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Research on the Application of Fast-steering Mirror in Stellar Interferometers
摘要： In stellar interferometers, the fast-steering mirror (FSM) is widely utilized to correct the wavefront tilt caused by the atmospheric turbulence and internal instrumental vibration, because of its high resolution and fast response frequency. In this study, the non-coplanar error between the FSM and the actuator deflection axis introduced by the manufacturing, assembly, and adjustment is analyzed systematically. Via a numerical method, the additional optical path difference (OPD) caused by the above factors is studied, and its effect on the fringe tracking accuracy of a stellar interferometer is also discussed. On the other hand, the starlight parallelism between the beams of two arms is one of the main factors for the loss of fringe visibility. By analyzing the influence of wavefront tilt caused by the atmospheric turbulence on fringe visibility, a simple and efficient real-time correction scheme of starlight parallelism is proposed based on a single array detector. The feasibility of this scheme is demonstrated by a laboratory experiment. The results show that after the correction of fast-steering mirror, the starlight parallelism meets preliminarily the requirement of a stellar interferometer on the wavefront tilt.
关键词： atmospheric effects,techniques: image processing,stars: imaging,instrumentation: interferometers,techniques: interferometric
更新于2025-09-19 17:15:36
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Clumpy dust clouds and extended atmosphere of the AGB star W Hydrae revealed with VLT/SPHERE-ZIMPOL and VLTI/AMBER
摘要： Aims. Our recent visible polarimetric images of the well-studied AGB star W Hya taken at pre-maximum light (phase 0.92) with VLT/SPHERE-ZIMPOL have revealed clumpy dust clouds close to the star at ～2 R(cid:63). We present second-epoch SPHERE-ZIMPOL observations of W Hya at minimum light (phase 0.54) as well as high-spectral resolution long-baseline interferometric observations with the AMBER instrument at the Very Large Telescope Interferometer (VLTI). Methods. We observed W Hya with VLT/SPHERE-ZIMPOL at three wavelengths in the continuum (645, 748, and 820 nm), in the Hα line at 656.3 nm, and in the TiO band at 717 nm. The VLTI/AMBER observations were carried out in the wavelength region of the CO ?rst overtone lines near 2.3 μm with a spectral resolution of 12 000. Results. The high-spatial resolution polarimetric images obtained with SPHERE-ZIMPOL have allowed us to detect clear time variations in the clumpy dust clouds as close as 34–50 mas (1.4–2.0 R(cid:63)) to the star. We detected the formation of a new dust cloud as well as the disappearance of one of the dust clouds detected at the ?rst epoch. The Hα and TiO emission extends to ～150 mas (～6 R(cid:63)), and the Hα images obtained at two epochs reveal time variations. The degree of linear polarization measured at minimum light, which ranges from 13 to 18%, is higher than that observed at pre-maximum light. The power-law-type limb-darkened disk ?t to the AMBER data in the continuum results in a limb-darkened disk diameter of 49.1 ± 1.5 mas and a limb-darkening parameter of 1.16±0.49, indicating that the atmosphere is more extended with weaker limb-darkening compared to pre-maximum light. Our Monte Carlo radiative transfer modeling shows that the second-epoch SPHERE-ZIMPOL data can be explained by a shell of 0.1 μm grains of Al2O3, Mg2SiO4, and MgSiO3 with a 550 nm optical depth of 0.6±0.2 and an inner and outer radii of 1.3 R(cid:63) and 10±2 R(cid:63), respectively. Our modeling suggests the predominance of small (0.1 μm) grains at minimum light, in marked contrast to the predominance of large (0.5 μm) grains at pre-maximum light. Conclusions. The variability phase dependence of the characteristic grain size implies that small grains might just have started to form at minimum light in the wake of a shock, while the pre-maximum light phase might have corresponded to the phase of e?cient grain growth.
关键词： stars: individual: W Hya,circumstellar matter,stars: imaging,stars: AGB and post-AGB,techniques: interferometric,techniques: polarimetric
更新于2025-09-12 10:27:22
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UVIT <i>–HST</i> – <i>GAIA</i> view of NGC 288: a census of the hot stellar population and its properties from UV
摘要： A complete census of the blue horizontal branch (BHB) and blue straggler star (BSS) population within a 10(cid:2) radius from the centre of the globular cluster NGC 288 is presented, based on images from the Ultraviolet Imaging Telescope (UVIT). The UV and UV?optical colour–magnitude diagrams (CMDs) are constructed by combining the UVIT, HST-ACS and ground data and are compared with the Bag of Stellar Tracks and Isochrones (BaSTI) isochrones generated for UVIT ?lters. We used stellar proper motion data from GAIA DR2 to select the cluster members. Our estimations of the temperature distribution of 110 BHB stars reveal two peaks, with the main peak at Teff ～ 10 300 K, and with the distribution extending up to Teff ～ 18 000 K. We identify the well-known photometric gaps, including the Grundahl jump (G-jump) in the BHB distribution, which are located between the peaks. We detect a plateau in the far-ultraviolet (FUV) magnitude for stars hotter than Teff ～ 11 500 K (G-jump), which could be caused by atomic diffusion. We detect two extreme horizontal branch (EHB) candidates, with temperatures ranging from 29 000 to 32 000 K. The radial distribution of 68 BSSs suggests that the bright BSSs are more centrally concentrated than the faint BSSs and the BHB stars. We ?nd that the BSSs have a mass range of 0.86–1.25 M(cid:4) and an age range of 2–10 Gyr, with peaks at 1 M(cid:4) and 4 Gyr respectively. This study showcases the importance of combining UVIT with HST, ground and GAIA data in deriving HB and BSS properties.
