AM CVn binaries - "Substellar stars" or substellar companions?

A topic worth of discussion since dealing with very low-mass secondaries to white dwarf binaries. The most recent paper provides a list of seven AM CVn binaries with substellar secondaries even well within planetary domain:

Discovery and characterization of five new eclipsing AM CVn systems

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AM CVn systems are ultra-compact, helium-rich, accreting binaries with degenerate or semi-degenerate donors. We report the discovery of five new eclipsing AM CVn systems with orbital periods of 61.5, 55.5, 53.3, 37.4, and 35.4 minutes. These systems were discovered by searching for deep eclipses in the Zwicky Transient Facility (ZTF) lightcurves of white dwarfs selected using Gaia parallaxes. We obtained phase-resolved spectroscopy to confirm that all systems are AM CVn binaries, and we obtained high-speed photometry to confirm the eclipse and characterize the systems. The spectra of two long-period systems (61.5 and 53.3 minutes) show many emission and absorption lines, indicating the presence of N, O, Na, Mg, Si, and Ca, and also the K and Zn, elements which have never been detected in AM CVn systems before. By modelling the high-speed photometry, we measured the mass and radius of the donor star, potentially constraining the evolutionary channel that formed these AM CVn systems. We determined that the average mass of the accreting white dwarf is ≈0.8M⊙, and that the white dwarfs in long-period systems are hotter than predicted by recently updated theoretical models. The donors have a high entropy and are a factor of ≈ 2 more massive compared to zero-entropy donors at the same orbital period. The large donor radius is most consistent with He-star progenitors, although the observed spectral features seem to contradict this. The discovery of 5 new eclipsing AM~CVn systems is consistent with the known observed AM CVn space density and estimated ZTF recovery efficiency. Based on this estimate, we expect to find another 1--4 eclipsing AM CVn systems as ZTF continues to obtain data. This will further increase our understanding of the population, but will require high precision data to better characterize these 5 systems and any new discoveries

A new system:

The magnetic system SMSS J1606-1000 as a period bouncer

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We report the discovery of a rare close binary system, SMSS J160639.78-100010.7, comprised of a magnetic white dwarf with a field of about 30 MG and a brown dwarf. We measured an orbital period of 92 min which is consistent with the photometric period. Minimum and maximum light occur at the orbital quadratures Phi=0.25 and 0.75, respectively, and cannot be caused by reflection on the brown dwarf, but, instead, by a spot on the synchronously rotating magnetic white dwarf. The brown dwarf does not fill its Roche lobe and the system may be in a low-accretion state or, more likely, in a detached state following episodes of mass transfer. SMSS J160639.78-100010.7 is the nearest known magnetic white dwarf plus brown dwarf system.

Primary mass 0.72 M_Sol, secondary 0.06 M_Sol with spectral type L8, nearly as in WD 0137-349's case.

Other cases are reported in the same paper, e.g. SDSS J1514+0744 and SDSS J1250+1549 but secondary masses are estimated to be lower than 0.085 M_Sol and spectral types L3 and M8 respectively, still compatible with low-mass stars.

Three new WDs with likely BD companions

System parameters of three short period cataclysmic variable stars

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Using photometric ULTRACAM observations of three new short period cataclysmic variables, we model the primary eclipse lightcurves to extract the orbital separation, masses, and radii of their component stars. We find donor masses of 0.060 ± 0.008 M_Sol, 0.042 ± 0.001 M_Sol, and 0.042 ± 0.004M_Sol, two being very low-mass sub-stellar donors, and one within 2σ of the hydrogen burning limit. All three of the new systems lie close to the modified, “optimal” model evolutionary sequence of Knigge et al. (2011). We briefly re-evaluate the long-standing discrepancy between observed donor mass and radius data, and theoretical CV evolutionary tracks. By looking at the difference in the observed period at each mass and the period predicted by the Knigge et al. (2011) evolutionary sequence, we qualitatively examine the form of excess angular momentum loss that is missing from the models below the period gap. We show indications that the excess angular momentum loss missing from CV models grows in importance relative to gravitational losses as the period decreases. Detailed CV evolutionary models are necessary to draw more quantitative conclusions in the future.

