly solar metallicity. Radial velocity measurements confirm that this is a compact binary with an orbital period of 2.6975 hrs and a velocity semi-amplitude of K = 108.7 km/s. Analysis of the hydrogen Balmer lines yields an effective temperature of Teff = 8380 K and a surface gravity of log g = 6.21 that in turn indicate a mass of M = 0.16 Msol and a cooling age of 4.2 Gyr. In addition, a detailed analysis of the observed metal lines yields abundances of log Mg/H = -3.90, log Ca/H = -5.80, log Ti/H = -6.10, log Cr/H = -5.60, and log Fe/H = -4.50, similar to the sun. We see no evidence of a debris disk from which these metals would be accreted though the possibility cannot entirely be ruled out. Other potential mechanisms to explain the presence of heavy elements are discussed. Finally, we expect this system to ultimately undergo unstable mass transfer and merge to form a ~0.3-0.6 Msol WD in a few Gyr.
We present the orbit and properties of 2MASS J050051.85-093054.9, establishing it as the closest (d ~ 71 pc) extremely low mass white dwarf to the Sun. We find that this star is hydrogen-rich with Teff ~ 10 500 K, log g ~ 5.9, and, following evolutionary models, has a mass of ~ 0.17 solar masses. Independent analysis of radial velocity and TESS photometric time series reveals an orbital period of ~ 9.5 h. Its high velocity amplitude (K ~ 144 km/s) produces a measurable Doppler beaming effect in the TESS light curve with an amplitude of 1 mmag. The unseen companion is most likely a faint white dwarf. J0500-0930 belongs to a class of post-common envelope systems that will most likely merge through unstable mass transfer and in specific circumstances lead to Type Ia supernova explosions.
Many low-mass white dwarfs are being discovered in the field of our galaxy and some of them exhibit $g$-mode pulsations, comprising the extremely low-mass variable (ELMV) stars class. Despite it is generally believed that these stars are characterized by thick H envelopes, from stellar evolution considerations, the existence of low-mass WDs with thin H envelopes is also possible. We have performed detailed asteroseismological fits to all the known ELMVs to search for a representative model by employing a set of fully evolutionary models that are representative of low-mass He-core white dwarf stars with a range of stellar masses $[0.1554-0.4352] M_{odot}$, effective temperatures $[6000-10000] $K, and also with a range of H envelope thicknesses $-5.8 lesssim log(M_{rm H}/M_{star}) lesssim -1.7$, hence expanding the space of parameters. We found that some of the stars under analysis are characterized by thick H envelopes, but others are better represented by models with thin H envelope.
At present, a large number of pulsating white dwarf (WD) stars is being discovered either from Earth-based surveys such as the Sloan Digital Sky Survey, or through observations from space (e.g., the Kepler mission). The asteroseismological techniques allow us to infer details of internal chemical stratification, the total mass, and even the stellar rotation profile. In this paper, we first describe the basic properties of WD stars and their pulsations, as well as the different sub-types of these variables known so far. Subsequently, we describe some recent findings about pulsating low-mass WDs.
We present analysis of a new pulsating helium-atmosphere (DB) white dwarf, EPIC~228782059, discovered from 55.1~days of {em K2} photometry. The long duration, high quality light curves reveal 11 independent dipole and quadruple modes, from which we derive a rotational period of $34.1 pm 0.4$~hr for the star. An optimal model is obtained from a series of grids constructed using the White Dwarf Evolution Code, which returns $M_{*} = 0.685 pm 0.003 M_{odot}$, $T_{rm{eff}}= 21{,}910 pm 23$,K and $log g = 8.14 pm0.01$,dex. These values are comparable to those derived from spectroscopy by Koester & Kepler ($20{,}860 pm 160$,K and $7.94 pm0.03$,dex). If these values are confirmed or better constrained by other independent works, it would make EPIC~228782059 one of the coolest pulsating DB white dwarf star known, and would be helpful to test different physical treatments of convection, and to further investigate the theoretical instability strip of DB white dwarf stars.
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