| 1. |
- Acharya, B. S., et al.
(författare)
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Introducing the CTA concept
- 2013
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Ingår i: Astroparticle physics. - : Elsevier BV. - 0927-6505 .- 1873-2852. ; 43, s. 3-18
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The Cherenkov Telescope Array (CTA) is a new observatory for very high-energy (VHE) gamma rays. CTA has ambitions science goals, for which it is necessary to achieve full-sky coverage, to improve the sensitivity by about an order of magnitude, to span about four decades of energy, from a few tens of GeV to above 100 TeV with enhanced angular and energy resolutions over existing VHE gamma-ray observatories. An international collaboration has formed with more than 1000 members from 27 countries in Europe, Asia, Africa and North and South America. In 2010 the CTA Consortium completed a Design Study and started a three-year Preparatory Phase which leads to production readiness of CTA in 2014. In this paper we introduce the science goals and the concept of CTA, and provide an overview of the project. (C) 2013 Elsevier B.V. All rights reserved.
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| 2. |
- Actis, M., et al.
(författare)
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Design concepts for the Cherenkov Telescope Array CTA : an advanced facility for ground-based high-energy gamma-ray astronomy
- 2011
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Ingår i: Experimental astronomy. - : Springer. - 0922-6435 .- 1572-9508. ; 32:3, s. 193-316
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Tidskriftsartikel (refereegranskat)abstract
- Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtained using systems of imaging atmospheric Cherenkov telescopes. Ground-based gamma-ray astronomy has a huge potential in astrophysics, particle physics and cosmology. CTA is an international initiative to build the next generation instrument, with a factor of 5-10 improvement in sensitivity in the 100 GeV-10 TeV range and the extension to energies well below 100 GeV and above 100 TeV. CTA will consist of two arrays (one in the north, one in the south) for full sky coverage and will be operated as open observatory. The design of CTA is based on currently available technology. This document reports on the status and presents the major design concepts of CTA.
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| 3. |
- Arnadottir, Anna, et al.
(författare)
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Exoplaneter : kretsande runt avlägsna stjärnor
- 2011
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Konstnärligt arbete (övrigt vetenskapligt/konstnärligt)abstract
- Orbiting our star, the Sun, there are eight planets and several dwarf planets. Astronomers have now discovered planets orbiting several nearby stars in our galaxy, the Milky Way. In this planetarium show we look up into the starry night sky, familiarize ourselves with the stars and the constellations, and together we discover the exoplanets out there and how they can be detected.
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| 4. |
- Arnadottir, Anna, et al.
(författare)
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The Meridian S02E04 : The European Southern Observatory & an eathquake on the mountain
- 2022
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Konstnärligt arbete (övrigt vetenskapligt/konstnärligt)abstract
- The European Southern Observatory, or ESO, is celebrating 60 years of scientific achievements in the southern hemisphere. Sweden is one of the founding members of ESO, and several astronomers from Lund have been actively involved over the years. In the fourth episode of the second season our podcast hosts, Nic Borsato and Rebecca Forsberg, invite professor emeritus Dainis Dravins to the microphone to tell us about both the Swedish and his own connection to the European Southern Observatory. In this second season of the Meridian we are also bringing you some field reporting from the Nordic Optical Telescope on La Palma, where a team of astronomers are trying to catch an ultra-hot Jupiter-sized exoplanet.
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| 5. |
- Barbieri, C., et al.
(författare)
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Astronomical applications of quantum optics for extremely large telescopes
- 2007
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Ingår i: Journal of Modern Optics. - 0950-0340. ; 54:2-3, s. 191-197
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Tidskriftsartikel (refereegranskat)abstract
- A programme has been started to investigate photon properties that are not currently exploited in astronomical instruments, namely second- and higher-order coherence functions encoded in their arrival time, and the orbital angular momentum. This paper expounds the first results achieved in the study of a novel astronomical photometer capable of pushing time tagging towards the picosecond region. This conceptual device has been developed as a possible focal plane instrument for the future OverWhelmingly Large Telescope (OWL) of the European Southern Observatory. This instrument has been named QuantEYE, that is, the Quantum Eye of OWL.
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| 6. |
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| 7. |
- Björn, Lars Olof, et al.
(författare)
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Detectability of life and photosynthesis on exoplanets.
- 2009
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Ingår i: Current Science. - 0011-3891. ; 96:9, s. 1171-1175
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Tidskriftsartikel (populärvet., debatt m.m.)abstract
- ‘Is there life on exoplanets?’. We refer to exoplanets as planets in other solar systems than our own. This often asked question can be further refined by asking ‘is there life on exoplanets which is so extensive that it may impact on its atmosphere, its biosphere and its optical properties?’. And if such a life exists, at astronomical distances from us, can we detect it with instruments on Earth-based or Earth-orbiting observatories? Will then, in that case, our advanced knowledge of present-day and early-day photosynthesis on Earth help us select appropriate biosignatures that may signal its presence? Here we elaborate further on these themes, based on the most recent literature, and from the point of view of photosynthesis. We also provide our considered views. Although search for chlorophyll is considered desirable, we conclude that our best bet is to look for and analyse photosynthesis-related gases, namely O2, CO2 and H2O vapour. We shall keep in mind that the evolutionary tree of life on our planet has its roots in autotrophy, and of the various forms of autotrophy, only oxygenic
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| 8. |
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| 9. |
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| 10. |
- Dravins, Dainis, et al.
