Marc G. Lacasse

Interferometric observations of the supergiant stars

We report the observations in the K band of the red supergiant star α Orionis and of the bright giant star α Herculis with the FLUOR beamcombiner at the IOTA interferometer. The high quality of the data allows us to estimate limb-darkening and derive precise diameters in the K band which combined with bolometric fluxes yield effective temperatures. In the case of Betelgeuse, data collected at high spatial frequency although sparse are compatible with circular symmetry and there is no clear evidence for departure from circular symmetry. We have combined the K band data with interferometric measurements in the L band and at 11.15 µm. The full set of data can be explained if a 2055 K layer with optical depths τK = 0.060 ± 0.003, τL = 0.026 ± 0.002 and τ11.15 µm = 2.33 ± 0.23 is added 0.33 Rabove the photosphere providing a first consistent view of the star in this range of wavelengths. This layer provides a consistent explanation for at least three otherwise puzzling observations: the wavelength variation of apparent diameter, the dramatic difference in limb darkening between the two supergiant stars, and the previously noted reduced effective temperature of supergiants with respect to giants of the same spectral type. Each of these may be simply understood as an artifact due to not accounting for the presence of the upper layer in the data analysis. This consistent picture can be considered strong support for the presence of a sphere of warm water vapor, proposed by Tsuji (2000) when interpreting the spectra of strong molecular lines.

\alpha

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.

Orionis and

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

\alpha

We have measured non-zero closure phases for about 29% of our sample of 56 nearby Asymptotic Giant Branch (AGB) stars, using the 3-telescope Infrared Optical Telescope Array (IOTA) interferometer at near-infrared wavelengths (H band) and with angular resolutions in the range 5-10 milliarcseconds. These nonzero closure phases can only be generated by asymmetric brightness distributions of the target stars or their surroundings. We discuss how these results were obtained, and how they might be interpreted in terms of structures on or near the target stars. We also report measured angular sizes and hypothesize that most Mira stars would show detectable asymmetry if observed with adequate angular resolution.We have measured nonzero closure phases for about 29% of our sample of 56 nearby asymptotic giant branch (AGB) stars, using the three-telescope Infrared Optical Telescope Array (IOTA) interferometer at near-infrared wavelengths (H band) and with angular resolutions in the range 5-10 mas. These nonzero closure phases can only be generated by asymmetric brightness distributions of the target stars or their surroundings. We discuss how these results were obtained and how they might be interpreted in terms of structures on or near the target stars. We also report measured angular sizes and hypothesize that most Mira stars would show detectable asymmetry if observed with adequate angular resolution.

Herculis with FLUOR at IOTA

The FLUOR project started in 1991 with a prototype fiber recombination unit that transformed a pair of independent 80 cm telescopes into a stellar interferometer. An improved version of this unit is now used as part of the instrumentation at the IOTA interferometer on Mt Hopkins (Arizona). The system is based on fluoride glass single-mode waveguides (non polarization-preserving) for observations at infrared wavelengths between 2 and 2.4 μm. A triple coupler performs the coherent recombination of the beams and extracts two calibration signals. A passive polarization control is sufficient to maintain the interferornetric efficiency above 80 %, with variations of the order of a few percents from one night to the next. The combination FLUOR/JOTA now routinely provides stellar interferograms on baselines ranging between 5 and 38 m, with an accuracy of 1 % or better in the fringe visibility measurements.

VLBA observations of SiO masers towards Mira variable stars

We report the first spatially resolved observations of the spectroscopic binaries λ Vir and WR 140, including the debut of aperture-synthesis imaging with the upgraded three-telescope IOTA interferometer. Using IONIC-3, a new integrated optics beam combiner capable of a precise closure phase measurement, short observations were sufficient to extract the angular separation and orientation of each binary system and the component brightness ratio. Most notably, the underlying binary in the prototypical colliding-wind source WR 140 (WC7 + O4/O5) was found to have a separation of ~13 mas with a position angle of 152°, consistent with previous interpretations of the 2001 dust shell ejection only if the Wolf-Rayet star is fainter than the O star at 1.65 μm. We also highlight λ Vir, whose peculiar stellar properties of the Am star components will permit direct testing of current theories of tidal evolution when the full orbit is determined.

