Astrometric Light-Travel Time signature of sources in nonlinear motion

Context:Very precise planned space astrometric missions and recent improvements on imaging capabilities require a detailed review of the assumptions of classical astrometric modeling. Aims:We show that Light-Travel Time must be taken into account to model the kinematics of astronomical objects in nonlinear motion, even at stellar distances. Methods:A closed expression to include Light-Travel Time in the actual astrometric models with nonlinear motion is provided. Using a perturbative approach the expression of the Light-Travel Time signature is derived. We propose a practical form of the astrometric modelling to be applied in astrometric data reduction of sources at stellar distances($d>1 pc$). Results :We show that the Light-Travel Time signature is relevant at \muas accuracy (or even at $mas$) depending on the time span of the astrometric measurements. We explain how information about the radial motion of a source can be obtained. Some estimative numbers are provided for known nearby binary systems Conclusions :In the light of the obtained results, it is clear that this effect must be taken into account to interpret any kind of precise astrometric measurements. The effect is particularly interesting in measurements performed by the planned astrometric space missions (GAIA, SIM, JASMINE, TPF/DARWIN). Finally an objective criterion is provided to quickly evaluate whether the Light-Travel Time modeling is required for a given source or system.Comment: 10 pages, 2 figure

Perspective acceleration and gravitational redshift. Measuring masses of individual white dwarfs using Gaia + SIM astrometry

According to current plans, the SIM/NASA mission will be launched just after the end of operations for the Gaia/ESA mission. This is a new situation which enables long term astrometric projects that could not be achieved by either mission alone. Using the well-known perspective acceleration effect on astrometric measurements, the true heliocentric radial velocity of a nearby star can be measured with great precision if the time baseline of the astrometric measurements is long enough. Since white dwarfs are compact objects, the gravitational redshift can be quite large (40-80 km/s), and is the predominant source of any shift in wavelength. The mismatch of the true radial velocity with the spectroscopic shift thus leads to a direct measure of the Mass--Radius relation for such objects. Using available catalog information about the known nearby white dwarfs, we estimate how many masses/gravitational redshift measurements can be obtained with an accuracy better than 2%. Nearby white dwarfs are relatively faint objects (10 < V < 15), which can be easily observed by both missions. We also briefly discuss how the presence of a long period planet can mask the astrometric signal of perspective acceleration.Comment: 3 pages, 2 Figures. Proceedings of the IAU Symposium 261 : Relativity in Fundamental Astronomy. 27 April - 1 May 2009, Virginia Beach, VA, USA. refereed and accepted versio

Study of the integration and structural verification for the 3CAT-4

This study talks about the process of integration and verification of a nanosatellite, the 3CAT-4, developed in the NanoSat Lab laboratory in the UPC. In the study the latest refurbishments of the satellite have been introduced, changes that lead to a series of structural analisis tests of every subsystem to qualify them, and finally to integrate a replica of the entire satellite to which a study of the vibrations profile has been made in preparation for the future launch

Comment on "Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581"

This document is the Accepted Manuscript Version of the following article: Guillem Anglada-Escude and Mikko Tuomi, 'Comment on "Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581"', Science, Vol 347 (6226), 2015, the final, published version is available online at doi: 10.1126/science.1260796. © 2015 The American Association for the Advancement of Science. All rights reserved.Robertson et al. (Reports, 25 July 2014, p. 440) claimed that activity-induced variability is responsible for the Doppler signal of the proposed planet candidate GJ 581d. We point out that their analysis using periodograms of residual data is inappropriate and promotes inadequate tools. Because the claim challenges the viability of the method to detect exo-Earths, we encourage reanalysis and a deliberation on what the field-standard methods should be.Peer reviewe

High Angular Resolution Radio Observations of the HL/XZ Tau Region: Mapping the 50 AU Protoplanetary Disk around HL Tau and Resolving XZ Tau S into a 13 AU Binary

We present new 7 mm and archive 1.3 cm high angular resolution observations of the HL/XZ Tau region made with the VLA. At 7 mm, the emission from HL Tau seems to be arising in a clumpy disk with radius of order 25 AU. The 1.3 cm emission from XZ Tau shows the emission from a binary system with 0"3 (42 AU) separation, known from previous optical/IR observations. However, at 7 mm, the southern radio component resolves into a binary with 0"09 (13 AU) separation, suggesting that XZ Tau is actually a triple star system. We suggest that the remarkable ejection of gas from the XZ Tau system observed with the HST may be related to a periastron passage of this newly discovered close binary system.Comment: Accepted by The Astrophysical Journal Letters (23 Jan 2009

Experiments i models relativistes per a l'astrometria òptica des de l'espai. Aplicació a la missió Gaia.

[spa] Gaia es un proyecto de la agencia espacial Europea (ESA) que pretende hacer un censo galáctico catalogando más de 1000 millones de objetos con una precisión astrometrica sin precedentes. La astrometria es la parte de la astronomia dedicada a la medida y al estudio del movimiento de los astros y ha sido uno de los principales motores de la física y las matemáticas de los últimos 2000 años. La precisión astrometrica de Gaia se estima en unos 10 microsegundos de arco, que es el tamaño angular de una moneda de dos euros en la superficie de la Luna vista desde la Tierra. A tal precisión, un montón de efectos en la propagación de la luz y la definición de los observables astrometricos entran en juego. Por ejemplo, la deflexión de la luz por el Sol pero también de los planetas del sistema solar, efectos de aberración en las imágenes adquiridas por la sonda debido a su rotación, o nuevos efectos astrométricos de segundo orden en el movimiento de las fuentes previamente indetectables. Este trabajo estudia estos aspectos desde un punto de vista relativista. Contiene tres partes. La primera se dedica a los efectos en la adquisición de datos astrometricos debidos a la propagación pro el interior de un sistema óptico en rotación (tipo Gaia). La segunda parte versa sobre la relaicón entre el intervalo de tiempo de emisión y de recepción, que causa movimiento aparente adicional sobre todo el movimiento de la sonda es no-lineal. La tercera parte estudia la deflexión dela luz relativista por parte de los planetas del sistema solar y se estudia la calidad así como el método de extracción de parámetros físicos a partir de la observaciones