adsabs/FOCAL huggingface dataset:
Close to the end of their formation, stars are surrounded by a gas and dust disk, from which planets form. Magnetic fields are known to play a key role in the star-disk system (Johns-Krull 2014). It is believed that the inner regions of the disk are significantly ionized by the stellar radiation and that accreting material flows along magnetic channels that connect the disk to the star (Koenigl 1991). Very long and intense X-ray flares in star-forming regions might occur in such long channels (Favata et al. 2005), but this is highly debated (Getman et al. 2008). These flux tubes might resemble those observed in the solar corona and diagnosed in the stellar coronae, but on a much larger scale. On the Sun we see the so-called coronal loops on the scale of several thousand kilometers in active regions, but some faint large-scale structures can extend up to ∼1 R⊙ (Reale 2014). Most solar flares occur in active region loops, but the long-lasting ones can involve more and more loops aligned in arcades. The other stars are so distant that we cannot resolve the flaring regions, but it is supposed that they occur in loops and even in arcades. Whereas the duration of solar flares typically ranges from a few minutes to several hours, stellar flares can be very intense, more than the solar bolometric luminosity, and long-lasting, including longer than one day, in very active stars. Several such gigantic coronal flares have been surveyed in star-forming regions (Favata et al. 2005) and where they occur is a big question. Magnetic instabilities in flux tubes were proposed to be the origin of the flaring activity also in T Tauri stars (Birk 1998; Birk et al. 2000), and long-lasting stellar flares might be expected to involve loop arcades (Getman et al. 2008), like those on the Sun. In long-lasting solar flares, the duration is mainly due to the progressive involvement of more and similar loops, therefore duration is not directly linked to the size of the flaring structures. This might also be the case for giant stellar flares. However, if a single stellar loop were flaring, the cooling time of the confined plasma would be proportional to the loop length (Serio et al. 1991; Reale 2014), and day-long flares would correspond to giant loops, as long as they possibly connected the star with the disk (Hartmann et al. 2016). There are ways to distinguish between a pure cooling in a single loop and a decay only due to progressive reduction of the energy release in a loop arcade (Reale et al. 1997), but the explanations are contested and the uncertainties are large (Getman et al. 2008). Several studies (Favata et al. 2005; Giardino et al. 2007) find results compatible with long magnetic channels in pre-main sequence (PMS) stars, but the derivation of the loop length is based on the assumption of a flare occurring in a single loop (Reale 2007).
Citation: Reale 2014
Citation function: Background
Cosmological hydro dynamical N-body simulations offer another possibility to investigate the origin of the bimodality in the ([Fe/H], [α/Fe]) plane. Earlier simulations, e.g. full N-body simulations by Loebman et al. (2011), Brook et al. (2012) or hybrid simulations in which a semi-analytic chemical evolution was added on top of a cosmological simulation (Minchev, Chiappini & Martig 2013, 2014), were able to show that the thin and thick discs lie along different tracks in the ([Fe/H], [α/Fe]) plane, with the thick disc being old metal poor and rich in [α/Fe] and the thin disc being young, metal-rich and poor in [α/Fe]. They also showed that migration was important to generate the two discs. However, a clear bimodality in the ([Fe/H], [α/Fe]) plane was not seen. In the past few years good progress has been made to improve the spatial resolution as well as the chemical enrichment prescriptions. The bimodality has now been observed in some simulations (Grand et al. 2018; Mackereth et al. 2018; Clarke et al. 2019), and some of the simulations, in addition to the bimodality, also reproduce the basic trends of the ([Fe/H], [α/Fe]) distribution with radius R (Buck 2020; Vincenzo & Kobayashi 2020). Unlike analytical models, such simulations cannot be fine tuned to reproduce the Milky Way data, hence, the focus of these simulations is to qualitatively reproduce the abundance trends seen in the Milky Way, to understand how frequently do we get the bimodality and what is the mechanism for it. However, there is a lack of consensus between the different studies. Clarke et al. (2019) and Buck (2020) suggest that bimodality should be common in disc galaxies, whereas Mackereth et al. (2018) suggest that it is rare. Each simulation suggests slightly different mechanisms for the existence of the bimodality. Clarke et al. (2019) attribute bimodality to vigorous star formation in clumps at high redshift. Grand et al. (2018) suggest two distinct pathways, a centralized starbust pathway induced by mergers and a shrinking gas disc pathway. Buck (2020) suggest that after the formation of the high-[α/Fe] sequence a gas-rich merger dilutes the metallicity of the ISM leading to the formation of the low-[α/Fe] sequence. Mackereth et al. (2018) attribute the bimodality to unusually rapid gas accretion at earlier times, which is also characterized by a short time-scale to convert gas to stars. While some simulations clearly identify migration as key process to shape the sequences, others do not. In spite of the differences, it seems that some of the simulations (e.g. Mackereth et al. 2018; Buck 2020; Vincenzo & Kobayashi 2020) are not inconsistent with the Schönrich & Binney (2009a) paradigm.
