arXiv subjects

A. Richichi

Publications and source records attributed to A. Richichi.

At least 19 recordsLinked to original sources

arXiv2020

Further milliarcsecond resolution results on cool giants and binary stars from lunar occultations at Devashtal

We report on 26 lunar occultation events observed in the context of a program started at Devasthal in 2017. In addition to the customary observations with the 1.3-m telescope, we report here also the first attempts performed with the TIRCAM2 camera at the 3.6-m DOT telescope in the near-IR. The results consist in the first-time angular diameters for two late-type giants, in a measurement of the well-known AGB pulsating variable SW~Vir, and in the measurement of eight small separation binaries one of which is detected for the first time (HR~1860). We also measured the wider binaries SAO~94431 and 55~Tau (twice). The remaining sources were found to be unresolved with upper limits as small as 1~milliarcsecond. We plan to develop further the high-speed capability of the TIRCAM2 instrument, so as to include in our program also more near-infrared, highly extincted sources.

astro-ph.SR↗

arXiv2019

Lunar occultations with Aqueye+ and Iqueye

We report the first-time use of the Aqueye+ and Iqueye instruments to record lunar occultation events. High-time resolution recordings in different filters have been acquired for several occultations taken from January 2016 through January 2018 with Aqueye+ at the Copernicus telescope and Iqueye at the Galileo telescope in Asiago, Italy. Light curves with different time bins were calculated in post-processing and analyzed using a least-square model-dependent method. A total of nine occultation light curves were recorded, including one star for which we could measure for the first time the size of the chromosphere ($μ$ Psc) and one binary star for which discrepant previous determinations existed in the literature (SAO 92922). A disappearance of Alf Tau shows an angular diameter in good agreement with literature values. The other stars were found to be unresolved, at the milliarcsecond level. We discuss the unique properties of Aqueye+ and Iqueye for this kind of observations, namely the simultaneous measurement in up to four different filters thanks to pupil splitting, and the unprecedented time resolution well exceeding the microsecond level. This latter makes Aqueye+ and Iqueye suitable to observe not just occultations by the Moon, but also much faster events such as e.g. occultations by artificial screens in low orbits. We provide an outlook of future possible observations in this context.

astro-ph.SR↗

arXiv2017

The structure of Chariklo's rings from stellar occultations

Two narrow and dense rings (called C1R and C2R) were discovered around the Centaur object (10199) Chariklo during a stellar occultation observed on 2013 June 3. Following this discovery, we planned observations of several occultations by Chariklo's system in order to better characterize the physical properties of the ring and main body. Here, we use 12 successful occulations by Chariklo observed between 2014 and 2016. They provide ring profiles (physical width, opacity, edge structure) and constraints on the radii and pole position. Our new observations are currently consistent with the circular ring solution and pole position, to within the $\pm 3.3$ km formal uncertainty for the ring radii derived by Braga-Ribas et al. The six resolved C1R profiles reveal significant width variations from $\sim 5$ to 7.5 km. The width of the fainter ring C2R is less constrained, and may vary between 0.1 and 1 km. The inner and outer edges of C1R are consistent with infinitely sharp boundaries, with typical upper limits of one kilometer for the transition zone between the ring and empty space. No constraint on the sharpness of C2R's edges is available. A 1$σ$ upper limit of $\sim 20$ m is derived for the equivalent width of narrow (physical width <4 km) rings up to distances of 12,000 km, counted in the ring plane.

astro-ph.EP↗

arXiv2017

Study of the plutino object (208996) 2003 AZ84 from stellar occultations: size, shape and topographic features

