adsabs/FOCAL huggingface dataset:
Turbulence in plasmas is a complex phenomenon that is characterized by different regimes in different ranges of spatial and temporal scales. Turbulence in the solar wind has been extensively studied, both by detailed analyses of in situ measurements and from a theoretical point of view; see Bruno & Carbone (2005) for a review. Such studies often adopt complementary views that the turbulence may be described either as a collection of waves that interact nonlinearly, so-called wave turbulence, or else as a collection of broadband, essentially zero-frequency eddies or flux tubes that form a hierarchy of coherent structures. These approaches have been extensively reviewed (Barnes 1979; Matthaeus et al. 2015), and we do not attempt a critical comparison in the present work. Instead, we adopt mainly a wave taxonomy of the fluctuations based on linear theory in order to address a specific set of questions. As motivation, we note that a variety of observations in the solar wind (Bale et al. 2005; Sahraoui et al. 2012) have suggested that fluctuations near the end of the magnetohydrodynamic (MHD) inertial cascade range, and approaching the kinetic plasma range, may consist primarily of kinetic Alfvén waves (KAWs). Here we address in particular the nature of fluctuations produced due to nonlinear interactions near the proton inertial length dp and investigate in some detail the basis for identifying them as KAWs. We show how phase mixing of large-scale parallel-propagating Alfvén waves is an efficient mechanism for the production of KAWs at wavelengths close to dp and at a large propagation angle with respect to the magnetic field. To support the interpretation as KAWs, we perform and analyze MHD, Hall magnetohydrodynamic (HMHD), and hybrid Vlasov–Maxwell (HVM) simulations that model the propagation of Alfvén waves and their fully nonlinear interaction with a nonuniform plasma background. We will be able to characterize fluctuations produced by this phase-mixing-like interaction as highly oblique KAWs.
Citation: Bale et al. 2005
Citation function: Motivation; Motivation
The spectral energy distribution of the flare determines the altitude range in the planet’s atmosphere that is affected by the flare. Intense flares on AU Mic display a strong continuum emission enhancement in the XUV (see Fig. 2c) which results in increased ionisation over a broad altitude range in the thermosphere. Large continuum enhancement during flares have also been seen in other wavebands. Strong increases in the FUV have been observed on AU Mic (Robinson et al. 2001), and also on another active M dwarf, AD Leo (Hawley & Pettersen 1991). More recently, Kowalski et al. (2010) showed that continuum emission may be the dominant luminosity source in the near ultraviolet (NUV) during flares. There have not been many simultaneous multi-wavelength studies of flaring stars (Hawley et al. 2003; Osten et al. 2005). There is a particular dearth of simultaneous data in the EUV which is critical for studies of exoplanetary upper atmospheres, but where measurements are difficult due to ISM absorption. More observations are needed, but if young, magnetically active stars display broadband continuum enhancements during flares, similar to AU Mic, this would mean that large altitude ranges in planetary atmospheres are affected. Programmes such as Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems (MUSCLES) have begun to provide more information on the high-energy spectral shape of K and M dwarf stars, including in the EUV (France et al. 2016; Youngblood et al. 2016; Loyd et al. 2016). However, in these studies, the EUV flux is computed using reconstructed Lyman alpha flux assuming a spectral energy distributio similar to the Sun. This approach could lead to incorrect results for active stars. Further development of coronal models driven by multi-wavelength observations would provide the most accurate high energy spectra for active low-mass stars. Such observations should be provided by the Multiwavelength Observations of an eVaporating Exoplanet and its Star (MOVES) program which has begun to undertake a long-term study of the HD 189733 system (Fares et al. 2017).
