123 research outputs found
Recherche d’étoiles jeunes de faible masse dans le voisinage solaire
Formées lors de l’effondrement gravitationnel d’un nuage de gaz moléculaire, les étoiles
naissantes auront différentes masses variant entre 0.08 et environ 100M . La majorité de
la population stellaire de la Galaxie est constituée d’étoiles dont la masse est inférieure
à environ 0.6 M . Le dernier évènement de formation stellaire dans le voisinage solaire
s’est produit dans la bulle locale il y a au plus 100 millions d’années, vraisemblablement
provoqué par le passage d’une onde de choc dans le bras local de la Galaxie. C’est ainsi
que se formèrent de jeunes associations d’étoiles dont les membres se caractérisent en
particulier par une vitesse spatiale et une position commune dans la Galaxie.
Les associations jeunes étant peu densément peuplées et relativement proches du
Soleil, leurs membres se font plutôt rares et dispersés sur toute la voûte céleste. Jusqu’Ã
présent, surtout les étoiles les plus massives (brillantes) ont été répertoriées. Les étoiles
jeunes de faible masse, constituant la majorité de la population, restent pour la plupart
à être identifiées. Les étoiles jeunes de faible masse représentent une population clef
pour contraindre les modèles évolutifs des étoiles M et des naines brunes. Elles sont
également d’excellentes candidates pour chercher des exoplanètes via les techniques
d’imagerie directe.
Ce mémoire présente une nouvelle méthode utilisant un modèle cinématique enrichi
d’une analyse statistique Bayesienne pour identifier des étoiles jeunes de faible
masse dans les associations beta Pictoris, Tucana-Horologium et AB Doradus. À partir
d’un échantillon de 1080 étoiles K et M, toutes comportant des indicateurs de jeunesse
tels l’émission Halpha et une forte luminosité dans les rayons X, leurs propriétés cinématiques
(mouvement propre) et photométriques sont analysées pour en extraire 98 candidates
hautement probables membres d’une des trois associations. Une confirmation de
leur statut comme membre nécessitera en particulier une mesure de leur vitesse radiale
(prédit par notre analyse) et une mesure de la largeur équivalente du lithium à 6708 Å
pour mieux contraindre leur âge.The gravitational collapse of a molecular gas cloud produces the incipient stars with
various masses between 0.08 and approximately 100 M . The majority of the stellar
galactic population is made up of stars with masses lower than approximately 0.6 M .
The last event of stellar formation in the solar neighborhood happened in the local bubble
no more than 100 million of years ago, probably caused by the propagation of a
shock wave in the galactic local arm. This is how young associations, also called moving
groups were formed. Their members are characterized by a common velocity and
position within the Galaxy.
Young associations, being sparsely populated and relatively close to the Sun, their
members are found all over the sky. So far, only the most massive members (luminous
ones) have been identified. Young low-mass stars, comprising the majority of the population,
remain to be identified. Those stars are expected to be excellent candidates to
find exoplanets through direct imaging techniques, while also forming a key population
to constrain M stars and brown dwarfs evolutionnary models.
This master thesis presents a new method using a kinematical model coupled with
a Bayesian statistic analysis to identify young low-mass stars in the beta Pictoris, Tucana-
Horologium and AB Doradus associations. Using a sample of 1080 K and M stars, all
showing youth indicators such as Halpha emission and X-rays luminosity, their photometric
and kinematic properties (proper motion) are analyzed to extract 98 highly probable
members distributed over the three associations. Status confirmation as members will
require measurement of their radial velocity (predicted by our analysis) and the lithium
at 6708 Ã… equivalent widths to better constrain their age
Recherche et caractérisation des étoiles jeunes de faible masse dans le voisinage solaire
L'outil développé dans le cadre de cette thèse est disponible à l'adresse suivante: www.astro.umontreal.ca/~malo/banyan.phpPrès de 70% des étoiles de la Galaxie ont une masse inférieure à ~0.8 Msun.