(författare)
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Absolute wavelength shifts - A new diagnostic for rapidly rotating stars
- 2004
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Ingår i: Stellar Rotation; IAU Symposium. - 0074-1809. ; :215, s. 27-32
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Konferensbidrag (refereegranskat)abstract
- Accuracies reached in space astrometry now permit. the accurate determination of astrometric radial velocities. without any use of spectroscopy. Knowing this true stellar motion, spectral shifts; intrinsic to stellar atmospheres can be identified, for instance gravitational redshifts and those caused by, velocity fields on stellar surfaces. The astrometric accuracy is independent. of any spectral complexity, such as the smeared-out line profiles of rapidly rotating stars. Besides a better determination of stellar velocities, this permits more precise studies of atmospheric dynamics, such as possible modifications of stellar surface convection (granulation) by rotation-induced forces, as well as a potential for observing meridional flows across stellar surfaces.
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| 11. |
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| 12. |
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| 13. |
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| 14. |
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| 15. |
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| 16. |
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| 17. |
- Dravins, Dainis, et al.
(författare)
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IAU Division IV: Stars
- 2007
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Ingår i: Transactions of the International Astronomical Union, Reports on Astronomy. - 1743-9221 .- 1743-9213. ; 26A, s. 191-192
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Bokkapitel (populärvet., debatt m.m.)
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| 18. |
- Dravins, Dainis
(författare)
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Intensity interferometry : Optical imaging with kilometer baselines
- 2016
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Ingår i: Optical and Infrared Interferometry and Imaging V. - : SPIE. - 9781510601932 ; 9907
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Konferensbidrag (refereegranskat)abstract
- Optical imaging with microarcsecond resolution will reveal details across and outside stellar surfaces but requires kilometer-scale interferometers, challenging to realize either on the ground or in space. Intensity interferometry, electronically connecting independent telescopes, has a noise budget that relates to the electronic time resolution, circumventing issues of atmospheric turbulence. Extents up to a few km are becoming realistic with arrays of optical air Cherenkov telescopes (primarily erected for gamma-ray studies), enabling an optical equivalent of radio interferometer arrays. Pioneered by Hanbury Brown and Twiss, digital versions of the technique have now been demonstrated, reconstructing diffraction-limited images from laboratory measurements over hundreds of optical baselines. This review outlines the method from its beginnings, describes current experiments, and sketches prospects for future observations.
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| 19. |
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| 20. |
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| 21. |
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| 22. |
- Dravins, Dainis, et al.
(författare)
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Long-baseline optical intensity interferometry Laboratory demonstration of diffraction-limited imaging
- 2015
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 580
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Tidskriftsartikel (refereegranskat)abstract
- Context. A long-held vision has been to realize diffraction-limited optical aperture synthesis over kilometer baselines. This will enable imaging of stellar surfaces and their environments, and reveal interacting gas flows in binary systems. An opportunity is now opening up with the large telescope arrays primarily erected for measuring Cherenkov light in air induced by gamma rays. With suitable software, such telescopes could be electronically connected and also used for intensity interferometry. Second-order spatial coherence of light is obtained by cross correlating intensity fluctuations measured in different pairs of telescopes. With no optical links between them, the error budget is set by the electronic time resolution of a few nanoseconds. Corresponding light-travel distances are approximately one meter, making the method practically immune to atmospheric turbulence or optical imperfections, permitting both very long baselines and observing at short optical wavelengths. Aims. Previous theoretical modeling has shown that full images should be possible to retrieve from observations with such telescope arrays. This project aims at verifying diffraction-limited imaging experimentally with groups of detached and independent optical telescopes. Methods. In a large optics laboratory, artificial stars (single and double, round and elliptic) were observed by an array of small telescopes. Using high-speed photon-counting solid-state detectors and real-time electronics, intensity fluctuations were cross-correlated over up to 180 baselines between pairs of telescopes, producing coherence maps across the interferometric Fourier-transform plane. Results. These interferometric measurements were used to extract parameters about the simulated stars, and to reconstruct their two-dimensional images. As far as we are aware, these are the first diffraction-limited images obtained from an optical array only linked by electronic software, with no optical connections between the telescopes. Conclusions. These experiments serve to verify the concepts for long-baseline aperture synthesis in the optical, somewhat analogous to radio interferometry.
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| 23. |
- Dravins, Dainis, et al.
(författare)
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Optical aperture synthesis with electronically connected telescopes.
