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The cosmological lithium problem, i.e. the discrepancy between the lithium abundance predicted by the Big Bang Nucleosynthesis and the one observed for the stars of the Spite plateau, is one of the long standing problems of modern astrophysics. A possible astrophysical solution involves lithium burning due to protostellar mass accretion on Spite plateau stars. In present work, for the first time, we investigate with accurate evolutionary computations the impact of accretion on the lithium evolution in the metal-poor regime, that relevant for stars in the Spite plateau.
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The cosmological lithium problem, that is, the discrepancy between the lithium abundance predicted by the Big Bang nucleosynthesis and the one observed for the stars of the Spite plateau, is one of the long standing problems of modern astrophysics. Recent hints for a possible solution involve lithium burning induced by protostellar mass accretion on Spite plateau stars. The purpose of this paper is to analyze the effect of protostellar accretion on low metallicity low-mass stars with a focus on PMS lithium evolution. We computed the evolution from the protostar to the MS phase of accreting models with final masses of 0.7 and 0.8 M$_odot$, and three metallicities Z=0.0001, Z=0.0010, and Z=0.0050. The effects of changing the main parameters affecting accreting models, that is the accretion energy (cold versus hot accretion), the initial seed mass $M_{seed}$ and radius $R_{seed}$, and the mass accretion rate $dot{m}$, have been investigated in detail. As for the main stellar properties and the surface $^7 Li$ abundance, hot accretion models converge to standard non-accreting ones within 1 Myr, regardless of the actual value of $M_{seed}$, $R_{seed}$, and $dot{m}$. Also, cold accretion models with a relatively large $M_{seed}$ ($gtrsim 10~M_{jup}$) or $R_{seed}$ ($gtrsim 1~R_odot$) converge to standard non-accreting ones in less than about 10-20~Myr. A drastically different evolution occurs whenever a cold protostellar accretion process starts from small values of $M_{seed}$ and $R_{seed}$ ($M_{seed}sim 1~M_{jup}$, $R_{seed} lesssim 1~R_odot$). These models almost entirely skip the standard Hayashi track evolution and deplete Li before the end of the accretion phase. The exact amount of depletion depends on the actual combination of the accretion parameters ($dot{m}$, $M_{seed}$, and $R_{seed}$), achieving in some cases the complete exhaustion of Li in the whole star.
We aim at studying the causal link between the knotty jet structure in CARMA 7, a young Class 0 protostar in the Serpens South cluster, and episodic accretion in young protostellar disks. We used numerical hydrodynamics simulations to derive the protostellar accretion history in gravitationally unstable disks around solar-mass protostars. We compared the time spacing between luminosity bursts Deltatau_mod, caused by dense clumps spiralling on the protostar, with the differences of dynamical timescales between the knots Deltatau_obs in CARMA 7. We found that the time spacing between the bursts have a bi-modal distribution caused by isolated and clustered luminosity bursts. The former are characterized by long quiescent periods between the bursts with Deltatau_mod = a few * (10^3-10^4) yr, whereas the latter occur in small groups with time spacing between the bursts Deltatau_mod= a few * (10-10^2) yr. For the clustered bursts, the distribution of Deltatau_mod in our models can be fit reasonably well to the distribution of Deltatau_obs in the protostellar jet of CARMA 7, if a certain correction for the (yet unknown) inclination angle with respect to the line of sight is applied. The K-S test on the model and observational data sets suggests the best-fit values for the inclination angles of 55-80 deg., which become narrower (75-80 deg.) if only strong luminosity bursts are considered. The dynamical timescales of the knots in the jet of CARMA 7 are too short for a meaningful comparison with the long time spacings between isolated bursts in our models. The exact sequences of time spacings between the luminosity bursts in our models and knots in the jet of CARMA 7 were found difficult to match. (abridged)
Building on our previous hydrodynamic study of the angular momenta of cloud cores formed during gravitational collapse of star-forming molecular gas in our previous work, we now examine core properties assuming ideal magnetohydrodynamics (MHD). Using the same sink-patch implementation for the emph{Athena} MHD code, we characterize the statistical properties of cores, including the mass accretion rates, specific angular momenta, and alignments between the magnetic field and the spin axis of the core on the $0.1 mathrm{pc}$ scale. Our simulations, which reproduce the observed relation between magnetic field strength and gas density, show that magnetic fields can help collimate low density flows and help seed the locations of filamentary structures. Consistent with our previous purely hydrodynamic simulations, stars (sinks) form within the heterogeneous environments of filaments, such that accretion onto cores is highly episodic leading to short-term variability but no long-term monotonic growth of the specific angular momenta. With statistical characterization of protostellar cores properties and behaviors, we aim to provide a starting point for building more realistic and self-consistent disk formation models, helping to address whether magnetic fields can prevent the development of (large) circumstellar disks in the ideal MHD limit.
We present the latest development of the disk gravitational instability and fragmentation model, originally introduced by us to explain episodic accretion bursts in the early stages of star formation. Using our numerical hydrodynamics model with improved disk thermal balance and star-disk interaction, we computed the evolution of protostellar disks formed from the gravitational collapse of prestellar cores. In agreement with our previous studies, we find that cores of higher initial mass and angular momentum produce disks that are more favorable to gravitational instability and fragmentation, while a higher background irradiation and magnetic fields moderate the disk tendency to fragment. The protostellar accretion in our models is time-variable, thanks to the nonlinear interaction between different spiral modes in the gravitationally unstable disk, and can undergo episodic bursts when fragments migrate onto the star owing to the gravitational interaction with other fragments or spiral arms. Most bursts occur in the partly embedded Class I phase, with a smaller fraction taking place in the deeply embedded Class 0 phase and a few possible bursts in the optically visible Class II phase. The average burst duration and mean luminosity are found to be in good agreement with those inferred from observations of FU-Orionis-type eruptions. The model predicts the existence of two types of bursts: the isolated ones, showing well-defined luminosity peaks separated with prolonged periods (~ 10^4 yr) of quiescent accretion, and clustered ones, demonstrating several bursts occurring one after another during just a few hundred years. Finally, we estimate that 40%--70% of the star-forming cores can display bursts after forming a star-disk system.
The abundance of primordial lithium is derived from the observed spectroscopy of metal-poor stars in the galactic halo. However, the observationally inferred abundance remains at about a factor of three below the abundance predicted by standard big bang nucleosynthesis (BBN). The resolution of this dilemma can be either astrophysical (stars destroy lithium after BBN), nuclear (reactions destroy lithium during BBN), or cosmological, i.e. new physics beyond the standard BBN is responsible for destroying lithium. Here, we overview a variety of possible cosmological solutions, and their shortcomings. On the one hand, we examine the possibility of physical processes that modify the velocity distribution of particles from the usually assumed Maxwell-Boltzmann statistics. A physical justification for this is an inhomogeneous spatial distribution of domains of primordial magnetic field strength as a means to reduce the primordial lithium abundance. Another possibility is that scattering with the mildly relativistic electrons in the background plasma alters the baryon distribution to one resembling a Fermi-Dirac distribution. We show that neither of these possibilities can adequately resolve the lithium problem. A number of alternate hybrid models are discussed including a mix of neutrino degeneracy, unified dark matter, axion cooling, and the presence of decaying and/or charged supersymmetric particles.
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