关键词： stars: blue stragglers,ultraviolet: stars,catalogues,stars: imaging,globular clusters: individual: NGC 288,stars: Hertzsprung–Russell and colour–magnitude diagrams,stars: horizontal branch
更新于2025-09-10 09:29:36
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A Study of the Point-spread Function in SDSS Images
摘要： We use SDSS imaging data in the ugriz passbands to study the shape of the point-spread function (PSF) pro?le and the variation of its width with wavelength and time. We ?nd that the PSF pro?le is well described by theoretical predictions based on von Kármán’s turbulence theory. The observed PSF radial pro?le can be parameterized by only two parameters, the pro?le’s full width at half maximum (FWHM) and a normalization of the contribution of an empirically determined “instrumental” PSF. The pro?le shape is very similar to the “double Gaussian plus power-law wing” decomposition used by the SDSS image-processing pipeline, but here it is successfully modeled with two free model parameters, rather than six as in the SDSS pipeline. The FWHM variation with wavelength follows the λ α power law, where α?≈??0.3 and is correlated with the FWHM itself. The observed behavior is much better described by von Kármán’s turbulence theory, with the outer-scale parameter in the range of 5–100 m, than by Kolmogorov’s turbulence theory. We also measure the temporal and angular structure functions for FWHM and compare them to simulations and results from the literature. The angular structure function saturates at scales beyond 0°. 5?1°. 0. The power spectrum of the temporal behavior is found to be broadly consistent with a damped random-walk model with a characteristic timescale in the range of ～5–30 minutes, though the data show a shallower high-frequency behavior. The latter is well ?t by a single power law with an index in the range of ?1.5 to ?1.0. A hybrid model is likely needed to fully capture both the low-frequency and high-frequency behavior of the temporal variations of atmospheric seeing.
关键词： surveys,atmospheric effects,stars: imaging,methods: observational,instrumentation: detectors
更新于2025-09-10 09:29:36
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Time-series Doppler imaging of the red giant HD 208472
摘要： Context. HD 208472 is among the most active RS CVn binaries with cool starspots. Decade-long photometry has shown that the spots seem to change their longitudinal appearance with a period of about six years, coherent with brightness variations. Aims. Our aim is to spatially resolve the stellar surface of HD 208472 and relate the photometric results to the true longitudinal and latitudinal spot appearance. Furthermore, we investigate the surface di?erential rotation pattern of the star. Methods. We employed three years of high-resolution spectroscopic data with a high signal-to-noise ratio (S/N) from the STELLA robotic observatory and determined new and more precise stellar physical parameters. Precalculated synthetic spectra were ?t to each of these spectra, and we provide new spot-corrected orbital elements. A sample of 34 absorption lines per spectrum was used to calculate mean line pro?les with a S/N of several hundred. A total of 13 temperature Doppler images were reconstructed from these line pro?les with the inversion code iMap. Di?erential rotation was investigated by cross-correlating successive Doppler images in each observing season. Results. Spots on HD 208472 are distributed preferably at high latitudes and less frequently around mid-to-low latitudes. No polar-cap like structure is seen at any epoch. We observed a ?ip-?op event between 2009 and 2010, manifested as a ?ip of the spot activity from phase 0.0 to phase 0.5, while the overall brightness of the star continued to increase and reached an all-time maximum in 2014. Cross-correlation of successive Doppler images suggests a solar-like di?erential rotation that is ≈15 times weaker than that of the Sun.
关键词： starspots,stars: imaging,stars: activity,stars: individual: HD 208472
更新于2025-09-09 09:28:46
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The LEECH Exoplanet Imaging Survey: Limits on Planet Occurrence Rates under Conservative Assumptions
摘要： We present the results of the largest L′ (3.8 μm) direct imaging survey for exoplanets to date, the Large Binocular Telescope Interferometer Exozodi Exoplanet Common Hunt (LEECH). We observed 98 stars with spectral types from B to M. Cool planets emit a larger share of their ?ux in L′ compared to shorter wavelengths, affording LEECH an advantage in detecting low-mass, old, and cold-start giant planets. We emphasize proximity over youth in our target selection, probing physical separations smaller than other direct imaging surveys. For FGK stars, LEECH outperforms many previous studies, placing tighter constraints on the hot-start planet occurrence frequency interior to ～20 au. For less luminous, cold-start planets, LEECH provides the best constraints on giant-planet frequency interior to ～20 au around FGK stars. Direct imaging survey results depend sensitively on both the choice of evolutionary model (e.g., hot- or cold-start) and assumptions (explicit or implicit) about the shape of the underlying planet distribution, in particular its radial extent. Arti?cially low limits on the planet occurrence frequency can be derived when the shape of the planet distribution is assumed to extend to very large separations, well beyond typical protoplanetary dust-disk radii (?50 au), and when hot-start models are used exclusively. We place a conservative upper limit on the planet occurrence frequency using cold-start models and planetary population distributions that do not extend beyond typical protoplanetary dust-disk radii. We ?nd that ?90% of FGK systems can host a 7–10 MJup planet from 5 to 50 au. This limit leaves open the possibility that planets in this range are common.
关键词： planetary systems,techniques: high angular resolution,planets and satellites: gaseous planets,stars: imaging
更新于2025-09-04 15:30:14