Short-period White Dwarf Research Articles

LISA Galactic binaries in the Roman Galactic Bulge Time-Domain Survey

ABSTRACT Short-period Galactic white dwarf binaries detectable by Laser Interferometer Space Antenna are the only guaranteed persistent sources for multimessenger gravitational-wave astronomy. Large-scale surveys in the 2020s present an opportunity to conduct preparatory science campaigns to maximize the science yield from future multimessenger targets. The Nancy Grace Roman Space Telescope Galactic Bulge Time-Domain Survey will (in its Reference Survey design) image seven fields in the Galactic Bulge approximately 40 000 times each. Although the Reference Survey cadence is optimized for detecting exoplanets via microlensing, it is also capable of detecting short-period white dwarf binaries. In this paper, we present forecasts for the number of detached short-period binaries the Roman Galactic Bulge Time-Domain Survey will discover and the implications for the design of electromagnetic surveys. Although population models are highly uncertain, we find a high probability that the baseline survey will detect of the order of ∼5 detached white dwarf binaries. The Reference Survey would also have a ${\gtrsim} 20\,{\rm per\,cent}$ chance of detecting several known benchmark white dwarf binaries at the distance of the Galactic Bulge.

Radio and optical observations of the possible AE Aqr twin, LAMOST J024048.51+195226.9

ABSTRACT It was recently proposed that the cataclysmic variable (CV) LAMOST J024048.51+195226.9 may be a twin to the unique magnetic propeller system AE Aqr. If this is the case, two predictions are that it should display a short period white dwarf spin modulation, and that it should be a bright radio source. We obtained follow-up optical and radio observations of this CV, in order to see if this holds true. Our optical high-speed photometry does not reveal a white dwarf spin signal, but lacks the sensitivity to detect a modulation similar to the 33 s spin signal seen in AE Aqr. We detect the source in the radio, and measure a radio luminosity similar to that of AE Aqr and close to the highest so far reported for a CV. We also find good evidence for radio variability on a time-scale of tens of minutes. Optical polarimetric observations produce no detection of linear or circular polarization. While we are not able to provide compelling evidence, our observations are all consistent with this object being a propeller system.

Simplified 3D GCM modelling of the irradiated brown dwarf WD 0137−349B

ABSTRACT White dwarf–brown dwarf short-period binaries (Porb ≲ 2 h) are some of the most extreme irradiated atmospheric environments known. These systems offer an opportunity to explore theoretical and modelling efforts of irradiated atmospheres different to typical hot Jupiter systems. We aim to investigate the three-dimensional (3D) atmospheric structural and dynamical properties of the brown dwarf WD 0137−349B. We use the 3D global circulation model (GCM) Exo-Flexible Modelling System (FMS) with a dual-band grey radiative transfer scheme to model the atmosphere of WD 0137−349B. The results of the GCM model are post-processed using the 3D Monte Carlo radiative transfer model cmcrt. Our results suggest inefficient day–night energy transport and a large day–night temperature contrast for WD 0137−349B. Multiple flow patterns are present, shifting energy asymmetrically eastward or westward depending on their zonal direction and latitude. Regions of overturning are produced on the western terminator. We are able to reproduce the start of the system near-infrared (IR) emission excess at ≳1.95 μm as observed by the Gemini Near-Infrared Spectrograph (GNIRS) instrument. Our model overpredicts the IR phase curve fluxes by factors of ≈1–3, but generally fits the shape of the phase curves well. Chemical kinetic modelling using vulcan suggests a highly ionized region at high altitudes can form on the dayside of the brown dwarf. We present a first attempt at simulating the atmosphere of a short-period white dwarf–brown dwarf binary in a 3D setting. Further studies into the radiative and photochemical heating from the ultraviolet irradiation are required to more accurately capture the energy balance inside the brown dwarf atmosphere. Cloud formation may also play an important role in shaping the emission spectra of the brown dwarf.

Evidence for very nearby hidden white dwarfs

We report the discovery of a tertiary component to the prominent and nearby (d = 14 pc) visual binary alpha Fornacis. The new invisible component is part of a 4-d short-period Ba-Bb subsystem, whose K-type alpha For Ba primary shows a striking [Ba/Fe] = +0.26 dex overabundance, at variance with a close to solar [Ba/Fe] = +0.04 dex abundance for its distant visual companion alpha For A. For the even nearer (d = 8 pc) multiple star xi Ursae Majoris and its alike 4-d Ba-Bb subsystem, we similarly find [Ba/Fe] = +0.37 dex for its G-type xi UMa Ba primary, compared to a significantly lower [Ba/Fe] = +0.12 dex for the visual companion xi UMa A. In both cases, the differential barium enrichment is direct evidence for short-period white dwarf systems and the creation of blue straggler stars through mass transfer and wind accretion, for their close and distant companions, respectively. These findings also imply that conventional stellar age datings for both alpha For and xi UMa are likely not reliable.

