Observations of Mira stars with the IOTA/FLUOR interferometer and comparison with Mira star models

We present K'-band observations of five Mira stars with the IOTA interferometer. The interferograms were obtained with the FLUOR fiber optics beam combiner, which provides high-accuracy visibility measurements in spite of time-variable atmospheric conditions. For the M-type Miras X Oph, R Aql, RU Her, R Ser, and the C-type Mira V CrB we derived the uniform-disk diameters 11.7mas, 10.9mas, 8.4mas, 8.1mas, and 7.9mas (+/- 0.3mas), respectively. Simultaneous photometric observations yielded the bolometric fluxes. The derived angular Rosseland radii and the bolometric fluxes allowed the determination of effective temperatures. For instance, the effective temperature of R Aql was determined to be 2970 +/- 110 K. A linear Rosseland radius for R Aql of (250 +100/-60) Rsun was derived from the angular Rosseland radius of 5.5mas +/- 0.2mas and the HIPPARCOS parallax of 4.73mas +/- 1.19mas. The observations were compared with theoretical Mira star models of Bessel et al. (1996) and Hofmann et al. (1998). The effective temperatures of the M-type Miras and the linear radius of R Aql indicate fundamental mode pulsation.Comment: 12 pages, 4 postscript figure

Abundances in intermediate-mass AGB stars undergoing third dredge-up and hot-bottom burning

High dispersion near-infrared spectra have been taken of seven highly-evolved, variable, intermediate-mass (4-6 Msun) AGB stars in the LMC and SMC in order to look for C, N and O variations that are expected to arise from third dredge-up and hot-bottom burning. The pulsation of the objects has been modelled, yielding stellar masses, and spectral synthesis calculations have been performed in order to derive abundances from the observed spectra. For two stars, abundances of C, N, O, Na, Al, Ti, Sc and Fe were derived and compared with the abundances predicted by detailed AGB models. Both stars show very large N enhancements and C deficiencies. These results provide the first observational confirmation of the long-predicted production of primary nitrogen by the combination of third dredge-up and hot-bottom burning in intermediate-mass AGB stars. It was not possible to derive abundances for the remaining five stars: three were too cool to model, while another two had strong shocks in their atmospheres which caused strong emission to fill the line cores and made abundance determination impossible. The latter occurrence allows us to predict the pulsation phase interval during which observations should be made if successful abundance analysis is to be possible.Comment: Accepted for publication in MNRA

Interferometric observations of the Mira star o Ceti with the VLTI/VINCI instrument in the near-infrared

We present K-band commissioning observations of the Mira star prototype o Cet obtained at the ESO Very Large Telescope Interferometer (VLTI) with the VINCI instrument and two siderostats. The observations were carried out between 2001 October and December, in 2002 January and December, and in 2003 January. Rosseland angular radii are derived from the measured visibilities by fitting theoretical visibility functions obtained from center-to-limb intensity variations (CLVs) of Mira star models (Bessel et al. 1996, Hofmann et al. 1998, Tej et al. 2003). Using the derived Rosseland angular radii and the SEDs reconstructed from available photometric and spectrophotometric data, we find effective temperatures ranging from T_eff=3192 +/- 200 K at phase phi=0.13 to 2918 +/- 183 K at phi=0.26. Comparison of these Rosseland radii, effective temperatures, and the shape of the observed visibility functions with model predictions suggests that o Cet is a fundamental mode pulsator. Furthermore, we investigated the variation of visibility function and diameter with phase. The Rosseland angular diameter of o Cet increased from 28.9 +/- 0.3 mas (corresponding to a Rosseland radius of 332 +/- 38 R_sun for a distance of D=107 +/- 12 pc) at phi=0.13 to 34.9 +/- 0.4 mas (402 +/- 46 R_sun) at phi=0.4. The error of the Rosseland linear radius almost entirely results from the error of the parallax, since the error of the angular diameter is only approximately 1 %.Comment: 12 pages, 8 figures, accepted for publication in Astronomy & Astrophysic

