Thomas Granzer

Calibrating the Cepheid period-luminosity relation from the infrared surface brightness technique I. The p-factor, the Milky Way relations, and a universal K-band relation

Aims. We determine period-luminosity relations for Milky Way Cepheids in the optical and near-IR bands. These relations can be used directly as reference for extra-galactic distance determination to Cepheid populations with solar metallicity, and they form the basis for a direct comparison with relations obtained in exactly the same manner for stars in the Magellanic Clouds, presented in an accompanying paper. In that paper we show that the metallicity effect is very small and consistent with a null effect, particularly in the near-IR bands, and we combine here all 111 Cepheids from the Milky Way, the LMC and SMC to form a best relation. Methods. We employ the near-IR surface brightness (IRSB) method to determine direct distances to the individual Cepheids after we have recalibrated the projection factor using the recent parallax measurements to ten Galactic Cepheids and the constraint that Cepheid distances to the LMC should be independent of pulsation period. Results. We confirm our earlier finding that the projection factor for converting radial velocity to pulsational velocity depends quite steeply on pulsation period, p = 1.550− 0.186 log(P) in disagrement with recent theoretical predictions. We find PL relations based on 70 Milky Way fundamental mode Cepheids of MK = −3.33(±0.09)(log(P) − 1.0) − 5.66(±0.03), WVI = −3.26(±0.11)(log(P) − 1.0) − 5.96(±0.04). Combining the 70 Cepheids presented here with the results for 41 Magellanic Cloud Cepheids which are presented in an accompanying paper, we find MK = −3.30(±0.06)(log(P) − 1.0) − 5.65(±0.02), WVI = −3.32(±0.08)(log(P) − 1.0) − 5.92(±0.03). Conclusions. We delineate the Cepheid PL relation using 111 Cepheids with direct distances from the IRSB analysis. The relations are by construction in agreement with the recent HST parallax distances to Cepheids and slopes are in excellent agreement with the slopes of apparent magnitudes versus period observed in the LMC.

Flip-flops of FK Comae Berenices

Context. FK Comae Berenices is a rapidly rotating magnetically active star, the light curve of which is modulated by cool spots on its surface. It was the first star where the “flip-flop” phenomenon was discovered. Since then, flip-flops in the spot activity have been reported in many other stars. Follow-up studies with increasing length have shown, however, that the phenomenon is more complex than was thought right after its discovery. Aims. Therefore, it is of interest to perform a more thorough study of the evolution of the spot activity in FK Com. In this study, we analyse 15 years of photometric observations with two different time series analysis methods, with a special emphasis on detecting flip-flop type events from the data. Methods. We apply the continuous period search and carrier fit methods on long-term standard Johnson-Cousins V-observations from the years 1995−2010. The observations were carried out with two automated photometric telescopes, Phoenix-10 and Amadeus T7 located in Arizona. Results. We identify complex phase behaviour in 6 of the 15 analysed data segments. We identify five flip-flop events and two cases of phase jumps, where the phase shift is Δ φ 0.031. Conclusions. The flip-flop cannot be interpreted as a single phenomenon, where the main activity jumps from one active longitude to another. In some of our cases the phase shifts can be explained by differential rotation: two spot regions move with different angular velocity and even pass each other. Comparison between the methods show that the carrier fit utility is better in retrieving slow evolution especially from a low amplitude light curve, while the continuous period search is more sensitive in case of rapid changes.

The STELLA robotic observatory : first two years of high-resolution spectroscopy

The STELLA project consists of two robotic 1.2m telescopes to simultaneously monitor stellar activity with a high resolution echelle spectrograph on one telescope, and a photometric imaging instrument on the other telescope. The STELLA observatory is located at the Observatorio del Teide on the Canary island of Tenerife. The STELLA Echelle spectrograph (SES) has been operated in robotic mode for two years now, and produced approximately 10,000 spectra of the entire optical range between 390 and 900 nm at a spectral resolution of 55,000 with a peak shutter-open time of 93%. Although we do not use an iodine cell nor an actively stabilized chamber, its average radial velocity precision over the past two years was 60 to 150m/s rms, depending on target. The Wide-Field STELLA Imaging Photometer (WIFSIP) is currently being tested and will enter operation early 2009. In this paper, we present an update report on the first two years of operation.

The STELLA Robotic Observatory on Tenerife

The Astrophysical Institute Potsdam (AIP) and the Instituto de Astrofisica de Canarias (IAC) inaugurated the robotic telescopes STELLA-I and STELLA-II (STELLar Activity) on Tenerife on May 18, 2006. The observatory is located on the Izana ridge at an elevation of 2400 m near the German Vacuum Tower Telescope. STELLA consists of two 1.2 m alt-az telescopes. One telescope fiber feeds a bench-mounted high-resolution echelle spectrograph while the other telescope feeds a wide-field imaging photometer. Both scopes work autonomously by means of artificial intelligence. Not only that the telescopes are automated, but the entire observatory operates like a robot, and does not require any human presence on site.

