MNRAS 000, 1–10 (2019) Preprint 18 January 2019 Compiled using MNRAS LATEX style file v3.0 Signs of accretion in the white dwarf + brown dwarf binary NLTT5306 E. S. Longstaff1⋆, S. L. Casewell1, G. A. Wynn1, K.L. Page1, P. K. G. Williams2, I. Braker1, P. F. L. Maxted3 1Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK 2Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA 3Department of Physics and Astrophysics, Keele University, Keele, Staffordshire, ST5 5BG, UK Accepted XXX. Received YYY; in original form ZZZ ABSTRACT We present new XSHOOTER spectra of NLTT5306, a 0.44 ± 0.04 M⊙white dwarf in a short period (101 min) binary system with a brown dwarf companion that is likely to have previously undergone common envelope evolution. We have confirmed the presence of Hα emission and discovered Na I absorption associated with the white dwarf. These observations are indicative of accretion. Accretion is typically evidenced by high energy emission in the UV and X-ray regime. However our Swift observations covering the full orbital period in three wavebands (uvw1, uvm2, uvw2) revealed no UV excess or modulation. We used the X-ray non-detection to put an upper limit −15 −1 on the accretion rate of 2×10 M⊙yr . We compare NLTT5306 to similar accreting binaries with brown dwarf donors and suggest the inferred accretion rate could be from wind accretion or accretion from a debris/dust disk. The lack of evidence for a disk implies NLTT5306 is magnetically funnelling a weak wind from a potentially low gravity brown dwarf. The upper limit on the accretion rate suggests a magnetic field as low as 0.45 kG would be sufficient to achieve this. If confirmed this would constitute the first detection of a brown dwarf wind and could provide useful constraints on mass loss rates. Key words: stars: brown dwarfs, white dwarfs, binaries: close, accretion, winds 1 INTRODUCTION core. Complete ejection of the envelope leaves a close white dwarf – brown dwarf binary (Politano 2004). At Brown dwarf companions to main sequence stars are rarely this point the two bodies (not necessarily interacting) detected in orbits closer than 3 AU (Metchev 2006). This is are known as a post common envelope binary (PCEB). thought to be due to the difficulty in forming a binary with Currently, there are nine known detached PCEBs con- such low mass ratios (q∼0.02–0.1) (Grether & Lineweaver taining a brown dwarf companion: GD1400 (WD+L6, arXiv:1901.05485v1 [astro-ph.SR] 16 Jan 2019 2007). Consequently white dwarf – brown dwarf binaries are P=9.98 hrs; Farihi & Christopher 2004; Dobbie et al. rare: Steele et al. (2011) estimate only 0.5% of white dwarfs 2005; Burleigh et al. 2011), WD0137-349 (WD+L6-L8, have a brown dwarf companion. This scarcity exists in both P=116 min; Maxted et al. 2006; Burleigh et al. 2006; detached and non-detached binaries. Casewell et al. 2015), WD0837+185 (WD+T8, P=4.2hrs; Their formation begins when the white dwarf pro- Casewell et al. 2012), SDSS J141126.20+200911.1 genitor evolves off the main sequence and expands up (WD+T5, P=121.73 min; Beuermann et al. 2013; the giant or asymptotic branch. This expansion leads to Littlefair et al. 2014), SDSS J155720.77+091624.6 Roche lobe overflow (RLOF), which causes the companion (WD+L3-5, P=2.27 hrs; Farihi et al. 2017), SDSS to become engulfed and the subsequent brief phase of J120515.80-024222.6 (WD+L0, P=71.2 mins; Parsons et al. binary evolution to take place in a common envelope 2017), SDSSJ123127.14+004132.9 (WD+BD, P=72.5 mins; (CE). The companion loses orbital angular momentum to Parsons et al. 2017) and EPIC212235321 (WD+BD, the envelope, which is expelled from the system, and the P=68.2 mins; Casewell et al. 2018). companion spirals in towards the white dwarf progenitor Eventually a detached PCEB may begin to interact via the loss of angular momentum from the companion’s orbit ⋆ E-mail: [email protected] (by means of e.g. magnetic braking or gravitational radia- © 2019 The Authors 2 E. S. Longstaff et al. WDmass BDmass WDT BD T Period Name Type eff eff Reference (M⊙) (M⊙) (K) (K) (Minutes) SDSS J121209.31 Burleigh et al. 2006 Polar 0.8 − 9500 < 1700 88.428 ± 0.001 +013627.7 Stelzer et al. 2017 Beuermann et al. 2000 Schwope et al. 2007 EF Eridani Polar > 0.65 < 0.06 > 9500 < 2000 80.64 Schwope & Christensen 2010 Szkody et al. 2010 SDSS J125044.42 Steele et al. 2011 LARP 0.60 0.081 ± 0.005 10 000 − 86 +154957.4 Breedt et al. 2012 SDSS J151415.65 Kulebi¨ et al. 2009 LARP ∼ 0.75 0.077 10 000 − 89 +074446.5 Breedt et al. 2012 Szkody et al. 2009 SDSS J105754.25 CV 0.800 ± 0.015 0.0436 ± 0.0020 13300 ± 1100 − 90.44 ± 0.06 Southworth et al. 2015 +275947.5 McAllister