- 2015
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 6
-
Tidskriftsartikel (refereegranskat)abstract
- Highest resolution imaging in astronomy is achieved by interferometry, connecting telescopes over increasingly longer distances and at successively shorter wavelengths. Here, we present the first diffraction-limited images in visual light, produced by an array of independent optical telescopes, connected electronically only, with no optical links between them. With an array of small telescopes, second-order optical coherence of the sources is measured through intensity interferometry over 180 baselines between pairs of telescopes, and two-dimensional images reconstructed. The technique aims at diffraction-limited optical aperture synthesis over kilometre-long baselines to reach resolutions showing details on stellar surfaces and perhaps even the silhouettes of transiting exoplanets. Intensity interferometry circumvents problems of atmospheric turbulence that constrain ordinary interferometry. Since the electronic signal can be copied, many baselines can be built up between dispersed telescopes, and over long distances. Using arrays of air Cherenkov telescopes, this should enable the optical equivalent of interferometric arrays currently operating at radio wavelengths.
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| 24. |
- Dravins, Dainis, et al.
(författare)
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Optical intensity interferometry with the Cherenkov Telescope Array
- 2013
-
Ingår i: Astroparticle physics. - : Elsevier BV. - 0927-6505 .- 1873-2852. ; 43, s. 331-347
-
Tidskriftsartikel (refereegranskat)abstract
- With its unprecedented light-collecting area for night-sky observations, the Cherenkov Telescope Array (CTA) holds great potential for also optical stellar astronomy, in particular as a multi-element intensity interferometer for realizing imaging with sub-milliarcsecond angular resolution. Such an order-of-magnitude increase of the spatial resolution achieved in optical astronomy will reveal the surfaces of rotationally flattened stars with structures in their circumstellar disks and winds, or the gas flows between close binaries. Image reconstruction is feasible from the second-order coherence of light, measured as the temporal correlations of arrival times between photons recorded in different telescopes. This technique (once pioneered by Hanbury Brown and Twiss) connects telescopes only with electronic signals and is practically insensitive to atmospheric turbulence and to imperfections in telescope optics. Detector and telescope requirements are very similar to those for imaging air Cherenkov observatories, the main difference being the signal processing (calculating cross correlations between single camera pixels in pairs of telescopes). Observations of brighter stars are not limited by sky brightness, permitting efficient CTA use during also bright-Moon periods. While other concepts have been proposed to realize kilometer-scale optical interferometers of conventional amplitude (phase-) type, both in space and on the ground, their complexity places them much further into the future than CTA, which thus could become the first kilometer-scale optical imager in astronomy.
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| 25. |
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| 26. |
- Dravins, Dainis, et al.
(författare)
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Photonic Astronomy and Quantum Optics
- 2008
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Ingår i: High Time Resolution Astrophysics (Astrophysics and Space Science Library). - 9781402065170 ; 353, s. 95-132
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Bokkapitel (övrigt vetenskapligt/konstnärligt)
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| 27. |
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| 28. |
- Dravins, Dainis, et al.
(författare)
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Quantum-Optical Signatures of Stimulated Emission
- 2001
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Ingår i: Eta Carinae and Other Mysterious Stars: The Hidden Opportunities of Emission Spectroscopy, ASP Conference Proceedings. San Francisco. ; 242, s. 339-339
-
Konferensbidrag (refereegranskat)
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| 29. |
- Dravins, Dainis
(författare)
-
Science with the Cherenkov Telescope Array
- 2019
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Bok (refereegranskat)abstract
- The Cherenkov Telescope Array (CTA), will be the major global observatoryfor very high-energy (VHE) gamma-ray astronomy over the next decadeand beyond. The scientific potential of CTA is extremely broad: fromunderstanding the role of relativistic cosmic particles to the search for darkmatter. CTA is an explorer of the extreme universe, probing environmentsfrom the immediate neighbourhood of black holes to cosmic voids on thelargest scales. Covering a huge range in photon energy from 20 GeV to300 TeV, CTA will improve on all aspects of performance with respect tocurrent instruments. Wider field of view and improved sensitivity will enableCTA to survey hundreds of times faster than previous TeV telescopes. Theangular resolution of CTA will approach 1 arc-minute at high energies —the best resolution of any instrument operating above the X-ray band —allowing detailed imaging of a large number of gamma-ray sources. A one totwo order-of-magnitude collection area improvement makes CTA a powerfulinstrument for time-domain astrophysics, three orders of magnitude moresensitive on hour timescales than Fermi-LAT at 30 GeV. The observatorywill operate arrays on sites in both hemispheres to provide full sky coverageand will hence maximise the potential for the rarest phenomena such asvery nearby supernovae, gamma-ray bursts, or gravitational wave transients.With 99 telescopes on the southern site and 19 telescopes on the northernsite, flexible operation will be possible, with sub-arrays available for specifictasks.
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| 30. |
- Dravins, Dainis, et al.
