[en] Spiral arms have been observed in more than a dozen protoplanetary disks, yet the origin of nearly all systems is under debate. Multi-epoch monitoring of spiral arm morphology offers a dynamical way to distinguish two leading arm formation mechanisms: companion-driven and gravitational instability induction, since these mechanisms predict distinct motion patterns. By analyzing multi-epoch J-band observations of the SAO 206462 system using the SPHERE instrument on the Very Large Telescope in 2015 and 2016, we measure the pattern motion for its two prominent spiral arms in polarized light. On one hand, if both arms are comoving, they can be driven by a planet at ${86}_{-<!--\; ???\; -->13}^{+18}$ au on a circular orbit, with gravitational instability motion ruled out. On the other hand, they can be driven by two planets at ${120}_{-<!--\; ???\; -->30}^{+30}$ au and ${49}_{-<!--\; ???\; -->5}^{+6}$ au, offering tentative evidence (3.0σ) that the two spirals are moving independently. The independent arm motion is possibly supported by our analysis of a re-reduction of archival observations using the NICMOS instrument on board the Hubble Space Telescope (HST) in 1998 and 2005, yet artifacts including shadows can manifest spurious arm motion in HST observations. We expect future re-observations to better constrain the motion mechanism for the SAO 206462 spiral arms.

[en] The Jeans instabilities in an unmagnetized, collisionless, isotropic self-gravitating matter system are investigated in the context of $\mathrm{\kappa <!--\; ??\; -->}$-deformed Kaniadakis distribution based on kinetic theory. The result shows that both the growth rates and critical wave numbers of Jeans instability are lower in the $\mathrm{\kappa <!--\; ??\; -->}$-deformed Kaniadakis distributed self-gravitating matter systems than the Maxwellian case. The standard Jeans instability for a Maxwellian case is recovered under the limitation $\mathrm{\kappa <!--\; ??\; -->}=0$. (paper)

[en] We illustrate the derivations of Jeans' criteria for the gravitational instabilities in a static homogeneous Newtonian system for pedagogical objectives. The critical Jeans density surface is presented in terms of dimensionless sound speeds and (characteristic) length scales

[en] We present a revised method for simultaneous determination of the pattern speed (Ω_P) and star formation timescale (t _SF) of spiral galaxies, which is originally proposed in our previous work. As this method utilizes offsets between molecular and young-stellar arms, we refer to it as the 'Offset Method'. Details of the method, its application, and results for CO and Hα images of 13 nearby spiral galaxies are described here. CO data are from our observations with the Nobeyama Millimeter Array for two galaxies, and from the BIMA SONG for the rest. Out of 13 galaxies, we were able to derive Ω_P and t _SF for five galaxies. We categorize them as 'C' galaxies as their offsets are clear. Our findings from these galaxies are as follows. (1) The corotation radius calculated by the derived Ω_P is close to the edge of the CO data, and is about half of the optical radius for three galaxies. (2) The derived t _SF is roughly consistent with the free-fall time of typical molecular clouds, which indicates that the gravitational instability is the dominant mechanism triggering star formation in spiral arms. (3) The t _SF is found to be almost independent of the surface density of molecular gas, metallicity, or spiral arm strengths. The number of 'C' galaxies and the quality of CO data, however, are not enough to confirm these relationships. We also find that two other galaxies show no offsets between CO and Hα, although their arms are clearly traced, and categorize them as 'N' galaxies. The presence of a bar could account for this feature, since these two galaxies are both barred. With one galaxy excluded from our analysis due to its poor rotation curve, offsets of the remaining five galaxies are found to be ambiguous. Either their dependence on the rotational frequency cannot be explained by our picture, or the number or quality of data is not sufficient for the analysis. We categorize them as 'A' galaxies. The possible reasons for this ambiguity are (1) the density wave is weaker, and/or (2) observational resolution and sensitivity are not enough to detect the spiral arms and their offsets clearly. The former is supported by our finding that the arm strengths of 'A' galaxies are slightly weaker than that of 'C' galaxies.

[en] The influence of dust charge gradient force and ion drag force on the fragmentation of unmagnetized, self-gravitating dust cloud has been studied. The thermal electrons satisfy the Boltzmann relation, while inertialess ions are affected by the ion-neutral collisions. The dynamics of dusty fluid are modified by ion drag, charge gradient, and gravitational forces. The onset criterion of pinching instability and gravitational instability is derived. The pinching instability depends upon the critical ion drag coefficient and dust charge variation parameter. In the laboratory complex plasma, with finite dust charge variations, the ion drag coefficient larger than the critical value causes pinching instability. This results in the fragmentation of the dusty cloud, which is affected due to the dust charge variations. The ion drag coefficient has destabilizing, while the dust charge variation parameter has stabilizing influence on the growth rate of the linear gravitational instability. The results have been discussed to understand the dust cloud collapse in the astrophysical system. (paper)