We present results derived from four stellar occultations by the plutino object (208996) 2003~AZ$_{84}$, detected at January 8, 2011 (single-chord event), February 3, 2012 (multi-chord), December 2, 2013 (single-chord) and November 15, 2014 (multi-chord). Our observations rule out an oblate spheroid solution for 2003~AZ$_{84}$'s shape. Instead, assuming hydrostatic equilibrium, we find that a Jacobi triaxial solution with semi axes $(470 \pm 20) \times (383 \pm 10) \times (245 \pm 8)$~km % axis ratios $b/a= 0.82 \pm 0.05$ and $c/a= 0.52 \pm 0.02$, can better account for all our occultation observations. Combining these dimensions with the rotation period of the body (6.75~h) and the amplitude of its rotation light curve, we derive a density $ρ=0.87 \pm 0.01$~g~cm$^{-3}$ a geometric albedo $p_V= 0.097 \pm 0.009$. A grazing chord observed during the 2014 occultation reveals a topographic feature along 2003~AZ$_{84}$'s limb, that can be interpreted as an abrupt chasm of width $\sim 23$~km and depth $> 8$~km or a smooth depression of width $\sim 80$~km and depth $\sim 13$~km (or an intermediate feature between those two extremes).

astro-ph.EP↗

arXiv2017

The VLTI/MIDI view on the inner mass loss of evolved stars from the Herschel MESS sample

The mass-loss process from evolved stars is a key ingredient for our understanding of many fields of astrophysics, including stellar evolution and the chemical enrichment of the interstellar medium via stellar yields. One the main unsolved questions is the geometry of the mass-loss process. Taking advantage of the results from the Herschel Mass loss of Evolved StarS (MESS) programme, we initiated a coordinated effort to characterise the geometry of mass loss from evolved red giants at various spatial scales. For this purpose we used the MID-infrared interferometric Instrument (MIDI) to resolve the inner envelope of 14 asymptotic giant branch stars (AGBs) in the MESS sample. In this contribution we present an overview of the interferometric data collected within the frame of our Large Programme, and we also add archive data for completeness. We studied the geometry of the inner atmosphere by comparing the observations with predictions from different geometric models. Asymmetries are detected for five O-rich and S-type, suggesting that asymmetries in the N band are more common among stars with such chemistry. We speculate that this fact is related to the characteristics of the dust grains. Except for one star, no interferometric variability is detected, i.e. the changes in size of the shells of non-mira stars correspond to changes of the visibility of less than 10%. The observed spectral variability confirms previous findings from the literature. The detection of dust in our sample follows the location of the AGBs in the IRAS colour-colour diagram: more dust is detected around oxygen-rich stars in region II and in the carbon stars in region VII. The SiC dust feature does not appear in the visibility spectrum of UAnt and SSct, which are two carbon stars with detached shells. This finding has implications for the theory of SiC dust formation.

astro-ph.SR↗

arXiv2016

Evidence of asymmetries in the Aldebaran photosphere from multi-wavelength lunar occultations

We have recorded three lunar occultations of Aldebaran (AlfTau) at different telescopes and using various band-passes, from the ultraviolet to the far red. The data have been analyzed 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 milliarcseconds 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 one and two milliarcsecond, 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 days time span of our measurements. Surface spots appear as a likely explanation for the differences between observations and the models.

astro-ph.SR↗

arXiv2015

Lunar Occultations of Eighteen Stellar Sources from the 2.4-m Thai National Telescope

We report further results from the program of lunar occultation (LO) observations started at the 2.4-m Thai National Telescope (TNT) in 2014. We have recorded LO events of 18 stellar sources, leading to the detection of four angular diameters and two binary stars. With two exceptions, these are first-time determinations. We could resolve angular diameters as small as 2 milliarcseconds (mas) and projected separations as small as 4mas. We discuss the individual results, in the context of previous observations when available. The first-time angular diameters for o Psc, HR 6196 and 75 Leo are in good agreement with expected values, while that of Pi Leo agrees with the average of previous determinations but has a higher accuracy. We find a new secondary in o Psc, as previously suspected from Hipparcos data. We also obtain an accuratemeasurement of the companion in 31 Ari, revealing inconsistencies in the currently available orbital parameters. The TNT, equipped with the fast ULTRASPEC imager, is the leading facility in Southeast Asia for high time resolution observations. The LO technique at this telescope achieves a sensitivity of i up to 10mag, with a potential to detect several hundreds of LO events per year.

astro-ph.SR↗

arXiv2015

A large, long-lived structure near the trojan L5 point in the post common-envelope binary SDSS J1021+1744

