Stellar Surface Imaging Research Articles

Model-free inverse method for transit imaging of stellar surfaces

Context. We present a model-free method for mapping surface brightness variations. Aims. We aim to develop a method that is not dependent on either stellar atmosphere models or limb-darkening equation. This method is optimized for exoplanet transit surveys such that a large database of stellar spot coverage can be created. Methods. The method uses light curves from several transit events of the same system. These light curves are phase-folded and median-combined to for a high-quality light curve without temporal local brightness variations. Stellar specific intensities are extracted from this light curve using a model-free method. We search individual light curves for departures from the median-combined light curve. Such departures are interpreted as brightness variations on the stellar surface. A map of brightness variations on the stellar surface is produced by finding the brightness distribution that can produce a synthetic light curve that fits observations well. No assumptions about the size, shape, or contrast of brightness variations are made. Results. We successfully reproduce maps of stellar disks from both synthetic data and archive observations from FORS2, the visual and near UV FOcal Reducer and low dispersion Spectrograph for the Very Large Telescope (VLT).

Report on the ESO Workshop "Imaging of Stellar Surfaces"

Report on the ESO Workshop "Imaging of Stellar Surfaces"

SPICA, Stellar Parameters and Images with a Cophased Array: a 6T visible combiner for the CHARA array.

High angular resolution studies of stars in the optical domain have highly progressed in recent years. After the results obtained with the visible instrument Visible spEctroGraph and polArimeter (VEGA) on the Center for High Angular Resolution Astronomy (CHARA) array and the recent developments on adaptive optics and fibered interferometry, we have started the design and study of a new six-telescope visible combiner with single-mode fibers. It is designed as a low spectral resolution instrument for the measurement of the angular diameter of stars to make a major step forward in terms of magnitude and precision with respect to the present situation. For a large sample of bright stars, a medium spectral resolution mode will allow unprecedented spectral imaging of stellar surfaces and environments for higher accuracy on stellar/planetary parameters. To reach the ultimate performance of the instrument in terms of limiting magnitude (Rmag≃8 for diameter measurements and Rmag≃4 to 5 for imaging), Stellar Parameters and Images with a Cophased Array (SPICA) includes the development of a dedicated fringe tracking system in the H band to reach "long" (200 ms to 30 s) exposures of the fringe signal in the visible.

Pathways for Observing Stellar Surfaces Using 3D Hydrodynamical Simulations of Evolved Stars

Evolved stars are among the largest and brightest stars and they are ideal targets for the new generation of sensitive, high resolution instrumentation that provides spectrophotometric, interferometric, astrometric, and imaging observables. The interpretation of the complex stellar surface images requires numerical simulations of stellar convection that take into account multi-dimensional time-dependent radiation hydrodynamics with realistic input physics. We show how the evolved star simulations are obtained using the radiative hydrodynamics code CO5BOLD and how the accurate observables are computed with the post-processing radiative transfer code Optim3D. The synergy between observations and theoretical work is supported by a proper and quantitative analysis using these simulations, and by strong constraints from the observational side.

3D hydrodynamical simulations to interpret observations of stellar surfaces of red supergiant stars

As red supergiants are the largest and brightest stars, they are ideal targets for the new generation of sensitive, high resolution instrumentation that provides spectrophotometric, interferometric, astrometric, and imaging observables. The interpretation of the complex stellar surface images requires numerical simulations of stellar convection that take into account multi-dimensional time-dependent radiation hydrodynamics with realistic input physics. We show the results obtained with the synergy between the radiative hydrodynamics code CO5BOLD and the post-processing radiative transfer code Optim3D. Such simulations support a proper and quantitative analysis of these observations, and the observations provide the theoretical work with strong constraints.

The Stellar Imager (SI) – A Mission to Resolve Stellar Surfaces, Interiors, and Magnetic Activity

The Stellar Imager (SI) is a space-based, UV/Optical Interferometer (UVOI) designed to enable 0.1 milli-arcsecond (mas) spectral imaging of stellar surfaces and of the Universe in general. It will also probe via asteroseismology flows and structures in stellar interiors. SI will enable the development and testing of a predictive dynamo model for the Sun, by observing patterns of surface activity and imaging of the structure and differential rotation of stellar interiors in a population study of Sun-like stars to determine the dependence of dynamo action on mass, internal structure and flows, and time. SI's science focuses on the role of magnetism in the Universe and will revolutionize our understanding of the formation of planetary systems, of the habitability and climatology of distant planets, and of many magneto-hydrodynamically controlled processes in the Universe. SI is a "Landmark/Discovery Mission" in the 2005 Heliophysics Roadmap, an implementation of the UVOI in the 2006 Astrophysics Strategic Plan, and a NASA Vision Mission ("NASA Space Science Vision Missions" (2008), ed. M. Allen). We present here the science goals of the SI Mission, a mission architecture that could meet those goals, and the technology development needed to enable this mission. Additional information on SI can be found at: http://hires.gsfc.nasa.gov/si/.