Citation: Mackereth et al. 2018
Citation function: Compare/Contrast; Compare/Contrast
SEP events with gigaelectronvolt particles are generally rare. Typically about a dozen events occur during each solar cycle, although only two GLEs were reported in cycle 24, probably due to the change in the state of the heliosphere (Gopalswamy et al. 2013a, 2014a; Thakur et al. 2014). It appears that the 2012 July 23 event would have been another GLE event if it had occurred on the front side of the Sun. The purpose of this paper is to examine the event from the perspectives of CME kinematics, SEP intensity and spectrum, and radio-burst association to see if the 2012 July 23 event can be considered as an extreme particle event. The reason for considering these properties is clear from the following facts. Particles up to gigaelectronvolt energies are accelerated by strong shocks driven by CMEs of very high speeds (∼2000 km s−1) and intense, soft X-ray flares (see Gopalswamy et al. 2010, 2012b). The high speed is typically attained very close to the Sun, so the density and magnetic field in the corona are high for efficient particle acceleration (e.g., Mewaldt et al. 2012; Gopalswamy et al. 2014a). The high CME speed implies that a fast-mode MHD shock forms close to the Sun, as indicated by the onset of metric type II radio bursts, typically at heights 1.5 solar radii (Rs). CMEs attaining high speeds near the Sun have to accelerate impulsively, so these events are characterized by high initial acceleration (∼2 km s−2, see Gopalswamy et al. 2012b). This is in contrast to slowly accelerating CMEs (from filament regions outside active regions) that form shocks at large distances from the Sun and do not accelerate particles to energies more than a few tens of megaelectronvolts (Gopalswamy et al. 2015a, 2015d). Accordingly, the SEP spectra of such events are very soft, as opposed to the hard spectra of GLE events. Whether an event has a soft or hard spectrum is important information because the hard-spectrum events have stronger space weather impacts (see, e.g., Reames 2013). SEP events with gigaelectronvolt components are accompanied by type II radio bursts from meter (m) wavelengths to kilometer (km) wavelengths (Gopalswamy et al. 2005b, 2010). Type II bursts occurring at such wide-ranging wavelengths imply strong shocks throughout the inner heliosphere (Gopalswamy et al. 2005a).
Citation: Gopalswamy et al. 2014a
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It is much harder to localize and track the formation of current sheets in realistic black hole accretion flows in a larger domain and for a longer period because of the effects of the more complicated global dynamics governed by the central object, and due to the turbulence induced by the MRI. Both the evolution of accretion flows and the formation of current sheets therein strongly depend on the magnetic field geometry. We model an accretion disk around a rotating black hole, varying the initial conditions to study current sheet formation in different scenarios of magnetic field geometry. In the magnetically arrested disk (MAD; Igumenshchev et al. 2003; Narayan et al. 2003) scenario, the MRI and subsequent turbulence in the inner accretion disk are suppressed due to large-scale magnetic flux (see, e.g., White et al. 2019). In axisymmetric simulations as considered here, the arrested inflow is regularly broken by frequent bursts of accretion, allowing for a macroscopic equatorial current sheet to form and break in a periodic fashion. In a full 3D setup, magnetically buoyant structures are interchanged with less-magnetized dense fluid (Igumenshchev 2008; White et al. 2019), resulting in a magnetic Rayleigh–Taylor instability (Kruskal & Schwarzschild 1954) potentially sourcing interchange-type magnetic reconnection. In the Standard And Normal Evolution (SANE; Narayan et al. 2012; Sadowski et al. 2013) state, a fully turbulent accretion disk can develop due to a smaller magnetic flux (see, e.g., Porth et al. 2019), and current sheets can ubiquitously form and interact with the turbulent flow. Polarized synchrotron radiation observed by the Event Horizon Telescope (Event Horizon Telescope Collaboration et al. 2019a) can probe the field-line structure at event-horizon scales and put tighter constraints on the magnetization and address whether the accretion is in a SANE or a MAD state (Event Horizon Telescope Collaboration et al. 2019b).
