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Saint-Martin-Vésubie, France

Ducourant C.,University of Bordeaux 1 | Ducourant C.,French National Center for Scientific Research | Ducourant C.,Institute Astronomia | Teixeira R.,University of Bordeaux 1 | And 10 more authors.
Astronomy and Astrophysics | Year: 2014

Context. The nearby TW Hydrae association (TWA) is currently a benchmark for the study of the formation and evolution of young low-mass stars, circumstellar disks, and the imaging detection of planetary companions. For these studies, it is crucial to evaluate the distance to group members in order to access their physical properties. Membership of several stars is strongly debated and age estimates vary from one author to another with doubts about coevality. Aims. We revisit the kinematic properties of the TWA in light of new trigonometric parallaxes and proper motions to derive the dynamical age of the association and physical parameters of kinematic members. Methods. Using observations performed with the New Technology Telescope (NTT) from ESO we measured trigonometric parallaxes and proper motions for 13 stars in TWA. Results. With the convergent point method we identify a co-moving group with 31 TWA stars. We deduce kinematic distances for seven members of the moving group that lack trigonometric parallaxes. A traceback strategy is applied to the stellar space motions of a selection of 16 of the co-moving objects with accurate and reliable data yielding a dynamical age for the association of t 7.5 ± 0.7 Myr. Using our new parallaxes and photometry available in the literature we derive stellar ages and masses from theoretical evolutionary models. Conclusions. With new parallax and proper motion measurements from this work and current astrometric catalogs we provide an improved and accurate database for TWA stars to be used in kinematical analysis. We conclude that the dynamical age obtained via traceback strategy is consistent with previous age estimates for the TWA, and is also compatible with the average ages derived in the present paper from evolutionary models for pre-main-sequence stars. © ESO, 2014.

Vidotto A.A.,University of St. Andrews | Jardine M.,University of St. Andrews | Morin J.,University of Gottingen | Donati J.-F.,Laboratoire dAstrophysique | And 2 more authors.
Astronomy and Astrophysics | Year: 2013

We investigate the effect of the magnetic fields of M dwarf (dM) stars on potentially habitable Earth-like planets. These fields can reduce the size of planetary magnetospheres to such an extent that a significant fraction of the planet's atmosphere may be exposed to erosion by the stellar wind. We used a sample of 15 active dM stars, for which surface magnetic-field maps were reconstructed, to determine the magnetic pressure at the planet orbit and hence the largest size of its magnetosphere, which would only be decreased by considering the stellar wind. Our method provides a fast means to assess which planets are most affected by the stellar magnetic field, which can be used as a first study to be followed by more sophisticated models. We show that hypothetical Earth-like planets with similar terrestrial magnetisation (~1 G) orbiting at the inner (outer) edge of the habitable zone of these stars would present magnetospheres that extend at most up to 6 (11.7) planetary radii. To be able to sustain an Earth-sized magnetosphere, with the exception of only a few cases, the terrestrial planet would either (1) need to orbit significantly farther out than the traditional limits of the habitable zone; or else, (2) if it were orbiting within the habitable zone, it would require at least a magnetic field ranging from a few G to up to a few thousand G. By assuming a magnetospheric size that is more appropriate for the young-Earth (3.4 Gyr ago), the required planetary magnetic fields are one order of magnitude weaker. However, in this case, the polar-cap area of the planet, which is unprotected from transport of particles to/from interplanetary space, is twice as large. At present, we do not know how small the smallest area of the planetary surface is that could be exposed and would still not affect the potential for formation and development of life in a planet. As the star becomes older and, therefore, its rotation rate and magnetic field reduce, the interplanetary magnetic pressure decreases and the magnetosphere of planets probably expands. Using an empirically derived rotation-activity/magnetism relation, we provide an analytical expression for estimating the shortest stellar rotation period for which an Earth-analogue in the habitable zone could sustain an Earth-sized magnetosphere. We find that the required rotation rate of the early- and mid-dM stars (with periods â‰3;37-202 days) is slower than the solar one, and even slower for the late-dM stars (â‰3;63-263 days). Planets orbiting in the habitable zone of dM stars that rotate faster than this have smaller magnetospheric sizes than that of the Earth magnetosphere. Because many late-dM stars are fast rotators, conditions for terrestrial planets to harbour Earth-sized magnetospheres are more easily achieved for planets orbiting slowly rotating early- and mid-dM stars. © ESO, 2013.

