We consider initial stage of the evolution of AM CVn type stars with white dwarf donors, which is characterized by thermonuclear explosions in the layer of accreted He. It is shown that accretion never results in detonation of He and accretors in AM CVn stars finish their evolution as massive WD. We found, for the first time, that in the outbursts the synthesis of n-rich isotopes, initiated by reaction 22 Ne(α, n) 25 Mg, becomes possible

CRISTALLO, Sergio

PIERSANTI, Luciano

Tornambé, A.

Yungelson, L. R.

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2019Publication Year2020-12-11T10:39:11ZAcceptance in OA@INAFThermonuclear oubursts of AM CVn starsTitleYungelson, L. R.; PIERSANTI, Luciano; Tornambé, A.; CRISTALLO, SergioAuthors10.26087/INASAN.2019.4.2.033DOIhttp://hdl.handle.net/20.500.12386/28788HandleINASAN Science ReportsJournal4NumberarXiv:1904.07662v1  [astro-ph.SR]  16 Apr 20191Thermonuclear oubursts of AM CVn starsYungelson L.R1, Piersanti L.2, Tornambé A.3, Cristallo S.21Institute of Astronomy of the RAS, Moscow, Russialev.yungelson@gmail.com2INAF-Osservatorio Astronomico d’Abruzzo, Teramo, Italy3INAF-Osservatorio Astronomico di Roma, Monte Porzio Catone, ItalyWe consider initial stage of the evolution of AM CVn type stars with whitedwarf donors, which is accompanied by thermonuclear explosions in the layerof accreted He. It is shown that the accretion never results in detonation ofHe and accretor s in AM CVn stars finish their evolution as massive WDs. Wefound, for the first time, that in the outbursts the synthesis of n-rich isotopes,initiated by the 22Ne(α,n)25Mg reaction becomes possible.1. IntroductionAM CVn type stars are rare (≃60 objects [1]) cataclysmic variable stars(CV) with helium white dwarf (WD) or low-mass helium star donors (see,e.g., [2]). In the present paper we consider the systems of the former kind.Evolution of AM CVn stars is governed by the loss of angular momentumdue to gravitational waves radiation [3]. Immediately after formation,the rate of mass-loss by the WD donors in the AM CVn stars can reach∼ 10−5 M⊙/yr; later it decreases to ∼ 10−11M⊙/yr [2]. Depending onthe accretion rate onto the WD Ṁacc, the burning of He in the accretedlayer may be steady, occur in quasi-cyclic mild or strong thermonuclearflashes (eventually accompanied by matter ejection). Also He-detonationis possible, producing an explosive event of SN Ia proportion [4]. Anextended envelope may form around the WD, if Ṁacc exceeds steady-state burning rate of He. Figure 1 shows the dependence of the burningregimes of He in the MWD-Ṁacc plane according to our calculations [5].Thermonuclear flashes accompany the earliest stage of the evolution ofAM CVn stars.We have investigated the flashes on the accretingWDs in the AM CVntype CVs with donors – helium WDs and the accompanying nucleosynthe-sis [6, 7]. The stars with the initial masses of components (Macc+Mdon)=(0.6+0.17), (0.92+0.15), (1.02+0.20), and (1.02+0.30)M⊙ were consid-ered. Such masses are characteristic for known detached binary WDs,possible ancestors of the AM CVn stars (see discussion in [6]) and fit themodel of the population of AM CVn stars [2].11Further, we will distinguish models by the initial masses of the components.2Figure 1. Helium burning regimes in the layer of accreted He depending on themass of the accretor MWD and the accretion rate Ṁacc [5]. Dashed lines mark theboundaries of the zones in which detonation (DET), strong (SF) and weak (MF)flashes, steady burning (SB) occur. In the RG-region of the diagram, extendedenvelopes around WDs are formed. Dotted lines show the evolutionary tracksof the stars considered in this paper. Filled circles on the tracks correspond tothe age of the system 105 and 106 years.In §2 we present the results of calculations, which are summarised in§3.2. Results of computationsEvolution. The