Citation: White et al. 2019
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Astrophysical processes involving radiative energy transfer are calculated by the balance between compressional heating and adiabatic cooling (Blondin et al. 1990; Taam, Fu & Fryxell 1991). Analytical prescriptions for the heating and cooling rates in complex environments are only possible under certain limits. However, the increasing computer power available today is allowing to model complex astrophysical scenarios efficiently and at a relatively low cost, including the dynamical update of the microphysics and chemistry. For instance, thermal instabilities, non-axisymmetric rotating objects, the relationships between ionization, molecular states, level populations, and kinetic temperatures of low-density environments are some of the ingredients that have no analytical counterparts and that can be calculated with highly efficient numerical algorithms. A particularly interesting question is related to the accretion of angular momentum, when the flow contains a density or velocity gradient perpendicular to the flow direction (Davies & Pringle 1980; Ruffert & Anzer 1995; Ruffert 1997, 1999). In particular, Proga & Begelman (2003) performed two-dimensional (2D) axisymmetric simulations of the accretion of slowly rotating gas on to a black hole. They found that for low angular momentum, the flow is Bondi-like, while for intermediate total angular momentum, the gas with higher angular momentum does not accrete and forms a dense torus about the black hole and, as a result, the accretion rate is substantially below the Bondi rate. For even higher total angular momentum, a dense torus coupled to a centrifugal barrier sets in, which further reduces the accretion rate. Proga & Begelman (2003) have argued that rotation may explain the difference between accretion rates estimated from observations and those predicted by the Bondi flow. Moreover, Perna et al. (2003) suggested that combining rotation and magnetic fields results in accretion rates on to isolated neutron stars that are much lower than those estimated using the Bondi formula. Later on, Krumholz, McKee & Klein (2005) considered the three-dimensional (3D) accretion of gas with constant vorticity and found that the resulting flow field is highly non-axisymmetric and time-dependent and that even a small amount of vorticity can substantially change the accretion rate. Considerable progress has been made over the past 20 yr in developing advanced numerical models to understand the physics of accretion on to compact objects. A thorough review of Bondi accretion theory and related earlier numerical simulations is given by Edgar (2004).
Citation: Ruffert; 1999
Citation function: Motivation
We used the publicly available code AROME (Boué et al. 2013) to analytically compute the RM RV anomaly as measured with the CCF method. This code, written in C, was given the orbital planetary and stellar parameters (the projected rotation velocity Vsin(i), the semi-major axis a, the radius of the star Rstar, expressed in solar radii units, the inclination angle i, the mutual inclination angle λ, and the planet orbital period P, all taken from Triaud et al. 2009, see Table 1), and the times of observation, which it subsequently used to calculate the CCFs and the expected RV measurements from a Gaussian fit to these. The fraction of flux blocked by a planet crossing the surface of its host star depends on the limb darkening of the star. For the calculation with AROME we chose the non-linear law of Claret (2000) to match those used in the analysis of Pont et al. (2008) and Sing et al. (2011b), that is, (3)\begin{equation} \frac{I \left( \mu \right)}{I \left( 1 \right)} \ = \ 1 - \sum_{k=1}^{4} a_{k} \left( 1 - \mu^{k/2} \right) \label{claret2000}. \end{equation}IμI(1)=1−∑k=14ak(1−μk/2).For the white-light RV time series we used the quadratic limb-darkening law coefficients as in Triaud et al. (2009), who studied the RM effect for this planet using the same data set. All the limb-darkening coefficients used in our analysis are given in Table 2. Since we are interested in the dependence of the RM effect on the planet effective size, for each passband we selected the limb-darkening law corresponding to those wavelengths and calculated the anomaly for 5000 values of the planet-star radius ratio (Rp/Rstar), between 0.105 and 0.205 with a step of 2 × 10-5. These boundaries were chosen arbitrarily with the purpose of including the measured planet-star radius ratio for this planet using the same HARPS data as Triaud et al. (2009) (0.1581 ± 0.0005). All other parameters were kept fixed. We note that for the analysis we are only interested in the relative change of the Rp/Rstar as function of wavelength, not in absolute values. Therefore it is not relevant to fit for parameters such as the projected rotation velocity Vsini, the mutual inclination angle λ, the macroturbulence z, and the scaled semi-major axis a/Rstar separately for each passband. These should be independent of wavelength and therefore can only result in a small vertical offset in the transmission spectrum, not in a change of slope.
