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10. May 2011 (Multifunktionsraum!): Mickael Bonnefoy: "A Very Cool Pair of Brown Dwarfs" review of the article "CFBDSIR J1458+1013B: A Very Cold (>T10) Brown Dwarf in a Binary System", Liu et al. 2011, ApJ (ApJ paper, ESO press release) 6. Apr 2011: Min Fang: "Brown dwarf formation by disk instability: do the required massive disks exist?" review of the article "The lower limits of disc fragmentation and the prospects for observing fragmenting discs", Stamatellos et al. 2011, MNRAS (arXiv1012.3455) 24. Nov 2010: Beth Biller: A Keck LGS AO Search for Planetary Mass Companions to USco Brown Dwarfs: Constraining the Evolution of BD Binarity as a Function of Age Abstract: Understanding multiplicity as a function of age for young brown dwarfs and planetary mass objects is critical for discriminating between the (numerous) models of brown dwarf formation. For instance, a small number of wide young brown dwarf binaries have recently been discovered (Chauvin et al. 2005, Konopacky et al., Close et al. 2007). If such binaries are common, they pose a challenge for ejection scenarios of brown dwarf formation, since such systems should be disrupted at a very early age according to these models. Here we discuss results from a binarity survey to detect wide young brown dwarf binaries in the nearby Upper Sco embedded cluster (age ~ 5 Myr). We are sensitive to separations of 0.07" to 5", corresponding to ~10-700 AU at an adopted distance of 145 pc to Upper Scorpius. We imaged 21 Upper Sco brown dwarfs (from Lodieu et al. 2008 and Slesnick et al. 2008) with the Keck II Laser Guide Star Adaptive Optics system. Previous binarity studies in Upper Sco concentrated on objects right at the stellar/substellar mass limit (Kraus et al. 2005, Kraus et al. 2008, Bouy et al. 2006); all our objects are clearly substellar and are the lowest mass objects surveyed in Upper Sco to date. Comparing our binary fraction with results from the literature for other young star-forming regions as well as older field brown dwarfs, we place constraints on the evolution of brown dwarf binarity as a function of age. Benoit Commercon & Wolfgang Brandner: Summary of the conference "The Origin of Stellar Masses", Tenerife, Oct 2010 21. July 2010: Carolina Bergfors: The AstraLux binary M dwarfs survey: First results from the southern sky While M dwarfs are the most abundant stars in the Milky Way, there is still large uncertainty about their basic physical properties (mass, luminosity, radius, etc.), as well as their formation environment. Precise knowledge of multiplicity characteristics and how they change in this transitional mass region, between Sun-like stars on the one side and very low mass stars and brown dwarfs on the other, provide constraints on low mass star and brown dwarf formation. I will present the first results from the AstraLux M dwarf survey, a high-resolution Lucky Imaging search for low-mass companions to ~800 active, nearby M dwarfs and the largest M dwarf multiplicity survey to date. For the first subsample, we have derived relative astrometry and photometry, and multiplicity characteristics such as multiplicity fraction and distributions of mass-ratio and separation. 30. June 2010: Bertrand Goldman: First T dwarf discovered with the PanSTARRS 1 Survey Abstract: PanSTARRS1 has started regular observations early May and we are striving to use the recent data and the commissioning data of the past year to prove the system efficiency. The search for nearby brown dwarfs, in particular bright enough but missed by previous surveys for a variety of reasons, or faint, extremely cool brown dwarfs, are crucial to our understanding of the Solar neighbourhood and the brown dwarf luminosity function. In this talk I'll report on the PS1 system and the first brown dwarf (5 so far!) confirmed discoveries. Benoit Commercon: Calculations of low mass star and brown dwarf formation by gravitational collapse Abstract: I will present ongoing work of 3D radiation magneto hydrodynamic (RMHD) calculations of low mass star formation, for a range of initial dense core mass ranging from 1 and 0.1 solar mass. I will study the effect of radiative transfer and magnetic field on the collapse and fragmentation. Eventually, I will compare my results to those of Machida et al 2009. 26. May 2010: Paul Clark (ITA): The formation of brown dwarfs in gravitationally-sheared filaments Abstract: We review a new model for the formation of BDs, that suggests that BDs and low-mass stars can form via the break-up of infalling gaseous filaments in stellar clusters. As the filaments fall towards the cluster centre, they are compressed by gravitational potential of the system, lowering the local Jeans mass in the filament. When the Jeans mass becomes sufficiently low, and when the associated density is above the limit required for the gas to be stable against tidal shear from the cluster potential, the filament is able to fragment, forming low-mass objects. Further, the tidal shear and high-velocity dispersion present in the cluster preclude any subsequent accretion. Ejections are not required as the BDs enter the cluster with high relative velocities, suggesting that their disk and binary properties should be similar to that of low-mass stars. This mechanism requires the presence of a strong gravitational potential due to the stellar cluster, implying that BD formation should be more frequent in stellar clusters than in distributed populations of young stars. 