algorithm of computations of Ṁdon is described in [2]. Theevolution of accretors was calculated using 1-D FUNS evolutionary code[8]. The nuclear network included about 700 isotopes from 1H to 210Bi,coupled by ≃1000 reactions [9]. Expansion of the WDs during the flasheswas limited by RLOF. It was assumed that the excess of the matter islost from the system with the specific angular momentum of the accretor.Accretion is accompanied by the release of gravitational energy, which“counteracts” the cooling of the WD. Heating of WD outer He-rich layercauses ignition of He. Since the matter in the burning zone is degenerate,a thermonuclear runaway occurs. Radiative transfer can not ensure theremoval of released energy and there arises a convective zone propagatingboth inward and outward with respect to the He-ignition point. Con-vection removes the heat from the He-burning zone, but also mixes intoit “fresh” He, stimulating burning. This process is accompanied by an31689432 57Figure 2. Left panel – evolutionary track of accreting WD in (1.02+0.3) M⊙system in the HRD. Right panel – evolution of temperature and density in theHe-burning zone. 1 – resumption of accretion after previous flash, t9−1=28 yr;2 – He-ignition, t1−2=870 yr; (2-3) – rapid increase of nuclear energy release,t2−3=5.93 yr; (3-4) – thermonuclear runaway, formation of convective zone,t3−4=214 day; 5 – maximum outward extension of convective zone, T ≈ 3·108 K,activation of 22Ne(α,n)25Mg reaction, onset of neutron-isotopes nucleosynthesis,t4−5=314 day; 6 – temperature maximum in the He-burning zone (3.8 · 108 K),t5−6=7.5 day; (6-8) – active burning of22Ne, t6−8=3.57 yr; (8-9) – RLOF stage,t8−9=8.63 yr.increase of the radius of the WD, which leads to the RLOF and ejectionof the matter enriched by the burning products into ISM.Evolution of the dwarf during a typical cycle triggered by a strongflash is shown in Fig. 2. Figure 3 presents the evolution of the charac-teristics of the burning layer for three selected models. In the system(0.60+0.17)M⊙ one weak flash occurs 5 ·104 yr after the start of accretionand accumulation of 0.03M⊙ of He. After the flash, for ≃ 2.5 · 104 yr therelease of gravitational energy prevails over cooling, but then, because ofthe fall of Ṁdon, the temperature in the He layer starts to decrease, newflashes do not occur, and evolution ends by the formation of a CO WDwith a massive He envelope. In the system (0.92+0.15)M⊙, the first flashoccurs 24000 yr after the start of accretion and accumulation of 0.01M⊙of He. The flash turns out to be “strong”, because Ṁacc is low (Fig. 1) andthe degeneracy of matter at the base of He layer by the time of the flashis higher than in the other considered systems. Afterward, 4 more flashesoccur. The time interval between flashes and their power are increasing,4Figure 3. Characteristics of the He-burning zone at the flashes stage as afunction of donor mass for the systems(from left to right) (0.62+0.17)M⊙ ,(0.92+0.15)M⊙ , (1.02+0.30)M⊙ . Topto bottom: luminosity due to He-burning, temperature and density inthe He-burning zone. For the sys-tem (1.02+0.30)M⊙ vertical dashedline separates mild and strong flashesregimes. The numbers along upper x-axis mark orbital periods of the system.because of the decline of Ṁacc. Evolution ends, like in the case of thesystem (0.60+0.17)M⊙, by transformation of the accretor into a massiveCO WD with a thick He-mantle. In the system (1.02 + 0.30)M⊙, imme-diately after the contact Ṁacc exceeds the rate of stationary He-burning,but the matter excess is only ≃0.01M⊙ and the common envelope whichcan be formed hardly influences the evolution of the system. Progressively5Figure 4. The factor of elemental en-richment for the matter lost by the sys-tem (1.02+0.3)M⊙ in strong flashes.stronger flashes begin 120 yr after contact. Altogether, 50 mild and 87flashes