Table excerpt from Discovery and characterization of five new eclipsing AM CVn systems: very interesting secondary masses, all substellar and many well within planetary domain.

A new paper about these elusive objects

Mass Transfer and Stellar Evolution of the White Dwarfs in AM CVn Binaries

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We calculate the stellar evolution of both white dwarfs (WDs) in AM CVn binaries with orbital periods of Porb≈5−70 minutes. We focus on the cases where the donor starts as a MHe<0.2M⊙ Helium WD and the accretor is a M_WD>0.6M⊙ WD. Using Modules for Experiments in Stellar Astrophysics (MESA), we simultaneously evolve both WDs assuming conservative mass transfer and angular momentum loss from gravitational radiation. This self-consistent evolution yields the important feedback of the properties of the donor on the mass transfer rate, M˙, as well as the thermal evolution of the accreting WD. Consistent with earlier work, we find that the high M˙'s at early times forces an adiabatic evolution of the donor for Porb<30 minutes so that its mass-radius relation depends primarily on its initial entropy. As the donor reaches M_He≈0.02−0.03M⊙ at Porb≃30 minutes, it becomes fully convective and could lose entropy and expand much less than expected under further mass loss. However, we show that the lack of reliable opacities for the donor's surface inhibit a secure prediction for this possible cooling. Our calculations capture the core heating that occurs during the first ≈10⁷ years of accretion and continue the evolution into the phase of WD cooling that follows. When compared to existing data for accreting WDs, as seen by Cheng and collaborators for isolated WDs, we also find that the accreting WDs are not as cool as we would expect given the amount of time they have had to cool.

Also known as OV Bootis...

SDSS J150722.30+523039.8: a CV formed directly from a detached white dwarf/brown dwarf binary?

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We present high-speed, three-colour photometry of the eclipsing cataclysmic variable SDSS J150722.30+523039.8 (hereafter SDSS J1507). This system has an orbital period of 66.61 minutes, placing it below the observed ``period minimum'' for cataclysmic variables. We determine the system parameters via a parameterised model of the eclipse fitted to the observed lightcurve by chi^2 minimisation. We obtain a mass ratio of q = 0.0623 +/- 0.0007 and an orbital inclination i = 83.63 +/- 0.05 degrees. The primary mass is M_w = 0.90 +/- 0.01 M_sun. The secondary mass and radius are found to be M_r =0.056 +/- 0.001 M_sun and R_r = 0.096 +/- 0.001 R_sun respectively. We find a distance to the system of 160 +/- 10 pc. The secondary star in SDSS J1507 has a mass substantially below the hydrogen burning limit, making it the second confirmed sub-stellar donor in a cataclysmic variable. The very short orbital period of SDSS J1507 is readily explained if the secondary star is nuclearly evolved, or if SDSS J1507 formed directly from a detached white dwarf/brown dwarf binary. Given the lack of any visible contribution from the secondary star, the very low secondary mass and the low HeI(6678AA)/Halpha emission line ratio, we argue that SDSS J1507 probably formed directly from a detached white dwarf/brown dwarf binary. If confirmed, SDSS J1507 will be the first such system identified. The implications for binary star evolution, the brown-dwarf desert and the common envelope phase are discussed.

Along with another one in Virgo

Detection of a Variable Infrared Excess Around SDSS 121209.31+013627.7

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We present near-infrared JHKs photometry and light curves of the candidate magnetic white dwarf+brown dwarf binary SDSS J121209.31+013627.7 and report on the detection of near-infrared excess and variability in the Ks−band. The observed near-infrared excess can be explained by the presence of an L7 brown dwarf and an extra emission source. The J and H light curves appear flat, which rules out eclipses deeper than 0.2 mag and the presence of an accretion hot spot on the white dwarf. From the variable Ks lightcurve, we get a refined period for the system of 88±1 minutes. We show that the observed variability in Ks−band can be explained by cyclotron emission, which can be modeled by a small spot on the surface of the white dwarf. SDSS 1212 exhibits similarities to the ultra-short period polar EF Eridani, however the lack of evidence for Roche-lobe overflow accretion suggests it may be a pre-polar.