SDSS J1021+1744 is a detached, eclipsing white dwarf / M dwarf binary discovered in the Sloan Digital Sky Survey. Outside the primary eclipse, the light curves of such systems are usually smooth and characterised by low-level variations caused by tidal distortion and heating of the M star component. Early data on SDSS J1021+1744 obtained in June 2012 was unusual in showing a dip in flux of uncertain origin shortly after the white dwarf's eclipse. Here we present high-time resolution, multi-wavelength observations of 35 more eclipses over 1.3 years, showing that the dip has a lifetime extending over many orbits. Moreover the "dip" is in fact a series of dips that vary in depth, number and position, although they are always placed in the phase interval 1.06 to 1.26 after the white dwarf's eclipse, near the L5 point in this system. Since SDSS J1021+1744 is a detached binary, it follows that the dips are caused by the transit of the white dwarf by material around the Lagrangian L5 point. A possible interpretation is that they are the signatures of prominences, a phenomenon already known from H-alpha observations of rapidly rotating single stars as well as binaries. What makes SDSS J1021+1744 peculiar is that the material is dense enough to block continuum light. The dips appear to have finally faded out around 2015 May after the first detection by Parsons et al. in 2012, suggesting a lifetime of years.

astro-ph.SR↗

arXiv2014

The HU Aqr planetary system hypothesis revisited

We study the mid-egress eclipse timing data gathered for the cataclysmic binary HU Aquarii during the years 1993-2014. The (O-C) residuals were previously attributed to a single ~7 Jupiter mass companion in ~5 au orbit or to a stable 2-planet system with an unconstrained outermost orbit. We present 22 new observations gathered between June, 2011 and July, 2014 with four instruments around the world. They reveal a systematic deviation of ~60 - 120 seconds from the older ephemeris. We re-analyse the whole set of the timing data available. Our results provide an erratum to the previous HU Aqr planetary models, indicating that the hypothesis for a third and fourth body in this system is uncertain. The dynamical stability criterion and a particular geometry of orbits rule out coplanar 2-planet configurations. A putative HU Aqr planetary system may be more complex, e.g., highly non-coplanar. Indeed, we found examples of 3-planet configurations with the middle planet in a retrograde orbit, which are stable for at least 1Gyr, and consistent with the observations. The (O-C) may be also driven by oscillations of the gravitational quadrupole moment of the secondary, as predicted by the Lanza et al. modification of the Applegate mechanism. Further systematic, long-term monitoring of HU Aqr is required to interpret the (O-C) residuals.

astro-ph.EP↗

arXiv2014

ULTRASPEC: a high-speed imaging photometer on the 2.4-m Thai National Telescope

ULTRASPEC is a high-speed imaging photometer mounted permanently at one of the Nasmyth focii of the 2.4-m Thai National Telescope (TNT) on Doi Inthanon, Thailand's highest mountain. ULTRASPEC employs a 1024x1024 pixel frame-transfer, electron-multiplying CCD (EMCCD) in conjunction with re-imaging optics to image a field of 7.7'x7.7' at (windowed) frame rates of up to ~200 Hz. The EMCCD has two outputs - a normal output that provides a readout noise of 2.3 e- and an avalanche output that can provide essentially zero readout noise. A six-position filter wheel enables narrow-band and broad-band imaging over the wavelength range 330-1000 nm. The instrument saw first light on the TNT in November 2013 and will be used to study rapid variability in the Universe. In this paper we describe the scientific motivation behind ULTRASPEC, present an outline of its design and report on its measured performance on the TNT.

astro-ph.IM↗

arXiv2014

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 $α$~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 ($Δ{\rm m} > 5$) 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.

