How primordial is the structure of comet 67P/C-G? Combined collisional and dynamical models suggest a late formation

There is an active debate about whether the properties of comets as observed today are primordial or, alternatively, if they are a result of collisional evolution or other processes. We investigate the effects of collisions on a comet with a structure like 67P/C-G. We develop scaling laws for the critical specific impact energies required for a significant shape alteration. These are then used in simulations of the combined dynamical and collisional evolution of comets in order to study the survival probability of a primordially formed object with a shape like 67P/C-G. The effects of impacts on comet 67P/C-G are studied using a SPH shock physics code. The resulting critical specific impact energy defines a minimal projectile size which is used to compute the number of shape-changing collisions in a set of dynamical simulations. These simulations follow the dispersion of the trans-Neptunian disk during the giant planet instability, the formation of a scattered disk, and produce 87 objects that penetrate into the inner solar system with orbits consistent with the observed JFC population. The collisional evolution before the giant planet instability is not considered here. Hence, our study is conservative in its estimation of the number of collisions. We find that in any scenario considered here, comet 67P/C-G would have experienced a significant number of shape-changing collisions, if it formed primordially. This is also the case for generic bi-lobe shapes. Our study also shows that impact heating is very localized and that collisionally processed bodies can still have a high porosity. Our study indicates that the observed bi-lobe structure of comet 67P/C-G may not be primordial, but might have originated in a rather recent event, possibly within the last 1 Gy. This may be the case for any kilometer-sized two-component cometary nuclei.Comment: Astronomy & Astrophysics, accepted pending minor revision

SHINeS : Space and High-Irradiance Near-Sun Simulator

We present SHINeS, a space simulator which can be used to replicate the thermal environment in the immediate neighborhood of the Sun down to a heliocentric distance r ~ 0.06 au. The system consists of three main parts: the solar simulator which was designed and constructed in-house, a vacuum chamber, and the probing and recording equipment needed to monitor the experimental procedures. Our motivation for building this experimental system was to study the effect of intense solar radiation on the surfaces of asteroids when their perihelion distances become smaller than the semi-major axis of the orbit of Mercury. Comparisons between observational data and recent orbit and size-frequency models of the population of near-Earth asteroids suggest that asteroids are super-catastrophically destroyed when they approach the Sun. Whereas the current models are agnostic about the disruption mechanism, SHINeS was developed to study the mechanism or mechanisms responsible. The system can, however, be used for other applications that need to study the effects of high solar radiation on other natural or artificial objects.Peer reviewe

Mestizaje y comunicación intercultural como analépticos frente a las crisis fronterizas transhistóricas en la novela The Rag Doll Plagues (1992) de Alejandro Morales

Este artículo se centra en la novela The Rag Doll Plagues (1992) de Alejandro Morales e investiga las dimensiones transhistóricas de la subordinación de las identidades indígenas y mestizas bajo el dominio de las fuerzas autoritarias coloniales y postcoloniales en las zonas fronterizas entre México y los Estados Unidos. El colonialismo español, el racismo estadounidense y la eco-destrucción, que ocurren en diferentes momentos de la historia del Nuevo Mundo, constituyen distintas formas de crisis fronterizas en los tres Libros que componen la novela. El mestizaje y la comunicación intercultural, así como también la recuperación de las tradiciones culturales indígenas y mexicanas, fomentan la creación continua de nuevas identidades de hibridación racial, étnica y cultural en los tres Libros y, así, surgen como analépticos frente al problema diacrónicamente persistente del racismo.This article focuses on Alejandro Morales’s novel The Rag Doll Plagues (1992) and explores the transhistorical dimensions of the subordination indigenous and mestiza/o identities experience against colonial and postcolonial authoritarian forces in the borderlands between Mexico and the United States. Spanish colonialism, US racism and eco-destruction, each transpiring in different moments of the New World history, are the diverse forms the borderland crises take up in the three Books comprising the novel. Mestizaje and intercultural communication, as well as the retrieval of the indigenous and Mexican cultural traditions, foster the ongoing creation of new hybrid racial, ethnic and cultural identities in all the three Books and, thus, emerge as the analeptics to the diachronically persistent plight of racism