Upper limit for BD mass at 0.06 M_Sun, that is 63 M_Jup.

More details on two WD+BD systems, the latter not yet listed in EPE

HST/WFC3 Complete Phase-resolved Spectroscopy of White Dwarf-Brown Dwarf Binaries WD 0137 and EPIC 2122

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Brown dwarfs in close-in orbits around white dwarfs offer an excellent opportunity to investigate properties of fast-rotating, tidally-locked, and highly-irradiated atmospheres. We present Hubble Space Telescope Wide Field Camera 3 G141 phase-resolved observations of two brown dwarf-white dwarf binaries: WD 0137-349 and EPIC 212235321. Their 1.1 to 1.7 μm phase curves demonstrate rotational modulations with semi-amplitudes of 5.27±0.02% and 29.1±0.1%; both can be well fit by multi-order Fourier series models. The high-order Fourier components have the same phase as the first order and are likely caused by hot spots located at the substellar points, suggesting inefficient day/night heat transfer. Both brown dwarfs' phase-resolved spectra can be accurately represented by linear combinations of their day- and night-side spectra. Fitting the irradiated brown dwarf model grids to the day-side spectra require a filling factor of ~50%, further supporting a hot spot dominating the emission of the day-sides. The night-side spectrum of WD 0137-349B is reasonably well fit by non-irradiated substellar models and the one of EPIC 212235321B can be approximated by a Planck function. We find strong spectral variations in the brown dwarfs' day/night flux and brightness temperature contrasts, which highlights the limitations of band-integrated measurements in probing heat transfer in irradiated objects. On the color-magnitude diagram, WD 0137-349B evolves along a cloudless model track connecting the early-L and mid-T spectral types, demonstrating that clouds and disequilibrium chemistry have a negligible effect on this object. A full interpretation of these high-quality phase-resolved spectra calls for new models that couple atmospheric circulation and radiative transfer under high-irradiation conditions.

Back to 2006-2007 for "unburying" SDSS 1035 (=SDSS J103533.02+055158.4) and SDSS J1507 (=SDSS J150722.30+523039.8 or OV Boo)

A brown dwarf mass donor in an accreting binary

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A long standing and unverified prediction of binary star evolution theory is the existence of a population of white dwarfs accreting from sub-stellar donor stars. Such systems ought to be common, but the difficulty of finding them, combined with the challenge of detecting the donor against the light from accretion means that no donor star to date has a measured mass below the hydrogen burning limit. Here we apply a technique which allows us to reliably measure the mass of the unseen donor star in eclipsing systems. We are able to identify a brown dwarf donor star, with a mass of 0.052+/-0.002 Msun. The relatively high mass of the donor star for its orbital period suggests that current evolutionary models may underestimate the radii of brown dwarfs.

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The most important result is the donor’s mass, Mc = 0.052±0.002 MSol. This is comfortably below the hydrogen burning limit of around ~0.072 MSol for solar metallicities (27), making the donor star in SDSS 1035 a confirmed brown-dwarf in a CV

SDSS J150722.30+523039.8: a cataclysmic variable formed directly from a detached white dwarf/brown dwarf binary?