Citation: Osten et al. 2005
Citation function: Motivation; Motivation
Having intermediate colours between the blue cloud and the red sequence, galaxies populating the so-called green-valley (e.g. Salim 2014; Schawinski et al. 2014) are generally considered as the transiting objects par excellence (Martin et al. 2007; Mendel et al. 2013; Salim 2014; Schawinski et al. 2014). Among these, the most interesting population certainly consists of those galaxies which have just entered the quenching phase (within a few Myr). Although hampered by the short duration of the quenching process, the search for galaxies in this critical phase of evolution has been carried on by several authors in the past decades. Galaxies characterized by both a tidally disturbed morphology and intermediate colours (e.g. Schweizer & Seitzer 1992; Tal et al. 2009) or low level of recent SF (Kaviraj 2010), young elliptical galaxies (Sanders et al. 1988; Genzel et al. 2001; Dasyra et al. 2006) and very recent post-merger remnants with strong morphological disturbances (Hibbard & van Gorkom 1996; Rothberg & Joseph 2004; Carpineti et al. 2012) have been considered as valid 'recent time' quenching candidates. Moreover, many attempts aimed at spectroscopically identifying quenching galaxies come from the investigations of the post-starburst (E+A or K+A) galaxies' UV and optical spectra, whose strong Balmer absorption lines and missing [O ii] λ3727 (hereafter [O ii]) and Hα emission lines (Couch & Sharples 1987; Poggianti et al. 2004; Quintero et al. 2004; Balogh et al. 2011; Muzzin et al. 2012; Mok et al. 2013; Wu et al. 2014) have been interpreted as signs of a recent halt of the SF (Dressler & Gunn 1983; Zabludoff et al. 1996; Quintero et al. 2004; Poggianti et al. 2008; Wild et al. 2009). The scarcity of galaxies that are in the transition phase suggests that, whatever mechanism may be responsible for the SF shut-off, it has to happen on short time-scales (Tinker, Wechsler & Zheng 2010; Salim 2014). The relatively short duration of the quenching process is also suggested by the surprising identification of a significant number of galaxies that look already quiescent at z ∼ 4–5, when the Universe was only ∼1–1.5 Gyr old (e.g. Mobasher et al. 2005; Wiklind et al. 2008; Juarez et al. 2009; Brammer et al. 2011; Marsan et al. 2015; Citro et al. 2016). However, a general and coherent picture concerning the SF quenching is still lacking. Very recently, evidence that the quenching of the SF could be a separated process with respect to the morphological transformation has come from the photometric and spectroscopic investigations of passive spiral galaxies (Fraser-McKelvie et al. 2016).
Citation: Schweizer & Seitzer 1992
Citation function: Background; Background
The 21 cm global spectrum experiments aim to measure the sky-averaged spectrum with high precision so as to probe the early epochs of the universe. There are a number of such ground-based experiments, including the Experiment to Detect the Global Epoch-of-Reionization Signature (EDGES; Bowman et al. 2008; Bowman & Rogers 2010; Monsalve et al. 2017), the Sonda Cosmológica de las Islas para la Detecciónde Hidrógeno Neutro (SCI-HI; Voytek et al. 2014), the Probing Radio Intensity at high-z from Marion (PRIzM; Philip et al. 2019), the Shaped Antenna measurement of the background RAdio Spectrum (SARAS; Patra et al. 2013; Singh et al. 2017, 2018), the Cosmic Twilight Polarimeter (CTP; Nhan et al. 2019), the Broadband Instrument for Global Hydrogen Reionization Signal (BIGHORNS; Sokolowski et al. 2015), the Large-Aperture Experiment to Detect the Dark Age (LEDA; Bernardi et al. 2016; Bernardi 2018; Price et al. 2018), and the Radio Experiment for the Analysis of Cosmic Hydrogen (REACH; de Lera Acedo 2019). Compared to the 21 cm tomography experiments, measurement of the global 21 cm emission has a higher raw sensitivity and requires smaller collecting area, so that it could be conducted even with a single antenna. The EDGES (Bowman et al. 2018) reported the detection of a strong absorption feature at ∼78 MHz, which has a cosmic dawn 21 cm spectrum interpretation, though it has an unexpectedly large amplitude (0.5 K) and an unusual flattened profile. If it originated from the cosmic 21 cm spectrum, this would suggest possibly new physics or astrophysics (Chen & Miralda-Escudé 2004, 2008; Creasey et al. 2011; Nelson et al. 2013; Barkana 2018; Fialkov et al. 2018; Fraser et al. 2018; Barkana et al. 2018; Slatyer & Wu 2018; Li et al. 2021; Houston et al. 2018; Hirano & Bromm 2018; Muñoz et al. 2018; Yang 2021), though this result was not confirmed by a recent measurement of the SARAS experiment (Singh et al. 2022). It is imperative to check this result and improve upon it with further and more precise observations.
