Manuel Guedel

BRITE-Constellation: Nanosatellites for Precision Photometry of Bright Stars

BRITE-Constellation (where BRITE stands for BRIght Target Explorer) is an international nanosatellite mission to monitor photometrically, in two colours, the brightness and temperature variations of stars generally brighter than mag(V) ≈ 4 with precision and time coverage not possible from the ground. The current mission design consists of six nanosats (hence Constellation): two from Austria, two from Canada, and two from Poland. Each 7 kg nanosat carries an optical telescope of aperture 3 cm feeding an uncooled CCD. One instrument in each pair is equipped with a blue filter; the other with a red filter. Each BRITE instrument has a wide field of view (≈24°), so up to about 15 bright stars can be observed simultaneously, sampled in 32 × 32 pixels sub-rasters. Photometry of additional fainter targets, with reduced precision but thorough time sampling, will be possible through onboard data processing. The BRITE sample is dominated by the most intrinsically luminous stars: massive stars seen at all evolutionary stages, and evolved medium-mass stars at the very end of their nuclear burning phases. The goals of BRITE-Constellation are to (1) measure p- and g-mode pulsations to probe the interiors and ages of stars through asteroseismology; (2) look for varying spots on the stars surfaces carried across the stellar disks by rotation, which are the sources of co-rotating interaction regions in the winds of the most luminous stars, probably arising from magnetic subsurface convection; and (3) search for planetary transits.

X-Ray Emission from the Sun in Its Youth and Old Age

We have obtained ROSAT PSPC (Roentgen Satellite Position Sensitive Proportional Counter) pointed observations of two nearby G stars of ages 70 Myr and 9.5 Gyr that are of unique importance as proxies for the Sun at the two extremes of its main-sequence evolutionary lifetime. The younger star, HD 129333 (EK Dra; G0 V), a rapid rotator with a 2.7 day period, is a strong source with an X-ray luminosity L(x)(0.2-2.4 keV) = (7.5-11.5) x 10(exp 29) erg/s. Modeling suggests a two-temperature corona with T(1) = (2.0 +/- 0.3) x 10(exp 6) K and T(2) = (9.7 +/- 0.3) x 10(exp 6) K (formal uncertainties). A continuous emission measure distribution, increasing to higher temperatures and with a cutoff at (20-30) x 10(exp 6) K, yields even better fits to the data. The old star, beta Hyi (HR 98; G2 IV), represents the Sun in the future, near the end of its hydrogen-core burning stage, when it should be rotating more slowly (present P(rot) = 25.4 day) and should have lower levels of activity. The ROSAT measurements yield L(x) = (0.9-3.0) x 10(exp 27) ergs/s and a rather cool, single coronal temperature of T = (1.7 +/- 0.4) x 10(exp 6) K. For comparison, the Sun has L(x) approx. equal to 2 x 10(exp 27) ergs/s and a coronal temperature of about T = 2 x 10(exp 6) K. These stars provide information on the decline of the stellar (and specifically solar) magnetic activity from extreme youth to old age. HD 129333 is also important in that it yields an estimate of the solar soft X-ray flux in the early solar system at the epoch of the terminal stages of planetary accretion.

The effect of external environment on the evolution of protostellar disks

Using numerical hydrodynamics simulations we studied the gravitational collapse of pre-stellar cores of sub-solar mass embedded into a low-density external environment. Four models with different magnitude and direction of rotation of the external environment with respect to the central core were studied and compared with an isolated model. We found that the infall of matter from the external environment can significantly alter the disk properties as compared to those seen in the isolated model. Depending on the magnitude and direction of rotation of the external environment, a variety of disks can form including compact (<= 200 AU) ones shrinking in size due to infall of external matter with low angular momentum, as well as extended disks forming due to infall of external matter with high angular momentum. The former are usually stable against gravitational fragmentation, while the latter are prone to fragmentation and formation of stellar systems with sub-stellar/very-low-mass companions. In the case of counterrotating external environment, very compact (< 5 AU) and short-lived (<= a few * 10^5 yr) disks can form when infalling material has low angular momentum. The most interesting case is found for the infall of counterrotating external material with high angular momentum, leading to the formation of counterrotating inner and outer disks separated by a deep gap at a few tens AU. The gap migrates inward due to accretion of the inner disk onto the protostar, turns into a central hole, and finally disappears giving way to the outer strongly gravitationally unstable disk. This model may lead to the emergence of a transient stellar system with sub-stellar/very-low-mass components counterrotating with respect to that of the star.

