•Channel morphological parameters have been automatically computed by shell scripts.•The free software open source GRASS GIS has been used for data processing.•Channel undergoes drastic morphological ...changes till the end of 20th century.•In the last decade the channel shows a substantial morphological stability.•The morphological changes are mainly due to human activity.
The quantitative analysis of the planform changes of the unconfined reach of the Taro River, in the Italian Northern Apennines, has allowed the channel evolution in the last 200years to be outlined.
Nine sets of maps and orthophotos, ranging from 1828 to 2011, have been used to evaluate the medium-term changes in channel morphology along the entire time interval, as well as the short-term changes in the most recent decade. Starting from the digitized channel limits and bars, a number of shell scripts based on GIS commands has been used for fast and automatic calculation of the main morphological parameters (channel length, width, braiding, sinuosity, centerline shifting) and for the drawing of graphs showing in detail their continuous variations along the entire study reach. The analysis of the differences in parameters on subsequent dates has revealed that, at least until the end of the 20th century, continuous reduction in channel width (up to a total of 75%) and braiding (43%), as well as continuous increase in channel length (13%) and sinuosity (29%), took place. This is in agreement with most of the previous studies on other rivers, both in Italy and abroad.
In contrast with the results of other studies, the most recent evolutionary trend of the Taro River shows substantial morphological stability with possible slight narrowing. The identification of the variations along the channel, facilitated by the analysis of the parameter curves and supported by the historical documentation, reveals that these variations can be substantially attributed to human activities. In particular, the continuous narrowing is largely due to the recurrent subtraction of riverbed areas to be used for agricultural and industrial purposes, as well as to the construction of 10 bridges with the relevant bank protections. The intense mining between 1950s and 1980s seems to have caused a sharp incision and partial narrowing only. The morphological changes due to the reduction in the flow regime, which seem to emerge from the sporadic and discontinuous hydrological data, are expected to be negligible and, in any case, are immaterial when compared with the remarkable changes due to human activities.
Aims. We demonstrate that reliable photometric distances of stellar clusters, and more generally of stars, can be obtained using pseudomagnitudes and rough spectral type without having to correct for ...visual absorption. Methods. We determine the mean absolute pseudomagnitude of all spectral (sub)types between B and K. Distances are computed from the difference between the star’s observed pseudomagnitude and its spectral type’s absolute pseudomagnitude. We compare the distances of 30 open clusters thus derived against the distances derived from TGAS parallaxes. Results. Our computed distances, up to distance modulus 12, agree within 0.1 mag rms with those obtained from TGAS parallaxes, proving excellent distance estimates. We show additionally that there are actually two markedly different distances in the cluster NGC 2264. Conclusions. We suggest that the pseudomagnitude distance estimation method, which is easy to perform, can be routinely used in all large-scale surveys where statistical distances on a set of stars, such as an open cluster, are required.
The concept of pseudomagnitude has recently been introduced to estimate apparent stellar diameters using a strictly observational methodology. Pseudomagnitudes are distance indicators which have the ...remarkable property of being reddening free. In this study, we use Hipparcos parallax measurements to compute the mean absolute pseudomagnitudes of solar neighbourhood dwarf stars as a function of their spectral type. To illustrate the use of absolute pseudomagnitudes, we derive the distance moduli of 360 Pleiades stars and find that the centroid of their distribution is 5.715 ± 0.018, corresponding to a distance of 139.0 ± 1.2 pc. We locate the subset of ~50 Pleiades stars observed by Hipparcos at a mean distance of 135.5 ± 3.7 pc, thus confirming the frequently reported anomaly in the Hipparcos measurements of these stars.
Context.
Surface brightness–colour relations (SBCRs) are used to derive the stellar angular diameters from photometric observations. They have various astrophysical applications, such as the distance ...determination of eclipsing binaries or the determination of exoplanet parameters. However, strong discrepancies between the SBCRs still exist in the literature, in particular for early and late-type stars.
Aims.
We aim to calibrate new SBCRs as a function of the spectral type and the luminosity class of the stars. Our goal is also to apply homogeneous criteria to the selection of the reference stars and in view of compiling an exhaustive and up-to-date list of interferometric late-type targets.
Methods.
We implemented criteria to select measurements in the JMMC Measured Diameters Catalog. We then applied additional criteria on the photometric measurements used to build the SBCRs, together with stellar characteristics diagnostics.
Results.
We built SBCRs for F5/K7–II/III, F5/K7–IV/V, M–II/III and M–V stars, with respective rms of
σ
F
V
= 0.0022 mag,
σ
F
V
= 0.0044 mag,
σ
F
V
= 0.0046 mag, and
σ
F
V
= 0.0038 mag. This results in a precision on the angular diameter of 1.0%, 2.0%, 2.1%, and 1.7%, respectively. These relations cover a large
V
−
K
colour range of magnitude, from 1 to 7.5. Our work demonstrates that SBCRs are significantly dependent on the spectral type and the luminosity class of the star. Through a new set of interferometric measurements, we demonstrate the critical importance of the selection criteria proposed for the calibration of SBCR. Finally, using the
Gaia
photometry for our samples, we obtained (
G
−
K
) SBCRs with a precision on the angular diameter between 1.1% and 2.4%.
Conclusions.
By adopting a refined and homogeneous methodology, we show that the spectral type and the class of the star should be considered when applying an SBCR. This is particularly important in the context of PLATO.