27. January 2010: Micaela Stumpf: IR-Spectral properties of methane brown dwarfs in general and Eps Ind BaBb in particular Wolfgang Brandner: The 800 K to 1100 K planets around HR 8799 - how do their spectral characteristics compare to methane brown dwarfs (T-dwarfs)? (For more details see MPIA Press Release from January 13, 2010 and ApJ Letter ) 25. November 2009: Cynthia Knight: Polarisation of field brown dwarfs Abstract: L-type brown dwarfs (Teff = 1400-2100K) are known for their dusty photospheres. Brown dwarfs may have asymmetric surfaces arising from rotationally-induced flattening or large-scale cloud cover and patchiness.This broken symmetry may prevent polarization by dust-scattering to cancel out, resulting in a non-zero polarization. Therefore, studying polarization provides useful insight into surface heterogeneities on brown dwarfs. Zapatero Osorio et al. (2005), Menard et al. (2002), and Goldman et al. (2009) report that some brown dwarfs have small polarization degrees on the order of 0.1%. Their results also suggest variability. In my project I am aiming at confirmation of past results through re-investigating previous targets as well as through expanding the sample size. I will also discuss the implications of a possible variable star - 2MASS J150747698722-1627386 - and compare our results to those of Tata et al. (2009), Zapatero Osorio et al.(2005), and Goldman et al. (2009). 28. October 2009: Hubert Klahr: review of "Giant Planet Formation by Disk Instability: A Comparison Simulation With An Improved Radiative Scheme" by Cai et al., ApJL (arXiv:0907.4213) Abstract of talk: "Giant planet formation in the standard scenario is considered to start with the formation of a solid core, which triggers run-away gas accretion as it reaches a critical mass of about 10-15 Earth masses. Alternatively to this core accretion scenario, it was suggested that giant planets can form by gravitational instabilities in a circumstellar disk. To form planets by disk instabilities within a few AU of the star requires efficient cooling. The long-standing question if this cooling can happen fast enough to form planets seems now to be answered with "no" in a recent paper by Cai et al., who apply an improved radiative scheme." 26. August 2009: Eva Meyer: Deriving the true mass of an unresolved Brown Dwarf companion with AO aided astrometry Abstract: The true masses of non-transiting, unresolved substellar companions to stars, detected through RV measurements, are only known for a few objects so far and they have all been determined by astrometry with the HST fine guidence sensor. From radial velocity detections alone one does not get all orbital parameters needed to derive the true mass of a substellar companion to a star. Additional astrometric measurements are needed to calculate the inclination and the longitude of the ascending node. The parallax and proper motion of the star must also be considered. We aim to derive the true mass of a brown dwarf candidate companion to an early M dwarf with groundbased high-resolution astrometry aided by adaptive optics. I will show the current status of the orbit determination and necessary astrometric correction need to be done for high precision astrometry. Eva Meyer: Short summary of the conference in ShangHai (19-24 July 09): New Technologies for Probing the Diversity of Brown Dwarfs and Exoplanets Kerstin Geissler: review of "First Direct Simulation of Brown Dwarf Formation in a Compact Cloud Core" by Machida et al. (arXiv:0907.3255) 22. July 2009: Christoph Mordasini: "On the maximal mass of companions: Giant Planets, Deuterium Burning Planets and Brown Dwarfs" Abstract: "Although rare, some companions to FGK stars with masses between 10 to 20 Jupiter masses have been detected in the past decade. In the standard picture of giant planet formation, gap formation in the protoplanetary gas disk limits planetary masses to about 5-10 Jupiter masses. Both theoretical and observational results however indicate that gap formation might not be a hard limit. I therefore study giant planet formation on the assumption that gap formation does not affect the planetary gas accretion rate. In this case it is found that core accretion can lead to the formation of planets well above the traditional planet - brown dwarf limit at about 13.6 Jupiter masses. I analyze how frequent such objects are, how their mass distribution looks like and how their formation depends on disk properties. Finally, I discuss the observational signatures of such objects." Patrick Weise: "Vsini distribution of nearby young stars" Abstract: "I will present the results of projected rotational velocity (vsini) measurements of our radial velocity survey with FEROS @ 2.2m ESO/MPG. A new calibration for vsini measurements with FEROS has been used to derive precise vsini values. I will compare the vsini distributions of accretors, stars with thick disks, stars with thin disks and stars without disks and try to find signs of evolution of stellar rotation." 