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

Aims. We undertook an H band interferometric examination of Arcturus, a star frequently used as a spatial and spectral calibrator. Methods. Using the IOTA 3 telescope interferometer, we performed spectro-interferometric observations (R ≈ 35) of Arcturus. Atmospheric models and prescriptions were fitted to the data to derive the brightness distribution of the photosphere. Image reconstruction was performed using two software algorithms: Wisard and Mira. Results. An achromatic power law proved to be a good model of the brightness distribution, with a limb darkening compatible with the one derived from atmospheric model simulations using our marcs model. A Rosseland diameter of 21.05 ± 0.21 was derived, corresponding to an effective temperature of Teff = 4295 ± 26 K. No companion was detected from the closure phases, with an upper limit on the brightness ratio of 8 × 10^−4 at 1 AU. The dynamic range at such distance from the photosphere was established as 1.5 × 10^−4 (1σrms). An upper limit of 1.7×10^−3 was also derived for the level of brightness asymmetries present in the photosphere.

First Surface-resolved Results with the Infrared Optical Telescope Array Imaging Interferometer: Detection of Asymmetries in Asymptotic Giant Branch Stars

We report imaging observations of the symbiotic long-period Mira variable R Aquarii (R Aqr) at near-infrared and radio wavelengths. The near-infrared observations were made with the IOTA imaging interferometer in three narrowband filters centered at 1.51, 1.64, and 1.78 μm, which sample mainly water, continuum, and water features, respectively. Our near-infrared fringe visibility and closure phase data are analyzed using three models. (1) A uniform disk model with wavelength-dependent sizes fails to fit the visibility data, and is inconsistent with the closure phase data. (2) A three-component model, consisting of a Mira star, water shell, and an off-axis point source, provide a good fit to all data. (3) A model generated by a constrained image reconstruction analysis provides more insight, suggesting that the water shell is highly nonuniform, i.e., clumpy. The VLBA observations of SiO masers in the outer molecular envelope show evidence of turbulence, with jetlike features containing velocity gradients.

FLUOR fibered instrument at the IOTA interferometer

Infrared interferometry of supergiant and Mira stars has recently been reinterpreted as revealing the presence of deep molecular layers. Empirical models for a photosphere surrounded by a simple molecular layer or envelope have led to a consistent interpretation of previously inconsistent data. The stellar photospheres are found to be smaller than previously understood, and the molecular layer is much higher and denser than predicted by hydrostatic equilibrium. However, the analysis was based on spatial observations with medium-band optical filters, which mixed the visibilities of different spatial structures. This paper reports spatial interferometry with narrow spectral bands, isolating near-continuum and strong molecular features, obtained for the supergiant µ Cep. The measurements confirm strong variation of apparent diameter across the K-band. A layer model shows that a stellar photosphere of angular diameter 14.11±0.60 mas is surrounded by a molecular layer of diameter 18.56±0.26 mas, with an optical thickness varying from nearly zero at 2.15 µm to >1 at 2.39 µm. Although µ Cep and α Ori have a similar spectral type, interferometry shows that they differ in their radiative properties. Comparison with previous broad-band measurements shows the importance of narrow spectral bands. The molecular layer or envelope appears to be a common feature of cool supergiants.

First Results with the IOTA3 Imaging Interferometer: The Spectroscopic Binaries λ Virginis and WR 140

ABSTRACT We describe the control and performance of a new near‐infrared camera based on a Rockwell PICNIC array detector for interferometry observations at the Infrared‐Optical Telescope Array (IOTA). The camera control uses a complex programmable logic device that allows fast and stable clocking of the PICNIC array and on‐the‐fly reconfiguration of the readout method. We measured a read noise as low as 12.4 e per correlated double sample. The read noise can be reduced even more through nondestructive readout, and decreases as the square root of the number of successive reads. We discuss the advantages of this system for near‐infrared interferometry.