Felix Hormuth

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. �

GRAVITY: getting to the event horizon of Sgr A*

We present the second-generation VLTI instrument GRAVITY, which currently is in the preliminary design phase. GRAVITY is specifically designed to observe highly relativistic motions of matter close to the event horizon of Sgr A*, the massive black hole at center of the Milky Way. We have identified the key design features needed to achieve this goal and present the resulting instrument concept. It includes an integrated optics, 4-telescope, dual feed beam combiner operated in a cryogenic vessel; near infrared wavefront sensing adaptive optics; fringe tracking on secondary sources within the field of view of the VLTI and a novel metrology concept. Simulations show that the planned design matches the scientific needs; in particular that 10µas astrometry is feasible for a source with a magnitude of K=15 like Sgr A*, given the availability of suitable phase reference sources.

Four new binary minor planets: (854) Frostia, (1089) Tama, (1313) Berna, (4492) Debussy

Aims. We present evidence that four minor planets of the main belt are binary systems. Methods. These discoveries are based on CCD photometric measurements made by many observers coordinated in a network of observatories. Results. Orbital and physical properties are derived from a total of 134 partial light curves involving 26 stations. (854) Frostia, (1089) Tama, (1313) Berna, and (4492) Debussy show mutual eclipses features on their light curves. In all cases, rotation and revolution are synchronous. Synodic periods are 37.728, 16.444, 25.464 and 26.606 h respectively. From a simple model, we have derived their bulk densities as follows: 0.89 ± 0.14, 2.52 ± 0.30, 1.22 ± 0.15 and 0.91 ± 0.10 g cm −3 respectively. Uncertainties in the bulk densities, arising from scattering and shadow effects are not taken into account. These could increase the density estimates by a factor up to 1.6. Our method of determining bulk density is completely independent of their mass and their diameter estimates. The low rotational periods and the low bulk densities clearly imply a collisional process to explain this kind of binary asteroid. Based on our database of a few thousand light curves of minor planets, the population of similar-sized objects in the main belt is estimated to 6 ± 3 percent in the 10–50 km diameter class.

The Rapid Outbursting Star GM Cep: An EXor in Tr 37?

We present optical, IR, and millimeter observations of the solar-type star 13-277, also known as GM Cep, in the 4 Myr old cluster Tr 37. GM Cep experiences rapid magnitude variations of more than 2 mag at optical wavelengths. We explore the causes of the variability, which seem to be dominated by strong increases in the accretion, being similar to EXor episodes. The star shows high, variable accretion rates (up to ~10−6 M☉ yr−1) and signs of powerful winds, and it is a very fast rotator (Vsin i ~ 43 km s−1). Its strong mid-IR excesses reveal a very flared disk and/or a remnant envelope, most likely out of hydrostatic equilibrium. The 1.3 mm fluxes suggest a relatively massive disk (MD ~ 0.1 M☉). Nevertheless, the millimeter mass is not enough to sustain increased accretion episodes over large timescales, unless the mass is underestimated due to significant grain growth. We finally explore the possibility of GM Cep having a binary companion, which could trigger disk instabilities producing the enhanced accretion episodes.

Massive binaries in the Cepheus OB2/3 region - Constraining the formation mechanism of massive stars

Context. Two different formation scenarios for stars of masses larger than 10 M exist. Although simulations within both scenarios are capable of producing stars up to the highest observed masses, the relevance of the two formation scenarios for massive star-formation is not yet clear. Aims. We aim to detect companions to massive stars to constrain the binary parameters of the multiple systems. These findings will help to constrain the formation of massive stars. Methods. We performed z′-band observations of massive and intermediate-mass stars in the Cep OB2/3 associations with the Lucky imaging camera AstraLux on the 2.2 m telescope of the Calar Alto observatory. The analysis aimed at detecting binary systems with separations out to 2. ′′2 (∼1700 AU), the inner limit depending on the contrast. The maximum contrast of 7 mag in z′ (corresponding to a mass contrast of 17:1 versus an O9V primary) requires a minimum separation of 0. ′′7 (∼550 AU). Results. We found 28 new companions for our sample of 148 intermediate-mass and massive stars in Cep OB2/3. The companion star fraction of the massive stars is 0.7; about 50% of the systems are triples. The mass function of the companions to the massive stars is strongly top-heavy. We found that the sample parameters closely agreed with those found in the Orion Trapezium cluster. Conclusions. The multiplicity of massive stars seems to be significantly higher than that of intermediate-mass stars, independently of the environment. The comparison of our findings with the theories of massive star-formation favor the formation of massive stars by the fragmentation of proto-stellar cores combined with competitive accretion.