(författare)
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Solar photospheric spectrum microvariability : I. Theoretical searches for proxies of radial-velocity jittering
- 2023
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Ingår i: Astronomy and Astrophysics. - 0004-6361. ; 679
-
Tidskriftsartikel (refereegranskat)abstract
- Context. Extreme precision radial-velocity spectrometers enable extreme precision in stellar spectroscopy. Searches for low-mass exoplanets around solar-type stars are limited by various types of physical variability in stellar spectra, such as the short-term jittering of apparent radial velocities on levels of ∼2 m s-1. Aims. To understand the physical origins of radial-velocity jittering, the solar spectrum is assembled, as far as possible, from basic principles. Solar surface convection is modeled with time-dependent 3D hydrodynamics, followed by the computation of high-resolution spectra during numerous instances of the simulation sequence. The behavior of different classes of photospheric spectral lines is monitored throughout the simulations to identify commonalities or differences between different classes of lines: weak or strong, neutral or ionized, high or low excitation, atomic or molecular. Methods. Synthetic spectra were examined. With a wavelength sampling λ/Δλ ∼ 1 000 000, the changing shapes and wavelength shifts of unblended and representative FeI and FeII lines were followed during the simulation sequences. The radial-velocity jittering over the small simulation area typically amounts to ±150 m s-1, scaling to ∼2 m s-1 for the full solar disk. Flickering within the G-band region and in photometric indices of the Strömgren uvby system were also measured, and synthetic G-band spectra from magnetic regions are discussed. Results. Most photospheric lines vary in phase, but with different amplitudes among different classes of lines. Amplitudes of radial-velocity excursions are greater for stronger and for ionized lines, decreasing at longer wavelengths. Matching precisely measured radial velocities to such characteristic patterns should enable us to remove a significant component of the stellar noise originating in granulation. Conclusions. The granulation-induced amplitudes in full-disk sunlight amount to ∼2 m s-1; the differences between various line groups are an order of magnitude less. To mitigate this jittering, a matched filter must recognize dissimilar lineshifts among classes of diverse spectral lines with a precision of ∼10 cm s-1 for each line group. To verify the modeling toward the filter, predictions of center-to-limb dependences of jittering amplitudes for different classes of lines are presented, testable with spatially resolving solar telescopes connected to existing radial-velocity instruments.
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| 31. |
- Dravins, Dainis, et al.
(författare)
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Spatially resolved spectroscopy across stellar surfaces : II. High-resolution spectra across HD 209458 (G0 V)
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 605
-
Tidskriftsartikel (refereegranskat)abstract
- Context. High-resolution spectroscopy across spatially resolved stellar surfaces aims at obtaining spectral-line profiles that are free from rotational broadening; the gradual changes of these profiles from disk center toward the stellar limb reveal properties of atmospheric fine structure, which are possible to model with 3D hydrodynamics. Aims. Previous such studies have only been carried out for the Sun but are now extended to other stars. In this work, profiles of photospheric spectral lines are retrieved across the disk of the planet-hosting star HD 209458 (G0 V). Methods. During exoplanet transit, stellar surface portions successively become hidden and differential spectroscopy provides spectra of small surface segments temporarily hidden behind the planet. The method was elaborated in Paper I, with observable signatures quantitatively predicted from hydrodynamic simulations. Results. From observations of HD 209458 with spectral resolution λ/ Δλ ~ 80 000, photospheric Fe I line profiles are obtained at several center-To-limb positions, reaching adequately high S/N after averaging over numerous similar lines. Conclusions. Retrieved line profiles are compared to synthetic line profiles. Hydrodynamic 3D models predict, and current observations confirm, that photospheric absorption lines become broader and shallower toward the stellar limb, reflecting that horizontal velocities in stellar granulation are greater than vertical velocities. Additional types of 3D signatures will become observable with the highest resolution spectrometers at large telescopes.
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| 32. |
- Dravins, Dainis, et al.
(författare)
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Spatially resolved spectroscopy across stellar surfaces : III. Photospheric FeI lines across HD 189733A (K1 V)
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 616
-
Tidskriftsartikel (refereegranskat)abstract
- Context. Spectroscopy across spatially resolved stellar surfaces reveals spectral line profiles free from rotational broadening, whose gradual changes from disk center toward the stellar limb reflect an atmospheric fine structure that is possible to model by 3D hydrodynamics. Aims. Previous studies of photospheric spectral lines across stellar disks exist for the Sun and HD 209458 (G0 V) and are now extended to the planet-hosting HD 189733A to sample a cooler K-type star and explore the future potential of the method. Methods. During exoplanet transit, stellar surface portions successively become hidden and differential spectroscopy between various transit phases uncovers spectra of small surface segments temporarily hidden behind the planet. The method was elaborated in Paper I, in which observable signatures were predicted quantitatively from hydrodynamic simulations. Results. From observations of HD 189733A with the ESO HARPS spectrometer at λ/Δλ∼ 115 000, profiles for stronger and weaker FeI lines are retrieved at several center-to-limb positions, reaching adequate S/N after averaging over numerous similar lines. Conclusions. Retrieved line profile widths and depths are compared to synthetic ones from models with parameters bracketing those of the target star and are found to be consistent with 3D simulations. Center-to-limb changes strongly depend on the surface granulation structure and much greater line-width variation is predicted in hotter F-type stars with vigorous granulation than in cooler K-types. Such parameters, obtained from fits to full line profiles, are realistic to retrieve for brighter planet-hosting stars, while their hydrodynamic modeling offers previously unexplored diagnostics for stellar atmospheric fine structure and 3D line formation. Precise modeling may be required in searches for Earth-analog exoplanets around K-type stars, whose more tranquil surface granulation and lower ensuing microvariability may enable such detections.