occur. Matter retention efficiency in flashes gradually decreasesfrom 1 to 0. After the 87th strong flash, Macc≈ 1.1M⊙, Mdon≈ 0.07M⊙.Further accretion does not lead to flashes, a massive WD forms, like inthe other cases.Nucleosynthesis. During strong flashes the physical conditions aresuitable for the activation of the intermediate neutron capture process(“i-process”), which occurs when the number density of neutrons is nn ≈(1014 − 1016) cm−3 [10]. Until recently, i-process was associated with thelate AGB or post-AGB stars, still hypothetical “rapidly accreting WDs”.As a source of neutrons, the 13C(α,n)16O reaction was considered, whichoccurs when protons are ingested during a He-flash by convection into thehot He- and C-rich zone, resulting in the appearance of regions with highconcentrations of neutrons. Since in the AM CVn stars WD accretes thematter that does not contain hydrogen, 13C is not synthesised. But as itis shown in Fig. 3, already at the stage of mild flashes the temperaturein the He-burning zone attains 3 · 108K and in the case of strong flashes– 109K. Under such conditions, the 22Ne(α, n)25Mg reaction is activated,which becomes the source of neutrons for the i-process, resulting in theformation of isotopes up to the stable 208Pb.During the flashes, WD loses all (or a part of) the layers with modifiedchemical composition. The latter can be characterized by the factor ofelemental enrichment Ψ = Nel/NISM , where Nel and NISM are relativeconcentrations of a certain element in the matter lost by the star andthe ISM, respectively. As an illustration, Fig. 4 shows the value of Ψ forthe matter lost by the system (1.02+0.30)M⊙ during strong flashes. Intotal, the system lost 0.051M⊙ of the matter with chemical composition ofaccretor and 0.052M⊙ of the matter enriched in α-elements and neutron-rich isotopes.3. ConclusionAM CVn stars with WD donors experience less than 100 flashes per sys-tem in the first ∼ 105 yr of existence. Formation rate of such AM CVn6stars is ∼ 10−3 per year [2]. The flashes occur predominantly on themost massive WDs, therefore, we can roughly estimate that currently inthe Galaxy may exist ∼ 10 AM CVn stars experiencing thermonuclearflashes.Even the most massive of the systems studied by us do not experiencethe “last”, dynamic flash that destroys accretor. Therefore, at least withinthe framework of our assumptions, the hypothetical optical transientsSN .Ia [11] do not exist.We found that the nuclear i-process triggered by the 22Ne(α, n)25Mgreaction, which is usually associated with massive stars, can occur duringthe outbursts of He burning on accreting WDs. However, rare AM CVnstars are vanishingly unimportant for the chemical evolution of the Galaxy.But their phenomenon shows that some exotic nuclear processes can occurin the depths of stars, hidden to the observers.References1. G. Ramsay, M. J. Green, T. R. Marsh, T. Kupfer, et al., A&A, 620, A141,2018.2. G. Nelemans, S. F. Portegies Zwart, F. Verbunt, and L. R. Yungelson, A&A,368, 939, 2001.3. B. Paczyński, Acta Astronom., 17, 287, 1967.4. K. Nomoto, ApJ, 253, 798, 1982.5. L. Piersanti, A. Tornambé, and L. R. Yungelson, MNRAS, 445, 3239, 2014.6. L. Piersanti, L. R. Yungelson, and A. Tornambé, MNRAS, 452, 2897, 2015.7. L. Piersanti, L. R. Yungelson, S. Cristallo, and A. Tornambé, MNRAS, 484,950, 2019.8. O. Straniero, R. Gallino, and S. Cristallo, Nuclear Physics A, 777, 311,2006.9. S. Cristallo, D. Karinkuzhi, A. Goswami, L. Piersanti, and D. Gobrecht,ApJ, 833, 181, 2016.10. J. J. Cowan and W. K. Rose, ApJ, 212, 149, 1977.11. L. Bildsten, K. J. Shen, N. N. Weinberg, and G. 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Thermonuclear oubursts of AM CVn stars

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Thermonuclear oubursts of AM CVn stars

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