Cependant, étant donné que ces étoiles sont plus difficilement observables en raison de leur plus faible luminosité, cette statistique ne reflète pas le recensement actuel de la population d'étoiles de faible masse dans le voisinage solaire, ni dans les groupes cinématiques d'étoiles jeunes. Cette population a une grande importance pour contraindre la forme de la fonction de masse Galactique, et aussi pour contraindre les modèles évolutifs.
Les étoiles de faible masse sont aussi d'excellentes cibles pour la recherche d'exoplanètes avec des techniques variées (imagerie directe, vitesse radiale, transit). La caractérisation des exoplanètes autour de ces étoiles est tributaire des connaissances fondamentales sur celles-ci, c'est-à -dire de leur luminosité bolométrique, température effective, rayon et âge.
Dans la présente thèse, dont le but est d'identifier et caractériser les étoiles de faible masse, une méthode statistique a été développée afin d'établir quantitativement l'appartenance d'une étoile à un groupe en dérivant une probabilité d'association. Cette méthode combine l'inférence Bayesienne et des modèles empiriques de plusieurs observables, dont la luminosité, vitesse spatiale et position galactique, de membres confirmés de 7 groupes d'étoiles jeunes (8-120 Mans) ainsi que d'étoiles vieilles du champ. Les étoiles ayant une probabilité d'association minimale de 90% sont considérées comme des candidates. L'analyse développée prédit aussi la vitesse radiale et la distance trigonométrique qu'une étoile aurait dans une association donnée. L'analyse a montré, pour les 177 membres confirmés, un excellent accord entre les paramètres prédits et observés, soit de 1.9 km/s et 10% respectivement, pour la vitesse radiale et la parallaxe. La mesure de ces paramètres pour les candidates est donc une bonne manière de confirmer leur appartenance à l'association. Cette méthode robuste a été appliquée sur un échantillon de 758 étoiles montrant des signes de jeunesse (émission H et rayons X).
L'analyse a permis d'identifier 214 candidates hautement probables, et le suivi spectroscopique de ces étoiles a permis, jusqu'à présent, de confirmer la justesse de la prédiction en vitesse radiale pour 130 étoiles. Ces observations spectroscopiques ont aussi permis de mesurer leur vitesse de rotation, qui s'est avérée élevée comparativement aux étoiles vieilles du champs. La mesure de la distance trigonométrique était aussi en accord avec la prédiction pour 18 candidates jeunes. Grâce aux membres dont l'appartenance à un groupe jeune a été confirmée, un modèle empirique de la luminosité en rayon X des étoiles a pu être établi. Cette luminosité s'est avérée significativement plus élevée (environ 4 fois plus) pour les étoiles des groupes les plus jeunes (~8-12 Mans) que pour celles des groupes plus vieux (~120 Mans). Cet observable constitue donc un bon indicateur d'âge.
La comparaison des spectres de 59 candidates à des modèles d'atmosphère a permis de déterminer trois paramètres fondamentaux: la luminosité bolométrique, la température effective et le rayon. Globalement, les candidates jeunes ont une luminosité plus élevée et un rayon plus grand que les étoiles vieilles. De récents modèles évolutifs incluant le traitement d'une dynamo de type rotationnel et générant un champ magnétique de surface de 1 à 2.5 kGauss ont été utilisés pour déterminer l'âge isochronal de ces étoiles.
Les âges ainsi déterminés pour les étoiles de l'association \beta Pictoris en utilisant des étoiles de types spectraux différents sont davantage cohérents (types K5V-M0V: 24 Mans, types M1V-M4V: 14 Mans) et sont aussi cohérents avec l'âge déterminé indépendamment pour le groupe en utilisant l'abondance du lithium des membres de faible masse (26 Mans).About 70% of the stars in the Galaxy have a mass inferior than ~0.8 Msun. However, this statistic does not reflect the current census population of low mass in the solar neighborhood
and in young kinematic groups, since their low luminosity make their observation more difficult. This population is of great interest to check the validity of the Galactic mass function, and also to constraint evolutionary models. The low-mass stars are also excellent targets for the search for exoplanets using various techniques (direct imaging, radial velocity, transit).