et al. 2017 SDSS J143317.78 Littlefair et al. 2008 +101123.3 Savoury et al. 2011 CV 0.868 ± 0.007 0.0571 ± 0.0007 12 800 ± 200 2000 ± 200 78.11 Littlefair et al. 2013 Hern´andez Santisteban et al. 2016 Cheng et al. 1997 Ciardi et al. 1998 WZ Sagittae CV 0.85 ± 0.04 0.078 ± 0.006 14 800 ∼ 1900 81.8 ± 0.90 Patterson et al. 1998 Steeghs et al. 2007 Harrison 2016 SDSS J103532.98 CV 0.94 ± 0.01 0.052 ± 0.002 10100 ± 200 − 82.0896 ± 0.0003 Littlefair et al. 2006 +144555.8 SDSS J150722.30 CV 0.90 ± 0.01 0.056 ± 0.001 11 000 ± 500 < 2450 66.61 Littlefair et al. 2007 +523039.8 Table 1. A summary of the current known polars, low accretion rate polars (LARPs), and CVs with brown dwarf donors. tion), causing it to fill or come close to filling its Roche Lobe. using the dusty atmospheric models (Chabrier et al. 2000; At this point, mass transfer from the companion (also known Baraffe et al. 2002). Follow up spectroscopy of this system as the donor or secondary star) onto the white dwarf will es- by Steele et al. (2013) revealed NLTT5306 to be the shortest tablish the system as a zero age cataclysmic variable (CV). period white dwarf - brown dwarf binary system (P = 101.88 If the white dwarf is magnetic, mass may be funnelled along ± 0.02 mins) known at the time, (although it has now been the field lines onto the magnetic poles of the white dwarf. superseded by SDSS J1205-0242, SDSS J1231+0041, and White dwarfs with magnetic fields strong enough to funnel EPIC212235321: Parsons et al. 2017; Casewell et al. 2018). mass along fieldlines through most of the distance between the binary components, preventing the formation of an ac- cretion disk, are known as ’polars’ (Ferrario et al. 2015). In Steele et al. (2013) found NLTT5306 to have a white the case of white dwarfs with less intense magnetic fields, the dwarf mass of 0.44 ± 0.04 M⊙ and effective temperature mass flow forms an accretion disk, which acts to redistribute (Teff) of 7756 ± 35 K. They estimated the brown dwarf spec- angular momentum within the gas and facilitate accretion tral type to be dL4 – dL7 with Teff ≈ 1700 K and a minimum onto the white dwarf surface. The detailed evolution of CVs mass of 56±3MJup. Observations of an Hα emission feature has been discussed extensively by e.g. Rappaport et al. 1983; with a radial velocity consistent with that of the Balmer Hellier 2001; Knigge et al. 2011; Kalomeni et al. 2016. lines from white dwarf led Steele et al. (2013) to suggest NLTT5306 is interacting, because the white dwarf is not The majority of CVs have stellar donors, but there hot enough to cause the emission via irradiation. are a few with sub-stellar donors and these are listed in Table 1. Of these nine interacting systems, six have a di- rect detection of the secondary, either from emission lines This paper presents spectra from XSHOOTER and pho- such as those seen in the detached PCEB WD0137-349AB tometry from The Neil Gehrels Swift Observatory (hereafter, (Longstaff et al. 2017) or via spectra of the brown dwarf Swift). We also obtained radio observations using the Karl (e.g. Hern´andez Santisteban et al. 2016). G. Jansky Very Large Array (VLA). Data reduction is de- NLTT5306 was identified by Girven et al. (2011) and scribed in Section 2 and the results are presented in Sec- Steele et al. (2011) who were searching for infrared excesses tion 3. In Section 4 we estimate an upper limit on the mass indicative of sub-stellar companions. Both Girven et al. accretion rate and in Section 5 we compare and contrast (2011) and Steele et al. (2011) estimated the brown dwarf to NLTT5306 with other weakly interacting systems. Finally, be of spectral type dL5 with a minimum mass of 58±2 MJup we present our conclusions in Section 6. MNRAS 000, 1–10 (2019) Signs of accretion in NLTT5306 3 Filter Target ID Start time Stop time Obs ID UVOT exposure All 34661 2016-08-09T16:06:58 2016-08-09T17:13:52 00034661001 1051.42 93147 2017-06-07T00:52:57 2017-06-07T01:46:57 00093147001 1618.32 uvw1 93147 2017-06-07T08:50:57 2017-06-07T09:44:01 00093147002 1674.38 93147 2017-06-07T16:52:57 2017-06-07T17:46:07 00093147003 1667.89 93147 2017-06-14T00:17:57 2017-06-14T01:11:40 00093147004 1635.64 uvm2 93147 2017-06-14T08:15:57 2017-06-14T09:09:53 00093147005 242.206 93147 2017-06-14T16:15:57 2017-06-14T17:09:41 00093147006 1634.65 34661 2017-07-24T16:07:57 2017-07-24T17:01:41 00034661002 1634.451 93147 2017-06-17T00:02:57 2017-06-17T00:57:52 00093147007 1562.78 uvw2 93147 2017-06-17T08:30:57 2017-06-17T12:06:43 00093147008 1501.512 93147 2017-06-17T17:36:57 2017-06-17T18:30:30 00093147009 1645.68 93147 2017-06-29T18:12:57 2017-06-29T19:48:20 00093147010 1634.27 Table 2.