Gilles Chagnon

VLBA observations of SiO masers towards Mira variable stars

We present new total intensity and linear polarization VLBA observations of the ν = 2a ndν = 1 J = 1−0 maser transitions of SiO at 42.8 and 43.1 GHz in a number of Mira variable stars over a substantial fraction of their pulsation periods. These observations were part of an observing program that also includes interferometric measurements at 2.2 and 3.6 micron (Mennesson et al. 2002); comparison of the results from different wavelengths allows studying the envelope independently of the poorly known distances to these stars. Nine stars were observed at from one to four epochs during 2001. The SiO emission is largely confined to rings which are smaller than the inner radius of the dust shells reported by Danchi et al. (1994). Two stars (U Orionis, R Aquarii) have maser rings with diameters corresponding to the size of the hot molecular layer as measured at 3.6 micron; in the other cases, the SiO rings are substantially larger. Variations of ring diameter for most, but not all stars, had an rms amplitude in agreement with the models of Humphreys et al. (2002) although the expected relationship between the diameter and pulsation phase was not seen. The ring diameter in U Orionis shows remarkably small variation. A correlation between the 2.2/3.6 µm diameter ratio with that of the SiO/3.6 µm diameter ratio is likely due to differences in the opacities at 2.2 and 3.6 µm in a molecular layer. A further correlation with the inner size of the dust shell reported by Danchi et al. (1994) suggest some differences in the temperature structure. Clear evidence is seen in R Aquarii for an equatorial disk similar to that reported by Hollis et al. (2001); rotation is possibly also detected in S Coronae Boralis.

EVIDENCE FOR VERY EXTENDED GASEOUS LAYERS AROUND O-RICH MIRA VARIABLES AND M GIANTS

Nine bright O-rich Mira stars and five semiregular variable cool M giants have been observed with the Infrared and Optical Telescope Array (IOTA) interferometer in both K 0 (� 2.15 lm) and L 0 (� 3.8 lm) broadband filters, in most cases at very close variability phases. All of the sample Mira stars and four of the semiregular M giants show strong increases, from ’20% to ’100%, in measured uniform-disk (UD) diameters between the K 0 and L 0 bands. (A selection of hotter M stars does not show such a large increase.) There is no evidence that K 0 and L 0 broadband visibility measurements should be dominated by strong molecular bands, and cool expanding dust shells already detected around some of these objects are also found to be poor candidates for producing these large apparent diameter increases. Therefore, we propose that this must be a continuum or pseudocontinuum opacity effect. Such an apparent enlargement can be reproduced using a simple two-component model consisting of a warm (1500–2000 K), extended (up to ’3 stellar radii), optically thin (� ’ 0:5) layer located above the classical photosphere. The Planck weighting of the continuum emission from the two layers will suffice to make the L 0 UD diameter appear larger than the K 0 UD diameter. This twolayer scenario could also explain the observed variation of Mira UD diameters versus infrared wavelength— outside of strong absorption bands—as already measured inside the H, K, L, and N atmospheric windows. This interpretation is consistent with the extended molecular gas layers (H2O, CO, etc.) inferred around some of these objects from previous IOTA K 0 -band interferometric observations obtained with the Fiber Linked Unit for Optical Recombination (FLUOR) and from Infrared Space Observatory and high-resolution ground-based FTS infrared spectra. The two-component model has immediate implications. For example, the Mira photosphere diameters are smaller than previously recognized—this certainly implies higher effective temperatures, and it may favor fundamental mode pulsation. Also, the UD model fails generally to represent the brightness distribution and has very limited applicability for Mira stars. The presence of a very extended gas layer extending up to ’3 stellar radii seems now well established on a fair sample of asymptotic giant branch stars ranging from late-type giants to long-period variables, with some probable impact on stellar model atmospheres and mass-loss mechanisms. Subject headings: circumstellar matter — instrumentation: interferometers — stars: atmospheres — stars: variables: other — techniques: interferometric

Further VLBA observations of SiO masers toward Mira variable stars

We present the results of continued monitoring of the SiO masers at 7 mm wavelength in several of the Mira variable stars reported in Cotton et al. (2004, A&A, 414, 275) (o Ceti = Mira, U Orionis = U Ori, R Aquarii = R Aqr) over a period of 16 months, extending the observations to several pulsation cycles. The observed size of the maser rings varied by 3–14% with time but show no clear correlation with pulsation phase. In all cases, the SiO masers appear just outside the dense molecular layer indicated by near-IR observations. Rotation (or large scale motion) is possibly detected in o Ceti with a period of 89 × sin(i) years. We find linear polarization up to ∼60% and at several epochs predominantly tangentially ordered polarization vectors indicate a radial magnetic field direction. Jet-like features are examined in o Ceti and R Aqr and in both cases, the magnetic field appears elongated with the masing structure. This suggests that the dynamic feature in the envelope is dragging the magnetic field or that the gas is constrained to follow magnetic field.

Thermal infrared stellar interferometry using single-mode guided optics: first scientific results from the IOTA

We report on first scientific observations of a few bright late type stars by direct long baseline interferometry in the thermal infrared (3.4 to 4.1 microns) obtained with the TISIS (Thermal Infrared Stellar Interferometric Set-up) experiment of the IOTA (Infrared and Optical Telescope Array) interferometer. Beam combination is provided by a single-mode fluoride glass coupler optimized for operation in that wavelength domain and yielding visibility measurements with 2% typical relative accuracy. First precise estimations of uniform disk diameters for (alpha) Orionis, (alpha) Herculis, o Ceti and R Leonis are presented in the L band. Very large increase (50 to 70%) in apparent angular diameters have been found for the 2 Mira stars o Ceti and R Leonis with respect to previous measurements obtained at shorter infrared wavelengths and same luminosity phase. Extended optically thin close-by dust shells characterized by Infrared Spatial Interferometer measurements are not found to play a significant role in the observed L band intensity distribution. Gas properties are likely to have a greater impact at these wavelengths. Our o Ceti interferometric observations look indeed in good agreement with the presence of very extended circumstellar gas layers (mostly H2O and SiO) derived from recent Infrared Space Observatory thermal infrared spectral data.

The FLUOR interferometric beam combiner

Abstract FLUOR stands for Fibered Linked Unit for Optical Recombination and is an interferometric instrument which started out as a technology demonstrator, demonstrated the potential of single-mode fiber optics for high precision visibility measurements, and has been operated as a focal instrument of the IOTA interferometer since 1995. After a presentation of the instrument, the programs carried out with FLUOR are reviewed, as well as the perspectives introduced by interferometric observations with a high dynamic range.

Catalog of reference stars for long baseline stellar interferometry

The calibration process of long baseline stellar interferometers requires the use of reference stars with accurately determined angular diameters. We present a catalog of 374 carefully chosen stars among the all-sky network of infrared sources provided by Ref. 1. The catalog benefits from a very good sky coverage and a median formal error on the angular diameters of only 1.2%. Besides, its groups together in a homogeneous handy set stellar coordinates, uniform and limb-darkened angular diameters, photometric measurements, and other parameters relevant to optical interferometry. In this paper, we describe the selection criteria applied to qualify stars as reference sources. Then, we discuss the catalogs statistical properties such as the sky coverage or the distributions of magnitudes and angular diameters. We study the number of available reference stars as a function of the baseline and the precision needed on the visibility measurements. Finally, we compare the angular diameters predicted in Ref. 1 with existing determinations in the literature, and find a very good agreement.