adsabs/FOCAL huggingface dataset:
Very small asteroids (VSAs) are objects with diameters D  150 m. They often rotate with periods shorter than 2 h enabling us to study their internal structure by comparing the centrifugal force with the material forces holding them together (Holsapple 2007). Because of their small sizes, VSAs are sensitive to the YORP effect (Rubincam 2000), which is a torque induced on the rotating body by the thermal radiation emitted by its surface complemented by a torque produced by scattered sunlight. YORP can either spin up or slow down the asteroid rotation as well as change the obliquity of its spin axis ϵ, which is an angle between the normal to the asteroid orbital plane and its rotation axis. While the fast rotation has been observed for many VSAs, their spin axes were not determined except for one object1, (54 509) YORP (whose name is the same as the name of the effect itself). (54 509) was the first asteroid for which the effect of YORP has been observed (Lowry et al. 2007; Taylor et al. 2007). The obliquity of the (54 509) spin axis is ϵ = 173°, which means it is nearly perpendicular to the asteroid orbital plane. Such an orientation of the spin axis was found as an end state of the YORP evolution in the simulations performed by Čapek & Vokrouhlický (2004) for objects with finite surface thermal conductivity. If their prediction is true, then for the VSAs that experienced a strong YORP effect for a long time (and the fastest rotating VSAs are such objects), we should observe spin axis obliquities close to 0° or 180°. Recently Golubov et al. (2021) have shown that for very small objects of a highly irregular shape, the transverse heat conduction (TYORP) can add new asymptotic states for the obliquity. For this reason it would be interesting to verify those predictions with observationsof VSAs. To do that, we should observe their lightcurves at many different positions on the sky to be able to determine their spin axes. For near-Earth objects (NEAs) this condition is met either by the Earth co-orbital asteroids – and (54 509) is an example of such objects – or by objects for which their close encounter with the Earth can be observed along a long arc on the sky.
Citation: Lowry et al. 2007
Citation function: Background
An alternative possibility to reduce the fraction of X-ray sources in the WISE R75 wedge is to allow a large fraction of heavily obscured AGN. This can be achieved for example, by increasing the fraction of Compton-thick AGN in the X-ray luminosity function above the current assumption of 34 per cent, or by increasing the scatter in the relation that links X-ray obscuration to optical extinction beyond the adopted value of 0.5 dex. Relaxing the above model assumptions would allow heavily obscured and hence, X-ray faint, sources to be selected by the WISE R75 criteria. There is indeed evidence for a potentially large population of heavily obscured, possibly Compton-thick AGN, among the WISE population (e.g. Assef et al. 2015; Mountrichas et al. 2017; Yan et al. 2019). The SDSS spectroscopic follow-up programme presented by LaMassa et al. (2019) also revealed a non-negligible number of WISE R75 sources that are optically faint ($r\gtrsim 22$ mag), lie at redshifts $z\lesssim 1$ and are spectroscopically identified by their prominent [O ii] 3727 emission lines. These sources are prime candidates for heavily obscured AGN. Our baseline model predicts that the most heavily obscured, Compton-thick, WISE AGN are at low redshift, $z\lesssim 0.6$ (see Fig. 19) and relatively optically bright. The top panel of Fig. 22 demonstrates the latter point by plotting the r-band distribution of the Compton-thick AGN population predicted by the model. Observationally, the identification of such AGN needs to account for the relatively high level of contamination of the WISE R75 AGN selection by star-forming galaxies at redshifts $z\lesssim 0.6$ (s1ee Fig. 14). One approach to achieve this is via diagnostic optical emission-line ratios (e.g. Kewley et al. 2001) to separate star-forming galaxies from Seyfert-2s. Observations at hard X-rays can also provide useful information on the nuclear activity of a galaxy and the level of line-of-sight obscuration. The bottom panel of Fig. 22 shows the expected 2–10 keV X-ray flux distribution of the Compton-thick AGN predicted by the model. The expected fluxes have already been reached in deep X-ray survey fields, for example, COSMOS-Legacy (Civano et al. 2016). Study of the X-ray spectral properties of WISE-selected AGN in such fields can test the baseline model predictions for the demographics of Compton-thick AGN in the WISE R75 wedge.