Citation: Singh et al. 2017
Citation function: Background; Background
The ORCA software (version 4.0.2) (Neese 2012) was used for all geometry minimizations, potential energy surface (PES), and vibrational frequency analyses using the global hybrid functional PW6B95 (Zhao & Truhlar 2005) coupled to the split valence triple-ζ def2-TZVPP basis set with two sets of polarization functions (Weigend & Ahlrichs 2005) and the atom-pairwise dispersion correction energy with Becke–Johnson damping (D3BJ) (Grimme et al. 2010; Grimme, Ehrlich & Goerigk 2011). The selected level of theory (PW6B95-D3BJ/def2-TZVPP) is a reliable and accurate theoretical tool in the estimation of general main group thermochemistry, kinetics, and non-covalent interactions after the double hybrid functionals (Goerigk et al. 2017). The reliability of the used method is also underlined by the good qualitative agreement between the Density Functional Theory (DFT) and MP2-F12 calculations as found in previous works (Fioroni & DeYonker 2016; Fioroni et al. 2018; Fioroni, Savage & DeYonker 2019). To speed up calculations, the RI (Resolution of the Identity) (Neese 2003) and RIJCOSX (Neese et al. 2009b) algorithms were used coupling the Coulomb-fitting basis sets def2/J (Weigend 2006). Because all the considered TSs involve a simple bond breaking/formation or a dihedral rotation, after a PES search performed on the desired reaction coordinate, the eigenvector following method (Schlegel 1982; Horn et al. 1991; Eckert, Pulay & Werner 1997), as implemented in ORCA, was used. Furthermore, the computed structures were verified to represent a minimum or a transition state by analysis of harmonic vibrational frequency calculations. Finally, the visualization of the normal mode associated to the TS was analysed. The obtained enthalpies: (HTot = [EEl. + EZPE + EVib. + ERot. + ETrans.] + kBT) and S values (STot = SEl. + SVib. + SRot. + STrans.) were used to estimate the free energies (G) at T = 200 K. The selected T = 200 K is lower bound to the HCN polymerization to progress efficiently. When referring to astronomical bodies, such temperatures can be experienced, for example, by comets where temperature rises periodically by surface heating to release HCN and H molecules (Hoang et al. 2019) or by dust particles or greater bodies in the turbulent phase of a proto-planetary disc and planetary system evolution.
Citation: Fioroni, Savage & DeYonker 2019
Citation function: Similarities
The early history of galaxy clusters is a poorly constrained aspect of galaxy and large-scale structure formation. Hierarchical clustering models predict that massive elliptical galaxies will form in the cores of what will become the most massive galaxy clusters today, but the epoch of the bulk of star formation for these galaxies is unclear. Observations of high redshift clusters (z = 1–1.5) by the ISCS project (IRAC Shallow Cluster Survey; Eisenhardt et al. 2008) suggest that this is at z > 3, and the presence of well-defined red sequences of galaxies in clusters out to z ∼ 2 (Andreon & Huertas-Company 2011; Gobat et al. 2011; Santos et al. 2011; Pierini et al. 2012) supports this conclusion. Theoretical models by Granato et al. (2004) suggest that forming clusters will go through a phase in which multiple members will undergo near-simultaneous massive bursts of star formation. The spectral energy distribution of these objects would be dominated by the far-IR, as is the case for local massive starbursts (e.g. Clements, Dunne & Eales 2010). A galaxy cluster or protocluster (we use the term protocluster to indicate a structure that has yet to become virialized) going through such a formative phase would appear as a clump of dusty protospheroidal galaxies, and might be detected by observations in the far-IR and submm bands. Hints of such objects may already have been found by Spitzer (Magliocchetti et al. 2007) and SCUBA (Ivison et al. 2000; Priddey, Ivison & Isaak 2008; Stevens et al. 2010 and references therein). A recent study by Ivison et al. (2013) has uncovered a group of HLIRGs and ULIRGs at z = 2.41 thought to be the progenitor of a 1014.6 M⊙ cluster. At still higher redshifts, the highest redshift protocluster currently known, at z ∼ 5.3 (Riechers et al. 2010; Capak et al. 2011), and a group of objects associated with a z = 5 quasar (Husband et al. 2013) both contain at least one submm-luminous object, while the best studied group of high z submm-luminous sources to date is probably the four objects associated with a source in the GOODS-North field designated GN20, all lying at z = 4 (Daddi et al. 2009; Carilli et al. 2011), though this group is extended over a broad range of redshifts Δz ∼ 0.1.