[en] Disk fragmentation resulting from the gravitational instability has been proposed as an efficient mechanism for forming giant planets. We use the planet Fomalhaut b, the triple-planetary system HR 8799, and the potential protoplanet associated with HL Tau to test the viability of this mechanism. We choose the above systems since they harbor planets with masses and orbital characteristics favored by the fragmentation mechanism. We do not claim that these planets must have formed as the result of fragmentation, rather the reverse: if planets can form from disk fragmentation, then these systems are consistent with what we should expect to see. We use the orbital characteristics of these recently discovered planets, along with a new technique to more accurately determine the disk cooling times, to place both lower and upper limits on the disk surface density-and thus mass-required to form these objects by disk fragmentation. Our cooling times are over an order of magnitude shorter than those of Rafikov, which makes disk fragmentation more feasible for these objects. We find that the required mass interior to the planet's orbital radius is ∼0.1 M_sun for Fomalhaut b, the protoplanet orbiting HL Tau, and the outermost planet of HR 8799. The two inner planets of HR 8799 probably could not have formed in situ by disk fragmentation.

[en] We study the properties of luminous stellar 'clumps' identified in deep, high-resolution Hubble Space Telescope NIC2/F160W imaging at 1.6 μm of six z ∼ 2 star-forming galaxies with existing near-infrared integral field spectroscopy from SINFONI at the Very Large Telescope. Individual clumps contribute ∼0.5%-15% of the galaxy-integrated rest-frame ∼5000 A emission, with median of ∼2%; the total contribution of clump light ranges from 10% to 25%. The median intrinsic clump size and stellar mass are ∼1 kpc and ∼10⁹ M_sun, in the ranges for clumps identified in rest-UV or line emission in other studies. The clump sizes and masses in the subset of disks are broadly consistent with expectations for clump formation through gravitational instabilities in gas-rich, turbulent disks given the host galaxies' global properties. By combining the NIC2 data with Advanced Camera for Surveys (ACS)/F814W imaging available for one source, and adaptive-optics-assisted SINFONI Hα data for another, we infer modest color, M/L, and stellar age variations within each galaxy. In these two objects, sets of clumps identified at different wavelengths do not fully overlap; NIC2-identified clumps tend to be redder/older than ACS- or Hα-identified clumps without rest-frame optical counterparts. There is evidence for a systematic trend of older ages at smaller galactocentric radii among the clumps, consistent with scenarios where inward migration of clumps transports material toward the central regions. From constraints on a bulge-like component at radii ∼< 1-3 kpc, none of the five disks in our sample appears to contain a compact massive stellar core, and we do not discern a trend of bulge stellar mass fraction with stellar age of the galaxy. Further observations are necessary to probe the buildup of stellar bulges and the role of clumps in this process.

[en] To determine the origin of the spiral structure observed in the dust continuum emission of Elias 2–27 we analyze multiwavelength continuum ALMA data with a resolution of ∼0.″2 (∼23 au) at 0.89, 1.3, and 3.3 mm. We also study the kinematics of the disk with ¹³CO and C¹⁸O ALMA observations in the J = 3–2 transition. The spiral arm morphology is recovered at all wavelengths in the dust continuum observations, where we measure contrast and spectral index variations along the spiral arms and detect subtle dust-trapping signatures. We determine that the emission from the midplane is cold and interpret the optical depth results as signatures of a disk mass higher than previous constraints. From the gas data, we search for deviations from Keplerian motion and trace the morphology of the emitting surfaces and the velocity profiles. We find an azimuthally varying emission layer height in the system, large-scale emission surrounding the disk, and strong perturbations in the channel maps, colocated with the spirals. Additionally, we develop multigrain dust and gas hydrodynamical simulations of a gravitationally unstable disk and compare them to the observations. Given the large-scale emission and highly perturbed gas structure, together with the comparison of continuum observations to theoretical predictions, we propose infall-triggered gravitational instabilities as the origin for the observed spiral structure.

[en] Direct imaging searches have begun to detect planetary and brown dwarf companions and to place constraints on the presence of giant planets at large separations from their host star. This work helps to motivate such planet searches by predicting a population of young giant planets that could be detectable by direct imaging campaigns. Both the classical core accretion and the gravitational instability model for planet formation are hard pressed to form long-period planets in situ. Here, we show that dynamical instabilities among planetary systems that originally formed multiple giant planets much closer to the host star could produce a population of giant planets at large (∼ 10²-10⁵ AU) separations. We estimate the limits within which these planets may survive, quantify the efficiency of gravitational scattering into both stable and unstable wide orbits, and demonstrate that population analyses must take into account the age of the system. We predict that planet scattering creates detectable giant planets on wide orbits that decreases in number on timescales of ∼ 10 Myr. We demonstrate that several members of such populations should be detectable with current technology, quantify the prospects for future instruments, and suggest how they could place interesting constraints on planet formation models.

[en] Gravitational instability of the distribution of stars in a galaxy is a well-known phenomenon in astrophysics. This report is an attempt to analyze this phenomenon by applying standard tools developed in accelerator physics. It is found that a nonrotating galaxy would become unstable if its size exceeds a certain limit that depends on its mass density and its velocity spread.