Search Results for « Disk Evolution »

It is found that a previously thought-to-be well established result of density wave theory, that there is no interaction between a quasi-stationary spiral density wave and the basic state (i.e. the axisymmetric part) of the galactic disk, is in fact false. When solved as a nonlinear and globally self-consistent problem, the presence of non-axisymmetric patterns such as spirals or bars is shown to lead to significant interaction of the basic state and the density wave, with the net result being the simultaneous acquirement of a quasi-steady wave amplitude and the secular redistribution of disk matter. The secular operation of this dynamical mechanism leads to the evolution of the Hubble type of a galaxy from late to early.

Although mutations drive the evolutionary process, the rates at which the mutations occur are themselves subject to evolutionary forces. Our purpose here is to understand the role of selection and random genetic drift in the evolution of mutation rates, and we address this question in asexual populations at mutation-selection equilibrium neglecting selective sweeps. Using a multitype branching process, we calculate the fixation probability of a rare nonmutator in a large asexual population of mutators, and find that a nonmutator is more likely to fix when the deleterious mutation rate of the mutator population is high. Compensatory mutations in the mutator population are found to decrease the fixation probability of a nonmutator when the selection coefficient is large. But, surprisingly, the fixation probability changes nonmonotonically with increasing compensatory mutation rate when the selection is mild. Using these results for the fixation probability and a drift-barrier argument, we find a novel relationship between the mutation rates and the population size. We also discuss the time to fix the nonmutator in an adapted population of asexual mutators, and compare our results with experiments.

We used the Disk Detective citizen science project and the BANYAN II Bayesian analysis tool to identify a new candidate member of a nearby young association with infrared excess. WISE J080822.18-644357.3, an M5.5-type debris disk system with significant excess at both 12 and 22 $μ$m, is a likely member ($\sim 90\%$ BANYAN II probability) of the $\sim 45$ Myr-old Carina association. Since this would be the oldest M dwarf debris disk detected in a moving group, this discovery could be an important constraint on our understanding of M dwarf debris disk evolution.

We present a brief discussion of the evolution of disk galaxy stellar masses, sizes, rotation velocities, and star formation rates over the last eight billion years. Recent observations have failed to detect significant evolution in the stellar mass Tully-Fisher relation, stellar mass-size relation, and the stellar mass function of disk galaxies. Yet, most z<1 star formation is in disks, and this star formation would be expected to drive a rapid growth of the total stellar mass (and therefore mass function) of disks in the last eight billion years. Such a build-up is not seen; instead, a rapid build-up in the total stellar mass in non-star-forming spheroid-dominated galaxies is observed. Large numbers of disk-dominated galaxies are systematically shutting off their star formation and building up a spheroid (or losing a disk) in the epoch 0<z<1.

We present Chandra X-ray observations of 6 previously-identified Peter Pan objects, rare 40 Myr systems with evidence of primordial disk retention. We observe X-ray luminosities (0.8-3.0 keV) ranging from log Lx 27.7-29.1. We find that our Peter Pan sample exhibits X-ray properties similar to that of weak-lined T-Tauri stars and do not exhibit evidence of stellar accretion induced X-ray suppression. Our observed Peter Pan X-ray luminosities are consistent with that measured for field dM stars of similar spectral type and age, implying their long primordial disk lifetimes are likely not a consequence of unusually faint X-ray host stars. Our derived X-ray photoevaporative mass loss rates predict our systems have passed the point of rapid gas dispersal and call into question the impact of this internal mechanism for primordial disk dispersal around dM stars. Our qualitative assessment of the surrounding Peter Pan environments also does not predict unusually low levels of external photoevaporation relative to other respective moving group members. Overall, our results suggest Peter Pan disks may be a consequence of the low FUV flux incident on the disk in low-mass DM stars given their relatively lower levels of accretion over the course of their pre-main-sequence evolution.