Citation: Clements, Dunne & Eales 2010
Citation function: Background
For the TRAPPIST-1 system, data obtained by HST provide initial constraints on the extent and composition of the planet’s atmospheres, suggesting that the four innermost planets do not have a cloud/haze-free H2-dominated atmosphere (de Wit et al. 2016, 2018). However, follow-up work by Moran et al. (2018) have shown that HST data can also be fit to a cloudy/hazy H2-dominated atmosphere. Complementary to HST, NASA’s Spitzer Space Telescope—which played a major role in the discovery and orbital determination of TRAPPIST-1d, e, f, and g (Gillon et al. 2017)—has also allowed us to put additional constraints on the atmospheric composition of TRAPPIST-1b. Transit observations with Spitzer (Delrez et al. 2018) have found a +208 ± 110 ppm difference between the 3.6 and 4.2 μm bands, suggesting CO2 absorption. Spitzer also showed that transit depth measurements do not show any hint of significant stellar contamination in the 4.5 μm spectral range. Morris et al. (2018) reached the same conclusion using a “self-contamination” approach based on the Spitzer data set. Spitzer's “Red Worlds” Program encompassed over 1000 hours of observations of the TRAPPIST-1 system, whose global results have been presented (Ducrot et al. 2020). HST and Spitzer measurements have also been combined with transit light curves obtained from space with K2 (Luger et al. 2017) and from the ground with the SPECULOOS-South Observatory (Burdanov et al. 2018; Gillon 2018) and Liverpool Telescope (Steele et al. 2004) where Ducrot et al. (2018) produced featureless transmission spectra for the planets in the 0.8–4.5 μm wavelength range, showing an absence of significant temporal variations of the transit depths in the visible. Additional ground-based observations with the United Kingdom Infra-Red Telescope, Anglo-Australian Telescope, and Very Large Telescope also show no substantial temporal variations of transit depths for TRAPPIST-1 b, c, e, and g (Burdanov et al. 2019). While the K2 optical data set detected a 3.3 day periodic 1% photometric modulation, it is not present in the Spitzer observations (Delrez et al. 2018). Further constraints on the molecular weight and presence/absence of atmospheres on the TRAPPIST-1 planets will require additional observations with future facilities.
Citation: Delrez et al. 2018
Citation function: Motivation; Background
Citation: Delrez et al. 2018
Citation function: Differences
As a class, the fastest stars in our Galaxy are expected to be hypervelocity stars (HVSs). These were first theoretically predicted by Hills (1988) as the result of a three-body interaction between a binary star and the massive black hole in the Galactic Centre (GC), Sagittarius A*. Following this close encounter, a star can be ejected with a velocity ∼1000 km s−1, sufficiently high to escape from the gravitational field of the MW (Kenyon et al. 2008; Brown 2015). The first HVS candidate was discovered by Brown et al. (2005); a B-type star with a velocity more than twice the Galactic escape speed at its position. Currently about ∼20 unbound HVSs with velocities ∼300–700 km s−1 have been discovered by targeting young stars in the outer halo of the MW (Brown, Geller & Kenyon 2014). In addition, tens of mostly bound candidates have been found at smaller distances but uncertainties prevent the precise identification of the GC as their ejection location (e.g. Hawkins et al. 2015; Vickers, Smith & Grebel 2015; Zhang, Smith & Carlin 2016; Marchetti et al. 2017; Ziegerer et al. 2017). HVSs are predicted to be ejected from the GC with an uncertain rate around 10−4 yr−1 (Yu & Tremaine 2003; Zhang, Lu & Yu 2013), two orders of magnitude larger than the rate of ejection of RSs with comparable velocities from the stellar disc (Brown 2015). Because of their extremely high velocities, HVS trajectories span a large range of distances, from the GC to the outer halo. Thus, HVSs have been proposed as tools to study the matter