An irradiated brown-dwarf companion to an accreting white dwarf

Abstract

Brown dwarfs and giant planets orbiting close to a host star are subjected to significant irradiation that can modify the properties of their atmospheres. In order to test the atmospheric models that are used to describe these systems, it is necessary to obtain accurate observational estimates of their physical properties (masses, radii, temperatures, albedos). Interacting compact binary systems provide a natural laboratory for studying strongly irradiated sub-stellar objects. As the mass-losing secondary in these systems makes a critical, but poorly understood transition from the stellar to the sub-stellar regime, it is also strongly irradiated by the compact accretor. In fact, the internal and external energy fluxes are both expected to be comparable in these objects, providing access to an unexplored irradiation regime. However, the atmospheric properties of such donors have so far remained largely unknown 3 . Here, we report the direct spectroscopic detection and characterisation of an irradiated sub-stellar donor in an accreting white dwarf binary system. Our near-infrared observations allow us to determine a model-independent mass estimate for the donor of M 2 = 0.055 ± 0.008M and an average spectral type of L1 ± 1, supporting both theoretical predictions and model-dependent observational constraints. Our time-resolved data also allow us to estimate the average irradiation-induced temperature difference between the day and night sides on the sub-stellar donor, ∆T 57 K, and the maximum difference between the hottest and coolest parts of its surface, of ∆T max 200 K. The observations are well described by a simple geometric reprocessing model with a bolometric (Bond) albedo of A B < 0.54 at the 2-σ confidence level, consistent with high reprocessing efficiency, but poor lateral heat redistribution in the donor's atmosphere 4, 5 .Only a single brown dwarf donor to an accreting white dwarf has been detected spectroscopically to date 3 , and no empirical information at all is available regarding the effect of irradiation on the donor's atmosphere. We have therefore carried out simultaneous optical and nearinfrared time-resolved spectroscopy of one such system with the X-Shooter instrument 6 at the Very 2 Large Telescope (ESO). Our target, SDSS J143317.78+101123.3 7 (J1433 hereafter), is an eclipsing system with a short orbital period (P orb = 78.1 min) and a likely donor mass well below the hydrogen-burning limit, as estimated from eclipse modelling (M 2 = 0.0571 ± 0.0007M 8, 9 ). The exceptionally wide wavelength coverage provided by our data set (0.35 µm -2.5 µm) allows us to confidently dissect the overall spectral energy distribution. This is illustrated in Fig. 1. Diskformed, double-peaked hydrogen emission lines can be seen across the entire spectral range. The presence of broad Balmer absorption lines between 0.3 µm and 0.5 µm shows that the white dwarf is the strongest emitter in this wave band. However, the donor star spectrum is clearly visible in the near-infrared and dominates between 1.0 µm and 2.5...

The direct detection of the irradiated brown dwarf in the white dwarf–brown dwarf binary SDSS J141126.20+200911.1

“…We calculated brightness temperatures for the dayside of the brown dwarf for the J band using a model white dwarf spectrum and the method detailed in Casewell et al (2015). For the H and K s bands where we have models of the system from lcurve we generated a temperature map of the surface of the brown dwarf as was done in Hernández Santisteban et al (2016). The average dayside and nightside temperatures are reported in Table 1, although, from the surface map of the brown dwarf we were also able to model the maximum and minimum temperatures present across the surface.…”

Signs of accretion in the white dwarf + brown dwarf binary NLTT5306

“…The detailed evolution of CVs has been discussed extensively by e.g. Rappaport et al 1983;Hellier 2001;Knigge et al 2011;Kalomeni et al 2016.…”

“…Of these nine interacting systems, six have a direct detection of the secondary, either from emission lines such as those seen in the detached PCEB WD0137-349AB (Longstaff et al 2017) or via spectra of the brown dwarf (e.g. Hernández Santisteban et al 2016).…”

A spider timing model: accounting for quadrupole deformations and relativity in close pulsar binaries

“…Thus, probing the internal structure of the companion stars would greatly help in understanding and constraining models of their past and future evolution. Additionally spider pulsar companions lie at the most extreme range of irradiated systems, beyond hot Jupiters and donor stars in cataclysmic variables (Hernández Santisteban et al 2016). The process through which their day side gets heated must considerably affect the structure and evolution of the star such as bloating them to the point of filling their Roche lobe Podsiadlowski (1991).…”