Fundamental Properties of very low mass star and brown dwarf binaries

This work presents the results of a detailed study of the statistical and physical properties of binary ultracool dwarfs and brown dwarfs (spectral type later than M7). As for the statistical properties, we found that the frequency of binaries among ultracool objects is significantly lower than for earlier type objects, with a lower limit at 10--15% in the field, and <10% in the Pleiades Open cluster. While we were sensitive to systems with separations up to 100 A.U, we did not find any multiple system with separation greater than 20 A.U. At even larger separations, no wide binaries were reported by the all sky surveys such as 2MASS, DENIS or SDSS. The distribution of separations looks similar to that of F and G dwarfs (Gaussian), but with a peak at 4--8 A.U. Although we were sensitive to mass ratios down to 0.65, we found most of the objects to have mass ratios larger than 0.85. This latter result needs to be confirmed by further statistical studies on well defined statistical samples. Although the sample of known binaries in the Pleiades is too small for a similar analysis, we note that the binary frequency, the distributions of mass ratio and separations are similar, indicating that the properties of binary brown dwarfs might not depend on the age and environment after 125 Myr. Finally, although we did not have the opportunity to perform a similar statistical study in a star forming region, we report the first detection of a young binary brown dwarf with a disk in the R-CrA association. These results provide strong constraints on the models of ultracool dwarf formation and evolution. The binary frequency is currently not reproduced by any of these models. The models of ejection could explain the lack of binaries wider than 20 A.U and the apparent preference for equal mass systems, but they predict a much lower binary frequency. The model assuming that brown dwarfs form in a smilar way than stars could reproduce the binary frequency we observe, but could not explain the distributions of separation and mass ratio. More efforts are required on the theoretical side in order to better explain the observed properties, and on the observational side to give new and improved constraints. As for the physical properties, our observations lead to the discovery of a short period binary L dwarf. Observations at high angular resolution spread over 4 years allowed us to follow the companion on 60% of its orbit. For the first time, we were then able to compute the orbital parameters and total mass of such a very low mass object. In the near future, similar studies should enable us to calibrate brown dwarf mass and luminosity models. Using high angular resolution spectroscopy, we were able to disentangle the spectra of the individual components of 4 binary ultracool dwarfs and to compute their spectral types. Two binaries have companions with spectral types significantly later than their primary (by 3 to 4 spectral subclasses), allowing us to compare the evolution of their effective temperature and atmosphere. Finally, using our high angular resolution images, we were able to detect a possible third component in one of the binaries of our sample.Diese Arbeit praesentiert die Ergebnisse einer detaillierten Untersuchung der statistischen und physikalischen Eigenschaften von ultrakuehlen Zwergsternen und braunen Zwergen (Spektraltyp spaeter als M7). Hinsichtlich der statistischen Eigenschaften fanden wir, dass, die Haeufigkeit doppelter Systeme unter ultrakuehlen Objekten mit 10--15% im Feld und <10% in dem offenen Sternhaufen Pleiaden signifikant niedriger ist als fuer Objekte mit frueherem Spektraltyp. Obwohl wir Systeme mit Abstaenden bis zu 100 A.U. haetten nachweisen koennen, fanden wir keine Mehrfachsysteme mit Abstaenden groesser als 20 A.U. Die Himmeldurchmusterungen 2MASS, DENIS oder SDSS entdeckten kein einziges weites Doppelsystem mit noch groesseren Abstaenden. Die Verteilung der Abstaende aehnelt der von F- und G-Zwergen (Gaussverteilung), allerdings mit einem Maximum bei 4--8 A.U. Die meisten Doppelsysteme weisen Massenverhaeltnisse groesser als 0.85 auf, obwohl wir Doppelsysteme mit Massenverhaeltnissen bis hinunter zu 0.6 haetten nachweisen koennen. Dieses Ergebnis muss, durch zukuenftige Untersuchungen statistisch wohldefinierter Stichproben ueberprueft werden. Obwohl die Stichprobe der bekannten Doppelsysteme in den Pleiaden zu klein ist fuer eine den Feldobjekten vergleichbare Untersuchung, weisen wir darauf hin, dass, die Haeufigkeit der Doppelsysteme und die Verteilung der Massenverhaeltnisse und der Abstaende aehnlich sind. Dies deutet an, dass, sich die Eigenschaften doppelter brauner Zwerge ab einem Alter von etwa 125 Myr nicht mehr als Funktion des Alters oder der Umgebung aendern. Obwohl keine Gelegenheit bestand, eine aehnliche statistisch signifikante Untersuchtung in einem Sternentstehungsgebiete durchzufuehren, fuehrte diese Arbeit zur Entdeckung des ersten jungen, doppelten braunen Zwerges in der CrA-Assoziation. Die Ergebnisse liefern starke Randbedingungen fuer Entstehungs- und Entwicklungsmodelle brauner Zwerge. Die Haeufigkeit der Doppelsysteme wird momentan von keinem der Modelle richtig wiedergegeben. Dynamische Ausstossmodelle koennen den Mangel an weiten Doppelsystemen mit Abstaenden groesser als 20 A.U. und das bevorzugte Auftreten von Doppelsystemen mit Komponenten gleicher