distribution in our Galaxy (e.g. Gnedin et al. 2005; Sesana, Haardt & Madau 2007; Kenyon et al. 2014; Fragione & Loeb 2017; Rossi et al. 2017; Contigiani, Rossi & Marchetti 2018) and the GC environment (e.g. Zhang et al. 2013; Madigan et al. 2014), but a larger and less observationally biased sample is needed in order to break degeneracies between the GC binary content and the Galactic potential parameters (Rossi et al. 2017). Using the fact that their angular momentum should be very close to zero, HVSs have also been proposed as tools to constrain the solar position and velocity (Hattori, Valluri & Castro 2018a). Other possible alternative mechanisms leading to the acceleration of HVSs are the encounter between a single star and a massive black hole binary in the GC (e.g. Yu & Tremaine 2003; Sesana, Haardt & Madau 2006, 2008), the interaction between a globular cluster with a single or a binary massive black hole in the GC (Capuzzo-Dolcetta & Fragione 2015; Fragione & Capuzzo-Dolcetta 2016), and the tidal interaction of a dwarf galaxy near the centre of the Galaxy (Abadi et al. 2009). Another possible ejection origin for HVSs and high-velocity stars in our Galaxy is the Large Magellanic Cloud (LMC; Boubert & Evans 2016; Boubert et al. 2017; Erkal et al. 2018), orbiting the MW with a velocity ∼380 km s−1 (van der Marel & Kallivayalil 2014).
Citation: Yu & Tremaine 2003
Citation function: Compare/Contrast
A color–color diagram in V − R versus B − V of the individual SSCs projected against the central spiral is shown in Figure 2 (left panel). The manner in which photometry of the individual stellar clusters was performed, including background subtraction, is described at length in Lim et al. (2020). In brief, we used the StarFinder algorithm (Diolaiti et al. 2000) to perform PSF-fitting photometry that includes subtraction of the local background. Only objects that exceed 4σ
local, where σ
local is the local root-mean-square noise level, are accepted as detections. Conversions to magnitude are based on the standard zero-points in the Vega system as described in the ACS Data Handbook (Lucas et al. 2018), and corrected for Galactic extinction. Two age loci, each of which is based on a model single stellar population (SSP; i.e., population of stars all having the same age and metallicity) as taken from Zackrisson et al. (2011), are plotted in Figure 2 for different metallicities of Z = 0.4 Z
⊙ (black) and Z = Z
⊙ (yellow). The model SSPs are based on those utilized in the Starburst99 software (Leitherer et al. 1999; Vázquez & Leitherer 2005), and their colors include both continuum and line emission from H ii regions corresponding to an adopted unity filling factor for the gas left over from star formation. The resulting line emission, in particular Hα+[N ii], that is contained in the R band causes the steep rise in V − R starting below B − V ≈ 0.0 when the star clusters are younger than ∼10 Myr. The two loci having different metallicities closely overlap, such that the inferred age at a given color differs little between the two loci. The  SSCs selected for study as indicated in Figure 1 (lower row) are concentrated around B − V ≈ 0.35, corresponding to an age around ∼500 Myr (more details below). The spread in their colors is larger than can be accounted for by photometric uncertainties alone, although there may well be a degree of contamination by stellar clusters associated with the HVS, as well as the chance projection of SSCs lying beyond 5 kpc but positioned along the sightline toward the central spiral. By contrast, as shown in the color–color diagram of Figure 2 (right panel), the SSCs located beyond the central spiral (omitting the region occupied by the HVS and the outer regions of NGC 1275 dominated by GCs, as described by Lim et al. 2020) span a continuous range of ages from a few Myr to at least ∼1 Gyr, beyond which they cannot be distinguished from the even more numerous GCs around NGC 1275.
Citation: Lucas et al. 2018
Citation function: Uses