Nucleosynthesis across the Galaxy: AGB Stars and Neutron Stars Mergers Diego Vescovi 1,2,3 , Sergio Cristallo 2,3 , and Marica Branchesi 1,4 1. Gran Sasso Science Instjtute (GSSI), L’Aquila, Italy 2. INFN – Sectjon of Perugia, Perugia, Italy 3. INAF – Osservatorio Astronomico d’Abruzzo, Teramo, Italy 4. INFN – Laboratori Nazionali del Gran Sasso, Assergi, Italy GSSI Admission to the 3 rd year GSSI - L’Aquila - Italy, 10 October 2019
The origin of heavy elements in the Solar System Locatjon of peaks indicates n -captures along valley of stability s -process Neutron captures processes : • r -process • s -process 1)Weak component (A<90) Massive Stars 2)Main component (from Sr to Bi) AGB stars Nucleosynthesis across the Galaxy: AGB Stars and NMS 2 Diego Vescovi - L’Aquila, 2019
H- and He-burning in TP-AGB stars • What? Low-Mass Stars • When? Asymptotjc Giant Branch (AGB) • How? Thermally Pulsing (TP) proton He-fmash 13 C-pocket penetratjon 12 C(p,γ) 13 N(β + ν) 13 C 22 Ne(α,n) heavy elements!! 13 C(α,n) Nucleosynthesis across the Galaxy: AGB Stars and NMS 3 Diego Vescovi - L’Aquila, 2019
The 13 C-pocket: formatjon ● Protons can penetrate into the He-rich region at each TDU (Third Dredge-Up) phenomenon Which is the physical mechanism? Classic models assume the 13 C-pocket formatjon Many recent physical approaches: ● Opacity induced overshoot (Cristallo+ 2009, 2011, 2015) ● Convectjve Boundary Mixing (Battjno+ 2016) ● Magnetjc fjelds (Trippella+ 2016; Palmerini+ 2018) botuom-up mechanism through magnetjc buoyancy 1a) Rotatjonal shears promote magnetjc fjelds? 1b) Fossil magnetjc fjelds? 2) Magnetjc structures reach the envelope 3) Protons are injested into the He-rich region Nucleosynthesis across the Galaxy: AGB Stars and NMS 4 Diego Vescovi - L’Aquila, 2019
Magnetjc buoyancy ● MagnetoHydroDynamics ( MHD ) solutjons (Nucci & Busso 2014): ➔ No numerical approximatjons (exact analytjc solutjon) ➔ Simple geometry: toroidal magnetjc fjeld Equatjons: Solutjons: where k is the exponent of the density distributjon: Nucleosynthesis across the Galaxy: AGB Stars and NMS 5 Diego Vescovi - L’Aquila, 2019
Implementatjon ● Exponentjal decay of the convectjve velocity (Straniero+ 2006, Cristallo+ 2009) : Parameters: ➔ Radius extentjon of the overshootjng region ➔ β ● Magnetjc contributjon ( this work ), actjng when the density distributjon is ρ ∝ r k : Parameters: ➔ Layer “ p ” at the deepest coordinate from which buoyancy starts (can be identjfjed from the corresponding critjcal toroidal B φ value) ➔ Startjng velocity v p of the buoyant material Calibratjon is needed! Nucleosynthesis across the Galaxy: AGB Stars and NMS 6 Diego Vescovi - L’Aquila, 2019
SiC Grains ● Stellar models with difgerent initjal mass and metallicity ➔ difgerent numbers of thermal pulses experienced ➔ difgerent extentjon of 13 C-pockets ➔ Isotopic ratjos of mainstream grains are quite well reproduced Nucleosynthesis across the Galaxy: AGB Stars and NMS 7 Diego Vescovi - L’Aquila, 2019
Intrinsic C-rich AGB Stars ● Stellar models with close-to-solar metallicity ➔ Low [hs/ls] ➔ High [s/Fe] ● Does magnetjsm fade out for low-to-intermediate mass (3 to 6 M ⊙ )? Nucleosynthesis across the Galaxy: AGB Stars and NMS 8 Diego Vescovi - L’Aquila, 2019
Post- and Intrinsic C-rich AGB Stars I ● Stellar models with low metallicity ➔ [hs/ls] vs. [s/Fe] consistent with observatjons ➔ Models with opacity-induced overshoot only fail Nucleosynthesis across the Galaxy: AGB Stars and NMS 9 Diego Vescovi - L’Aquila, 2019
Post- and Intrinsic C-rich AGB Stars II ● Stellar models at difgerent metallicitjes ➔ [hs/ls] vs. [Fe/H] consistent with observatjons ➔ Models with opacity-induced overshoot only fail again Weak magnetjsm Strong magnetjsm ➔ Variable effjciency of the MHD-induced mixing? ➔ Mass-dependent effjciency ? Nucleosynthesis across the Galaxy: AGB Stars and NMS 10 Diego Vescovi - L’Aquila, 2019
Summary I ➔ Most of what we know has been learned through a lengthy work with parameterized models, trying to constrain the parameters gradually, from the increasing accuracy of observatjons ➔ This allowed recently the development of physical models for the mixing mechanisms required to produce the 13 C neutron source. ➔ Taking into account magnetjc fjelds in radiatjve regions might be crucial in modeling the mixing episodes (e.g. through magnetjc buoyancy ). ➔ First outcomes confjrms recent results from Trippella+ (2016), Palmerini+ (2018), and Liu+ (2018, 2019) ➔ More extended and fmatuer 13 C-pocket ➔ The majority of isotopic ratjos of mainstream grains are quite well reproduced ➔ [hs/ls] vs. [s/Fe] and [hs/ls] vs. [Fe/H] consistent with observatjons of post-AGB and intrinsic AGB stars ➔ Magnetjsm has (most problably) variable intensity Nucleosynthesis across the Galaxy: AGB Stars and NMS 11 Diego Vescovi - L’Aquila, 2019
