Infrared and visual lunar occultations measurements of stellar diameters and new binary stars detections at the Calar Alto 1.5m telescope

We present a program of routine lunar occultations, at optical and near-IR wavelengths, recently started at the 1.5m Spanish telescope at the Calar Alto Observatory. Both a CCD and an infrared array detector are used. The program is aimed mainly at the detection and investigation of binary systems, although results in other areas of stellar research are also anticipated. Occultations are reported for a total of 40 stars. Among these, SAO 164567, SAO 78258 and AG+24 788 have been discovered to be binaries, with projected separations as small as 0.006". Furthermore, binarity is suspected in the case of SAO 78119 and SAO 79251. Additionally, the angular diameter of the late-type giant 30 Psc and of the infrared star V349 Gem have been accurately measured, this latter for the first time. We finally evaluate the instrumentation performance in terms of limiting magnitude and angular resolution, and discuss applications to larger telescopes.Comment: 15 pages, 3 figures, LaTeX, uses aa.cls. Accepted for publication in A&

Aldebaran's angular diameter: how well do we know it?

The bright, well-known K5 giant Aldebaran, alpha Tau, is probably the star with the largest number of direct angular diameter determinations, achieved over a long time by several authors using various techniques. In spite of this wealth of data, or perhaps as a direct result of it, there is not a very good agreement on a single angular diameter value. This is particularly unsettling if one considers that Aldebaran is also used as a primary calibrator for some angular resolution methods, notably for optical and infrared long baseline interferometry. Directly connected to Aldebaran's angular diameter and its uncertainties is its effective temperature, which also has been used for several empirical calibrations. Among the proposed explanations for the elusiveness of an accurate determination of the angular diameter of Aldebaran are the possibility of temporal variations as well as a possible dependence of the angular diameter on the wavelength. We present here a few, very accurate new determinations obtained by means of lunar occultations and long baseline interferometry. We derive an average value of 19.96+-0.03 milliarcseconds for the uniform disk diameter. The corresponding limb-darkened value is 20.58+-0.03 milliarcseconds, or 44.2+-0.9 R(sun). We discuss this result, in connection with previous determinations and with possible problems that may affect such measurements.Comment: 8 pages, 4 figures, accepted for publication in A&

New high-sensitivity, milliarcsecond resolution results from routine observations of lunar occultations at the ESO VLT

(Abridged) Lunar occultations (LO) are a very efficient and powerful technique, that achieves the best combination of high angular resolution and sensitivity possible today at near-infrared wavelengths. Given that the events are fixed in time, that the sources are occulted randomly, and that the telescope use is minimal, the technique is very well suited for service mode observations. We have established a program of routine LO observations at the VLT observatory, especially designed to take advantage of short breaks available in-between other programs. We have used the ISAAC instrument in burst mode, capable of producing continuous read-outs at millisecond rates on a suitable subwindow. Given the random nature of the source selection, our aim has been primarily the investigation of a large number of stellar sources at the highest angular resolution in order to detect new binaries. Serendipitous results such as resolved sources and detection of circumstellar components were also anticipated. We have recorded the signal from background stars for a few seconds, around the predicted time of occultation by the Moon's dark limb. At millisecond time resolution, a characteristic diffraction pattern can be observed. Patterns for two or more sources superimpose linearly, and this property is used for the detection of binary stars. The detailed analysis of the diffraction fringes can be used to measure specific properties such as the stellar angular size and the presence of extended light sources such as a circumstellar shell. We present a list of 191 stars for which LO data could be recorded and analyzed. Results include the detection of 16 binary and 2 triple stars, all but one of which were previously unknown. The projected angular separations are as small as 4 milliarcseconds and magnitude differences as high as ?K=5.8 mag...Comment: 10 pages, 3 figures, to be published in A&

Detection of a sub-arcsecond dust shell around the Wolf-Rayet star WR 112

A lunar occultation event of the Wolf-Rayet star WR 112 (type WC9) has been observed simultaneously from two independent telescopes at lambda = 2.2microns, allowing us to investigate this source with an angular resolution of approx 0.003 arc-seconds. We have detected a circumstellar dust envelope whose brightness distribution can be approximately fitted by a gaussian with a FWHM of approx 0.06 arc-seconds (approx 10^15 cm). We present and discuss the reconstructed brightness profile, which shows an asymmetry in the radial dust distribution. The derived dust grain temperature at the inner dust zone of approx 1150 K is consistent with available model calculations. There is no signature of the central star from our observations, providing a direct confirmation that the circumstellar shell emission dominates over the photospheric emission at 2.2microns as predicted by fits to the spectral energy distribution. Further lunar occultation observations at different position angles are essential to reconstruct the 2--D image of the dust shell around WR 112. The current series of lunar occultations of WR 112 will continue to the end of 1999 and will be visible for all equatorial and southern latitude observatories.Comment: 5 pages, 1 figur

Evidence of asymmetries in the Aldebaran photosphere from multiwavelength lunar occultations

We have recorded three lunar occultations of Aldebaran (α Tau) at different telescopes and using various band-passes, from the ultraviolet to the far red. The data have been analysed using both model-dependent and model-independent methods. The derived uniform-disc angular diameter values have been converted to limb-darkened values using model atmosphere relations and are found in broad agreement among themselves and with previous literature values. The limb-darkened diameter is about 20.3 mas on average. However, we have found indications that the photospheric brightness profile of Aldebaran may have not been symmetric, a finding already reported by other authors for this and for similar late-type stars. At the sampling scale of our brightness profile, between 1 and 2 mas, the uniform and limb-darkened disc models may not be a good description for Aldebaran. The asymmetries appear to differ with wavelength and over the 137-d time span of our measurements. Surface spots appear as a likely explanation for the differences between observations and the models

First Lunar Occultation Results from the 2.4 m Thai National Telescope equipped with ULTRASPEC

The recently inaugurated 2.4\,m Thai National Telescope (TNT) is equipped, among other instruments, with the ULTRASPEC low-noise, frame-transfer EMCCD camera. At the end of its first official observing season, we report on the use of this facility to record high time resolution imaging using small detector subarrays with sampling as fast as several $10^2$\,Hz. In particular, we have recorded lunar occultations of several stars which represent the first contribution to this area of research made from South-East Asia with a telescope of this class. Among the results, we discuss an accurate measurement of $\alpha$~Cnc, which has been reported previously as a suspected close binary. Attempts to resolve this star by several authors have so far met with a lack of unambiguous confirmation. With our observation we are able to place stringent limits on the projected angular separation (<0\farcs003) and brightness ($\Delta{\rm m} > 5$) of a putative companion. We also present a measurement of the binary {HR~7072}, which extends considerably the time coverage available for its yet undetermined orbit. We discuss our precise determination of the flux ratio and projected separation in the context of other available data. We conclude by providing an estimate of the performance of ULTRASPEC at TNT for lunar occultation work. This facility can help to extend the lunar occultation technique in a geographical area where no comparable resources were available until now.Comment: Accepted for publication in Astronomical Journa