White dwarfs: perfect laboratories for understanding non-radial pulsations and revealing secrets on the single degenerate pathway towards Supernova type Ia

White dwarfs are elderly stars that represent the endpoint of stars with masses lower than ≈9MO, which comprise 95%–98% of all stars in our Galaxy, including our Sun. Hence, the motivation of their study is to reveal important insight about the future of our Solar system. In this thesis I present three main projects that are linked because the analysed systems host white dwarfs. I begin by introducing in Chapter 1 the evolution and properties of single white dwarfs and in cataclysmic variables. Most of the light emitted by the white dwarf is detected in the ultraviolet, therefore the spectrographs mounted in the Hubble Space Telescope are ideal for the white dwarf science. In this thesis I performed the analysis of spectroscopic data taken with the Cosmic Origins Spectrograph and the Space Telescope Imaging Spectrograph, therefore I explain their performance and capabilities in Chapter 2. The analysis of the Hubble spectroscopy consists mainly in determining the white dwarf atmospheric parameters. The technique used in this thesis is fitting the data with synthetic white dwarf atmospheres using the Markov Chain Monte Carlo for Bayesian inference, which I explain in Chapter 3. G29-38 is a non-radial pulsating white dwarf that shows infrared excess due to a dusty debris disc that formed from the tidal disruption of a planetesimal. The ongoing accretion from this debris disc pollutes the atmosphere of G29-38. The analysis of the photospheric contamination provides a direct measurement of the bulk composition of the disrupted planetesimal. However, the geometry and the process of the accretion from the debris disc onto the white dwarf are not yet well understood. In Chapter 4, I make use of the pulsations as a spotlight to investigate the metal distribution across the white dwarf surface which provides constraints on the geometry of the accretion process. In Chapter 5 I present my work on the dwarf nova GWLibrae in which the white dwarf drives nonradial pulsations. GWLibrae stands out by having a well-established observational record of three independent pulsation modes that disappeared when the white dwarf temperature rose dramatically following its 2007 outburst. Therefore, GWLibrae offers the opportunity to investigate the response of these modes to changes in the white dwarf temperature. Here I report the presence of a high-amplitude variability on a ' 4:4 h time-scale, which I demonstrate to be the result of an increase of the temperature of a relatively small region on the white dwarf surface. Cataclysmic variables undergoing thermal time scale mass transfer are known as super-soft X-rays sources which provide a pathway towards Supernova type Ia through the single degenerate channel. The study of these systems is very difficult due to the small number of systems known, moreover the extremely hot white dwarfs outshine their companions. In Chapter 6 I present the analysis of HS0218+3229 and QZSer which present low abundance ratios of carbon to nitrogen which is the signature seen in descendants of super-soft X-rays sources. I also present MESA simulations that constrain the parameter space for the formation of these failed supernova type Ia. Finally, I present the concluding remarks of this thesis in Chapter 7

Cold giant planets evaporated by hot white dwarfs

Atmospheric escape from close-in Neptunes and hot Jupiters around Sun-like stars driven by extreme ultraviolet (EUV) irradiation plays an important role in the evolution of exoplanets and in shaping their ensemble properties. Intermediate and low mass stars are brightest at EUV wavelengths at the very end of their lives, after they have expelled their envelopes and evolved into hot white dwarfs. Yet the effect of the intense EUV irradiation of giant planets orbiting young white dwarfs has not been assessed. We show that the giant planets in the solar system will experience significant hydrodynamic escape caused by the EUV irradiation from the white dwarf left behind by the Sun. A fraction of the evaporated volatiles will be accreted by the solar white dwarf, resulting in detectable photospheric absorption lines. As a large number of the currently known extrasolar giant planets will survive the metamorphosis of their host stars into white dwarfs, observational signatures of accretion from evaporating planetary atmospheres are expected to be common. In fact, one-third of the known hot single white dwarfs show photospheric absorption lines of volatile elements, which we argue are indicative of ongoing accretion from evaporating planets. The fraction of volatile contaminated hot white dwarfs strongly decreases as they cool. We show that accretion from evaporating planetary atmospheres naturally explains this temperature dependence if more than 50% of hot white dwarfs still host giant planets

SDSS J124043.01+671034.68 : the partially burned remnant of a low-mass white dwarf that underwent thermonuclear ignition?