Hot subdwarf binaries from the MUCHFUSS project

The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (MUCHFUSS) aims at finding hot subdwarf stars with massive compact companions like massive white dwarfs (M>1.0 M$_\odot$), neutron stars, or stellar-mass black holes. We present orbital and atmospheric parameters and put constraints on the nature of the companions of 12 close hot subdwarf B star (sdB) binaries found in the course of the MUCHFUSS project. The systems show periods between 0.14 and 7.4 days. Three systems most likely have white dwarf companions. SDSS J083006.17+475150.3 is likely to be a rare example of a low-mass helium-core white dwarf. SDSS J095101.28+034757.0 shows an excess in the infrared that probably originates from a third companion in a wide orbit. SDSS J113241.58-063652.8 is the first helium deficient sdO star with a confirmed close companion. This study brings to 142 the number of sdB binaries with orbital periods of less than 30 days and with measured mass functions. We present an analysis of the minimum companion mass distribution and show that it is bimodal. One peak around 0.1 M$_\odot$ corresponds to the low-mass main sequence and substellar companions. The other peak around 0.4 M$_\odot$ corresponds to the white dwarf companions. The derived masses for the white dwarf companions are significantly lower than the average mass for single carbon-oxygen white dwarfs. In a T$_{\rm eff}$-log(g) diagram of sdB+dM companions, we find signs that the sdB components are more massive than the rest of the sample. The full sample was compared to the known population of extremely low-mass white dwarf binaries as well as short-period white dwarfs with main sequence companions. Both samples show a significantly different companion mass distribution. We calculate merger timescales and timescales when the companion will fill its Roche Lobe and the system evolves into a cataclysmic variable.

A circumbinary planet in orbit around the short-period white dwarf eclipsing binary RR Cae

Abstract By using six newly determined mid-eclipse times together with those collected from the literature, we have found that the observed minus calculated (O−C) curve of RR Cae shows a cyclic change with a period of 11.9 yr and an amplitude of 14.3 s while it undergoes an upward parabolic variation [revealing a long-term period increase at a rate of ]. The cyclic change was analysed for the light-travel-time effect that arises from the gravitational influence of a third companion. The mass of the third body was determined to be M3sin i′= 4.2(± 0.4) MJup, suggesting that it is a circumbinary giant planet when its orbital inclination is larger than 17°.6. The orbital separation of the circumbinary planet from the central eclipsing binary is about 5.3(± 0.6) au. The period increase is opposite to the changes caused by angular momentum loss via magnetic braking or/and gravitational radiation; and it cannot be explained by the mass transfer between both components because of its detached configuration. These indicate that the observed upward parabolic change is only a part of a long-period (longer than 26.3 yr) cyclic variation, which may reveal the presence of another giant circumbinary planet in a wide orbit.

A giant planet in orbit around a magnetic-braking hibernating cataclysmic variable

Abstract Detections of Jupiter-like giant planets in orbit around short-period white dwarf binaries should provide insight into the formation and evolution of circumbinary planets (planets orbiting both components of short-period binaries), as well as into the ultimate fate of planets and the late evolutionary stage of binary stars (e.g. the evolution of the common envelope). However, to date no planets have been detected as companions to such close binaries. Here, we report the discovery of a giant planet orbiting the only known hibernating cataclysmic variable (CV), QS Vir, with a period of 7.86 yr. We analysed the variations of the orbital period of the eclipsing white dwarf–red dwarf binary, and a very small-amplitude cyclic change is found to be superimposed on a long-term period decrease. The period oscillation has the smallest amplitude among close binary stars and can be plausibly interpreted as the light-travel time effect via the presence of a third body. We found that the tertiary component is a giant planet with a mass of ∼6.4 MJupiter at a distance of ∼4.2 astronomical units (au) from the binary. The continuous decrease is explained as angular momentum loss via magnetic braking which is driving the evolution of the hibernating CV into a normal cataclysmic binary.