Citation: Assef et al. 2015
Citation function: Background
Similarly to long bursts, short GRBs are thought to be produced by a relativistic jet with a typical half-opening angle θjet ~ 5–15 deg (Fong et al. 2016). However, whether or not BNS mergers can always efficiently produce a relativistic jet is still debated (Paschalidis et al. 2015; Ruiz et al. 2016; Kawamura et al. 2016). Given the small probability that our line of sight was within the jet half-opening angle, 1 − cos(θjet), it is unlikelythat the first short GRB associated to a GW event had a jet pointing towards the Earth. The extremely low γ-ray luminosity of GRB 170817A has been interpreted as being due to (i) the debeamed radiation of a jet observed off-beam (i.e. viewing angle θview > θjet), provided that the jet bulk Lorentz factor is significantly smaller than usually assumed (see, e.g. Pian et al. 2017). Alternatively, the jet could be (ii) structured, with a fast and energetic inner core surrounded by a slower, less energetic layer/sheath/cocoon (first proposed for long GRBs – Lipunov et al. 2001; Rossi et al. 2002; Salafia et al. 2015 – and only recently extended to short GRBs – Kathirgamaraju et al. 2018; Lazzati et al. 2017a; Gottlieb et al. 2017; Lazzati et al. 2017b; Lyman et al. 2018; Margutti et al. 2018; Troja et al. 2018a). In this scenario the faint, off-beam emission is due to the slower component, which originates from the interaction of the jet with the merger dynamical ejecta or the post-merger winds. Recently, Mooley et al. (2018) suggested the possibility that (iii) the jet was not successful in excavating its way through the ambient medium and that GRB 170817A was due to its vestige, a quasi-isotropic cocoon with a velocity profile. Last but not least, (iv) a jet-less interpretation of GRB 17017A could still be viable: an isotropic fireball, expanding ahead of the kilonova ejecta, which could account for both the low luminosity of the γ-ray event and the properties of the EM component in the radio and X-ray bands (Salafia et al. 2017). In this case, all BNS mergers should have this kind of faint, hard X-ray counterpart. All of the above scenarios have relatively clear predictions for the temporal and spectral evolution of the electromagnetic emission from X-rays to the radioband. A comprehensive discussion of the possible physical scenarios leading to the observed broad-band emission of GW 170817/GRB 170817A can be found in Nakar & Piran (2018). Recent radio and X-ray observations(Mooley et al. 2018; Margutti et al. 2018; Ruan et al. 2018; Troja et al. 2018a), carried out until ~ 110−115 d after the event, indicate that the source flux is steadily rising and that the spectral energy distribution (SED) over these bands is consistent with a single power-law component. These results disfavour interpretation (i) reported above (an off-beam homogeneous jet).