The white dwarf SDSS J124043.01+671034.68 (SDSS J1240+6710) was previously found to have an oxygen-dominated atmosphere with significant traces of neon, magnesium, and silicon. A possible origin via a violent late thermal pulse or binary interactions has been suggested to explain this very unusual photospheric composition. We report the additional detection of carbon, sodium, and aluminium in far-ultraviolet and optical follow-up spectroscopy. No iron-group elements are detected, with tight upper limits on titanium, iron, cobalt, and nickel, suggesting that the star underwent partial oxygen burning, but failed to ignite silicon burning. Modelling the spectral energy distribution and adopting the distance based on the Gaia parallax, we infer a low white dwarf mass, Mwd = 0.41 ± 0.05 M. The large space velocity of SDSS J1240+6710, computed from the Gaia proper motion and its radial velocity, is compatible with a Galactic rest-frame velocity of 250 km s−1 in the opposite direction with respect to the Galactic rotation, strongly supporting a binary origin of this star. We discuss the properties of SDSS J1240+6710 in the context of the recently identified survivors of thermonuclear supernovae, the D6 and LP 40−365 stars, and conclude that it is unlikely related to either of those two groups. We tentatively suggest that SDSS J1240+6710 is the partially burned remnant of a low-mass white dwarf that underwent a thermonuclear event

SDSS J124043.01+671034.68 : the partially burned remnant of a low-mass white dwarf that underwent thermonuclear ignition?

The white dwarf SDSS J124043.01+671034.68 (SDSS J1240+6710) was previously found to have an oxygen-dominated atmosphere with significant traces of neon, magnesium, and silicon. A possible origin via a violent late thermal pulse or binary interactions have been suggested to explain this very unusual photospheric composition. We report the additional detection of carbon, sodium, and aluminium in far-ultraviolet and optical follow-up spectroscopy. No iron-group elements are detected, with tight upper limits on iron, cobalt and nickel, suggesting that the star underwent partial oxygen burning, but failed to ignite silicon burning. Modelling the spectral energy distribution and adopting the distance based on the Gaia parallax, we infer a low white dwarf mass, M(wd)=0.41+/-0.05Msun. The large space velocity of SDSS J1240+6710, computed from the Gaia proper motion and its radial velocity, is compatible with a Galactic rest-frame velocity of ~250km/s in the opposite direction with respect to the Galactic rotation, strongly supporting a binary origin of this star. We discuss the properties of SDSS J1240+6710 in the context of the recently identified survivors of thermonuclear supernovae, the D6 and LP 40-365 stars, and conclude that it is unlikely related to either of those two groups. We tentatively suggest that SDSS J1240+6710 is the partially burned remnant of a low-mass white dwarf that underwent a thermonuclear event.Comment: Accepted for publication in MNRA

Calibration of the mixing-length theory for structures of helium-dominated atmosphere white dwarfs

We perform a calibration of the mixing-length parameter at the bottom boundary of the convection zone for helium-dominated atmospheres of white dwarfs. This calibration is based on a grid of 3D DB (pure-helium) and DBA (helium-dominated with traces of hydrogen) model atmospheres computed with the co5bold radiation-hydrodynamics code, and a grid of 1D DB and DBA envelope structures. The 3D models span a parameter space of hydrogen-to-helium abundances in the range −10.0 ≤ log (H/He) ≤−2.0, surface gravities in the range 7.5 ≤ log g ≤ 9.0, and effective temperatures in the range 12 000 K ≲ Teff ≲ 34 000 K. The 1D envelopes cover a similar atmospheric parameter range, but are also calculated with different values of the mixing-length parameter, namely 0.4 ≤ ML2/α ≤ 1.4. The calibration is performed based on two definitions of the bottom boundary of the convection zone: the Schwarzschild and the zero convective flux boundaries. Thus, our calibration is relevant for applications involving the bulk properties of the convection zone including its total mass, which excludes the spectroscopic technique. Overall, the calibrated ML2/α is smaller than what is commonly used in evolutionary models and theoretical determinations of the blue edge of the instability strip for pulsating DB and DBA stars. With calibrated ML2/α we are able to deduce more accurate convection zone sizes needed for studies of planetary debris mixing and dredge-up of carbon from the core. We highlight this by calculating examples of metal-rich 3D DBAZ models and finding their convection zone masses. Mixing-length calibration represents the first step of in-depth investigations of convective overshoot in white dwarfs with helium-dominated atmospheres

