T. Appourchaux

Asteroseismology of Solar-Type Stars with K2: Detection of Oscillations in C1 Data

We present the first detections by the NASA K2 mission of oscillations in solar-type stars, using short-cadence data collected during K2 Campaign 1 (C1). We understand the asteroseismic detection thresholds for C1-like levels of photometric performance, and we can detect oscillations in subgiants having dominant oscillation frequencies around 1000 μHz. Changes to the operation of the fine-guidance sensors are expected to give significant improvements in the high-frequency performance from C3 onwards. A reduction in the excess high-frequency noise by a factor of 2.5 in amplitude would bring main-sequence stars with dominant oscillation frequencies as high as 2500 μHz into play as potential asteroseismic targets for K2.

AsteroFLAG ? from the Sun to the stars

We stand on the threshold of a critical expansion of asteroseismology of Sun-like stars, the study of stellar interiors by observation and analysis of their global acoustic modes of oscillation. The Sun-like oscillations give a very rich spectrum allowing the internal structure and dynamics to be probed down into the stellar cores to very high precision. Asteroseismic observations of many stars will allow multiple-point tests of crucial aspects of stellar evolution and dynamo theory. The aims of the asteroFLAG collaboration are to help the community to refine existing, and to develop new, methods for analysis of the asteroseismic data on the Sun-like oscillators.

Advances in the development of a Mach-Zehnder interferometric Doppler imager for seismology of giant planets

The measurements of radial velocity fields on planets with a Doppler Spectro-Imager allow the study of atmospheric dynamics of giant planets and the detection of their acoustic oscillations. The frequencies of these oscillations lead to the determination of the internal structure by asteroseismology. A new imaging tachometer, based on a Mach-Zehnder interferometer, has been developed to monitor the Doppler shift of solar lines reflected at the surface of the planets. We present the principle of this instrument. A prototype was designed and built, following the specifications of a future space mission. The performance of the prototype, both at the laboratory and on the sky, is presented here.

Echoes: a new instrumental concept of spectro-imaging for Jovian seismology

Echoes is a project of a space-borne instrument which has been proposed as part of the JUICE mission which is selected in the Cosmic Vision program of the European Space Agency (ESA) to perform seismic and dynamics studies of Jupiters interior and atmosphere. Based on an original Mach-Zehnder design, the instrument aims to measure Doppler shifts of solar spectral lines, which are reflected by cloud layers of Jupiters upper troposphere, coupled with imaging capabilities. It is specified to detect global oscillations with degree up to l = 50 and amplitude as low as 1 cm/s at the surface of Jupiter. In order to check the compliance of the instrument, and its capability to operate in representative environment (TRL5), we build a prototype to perform tests. In this paper, we present the prototype implemented at Observatoire de la Côte dAzur in collaboration with Institut dAstrophysique Spatiale. We describe the design of the Mach-Zehnder and the procedure of control and adjustment. We present the necessary tests and we show on simulation that the measurements will provide the required precision. In conclusion, we will explain the perspective for such a new instrument.

Echoes: a new instrumental concept of Doppler Spectro-Imager for the ESA mission project JUICE

Fourier transform spectrometry allows us to detect small Doppler velocity shifts of spectral lines by measuring phase shifts of their associated interference patterns. In this paper, we present the project of space-borne Doppler Spectro Imager (DSI), Echoes, dedicated to Jovian seismology and aeronomy, which is proposed as payload to the JUICE mission project (ex-EJSM/Laplace), which competes in the Cosmic Vision program of the European Space Agency (ESA). The instrumental principle inherits from the ground based experiment SYMPA (Schmider et al, 2007, Gaulme et al, 2008): it is a Mach-Zehnder interferometer working in the visible domain that is sensitive to Doppler shifts of solar spectral lines reflected on the planetary atmosphere. It aims to detect small periodic movements of Jupiters upper troposphere generated by internal acoustic modes, and to measure their temporal frequencies and spatial geometry. Such measurement would bring unprecedented knowledge on the internal structure of Jupiter, especially on the central region, and would provide unique constraints on giant planet formation models. We are currently realizing a prototype to measure the real instrumental performance in laboratory and to demonstrate the capacity to reach the Technology Readiness Level 5. We describe the experimental set-up and the expected results.

Helioseismology: Observations and Space Missions

The great success of Helioseismology resides in the remarkable progress achieved in the understanding of the structure and dynamics of the solar interior. This success mainly relies on the ability to conceive, implement, and operate specific instrumentation with enough sensitivity to detect and measure small fluctuations (in velocity and/or intensity) on the solar surface that are well below one meter per second or a few parts per million. Furthermore the limitation of the ground observations imposing the day-night cycle (thus a periodic discontinuity in the observations) was overcome with the deployment of ground-based networks --properly placed at different longitudes all over the Earth-- allowing longer and continuous observations of the Sun and consequently increasing their duty cycles. In this chapter, we start by a short historical overview of helioseismology. Then we describe the different techniques used to do helioseismic analyses along with a description of the main instrumental concepts. We in particular focus on the instruments that have been operating long enough to study the solar magnetic activity. Finally, we give a highlight of the main results obtained with such high-duty cycle observations (>80%) lasting over the last few decades.

Optical characterization of the breadboard narrowband prefilters for Solar Orbiter PHI

The Polarimetric and Helioseismic Imager (PHI) on board of Solar Orbiter will observe the Sun to measure the photospheric vector magnetic field and the line-of-sight velocity. It will employ a narrowband filtergraph (FG) to scan the FeI 6173 Å absorption line. At different spectral positions, the polarization state of the incoming light will be analyzed. The FG will provide a tuning range to scan the line, the continuum, and to compensate for the spacecraft radial velocity, as it will approach to the Sun down to 0.28 AU. The FG includes a Fabry-Perot etalon and two narrowband prefilters. The bandpass of the narrowest one has a nominal Full Width at Half Maximum (FWHM) of 2.7 Å. The measurement of the prefilters characteristics is essential for the instrument calibration. Here we present the results of the breadboard prefilters characterization, which is an important milestone in the development of the instrument.