The characterization of the exoplanets orbiting these stars depends mostly on our basic knowledge of the host star, that is their bolometric luminosity, effective temperature, radius and age.
The present thesis aim to identify and characterize low-mass stars. Toward that end, a statistical method has been developed to determine quantitatively the membership probability of a star to a young kinematic group. This method combines the Bayesian inference and empirical models of several observables such as the brightness, Galactic space velocity and position of bona fide members of 7 young stars groups (8-120 Mans), as well as old field stars. Stars with a membership probability greater than 90% are considered candidate members. The analysis also predicts the radial velocity and distance that a star would have if it was an actual member. For the 177 previously-known members, an excellent agreement was found between the predicted and observed parameters (1.9 km/s
and 10% for the radial velocity and parallax, respectively). Measuring these observables for the candidates stars is thus a good way to confirm their membership.
This robust method was applied to a sample of 758 stars which showed signs of youth (H and X-ray emission). It allowed to identify 214 highly probable candidates.
The spectroscopic follow-up yields a radial velocity in agreement with predictions for 130 stars. These spectroscopic observations also allowed to measure their projected rotational velocity, which turned out to be higher than that of the old population of stars.
Trigonometric distance measurements were also obtained and were coherent with predictions for 18 young candidates. Using the confirmed members, a new empirical model of the X-ray luminosity was developed. The X-ray luminosity was found to be about 4 times higher for stars around ~8-12Myr than for older, ~120Myr stars, thus, this observable is a good age indicator in this range.
Comparing the spectra of 59 young candidate members to atmosphere models allowed to determine three basic parameters: the bolometric luminosity, the effective temperature and the radius. Overall, these candidates are more luminous and have a greater radius than old stars. Recent evolutionary models that include the rotational dynamo-type treatment and produce magnetic field strength of 1 to 2.5 kGauss were used to derive an isochronal age for each star. The ages determined for \beta Pictoris moving group members using stars of different spectral types are coherent with one another (types K5V-M0V: 24 Mans, types M1V-M4V: 14 Mans) and are also coherent with age determined independently using lithium abundance of the low-mass members (26 Mans)
BANYAN. II. Very Low Mass and Substellar Candidate Members to Nearby, Young Kinematic Groups With Previously Known Signs of Youth
We present Bayesian Analysis for Nearby Young AssociatioNs II (BANYAN II), a
modified Bayesian analysis for assessing the membership of later-than-M5
objects to any of several Nearby Young Associations (NYAs). In addition to
using kinematic information (from sky position and proper motion), this
analysis exploits 2MASS-WISE color-magnitude diagrams in which old and young
objects follow distinct sequences. As an improvement over our earlier work, the
spatial and kinematic distributions for each association are now modelled as
ellipsoids whose axes need not be aligned with the Galactic coordinate axes,
and we use prior probabilities matching the expected populations of the NYAs
considered versus field stars. We present an extensive contamination analysis
to characterize the performance of our new method. We find that Bayesian
probabilities are generally representative of contamination rates, except when
a parallax measurement is considered. In this case contamination rates become
significantly smaller and hence Bayesian probabilities for NYA memberships are
pessimistic. We apply this new algorithm to a sample of 158 objects from the
literature that are either known to display spectroscopic signs of youth or
have unusually red near-infrared colors for their spectral type. Based on our
analysis, we identify 25 objects as new highly probable candidates to NYAs,
including a new M7.5 bona fide member to Tucana-Horologium, making it the
latest-type member. In addition, we reveal that a known L2{\gamma} dwarf is
co-moving with a bright M5 dwarf, and we show for the first time that two of
the currently known ultra red L dwarfs are strong candidates to the AB Doradus
moving group. Several objects identified here as highly probable members to
NYAs could be free-floating planetary-mass objects if their membership is
confirmed.Comment: 35 pages, 10 figures; accepted for publication in The Astrophysical