Accretion of a giant planet onto a white dwarf star

The detection of a dust disc around G29-38 and transits from debris orbiting WD1145+017 confirmed that the photospheric trace metals found in many white dwarfs arise from the accretion of tidally disrupted planetesimals. The composition of these planetesimals is similar to that of rocky bodies in the inner solar system. Gravitationally scattering planetesimals towards the white dwarf requires the presence of more massive bodies, yet no planet has so far been detected at a white dwarf. Here we report optical spectroscopy of a $\simeq27\,750$K hot white dwarf that is accreting from a circumstellar gaseous disc composed of hydrogen, oxygen, and sulphur at a rate of $\simeq3.3\times10^9\,\mathrm{g\,s^{-1}}$. The composition of this disc is unlike all other known planetary debris around white dwarfs, but resembles predictions for the makeup of deeper atmospheric layers of icy giant planets, with H$_2$O and H$_2$S being major constituents. A giant planet orbiting a hot white dwarf with a semi-major axis of $\simeq15$ solar radii will undergo significant evaporation with expected mass loss rates comparable to the accretion rate onto the white dwarf. The orbit of the planet is most likely the result of gravitational interactions, indicating the presence of additional planets in the system. We infer an occurrence rate of spectroscopically detectable giant planets in close orbits around white dwarfs of $\simeq10^{-4}$.Comment: Nature, December 5 issu

GW Librae: Still Hot Eight Years Post-Outburst

We report continued Hubble Space Telescope (HST) ultraviolet spectra and ground-based optical photometry and spectroscopy of GW Librae eight years after its largest known dwarf nova outburst in 2007. This represents the longest cooling timescale measured for any dwarf nova. The spectra reveal that the white dwarf still remains about 3000 K hotter than its quiescent value. Both ultraviolet and optical light curves show a short period of 364-373 s, similar to one of the non-radial pulsation periods present for years prior to the outburst, and with a similar large UV/optical amplitude ratio. A large modulation at a period of 2 h (also similar to that observed prior to outburst) is present in the optical data preceding and during the HST observations, but the satellite observation intervals did not cover the peaks of the optical modulation so it is not possible to determine its corresponding UV amplitude. The similarity of the short and long periods to quiescent values implies the pulsating, fast spinning white dwarf in GW Lib may finally be nearing its quiescent configuration.Comment: 6 figures, accepted in A

Rotation plays a role in the generation of magnetic fields in single white dwarfs

Recent surveys of close white dwarf binaries as well as single white dwarfs have provided evidence for the late appearance of magnetic fields in white dwarfs, and a possible generation mechanism a crystallization and rotation-driven dynamo has been suggested. A key prediction of this dynamo is that magnetic white dwarfs rotate, at least on average, faster than their non-magnetic counterparts and/or that the magnetic field strength increases with rotation. Here we present rotation periods of ten white dwarfs within 40 pc measured using photometric variations. Eight of the light curves come from TESS observations and are thus not biased towards short periods, in contrast to most period estimates that have been reported previously in the literature. These TESS spin periods are indeed systematically shorter than those of non-magnetic white dwarfs. This means that the crystallization and rotation-driven dynamo could be responsible for a fraction of the magnetic fields in white dwarfs. However, the full sample of magnetic white dwarfs also contains slowly rotating strongly magnetic white dwarfs which indicates that another mechanism that leads to the late appearance of magnetic white dwarfs might be at work, either in addition to or instead of the dynamo. The fast-spinning and massive magnetic white dwarfs that appear in the literature form a small fraction of magnetic white dwarfs, and probably result from a channel related to white dwarf mergers.Comment: 20 pages, 10 figures, 8 tables, accepted for publication in MNRA

Hubble Space Telescope ultraviolet light curves reveal interesting properties of CC Sculptoris and RZ Leonis

Time-tag ultraviolet data obtained on the Hubble Space Telescope in 2013 reveal interesting variability related to the white dwarf spin in the two cataclysmic variables RZ Leo and CC Scl. RZ Leo shows a period at 220 s and its harmonic at 110 s, thus identifying it as a likely Intermediate Polar (IP). The spin signal is not visible in a short single night of ground-based data in 2016, but the shorter exposures in that data set indicate a possible partial eclipse. The much larger UV amplitude of the spin signal in the known IP CC Scl allows the spin of 389 s, previously only seen at outburst, to be visible at quiescence. Spectra created from the peaks and troughs of the spin times indicate a hotter temperature of several thousand degrees during the peak phases, with multiple components contributing to the UV light