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We present high-speed, three-colour photometry of the eclipsing cataclysmic variable SDSS J150722.30+523039.8 (hereafter SDSS J1507). This system has an orbital period of 66.61 min, placing it below the observed `period minimum' for cataclysmic variables. We determine the system parameters via a parametrized model of the eclipse fitted to the observed lightcurve by χ2 minimization. We obtain a mass ratio of q = 0.0623 +/- 0.0007 and an orbital inclination . The primary mass is Mw = 0.90 +/- 0.01Msolar. The secondary mass and radius are found to be Mr = 0.056 +/- 0.001Msolar and Rr = 0.096 +/- 0.001Rsolar, respectively. We find a distance to the system of 160 +/- 10pc. The secondary star in SDSS J1507 has a mass substantially below the hydrogen burning limit, making it the second confirmed substellar donor in a cataclysmic variable. The very short orbital period of SDSS J1507 is readily explained if the secondary star is nuclearly evolved, or if SDSS J1507 formed directly from a detached white dwarf/brown dwarf binary. Given the lack of any visible contribution from the secondary star, the very low secondary mass and the low HeI λ6678/Hα emission-line ratio, we argue that SDSS J1507 probably formed directly from a detached white dwarf/brown dwarf binary. If confirmed, SDSS J1507 will be the first such system identified. The implications for binary star evolution, the brown dwarf desert and the common envelope phase are discussed.

Outstanding system

Gaia0007-1605: an old triple system with an inner brown dwarf-white dwarf binary and an outer white dwarf companion

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We identify Gaia0007-1605AC as the first inner brown dwarf-white dwarf binary of a hierarchical triple system in which the outer component is another white dwarf (Gaia0007-1605B). From optical/near-infrared spectroscopy obtained at the Very Large Telescope with the X-Shooter instrument and/or from Gaia photometry plus SED fitting, we determine the effective temperatures and masses of the two white dwarfs (12018+-68 K, 0.54+-0.01 Msun for Gaia0007-1605A and 4445+-116 K, 0.56+-0.05 Msun for Gaia0007-1605B) and the effective temperature of the brown dwarf (1850+-50 K; corresponding to a spectral type L3+-1). By analysing the available TESS light curves of Gaia0007-1605AC we detect a signal at 1.0446+-0.0015 days with an amplitude of 6.25 ppt, which we interpret as the orbital period modulated from irradiation effects of the white dwarf on the brown dwarf's surface. This drives us to speculate that the inner binary evolved through a common envelope phase in the past. Using the outer white dwarf as a cosmochronometer and analysing the kinematic properties of the system, we conclude that the triple system is about 10 Gyr old.

Sunbathing under white light -- 3D modelling of brown dwarf - white dwarf atmospheres with strong UV irradiation

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The atmospheres of brown dwarfs orbiting in close proximity to their parent white dwarf represent some of the most extreme irradiated environments known. Understanding their complex dynamical mechanisms pushes the limits of theoretical and modelling efforts, making them valuable objets to study to test contemporary understanding of irradiated atmospheres. We use the Exo-FMS GCM to simulate the brown dwarfs WD0137-349B, SDSS J141126.20+200911.1B and EPIC212235321B, first coupled to a multi-banded grey radiative-transfer scheme then a spectral correlated-k scheme with high temperature opacity tables. We then post-process the GCM results using gCMCRT to compare to available observational data. Our GCM models predict strongly temperature inverted atmospheres, spanning many decades in pressure due to impact of UV band heating. Post-processing of our models suggest that the day-night contrast is too small in the GCM results. We therefore suggest that the formation of cloud particles as well as atmospheric drag effects such as magnetic drag are important considerations in setting the day-night temperature contrast for these objects.

Period bouncer cataclysmic variable EZ Lyn in quiescence

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We report the study of the accretion disk structure of the period bouncer cataclysmic variable EZ Lyn (SDSS J080434.20+510349.2) in quiescence based on our new time-resolved photometric and spectroscopic observations and data extracted from archives. The object magnitude now is V = 17.95(5), close to its brightest before the first superoutburst in 2006. We confirmed the presence of the small eclipse in the optical light curve. The spectra obtained in quiescence at different epochs look similar. However, the contribution of the disk and intensities of emission lines are strongly varied. We singled out pure accretion disk spectra and found that the Balmer decrement Hα:Hβ:Hγ:Hδ = 1.61:1.0:0.76:0.59 is comparable with one at bright accretion disks in longer period cataclysmic variables. The decrement suggests that emission lines are excited collisionally in an optical thin part of the disk with average density and temperature of logN0 = 12.5(2) and T = (10–15) × 103 K. Based on the photometric data and our modeling techniques, we redetermined the mass of MWD = 0.85(1) M⊙ and the current effective temperature TWD,eff = 11,250(50) K of the white dwarf. The secondary has mass of M₂ = 0.042(14) M⊙. The system inclination is 79.0°(2). The mass accretion rate is about $\dot{M}\approx (0.3\mbox{--}3.0)\times {10}^{-12}\ {M}_{\odot }$ yr−1. The disk luminosity, together with results from light-curve modeling, suggests a low effective temperature ∼2500 K of the continuum emitting region, where also the spiral arm pattern is hosted.

