Sebastian Daemgen

Lucky Imaging survey for southern M dwarf binaries

Context. 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. Aims. In the largest M dwarf binary survey to date, we search for companions to active, and thus preferentially young, M dwarfs in the solar neighbourhood. We study their binary/multiple properties, such as the multiplicity frequency and distributions of mass-ratio and separation, and identify short period visual binaries, for which orbital parameters and hence dynamical mass estimates can be derived in the near future. �

Stellar companions to exoplanet host stars: Lucky Imaging of transiting planet hosts

Observed properties of stars and planets in binary/multiple star systems provide clues to planet formation and evolution. We extended our survey for visual stellar companions to the hosts of transiting exoplanets by 21 stars, using the Lucky Imaging technique with the two AstraLux instruments: AstraLux Norte at the Calar Alto 2.2-m telescope, and AstraLux Sur at the ESO 3.5-m New Technology Telescope at La Silla. We present observations of two previously unknown binary candidate companions, to the transiting planet host stars HAT-P-8 and WASP-12, and derive photometric and astrometric properties of the companion candidates. The common proper motions of the previously discovered candidate companions with the exoplanet host stars TrES-4 and WASP-2 are confirmed from follow-up observations. A Bayesian statistical analysis of 31 transiting exoplanet host stars observed with AstraLux suggests that the companion star fraction of planet hosts is not significantly different from that of solar-type field stars, but that the binary separation is on average larger for planet host stars.

Discovery of nine new companions to nearby young M stars with the altair ao system

We present results of a high-resolution, near-infrared survey of 41 nearby, young (300 Myr) M0-M5.0 dwarfs using the Altair natural guide star adaptive optics system at the Gemini North telescope. Twelve of the objects appear to be binaries, seven of which are reported here for the first time. One triple system was discovered. Statistical properties are studied and compared with earlier (F to K) and later (≥M6 very low mass [VLM]) populations. We find that the separation distribution of the binaries in this sample peaks at 13 AU, which is consistent with previous measurements of early M binaries. Hence, early M binaries seem to occur in—on average—tighter systems than G binaries. At the same time they are significantly wider than field VLM binary stars. The distribution of mass ratios q of primary and secondary stars was found to show an intermediate distribution between the strongly q → 1 peaked distribution of field VLM systems and the almost flat distribution of earlier type stars. Consequently, we show evidence for relatively young, early M binaries representing a transition between the well-known earlier star distributions and the recently examined field VLM population characteristics. Despite the fact that this survey was dedicated to the search for faint brown dwarf and planetary mass companions, all planetary mass candidates were background objects. We exclude the existence of physical companions with masses greater than 10 Jupiter masses (MJ) at separations of 40 AU and masses greater than 24MJ for separations 10 AU around 37 of the 41 observed objects.

SUB-STELLAR COMPANIONS AND STELLAR MULTIPLICITY IN THE TAURUS STAR-FORMING REGION

We present results from a large, high-spatial-resolution near-infrared imaging search for stellar and sub-stellar companions in the Taurus-Auriga star-forming region. The sample covers 64 stars with masses between those of the most massive Taurus members at ~3 M_sun and low-mass stars at ~0.2 M_sun. We detected 74 companion candidates, 34 of these reported for the first time. Twenty-five companions are likely physically bound, partly confirmed by follow-up observations. Four candidate companions are likely unrelated field stars. Assuming physical association with their host star, estimated companion masses are as low as ~2 M_Jup. The inferred multiplicity frequency within our sensitivity limits between ~10-1500 AU is 26.3(+6.6/-4.9)%. Applying a completeness correction, 62(+/-14)% of all Taurus stars between 0.7 and 1.4 M_sun appear to be multiple. Higher order multiples were found in 1.8(+4.2/-1.5)% of the cases, in agreement with previous observations of the field. We estimate a sub-stellar companion frequency of ~3.5-8.8% within our sensitivity limits from the discovery of two likely bound and three other tentative very low-mass companions. This frequency appears to be in agreement with what is expected from the tail of the stellar companion mass ratio distribution, suggesting that stellar and brown dwarf companions share the same dominant formation mechanism. Further, we find evidence for possible evolution of binary parameters between two identified sub-populations in Taurus with ages of ~2 Myr and ~20 Myr, respectively.