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| 33. |
- Dravins, Dainis, et al.
(författare)
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Spatially resolved spectroscopy across stellar surfaces : IV. F, G, and K-stars: Synthetic 3D spectra at hyper-high resolution
- 2021
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 649
-
Tidskriftsartikel (refereegranskat)abstract
- Context. High-precision stellar analyses require hydrodynamic 3D modeling. Such models predict changes across stellar disks of spectral line shapes, asymmetries, and wavelength shifts. For testing models in stars other than the Sun, spatially resolved observations are feasible from differential spectroscopy during exoplanet transits, retrieving spectra of those stellar surface segments that successively become hidden behind the transiting planet, as demonstrated in Papers I, II, and III. Aims. Synthetic high-resolution spectra over extended spectral regions are now available from 3D models. Similar to other ab initio simulations in astrophysics, these data contain patterns that have not been specifically modeled but may be revealed after analyses to be analogous to those of a large volume of observations. Methods. From five 3D models spanning Teff = 3964-6726 K (spectral types ~K8 V-F3 V), synthetic spectra at hyper-high resolution (λ/Δλ >1 000 000) were analyzed. Selected Fe » I and Fe » II lines at various positions across stellar disks were searched for characteristic patterns between different types of lines in the same star and for similar lines between different stars. Results. Spectral-line patterns are identified for representative photospheric lines of different strengths, excitation potentials, and ionization levels, thereby encoding the hydrodynamic 3D structure. Line profiles and bisectors are shown for various stars at different positions across stellar disks. Absolute convective wavelength shifts are obtained as differences to 1D models, where such shifts do not occur. Conclusions. Observable relationships for line properties are retrieved from realistically complex synthetic spectra. Such patterns may also test very detailed 3D modeling, including non-LTE effects. While present results are obtained at hyper-high spectral resolution, the subsequent Paper V examines their practical observability at realistically lower resolutions, and in the presence of noise.
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| 34. |
- Dravins, Dainis, et al.
(författare)
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Spatially resolved spectroscopy across stellar surfaces : V. Observational prospects: Toward Earth-like exoplanet detection
- 2021
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 649
-
Tidskriftsartikel (refereegranskat)abstract
- Context. High-precision stellar analyses require hydrodynamic 3D modeling. Testing such models is feasible by retrieving spectral line shapes across stellar disks, using differential spectroscopy during exoplanet transits. Observations were presented in Papers I, II, and III, while Paper IV explored synthetic data at hyper-high spectral resolution for different classes of stars, identifying characteristic patterns for Fe » I and Fe » II lines. Aims. Anticipating future observations, the observability of patterns among photospheric lines of different strength, excitation potential and ionization level are examined from synthetic spectra, as observed at ordinary spectral resolutions and at different levels of noise. Time variability in 3D atmospheres induces changes in spectral-line parameters, some of which are correlated. An adequate calibration could identify proxies for the jitter in apparent radial velocity to enable adjustments to actual stellar radial motion. Methods. We used spectral-line patterns identified in synthetic spectra at hyper-high resolution in Paper IV from 3D models spanning Teff = 3964-6726 K (spectral types ~K8 V-F3 V) to simulate practically observable signals at different stellar disk positions at various lower spectral resolutions, down to λ/Δλ = 75 000. We also examined the center-to-limb temporal variability. Results. Recovery of spatially resolved line profiles with fitted widths and depths is shown for various noise levels, with gradual degradation at successively lower spectral resolutions. Signals during exoplanet transit are simulated. In addition to Rossiter-McLaughlin type signatures in apparent radial velocity, analogous effects are shown for line depths and widths. In a solar model, temporal variability in line profiles and apparent radial velocity shows correlations between jittering in apparent radial velocity and fluctuations in line depth. Conclusions. Spatially resolved spectroscopy using exoplanet transits is feasible for main-sequence stars. Overall line parameters of width, depth and wavelength position can be retrieved already with moderate efforts, but a very good signal-to-noise ratio is required to reveal the more subtle signatures between subgroups of spectral lines, where finer details of atmospheric structure are encoded. Fluctuations in line depth correlate with those in wavelength, and because both can be measured from the ground, searches for low-mass exoplanets should explore these to adjust apparent radial velocities to actual stellar motion.
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| 35. |
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| 36. |
- Dravins, Dainis, et al.