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Assuming the mass ratio q = 0.056 the mass of the secondary is M₂ = 0.028-0.056 M☉ ≈ 42 M_J mass of Jupiter. The separation of the stellar components is a = 0.59 R☉ = 4.1 × 10¹⁰ cm and the secondary’s radius is R₂ = 0.103 R☉ = 1.02 R_J. The tidal limitation radius of the disk is R_disk,max = 0.34 R☉ and the minimal possible Keplerian velocity in the disk is υ_disk,out = 650 km s^-1

The size of the secondary looks rather compatible with a brown dwarf.

CSS1603+19: a low-mass polar near the cataclysmic variable period minimum

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CSS1603+19 is a cataclysmic variable (CV) with an orbital period of 81.96 min, near the minimal period of cataclysmic variables. It is unusual in having a strong mid-infrared excess inconsistent with thermal emission from a brown dwarf companion. Here we present time-resolved multi-wavelength observations of this system. WISE photometry indicates that the mid-infrared excess displays a one-magnitude eclipsing-like variability during the orbit. We obtained near-infrared and optical spectroscopy using Gemini, MDM and APO telescopes. Near-infrared spectra show possible cyclotron features indicating that the white dwarf has a magnetic field of about 5MG. Optical and near-infrared spectra display double-peaked emission lines, with both components showing strong radial velocity variations during the orbital period and with the broad component leading the narrow component stably by about 0.2 of the orbital phase. We construct a physical model informed by existing observations of the system and determine that one component likely originates from the accretion column onto the magnetized white dwarf in synchronous rotation with the orbital motion and the other from the Roche overflow point. This allows us to constrain the masses of the binary components to be M₁>0.24M⊙ for the white dwarf accretor and M₂=0.0644±0.0074M⊙ for the donor. We classify the system as an AM Herculis star, or a polar. It has likely completed its stint on the period gap, but has not yet gone through the period bounce

Detection of the brown dwarf donor in the period-bouncer BW Sculptoris

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We present an analysis of multi-epoch spectroscopic and photometric observations of BW Scl, which is believed to be one of the best period-bouncer candidates. We detected multiple irradiation-induced emission lines from the donor star allowing the radial velocity variations to be measured with high accuracy. Also, using the absorption Mgii 4481 line originated in the photosphere of the accreting white dwarf (WD), we measured the radial velocity semi-amplitude of the WD and its gravitational redshift. We find that the WD has a mass of 0.92±0.04 M⊙, while the donor is a low-mass object with a mass of 0.054±0.008 M⊙, well below the hydrogen-burning limit. Using NIR data, we put an upper limit on the effective temperature of the donor to be ≤1600 K, corresponding to a brown dwarf of T spectral type. The optically thin accretion disc in BW Scl has a very low luminosity ≲4 ×1030 erg s−1 which corresponds to a very low mass accretion rate of ≲6 ×10−13 M⊙ year−1. The outer parts of the disc have a low density allowing the stream to flow down to the inner disc regions. The brightest part of the hotspot is located close to the circularization radius of the disc. The hotspot is optically thick and has a complex, elongated structure. Despite the relatively high temperature of the WD (14750-15000 K), we suggest that BW Scl has already passed the minimum period and is now evolving back towards longer periods. Thus, BW Scl is a period bouncer.