Probing Red Giant Atmospheres with Gravitational Microlensing

Gravitational microlensing provides a new technique for studying the surfaces of distant stars. Microlensing events are detected in real time and can be followed up with precision photometry and spectroscopy. This method is particularly adequate for studying red giants in the Galactic bulge. Recently we developed an efficient method capable of computing the lensing effect for thousands of frequencies in a high-resolution stellar spectrum. Here we demonstrate the effects of microlensing on synthesized optical spectra of red giant model atmospheres. We show that different properties of the stellar surface can be recovered from time-dependent photometry and spectroscopy of a point-mass microlensing event with a small impact parameter. In this study we concentrate on center-to-limb variation of spectral features. Measuring such variations can reveal the depth structure of the atmosphere of the source star.Comment: 23 pages with 11 Postscript figures, submitted to ApJ; Section 2 expanded, references added, text revise

The Keck aperture masking experiment: multiwavelength observations of six Mira variables

The angular diameters of six oxygen-rich Mira-type long-period variables have been measured at various NIR wavelengths using the aperture-masking technique in an extensive observing program from 1997 January to 2004 September. These data sets span many pulsation cycles of the observed objects and represent the largest study of multiwavelength, multiepoch interferometric angular diameter measurements on Mira stars to date. The calibrated visibility data of ο Cet, R Leo, R Cas, W Hya, χ Cyg, and R Hya are fitted using a uniform disk brightness distribution model to facilitate comparison between epochs, wavelengths, and with existing data and theoretical models. The variation of angular diameter as a function of wavelength and time is studied, and cyclic diameter variations are detected for all objects in our sample. These variations are believed to stem from time-dependent changes of density and temperature (and hence varying molecular opacities) in different layers of these stars. The similarities and differences in behavior between these objects are analyzed and discussed in the context of existing theoretical models. Furthermore, we present time-dependent 3.08 μm angular diameter measurements, probing for the first time these zones of probable dust formation, which show unforeseen sizes and are consistently out of phase with other NIR layers shown in this study. The S-type Mira χ Cyg exhibits significantly different behavior compared to the M-type Mira variables in this study, in both its NIR light curves and its diameter pulsation signature. Our data show that the NIR diameters predicted by current models are too small and need to incorporate additional and/or enhanced opacity mechanisms. Also, new tailored models are needed to explain the behavior of the S-type Mira χ Cyg

Abundance analysis for long period variables. Velocity effects studied with O-rich dynamic model atmospheres

(abbreviated) Measuring the surface abundances of AGB stars is an important tool for studying the effects of nucleosynthesis and mixing in the interior of low- to intermediate mass stars during their final evolutionary phases. The atmospheres of AGB stars can be strongly affected by stellar pulsation and the development of a stellar wind, though, and the abundance determination of these objects should therefore be based on dynamic model atmospheres. We investigate the effects of stellar pulsation and mass loss on the appearance of selected spectral features (line profiles, line intensities) and on the derived elemental abundances by performing a systematic comparison of hydrostatic and dynamic model atmospheres. High-resolution synthetic spectra in the near infrared range were calculated based on two dynamic model atmospheres (at various phases during the pulsation cycle) as well as a grid of hydrostatic COMARCS models. Equivalent widths of a selection of atomic and molecular lines were derived in both cases and compared with each other. In the case of the dynamic models, the equivalent widths of all investigated features vary over the pulsation cycle. A consistent reproduction of the derived variations with a set of hydrostatic models is not possible, but several individual phases and spectral features can be reproduced well with the help of specific hydrostatic atmospheric models. In addition, we show that the variations in equivalent width that we found on the basis of the adopted dynamic model atmospheres agree qualitatively with observational results for the Mira R Cas over its light cycle. The findings of our modelling form a starting point to deal with the problem of abundance determination in strongly dynamic AGB stars (i.e., long-period variables).Comment: 13 pages, 22 figures, accepted for publication in A&