Rotation and magnetic activity of the Hertzsprung-gap giant 31 Comae

Context. The single rapidly-rotating G0 giant 31 Comae has been a puzzle because of the absence of photometric variability despite its strong chromospheric and coronal emissions. As a Hertzsprung-gap giant, it is expected to be at the stage of rearranging its moment of inertia, hence likely also its dynamo action, which could possibly be linked with its missing photospheric activity. Aims. Our aim is to detect photospheric activity, obtain the rotation period, and use it for a first Doppler image of the star’s surface. Its morphology could be related to the evolutionary status. Methods. We carried out high-precision, white-light photometry with the MOST satellite, ground-based Stromgren photometry with automated telescopes, and high-resolution optical echelle spectroscopy with the new STELLA robotic facility. Results. The MOST data reveal, for the first time, light variations with a full amplitude of 5 mmag and an average photometric period of 6.80 ± 0.06 days. Radial-velocity variations with a full amplitude of 270 m s −1 and a period of 6.76 ± 0.02 days were detected from our STELLA spectra, which we also interpret as due to stellar rotation. The two-year constancy of the average radial velocity of +0.10 ± 0.33 km s −1 confirms the star’s single status, as well as the membership in the cluster Melotte 111. A spectrum synthesis gives Teff = 5660 ± 42 K, log g = 3.51 ± 0.09, and [Fe/H] = −0.15 ± 0.03, which together with the revised Hipparcos distance, suggests a )

The STELLA robotic observatory on Tenerife

The STELLA project is made up of two 1.2m robotic telescopes to simultaneously monitor stellar activity using a high-resolution spectrograph on one telescope, and an imaging instrument on the other telescope. The STELLA Echelle spectrograph (SES) along with the building has been in operation successfully since 2006, and is producing spectra covering the visual wavelength range between 390 and 900 nm at a resolution of 55 000. The stability of the spectrograph over the entire two year span, measured by monitoring 15 radial velocity standard stars, is 30 to 150 m/s rms. The Wide-field stellar imager and photometer (WIFSIP) was put into operation in 2010, when the SES-lightfeed was physically moved to the second telescope. We describe the final instrument conguration in use now, and on the efficiency of the robotic scheduling in use at the observatory.

Wolfgang-Amadeus: the University of Vienna twin robotic photoelectric telescopes

We describe the operations model for our two robotic photoelectric telescopes and give a brief status report after more than three years of routine operation in southern Arizona. The telescopes operate fully unattended, also the observatory itself is automatic. A site-control computer monitors weather sensors and operates the roof while the telescope control computers operate the photometers and accept input files from and provides nightly observations to the astronomer in Vienna. In the first three years of operation a total of 3.3 million individual scientific measurements were made.

All-sky mid-infrared imagery to characterize sky conditions and improve STELLA's observational performance

The All Sky Infrared Visible Analyzer (ASIVA) is an instrument principally designed to characterize sky con- ditions for purposes of improving ground-based astronomical observational performance. The ASIVAs primary functionality is to provide radiometrically calibrated imagery across the entire sky over the mid-infrared (IR) spectrum (8-13 μm). Calibration procedures have been developed for purposes of quantifying the photometric quality of the sky. These data products are used to provide the STELLA scheduler with real-time measured conditions of the sky/clouds, including thin cirrus to better optimize observing strategy. We describe how this can be used in the denition of the observing programs to make best use of the telescope time. Additional research is underway to correlate infrared spectral radiance with visible-spectrum extinction.

GANS: a nighttime spectrograph for the GREGOR solar telescope

GREGOR at night spectrograph (GANS) is a high-resolution thermally-stabilised vacuum-enclosed fixed-format fiber-fed Echelle spectrograph. GANS will be installed starting 2018 alongside the daytime instrumentation in the building of the 1,5m Gregor Solar Telescope at the Observatorio del Teide at Izan˜a, Tenerife. Specified resolving power is R~55k with wavelength coverage from 470 to 680 nm in single shot on 2k 2k CCD with 3”, 50μm fiber on sky, and with space between orders for simultaneous calibration light in the form of a Fabry-Perot Etalon or a Laser-comb spectrum. The end-to-end simulated radial velocity precision performance estimate is 2 ms−1. The main observing project of GANS will be the ground-based follow-up survey of TESS and PLATO2.0 exoplanet candidates. GANS will observe its targets in autonomous operation without human intervention using the normally human-operated day-time observatory. Limited operations will begin in first half of 2019 with first science-light planned for summer 2019.

RS CVn binary IM Peg – investigation of stellar activity and surface flows

We have obtained high resolution, high S/N spectra of the RS CVn binary IM Peg using UVES spectrograph at Kueyen 8.2m telescope of ESO VLT. We use Doppler imaging techniques to obtain stellar surface temperature maps from the UVES data. The TempMap code allows us to use surface differential rotation as an input parameter and thus to try to construct the rotation pattern on the stellar surface as part of the inversion process. The UVES observations are combined with spectroscopic observations from another time period obtained at the STELLA observatory. We obtain stellar surface temperature maps also from these spectra. These Doppler images are used to study the magnetic activity and surface differential rotation on IM Peg.