The first rotation periods in Praesepe

The cluster Praesepe (age 650 Myr) is an ideal laboratory to study stellar evolution. Specifically, it allows us to trace the long-term decline of rotation and activity on the main-sequence. Here we present rotation periods measured for five stars in Praesepe with masses of 0.1-0.5Ms -- the first rotation periods for members of this cluster. Photometric periodicities were found from two extensive monitoring campaigns, and are confirmed by multiple independent test procedures. We attribute these variations to magnetic spots co-rotating with the objects, thus indicating the rotation period. The five periods, ranging from 5 to 84h, show a clear positive correlation with object mass, a trend which has been reported previously in younger clusters. When comparing with data for F-K stars in the coeval Hyades, we find a dramatic drop in the periods at spectral type K8-M2 (corresponding to 0.4-0.6Ms). A comparison with periods of VLM stars in younger clusters provides a constraint on the spin-down timescale: We find that the exponential rotational braking timescale is clearly longer than 200 Myr, most likely 400-800 Myr. These results are not affected by the small sample size in the rotation periods. Both findings, the steep drop in the period-mass relation and the long spin-down timescale, indicate a substantial change in the angular momentum loss mechanism for very low mass objects, possibly the breakdown of the solar-type (Skumanich) rotational braking. While the physical origin for this behaviour is unclear, we argue that parts of it might be explained by the disappearance of the radiative core and the resulting breakdown of an interface-type dynamo in the VLM regime. Rotational studies in this mass range hold great potential to probe magnetic properties and interior structure of main-sequence stars.Comment: 10 pages, 3 figures, accepted for publication in MNRA

Variable accretion and outflow in young brown dwarfs

We report on the first dedicated monitoring campaign of spectroscopic variability in young brown dwarfs. High-resolution optical spectra of six targets in nearby star-forming regions were obtained over 11 nights between 2005 January-March on the Magellan 6.5m telescope. We find significant variability in Halpha and a number of other emission lines related to accretion and outflow processes on a variety of timescales ranging from hours to weeks to years. The most dramatic changes are seen for 2M1207, 2M1101 and ChaI-ISO217. We observe possible accretion rate changes by about an order of magnitude in two of these objects, over timescales of weeks (2M1207) or hours (2M1101). The accretion 'burst' seen in 2M1101 could be due to a 'clumpy' flow. We also see indications for changes in the outflow rate in at least three objects. In one case (ISO217), there appears to be a ~1-hour time lag between outflow and accretion variations, consistent with a scenario in which the wind originates from the inner disk edge. Our variability study supports a close to edge-on inclination for the brown dwarf LS-RCrA 1. The fact that all targets in our sample show variations in accretion and/or outflow indicators suggests that studies of young brown dwarf properties should be based either on large samples or time series. As an example, we demonstrate that the large scatter in the recently found accretion rate vs. mass relationship can be explained primarily with variability. The observed profile variations imply asymmetric accretion flows in brown dwarfs, which, in turn, is evidence for magnetic funneling by large-scale fields. We show that accreting sub-stellar objects may harbor magnetic fields with ~kG strength (abridged).Comment: 33 pages, 5 figures, accepted for publication in Ap

The rotation of very low-mass stars and brown dwarfs

The evolution of angular momentum is a key to our understanding of star formation and stellar evolution. The rotational evolution of solar-mass stars is mostly controlled by magnetic interaction with the circumstellar disc and angular momentum loss through stellar winds. Major differences in the internal structure of very low-mass stars and brown dwarfs -- they are believed to be fully convective throughout their lives, and thus should not operate a solar-type dynamo -- may lead to major differences in the rotation and activity of these objects. Here, we report on observational studies to understand the rotational evolution of the very low-mass stars and brown dwarfs.Comment: 8 pages, 5 figures, Proceedings for IAU Symposium No. 243, 2007, 'Star-disk interaction in young stars

Interplay between spin-orbit interactions and a time-dependent electromagnetic field in monolayer graphene

We apply a circularly and linearly polarized terahertz field on a monolayer of graphene taking into account spin-orbit interactions of the intrinsic and Rashba type. It turns out that the field can not only be used to induce a gap in the energy spectrum, but also to close an existing gap due to the different reaction of the spin components on circularly polarized light. Signatures of spin-orbit coupling on the density of states of the driven system can be observed even for energies where the static density of states is independent of spin-orbit interactions. Furthermore it is shown that the time evolution of the spin polarization and the orbital dynamics of an initial wave packet can be modulated by varying the ratio of the spin-orbit coupling parameters. Assuming that the system acquires a quasi stationary state, the optical conductivity of the irradiated sample is calculated. Our results confirm the multi step nature of the conductivity obtained recently, where the number of intermediate steps can be changed by adjusting the spin-orbit coupling parameters and the orientation of the field.Comment: 10 pages, 11 figures; typos corrected, references adde