Masse erklaeren. Diese Modelle sagen allerdings eine noch geringere Haeufigkeit von doppelten Systemen voraus. Der Sternentstehung analoge Fragmentations- und Kollapsmodelle koennen die beobachtete Haeufigkeit der Doppelsysteme erklaeren, allerdings nicht die Verteilung der Abstaende und Massenverhaeltnisse. Weiterreichende theoretische Modelle sind nun notwendig, um die beobachteten Eigenschaften zu erklaeren. Von Seiten der Beobachtung sollten die Randbedingungen weiter eingeschraenkt werden. Im Hinblick auf die physikalischen Eigenschaften der untersuchten Systeme, fuehrten unsere Beobachtungen zur Entdeckung eines doppelten L-Zwerges mit kurzer Umlaufperiode. Ueber vier Jahre verteilte Beobachtungen mit hoher raeumlichen Aufloesung ermoeglichten uns, 60% der Umlaufbahn des Doppelsystems zu vermessen. Zum ersten Mal waren wir hiermit in der Lage die Bahnparameter und die Gesamtmasse eines derartig massearmen Doppelsystems zu bestimmen. In naher Zukunft werden aehnliche Studien die Eichung der Masse- und Leuchtkraftmodelle von braunen Zwergen ermoeglichen. Spektroskopische Beobachtungen mit hoher Winkelaufloesung erlaubten uns die Spektraltypen der individuellen Komponenten in vier Doppelsystemen zu bestimmen. Zwei der Begleiter weisen deutlich spaetere Spektraltypen auf als die Primaerkomponenten (3 bis 4 spektrale Unterklassen), was vergleichende Untersuchungen des Abkuehlverhaltens und der Atmosphaereneigenschaften ermoeglicht. Darueber hinaus konnten wir in unseren hochaufloesenden Bildern ein moegliches Dreifachsystem identifizieren.Ce travail présente les résultats d'une étude détaillée des propriétés statistiques et physiques des binaires de naines brunes et ultra-froides (type spectral plus tardif que M7). Concernant les propriétés statistiques, nous constatons que la fréquence de binaires parmi les objets ultra-froids est inférieure a celle des objets de types spectraux plus precoces, avec une limite inférieure à 10~15% pour les objets du champ, et <10% dans l'amas ouvert des Pleiades. Alors que nous étions capables d'identifier les systèmes binaires jusqu'a des séparations de ~100 U.A, nous n'avons trouvé aucun système mutliple aux séparations superieures a 20~U.A. A plus grande séparation encore, aucune binaire n'a été trouvée par les surveys 2MASS, DENIS ou SDSS. La distribution des séparations est semblable à celle des naines F et G (gaussienne), mais avec un maximum autour de 4~8 U.A. Bien que nous étions sensibles aux binaires avec des rapports de masse allant jusqu'a 0,6, nous n'avons detecte aucune binaire avec un rapport de masse inferieur a ~0,6; la plupart des objets ayant un rapport de masse plus grand que 0,85. Ce résultat devra être confirmé par d'autres études sur des échantillons mieux definis statistiquement. Bien que l'échantillon de binaires connues dans les Pleiades soit trop petit pour nous permettre de faire une analyse semblable, nous obervons que la fréquence de binaires, les distributions de rapport de masse et de séparations sont semblables, indiquant que les propriétés des naines brunes binaires ne dépendent probablement pas de l'âge et de l'environnement passe 125 millions d'annees. Enfin, bien que nous n'ayons pas eu l'occasion de pouvoir mener une etude statistique similaire dans une region de formation stellaire, nous presentons la première détection d'une jeune naine brune binaire entouree d'un disque dans la region de la Couronne Australe (R-CrA). Ces résultats apportent de fortes contraintes sur les modèles de formation et d'évolution. La fréquence de binaires n'est actuellement reproduite correctement par aucun des modèles. Les modèles d'éjection pourraient expliquer le manque de binaires aux separations superieures a 20 U.A ainsi que le manque apparent de systèmes de faibles rapports de masses, mais elle prévoit une fréquence de binaires beaucoup trop faible. Le modele prevoyant que les naines brunes se forment d'une maniere analogue aux etoiles pourrait quant a lui reproduire la fréquence de binaires que nous observons, mais ne pourrait pas expliquer les distributions de rapport de séparation et de masse. Plus d'efforts sont donc necessaires a la fois du côté théorique afin de pouvoir expliquer les propriétés observées, et du côté observationnel pour apporter des contraintes nouvelles et plus precises. Concernant les propriétés physiques des binaires de naines ultra-froides, nos observations ont mene à la découverte d'une binaire de naines L de courte periode. Les observations à haute résolution angulaire réparties sur quatre ans nous ont permis de suivre le compagnon sur 60% de son orbite. Pour la première fois, nous avons pu calculer les paramètres orbitaux et la masse totale d'un objet de ce type. A moyen terme, ce genre de mesures devraient nous permettre de calibrer la très importante relation masse-luminosite. En utilisant la spectroscopie a haute résolution angulaire, nous avons ete en mesure de separer les spectres des composantes individuelles de quatres binaires de naines ultra-froides, et de calculer leurs types spectraux. Deux de ces binaries ont des compagnons de types spectraux relativement plus tardifs que leurs primaires (3 et 4 sous-classes spectrales de plus), nous permettant de comparer l'evolution de leurs temperatures effectives et leurs atmospheres. Enfin, utilisant nos images a haute résolution angulaire, nous avons détecte une possible troisième composante dans une des binaries de notre échantillon