Common envelope evolution and triple dynamics as potential pathways to form the inner white dwarf + brown dwarf binary of the triple star system Gaia 0007-1605

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The recently discovered system Gaia 0007-1605 consisting of a white dwarf with a close brown dwarf companion and a distant white dwarf tertiary very much resembles the triple system containing the first transiting planet candidate around a white dwarf ever discovered: WD 1856+534. We have previously argued that the inner binary in WD 1856+534 most likely formed through common envelope evolution but triple star dynamics represent an alternative scenario. Here we analyze different formation scenarios for Gaia 0007-1605. We reconstructed the potential common envelope evolution of the system and find that assuming standard parameters for the energy budget provides a reasonable solution. In agreement with other close white dwarf + brown dwarf binaries, and in contrast to WD 1856+534, no energy sources other than orbital energy during common envelope evolution are required to understand the current configuration of the system. In addition, using analytical prescriptions for triple dynamics, we show that Von Zeipel-Lidov-Kozai oscillations might have trigger tidal migration due to high eccentricity incursions (e \gtrsim 0.997). We conclude that the inner binary in Gaia 0007-1605, as its sibling WD 1856+534, formed either through common envelope evolution, triple dynamics or a combination of both mechanisms.

Recent humps and superhumps observations and an estimation of outburst parameters of the AM CVn star CR Boo
https://arxiv.org/abs/2212.07189

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The masses of two components were estimated to be in ranges of: M₁ = 0.7 − 1.1M⊙ for the mass of the primary and M₂ = 0.044−0.09M⊙ for the mass of the secondary

Interesting discussion about substellar “white dwarfs”. As long as commonly accepted stellar-substellar boundary (ca. 0.07 M_Sol) is recognized, the model works enough, but below that threshold are we sure we are still dealing with albeit degenerate “stars”?

Ultra low-mass and small-radius white dwarfs made of heavy elements

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Seven ultra low-mass and small-radius white dwarfs (LSPM J0815+1633, LP 240-30, BD+20 5125B, LP 462-12, WD J1257+5428, 2MASS J13453297+4200437, and SDSS J085557.46+053524.5) have been recently identified with masses ranging from ∼0.02 M⊙ to ∼0.08 M⊙ and radii from ∼ 4270 km to 10670 km. The mass-radius measurements of these white dwarfs pose challenges to traditional white dwarf models assuming they are mostly made of nuclei lighter than 56Fe. In this work we consider the possibility that those white dwarfs are made of heavier elements. Due to the small charge-to-mass ratios in heavy elements, the electron number density in white dwarf matter is effectively reduced, which reduces the pressure with additional contributions of lattice energy and electron polarization corrections. This consequently leads to white dwarfs with much smaller masses and radii, which coincide with the seven ultra low-mass and small-radius white dwarfs. The corresponding equation of state and matter contents of dense stellar matter with and without reaching the cold-catalyzed ground state are presented, which are obtained using the latest Atomic Mass Evaluation (AME 2020). Further observations are necessary to unveil the actual matter contents in those white dwarfs via, e.g., spectroscopy, asteroseismology, and discoveries of other ultra low-mass and small-radius white dwarfs.

In my very humble opinion a ca. 20-30 M_Jup “star”, even about the size of the Earth and high-density, is not enough massive for nucleosynthesis. It is theorized but how solid is this assumption?

Authors indeed assume some kind of exotic nature and composition, like heavy element-rich stellar remnants, even very hot (4000 K). This indeed defies the definition itself of star and planets. The broad “exotic objects” class is the simplest way to define these oddballs but it is certainly not enough.

A bit misleading title, by the way...

