Search

Binaries provide empirical key constraints for star formation theories, like the overall binary fraction, mass ratio distribution and the separation distribution. They play a crucial role to calibrate the output of theoretical models, like absolute magnitudes, colors and effective temperature depending on mass, metallicity and age. We present first results of our on-going high-resolution imaging survey of late type brown dwarfs. The survey aims at resolving tight brown dwarf binary systems to better constrain the T dwarf binary fraction. We intent to follow-up the individual binaries to determine orbital parameters. Using NACO at the VLT we performed AO-assisted near-infrared observations of SDSS J2052-1609. High-spatial resolution images of the T1 dwarf were obtained in H and Ks filters. We resolved SDSS J2052-1609 into a binary system with a separation of 0.101" \pm 0.001". Archival data from HST/NICMOS taken one year previous to our observations proves the components to be co-moving. Using the flux ratio between the components we infer J, H and Ks magnitudes for the resolved system. From the near-IR colors we estimate spectral types of T1 +1 -4 and T2.5 \pm 1 for component A and B, respectively. A first estimate of the total system mass yields Mtot > 78 Mjup, assuming a circular orbit., Comment: 5 pages, 3 figures, 3 tables, accepted for publication by A&A

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. 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. The observations are carried out in the SDSS i' and z' band using the Lucky Imaging camera AstraLux Sur at the ESO 3.5 m New Technology Telescope. In the first part of the survey, we observed 124 M dwarfs of integrated spectral types M0-M6 and identified 34 new and 17 previously known companions to 44 stars. We derived relative astrometry and component photometry for these systems. More than half of the binaries have separations smaller than 1 arcsec and would have been missed in a simply seeing-limited survey. Correcting our sample for selection effects yields a multiplicity fraction of 32+/-6% for 108 M dwarfs within 52 pc and with angular separations of 0.1-6.0 arcsec, corresponding to projected separation 3-180 AU at median distance 30 pc. Compared to early-type M dwarfs (M>0.3M_Sun), later type (and hence lower mass) M dwarf binaries appear to have closer separations, and more similar masses., Comment: 18 pages, 9 figures. Minor corrections and changes. Revised to match accepted A&A version

Due to the recent interest shown by astronomers towards the Antarctic Plateau as a potential site for large astronomical facilities, we assisted in the last years to a strengthening of site testing activities in this region, particularly at Dome C. Most of the results collected so far concern meteorologic parameters and optical turbulence measurements based on different principles using different instruments. At present we have several elements indicating that, above the first 20-30 meters, the quality of the optical turbulence above Dome C is better than above whatever other site in the world. The challenging question, crucial to know which kind of facilities to build on, is to establish how much better the Dome C is than a mid-latitude site. In this contribution we will provide some complementary elements and strategies of analysis aiming to answer to this question. We will try to concentrate the attention on critical points, i.e. open questions that still require explanation/attention., Comment: 3 figures, EAS Publications Series, Volume 25, 2007, pp.57

Eps Ind A is one of the nearest sun-like stars, located only 3.6 pc away. It is known to host a binary brown dwarf companion, Eps Ind Ba/Bb, at a large projected separation of 6.7", but radial velocity measurements imply that an additional, yet unseen component is present in the system, much closer to Eps Ind A. Previous direct imaging has excluded the presence of any stellar or high-mass brown dwarf companion at small separations, indicating that the unseen companion may be a low-mass brown dwarf or high-mass planet. We present the results of a deep high-contrast imaging search for the companion, using active angular differential imaging (aADI) at 4 micron, a particularly powerful technique for planet searches around nearby and relatively old stars. We also develop an additional PSF reference subtraction scheme based on locally optimized combination of images (LOCI) to further enhance the detection limits. No companion is seen in the images, although we are sensitive to significantly lower masses than previously achieved. Combining the imaging data with the known radial velocity trend, we constrain the properties of the companion to within approximately 5-20 Mjup, 10-20 AU, and i > 20 deg, unless it is an exotic stellar remnant. The results also imply that the system is probably older than the frequently assumed age of ~1 Gyr., Comment: 10 pages, 6 figures, accepted for publication in MNRAS

Context: Brown dwarfs exhibit complex atmospheric signatures, and their properties depend sensitively on effective temperature, surface gravity, and metallicity. Several physical properties of brown dwarfs in binary systems can be well inferred from the primary, and therefore allow to better constrain their atmospheres. Aims: We want to constrain atmospheric models of brown dwarfs in binary systems using narrow-band mid-infrared photometry. Methods: High spatial resolution and sensitivity is required to resolve the components. Therefore we have obtained deep mid-infrared images of four close binary systems with brown dwarf companions using VISIR at the VLT in three narrow-band filters at 8.6, 10.5 and 11.25 micron. Results: Three brown dwarfs companions (GJ 229B, HD 130948BC and HR 7329B) were detected at 8.6micron. HD 130948BC was also observed at 10.5micron. We finally place upper flux limits for the other narrow band filters with null detections. Conclusions: Our results are in general compatible with previous observations and model expectations for these objects. For HD 130948BC, we conclude photometric variability on a significance level of 2.8sigma based on repeated observations. The bandpass around 10.5micron appears specifically well suited for variability studies, and we speculate that either inhomogeneities in the atmospheric NH3 distribution, or silicate absorption might cause its time-variability., Comment: accepted by A&A

We have carried out a direct and differential imaging search for sub-stellar companions to eps Indi A using the adaptive optics system NACO at the ESO VLT. The observations were carried out in September 2004 with NACO/SDI as well as with NACO's S27 camera in the H and Ks filters. The SDI data cover an area of \~2.8" around eps Indi A. No detection was achieved in the inner neighbourhood down to 53 Mj (5 sigma confidence level) at a separation > 0.4" (1.45 AU) and down to 21 Mj for separations > 1.3" (4.7 AU). To cover a wider field of view, observations with the S27 camera and a coronagraphic mask were obtained. We detected a faint source at a separation of (7.3 +/- 0.1)" and a position angle of (302.9 +/- 0.8) degree. The photometry for the candidate companion yields m(H)=(16.45 +/- 0.04)mag and m(Ks) = (15.41 +/- 0.06)mag, respectively. Those magnitudes and the resulting color (H-Ks) = (1.04 +/- 0.07)mag fit best to a spectral type of L5 - L9.5 if it is bound. Observations done with HST/NICMOS by M. Endl have shown the source to be a background object., Comment: 6 pages, 5 figures, accepted by A&A

In this paper we present the characterization of all the principal meteorological parameters (wind speed and direction, pressure, absolute and potential temperature) extended over 25 km from the ground and over two years (2003 and 2004) above the Antarctic site of Dome C. The data set is composed by 'analyses' provided by the General Circulation Model (GCM) of the European Center for Medium Weather Forecasts (ECMWF) and they are part of the catalog MARS. A monthly and seasonal (summer and winter time) statistical analysis of the results is presented. The Richardson number is calculated for each month of the year over 25 km to study the stability/instability of the atmosphere. This permits us to trace a map indicating where and when the optical turbulence has the highest probability to be triggered on the whole troposphere, tropopause and stratosphere. We finally try to predict the best expected isoplanatic angle and wavefront coherence time employing the Richardson number maps, the wind speed profiles and simple analytical models of CN2 vertical profiles., Comment: 28 pages, 14 figures, pdf file, to be published on July 2006 - PASP, see also http://www.arcetri.astro.it/~masciadr