Imaging of stellar surfaces with the Occamian approach and the least-squares deconvolution technique

Context. We present in this paper a new technique for the indirect imaging of stellar surfaces (Doppler imaging, DI), when low signalto-noise spectral data have been improved by the least-squares deconvolution (LSD) method and inverted into temperature maps with the Occamian approach. We apply this technique to both simulated and real data and investigate its applicability for different stellar rotation rates and noise levels in data. Aims. Our goal is to boost the signal of spots in spectral lines and to reduce the effect of photon noise without loosing the temperature information in the lines. Methods. We simulated data from a test star, to which we added different amounts of noise, and employed the inversion technique based on the Occamian approach with and without LSD. In order to be able to infer a temperature map from LSD profiles, we applied the LSD technique for the first time to both the simulated observations and theoretical local line profiles, which remain dependent on temperature and limb angles. We also investigated how the excitation energy of individual lines effects the obtained solution by using three submasks that have lines with low, medium, and high excitation energy levels. Results. We show that our novel approach enables us to overcome the limitations of the two-temperature approximation, which was previously employed for LSD profiles, and to obtain true temperature maps with stellar atmosphere models. The resulting maps agree well with those obtained using the inversion code without LSD, provided the data are noiseless. However, using LSD is only advisable for poor signal-to-noise data. Further, we show that the Occamian technique, both with and without LSD, approaches the surface temperature distribution reasonably well for an adequate spatial resolution. Thus, the stellar rotation rate has a great influence on the result. For instance, in a slowly rotating star, closely situated spots are usually recovered blurred and unresolved, which affects the obtained temperature range of the map. This limitation is critical for small unresolved cool spots and is common for all DI techniques. Finally the LSD method was carried out for high signal-to-noise observations of the young active star V889 Her: the maps obtained with and without LSD are found to be consistent. Conclusions. Our new technique provides meaningful information on the temperature distribution on the stellar surfaces, which was previously inaccessible in DI with LSD. Our approach can be easily adopted for any other multi-line techniques.

The Stellar Imager (SI) project: a deep space UV/Optical Interferometer (UVOI) to observe the Universe at 0.1 milli-arcsec angular resolution

The Stellar Imager (SI) is a space-based, UV/Optical Interferometer (UVOI) designed to enable 0.1 milli-arcsecond (mas) spectral imaging of stellar surfaces and of the Universe in general. It will also probe via asteroseismology flows and structures in stellar interiors. SI’s science focuses on the role of magnetism in the Universe and will revolutionize our understanding of the formation of planetary systems, of the habitability and climatology of distant planets, and of many magneto-hydrodynamically controlled processes, such as accretion, in the Universe. The ultra-sharp images of SI will revolutionize our view of many dynamic astrophysical processes by transforming point sources into extended sources, and snapshots into evolving views. SI is a “Flagship and Landmark Discovery Mission” in the 2005 Heliophysics Roadmap and a potential implementation of the UVOI in the 2006 Science Program for NASA’s Astronomy and Physics Division. We present here the science goals of the SI Mission, a mission architecture that could meet those goals, and the technology development needed to enable this mission. Additional information on SI can be found at: http://hires.gsfc.nasa.gov/si/.

Tomography of stellar non‐radial pulsations

AbstractThe stellar surface imaging technique is used for studying stellar non‐radial pulsations on the basis of inversions of time series of variable line profiles without making assumptions on the specific shape of the pulsations. The inversion results in an image of the stellar surface in which sectoral and tesseral modes can be distinguished in many cases and the pulsational degree and the azimuthal order can be determined. The capability of the technique is studied with simulated data. Then, the surface imaging technique is applied to high‐resolution spectra of the rapidly rotating Beta Cep‐type star ω1 Sco, which shows strong line‐profile variations. Stellar surface imaging is concluded to be a useful technique for pulsation‐mode identification. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Stellar surface imaging: mapping brightness and magnetic fields

AbstractRapidly rotating late‐type stars typically display signs of magnetic activity that exceed those seen on the Sun by over two orders of magnitude. The techniques of Doppler imaging and Zeeman Doppler imaging have been instrumental in unveiling magnetic activity patterns at the photospheres of these active stars. Essentially, these techniques work by inverting time‐series of high resolution spectra to produce temperature, brightness and/or magnetic field maps at the surfaces of stars. I will describe how these techniques work and review what they have taught us about the nature of magnetic activity in rapid rotators over the last 20 years. Finally, I will conclude by outlining the capabilities of these techniques in light of new instrumentation that is now becoming available. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface imaging of stellar non-radial pulsations

We investigate capability of the stellar surface imaging technique for studying stellar non-radial pulsations on the basis of inversions of time series of variable line profiles without making assumptions on the specific shape of the pulsations. The inversion results in an image of the stellar surface in which sectoral and tesseral modes can be distinguished in many cases, and the pulsational degree and the azimuthal order can be determined. We find that sectoral and tesseral modes with (or any even number) can be successfully restored under various conditions (different inclinations of the rotational axis, surface temperature or velocity fluctuations due to pulsations). Tesseral modes with (or any odd number) do not show significant line profile variations at higher inclinations of the rotational axis, and, thus, no information can be recovered from line profiles. At lower inclinations, only the azimuthal order of such modes can be recovered. We conclude that stellar surface imaging is a useful technique for pulsation-mode identification.

