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Limits of ultra-high-precision optical astrometry: stellar surface structures
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Physics Didactics. Kristianstad University. (Physics Education Research, Fysikens didaktik)ORCID iD: 0000-0001-6638-1246
Lund University.
2007 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 476, no 3, 1389-1400 p.Article in journal (Refereed) 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.

Place, publisher, year, edition, pages
2007. Vol. 476, no 3, 1389-1400 p.
Keyword [en]
Stars : general, starspots, planetary systems, techniques : interferometric, methods : statistical
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:uu:diva-234619OAI: oai:DiVA.org:uu-234619DiVA: diva2:757310
Available from: 2014-10-21 Created: 2014-10-21 Last updated: 2017-12-05
In thesis
1. Stellar Surface Structures and the Astrometric Serach for Exoplnaets
Open this publication in new window or tab >>Stellar Surface Structures and the Astrometric Serach for Exoplnaets
2007 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Measuring stellar parallax, position and proper motion is the task of astrometry. With the development of new and much more accurate equipment, different noise sources are likely to affect the very precise measurements made with future instruments. Some of these sources are: stellar surface structures, circumstellar discs, multiplicity and weak microlensing. Also exoplanets may act as a source of perturbation.

In this thesis I present an investigation of stellar surface structures as a practical limitation to ultra-high-precision astrometry. The expected effects in different regions of the HR-diagram are quantified. I also investigate the astrometric effect of exoplanets, since their astrometric detection will be possible with future projects such as Gaia and SIM PlanetQuest.

Stellar surface structures like spots, plages and granulation produce small surface areas of different temperatures, i.e. of different brightness, which will influence integrated properties such as the total flux (zeroth moment of the brightness distribution), radial velocity and photocenter position (first moments of the brightness distribution). Also the third central moment of the brightness distribution, interferometrically observable as closure phase, will vary due to irregularities in the brightness distribution. All these properties have been modelled, using both numerical simulations and analytical methods, and statistical relations between the variations of the different properties have been derived.

Bright and/or dark surface areas, randomly spread over the stellar surface, will lead to a binomial distribution of the number of visible spots and the dispersion of such a model will be proportional topN, where N is the number of spots or surface structures. The dispersion will also be proportional to the size of each spot, A. The dispersions of the integrated properties are therefore expected to be/ ApN. It is noted that the commonly used spot filling factor, f / AN, is notthe most relevant characteristic of spottiness for these effects.

Both the simulations and the analytic model lead to a set of statistical relations for the dispersions or variations of the integrated properties. With ,e.g. knowledge of the photometric variation, m, it is possible to statistically estimate the dispersions for the other integrated properties. Especially interesting is the variation of the observed photocenter, i.e. the astrometric jitter. A literature review was therefore made of the observed photometric and radial-velocity variations for various types of stars. This allowed to map the expected levels of astrometric jitter across the HR diagram.

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 of a long-lived main-sequence star, which is about 1–4 μAU. 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 order 1 μAU, sufficient to allow 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 negligible impact on the detection of large (Jupiter-size) planets.

Place, publisher, year, edition, pages
Lund University, 2007
Keyword
Astrometry, astronomy
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-235229 (URN)
Presentation
2007-11-02, Lund, 13:00
Supervisors
Available from: 2014-10-30 Created: 2014-10-29 Last updated: 2014-10-30Bibliographically approved
2. Reading the Sky: From Starspots to Spotting Stars
Open this publication in new window or tab >>Reading the Sky: From Starspots to Spotting Stars
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
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.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. 229 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1196
Keyword
Astrometry, Astronomy Education Research, Disciplinary Discernment, Extrapolating three-dimensionality, Reading the Sky
National Category
Didactics Astronomy, Astrophysics and Cosmology
Research subject
Physics with specialization in Physics Education
Identifiers
urn:nbn:se:uu:diva-234636 (URN)978-91-554-9086-7 (ISBN)
Public defence
2014-12-11, Polhemsalen (Å10134), Ångströmlaboratoriet, Uppsala, 09:00 (English)
Opponent
Supervisors
Available from: 2014-11-20 Created: 2014-10-21 Last updated: 2015-07-10

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