Near-infrared IOTA interferometry of the symbiotic star CH Cyg

We present observations of the symbiotic star CH Cyg with a new JHK-band beam combiner mounted to the IOTA interferometer. The new beam combiner consists of an anamorphic cylindrical lens system and a grism, and allows the simultaneous recording of spectrally dispersed J-, H- and K-band Michelson interferograms. The observations of CH Cyg were conducted on 5, 6, 8 and 11 June 2001 using baselines of 17m to 25m. From the interferograms of CH Cyg, J-, H-, and K-band visibility functions can be determined. Uniform-disk fits to the visibilities give, e.g., stellar diameters of (7.8 ± 0.6) mas and (8.7 ± 0.8) mas in H and K, respectively. Angular stellar filter radii and Rosseland radii are derived from the measured visibilities by fitting theoretical center-to-limb intensity variations (CLVs) of Mira star models. The available HIPPARCOS parallax of CH Cyg allows us to determine linear radii. For example, on the basis of the K-band visibility, Rosseland radii in the range of 214 to 243 solar radii can be derived utilizing CLVs of different fundamental mode Mira models as fit functions. These radii agree well within the error bars with the corresponding theoretical model Rosseland radii of 230 to 282 solar radii. Models of first overtone pulsators are not in good agreement with the observations. The wavelength dependence of the stellar diameter can be well studied by using visibility ratios V(λ1)/V(λ2) since ratios of visibilities of different spectral channels can be measured with higher precision than absolute visibilities. We found that the 2.03 μm uniform disk diameter of CH Cyg is approximately 1.1 times larger than the 2.15 μm and 2.26 μm uniform-disk diameter

Observations of MIRA stars with the IOTA/FLUOR interferometer and comparison with MIRA star models

We present K-band observations of five Mira stars with the IOTA interferometer. The interferograms were obtained with the FLUOR fiber optics beam combiner which provides high- accuracy visibility measurements in spite of time-variable atmospheric conditions. For the Mira stars X Oph, R Aql, RU Her, R Ser, and V CrB we derived the uniform-disk diameters 11.7 mas, 10.9 mas, 8.4 mas, 8.1 mas, and 7.9 mas (+/- 0.3 mas), respectively. Simultaneous photometric observations yielded the bolometric fluxes. The derived angular Rosseland radii and the bolometric fluxes allowed the determination of effective temperatures. For instance, the effective temperature of R Aql was determined to be 3072 K +/- 161 K. A Rosseland radius for R Aql of 250 R⊙ +/- 63 R⊙ was derived from the angular Rosseland radius of 5.5 mas +/- 0.2 mas and the HIPPARCOS parallax of 4.73 mas +/- 1.19 mas. The observations were compared with theoretical Mira star models (D/P model Rosseland radius equals 255 R⊙ measured R Aql Rosseland radius equals 250 R⊙ +/- 63 R.)

VLTI/AMBER spectro-interferometry of the Herbig Be star MWC 297 with spectral resolution 12 000

Circumstellar disks and outflows play a fundamental role in star formation. Infrared spectro-interferometry allows the inner accretion-ejection region to be resolved. We measured interferometric visibilities, wavelength-differential phases, and closure phases of MWC 297 with a spectral resolution of 12000. To interpret our MWC 297 observations, we employed disk-wind models. The measured continuum visibilities confirm previous results that the continuum-emitting region of MWC 297 is remarkably compact. We derive a continuum ring-fit radius of ~2.2 mas (~0.56 AU at a distance of 250 pc), which is ~5.4 times smaller than the 3 AU dust sublimation radius expected for silicate grains (in the absence of radiation-shielding material). The strongly wavelength-dependent and asymmetric Br gamma-emitting region is more extended (~2.7 times) than the continuum-emitting region. At the center of the Br gamma line, we derive a Gaussian fit radius of ~6.3 mas HWHM (~1.6 AU). To interpret the observations, we employ a magneto-centrifugally driven disk-wind model consisting of an accretion disk, which emits the observed continuum radiation, and a disk wind, which emits the Br gamma line. The calculated wavelength-dependent model intensity distributions and Br gamma line profiles are compared with the observations (i.e., K-band spectrum, visibilities, differential phases, and closure phases). The closest fitting model predicts a continuum-emitting disk with an inner radius of ~0.3 AU and a disk wind ejection region with an inner radius of ~0.5 AU (~17.5 stellar radii). We obtain a disk-wind half-opening angle (the angle between the rotation axis and the innermost streamline of the disk wind) of ~80 degrees, which is larger than in T Tau models, and a disk inclination angle of ~20 degrees (i.e., almost pole-on).Comment: Astron. & Astrophys. accepte