NGC 346 IN THE SMALL MAGELLANIC CLOUD. IV. TRIGGERED STAR FORMATION IN THE H II REGION N66

Stellar feedback, expanding H II regions, wind-blown bubbles, and supernovae are thought to be important triggering mechanisms of star formation. Stellar associations, being hosts of significant numbers of early-type stars, are the loci where these mechanisms act. In this part of our photometric study of the star-forming region NGC 346/N66 in the Small Magellanic Cloud, we present evidence based on previous and recent detailed studies that it hosts at least two different events of triggered star formation, and we reveal the complexity of its recent star formation history. In our earlier studies of this region (Papers I and III) we find that besides the central part of N66, where the bright OB stellar content of the association NGC 346 is concentrated, an arclike nebular feature, north of the association, hosts recent star formation. This feature is characterized by a high concentration of emission-line stars and young stellar objects, as well as embedded sources seen as IR emission peaks that coincide with young compact clusters of low-mass pre-main-sequence stars. All these objects indicate that the northern arc of N66 encompasses the most current star formation event in the region. We present evidence that this star formation is the product of a different mechanism than that in the general area of the association, and that it is triggered by a wind-driven expanding H II region (or bubble) blown by a massive supernova progenitor, and possibly other bright stars, a few megayears ago. We propose a scenario according to which this mechanism triggered star formation away from the bar of N66, while in the bar of N66 star formation is introduced by the photoionizing OB stars of the association itself.

ORBITAL MONITORING OF THE ASTRALUX LARGE M-DWARF MULTIPLICITY SAMPLE

Orbital monitoring of M-type binaries is essential for constraining their fundamental properties. This is particularly useful in young systems, where the extended pre-main sequence evolution can allow for precise isochronal dating. Here, we present the continued astrometric monitoring of the more than 200 binaries of the AstraLux Large Multiplicity Survey, building both on our previous work, archival data, and new astrometric data spanning the range of 2010-2012. The sample is very young overall -- all included stars have known X-ray emission, and a significant fraction (18%) of them have recently also been identified as members of young moving groups in the Solar neighborhood. We identify ~30 targets that both have indications of being young and for which an orbit either has been closed or appears possible to close in a reasonable timeframe (a few years to a few decades). One of these cases, GJ 4326, is however identified as probably being substantially older than has been implied from its apparent moving group membership, based on astrometric and isochronal arguments. With further astrometric monitoring, these targets will provide a set of empirical isochrones, against which theoretical isochrones can be calibrated, and which can be used to evaluate the precise ages of nearby young moving groups.

Atmosphere-like turbulence generation with surface-etched phase-screens

We built and characterized an optical system that emulates the optical characteristics of an 8m-class telescope like the Very Large Telescope. The system contains rotating glass phase-screens to generate realistic atmosphere-like optical turbulence, as needed for testing multiconjugate adaptive optics systems. In this paper we present an investigation of the statistical properties of two phase-screens etched on glass-plate surfaces, obtained from Silios Technologies. Those etched screens are highly transmissive (above 85%) from 0.45 to 2.5 microm. From direct imaging, their Fried parameter r0 values (0.43+/-0.04 microm and 0.81+/-0.03 microm, respectively, at 0.633 microm) agree with the expectation to within 10%. This is also confirmed by a comparison of measured and expected Zernike coefficient variances. Overall, we find that those screens are quite reproducible, allowing sub-millimetre r0 values, which were difficult to achieve in the past. We conclude that the telescope emulator and phase-screens form a powerful atmospheric turbulence generator allowing systematic testing of different kinds of AO instrumentation.

The MPIA multipurpose laboratory atmospheric turbulence simulator MAPS

We built an optical system that emulates the optical characteristics of an 8m-class telescope like the VLT and that contains rotating glass plates phase screens to generate realistic atmosphere-like optical turbulence. Together with an array of single mode fibers fed from white light sources to simulate various stellar configurations, we can investigate the behavior of different single or multi-conjugate adaptive optics setups. In this paper we present the characteristics of phase screens etched on glass plates surfaces obtained from Silios Technologies.

Catalogue of ISO LWS observations of asteroids

Context. The long wavelength spectrometer (LWS) onboard the infrared space observatory (ISO) observed the four large main-belt asteroids (1) Ceres, (2) Pallas, (4) Vesta, and (10) Hygiea multiple times. The photometric and spectroscopic data cover the wavelength range between 43 and 197 μm, and are a unique dataset for future investigations and detailed characterisations of these bodies. Aims. The standard ISO archive products, produced through the last post-mission LWS pipeline, were still affected by instrument artefacts. Our goal was to provide the best possible data products to exploit the full scientific potential of these observations. Methods. For all asteroid observations we analysed in detail the dark current, the calibration reference flashes, the space environment effects (glitches), memory effects, tracking influences, and various other sources of uncertainty. We performed a refined reduction of all measurements, corrected for the various effects, and re-calibrated the data. We outline the data reduction process and give an overview of the available data and the quality of the observations. We apply a thermophysical model to the flux measurements to derive far-IR based diameter and albedo values of the asteroids. The measured thermal rotational lightcurve of (4) Vesta is compared to model predictions. Results. The catalogue of LWS (long wavelength spectrometer) observations of asteroids contains 57 manually reduced datasets, including seven non-standard observations, which as such did not have final pipeline products available before. In total, the archive now contains 11 spectral scans and 46 fixed grating measurements with a simultaneous observation at 10 key wavelengths distributed over the full LWS range. The new data products are now accessible via the ISO data archive as highly processed data products (HPDP). Conclusions. The quality of the data products was checked against state-of-the-art thermophysical model predictions and an excellent agreement was found. The absolute photometric accuracy is better than 10%. The calibrated spectra will serve as a source for future mineralogical studies of dwarf planets and dwarf planet candidates.