Citation: Pont et al. (2008)
Citation function: Uses
Here, we relate the observed morphology of the dust continuum emission to the physical processes taking place within star-forming galaxies around the peak of cosmic star formation. As discussed by Cheng et al. (2020), the extended dust continuum emission observed in the less-FIR luminous SHiZELS galaxies suggests a dominant component of extended, disc-wide star formation; in contrast, the emission from sub-millimetre selected galaxies appears to be dominated by a compact, nuclear starburst. SHiZELS-14 is an outlier in the sense that it has both a submillimetre bright compact core and very extended emission. Tadaki et al. (2020) show that the most compact galaxies in their sample tend to have high gas fractions (derived via $S_{870\mu \rm {m}}$), and argue that this reflects efficient radial gas inflows. Numerical simulations have long shown that galaxy mergers are capable of triggering tidally driven gas inflows (Hernquist 1989; Barnes & Hernquist 1991), which can cause strong nuclear starbursts (e.g. Mihos & Hernquist 1994, 1996; Hopkins et al. 2013; Moreno et al. 2015). However, observations of local galaxies such as the Antennae system demonstrate that galaxy interactions can also trigger widespread star formation that is not limited to a compact, nuclear region (Wang et al. 2004). More recent, high resolution simulations show that these observations can be explained via merger-driven injections of turbulence into the ISM: extended compression results in fragmentation into dense, star-forming gas, and spatially extended starburst activity (Renaud et al. 2014; Renaud, Bournaud & Duc 2015). Renaud et al. (2015) argue that this process is particularly important in the early and mid-stages of a galaxy merger: during the first two simulated pericentre passages, star clusters form kiloparsecs from the galactic nucleus, with the central starburst dominating only from the beginning of the final coalescence. This progression of star formation from extended to compact as the merger unfolds is also consistent with observations of local galaxies (Pan et al. 2019). The extended star formation observed in SHiZELS-14 may therefore suggest that we are viewing the short-lived mid-stages of a merger; this would be consistent with its complex, irregular morphology and dispersion-dominated $\rm {H}\,\alpha$ kinematics. The similarly TIR-luminous but more compact sources within the AS2UDS samples may comprise galaxies experiencing a wider range of evolutionary stages, including some later-stage mergers.