On the Relevance of ther‐Mode Instability for Accreting Neutron Stars and White Dwarfs

We present a case study for the relevance of the r-mode instability for accreting compact stars. Our estimates are based on approximations that facilitate back-of-the-envelope calculations. We discuss two different cases: 1) For recycled millisecond pulsars we argue that the r-mode instability may be active at rotation periods longer than the Kepler period (which provides the dynamical limit on rotation) as long as the core temperature is larger than about $2\times10^5$ K. Our estimates suggest that the instability may have played a role in the evolution of the fastest spinning pulsars, and that it may be presently active in the recently discovered 2.49 ms X-ray pulsar SAX J1808.4-3658 as well as the rapidly spinning neutron stars observed in low mass X-ray binaries. This provides a new explanation for the remarkably similar rotation periods inferred from kHz quasi-periodic oscillations in the LMXBs. The possibility that the rotation of recycled pulsars may be gravitational-radiation limited is interesting because the gravitational waves from a neutron star rotating at the instability limit may well be detectable with the new generation of interferometric detectors. 2) We also consider white dwarfs, and find that the r-mode instability may possibly be active in short period white dwarfs. Our order of magnitude estimates (for a white dwarf of $M=M_\odot$ and $R=0.01 R_\odot$ composed of $C^{12}$) show that the instability could be operating for rotational periods shorter than $P\approx 27-33$ s. This number is in interesting agreement with the observed periods ($>28$ s) of the rapidly spinning DQ Herculis stars. However, we find that the instability grows too slowly to affect the rotation of these stars significantly .

Gravitational helioseismology?

The magnitudes of the external gravitational perturbations associated with the normal modes of the Sun are evaluated to determine whether these solar oscillations could be observed with the proposed laser interferometer space antenna (LISA), a network of satellites designed to detect gravitational radiation. The modes of relevance to LISA---the $l=2$, low-order $p$, $f$, and $g$ modes---have not been conclusively observed to date. We find that the energy in these modes must be greater than about ${10}^{30}$ ergs in order to be observable above the LISA detector noise. These mode energies are larger than generally expected, but are much smaller than the current observational upper limits. LISA may be confusion limited at the relevant frequencies due to the galactic background from short-period white dwarf binaries. Present estimates of the number of these binaries would require the solar modes to have energies above about ${10}^{33}$ ergs to be observable by LISA.

Rotation and magnetism in white dwarfs

view Abstract Citations (62) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Rotation and Magnetism in White Dwarfs Schmidt, Gary D. ; Norsworthy, Jane E. Abstract New rotational ephemerides for four isolated magnetic white dwarfs are derived from more than a decade of polarimetric monitoring. The stars span the period range 99 min-17.9 days and effectively double the number of white dwarfs with measured rotation periods. In addition, five stars whose polarization is constant on time scales up to at least 10 years may have very long rotation periods (P greater than about 100 yr). The lack of very short period white dwarfs attests to the coupling of angular momentum into the giant envelope or interstellar medium during late stages of evolution, but thus far there is no evidence that this process involves magnetic braking. With few exceptions, the disk-averaged fields of magnetic degenerates are dominated by dipolar patterns (as opposed to higher order multipoles or spots), and cover a broad range of obliquity to the rotation axis. Publication: The Astrophysical Journal Pub Date: January 1991 DOI: 10.1086/169559 Bibcode: 1991ApJ...366..270S Keywords: Magnetic Stars; Stellar Rotation; White Dwarf Stars; Polarimetry; Polarized Radiation; Stellar Envelopes; Stellar Magnetic Fields; Stellar Structure; Astrophysics; STARS: MAGNETIC; STARS: ROTATION; STARS: WHITE DWARFS full text sources ADS | data products SIMBAD (10)

IUE spectrophotometry of the DA4 primary in the short-period white dwarf-red dwarf spectroscopic binary Case 1

view Abstract Citations (26) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS IUE spectrophotometry of the DA4 primary in the short-period white dwarf red dwarf spectroscopic binary Case 1. Sion, E. M. ; Wesemael, F. ; Guinan, E. F. Abstract Low-resolution ultraviolet International Ultraviolet Explorer spectra of the DA white dwarf Case 1 are presented. The spectra show the presence of the 1400 A feature, already discovered in several other DA stars, and of a shallower trough in the 1550-1700 A range. A model atmosphere analysis of the ultraviolet energy distribution of the Ly-alpha red wing yields T(e) = 13,000 + or - 500 K. Possible interpretations of the 1400 A feature are reviewed. Case 1 is the coolest white dwarf found in a short-period, detached white dwarf-red dwarf binary, and its cooling time is consistent with estimates of the efficiency of angular momentum removal mechanisms in the phases subsequent to common envelope binary evolution. Publication: The Astrophysical Journal Pub Date: April 1984 DOI: 10.1086/161944 Bibcode: 1984ApJ...279..758S Keywords: Binary Stars; Red Dwarf Stars; Satellite Observation; Stellar Spectrophotometry; Ultraviolet Spectra; White Dwarf Stars; Angular Momentum; Iue; Spectral Energy Distribution; Stellar Evolution; Stellar Spectra; Stellar Temperature; Ultraviolet Astronomy; Astrophysics full text sources ADS | data products SIMBAD (18) MAST (1) INES (1)