Imaging stars and their environments with the VLTI

The opportunity to deliver high-angular resolution model-independent images is one of the most attractive prospects for optical/infrared interferometry. In this paper I use simple imaging simulations to identify some of the practical difficulties that may arise in achieving this goal with the VLTI. For the types of studies investigated here – stellar surface imaging and the mapping of emission line disks – it is likely that the key challenges will be to measure the visibility function on a sufficiently large range of baselines, and to secure accurate and reliable phase information.

Stellar surface imaging using closure phase

We describe the use of interferometry to make images of features on the surfaces of cool stars. We describe observations of Betelgeuse ( α Ori) and use this example to illustrate the requirements for astrophysical imaging with interferometers. We emphasise the importance of obtaining closure phase measurements and good Fourier plane coverage when imaging complex objects.

Doppler imaging of stellar surfaces - techniques and issues

The development of Doppler imaging has allowed us to observe stellar activity on the surface of stars other than the Sun for over a decade. We are now in a position to compare activity on rapidly rotating T Tauri, RS CVn and young main-sequence stars and to compare the activity on those, in turn, with that of the Sun. The images produced show some startling differences between stellar and solar activity. The strengths and weakness of the Doppler imaging technique must be reviewed regularly to remind us of what observed features are reliable and of when we should have doubts. This review is a general survey of the technique emphasizing the issue of testing and of potential artifacts without attempting excessive detail on variations in application or results. The difficulties faced as the technique is extended from images of surface temperature or abundance to images that include magnetic information are briefly surveyed.

Sunspot and starspot interiors as seen from molecular lines

Molecular lines represent powerful diagnostics of stellar and solar magnetic fields. They are currently successfully used for studying the thermal and magnetic structure of sunspots. The present calculations show how molecular lines can been used for testing stellar surface images and measuring magnetic fields inside spatially unresolved starspots. Particularly, observational capabilities for the registration of the Stokes V signal from starspots in the TiO band at 7055 Å are estimated.

Tuning Pulsation Modes with Stellar Interferometry

AbstractStellar interferometry represents a qualitative jump toward stellar surface imaging. We propose a method that tunes and filters out high degree non-radial pulsation (NRP) modes from the Fourier plane associated to the stellar brightness distribution. The proposed method can be applied to continuum flux, slow stellar rotation and high degree multi mode NRP stars.

Doppler Imaging of Stars with Surface Inhomogeneities

AbstractI review recent progress in the field of stellar surface imaging, with particular reference to advanced methods for mapping surface-brightness distributions on magnetically active late-type stars. New signal enhancement techniques, utilising profile information from hundreds or thousands of photospheric lines simultaneously, allow images to be derived for stars several magnitudes fainter than was previously possible. For brighter stars, the same techniques make it possible to map features as small as two or three degrees in extent on the stellar surface. While this opens up whole new areas of research, such as the ability to use starspot tracking to study surface differential rotation patterns on single and binary stars, caution must be exercised in the treatment of “nuisance” parameters such as the stellar rotation rate, surface abundances and radial velocity. At the very high S:N levels we now use, the effects of systematic errors in these parameters are easier to identify, isolate and eliminate. This leads to the possibility of making precise radial velocity variations (at the few hundred m s−2 level or better) in late-type stars even with equatorial rotation speeds as high as 100 km s−1. This is particularly topical given the recent discovery that one of our prime imaging targets, the young southern K0 dwarf AB Doradus, has an astrometric companion in a highly eccentric orbit with an inferred mass close to the H-burning limit.

Reality and the speckle imaging of stellar surfaces -- I. Methodology

Reality and the speckle imaging of stellar surfaces -- I. Methodology

Reality and the speckle imaging of stellar surfaces -- II. The asymmetry of Alpha Orionis

Reality and the speckle imaging of stellar surfaces -- II. The asymmetry of Alpha Orionis

Interferometric imaging of stellar surfaces

Until recently, all study of stellar surface structure, except for the sun, has been limited to indirect methods. This state of affairs is rapidly changing. With the introduction of interferometric techniques to optical astronomy, direct imaging of stellar surfaces is finally possible. Within a few years we will have images with sub-milliarcsecond resolution and 10 or more resolution elements across the stellar surface.In this talk, I will describe the technique of optical interferometry and explain how it can be made to work through the earth's turbulent atmosphere. I will show some actual data and describe what can be expected in the near future.