(författare)
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Stellar intensity interferometry over kilometer baselines: Laboratory simulation of observations with the Cherenkov Telescope Array
- 2014
-
Ingår i: Optical and Infrared Interferometry IV. - : SPIE. - 1996-756X .- 0277-786X. ; 9146, s. 91460-91460
-
Konferensbidrag (refereegranskat)abstract
- A long-held astronomical vision is to realize diffraction-limited optical aperture synthesis over kilometer baselines. This will enable imaging of stellar surfaces and their environments, show their evolution over time, and reveal interactions of stellar winds and gas flows in binary star systems. An opportunity is now opening up with the large telescope arrays primarily erected for measuring Cherenkov light in air induced by gamma rays. With suitable software, such telescopes could be electronically connected and used also for intensity interferometry. With no optical connection between the telescopes, the error budget is set by the electronic time resolution of a few nanoseconds. Corresponding light-travel distances are on the order of one meter, making the method practically insensitive to atmospheric turbulence or optical imperfections, permitting both very long baselines and observing at short optical wavelengths. Theoretical modeling has shown how stellar surface images can be retrieved from such observations and here we report on experimental simulations. In an optical laboratory, artificial stars (single and double, round and elliptic) are observed by an array of telescopes. Using high-speed photon-counting solid-state detectors and real-time electronics, intensity fluctuations are cross correlated between up to a hundred baselines between pairs of telescopes, producing maps of the second-order spatial coherence across the interferometric Fourier-transform plane. These experiments serve to verify the concepts and to optimize the instrumentation and observing procedures for future observations with (in particular) CTA, the Cherenkov Telescope Array, aiming at order-of-magnitude improvements of the angular resolution in optical astronomy.
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| 37. |
- Dravins, Dainis, et al.
(författare)
-
Stellar intensity interferometry: Prospects for sub-milliarcsecond optical imaging
- 2012
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Ingår i: New Astronomy Reviews. - : Elsevier BV. - 1872-9630 .- 1387-6473. ; 56:5, s. 143-167
-
Tidskriftsartikel (refereegranskat)abstract
- Using kilometric arrays of air Cherenkov telescopes at short wavelengths, intensity interferometry may increase the spatial resolution achieved in optical astronomy by an order of magnitude, enabling images of rapidly rotating hot stars with structures in their circumstellar disks and winds, or mapping out patterns of nonradial pulsations across stellar surfaces. Intensity interferometry (once pioneered by Hanbury Brown and Twiss) connects telescopes only electronically, and is practically insensitive to atmospheric turbulence and optical imperfections, permitting observations over long baselines and through large air-masses, also at short optical wavelengths. The required large telescopes (similar to 10 m) with very fast detectors (similar to ns) are becoming available as the arrays primarily erected to measure Cherenkov light emitted in air by particle cascades initiated by energetic gamma rays. Planned facilities (e.g., CTA, Cherenkov Telescope Array) envision many tens of telescopes distributed over a few square km. Digital signal handling enables very many baselines (from tens of meters to over a kilometer) to be simultaneously synthesized between many pairs of telescopes, while stars may be tracked across the sky with electronic time delays, in effect synthesizing an optical interferometer in software. Simulated observations indicate limiting magnitudes around m(v) = 8, reaching angular resolutions similar to 30 mu arcsec in the violet. The signal-to-noise ratio favors high-temperature sources and emission-line structures, and is independent of the optical passband, be it a single spectral line or the broad spectral continuum. Intensity interferometry directly provides the modulus (but not phase) of any spatial frequency component of the source image; for this reason a full image reconstruction requires phase retrieval techniques. This is feasible if sufficient coverage of the interferometric (u, v)-plane is available, as was verified through numerical simulations. Laboratory and field experiments are in progress; test telescopes have been erected, intensity interferometry has been achieved in the laboratory, and first full-scale tests of connecting large Cherenkov telescopes have been carried out. This paper reviews this interferometric method in view of the new possibilities offered by arrays of air Cherenkov telescopes, and outlines observational programs that should become realistic already in the rather near future. (C) 2012 Elsevier B.V. All rights reserved.
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| 38. |
- Dravins, Dainis
(författare)
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The Post-CCD Era in Optical Astronomy
- 2002
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Ingår i: European Astronomical Society Newsletter. ; 23, s. 2-3
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 39. |
- Dravins, Dainis, et al.
(författare)
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Toward a diffraction-limited square-kilometer optical telescope: Digital revival of intensity interferometry - art. no. 698609
- 2008
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Ingår i: Extremely Large Telescopes: Which Wavelengths? Retirement Symposium for Arne Ardeberg. - : SPIE. - 0277-786X .- 1996-756X. ; 6986, s. 98609-98609
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Konferensbidrag (refereegranskat)abstract
- Much of the progress in astronomy follows imaging with improved resolution. In observing stars, current capabilities are only marginal in beginning to image the disks of a few, although many stars will appear as surface objects for baselines of hundreds of meters. Since atmospheric turbulence makes ground-based phase interferometry challenging for such long baselines, kilometric space telescope clusters have been proposed for imaging stellar surface details. The realization of such projects remains uncertain, but comparable imaging could be realized by ground-based intensity interferometry. While insensitive to atmospheric turbulence and imperfections in telescope optics, the method requires large flux collectors, such as being set up as arrays of atmospheric Cherenkov telescopes for studying energetic gamma rays. High-speed detectors and digital signal handling enable very many baselines to be synthesized between pairs of telescopes, while stars may be tracked across the sky by electronic time delays. First observations with digitally combined optical instruments have now been made with pairs of 12-meter telescopes of the VERITAS array in Arizona. Observing at short wavelengths adds no problems, and similar techniques on an extremely large telescope could achieve diffraction-limited imaging down to the atmospheric cutoff, achieving a spatial resolution significantly superior by that feasible by adaptive optics operating in the red or near-infrared.