The Mid-infrared E-ELT Imager and Spectrograph (METIS)

METIS will be among the first generation of scientific instruments on the E-ELT. Focusing on highest angular resolution and high spectral resolution, METIS will provide diffraction limited imaging and coronagraphy from 3-14μm over an 20x20uf0b2 field of view, as well as integral field spectroscopy at R ~ 100,000 from 2.9-5.3μm. In addition, METIS provides medium-resolution (R ~ 5000) long slit spectroscopy, and polarimetric measurements at N band. While the baseline concept has already been discussed at previous conferences, this paper focuses on the significant developments over the past two years in several areas: The science case has been updated to account for recent progress in the main science areas circum-stellar disks and the formation of planets, exoplanet detection and characterization, Solar system formation, massive stars and clusters, and star formation in external galaxies. We discuss the developments in the adaptive optics (AO) concept for METIS, the telescope interface, and the instrument modelling. Last but not least we provide an overview of our technology development programs, which ranges from coronagraphic masks, immersed gratings, and cryogenic beam chopper to novel approaches to mirror polishing, background calibration and cryo-cooling. These developments have further enhanced the design and technology readiness of METIS to reliably serve as an early discovery machine on the E-ELT.

Astrophysical Conditions for Planetary Habitability

With the discovery of hundreds of exoplanets and a potentially huge number of Earth-like planets waiting to be discovered, the conditions for their habitability have become a focal point in exoplanetary research. The classical picture of habitable zones primarily relies on the stellar flux allowing liquid water to exist on the surface of an Earth-like planet with a suitable atmosphere. However, numerous further stellar and planetary properties constrain habitability. Apart from geophysical processes depending on the internal structure and composition of a planet, a complex array of astrophysical factors additionally determine habitability. Among these, variable stellar UV, EUV, and X-ray radiation, stellar and interplanetary magnetic fields, ionized winds, and energetic particles control the constitution of upper planetary atmospheres and their physical and chemical evolution. Short- and long-term stellar variability necessitates full time-dependent studies to understand planetary habitability at any point in time. Furthermore, dynamical effects in planetary systems and transport of water to Earth-like planets set fundamentally important constraints. We will review these astrophysical conditions for habitability under the crucial aspects of the long-term evolution of stellar properties, the consequent extreme conditions in the early evolutionary phase of planetary systems, and the important interplay between properties of the host star and its planets.

METIS: the thermal infrared instrument for the E-ELT

The ‘Mid-infrared ELT Imager and Spectrograph’ (METIS) will be the third instrument on the European Extremely Large Telescope (E-ELT). METIS will provide diffraction limited imaging in the atmospheric L/M and N-band from 3 to 14 μm over an 18˝×18˝ field of view, as well as high contrast coronagraphy, medium-resolution (R ≤ 5000) long slit spectroscopy, and polarimetry. In addition, an integral field spectrograph will provide a spectral resolution of R ~ 100,000 at L/M band. Focusing on highest angular resolution and high spectral resolution, METIS will deliver unique science, in particular in the areas of exo-planets, proto-planetary-disks and high-redshift galaxies, which are illustrated in this paper. The reduction of the E-ELT aperture size had little impact on the METIS science case. With the recent positive developments in the area of detectors, the METIS instrument concept has reached a high level of technology readiness. For some key components (cryogenic chopping mirror, immersed grating, sorption cooler and cryogenic derotator) a development and test program has been launched successfully.

Progress with the design and development of MIRI, the mid-IR instrument for JWST

MIRI is one of four instruments to be built for the James Webb Space Telescope. It provides imaging, coronography and integral field spectroscopy over the 5-28.5um wavelength range. MIRI is the only instrument which is cooled to 7K by a dedicated cooler, much lower than the passively cooled 40K of the rest of JWST, and consists of both an Optical System and a Cooler System. This paper will describe the key features of the overall instrument design and then concentrate on the status of the MIRI Optical System development. The flight model design and manufacture is complete, and final assembly and test of the integrated instrument is now underway. Prior to integration, all of the major subassemblies have undergone individual environmental qualification and performance tests and end-end testing of a flight representative model has been carried out. The paper will provide an overview of results from this testing and describe the current status of the flight model build and the plan for performance verification and ground calibration.

The roAp star α Circinus as seen by BRITE-Constellation

We report on an analysis of high-precision, multi-colour photometric observations of the rapidly-oscillating Ap (roAp) star

Effect of accretion on the pre-main-sequence evolution of low-mass stars and brown dwarfs

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The nature of very low luminosity objects (VeLLOs)

Cir. These observations were obtained with the BRITE-Constellation, which is a coordinated mission of five nanosatellites that collects continuous millimagnitude-precision photometry of dozens of bright stars for up to 180 days at a time in two colours (Johnson B and R). BRITE stands for BRight Target Explorer. The object