An irradiated-Jupiter analogue hotter than the Sun

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Planets orbiting close to hot stars experience intense extreme-ultraviolet radiation, potentially leading to atmosphere evaporation and to thermal dissociation of molecules. However, this extreme regime remains mainly unexplored due to observational challenges. Only a single known ultra-hot giant planet, KELT-9b, receives enough ultraviolet radiation for molecular dissociation, with a day-side temperature of ~4,600K. An alternative approach uses irradiated brown dwarfs as hot-Jupiter analogues. With atmospheres and radii similar to those of giant planets, brown dwarfs orbiting close to hot Earth-sized white-dwarf stars can be directly detected above the glare of the star. Here we report observations revealing an extremely irradiated low-mass companion to the hot white dwarf WD0032-317. Our analysis indicates a day-side temperature of ~8,000K, and a day-to-night temperature difference of ~6,000K. The amount of extreme-ultraviolet radiation (with wavelengths 100-912Å) received by WD0032-317B is equivalent to that received by planets orbiting close to stars as hot as a late B-type stars, and about 5,600 times higher than that of KELT-9b. With a mass of ~75-88 Jupiter masses, this near-hydrogen-burning-limit object is potentially one of the most massive brown dwarfs known.

SRGeJ045359.9+622444: A 55-min Period Eclipsing AM CVn Discovered from a Joint SRG/eROSITA + ZTF Search
https://arxiv.org/abs/2306.13133

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AM CVn systems are ultra-compact binaries where a white dwarf accretes from a helium-rich degenerate or semi-degenerate donor. Some AM CVn systems will be among the loudest sources of gravitational waves for the upcoming Laser Interferometer Space Antenna (LISA), yet the formation channel of AM CVns remains uncertain. We report the study and characterisation of a new eclipsing AM CVn, SRGeJ045359.9+622444 (hereafter SRGeJ0453), discovered from a joint SRG/eROSITA and ZTF program to identify cataclysmic variables (CVs). We obtained optical photometry to confirm the eclipse of SRGeJ0453 and determine the orbital period to be Porb=55.0802±0.0003 min. We constrain the binary parameters by modeling the high-speed photometry and radial velocity curves and find Mdonor=0.044±0.024M⊙ and Rdonor=0.078±0.012R⊙. The X-ray spectrum is approximated by a power-law model with an unusually flat photon index of Γ∼1 previously seen in magnetic CVs with SRG/eROSITA, but verifying the magnetic nature of SRGeJ0453 requires further investigation. Optical spectroscopy suggests that the donor star of SRGeJ0453 could have initially been a He star or a He white dwarf. SRGeJ0453 is the ninth eclipsing AM CVn system published to date, and its lack of optical outbursts have made it elusive in previous surveys. The discovery of SRGeJ0453 using joint X-ray and optical surveys highlights the potential for discovering similar systems in the near future.

A brown dwarf donor and an optically thin accretion disc with a complex stream impact region in the period-bouncer candidate BW Sculptoris
https://arxiv.org/abs/2212.03264

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We present an analysis of multi-epoch spectroscopic and photometric observations of the WZ Sge-type dwarf nova BW Scl, a period-bouncer candidate. We detected multiple irradiation-induced emission lines from the donor star allowing the radial velocity variations to be measured with high accuracy. Also, using the absorption lines Mgii 4481 and Caii K originated in the photosphere of the accreting white dwarf (WD), we measured the radial velocity semi-amplitude of the WD and its gravitational redshift. We find that the WD has a mass of 0.85±0.04 M⊙, while the donor is a low-mass object with a mass of 0.051±0.006 M⊙, well below the hydrogen-burning limit. Using NIR data, we put an upper limit on the effective temperature of the donor to be ≲1600 K, corresponding to a brown dwarf of T spectral type. The optically thin accretion disc in BW Scl has a very low luminosity ≲4 ×1030 erg s−1 which corresponds to a very low mass accretion rate of ≲7 ×10−13 M⊙ year−1. The outer parts of the disc have a low density allowing the stream to flow down to the inner disc regions. The brightest part of the hotspot is located close to the circularization radius of the disc. The hotspot is optically thick and has a complex, elongated structure. Based on the measured system parameters, we discuss the evolutionary status of the system.