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| 42. |
- Dravins, Dainis
(författare)
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'Ultimate' information content in solar and stellar spectra: Photospheric line asymmetries and wavelength shifts
- 2008
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 492:1, s. 98-199
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Tidskriftsartikel (refereegranskat)abstract
- Context. Spectral-line asymmetries (displayed as bisectors) and wavelength shifts are signatures of the hydrodynamics in solar and stellar atmospheres. Theory may precisely predict idealized lines, but accuracies in real observed spectra are limited by blends, few suitable lines, imprecise laboratory wavelengths, and instrumental imperfections. Aims. We extract bisectors and shifts until the "ultimate" accuracy limits in highest-quality solar and stellar spectra, so as to understand the various limits set by (i) stellar physics (number of relevant spectral lines, effects of blends, rotational line broadening); by (ii) observational techniques (spectral resolution, photometric noise); and by (iii) limitations in laboratory data. Methods. Several spectral atlases of the Sun and bright solar-type stars were examined for those thousands of "unblended" lines with the most accurate laboratory wavelengths, yielding bisectors and shifts as averages over groups of similar lines. Representative data were obtained as averages over groups of similar lines, thus minimizing the effects of photometric noise and of random blends. Results. For the solar-disk center and integrated sunlight, the bisector shapes and shifts were extracted for previously little-studied species (Fe II, Ti I, Ti II, Cr II, CaI, CI), using recently determined and very accurate laboratory wavelengths. In Procyon and other F-type stars, a sharp blueward bend in the bisector near the spectral continuum is confirmed, revealing line saturation and damping wings in upward-moving photospheric granules. Accuracy limits are discussed: "astrophysical" noise due to few measurable lines, finite instrumental resolution, superposed telluric absorption, inaccurate laboratory wavelengths, and calibration noise in spectrometers, together limiting absolute lineshift studies to approximate to 50-100 m s(-1). Conclusions. Spectroscopy with resolutions lambda/Delta lambda approximate to 300 000 and accurate wavelength calibration will enable bisector studies for many stars. Circumventing remaining limits of astrophysical noise in line-blends and rotationally smeared profiles may ultimately require spectroscopy across spatially resolved stellar disks, utilizing optical interferometers and extremely large telescopes of the future.
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| 43. |
- Dravins, Dainis, et al.
(författare)
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Wavelength shifts in solar-type spectra
- 2005
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Ingår i: European Space Agency, (Special Publication) ESA SP. - 1609-042X .- 0379-6566. ; 560, s. 113-119
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Konferensbidrag (refereegranskat)abstract
- Spectral-line displacements away from the wavelengths naively expected from the Doppler shift caused by stellar radial motion may originate as convective shifts (correlated velocity and brightness patterns in the photosphere), as gravitational redshifts, or perhaps be induced by wave motions. Absolute lineshifts, in the past studied only for the Sun, are now accessible also for other stars thanks to astrometric determination of stellar radial motion, and spectrometers with accurate wavelength calibration. Comparisons between spectroscopic apparent radial velocities and astrometrically determined radial motions reveal greater spectral blueshifts in F-type stars than in the Sun (as theoretically expected from their more vigorous convection), further increasing in A-type stars (possibly due to atmospheric shockwaves). Work is in progress to survey the spectra of the Sun and several solar-type stars for "unblended" photospheric lines of most atomic species with accurate laboratory wavelengths available. One aim is to understand the ultimate information content of stellar spectra, and in what detail it will be feasible to verify models of stellar atmospheric hydrodynamics. These may predict bisectors and shifts for widely different classes of lines, but there will not result any comparison with observations if such lines do not exist in real spectra, or are too blended for meaningful measurement. An important near-future development to enable a further analysis of stellar surface structure will be the study of wavelength variations across spatially resolved stellar disks, e.g., the center-to-limb wavelength changes along a stellar diameter, and their spatially resolved time variability.
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| 44. |
- Hawarden, Timothy G., et al.