Journal (in press); Several typographic correction
Optical and Near-Infrared Radial Velocity Content of M Dwarfs: Testing Models with Barnard's Star
High precision radial velocity (RV) measurements have been central in the
study of exoplanets during the last two decades, from the early discovery of
hot Jupiters, to the recent mass measurements of Earth-sized planets uncovered
by transit surveys. While optical radial-velocity is now a mature field, there
is currently a strong effort to push the technique into the near-infrared (nIR)
domain (chiefly , , and band passes) to probe planetary systems
around late-type stars. The combined lower mass and luminosity of M dwarfs
leads to an increased reflex RV signal for planets in the habitable zone
compared to Sun-like stars. The estimates on the detectability of planets rely
on various instrumental characteristics, but also on a prior knowledge of the
stellar spectrum. While the overall properties of M dwarf spectra have been
extensively tested against observations, the same is not true for their
detailed line profiles, which leads to significant uncertainties when
converting a given signal-to-noise ratio to a corresponding RV precision as
attainable on a given spectrograph. By combining archival CRIRES and HARPS data
with ESPaDOnS data of Barnard's star, we show that state-of-the-art atmosphere
models over-predict the and -band RV content by more than a factor of
, while under-predicting the and -band content by half.Comment: accepted for publication in A
The Coolest Isolated Brown Dwarf Candidate Member of TWA
We present two new late-type brown dwarf candidate members of the TW Hydrae
association (TWA) : 2MASS J12074836-3900043 and 2MASS J12474428-3816464, which
were found as part of the BANYAN all-sky survey (BASS) for brown dwarf members
to nearby young associations. We obtained near-infrared (NIR) spectroscopy for
both objects (NIR spectral types are respectively L1 and M9), as well as
optical spectroscopy for J1207-3900 (optical spectral type is L0{\gamma}), and
show that both display clear signs of low-gravity, and thus youth. We use the
BANYAN II Bayesian inference tool to show that both objects are candidate
members to TWA with a very low probability of being field contaminants,
although the kinematics of J1247-3816 seem slightly at odds with that of other
TWA members. J1207-3900 is currently the latest-type and the only isolated
L-type candidate member of TWA. Measuring the distance and radial velocity of
both objects is still required to claim them as bona fide members. Such
late-type objects are predicted to have masses down to 11-15 MJup at the age of
TWA, which makes them compelling targets to study atmospheric properties in a
regime similar to that of currently known imaged extrasolar planets.Comment: 8 pages, 4 figures, accepted for publication in the ApJ Letter
BANYAN. IV. Fundamental parameters of low-mass star candidates in nearby young stellar kinematic groups - Isochronal Age determination using Magnetic evolutionary models
Based on high resolution optical spectra obtained with ESPaDOnS at CFHT, we
determine fundamental parameters (\Teff, R, \Lbol, \logg\ and metallicity) for
59 candidate members of nearby young kinematic groups. The candidates were
identified through the BANYAN Bayesian inference method of \citet{2013malo},
which takes into account the position, proper motion, magnitude, color, radial
velocity and parallax (when available) to establish a membership probability.
The derived parameters are compared to Dartmouth Magnetic evolutionary models
and to field stars with the goal to constrain the age of our candidates. We
find that, in general, low-mass stars in our sample are more luminous and have
inflated radii compared to older stars, a trend expected for pre-main sequence
stars. The Dartmouth Magnetic evolutionary models show a good fit to
observations of field K and M stars assuming a magnetic field strength of a few
kG, as typically observed for cool stars. Using the low-mass members of
Pictoris moving group, we have re-examined the age inconsistency problem
between Lithium Depletion age and isochronal age (Hertzspring-Russell diagram).
We find that the inclusion of the magnetic field in evolutionary models
increase the isochronal age estimates for the K5V-M5V stars. Using these models
and field strengths, we derive an average isochronal age between 15 and 28 Myr
and we confirm a clear Lithium Depletion Boundary from which an age of
263~Myr is derived, consistent with previous age estimates based on this
method.Comment: Accepted for publication in Ap
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