Immunohistochemical expression of CD44 in thyroid gland lesions

BACKGROUND: CD44 is a polymorphic family of cell surface proteoglycans and glycoproteins implicated in cell-to-cell and cell-to-matrix adhesion interactions and tumor metastasis. Its expression appears to be an indicator of invasive and metastatic behavior in carcinomas. The purpose of our study is to investigate the immunohistochemical expression of CD44 protein in thyroid lesions and its association to other histopathological parameters. METHODS: Samples from thyroid lesions were obtained from 40 patients treated in our hospital. The material consisted of 5 cases of multinodular goiter, 7 cases of thyroiditis (Hashimoto type), 5 cases of follicular adenoma, 4 cases of Hurthle cell tumor, 15 cases of thyroid carcinoma (11 papillary carcinomas, and 4 myeloid tumors), and 4 cases of normal thyroid tissue. Immunostaining was performed using the Ventana ES automated immunostainer. A monoclonal antibody was used and avidin-biotin method was applied to paraffin-embedded samples. A membranous immunostaining pattern was considered positive. RESULTS: CD44 expression was detected in three adenomas (60%), mostly of follicular type, and in eight carcinomas (72%). The CD44 immunostaining was especially apparent in papillary type of carcinomas which showed high expression. In normal thyroid tissue, a reduced CD44 expression was observed. CONCLUSION: The results of our study indicate that deregulated expression of CD44 contributes to the ability of thyroid carcinomas for invasion and metastasis and may constitute a prognostic factor for malignant biological behavior

Dynamical evolution and thermal history of asteroids (3200) Phaethon and (155140) 2005 UD

The near-Earth objects (NEOs) (3200) Phaethon and (155140) 2005 UD are thought to share a common origin, with the former exhibiting dust activity at perihelion that is thought to directly supply the Geminid meteor stream. Both of these objects currently have very small perihelion distances (0.140 au and 0.163 au for Phaethon and 2005 UD, respectively), which results in them having perihelion temperatures around 1000 K. A comparison between NEO population models to discovery statistics suggests that low-perihelion objects are destroyed over time by a, possibly temperature-dependent, mechanism that is efficient at heliocentric distances less than 0.3 au. By implication, the current activity from Phaethon is linked to the destruction mechanism of NEOs close to the Sun. We model the past thermal characteristics of Phaethon and 2005 UD using a combination of a thermophysical model (TPM) and orbital integrations of each object. Temperature characteristics such as maximum daily temperature, maximum thermal gradient, and temperature at different depths are extracted from the model, which is run for a predefined set of semi-major axis and eccentricity values. Next, dynamical integrations of orbital clones of Phaethon and 2005 UD are used to estimate the past orbital elements of each object. These dynamical results are then combined with the temperature characteristics to model the past evolution of thermal characteristics such as maximum (and minimum) surface temperature and thermal gradient. The orbital histories of Phaethon and 2005 UD are characterized by cyclic changes in.., resulting in perihelia values periodically shifting between present-day values and 0.3 au. Currently, Phaethon is experiencing relatively large degrees of heating when compared to the recent 20, 000 yr. We find that the subsurface temperatures are too large over this timescale for water ice to be stable, unless actively supplied somehow. The near-surface thermal gradients strongly suggest that thermal fracturing may be very effective at breaking down and ejecting dust particles. Observations by the DESTINY+ flyby mission will provide important constraints on the mechanics of dust-loss from Phaethon and, potentially, reveal signs of activity from 2005 UD. In addition to simulating the recent dynamical evolution of these objects, we use orbital integrations that start from the Main Belt to assess their early dynamical evolution (origin and delivery mechanism). We find that dwarf planet (2) Pallas is unlikely to be the parent body for Phaethon and 2005 UD, and it is more likely that the source is in the inner part of the asteroid belt in the families of, e.g., (329) Svea or (142) Polana.Peer reviewe