(författare)
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Critical science with the largest telescopes: science drivers for a 100m ground-based optical-IR telescope
- 2003
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Ingår i: Future Giant Telescopes (Proceedings of the SPIE). - : SPIE. - 081944619X ; 4840, s. 299-308
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Konferensbidrag (refereegranskat)abstract
- Extremely large filled-aperture ground-based optical-IR telescopes, or ELTs, ranging from 20 to 100m in diameter, are now being proposed. The all-important choice of the aperture must clearly be driven by the potential science offered. We here highlight science goals from the Leiden Workshop in May 2001 suggesting that for certain critical observations the largest possible aperture - assumed to be 100m (theproposed European OverWhelmingly Large telescope (OWL) - is strongly tobe desired. Examples from a long list include: COSMOLOGY: Identifying the first sources of ionisation in the universe, out to z >=14 Identifying and studying the first generation of dusty galaxies More speculatively, observing the formation of the laws of physics, via the evolution of the fundamental physical contants in the very early Universe, by high-resolution spectroscopy of very distant quasars. NEARER GALAXIES: Determining detailed star-formation histories of galaxies out to the Virtgo Cluster, and hence for all major galaxy types (not just those available close to the Local Group of galaxies). THE SOLAR SYSTEM: A 100-m telescope would do the work of a flotilla of fly-by space probes for investigations ranging from the evolution ofplanetary sutfaces and atmospheres to detailed surface spectroscopy of Kuiper Belt Objects. (Such studies could easily occupy it full-time.) EARTHLIKE PLANETS OF NEARBY STARS: A prospect so exciting as perhaps to justify the 100-m telescope on its own, is that of the direct detectionof earthlike planets of solar-type stars by imaging, out to at least 25 parsecs (80 light years) from the sun, followed by spectroscopic and photometric searches for the signature of life on the surfaces of nearer examples.
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| 48. |
- Lindegren, Lennart, et al.
(författare)
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Astrometric radial velocities for nearby stars
- 2021
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 652
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Tidskriftsartikel (refereegranskat)abstract
- Context. Under certain conditions, stellar radial velocities can be determined from astrometry, without any use of spectroscopy. This enables us to identify phenomena, other than the Doppler effect, that are displacing spectral lines. Aims. The change of stellar proper motions over time (perspective acceleration) is used to determine radial velocities from accurate astrometric data, which are now available from the Gaia and » HIPPARCOS missions. Methods. Positions and proper motions at the epoch of » HIPPARCOS are compared with values propagated back from the epoch of the Gaia Early Data Release 3. This propagation depends on the radial velocity, which obtains its value from an optimal fit assuming uniform space motion relative to the solar system barycentre. Results. For 930 nearby stars we obtain astrometric radial velocities with formal uncertainties better than 100 km s-1; for 55 stars the uncertainty is below 10 km s-1, and for seven it is below 1 km s-1. Most stars that are not components of double or multiple systems show good agreement with available spectroscopic radial velocities. Conclusions. Astrometry offers geometric methods to determine stellar radial velocity, irrespective of complexities in stellar spectra. This enables us to segregate wavelength displacements caused by the radial motion of the stellar centre-of-mass from those induced by other effects, such as gravitational redshifts in white dwarfs.
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| 49. |
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| 50. |
- Lindegren, Lennart, et al.
(författare)
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The fundamental definition of "radial velocity''
- 2003
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 401:3, s. 1185-1201
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Tidskriftsartikel (refereegranskat)abstract
- Accuracy levels of metres per second require the fundamental concept of``radial velocity'' for stars and other distant objects to be examined,both as a physical velocity, and as measured by spectroscopic andastrometric techniques. Already in a classical (non-relativistic)framework the line-of-sight velocity component is an ambiguous concept,depending on whether, e.g., the time of light emission (at the object)or that of light detection (by the observer) is used for recording thetime coordinate. Relativistic velocity effects and spectroscopicmeasurements made inside gravitational fields add further complications,causing wavelength shifts to depend, e.g., on the transverse velocity ofthe object and the gravitational potential at the source. Aiming atdefinitions that are unambiguous at accuracy levels of 1 ms-1, we analyse different concepts of radial velocity andtheir interrelations. At this accuracy level, a strict separation mustbe made between the purely geometric concepts on one hand, and thespectroscopic measurement on the other. Among the geometric concepts wedefine kinematic radial velocity, which corresponds most closely to the``textbook definition'' of radial velocity as the line-of-sightcomponent of space velocity; and astrometric radial velocity, which canbe derived from astrometric observations. Consistent with thesedefinitions, we propose strict definitions also of the complementarykinematic and astrometric quantities, namely transverse velocity andproper motion. The kinematic and astrometric radial velocities depend onthe chosen spacetime metric, and are accurately related by simplecoordinate transformations. On the other hand, the observationalquantity that should result from accurate spectroscopic measurements isthe barycentric radial-velocity measure. This is independent of themetric, and to first order equals the line-of-sight velocity. However,it is not a physical velocity, and cannot be accurately transformed to akinematic or astrometric radial velocity without additional assumptionsand data in modelling the process of light emission from the source, thetransmission of the signal through space, and its recording by theobserver. For historic and practical reasons, the spectroscopicradial-velocity measure is expressed in velocity units asczB, where c is the speed of light and zB is theobserved relative wavelength shift reduced to the solar-systembarycentre, at an epoch equal to the barycentric time of light arrival.The barycentric radial-velocity measure and the astrometric radialvelocity are defined by recent resolutions adopted by the InternationalAstronomical Union (IAU), the motives and consequences of which areexplained in this paper.
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