We present the first mid-IR long baseline interferometric observations of the circumstellar matter around binary post-AGB stars. Two objects, SX Cen and HD 52961, were observed using the VLTI/MIDI ...instrument during Science Demonstration Time. Both objects are known binaries for which a stable circumbinary disc is proposed to explain the SED characteristics. This is corroborated by our N-band spectrum showing a crystallinity fraction of more than 50% for both objects, pointing to a stable environment where dust processing can occur. Surprisingly, the dust surrounding SX Cen is not resolved in the interferometric observations providing an upper limit of 11 mas (or 18 AU at the distance of this object) on the diameter of the dust emission. This confirms the very compact nature of its circumstellar environment. The dust emission around HD 52961 originates from a very small but resolved region, estimated to be 635 mas at 8 km and 655 mas at 13 km. These results confirm the disc interpretation of the SED of both stars. In HD 52961, the dust is not homogeneous in its chemical composition: the crystallinity is clearly concentrated in the hotter inner region. Whether this is a result of the formation process of the disc, or due to annealing during the long storage time in the disc is not clear.
We present the results of N-band spectro-interferometric observations of the silicate carbon star IRAS08002-3803 with the MID-infrared Interferometric instrument (MIDI) at the Very Large Telescope ...Interferometer (VLTI) of the European Southern Observatory (ESO). The observations were carried out using two unit telescopes (UT2 and UT3) with projected baseline lengths ranging from 39 to 47 m. Our observations of IRAS08002-3803 have spatially resolved the dusty environment of a silicate carbon star for the first time and revealed an unexpected wavelength dependence of the angular size in the N band: the uniform-disk diameter is found to be constant and 636 mas (72 R*) between 8 and 10 km, while it steeply increases longward of 10 km to reach 653 mas (106 R*) at 13 km. Model calculations with our Monte Carlo radiative transfer code show that neither spherical shell models nor axisymmetric disk models consisting of silicate grains alone can simultaneously explain the observed wavelength dependence of the visibility and the spectral energy distribution (SED). We propose that the circumstellar environment of IRAS08002-3803 may consist of two grain species coexisting in the disk: silicate and a second grain species, for which we consider amorphous carbon, large silicate grains, and metallic iron grains. Comparison of the observed visibilities and SED with our models shows that such disk models can fairly - though not entirely satisfactorily - reproduce the observed SED and N-band visibilities. Our MIDI observations and the radiative transfer calculations lend support to the picture where oxygen-rich material around IRAS08002-3803 is stored in a circumbinary disk surrounding the carbon-rich primary star and its putative low-luminosity companion.
We present near-infrared K-band interferometric measurements of the Mira star R Leonis obtained in April 2001 and January 2002 with the VLTI, the commissioning instrument VINCI, and the two test ...siderostats. These epochs correspond to near-maximum stellar variability phases ~0.08 and ~1.02 (one cycle later), respectively. The April 2001 data cover a range of spatial frequencies (31–35 cycles/arcsecond) within the first lobe of the visibility function. These measurements indicate a center-to-limb intensity variation (CLV) that is clearly different from a uniform disk (UD) intensity profile. We show that these measured visibility values are consistent with predictions from recent self-excited dynamic Mira model atmospheres that include molecular shells close to continuum-forming layers. We derive high-precision Rosseland diameters of $28.5 \pm 0.4$ mas and $26.2 \pm 0.8$ mas for the April 2001 and January 2002 data, respectively. Together with literature estimates of the distance and the bolometric flux, these values correspond to linear radii of $350^{+50}_{-40}$ $R_\odot$ and $320^{+50}_{-40}$ $R_\odot$, and to effective temperatures of $2930 \pm 270$ K and $3080 \pm 310$ K, respectively.
We present the results of the first mid-infrared interferometric observations of the Mira variable RR Sco with the MID-infrared Interferometer (MIDI) coupled to the European Southern Observatory's ...(ESO) Very Large Telescope Interferometer (VLTI), together with K-band observations using VLTI VINCI. The observations were carried out in June 2003, when the variability phase of the object was 0.6, using two unit telescopes (UT1 and UT3), as part of the Science Demonstration Time (SDT) program of the instrument. Projected baseline lengths ranged from 73 to 102 m, and a spectral resolution of 30 was employed in the observations, which enabled us to obtain the wavelength dependence of the visibility in the region between 8 and 13 mu m. The uniform-disk diameter was found to be 18 mas between 8 and 10 mu m, while it gradually increases at wavelengths longer than 10 mu m to reach 24 mas at 13 mu m. The uniform-disk diameter between 8 and 13 mu m is significantly larger than the K-band uniform-disk diameter of 10.2 plus or minus 0.5 mas measured using VLTI VINCI with projected baseline lengths of 15-16 m, three weeks after the MIDI observations. Our model calculations show that optically thick emission from a warm molecular envelope consisting of H sub(2)O and SiO can cause the apparent mid-infrared diameter to be much larger than the continuum diameter. We find that the warm molecular envelope model extending to similar to 2.3 R* with a temperature of similar to 1400 K and column densities of H sub(2)O and SiO of 3 x 10 super(21) cm super(-2) and 1 x 10 super(20) cm super(-2), respectively, can reproduce the observed uniform-disk diameters between 8 and 10 mu m. The observed increase of the uniform-disk diameter longward of 10 mu m can be explained by an optically thin dust shell consisting of silicate and corundum grains. The inner radius of the optically thin dust shell is derived to be 7-8 R* with a temperature of similar to 700 K, and the optical depth at 10 mu m is found to be similar to 0.025.