Citation: Salafia et al. 2015
Citation function: Uses
The period candidates of other three ULXs may range from ∼100 to ∼600 d. Apart from noise and artefacts, all the candidate periods are only significant in a specific epoch. This suggests that they are not associated with any stable mechanism such as orbital motion. Instead, such long-term (> 100 d) X-ray quasi-periodic variations are likely related to superorbital periods that are thought to be due to radiation-driven warping of accretion discs (Ogilvie & Dubus 2001) or tidal interaction-induced disc precession (Whitehurst & King 1991). Alternatively, mass transfer rate-related events such as X-ray state changes and disc instability can also cause long-term modulations (Kotze & Charles 2012). In particular, there are two intermittent quasi-periodicities for both NGC 5408 X-1 and M81 X-6, suggesting that the quasi-periods are changing or evolving. They resemble some Galactic X-ray binaries that show similar behaviour (e.g. Cyg X–2 and SMC X–1; Kotze & Charles 2012) and it has been suggested that a warped disc could lead to an unstable steadily precessing disc, causing quasi-periodic behaviour (Ogilvie & Dubus 2001). We note that there are many uncertainties on the physical parameters of ULXs. To determine the origin of superorbital periods of ULXs, one has to know at least the mass ratio between the companion and the compact star (q = MC/MX) and the binary separation. Unfortunately, it is very difficult to get these parameters for ULXs. For the three ULXs discussed here (i.e. excluding ESO 243-49 HLX-1), only M81 X-6 has better constraints on the black hole mass and the nature of the companion. The masses of the black hole and companion star are estimated (MX = 18 M⊙, MC = 23 M⊙) such that q can be derived. In this case, we can rule out a tidal interaction-induced disc precession scenario that requires q  0.25–0.33 (Whitehurst & King 1991). For a warped disc, the binary separation and the mass ratio suggest that M81 X-6 lies in the intermediate instability zone for radiation-driven warping in X-ray binaries (see fig. 1 of Kotze & Charles 2012). The quasi-periodic variability may represent the switching time-scale between a warped disc and a flat disc.
Citation: Kotze & Charles 2012
Citation function: Compare/Contrast
Citation: Kotze & Charles 2012
Citation function: Compare/Contrast
Citation: Kotze & Charles 2012
Citation function: Similarities
The year 2020 marks the 350 yr anniversary of the discovery of the eruption of Nova 1670 (or CK Vul) made by European astronomers (Shara et al. 1985). Their observations, predominantly performed with a naked eye, traced the object’s evolution on the sky in 1670−1672. From the archive records, we know that the eruption was rather unusual, in particular it was very much unlike classical novae. The light curve of CK Vul displayed three peaks and the star was described as reddish in the later stages of the eruption (Hevelius 1671; Shara et al. 1985). These characteristics resemble closest the behavior often observed in (luminous) red novae (Kato 2003; Tylenda et al. 2013), a modern category of eruptive stars known from our and other galaxies (e.g. Pastorello et al. 2019). Red novae are recognized as manifestations of on-going mergers of non-compact stars such as main-sequence dwarfs, sub-giants, or red giants (Soker & Tylenda 2003; Tylenda & Soker 2006; Tylenda et al. 2011; Pastorello et al. 2019). While the number of known red novae, mainly extragalactic ones, is quickly rising (e.g., Stritzinger et al. 2020), we know only a few red-nova remnants that are decades old (Kamiński et al. 2018a). The remnant of the 1670 eruption of CK Vul, as a candidate post-merger site, could be the oldest (counting from the onset of the eruption) known object of this type and as such offers the opportunity to investigate a merger aftermath centuries after the stellar coalescence. The nature of the progenitor system of CK Vul has been debated. Eyres et al. (2018) proposed that the seventeenth-century merger took place between a white dwarf and a brown dwarf, but there is little quantitative evidence to support this. Based on the analysis of the source’s chemical and isotopic composition, including the unique presence of the radioactive isotope of 26Al, Kamiński et al. (2018b) found that the progenitor system of CK Vul included at least one red-giant-branch (RGB) star with a fully developed helium core.
