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Limits of ultra-high-precision optical astrometry: stellar surface structures
Högskolan Kristianstad, Sektionen för lärande och miljö, Avdelningen för Naturvetenskap. Högskolan Kristianstad, Forskningsmiljön Learning in Science and Mathematics (LISMA). (LISMA)ORCID-id: 0000-0001-6638-1246
Lund Observatory, Lund University.
2007 (engelsk)Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 476, nr 3, s. 1389-1400Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

Aims. To investigate the astrometric effects of stellar surface structures as a practical limitation to ultra-high-precision astrometry (e.g. in the context of exoplanet searches) and to quantify the expected effects in different regions of the HR-diagram. Methods. Stellar surface structures (spots, plages, granulation, non-radial oscillations) are likely to produce fluctuations in the integrated flux and radial velocity of the star, as well as a variation of the observed photocentre, i.e. astrometric jitter. We use theoretical considerations supported by Monte Carlo simulations (using a starspot model) to derive statistical relations between the corresponding astrometric, photometric, and radial velocity effects. Based on these relations, the more easily observed photometric and radial velocity variations can be used to predict the expected size of the astrometric jitter. Also the third moment of the brightness distribution, interferometrically observable as closure phase, contains information about the astrometric jitter. Results. For most stellar types the astrometric jitter due to stellar surface structures is expected to be of the order of 10 micro-AU or greater. This is more than the astrometric displacement typically caused by an Earth-size exoplanet in the habitable zone, which is about 1-4 micro-AU for long-lived main-sequence stars. Only for stars with extremely low photometric variability (< 0.5 mmag) and low magnetic activity, comparable to that of the Sun, will the astrometric jitter be of the order of 1 micro-AU, sufficient to allow the astrometric detection of an Earth-sized planet in the habitable zone. While stellar surface structure may thus seriously impair the astrometric detection of small exoplanets, it has in general a negligible impact on the detection of large (Jupiter-size) planets and on the determination of stellar parallax and proper motion. From the starspot model we also conclude that the commonly used spot filling factor is not the most relevant parameter for quantifying the spottiness in terms of the resulting astrometric, photometric and radial velocity variations.

sted, utgiver, år, opplag, sider
2007. Vol. 476, nr 3, s. 1389-1400
Emneord [en]
Stars : general, starspots, planetary systems, techniques : interferometric, methods : statistical
HSV kategori
Identifikatorer
URN: urn:nbn:se:hkr:diva-50DOI: 10.1051/0004-6361:20078031ISI: 000251507500032OAI: oai:DiVA.org:hkr-50DiVA, id: diva2:132647
Tilgjengelig fra: 2008-12-23 Laget: 2008-12-23 Sist oppdatert: 2017-12-14bibliografisk kontrollert
Inngår i avhandling
1. Reading the sky: from starspots to spotting stars
Åpne denne publikasjonen i ny fane eller vindu >>Reading the sky: from starspots to spotting stars
2014 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Abstract [en]

This thesis encompasses two research fields in astronomy: astrometry and astronomy education and they are discussed in two parts. These parts represent two sides of a coin; astrometry, which is about constructing 3D representations of the Universe, and AER, where for this thesis, the goal is to investigate university students’ and lecturers’ disciplinary discernment vis-à-vis the structure of the Universe and extrapolating three-dimensionality.

Part I presents an investigation of stellar surface structures influence on ultra-high-precision astrometry. The expected effects in different regions of the HR-diagram were quantified. I also investigated the astrometric effect of exoplanets, since astrometric detection will become possible with projects such as Gaia. Stellar surface structures produce small brightness variations, influencing integrated properties such as the total flux, radial velocity and photocenter position. These properties were modelled and statistical relations between the variations of the different properties were derived. From the models it is clear that for most stellar types the astrometric jitter due to stellar surface structures is expected to be of order 10 μAU or greater. This is more than the astrometric displacement typically caused by an Earth-sized exoplanet in the habitable zone, which is about 1–4 μAU, making astrometric detection difficult.

Part II presents an investigation of disciplinary discernment at the university level. Astronomy education is a particularly challenging experience for students because discernment of the ‘real’ Universe is problematic, making interpretation of the many disciplinary-specific representations used an important educational issue. The ability to ‘fluently’ discern the disciplinary affordances of these representations becomes crucial for the effective learning of astronomy. To understand the Universe I conclude that specific experiences are called. Simulations could offer these experiences, where parallax motion is a crucial component. In a qualitative study, I have analysed students’ and lecturers’ discernment while watching a simulation video, and found hierarchies that characterize the discernment in terms of three-dimensionality extrapolation and an Anatomy of Disciplinary Discernment. I combined these to define a new construct: Reading the Sky. I conclude that this is a vital competency needed for learning astronomy and suggest strategies for how to implement this in astronomy education.

sted, utgiver, år, opplag, sider
Uppsala: Acta Universitatis Upsaliensis, 2014. s. 229
Serie
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1196
Emneord
Astrometry, Astronomy Education Research, Disciplinary Discernment, Extrapolating three-dimensionality, Reading the Sky
HSV kategori
Identifikatorer
urn:nbn:se:hkr:diva-13268 (URN)9789155490867 (ISBN)
Disputas
2014-12-11, 09:00 (engelsk)
Opponent
Veileder
Tilgjengelig fra: 2014-12-05 Laget: 2014-12-05 Sist oppdatert: 2014-12-05bibliografisk kontrollert

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