AA Tau's sudden and long-lasting deepening: enhanced extinction by its circumstellar disk

AA Tau has been monitored for more than 20 years since 1987, exhibiting a nearly constant brightness level of V=12.5 mag. We report here that in 2011 it suddenly faded away, becoming 2 magnitudes fainter in the V-band, and has remained in this deep state since then. We report new optical and near-IR photometry and spectroscopy obtained during the fading event. The system appears much redder and fainter than it was in the bright state. Also, the 8.2d photometric period continuously observed for more than 20 years is not seen during most of the deep state. The analysis of the system's brightness and colors suggests that the visual extinction on the line of sight has increased by about 3-4 magnitudes in the deep state. At optical wavelengths, the system appears to be dominated by scattered light, probably originating from the upper surface layers of a highly inclined circumstellar disk. The profiles of the Balmer lines have significantly changed as well, with the disappearance of a central absorption component regularly observed in the bright state. We ascribe this change to the scattering of the system's spectrum by circumstellar dust. Noticeably, the mass accretion rate in the inner disk and onto the central star has not changed as the system faded away. We conclude that the deepening of the AA Tau system is due to a sudden increase of circumstellar dust extinction on the line of sight without concomitant change in the accretion rate. We suggest that the enhanced obscuration may be produced by a non-axisymmetric overdense region in the disk, located at a distance of 7.7 AU or more, that was recently brought on the line of sight by its keplerian motion around the central star.Comment: 9 pages, 6 figures, Astronomy & Astrophysics; english language edited, email address update

A search for passive protoplanetary disks in the Taurus-Auriga star-forming region

We conducted a 12-month monitoring campaign of 33 T Tauri stars (TTS) in Taurus. Our goal was to monitor objects that possess a disk but have a weak Halpha line, a common accretion tracer for young stars, to determine whether they host a passive circumstellar disk. We used medium-resolution optical spectroscopy to assess the objects' accretion status and to measure the Halpha line. We found no convincing example of passive disks; only transition disk and debris disk systems in our sample are non-accreting. Among accretors, we find no example of flickering accretion, leading to an upper limit of 2.2% on the duty cycle of accretion gaps assuming that all accreting TTS experience such events. Combining literature results with our observations, we find that the reliability of traditional Halpha-based criteria to test for accretion is high but imperfect, particularly for low-mass TTS. We find a significant correlation between stellar mass and the full width at 10 per cent of the peak (W10%) of the Halpha line that does not seem to be related to variations in free-fall velocity. Finally, our data reveal a positive correlation between the Halpha equivalent width and its W10%, indicative of a systematic modulation in the line profile whereby the high-velocity wings of the line are proportionally more enhanced than its core when the line luminosity increases. We argue that this supports the hypothesis that the mass accretion rate on the central star is correlated with the Halpha W10% through a common physical mechanism.Comment: accepted for publication in MNRAS; 26 pages, 9 figures, 3 table

Stellar population of the Rosette Nebula and NGC 2244: application of the probabilistic random forest