24. June 2009: Sylvain Marsat: "Search for cool brown dwarfs with methane absorption" 25. February 2009: Mathias Zechmeister: "Planets around M dwarfs" Abstract: "I present results from the high precision radial velocity survey with VLT+UVES in search for planetary companions around M dwarfs. The lack of Jupiter-mass companions in this survey is compared with results from other search programs. Upper limits to the mass of hypothetical planets can be placed down to the Earth mass regime in the habitable zones of our better observed stars. Our sample includes the first genuine brown dwarf desert object and several low-mass companion candidates around early-type M dwarfs. Furthermore, I will explain the effect of secular acceleration which we observed in high proper motion stars." 21. January 2009: Patrick Weise: "Search for sub-stellar companions around very young stars with the radial-velocity technique" Abstract: "Young stars are known as very active stars and stellar activity can cause radial-velocity variations with amplitudes up to a few hundred m/s. In my PhD thesis I extend the current spectral analysis methods for old stars to be applicable for young stars. I'll briefly talk about these methods, but will focus on a few first results. After one year of observations with FEROS, there are probably 2 targets with a brown-dwarf companion orbiting within the circumstellar disk." 26. November 2008: Viki Joergens, Joe Carson, and Wolfgang Brandner: "Have we seen the first direct image of an exoplanet ?" Overviews of recent claims of the first direct images of planets orbiting other stars than our Sun, namely Fomalhaut, HR8799, and Beta Pic. Relevant articles: Kalas et al. Science; Marois et al. Science; Lagrange et al.ApJ Letter. Christian Thalmann will shortly present his ideas about follow-up observations of Fomalhaut b. Hubert Klahr will discuss some theoretical aspects of the detections. 25. June 2008: Steve Boudreault: "The radial dependence of the mass function of the young open cluster IC 2391 down to the hydrogen burning limit" Abstract: "I will present the results of our study of the mass function of IC2391 and its radial dependence. We find no convincing evidence for a variation in the substellar mass function with cluster radius, which is in agreement with formation via the premature ejection of accreting embryos. However, we further observe that there is no variation in the mass function over the stellar/substellar boundary at all three radius intervals analysed. From this lack of discontinuity, we conclude that the ejection mechanism cannot be the unique brown dwarf formation path. A radial variation is observed in the mass function from 0.15 to 0.5Msol and we conclude that this is a signature of mass segregation via dynamical evolution." 9. April 2008: Martin Kuerster (MPIA) & Sabine Reffert (LSW): "An Oasis in the Brown Dwarf Desert" Abstract: "This is the story about the discovery of a brown dwarf in a close orbit around an early M-type star, i.e. in that separation and companion mass range, which is known to be rather deserted for solar-type stars. The radial velocity detection of this companion was combined with data from the new edition of the HIPPARCOS catalog. This allowed to determine an absolute mass in the substellar mass regime." 12. March 2008, Joined Disk and Brown Dwarf Splinter: Aurora Sicilia Aguilar: "The realm of transitional disks: Very low-mass objects in the Coronet Cluster" Abstract: "The Coronet Cluster is a young (~1 Myr), low-mass star forming region, containing a numerous population of very low-mass objects. Multiwavelength observations reveal that the fraction of transitional and flattened disks in the Coronet Cluster is higher than in other regions. Is this an effect of the low-mass of the objects, or may be related to the initial conditions and formation in the cloud?" 23. January 2008: Coryn Bailer-Jones: "Atmospheric variability in ultra cool dwarfs" Juliet Datson: review of "Brown dwarfs and very low mass stars in the Hyades cluster: a dynamically evolved mass function" by Bouvier et al. 2008 (http://arxiv.org/abs/0801.0670) 28. November 2007: Micaela Stumpf: "PARSEC: The first spatially resolved H+K- band spectra of the binary Brown Dwarf Eps Ind Bab" Steve Boudreault: review of "A discontinuity in the low-mass initial mass function" by Thies & Kroupa (http://de.arxiv.org/abs/0708.1764) Markus Schmalzl: review of "Brown Dwarf formation by gravitational fragmentation of massive, extended protostellar discs" by Stamatellos et al. (http://de.arxiv.org/abs/0708.2827) Viki Joergens / viki @ mpia.de
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