Citation: Mihos & Hernquist; 1996
Citation function: Background; Background
An important reason for modeling the THS plasma is identifying the pathways of chemical growth. Tables 3 and 4 list the dominant production and loss reactions, respectively, for a set of species. The neutral versions of these species have each been identified in Titan's upper atmosphere (Waite et al. 2007; Cui et al. 2009). The values of the calculated reaction rates are listed if larger than 1 × 10−6 mol m−3 s−1. For the experimental conditions considered, the model predicts that neutral C2H2 production is only appreciable in the mixture that already contains an acetylene precursor through the reaction of C4H2 with C4H3+, which are themselves reaction products that, at Titan, might be derived through other chemical pathways. C2H2 is destroyed mainly by reactions with C2H2+ ions or CHx+ fragments. Neutral C2H4 exhibits large production rates for all the precursor mixtures since its formation pathway involves CH4 and the CH fragment, which are both abundant. Nitrogen incorporation is an important part of the haze chemistry of Titan (Imanaka & Smith 2010). HCN has been detected in Cassini Composite Infrared Spectrometer observations at 713 cm−1 (Coustenis et al. 2008). Although its abundance is seasonal, modeled concentrations of HCN nominally peak near the mesopause at around 108 cm−3 (Liang et al. 2007), and HCN mixing ratios are highest in the ionosphere where they are between 0.1% and 1% (Hébrard et al. 2012). According to Hébrard et al.'s network, the dominant pathways for HCN production in the upper atmosphere are HCNH+H





HCN+H2 and N+CH3





HCN+2H. In the THS plasma, various paths for nitrogen incorporation are included in the chemical network including N+ reactions with CH and CH4 leading to CN+ and HCNH+ as well as the dominant neutral mechanisms highlighted by Hébrard et al. The model suggests that the neutral pathway, N+CH3, is a dominant source of HCN for each of the N2/CH4/CxHy mixtures. The reason is that atomic nitrogen and CH3 are abundant products from the precursor fragmentation. The HCNH+H pathway is less productive since HCNH must first be formed from the fragments. The model also suggests that once formed, HCN is stable, subject only to small rates of charge transfer with N2(X, v = 0)+ or reaction with the C2H5+ ion.
Citation: Imanaka & Smith 2010
Citation function: Background
A growing body of work suggests that planet formation is strongly dependent on the birth environment of the host star. Stars preferentially form in groups (Lada & Lada 2003), and in sufficiently dense environments the evolution of a PPD can be significantly influenced by neighbours (de Juan Ovelar et al. 2012). Close star–disc encounters are one such environmental influence on PPDs that can result in enhanced accretion and hasten disc depletion (Clarke & Pringle 1993; Ostriker 1994; Pfalzner et al. 2005; Olczak, Pfalzner & Spurzem 2006; Bate 2018; Winter et al. 2018a; Cuello et al. 2019). However, the stellar number densities required for tidal truncation are high, and in practice few observed regions satisfy this condition (Winter et al. 2018b, 2019a). The influence of tidal truncation is therefore limited to stellar multiples, either in bound systems (Dai et al. 2015; Kurtovic et al. 2018) or during the decay of higher order multiplicity (Winter, Booth & Clarke 2018c). Since stellar multiplicity does not appear to be strongly dependent on environment (see Duchêne & Kraus 2013, for a review), this suggests that encounters are not an environmental influence, but may set disc initial conditions during the early phases of cluster evolution (Bate 2018). Discs can also be externally depleted via thermal winds driven by far-ultraviolet (FUV) and extreme ultraviolet (EUV) photons from neighbouring massive stars (Johnstone, Fabian & Taylor 1998; Störzer & Hollenbach 1999; Adams et al. 2004; Facchini, Clarke & Bisbas 2016; Haworth et al. 2018; Haworth & Clarke 2019). This process of external photoevaporation dominates over dynamical encounters in observed environments, and can deplete PPDs rapidly for many stars that are born in massive and dense clustered environments (Scally & Clarke 2001; Winter et al. 2018b). Many stars in the solar neighbourhood are born in regions where UV fields are sufficient to significantly shorten disc lifetimes (Fatuzzo & Adams 2008; Winter et al. 2018b), and the fraction of stars born in such environments may be much greater outside of this region, dependent on galactic environment (Winter et al. 2019a). From an observational perspective, Guarcello et al. (2016) report disc survival fractions that decrease with increasing FUV flux in Cygnus OB2 (see also Winter, Clarke & Rosotti 2019b), and Ansdell et al. 2017 find a correlation between the dust mass in PPDs and separation from σ Ori. However, Richert et al. (2015) find no correlation of disc fraction with distance from OB stars. Reconciling these contradictory findings may require appealing to the inefficiency of external photoevaporation at small radii within the disc, dynamical and projection effects, or the stellar age gradient apparent in many star forming regions (Getman et al. 2018).
Citation: Winter et al. 2018b
Citation function: Background