Disk or Companion: Characterizing Excess Infrared Flux in Seven White Dwarf Systems with Near-Infrared Spectroscopy
https://arxiv.org/abs/2303.16330

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Excess infrared flux from white dwarf stars is likely to arise from a dusty debris disk or a cool companion. In this work, we present near-infrared spectroscopic observations with Keck/MOSFIRE, Gemini/GNIRS, and Gemini/Flamingos-2 of seven white dwarfs with infrared excesses identified in previous studies. We confirmed the presence of dust disks around four white dwarfs (Gaia J0611-6931, Gaia J0006+2858, Gaia J2100+2122, and WD 0145+234) as well as two new white dwarf brown dwarf pairs (Gaia J0052+4505 and Gaia J0603+4518). In three of the dust disk systems, we detected for the first time near-infrared metal emissions (Mg I, Fe I, and Si I) from a gaseous component of the disk. We developed a new Markov Chain Monte Carlo framework to constrain the geometric properties of each dust disk. In three systems, the dust disk and the gas disk appear to coincide spatially. For the two brown dwarf white dwarf pairs, we identified broad molecular absorption features typically seen in L dwarfs. The origin of the infrared excess around Gaia J0723+6301 remains a mystery. Our study underlines how near-infrared spectroscopy can be used to determine sources of infrared excess around white dwarfs, which has now been detected in hundreds of systems photometrically.

The period bouncer system SDSS J105754.25+275947.5: first radial velocity study

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We report the first radial velocity spectroscopic study of the eclipsing period bouncer SDSS J105754.25+275947.5. Together with eclipse light curve modeling, we redetermined the system parameters and studied the accretion disk structure. We confirm that the system contains a white dwarf with MWD=0.83(3)M⊙ and an effective temperature of 11,500(400)K. The mass of the secondary is M2=0.056M⊙ with an effective temperature of T2=2,100K or below. The system inclination is i=84.3(6). The data is in good agreement with our determination of K1 = 33(4) km s−1. We estimate the mass transfer rate as M˙=1.9(2)×10−11M⊙yr−1. Based on an analysis of the SDSS and OSIRIS spectra, we conclude that the optical continuum is formed predominantly by the radiation from the white dwarf. The contribution of the accretion disk is low and originates from the outer part of the disk. The Balmer emission lines are formed in a plasma with log N0 = 12.7 [cm−1] and a kinetic temperature of T∼10,000K. The size of the disk, where the emission lines are formed, expands up to Rd,out=0.29R⊙. The inner part of the emission line forming region goes down to Rd,in≈2RWD . The Doppler tomography and trailed spectra show the presence of a hot spot and a clumpy structure in the disk, with variable intensity along the disk position angle. There is an extended region at the side opposite the hot spot with two bright clumps caused more probably by non-Keplerian motion there.

Period bouncers as detached magnetic cataclysmic variables
https://arxiv.org/abs/2310.17276

Substellar donors in multiple systems su h ad GW Lib, EZ Lyn and others.

Time-resolved Hubble Space Telescope Wide Field Camera 3 Spectrophotometry Reveals Inefficient Day-to-Night Heat Redistribution in the Highly Irradiated Brown Dwarf SDSS 1557B

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Brown dwarfs in ultra-short period orbits around white dwarfs offer a unique opportunity to study the properties of tidally-locked, fast rotating (1-3 hr), and highly-irradiated atmospheres. Here, we present phase-resolved spectrophotometry of the white dwarf-brown dwarf (WD-BD) binary SDSS 1557, which is the fifth WD-BD binary in our six-object sample. Using the Hubble Space Telescope Wide Field Camera 3 Near-infrared G141 instrument, the 1.1 to 1.7 μm phase curves show rotational modulations with semi-amplitudes of 10.5±0.1%. We observe a wavelength dependent amplitude, with longer wavelengths producing larger amplitudes, while no wavelength dependent phase shifts were identified. The phase-resolved extracted BD spectra exhibit steep slopes and are nearly featureless. A simple radiative energy redistribution atmospheric model recreates the hemisphere integrated brightness temperatures at three distinct phases and finds evidence for weak redistribution efficiency. Our model also predicts a higher inclination than previously published. We find that SDSS 1557B, the second most irradiated BD in our sample, is likely dominated by clouds on the night side, whereas the featureless day side spectrum is likely dominated by H− opacity and a temperature inversion, much like the other highly-irradiated BD EPIC2122B.