Tapered diblock copolymers are similar to typical AB diblock copolymers but have an added transition region between the two blocks which changes gradually in composition from pure A to pure B. This tapered region can be varied from 0% (true diblock) to 100% (gradient copolymer) of the polymer length, and this allows some control over the microphase separated domain spacing and other material properties. We perform molecular dynamics simulations of linearly tapered block copolymers with tapers of various lengths, initialized from fluids density functional theory predictions. To investigate the effect of sequence dispersity, we compare systems composed of identical polymers, whose taper has a fixed sequence that most closely approximates a linear gradient, with sequentially disperse polymers, whose sequences are created statistically to yield the appropriate ensemble average linear gradient. Especially at high segregation strength, we find clear differences in polymer conformations and microstructures between these systems. Importantly, the statistical polymers are able to find more favorable conformations given their sequence, for instance, a statistical polymer with a larger fraction of A than the median will tend towards the A lamellae. The conformations of the statistically different polymers can thus be less stretched, and these systems have higher overall density. Consequently, the lamellae formed by statistical polymers have smaller domain spacing with sharper interfaces. Published by AIP Publishing. [http://dx
The first interstellar object, 1I/2017 U1 (‘Oumuamua), exhibited several unique properties, including an extreme aspect ratio, a lack of typical cometary volatiles, and a deviation from a Keplerian trajectory. Several authors have hypothesized that the non-gravitational acceleration was caused by either cometary outgassing or radiation pressure. Here, we investigate the spin dynamics of ‘Oumuamua under the action of high-surface-area fractional activity and radiation pressure. We demonstrate that a series of transient jets that migrate across the illuminated surface will not produce a secular increase in the spin rate. We produce 3D tumbling simulations that approximate the dynamics of a surface-covering jet and show that the resulting synthetic light curve and periodogram are reasonably consistent with the observations. Moreover, we demonstrate that radiation pressure also produces a steady spin state. While carbon monoxide (CO) has been dismissed as a possible accelerant because of its non-detection in emission by Spitzer, we show that outgassing from a surface characterized by a modest covering fraction of CO ice can satisfy the non-ballistic dynamics for a plausible range of assumed bulk densities and surface albedos. Spitzer upper limits on CO emission are, however, inconsistent with the CO production necessary to provide the acceleration. Nonetheless, an ad hoc but physically plausible explanation is that the activity level varied greatly during the time that the trajectory was monitored. We reproduce the astrometric analysis presented in Micheli et al., and verify that the non-gravitational acceleration was consistent with stochastic changes in outgassing.
Based on the occurrence rates implied by the discoveries of 1I/‘Oumuamua and 2I/Borisov, the forthcoming Rubin Observatory Legacy Survey of Space and Time (LSST) should detect ≥one interstellar object every year. We advocate for future measurements of the production rates of H2O, CO2, and CO in these objects to estimate their carbon-to-oxygen ratios, which trace formation locations within their original protoplanetary disks. We review similar measurements for solar system comets, which indicate formation interior to the CO snow line. By quantifying the relative processing in the interstellar medium and solar system, we estimate that production rates will not be representative of primordial compositions for the majority of interstellar comets. Preferential desorption of CO and CO2 relative to H2O in the interstellar medium implies that measured C/O ratios represent lower limits on the primordial ratios. Specifically, production rate ratios of Q(CO)/Q(H2O) < 0.2 and Q(CO)/Q(H2O) > 1 likely indicate formation interior and exterior to the CO snow line, respectively. The high C/O ratio of 2I/Borisov implies that it formed exterior to the CO snow line. We provide an overview of the currently operational facilities capable of obtaining these measurements that will constrain the fraction of ejected comets that formed exterior to the CO snow line. This fraction will provide key insights into the efficiency of and mechanisms for cometary ejection in exoplanetary systems.
The compositional and morphological evolution of minor bodies in the solar system is primarily driven by the evolution of their heliocentric distances, as the level of incident solar radiation regulates cometary activity. We investigate the dynamical transfer of Centaurs into the inner solar system, facilitated by mean motion resonances with Jupiter and Saturn. The recently discovered object P/2019 LD2 will transition from the Centaur region to the inner solar system in 2063. In order to contextualize LD2, we perform N-body simulations of a population of Centaurs and Jupiter-family comets. Objects between Jupiter and Saturn with Tisserand parameter T J ∼ 3 are transferred onto orbits with perihelia q < 4 au within the next 1000 yr with notably high efficiency. Our simulations show that there may be additional LD2-like objects transitioning into the inner solar system in the near future, all of which have low ΔV with respect to Jupiter. We calculate the distribution of orbital elements resulting from a single Jovian encounter and show that objects with initial perihelia close to Jupiter are efficiently scattered to q < 4 au. Moreover, approximately 55% of the transitioning objects in our simulated population experience at least one Jovian encounter prior to reaching q < 4 au. We demonstrate that a spacecraft stationed near Jupiter would be well positioned to rendezvous, orbit-match, and accompany LD2 into the inner solar system, providing an opportunity to observe the onset of intense activity in a pristine comet in situ. Finally, we discuss the prospect of identifying additional targets for similar measurements with forthcoming observational facilities.
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