Aims. We present aperture-synthesis imaging of the red supergiant Antares (α Sco) in the CO first overtone lines. Our goal is to probe the structure and dynamics of the outer atmosphere. Methods. ...Antares was observed between 2.28 μm and 2.31 μm with VLTI/AMBER with spectral resolutions of up to 12 000 and angular resolutions as high as 7.2 mas at two epochs with a time interval of one year. Results. The reconstructed images in individual CO lines reveal that the star appears differently in the blue wing, line center, and red wing. In 2009, the images in the line center and red wing show an asymmetrically extended component, while the image in the blue wing shows little trace of it. In 2010, however, the extended component appears in the line center and blue wing, and the image in the red wing shows only a weak signature of the extended component. Our modeling of these AMBER data suggests that there is an outer atmosphere (MOLsphere) extending to 1.2–1.4 R⋆ with CO column densities of (0.5–1) × 1020 cm-2 and a temperature of ~2000 K. The CO line images observed in 2009 can be explained by a model in which a large patch or clump of CO gas is infalling at only 0–5 km s-1, while the CO gas in the remaining region is moving outward much faster at 20–30 km s-1. The images observed in 2010 suggest that a large clump of CO gas is moving outward at 0–5 km s-1, while the CO gas in the remaining region is infalling much faster at 20–30 km s-1. In contrast to the images in the CO lines, the AMBER data in the continuum show only a slight deviation from limb-darkened disks and only marginal time variations. We derive a limb-darkened disk diameter of 37.38 ± 0.06 mas and a power-law-type limb-darkening parameter of (8.7 ± 1.6) × 10-2 (2009) and 37.31 ± 0.09 mas and (1.5 ± 0.2) × 10-1 (2010). We also obtain an effective temperature of 3660 ± 120 K (the error includes the effects of the temporal flux variation that is assumed to be the same as Betelgeuse) and a luminosity of log L⋆/L⊙ = 4.88 ± 0.23. Comparison with theoretical evolutionary tracks suggests a mass of 15 ± 5 M⊙ with an age of 11–15 Myr, which is consistent with the recently estimated age for the Upper Scorpius OB association. Conclusions. The properties of the outer atmosphere of Antares are similar to those of another well-studied red supergiant, Betelgeuse. The density of the extended outer atmosphere of Antares and Betelgeuse is higher than predicted by the current 3D convection simulations by at least six orders of magnitude, implying that convection alone cannot explain the formation of the extended outer atmosphere.
Aims. We present one-dimensional aperture synthesis imaging of the red supergiant Betelgeuse (α Ori) with VLTI/AMBER. We reconstructed for the first time one-dimensional images in the individual CO ...first overtone lines. Our aim is to probe the dynamics of the inhomogeneous atmosphere and its time variation. Methods. Betelgeuse was observed between 2.28 and 2.31 μm with VLTI/AMBER using the 16-32-48 m telescope configuration with a spectral resolution up to 12 000 and an angular resolution of 9.8 mas. The good nearly one-dimensional uv coverage allows us to reconstruct one-dimensional projection images (i.e., one-dimensional projections of the object’s two-dimensional intensity distributions). Results. The reconstructed one-dimensional projection images reveal that the star appears differently in the blue wing, line center, and red wing of the individual CO lines. The one-dimensional projection images in the blue wing and line center show a pronounced, asymmetrically extended component up to ~1.3 R⋆, while those in the red wing do not show such a component. The observed one-dimensional projection images in the lines can be reasonably explained by a model in which the CO gas within a region more than half as large as the stellar size is moving slightly outward with 0–5 km s-1, while the gas in the remaining region is infalling fast with 20–30 km s-1. A comparison between the CO line AMBER data taken in 2008 and 2009 shows a significant time variation in the dynamics of the CO line-forming region in the photosphere and the outer atmosphere. In contrast to the line data, the reconstructed one-dimensional projection images in the continuum show only a slight deviation from a uniform disk or limb-darkened disk. We derive a uniform-disk diameter of 42.05 ± 0.05 mas and a power-law-type limb-darkened disk diameter of 42.49 ± 0.06 mas and a limb-darkening parameter of (9.7 ± 0.5) × 10-2. This latter angular diameter leads to an effective temperature of 3690 ± 54 K for the continuum-forming layer. These diameters confirm that the near-IR size of Betelgeuse was nearly constant over the last 18 years, in marked contrast to the recently reported noticeable decrease in the mid-IR size. The continuum data taken in 2008 and 2009 reveal no or only marginal time variations, much smaller than the maximum variation predicted by the current three-dimensional convection simulations. Conclusions. Our two-epoch AMBER observations show that the outer atmosphere extending to ~1.3–1.4 R⋆ is asymmetric and its dynamics is dominated by vigorous, inhomogeneous large-scale motions, whose overall nature changes drastically within one year. This is likely linked to the wind-driving mechanism in red supergiants.
We investigate the possiblity to detect Earth-like planets, in the visible and the near infrared domains, with ground based Extremely Large Telescopes equipped with adaptive systems capable of ...providing high Strehl ratios. From a detailed analysis of the speckle noise, we derive analytical expressions of the signal to noise ratio on the planet flux, for direct and differential imaging, in the presence of the speckle noise and the photon noise of the residual stellar halo. We find that a 100 m telescope would detect an Earth at a distance of 10 pc, with a signal to noise ratio of 5, in an integration time of 12 h. This requires to control the instrumental aberrations with a precision better than 1 nanometer rms, and to reach an image dynamics of $1.2 \times 10^{6}$ at $0\farcs 1$ radius. Under the same conditions, a telescope of 30 m would require a dynamics of $1.3 \times 10^{7}$ for a positive detection.
PDF