Visual orbit for the low-mass binary Gliese 22 AC from speckle interferometry

Based on 14 data points obtained with near-infrared speckle interferometry and covering an almost entire revolution, we present a first visual orbit for the low-mass binary system Gliese 22 AC. The quality of the orbit is largely improved with respect to previous astrometric solutions. The dynamical system mass is 0.592 +- 0.065 solar masses, where the largest part of the error is due to the Hipparcos parallax. A comparison of this dynamical mass with mass-luminosity relations on the lower main sequence and theoretical evolutionary models for low-mass objects shows that both probably underestimate the masses of M dwarfs. A mass estimate for the companion Gliese 22 C indicates that this object is a very low-mass star with a mass close to the hydrogen burning mass limit.Comment: Accepted by Astronomy and Astrophysics, 6 pages, 2 figure

The dusty disk around VV Ser

We have carried out observations at millimeter and centimeter wavelengths towards VV Ser using the Plateau de Bure Interferometer and the Very Large Array. This allows us to compute the SED from near infrared to centimeter wavelengths. The modeling of the full SED has provided insight into the dust properties and a more accurate value of the disk mass. The mass of dust in the disk around VV Ser is found to be about 4 10^(-5) Msun, i.e. 400 times larger than previous estimates. Moreoever, the SED can only be accounted for assuming dust stratification in the vertical direction across the disk. The existence of small grains (0.25--1 micron) in the disk surface is required to explain the emission at near- and mid-infrared wavelengths. The fluxes measured at millimeter wavelengths imply that the dust grains in the midplane have grown up to very large sizes, at least to some centimeters.Comment: To appear in Ap

Constraints on the Space Density of Methane Dwarfs and the Substellar Mass Function from a Deep Near-Infrared Survey

We report preliminary results of a deep near-infrared search for methane-absorbing brown dwarfs; almost five years after the discovery of Gl 229b, there are only a few confirmed examples of this type of object. New J band, wide-field images, combined with pre-existing R band observations, allow efficient identification of candidates by their extreme (R-J) colours. Follow-up measurements with custom filters can then confirm objects with methane absorption. To date, we have surveyed a total of 11.4 square degrees to J~20.5 and R~25. Follow-up CH_4 filter observations of promising candidates in 1/4 of these fields have turned up no methane absorbing brown dwarfs. With 90% confidence, this implies that the space density of objects similar to Gl 229b is less than 0.012 per cubic parsec. These calculations account for the vertical structure of the Galaxy, which can be important for sensitive measurements. Combining published theoretical atmospheric models with our observations sets an upper limit of alpha <= 0.8 for the exponent of the initial mass function power law in this domain.Comment: 11 pages + 2 figures To be published in Astrophysical Journal Letter

Multi-wavelength observations of the young binary system Haro 6-10: The case of misaligned discs

Context. We present a multi-wavelength, high-resolution observational survey of the young binary system Haro 6-10 (GV Tau, IRAS 04263+2426), which is harbouring one of the few known infrared companions. Aims. The primary goal of this project is to determine the physical and geometrical properties of the circumstellar and circumbinary material in the Haro 6-10 system. Methods. High-resolution optical (HST/WFPC2) and near-infrared (VLT/NACO) images in different bands were analysed to investigate the large-scale structures of the material around the binary.Mid-infrared interferometry (VLTI/MIDI) and spectroscopy (TIMMI2 at the 3.6m ESO telescope) were carried out to determine the structure and optical depth of the circumstellar material around the individual components. Results. The multi-wavelength observations suggest that both components of the binary system Haro 6-10 are embedded in a common envelope. The measured extinction indicates a dust composition of the envelope similar to that of the interstellar medium. Each component of the system has a circumstellar disc-like structure typical of young stars. The discs are highly misaligned: the northern component is seen almost edge-on and the southern component is an almost face-on disc. Conclusions. The two main formation scenarios of binary systems with misaligned discs are the gravitational capture of a passing object in a dense environment, and the fragmentation of the collapsing molecular cloud. Given the low-density environment of the Taurus-Aurigae star-forming region, the first scenario is unlikely for Haro 6-10. The binary system most probably formed via fragmentation of two different parts of the collapsing molecular cloud combined with other dynamical processes related to the cloud and/or the protostars. This can be the explanation also for other binary systems with an infrared companion.Comment: accepted for publication in A&A on July 15, 201

VLTI/MIDI 10 micron interferometry of the forming massive star W33A

We report on resolved interferometric observations with VLTI/MIDI of the massive young stellar object (MYSO) W33A. The MIDI observations deliver spectrally dispersed visibilities with values between 0.03 and 0.06, for a baseline of 45m over the wavelength range 8-13 micron. The visibilities indicate that W33A has a FWHM size of approximately 120AU (0.030'') at 8 micron which increases to 240AU at 13 micron, scales previously unexplored among MYSOs. This observed trend is consistent with the temperature falling off with distance. 1D dust radiative transfer models are simultaneously fit to the visibility spectrum, the strong silicate feature and the shape of the mid infrared spectral energy distribution (SED). For any powerlaw density distribution, we find that the sizes (as implied by the visibilities) and the stellar luminosity are incompatible. A reduction to a third of W33A's previously adopted luminosity is required to match the visibilities; such a reduction is consistent with new high resolution 70 micron data from Spitzer's MIPSGAL survey. We obtain best fits for models with shallow dust density distributions of r^(-0.5) and r^(-1.0) and for increased optical depth in the silicate feature produced by decreasing the ISM ratio of graphite to silicates and using optical grain properties by Ossenkopf et al. (1992).Comment: 4 pages, 4 figures. Accepted for ApJ letter