Protoplanetary disks of T Tauri binary systems in the Orion nebula cluster

Aims. We present a study of protoplanetary disks in spatially resolved low-mass binary stars in the well-known Orion Nebula Cluster (ONC) to assess the impact of binarity on the properties of circumstellar disks. This is currently the largest such study in a clustered high-stellar-density star-forming environment, as opposed to previous studies, which have mostly been focussed on the young, lowstellar-density Taurus association. We particularly aim t o determine the presence of magnetospheric accretion and dust disks for each binary component, and measure the overall disk frequency. Methods. We carried out spatially resolved adaptive-optics-assist ed observations to acquire near-infrared photometry and spectroscopy of 26 binaries in the ONC, and determine stellar parameters such as effective temperatures, spectral types, luminosities, and masses, as well as accretion properties and near-infrared excesses for the individual binary components. On the basis of our medium resolution K-band spectroscopy, we infer the presence of magnetospheric accretion around each binary component by measuring the strength of the Brackett-γ emission. The accretion disk frequency among the ONC binaries is then estimated from Bayesian statistics. The observed disk signatures, measured accret ion luminosities, and mass accretion rates are investigate d with respect to the binary separation, mass ratios, and distance to the center o f the ONC. Results. A fraction of 40 +10 −9 % of the binary components in the sample can be inferred to be T Tauri stars possessing an accretion disk. This is only marginally smaller than the disk fraction of single stars of∼50% in the ONC. We find that disks in wide binaries of >200 AU separation are consistent with random pairing, while the evolution of circumprimary and circumsecondary disks is observed to be synchronized in close binaries (separations <200AU). Circumbinary disks appear to be unsuitable to explain this difference. Furthermore, we identify several mixed pairs of accreting and non-accreting components, suggesting that these systems are common and that there is no preference for either the more or less massive component to evolve faster. The derived accretion luminosities and mass accretion rates of the ONC binary components are of similar magnitude as those for both ONC single stars and binaries in the Taurus star-forming region. The paper concludes with a discussion of the (presumably weak) connection between the presence of inner accretion disks in young binary systems and the existence of planets in stellar multiples.

Brown dwarf disks with Herschel: Linking far-infrared and (sub)-mm fluxes

Brown dwarf disks are excellent laboratories to test our understanding of disk physics in an extreme parameter regime. In this paper we investigate a sample of 29 well-characterized brown dwarfs and very low mass stars, for which Herschel far-infrared fluxes as well as (sub)-mm fluxes are available. We have measured new Herschel PACS fluxes for 11 objects and complement these with (sub)-mm data and Herschel fluxes from the literature. We analyze their spectral energy distributions in comparison with results from radiative transfer modeling. Fluxes in the far-infrared are strongly affected by the shape and temperature of the disk (and hence stellar luminosity), whereas the (sub)-mm fluxes mostly depend on disk mass. Nevertheless, there is a clear correlation between far-infrared and (sub)-mm fluxes. We argue that the link results from the combination of the stellar mass-luminosity relation and a scaling between disk mass and stellar mass. We find strong evidence of dust settling to the disk midplane. The spectral slopes between near- and far-infrared are mostly between

A New Sub-stellar Companion around the Young Star HD 284149

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RESOLVING THE PLANET-HOSTING INNER REGIONS OF THE LkCa 15 DISK

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The frequency of accretion disks around single stars: Chamaeleon I

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Stellar and circumstellar properties of visual binaries in the Orion Nebula Cluster

in our sample, comparable to more massive T Tauri stars, which may imply that the disk shapes are similar as well, though highly-flared disks are rare among brown dwarfs. We find that dust temperatures in the range of 7-15 K, calculated with