astro-ph.SR↗

arXiv2013

Twenty-Five Sub-Arcsecond Binaries Discovered By Lunar Occultations

We report on 25 sub-arcsecond binaries, detected for the first time by means of lunar occultations in the near-infrared as part of a long-term program using the ISAAC instrument at the ESO Very Large Telescope. The primaries have magnitudes in the range K=3.8 to 10.4, and the companions in the range K=6.4 to 12.1. The magnitude differences have a median value of 2.8, with the largest being 5.4. The projected separations are in the range 6 to 748 milliarcseconds and with a median of 18 milliarcseconds, or about 3 times less than the diffraction limit of the telescope. Among our binary detections are a pre-main sequence star and an enigmatic Mira-like variable previously suspected to have a companion. Additionally, we quote an accurate first-time near-IR detection of a previously known wider binary. We discuss our findings on an individual basis as far as made possible by the available literature, and we examine them from a statistical point of view. We derive a typical frequency of binarity among field stars of ~10%, in the resolution and sensitivity range afforded by the technique (~0.003" to ~0.5", and K~12mag, respectively). This is in line with previous results by the same technique but we point out interesting differences that we can trace up to sensitivity, time sampling, and average distance of the targets. Finally, we discuss the prospects for further follow-up studies.

astro-ph.SR↗

arXiv2012

A catalog of near-IR sources found unresolved with milliarcsecond resolution

Calibration is one of the long-standing problems in optical interferometric measurements, particularly with long baselines which demand stars with angular sizes on the milliarcsecond scale and no detectable companions. While systems of calibrators have been generally established for the near-infrared in the bright source regime (K$\la 3$\,mag), modern large interferometers are sensitive to significantly fainter magnitudes. We aim at providing a list of sources found unresolved from direct observations with high angular resolution and dynamic range, which can be used to choose interferometric calibrators. To this purpose, we have used a large number of lunar occultations recorded with the ISAAC instrument at the VLT to select sources found to be unresolved and without close companions. An algorithm has been used to determine the limiting angular resolution achieved for each source, taking into account a noise model built from occulted and unocculted portions of the light curves. We have obtained upper limits on the angular sizes of 556 sources, with magnitudes ranging from K$_{\rm s} \approx$4 to 10, with a median of 7.2\,mag. The upper limits on possible undetected companions (within $\approx 0\farcs5$) range from K$_{\rm s} \approx$8 to 13, with a median of 11.5\,mag. One-third of the sources have angular sizes $\le 1$, and two-thirds $\le 2$ milliarcseconds. This list of unresolved sources matches well the capabilities of current large interferometric facilities. We also provide available cross-identifications, magnitudes, spectral types, and other auxiliary information. A fraction of the sources are found to be potentially variable. The list covers parts of the Galactic Bulge and in particular the vicinity of the Galactic Center, where extinction is very significant and traditional lists of calibrators are often insufficient.

astro-ph.IM↗

arXiv2011

AMBER/VLTI observations of 5 giant stars

While the search for exoplanets around main sequence stars more massive than the Sun have found relatively few such objects, surveys performed around giant stars have led to the discovery of more than 30 new exoplanets. The interest in studying planet hosting giant stars resides in the possibility of investigating planet formation around stars more massive than the Sun. Masses of isolated giant stars up to now were only estimated from evolutionary tracks, which led to different results depending on the physics considered. To calibrate the theory, it is therefore important to measure a large number of giant star diameters and masses as much as possible independent of physical models. We aim in the determination of diameters and effective temperatures of 5 giant stars, one of which is known to host a planet. AMBER/VLTI observations with the ATs were executed in low resolution mode on 5 giant stars. In order to measure high accurate calibrated squared visibilities, a calibrator-star-calibrator observational sequence was performed. We measured the uniform disk and limb-darkened angular diameters of 4 giant stars. The effective temperatures were also derived by combining the bolometric luminosities and the interferometric diameters. Lower effective temperatures were found when compared to spectroscopic measurements. The giant star HD12438 was found to have an unknown companion star at an angular separation of ~ 12 mas. Radial velocity measurements present in the literature confirm the presence of a companion with a very long orbital period (P ~ 11.4 years).}

astro-ph.SR↗

arXiv2011

Lunar occultations of 184 stellar sources in two crowded regions towards the galactic bulge