Lattelais M.,CNRS Theoretical Chemistry Laboratory | Pauzat F.,CNRS Theoretical Chemistry Laboratory | Pilme J.,CNRS Theoretical Chemistry Laboratory | Ellinger Y.,CNRS Theoretical Chemistry Laboratory | Ceccarelli C.,Laboratoire dAstrophysique
Astronomy and Astrophysics | Year: 2011

Context. Glycine, the simplest of aminoacids, has been found in several carbonaceous meteorites. It remains unclear, however, wether glycine is formed in the interstellar medium (ISM) and therefore available everywhere in the Universe. For this reason, radioastronomers have searched for many years unsuccessfully to detect glycine in the ISM. Aims. We provide possible guidelines to optimize the return of these searches. Since, for most of the species observed so far in the ISM, the most abundant isomer of a given generic chemical formula is the most stable one (minimum energy principle (MEP)), we assess whether neutral glycine is the best molecule to search for or whether one of its isomers/conformers or ionic, protonated, or zwitterionic derivatives would have a higher probability of being detected. Methods. The question of the relative stability of these different species is addressed by means of quantum density functional theory (DFT) simulations within the hybrid B3LYP formalism. Each fully optimized structure is verified as a stationary point by means of a vibrational analysis. A comprehensive screening of 32 isomers/conformers of the C2H5O2N chemical formula (neutral, negative, and positive ions together with the corresponding protonated species and the possible zwitterionic structures) is carried out. In the sensitive case of the neutral compounds, more accurate relative energies were obtained by means of high level post Hartree-Fock coupled cluster calculations with large basis sets (CCSD(T)/cc-pVQZ). Results. We find that neutral glycine is not the most stable isomer and, therefore, probably not the most abundant one, which might explain why it has escaped detection so far. We find instead that N-methyl carbamic acid and methyl carbamate are the two most stable isomers and, therefore, probably the two most abundant ones. Among the non-neutral forms, we found that glycine is the most stable isomer only if protonated or zwitterionic if present in interstellar ices. Conclusions. Assuming that MEP can be applied to optimize our search for glycine, our conclusion is that this search will remain extremely difficult with the present instruments and we propose searching instead for other examples among the most stable isomers. © 2011 ESO.

Raiter A.,European Southern Observatory | Schaerer D.,University of Geneva | Schaerer D.,Laboratoire dAstrophysique | Fosbury R.A.E.,ST ECF
Astronomy and Astrophysics | Year: 2010

Aims: We study the expected properties of starburst galaxies in order to provide the point of reference for interpretation of high-z galaxy surveys and of very metal-poor galaxies. We concentrate mainly on the UV characteristics such as the ionizing spectra, the UV continuum, the Lyα and He II λ1640 line and two-photon continuum emission. Methods: We use evolutionary synthesis models covering metallicities from Pop III to solar and a wide range of IMFs. We also combine the synthetic SEDs with the CLOUDY photoionization code for more accurate predictions of nebular emission, and to study possible departures from case B assumed in the synthesis models. Results: The ionizing fluxes, UV continuum properties, and predicted Lyα and He II λ1640 line strengths are presented for synthesis models covering a wider range of parameter space than our earlier calculations. Strong departures from case B predictions are obtained for Lyα and 2γ continuum at low metallicities. At low nebular densities both are shown to be enhanced proportionally to the mean energy carried by the Lyman continuum photons emitted by the ionizing source. Larger Lyα equivalent widths are therefore predicted at low metallicity. The He II λ1640 line can be weaker than case B predicts (in terms of flux as well as the equivalent width) due to its ionization parameter dependence and to the enhanced underlying 2γ continuum. Conclusions: Our results have implications for the interpretation of star-forming metal-poor and/or high redshift galaxies, for galaxies among the Lyα emitters (LAE) and Lyman Break galaxy (LBG) populations, and for searches of Population III stars in the distant Universe. © ESO 2010.

Lattelais M.,CNRS Theoretical Chemistry Laboratory | Pauzat F.,CNRS Theoretical Chemistry Laboratory | Ellinger Y.,CNRS Theoretical Chemistry Laboratory | Ceccarelli C.,Laboratoire dAstrophysique
Astronomy and Astrophysics | Year: 2010

Context. The hunt for the interstellar complex organic molecules (COMs) supposed to be the building blocks of the molecules at the origin of life is a challenging but very expensive task. It starts with laboratory experiments, associated with theoretical calculations, that give the line frequencies and strengths of the relevant molecules to be identified and finishes with observations at the telescopes. Aims.The present study aims to suggest possible guidelines to optimize this hunt. Levering on the minimum energy principle (MEP) presented in a previous study, we discuss the link between thermodynamic stability and detectability of a number of structures in the important families of amides, sugars and aminonitriles. Methods.The question of the relative stability of these different species is addressed by means of quantum density functional theory simulations. The hybrid B3LYP formalism was used throughout. All 72 molecules part of this survey were treated on an equal footing. Each structure, fully optimized, was verified to be a stationary point by vibrational analysis. Results.A comprehensive screening of 72 isomers of CH3NO, C2H5NO, C3H7NO, C2H 4O2, C3H6O3 and C2H4N2 chemical formula has been carried out. We found that formamide, acetamide and propanamide (the first two identified in the Inter-Stellar Medium) are the most stable compounds in their families demonstrating at the same time that the peptide bond > N-C=O at the origin of life is the most stable bond that can be formed. Dihydroxyacetone, whose detection awaits for confirmation, is far from being the most stable isomer of its family while aminoacetonitrile, that has been recently identified, is effectively the most stable species. Conclusions. The MEP appears to be a useful tool for optimizing the hunt for new species by identifying the potentially more abundant isomers of a given chemical formula. © 2010 ESO.

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