(Abridged) In this work, we study the 2.8x2.6 deg2 region in the emblematic Rosette Nebula, centred at the young cluster NGC 2244, with the aim of constructing the most reliable candidate member list to date, determining various structural and kinematic parameters, and learning about the past and the future of the region. Starting from a catalogue containing optical to mid-infrared photometry, as well as positions and proper motions from Gaia EDR3, we apply the Probabilistic Random Forest algorithm and derive membership probability for each source. Based on the list of almost 3000 probable members, of which about a third are concentrated within the radius of 20' from the centre of NGC 2244, we identify various clustered sources and stellar concentrations, and estimate the average distance of 1489+-37 pc (entire region), 1440+-32 pc (NGC 2244) and 1525+-36 pc (NGC 2237). The masses, extinction, and ages are derived by SED fitting, and the internal dynamic is assessed via proper motions relative to the mean proper motion of NGC 2244. NGC 2244 is showing a clear expansion pattern, with an expansion velocity that increases with radius. Its IMF is well represented by two power laws (dN/dM\propto M^{-\alpha}), with slopes \alpha = 1.05+-0.02 for the mass range 0.2 - 1.5 MSun, and \alpha = 2.3+-0.3 for the mass range 1.5 - 20 MSun, in agreement with other star forming regions. The mean age of the region is ~2 Myr. We find evidence for the difference in ages between NGC 2244 and the region associated with the molecular cloud, which appears slightly younger. The velocity dispersion of NGC 2244 is well above the virial velocity dispersion derived from the total mass (1000+-70 MSun) and half-mass radius (3.4+-0.2 pc). From the comparison to other clusters and to numerical simulations, we conclude that NGC 2244 may be unbound, and possibly even formed in a super-virial state.Comment: 30 pages, 28 figures. Accepted for publication in Astronomy & Astrophysic

Cygnus OB2 DANCe: A high-precision proper motion study of the Cygnus OB2 association

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. © 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.We present a high-precision proper motion study of 873 X-ray and spectroscopically selected stars in the massive OB association Cygnus OB2 as part of the DANCe project. These were calculated from images spanning a 15 yr baseline and have typical precisions <1 mas yr−1. We calculate the velocity dispersion in the two axes to be σα(c) = 13.0+0.8 −0.7 and σδ (c) = 9.1+0.5 −0.5 km s−1, using a two-component, two-dimensional model that takes into account the uncertainties on the measurements. This gives a three-dimensional velocity dispersion of σ3D = 17.8 ± 0.6 km s−1 implying a virial mass significantly larger than the observed stellar mass, confirming that the association is gravitationally unbound. The association appears to be dynamically unevolved, as evidenced by considerable kinematic substructure, non-isotropic velocity dispersions and a lack of energy equipartition. The proper motions show no evidence for a global expansion pattern, with approximately the same amount of kinetic energy in expansion as there is in contraction, which argues against the association being an expanded star cluster disrupted by process such as residual gas expulsion or tidal heating. The kinematic substructures, which appear to be close to virial equilibrium and have typical masses of 40– 400 M, also do not appear to have been affected by the expulsion of the residual gas. We conclude that Cyg OB2 was most likely born highly substructured and globally unbound, with the individual subgroups born in (or close to) virial equilibrium, and that the OB association has not experienced significant dynamical evolution since then.Peer reviewe

An HST/WFPC2 Survey for Brown Dwarf Binaries in the alpha Per and the Pleiades Open Clusters

We present the results of a high-resolution imaging survey for brown dwarf (BD) binaries in two open clusters. The observations were carried out with WFPC2 onboard HST. Our sample consists of 8 BD candidates in the alpha Per cluster and 25 BD candidates in the Pleiades. We have resolved 4 binaries in the Pleiades with separations in the range 0".094--0".058, corresponding to projected separations between 11.7~AU and 7.2~AU. No binaries were found among the alpha Per targets. Three of the binaries have proper motions consistent with cluster membership in the Pleiades cluster, and for one of them we report the detection of Halpha in emission and LiI absorption obtained from Keck~II/ESI spectroscopy. One of the binaries does not have a proper motion consistent with Pleiades membership. We estimate that BD binaries wider than 12~AU are less frequent than 9% in the alphaPer and Pleiades clusters. This is consistent with an extension to substellar masses of a trend observed among stellar binaries: the maximum semimajor axis of binary systems decreases with decreasing primary mass. We find a binary frequency of 2 binaries over 13 BDs with confirmed proper motion membership in the Pleiades, corresponding to a binary fraction of 15%(1 sigma error bar +15%/-5%). These binaries are limited to the separation range 7-12~AU and their mass ratios are larger than 0.7. The relatively high binary frequency (>10%), the bias to separations smaller than about 15 AU and the trend to high mass ratios (q>0.7) are fundamental properties of BDs. Current theories of BD formation do not appear to provide a good description of all these properties.Comment: Accepted by ApJ (scheduled publication in volume 594, September 1, 2003