Lunar occultations (LO) provide a unique combination of high angular resolution and sensitivity at near-infrared wavelenghts. At the ESO Very Large Telescope, it is possible to achieve about 1 milliarcsecond (mas) resolution and detect sources as faint as K$\approx$12\,mag. We have taken advantage of a passage of the Moon over two crowded and reddened regions in the direction of the inner part of the galactic bulge, in order to obtain a high number of occultation light curves over two half nights. Our goal was to detect and characterize new binary systems, and to investigate highly extincted and relatively unknown infrared sources in search of circumstellar shells and similar peculiarities. Our target list included a significant number of very late-type stars, but in fact the majority of the sources was without spectral classification. A LO event requires the sampling of the light curve at millisecond rates in order to permit a detailed study of the diffraction fringes. For this, we used the so-called burst mode of the ISAAC instrument at the Melipal telescope. Our observing efficiency was ultimately limited by overheads for telescope pointing and data storage, to about one event every three minutes. We could record useful light curves for 184 sources. Of these, 24 were found to be binaries or multiples, all previously unknown. The projected separations are as small as 7.5\,mas, and the magnitude differences as high as $Δ$K=6.5\,mag. Additionally we could establish, also for the first time, the resolved nature of at least two more stars, with indication of circumstellar emission. We could also put upper limits on the angular size of about 165 unresolved stars, an information that combined with previous and future observations will be very helpful in establishing a list of reliable calibrators for long baseline interferometers.

astro-ph.SR↗

arXiv2010

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...

astro-ph.SR↗

arXiv2009

A List of Bright Interferometric Calibrators measured at the ESO VLTI

In a previous publication (Richichi & Percheron 2005) we described a program of observations of candidate calibrator stars at the ESO Very Large Telescope Interferometer (VLTI), and presented the main results from a statistical point of view. In the present paper, we concentrate on establishing a new homogeneous group of bright interferometric calibrators, based entirely on publicly available K-band VLTI observations carried out with the VINCI instrument up to July 2004. For this, we have defined a number of selection criteria for the quality and volume of the observations, and we have accordingly selected a list of 17 primary and 47 secondary calibrators. We have developed an approach to a robust global fit for the angular diameters using the whole volume of quality-controlled data, largely independent of a priori assumptions. Our results have been compared with direct measurements, and indirect estimates based on spectrophotometric methods, and general agreement is found within the combined uncertainties. The stars in our list cover the range K=-2.9 to +3.0 mag in brightness, and 1.3 to 20.5 milliarcseconds in uniform-disk diameter. The relative accuracy of the angular diameter values is on average 0.4% and 2% for the primary and secondary calibrators respectively. Our calibrators are well suited for interferometric observations in the near-infrared on baselines between ~20m and ~200m, and their accuracy is superior, at least for the primary calibrators, to other similar catalogues. Therefore, the present list of calibrators has the potential to lead to significantly improved interferometric scientific results.

astro-ph.IM↗

arXiv2009

Tracing the potential planet-forming regions around seven pre-main-sequence stars

We investigate the nature of the innermost regions of seven circumstellar disks around pre-main-sequence stars. Our object sample contains disks apparently at various stages of their evolution. Both single stars and spatially resolved binaries are considered. In particular, we search for inner disk gaps as proposed for several young stellar objects. When analyzing the underlying dust population in the atmosphere of circumstellar disks, the shape of the 10um feature is investigated. We performed interferometric observations in N band 8-13um with MIDI using baseline lengths of between 54m and 127m. The data analysis is based on radiative-transfer simulations using the Monte Carlo code MC3D by modeling simultaneously the SED, N band spectra, and interferometric visibilities. Correlated and uncorrelated N band spectra are compared to investigate the radial distribution of the dust composition of the disk atmosphere. Spatially resolved mid-infrared emission was detected in all objects. For four objects, the observed N band visibilities and corresponding SEDs could be simultaneously simulated using a parameterized active disk-model. For the more evolved objects of our sample, a purely passive disk-model provides the closest fit. The visibilities inferred for one source allow the presence of an inner disk gap. For another object, one of two visibility measurements could not be simulated by our modeling approach. All uncorrelated spectra reveal the 10um silicate emission feature. In contrast to this, some correlated spectra of the observations of the more evolved objects do not show this feature, indicating a lack of small silicates in the inner versus the outer regions of these disks. We conclude from this observational result that more evolved dust grains can be found in the more central disk regions.

astro-ph.SR↗