We present the first VLTI/MIDI observations of the Be star alpha Ara (HD 158 427), showing a nearly unresolved circumstellar disk in the N band. The interferometric measurements made use of the UT1 ...and UT3 telescopes. The projected baselines were 102 and 74 meters with position angles of 7° and 55°, respectively. These measurements put an upper limit on the envelope size in the N band under the uniform disk approximation of $\phi_{\rm max}= 4\pm1.5$ mas, corresponding to 14 $R_{\star}$, assuming $R_{\star}=4.8~R_\odot$ and the Hipparcos distance of 74 pc. On the other hand the disk density must be large enough to produce the observed strong Balmer line emission. In order to estimate the possible circumstellar and stellar parameters we have used the SIMECA code developed by Stee et al. (1995, A&A, 300, 219) and Stee & Bittar (2001, A&A, 367, 532). Optical spectra taken with the échelle instrument Heros and the ESO-50 cm telescope, as well as infrared ones from the 1.6m Brazilian telescope were used together with the MIDI spectra and visibilities. These observations place complementary constraints on the density and geometry of the alpha Ara circumstellar disk. We discuss the potential truncation of the disk by a companion and we present spectroscopic indications of a periodic perturbation of some Balmer lines.
We present the first detection of the envelope which surrounds the post-AGB binary source HR 4049. VINCI-VLTI K-band interferometric observations of this source imply the existence of a large ...structure with a Gaussian angular FWHM $22.4 \pm 1.4$ mas or uniform disk diameter of $34.9 \pm 1.9$ mas. With the Hipparcos parallax of $1.50 \pm 0.64$ mas these values correspond to a physical size of $14.9^{+11.1}_{-4.4}$ AU and $23.3^{+17.3}_{-7.0}$ AU, respectively. Our measurements, covering an azimuth range of ~60° for the sky-projected baseline, provide information on the geometry of the emitting region and show that there is only a slight variation of the measured angular values along the different directions sampled. Thus, our results are consistent with a spherical geometry of the envelope. However, we cannot completely rule out the existence of an asymmetric envelope (like the circumbinary disk envisaged by some recent models) because of the limited spatial frequency and azimuth range covered by the observations.
We report on the discovery of a surprising observed correlation between the slope of the low-mass stellar global mass function (GMF) of globular clusters (GCs) and their central concentration ...parameter c = log (r sub(t)/r sub(c)), i.e., the logarithmic ratio of tidal and core radii. This result is based on the analysis of a sample of 20 Galactic GCs with solid GMF measurements from deep HST or VLT data. All the high-concentration clusters in the sample have a steep GMF, most likely reflecting their initial mass function. Conversely, low-concentration clusters tend to have a flatter GMF, implying that they have lost many stars via evaporation or tidal stripping. No GCs are found with a flat GMF and high central concentration. This finding appears counterintuitive, since the same two-body relaxation mechanism that causes stars to evaporate and the cluster to eventually dissolve should also lead to higher central density and possibly core collapse. Therefore, more concentrated clusters should have lost proportionately more stars and have a shallower GMF than low-concentration clusters, contrary to what is observed. It is possible that severely depleted GCs have also undergone core collapse and have already recovered a normal radial density profile. It is, however, more likely that GCs with a flat GMF have a much denser and smaller core than that suggested by their surface brightness profile and may well be undergoing collapse at present. In either case, we may have so far seriously underestimated the number of post-core collapse clusters, and many may be lurking in the Milky Way.
Very deep images of the Galactic globular cluster M4 (NGC 6121) through the F606W and F814W filters were taken in 2001 with the WFPC2 on board the HST. A first published analysis of this data set ...produced the result that the age of M4 is 12.7 plus or minus 0.7 Gyr, thus setting a robust lower limit to the age of the universe. In view of the great astronomical importance of getting this number right, we have subjected the same data set to the simplest possible photometric analysis that completely avoids uncertain assumptions about the origin of the detected sources. This analysis clearly reveals both a thin main sequence, from which can be deduced the deepest statistically complete mass function yet determined for a globular cluster, and a white dwarf (WD) sequence extending all the way down to the 5 sigma detection limit at I 27. The WD sequence is abruptly terminated at exactly this limit as expected by detection statistics. Using our most recent theoretical WD models to obtain the expected WD sequence for different ages in the observed bandpasses, we find that the data so far obtained do not reach the peak of the WD luminosity function, thus only allowing one to set a lower limit to the age of M4 of similar to 9 Gyr. Thus, the problem of determining the absolute age of a globular cluster and, therefore, the onset of GC formation with cosmologically significant accuracy remains completely open. Only observations several magnitudes deeper than the limit obtained so far would allow one to approach this objective.
Aims.We aim to study the geometry and kinematics of the disk around the Be star α Arae as a function of wavelength, especially across the Brγ emission line. The main purpose of this paper is to ...understand the nature of the disk rotation around Be stars. Methods.We use the AMBER/VLTI instrument operating in the K-band, which provides a gain by a factor of 5 in spatial resolution compared to previous MIDI/VLTI observations. Moreover, it is possible to combine the high angular resolution provided with the (medium) spectral resolution of AMBER to study the kinematics of the inner part of the disk and to infer its rotation law. Results.For the first time, we obtain direct evidence that the disk is in Keplerian rotation, answering a question that has existed since the discovery of the first Be star γ Cas by Father Secchi in 1866. We also present the global geometry of the disk, showing that it is compatible with a thin disk and polar enhanced winds modeled with the SIMECA code. We found that the disk around α Arae is compatible with a dense equatorial matter confined to the central region, whereas a polar wind is contributing along the rotational axis of the central star. Between these two regions, the density must be low enough to reproduce the large visibility modulus (small extension) obtained for two of the four VLTI baselines. Moreover, we obtain that α Arae is rotating very close to its critical rotation. This scenario is also compatible with the previous MIDI measurements.
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