DAAD Project D

»Sun-as-a-Star Spectroscopy with PEPSI«

apl. Prof. Dr. Carsten Denker (AIP, advisor) and Prof. Dr. Klaus G. Strassmeier (AIP, co-advisor)


Background:The connectivity between daytime solar physics and nighttime stellar physics is an old obstacle. Even when observed with the largest telescopes, almost all stars remain point sources. How can we link spatially-resolved solar data to global stellar data? The Solar Disk- Integrated (SDI) telescope observes the »Sun-as-a-Star«. This tiny, 10-millimeter diameter, double solar telescope feeds the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) [1] installed at the Large Binocular Telescope (LBT) at Mt. Graham, Arizona. Matching spectral features in SDI spectra to solar activity and rotation displayed in synoptic full-disk images will facilitate their identification in stellar spectra of solar-type stars.

Aims: The objective of the doctoral project is to identify signatures of solar activity and rotation in Sun-as-a-star spectra and to match them to features observed in solar full-disk images, magnetograms, and dopplergrams. Using the Sun as a proxy allows us to interpret similar signatures discovered in stellar spectra. Even though the full scientific potential of SDI is uncovered only after monitoring a full 11-year magnetic cycle of the Sun, early-on spectra will be used to test 3D dynamic model atmospheres, extract differential rotation signatures, and retrieve indices of chromospheric activity.

Methods: The SDI telescope takes several tens of Sun-as-a-star spectra every day. Combining individual spectra yields a daily spectrum with a signal-to-noise ratio of about 5000 : 1 at a spectral resolution of R 300.000 (1 km/s) covering the entire optical range from 380 to 910 nm. Individual spectra will be retained to search for transient features such as the emission of major flares. Detecting sunspots in photospheric lines and bright plages in Ca II H & K and its infrared triplet reveals the rotational modulation due to these features. Cloud modeling [2] of strong chromospheric absorption lines, e.g., the Balmer H line, yields parameters of cool plasma (filaments and prominences) suspended in the magnetic field above the photosphere. Ground- and space-based instruments deliver full-disk data as context. The Chromospheric Telescope (ChroTel) [3] provides full-disk filtergrams and dopplergrams in strong chromospheric absorption lines. The Solar Dynamics Observatory (SDO) [4] supplies information of the solar (vector) magnetic field, global flow field, and extreme ultra-violet images, which cover the chromosphere, transition region, and corona. Cross-calibrating SDI spectra in selected wavelength bands with data of the Integrated Sunlight Spectrometer (ISS) of the Solar Long-Term Investigation of the Sun (SOLIS) [5] facility at Kitt Peak National Observatory offers an opportunity for collaborations with the U.S. community for solar and stellar astrophysics.




  1. K.G. Strassmeier, M. Woche, I. Ilyin, et al., 2008: PEPSI – The Potsdam Echelle Polarimetric and Spectroscopic Instrument for the LBT. In: Ground-Based and Airborne Instrumentation for Astronomy II, I.S. McLean, M.M. Casali (eds.), Proceedings of the SPIE, 7014, 70140N
  2. J.M. Beckers, 1964: A Study of the Fine structures in the Solar Chromosphere. PhD Thesis, University of Utrecht
  3. C. Bethge, H. Peter, T.J. Kentischer, et al., 2011: The Chromospheric Telescope. Astronomy and Astrophysics, 534, A105
  4. W.D. Pesnell, B.J. Thompson, P.C. Chamberlin, 2012: The Solar Dynamics Observatory (SDO). Solar Physics, 275, 3
  5. C.U. Keller, J.W. Harvey, M.S. Giampapa, 2003: SOLIS – An Innovative Suite of Synoptic Instruments. In: Innovative Telescopes and Instrumentation for Solar Astrophysics, S.L. Keil, S.V. Avakyan (eds), Proceedings of the SPIE, 4853, 194