DiscoveryofanovashellaroundthecataclysmicvariableV1315Aql

D.I.Sahmana,V.S.Dhillona,S.Littlefaira,G.Hallinanb

1.  Introduction The canonical theory2 of the evolution of cataclysmic variables (CVs) cannot explain why dwarf novae (DN) and nova-likes (NLs) co-exist at the same orbital period. One theory is the nova-induced cycle (or hibernation theory3). The hot white dwarf (WD) resulting from the nova explosion irradiates the secondary, causing it to expand. This increases the mass transfer rate and the system appears as a NL. As the WD cools, the secondary shrinks, reducing the mass transfer rate to DN levels, or possibly ceasing altogether, and going into hibernation (see Figure 1). Hence CVs are expected to cycle between nova, DN and NL states on timescales of 104 − 105 years. Supporting evidence comesfromthediscoveryofnovashellsaroundtheDNeZCam4zndATCnc5,andthediscoverythatBKLynhasevolvedthroughallthreephases6.FindingshellsaroundNLswouldlendfurthersupporttotheexistenceofnova-inducedcycles.Wereported1ourdiscoveryof a possible shell around the nova-like V1315 Aql and thenembarked on a second campaign to obtain deeper images andspectroscopyoftheshell.

DaveSahmand.sahman@sheffield.ac.uk

Abstract Following our tentative discovery1 of a faint shell around the cataclysmic variable V1315 Aql, we undertook deep Hα imaging and high resolutionspectroscopyoftheshell.WefindthattheshellhasitsgeometriccentrelocatedonV1315Aql.Themass,spectralfeaturesanddensityoftheshellareconsistentwithothernovashells,ratherthanplanetarynebulaeorsupernovaremnants.Theradialvelocityoftheshell isconsistentwiththesystemicvelocityofV1315Aql.WebelievethisevidencestronglysuggeststhattheshelloriginatesfromanearliernovaeruptionfromV1315Aql.Thisisthefirstnovashelldiscoveredaroundanova-likevariable,andsupportsthetheoryofnova-inducedevolutionarycyclesfirstproposedbySharaetal.(1986).

4. Discussion We present images and spectra of the nova shell sur-rounding V1315 Aql. The shell consists of spherical arcs ofemission,centredonV1315Aql.IthasspectralfeaturesofHαandforbiddenN[II]consistentwithanovaorigin.ThesystemicvelocityoftheshellisconsistentwiththeinnerbinaryandtheGalactic rotational velocity.Ourmeasured values of the fluxand H+mass are alsowithin typical ranges for nova shells11.The shell is very faint and lacks other key spectral featuresthat would have allowed us to accurately determine othercharacteristicseg.electrondensity,temperatureetc.Theshellis relativelyold tobestill visible:mostnovashellsdisappearafter100yrsorso(REF?),andisthereforedifficulttomeasureitsageaccurately.In summary, our results strongly suggest thatthe nova shell originated from the CV. Thisdiscovery of the first ever nova shell foundaroundanova-likevariableaddsfurthersupportto the theory of nova-induced evolutionarycycles.

2. Observations The images were obtained using the Wide Field Camera on the 2.5m Isaac Newton Telescope on La Palma, giving aplatescaleof0.33ʹʹ/pixelandafieldviewofapprox.34ʹ×34ʹ.Weused a narrow-band Hα filter (95Å FWHM = 4300 km s−1). Wetookeight900sexposures.ThespectrawereobtainedusingtheDEIMOSmulti-slitspectrographonthe10mKeck II telescopeonHawaii. The positions of the slits are shown in Figure 5. Weobtained 39 spectra of 300s duration each, using the 1200Ggrating centred on 6000Å. This gave a wavelength coverage ofλλ4550–7500Å,withaFWHMresolutionof1.6Å.

Figure 2: Spectraof thesevenshell slits,andtheblankskyslitfrom 6540-6600Å. Flux is μJy arcsec−2. The slit spectra all showthepresenceofHα andN[II] 6548Å andN[II] 6583Å.Hα is alsopresentintheblankskyslits.

Figure 4: PlotofthespectrumoftheskyfromtheSouth-EastsideofV1315Aql slit. Theerrorbars show thenoiseon thebackgroundsky.

Table 1. Hα and N[II] flux (ergs/s/cm2/arcsec2 × 10−18) fromthesevenshellslits.Theerrorsonthefluxvaluesare±25%.

References: 1. Sahman D. I. Dhillon V. S.,Knigge C., Marsh T. R., 2015, MNRAS, 451, 2863 2. Knigge C., Baraffe I., Patterson J., 2011, ApJS, 194, 28 3. Shara M. M., Livio M., Moffat A. F. J., Orio M., 1986, ApJ, 311, 163 4. Shara M. M., et al. 2007, Nature, 446, 159 5. Shara M. M., et al. 2012, ApJ, 758, 121

6. Patterson J. et al. 2013, MNRAS,434, 1902 7. Osterbrock D. E. ,Astrophysics of gaseous nebulae and active galactic nuclei, University Science Books, Sausolito, California 8. Corradi R. L. M., García-Rojas J., Jones D., Rodríguez-Gil P., 2015, ApJ, 803, 99 9. Downes A. R. et al 1986, ApJ, 301, 240 10.  Stephenson F. R., 1976, QJRAS, 17, 121 11.  Bode & Evans, 2008, Classical Novae, Cambridge University Press

Figure 3. Image of the shell surrounding V1315 Aql.

Figure 5: Imageof the shell showing the slitpositionswithoverlaid circles centredonV1315Aqlof radii 100ʹʹ, 180ʹʹand240ʹʹ.NorthisupandEastisleft.

3. Results In Figure 3. we show the combined image of the shell surrounding V1315 Aql. The shell is roughly circular with the most prominent emission to the West. In Figure 5. we have over-plotted concentric circles of radii 100ʹʹ,180ʹʹand 240ʹʹ to demonstrate that the arcs of emittingmaterialarecentredonV1315Aql.InFigure2.weshowthespectra of the seven slits placed around the shell, in therange 6540-6600Å. They show single peaked emission linesofHα,N[II]6548ÅandN[II]6583Å.Hαisalsopresentintheblank sky slit. We also found Hβ emission in those spectrathatcover4861Å.Usingtheradialvelocityof theHα line inthe sky portion of the slit placed on V1315 Aql (Figure 4.),andthesevenshellslits,wederiveasystemicvelocityof-10km s−1. This is comparable to the Galactic rotation at thelocationofV1315Aql.Wemeasuredthefluxineachlineandestimate an Hα luminosity of 7.1 x 1030 ergs. Measuringotherfeaturesoftheshellprovedtobedifficult.Wecanonlyplace anupper limit on the temperaturebecause theusualtechnique7 of using emission line ratios from specific ionswas not possible eg, we did not detect any N[II] 5755Åemissionwhich is usedwith theN[II] 6548Å&6583Å lines.NorwereweabletodetectanyO[III].Thetemperatureisnohigher than 5,000K. The H+ mass of the shell can becalculatedusingtheHβflux8,whichinturnwederivedusingourHαflux.WeobtainanH+massof≈7.8x10-5M¤. Usingtheexpansionvelocityandluminosityoftheshell9wederiveanageof≈500-1,200yrs.WesearchedancientFarEasternrecords10 but no nova sighting is recorded in theconstellationofAquiladuringthisperiod.

Figure 1. Pictorial representation of the nova induced cycle

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