A catalog of bright calibrator stars for 200-meter baseline near-infrared stellar interferometry

We present in this paper a catalog of reference stars suitable for calibrating infrared interferometric observations. In the K band, visibilities can be calibrated with a precision of 1% on baselines up to 200 meters for the whole sky, and up to 300 meters for some part of the sky. This work, extending to longer baselines a previous catalog compiled by Borde et al. (2002), is particularly well adapted to hectometric-class interferometers such as the Very Large Telescope Interferometer (VLTI, Glindemann et al. 2003) or the CHARA array (ten Brummelaar et al. 2003) when observing well resolved, high surface brightness objects (K<8). We use the absolute spectro-photometric calibration method introduced by Cohen et al. (1999) to derive the angular diameters of our new set of 948 G8--M0 calibrator stars extracted from IRAS, 2MASS and MSX catalogs. Angular stellar diameters range from 0.6 mas to 1.8 mas (median is 1.1 mas) with a median precision of 1.35%. For both the northern and southern hemispheres, the closest calibrator star is always less than 10 degree away.Comment: 9 pages, 7 figures, submitted to A&A. The full catalog can be found in http://calys.obspm.fr/~merand/Files/MerandEtAlCatalogue.tx

Extending the limits of globule detection -- ISOPHOT Serendipity Survey Observations of interstellar clouds

A faint $I_{\rm 170}=4$ MJysr$^{-1}$ bipolar globule was discovered with the ISOPHOT 170 $\mu$m Serendipity Survey (ISOSS). ISOSS J 20246+6541 is a cold ($T_{\rm d}\approx 14.5$ K) FIR source without an IRAS pointsource counterpart. In the Digitized Sky Survey B band it is seen as a 3\arcmin size bipolar nebulosity with an average excess surface brightness of $\approx 26$ mag/$\square$\arcsec . The CO column density distribution determined by multi-isotopic, multi-level CO measurements with the IRAM-30m telescope agrees well with the optical appearance. An average hydrogen column density of $\approx 10^{21}$cm$^{-2}$ was derived from both the FIR and CO data. Using a kinematic distance estimate of 400 pc the NLTE modelling of the CO, HCO$^+$, and CS measurements gives a peak density of $\approx 10^4$cm$^{-3}$. The multiwavelength data characterise ISOSS 20246+6541 as a representative of a class of globules which has not been discovered so far due to their small angular size and low 100$\mu$m brightness. A significant overabundance of $^{13}$CO is found $X(^{13}CO) \ge 150\times X(C^{18}O)$. This is likely due to isotope selective chemical processes.Comment: 5 pages, 3 figure

Tracing the envelopes around embedded low-mass young stellar objects with HCO+ and millimeter-continuum observations

Interferometer observations of millimeter-continuum (OVRO) and single-dish observations of HCO+ and H13CO+ J=1-0, 3-2, and 4-3 (JCMT, IRAM 30m) are presented of nine embedded low-mass young stellar objects (YSOs) in Taurus. All nine objects are detected at 3.4 and 2.7 mm, with fluxes of 4-200 mJy, and consist of unresolved (<3 arcsec) point sources, plus, toward about half of the objects, an extended envelope. The point sources likely are circumstellar disks, showing that these are established early in the embedded phase. Literature values of 1.1 mm continuum emission are used to trace the envelopes, carrying 0.001-0.26 M(sol). In HCO+, the 1-0 lines trace the surrounding clouds, while the 3-2 and 4-3 are concentrated toward the sources with intensities well correlated with the envelope flux. An HCO+/H2 abundance of 1.2e-8 is derived. The HCO+ line strengths and envelope fluxes can be fit simultaneously with the simple collapse model of Shu (1977), and related density power laws with slopes p=1-3. As an indicator of the relative evolutionary phase of a YSO, the ratio of HCO+ 3-2 line intensity over bolometric luminosity is proposed, which is roughly proportional to the current ratio of envelope over stellar mass. It is concluded that HCO+ 3-2 and 4-3 are excellent tracers of the early embedded phase of star formation.Comment: 45 pages, 10 figures, ApJ/AASLaTeX. To be published in The Astrophysical Journa

Data reduction methods for single-mode optical interferometry - Application to the VLTI two-telescopes beam combiner VINCI

The interferometric data processing methods that we describe in this paper use a number of innovative techniques. In particular, the implementation of the wavelet transform allows us to obtain a good immunity of the fringe processing to false detections and large amplitude perturbations by the atmospheric piston effect, through a careful, automated selection of the interferograms. To demonstrate the data reduction procedure, we describe the processing and calibration of a sample of stellar data from the VINCI beam combiner. Starting from the raw data, we derive the angular diameter of the dwarf star Alpha Cen A. Although these methods have been developed specifically for VINCI, they are easily applicable to other single-mode beam combiners, and to spectrally dispersed fringes.Comment: Accepted for publication in Astronomy & Astrophysics, 17 pages, 19 figure

Cometary Dust in the Debris Disks of HD 31648 and HD 163296: Two ``Baby'' beta Pics

The debris disks surrounding the pre-main sequence stars HD 31648 and HD 163296 were observed spectroscopically between 3 and 14 microns. Both possess a silicate emission feature at 10 microns which resembles that of the star beta Pictoris and those observed in solar system comets. The structure of the band is consistent with a mixture of olivine and pyroxene material, plus an underlying continuum of unspecified origin. The similarity in both size and structure of the silicate band suggests that the material in these systems had a processing history similar to that in our own solar system prior to the time that the grains were incorporated into comets.Comment: 17 pages, AASTeX, 5 eps figures, accepted for publication in Ap.