Citation: Pastorello et al. 2019
Citation function: Background
Citation: Pastorello et al. 2019
Citation function: Background; Background
The leading edges of the transients normally leave bright traces in the images of visible light, inspiring many methods that were developed to derive their locations and velocities, such as the icecream cone model (Fisher & Munro 1984), the graduated cylindrical shell (GCS) model (Thernisien 2011), geometric triangulation methods (Liu et al. 2010), mask-fitting methods (Feng et al. 2012), and trace-fitting methods including the point-p, fixed-Φ, harmonic mean, and self-similar expansion fitting methods (e.g., Sheeley et al. 1999; Howard et al. 2006; Davies et al. 2012; Möstl & Davies 2013). To derive the velocity distribution inside one transient rather than only at its leading edge, some other techniques have been proposed. Colaninno & Vourlidas (2006) applied an optical flow tool to extract the velocity vector of a coronal mass ejection (CME) in digital images. Feng et al. (2015) derived the radial velocity profiles of the whole CME from the spatial distribution of its density given by the mass continuum equation. A cross-correlation method was applied to derive continuous 2D speed maps of a CME from coronagraphic images by Bemporad et al. (2018). In their work, the radial shift pixel by pixel is determined by maximizing the cross correlation between the signal in a radial window at one frame and the signal in a radial shifted window at the previous frame, and the radial speed just equals the radial shift over the time interval between the two frames. Ying et al. (2019) improved this cross-correlation method by analyzing data in three steps: forward step (FS), backward step (BS), and average step (AS). In the FS (BS), the 2D velocity map between the current and the previous (next) frame is constructed with almost the same method as Bemporad et al. (2018). In the AS the average, velocity is obtained from the FS and BS. The velocities derived by all these methods are the component of the flow velocity vector projected onto the POS. This may underestimate the velocity especially for transients that do not propagate in the POS. Methods such as the polarizaition ratio technique (Moran & Davila 2004; DeForest et al. 2017) or the local correlation tracking (LCT) method (Mierla et al. 2009) can derive the 3D geometric information of the whole transients, but not the velocity distribution. Bemporad et al. (2018) chose the propagating direction averaged over the whole CME derived by the polarization ratio technique to correct the radial speed in the 2D maps, but the key information along the LOS is still lacking.
Citation: Bemporad et al. (2018)
Citation function: Uses
Citation: Bemporad et al. (2018)
Citation function: Uses
Citation: Bemporad et al. (2018)
Citation function: Compare/Contrast
In Figure 6, we plot the histogram distributions of the calculated number of Galactic BH–He XRBs as a function of the companion mass, BH mass, orbital period, and orbital eccentricity for Models A–D. Since the progenitor systems containing a BH and an OB star possess similar distributions of the OB star masses and the orbital periods in all models, the BH–He XRBs also have similar distributions of helium masses and orbital periods under the assumptions of different models. The masses of the helium stars are mainly distributed in the range of 





, and the orbital periods are distributed in a wide range (∼0.1–1000 days) with a peak at ∼30–100 days. Some recent investigations showed that the mass-transfer process in the lobe-filling BH binaries is more stable than previously expected (e.g., Pavlovskii et al. 2017; Shao & Li 2018); the maximal mass ratio of the donor star to the BH for stable mass transfer can reach as high as ∼6. Hence only a few BH–He XRBs with orbital periods of ≲1 day are produced in our calculations. Remarkably, such wind-fed XRBs in close orbits may be bright enough to be detected. In nearby galaxies, there also exist a couple such close XRBs (Esposito et al. 2015, and references therein). During the evolution of the progenitor binaries, mass accretion of the BH can increase its mass. We see that the BH masses in the BH–He XRBs are distributed in a broad range of 





. Model A predicts that BHs are more likely to possess masses of 





 since more binaries survive from BH formation via direct collapse without natal kicks, while Models B–D tend to produce light BHs with mass distributions that peak at 





 due to the IMF. Differently from the BH systems with an OB star, almost all BH–He XRBs have relatively low eccentricities of ≲0.4, whose distribution has two distinct peaks at ≲0.1 and ∼0.3. Tides and mass transfer between binary components tend to circularize the orbits during the progenitor system evolution. Our obtained low eccentricities can coincide with the observations of Galactic W-R−O binaries (van der Hucht 2001) that are also post-mass-transfer systems; although, we do not include a detailed treatment for the orbital evolution of mass-transferring eccentric binaries (e.g., Sepinsky et al. 2009; Dosopoulou & Kalogera 2016).