Tumor angiogenesis and anti-angiogenic therapy in malignant gliomas revisited

The cellular and molecular mechanisms of tumor angiogenesis and its prospects for anti-angiogenic cancer therapy are major issues in almost all current concepts of both cancer biology and targeted cancer therapy. Currently, (1) sprouting angiogenesis, (2) vascular co-option, (3) vascular intussusception, (4) vasculogenic mimicry, (5) bone marrow-derived vasculogenesis, (6) cancer stem-like cell-derived vasculogenesis and (7) myeloid cell-driven angiogenesis are all considered to contribute to tumor angiogenesis. Many of these processes have been described in developmental angiogenesis; however, the relative contribution and relevance of these in human brain cancer remain unclear. Preclinical tumor models support a role for sprouting angiogenesis, vascular co-option and myeloid cell-derived angiogenesis in glioma vascularization, whereas a role for the other four mechanisms remains controversial and rather enigmatic. The anti-angiogenesis drug Avastin (Bevacizumab), which targets VEGF, has become one of the most popular cancer drugs in the world. Anti-angiogenic therapy may lead to vascular normalization and as such facilitate conventional cytotoxic chemotherapy. However, preclinical and clinical studies suggest that anti-VEGF therapy using bevacizumab may also lead to a pro-migratory phenotype in therapy resistant glioblastomas and thus actively promote tumor invasion and recurrent tumor growth. This review focusses on (1) mechanisms of tumor angiogenesis in human malignant glioma that are of particular relevance for targeted therapy and (2) controversial issues in tumor angiogenesis such as cancer stem-like cell-derived vasculogenesis and bone-marrow-derived vasculogenesis

La ingesta de ácidos grasos trans y su asociación con sobrepeso y obesidad en niños de 4-5 años

El sobrepeso y la obesidad infantil se han convertido en un grave problema de salud pública a nivel mundial en las últimas décadas debido al aumento de su prevalencia. Según la definición aplicada, la edad y la región estudiada, la prevalencia de sobrepeso y obesidad infantil oscila entre 10-40%. Asimismo, el sobrepeso y la obesidad en la infancia se asocian a sobrepeso y obesidad en la adolescencia y en la vida adulta, así como a un aumento de la morbilidad y mortalidad. Además, los niños con sobrepeso y obesidad son más propensos a desarrollar enfermedades como diabetes mellitus tipo 2, enfermedades cardiovasculares, enfermedades musculoesqueléticas, depresión y ansiedad. Hasta la fecha, se han descrito numerosos factores de riesgo para el desarrollo de obesidad infantil, aunque solamente existe evidencia consistente para la asociación entre obesidad infantil y factores genéticos, así como bajos niveles de actividad física pero no para factores dietéticos. No obstante, en población adulta, la ingesta de tipos específicos de grasa se ha asociado a obesidad, en particular, la ingesta de ácidos grasos trans (AGT). AGT son isómeros geométricos de ácidos grasos insaturados que poseen al menos un doble enlace carbono-carbono no conjugado en configuración trans y pueden ser de origen natural o industrial. Recientemente ha crecido la evidencia que la ingesta de AGT de origen natural o industrial puede tener diferentes efectos. En este sentido, se ha mostrado que una mayor ingesta de AGT industriales está asociada a obesidad visceral y a un mayor riesgo cardiovascular en adultos. Sin embargo, la ingesta de AGT naturales no se ha asociado a cambios en el tamaño, la forma o la composición corporal. Debido a los efectos negativos de AGT descritos, muchos países han limitado legalmente el contenido de productos dietéticos de AGT y la Organización Mundial de la Salud (OMS) recomienda que menos del 1% de la energía ingerida provenga de AGT. Sin embargo, la información publicada sobre las fuentes y las cantidades de la ingesta de AGT industriales y naturales en menores que se encuentran en etapa preescolar, así como su asociación con sobrepeso y obesidad en esta población es escasa. Por ello, el objetivo de esta tesis es describir la ingesta de AGT en total, industriales y naturales en niños de 4-5 años en España y explorar la asociación entre la ingesta de AGT y sus subtipos y sobrepeso, incluyendo obesidad, en esta población. MÉTODOS: Se han analizado de forma transversal los datos obtenidos entre 2009 y 2012 de 1829 menores de 4-5 años del estudio INfancia y Medio Ambiente (INMA), un estudio de cohorte multicéntrico prospectivo de madres e hijos en España. A fin de recoger los datos, profesionales de la salud realizaron entrevistas con los niños y los padres y tomaron mediciones antropométricas directas del peso corporal y de la estatura de los niños mediante procedimientos estandarizados. Se calculó el índice de masa corporal y se definió sobrepeso incluyendo obesidad de acuerdo con los criterios de la International Obesity Task Force (IOTF). Además, se recogieron al inicio del estudio variables sociodemográficas de las familias. Por otro lado, se estimó la ingesta dietética de alimentos de los niños con un cuestionario semicuantitativo de frecuencia de alimentos validado. Se calculó la ingesta diaria habitual de cada nutriente incluyendo AGT en total, y por su origen industrial o natural, multiplicando la frecuencia de la ingesta de cada alimento por el contenido de nutrientes de la porción especificada en el cuestionario teniendo en cuenta tablas de composición de alimentos actuales publicados. En un primer estudio se realizaron análisis descriptivos para comparar la ingesta de AGT en g/día y en porcentaje de energía ingerida entre las cohortes del estudio. Además, se aplicaron análisis de regresión lineal múltiple para explorar la asociación entre las características de los padres y los niños y la ingesta de AGT en total, industriales y naturales calculando los coeficientes de regresión ß y sus correspondientes intervalos de confianza. Los modelos se ajustaron por posibles variables de confusión. En un segundo estudio se describieron las características de los niños con peso normal y de los que tenían sobrepeso incluyendo obesidad. Asimismo, se emplearon análisis de regresión logística múltiple para investigar la asociación entre la ingesta de AGT en total, industriales y naturales y sobrepeso, incluyendo obesidad, calculando las odds ratios (OR) y sus correspondientes intervalos de confianza. Se emplearon dos modelos diferentes ajustados por distintas variables de confusión, además de realizar múltiples análisis de sensibilidad. RESULTADOS: Las ingestas diarias medias de AGT en total, industriales y naturales fueron 1.36, 0.60 y 0.71 g/día respectivamente. Un 10% de los niños obtuvieron >1% de su ingesta de energía de AGT. Las principales fuentes de los AGT industriales fueron comida rápida, pan blanco y productos horneados procesados. Leche, carne roja y procesada y productos horneados procesados fueron las principales fuentes de AGT naturales. Tener padres de países distintos de España se asoció significativamente a una mayor ingesta de AGT (en mg/día) naturales (ß 45.5) y ver televisión (en horas/día) se asoció de manera significativa a una mayor ingesta de AGT industriales (ß 18.3). Una mayor ingesta de frutas y verduras se asoció significativamente a una menor ingesta de AGT en total (ß -85.5), industriales (ß -46.6) y naturales (ß -44.9), mientras que una mayor ingesta de bebidas azucaradas se asoció a una menor ingesta de AGT en total (ß -27.6) y naturales (ß -34.8). Además, tras ajustar por los principales factores de confusión, el cuartil más alto de la ingesta de AGT industriales (> 0.7 g/día) se asoció a un mayor riesgo de estar en sobrepeso y obesidad (OR 1.57, IC 95% 1.13¿2.21, p trend para cuartiles 0.01). Por el contrario, no se observaron asociaciones significativas entre la ingesta de AGT naturales y sobrepeso incluyendo obesidad. CONCLUSIONES: En menores españoles en edad preescolar, la ingesta de AGT en total fue relevante, teniendo en cuenta que el 10% de los niños estudiados sobrepasaron la ingesta recomendada por la OMS. Además, la ingesta de AGT en total e industriales se asoció a patrones alimentarios y estilos de vida menos saludables. Por último, una mayor ingesta de AGT industriales, pero no de AGT naturales, se asoció a sobrepeso y obesidad