r-process: basic ideas ● key reactjons: ( A , Z) + n ↔ ( A + 1, Z) + γ ● r -process requires initjal high n n and T ➔ high n n : τ (n,γ) << τ β-decay ➔ high n n and T : (n, γ) ↔(γ, n) along isotopic chain ➔ steady abundances intra-chain with one dominant nucleus ● β -decay rates of dominant nuclei regulate inter-chain fmow ● equilibrium freeze-out: n n drops and β -decays take over Nucleosynthesis across the Galaxy: AGB Stars and NMS 12 Diego Vescovi - L’Aquila, 2019
Neutron star mergers as r -process site ● r -process requires free n and seed nuclei (< A >, <Z>) ● seed propertjes/abundances depend on nuclear-statjstjcal equilibrium (NSE) freeze-out ● in adiabatjc expansion, neutron-to-seed ratjo depends on three parameters: 1) entropy s ~ T 3 / ρ n n / n seed ∝ s 3 / ( τ dyn Y e 3 ) 2) Y e ~ n p /( n n + n p ) 3) τ dyn ( T (t) ≈ T 0 exp(−t/ τ dyn )) high entropy r -process low entropy r -process Possible scenarios hot CCSN winds BNS and BHNS mergers MHD supernovae First evidences of r -process nucleosynthesis in kilonova from GW170817 Matteucci +14 Nucleosynthesis across the Galaxy: AGB Stars and NMS 13 Diego Vescovi - L’Aquila, 2019
BNS merger + kilonova Basic ideas: ● radioactjve decay of freshly sinthetjzed r -process elements in ejecta: release of nuclear energy ● thermalizatjon of high energy decay products with ejecta ● difgusion of thermal photons during ejecta expansion ● thermal emission of photons at photosphere Metzger & Berger 12 Nucleosynthesis across the Galaxy: AGB Stars and NMS 14 Diego Vescovi - L’Aquila, 2019
Propertjes of GW170817/AT2017gfo ● 17/08/17, GW+EM detectjon of an event compatjble with BNS merger (LVC PRL 2017) ● rather bright, nIR component, with a peak at 5 days (red component) ∼ ● bright, UV/O component, with a peak at 1 day (blue component) ∼ Light curves; Pian, D’Avanzo+ 2017 (left); Tanvir+ 2017 (right) ➔ Kilonova models fail in explaining the early behavior of the UV and visible light curves ➔ The presence of a larger nuclear heatjng rate at t 1 day can increase the light ≲ curves by half a magnitude during the fjrst day Nucleosynthesis across the Galaxy: AGB Stars and NMS 15 Diego Vescovi - L’Aquila, 2019
Heatjng rate vs. electron fractjon Y e − 1.3 erg g − 1 s − 1 ➔ is usually approximated by an analytjc fjttjng formula as Q fit ( t )= 10 10 t d Q ˙ ˙ ➔ Detailed nucleosynthesis calculatjons show a complex dependence ➔ Heatjng rates normalized to point out that all the normalized heatjng rates Q fit ˙ show considerable excess at difgerent tjmes Nucleosynthesis across the Galaxy: AGB Stars and NMS 16 Diego Vescovi - L’Aquila, 2019
Implementatjon and fjrst tests ➔ Inclusion of new detailed nuclear heatjng rates obtained by nuclear network calculatjons in an anisotropic, multjcomponent kilonova model (Perego+ 2017) ➔ Coupled with a parallelized Monte Carlo Markov Chain (MCMC) algorithm. ➔ Goal: re-analize AT2017gfo data by computjng the posterior distributjons associated to several difgerent models ● First outcome (simple isotropic dynamical ejecta) : ➔ brighter lightcurve [erg/s] Next steps: 1) different matter ejection mechanisms (multi-component) 2) angular dependence (anisotropy) [days] Nucleosynthesis across the Galaxy: AGB Stars and NMS 17 Diego Vescovi - L’Aquila, 2019
Summary II ➔ Kilonova from GW170817 originates from the radioactjve decay of heavy elements ➔ Signature of r -process nucleosynthesis in ejecta from neutron star mergers ➔ Astrophysical site of the r -process is identjfjed, but further observatjons are necessary ➔ Having identjfjed the astrophysical site it becomes fundamental to reduce the nuclear physics uncertaintjes ➔ Lanthanide-rich for Y e 0.25 ≲ ➔ Insensitjvity of the abundance patuern to the parameters of the merging system because of an extremely Y e environment, which guarantees the occurrence of several fjssion cycles before the r -process freezes out ➔ Nuclear heatjng rates are , at the tjmes relevant for the kilonova emission, uncertain for a factor a few ➔ Kilonova emission seems to be strongly afgected by non-approximated heatjng rates Nucleosynthesis across the Galaxy: AGB Stars and NMS 18 Diego Vescovi – L’Aquila, 2019
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