Citation: Shao & Li 2018
Citation function: Uses; Uses
The opacity file used for the PAH/VSG population (draine_opac_new.dat) was computed by Bruce Draine (Draine & Li 2007) and is available at https://github.com/hyperion-rt/hyperion-pah/tree/master/PAH-Legacy/input. It was computed taking two lognormal size distributions, one with grain radii below 20 Å and another for grains with radii from 20 to 200 Å. PAHs constitute a mass fraction of 4.5 per cent of this grain population and they have the size-dependent fractional ionization of the diffuse ISM (Wood et al. 2008). The presence of the 8.6-μm feature in the Spitzer IRS spectrum of MPI13 implies the existence of ionized PAHs. The PAH/VSG grain population is not in thermal equilibrium with the radiation field. However, their emissivities can be computed based on specific energy absorbed in each grid cell and assuming that the emissivity is the function of the mean intensity of the radiation field, which approximates the spectral shape to the first order (Robitaille et al. 2012; Whitney et al. 2013). The code uses the pre-computed emissivity tables of Draine & Li (2007) for different specific energy. As a Monte Carlo energy pocket is absorbed by a PAH/VSG, the pocket is reprocessed, sampling a new frequency from pre-computed emissivity tables. A detailed discussion on this can be found in Wood et al. (2008). The updated version of Hochunk3D is hence quite useful in modelling the circumstellar shells around evolved stars, which have thermally fluctuating dust. The code uses the method of Lucy (1999) to calculate the dust temperature very efficiently by summing up photon path-lengths. The temperature of the grid cell is updated when an iteration of simulation is completed, and the temperature converges in three to five iterations; this efficiency is due to the fact that flux is conserved exactly across all surfaces (Lucy 1999). By constructing Monte Carlo radiation fields that are rigorously divergence-free, rapid convergence is achieved with the temperature correction procedure in this method. We have performed the Lucy temperature correction of dust with five iterations.
Citation: Robitaille et al. 2012
Citation function: Uses
Our group is addressing this problem by combining a variety of complementary perspectives: (i) by constructing a new generation of high-quality star density profiles derived from star counts instead of surface brightness (see Lanzoni et al. 2007a, 2010, 2019; Miocchi et al. 2013; Pallanca et al. 2021); (ii) by investigating the population of stellar exotica (Ferraro et al. 2001, 2003, 2015, 2016; Pallanca et al. 2010, 2013,2014, 2017; Cadelano et al. 2017, 2018, 2020) and their connection with the dynamical evolution of the parent cluster (see Ferraro et al. 2009, 2012, 2018a, 2019; Lanzoni et al. 2016); (iii) by characterizing the three-dimensional (3D) global velocity space through the analysis of the velocity dispersion profile and rotation curve from resolved star spectroscopy (Lanzoni et al. 2013, 2018a, 2018b; Ferraro et al. 2018b) and proper motions (PMs; see Raso et al. 2020). The determination of GGC internal kinematics from resolved star velocities is particularly relevant and challenging. In this context we promoted the ESO-VLT Multi-Instrument Kinematic Survey (hereafter the MIKiS survey; Ferraro et al. 2018b, 2018c), a project specifically designed to characterize the kinematical properties of a sample of GGCs in different dynamical evolutionary stages from the radial velocities (RVs) of hundreds of individual stars distributed over the entire radial range of each stellar system. To this end, the survey fully exploits the spectroscopic capabilities of different instruments currently available at the ESO Very Large Telescope (VLT): originally designed to use the adaptive optics (AO) assisted integral-field spectrograph SINFONI, the multiobject integral-field spectrograph KMOS, and the multiobject fiber-fed spectrograph FLAMES/GIRAFFE, it has been recently complemented with individual projects and an ongoing large program (PI: Ferraro) fully exploiting the remarkable performances of the AO-assisted integral-field spectrograph MUSE.
Citation: Lanzoni et al. 2013
Citation function: Uses; Uses
Citation: Ferraro et al. 2018b
Citation function: Motivation; Motivation