Photoinduced pseudospin effects in silicene beyond the off resonant condition

We study the photoinduced manipulation of charge carriers in monolayer silicene subject to intense electromagnetic terahertz radiation. Considering the Dirac cone approximation and going beyond the off resonant condition for large frequencies of the radiation field, where only virtual photon processes are allowed, we present the exact zero-momentum pseudospin polarization and numerical results for the quasienergy band structure and time-averaged density of states. We find that resonant processes, due to real photon emission and absorbtion processes, induce a band inversion that qualitatively modifies the quasienergy spectrum. These band structure changes manifest themselves as an inversion of the averaged pseudospin polarization. Through the analysis of the time-averaged density of states we find that effective photoinduced gap manipulation can only be achieved in the intermediate and strong matter-radiation coupling regime where the off resonant approximation breaks down.Comment: 8 pages, 3 figures, Accepted in PRB (in press

The Herschel view of circumstellar discs: a multi-wavelength study of Chamaeleon I

We present the results of a multi-wavelength study of circumstellar discs around 44 young stellar objects in the 3 Myr old nearby Chamaeleon I star-forming region. In particular, we explore the far-infrared/submm regime using Herschel fluxes. We show that Herschel fluxes at 160-500$\,\mu$m can be used to derive robust estimates of the disc mass. The median disc mass is 0.005$M_{\odot}$ for a sample of 28 Class IIs and 0.006$M_{\odot}$ for 6 transition disks (TDs). The fraction of objects in Chamaeleon-I with at least the `minimum mass solar nebula' is 2-7%. This is consistent with previously published results for Taurus, IC348, $\rho$ Oph. Diagrams of spectral slopes show the effect of specific evolutionary processes in circumstellar discs. Class II objects show a wide scatter that can be explained by dust settling. We identify a continuous trend from Class II to TDs. Including Herschel fluxes in this type of analysis highlights the diversity of TDs. We find that TDs are not significantly different to Class II discs in terms of far-infrared luminosity, disc mass or degree of dust settling. This indicates that inner dust clearing occurs independently from other evolutionary processes in the discs.Comment: 16 pages, 13 figures. Accepted for publication in MNRA