ETHICS, ECONOMICS, AND INSTITUTIONAL INNOVATIONS

Institutional and Behavioral Economics,

A magnetic cycle of tau Bootis? The coronal and chromospheric view

Tau Bootis is a late F-type main sequence star orbited by a Hot Jupiter. During the last years spectropolarimetric observations led to the hypothesis that this star may host a global magnetic field that switches its polarity once per year, indicating a very short activity cycle of only one year duration. In our ongoing observational campaign, we have collected several X-ray observations with XMM-Newton and optical spectra with TRES/FLWO in Arizona to characterize tau Boo's corona and chromosphere over the course of the supposed one-year cycle. Contrary to the spectropolarimetric reconstructions, our observations do not show indications for a short activity cycle.Comment: 4 pages, 2 figures, appeared in Astronomical Notes 333, 1, 26-29 (2012

The Chandra X-ray view of the power sources in Cepheus A

The central part of the massive star-forming region Cepheus A contains several radio sources which indicate multiple outflow phenomena, yet the driving sources of the individual outflows have not been identified. We present a high-resolution Chandra observation of this region that shows the presence of bright X-ray sources, consistent with active pre-main sequence stars, while the strong absorption hampers the detection of less luminous objects. A new source has been discovered located on the line connecting H_2 emission regions at the eastern and western parts of Cepheus A. This source could be the driving source of HH 168. We present a scenario relating the observed X-ray and radio emission.Comment: 7 pages, 6 figures, accepted for publication in A&

The evolution of the X-ray emission of HH 2 - Investigating heating and cooling processes

Young stellar objects often drive powerful bipolar outflows which evolve on time scales of a few years. An increasing number of these outflows has been detected in X-rays implying the existence of million degree plasma almost co-spatial with the lower temperature gas observed in the optical and near-IR. The details of the heating and cooling processes of the X-ray emitting part of these so-called Herbig-Haro objects are still ambiguous, e.g., whether the cooling is dominated by expansion, radiation or thermal conduction. We present a second epoch Chandra observation of the first X-ray detected Herbig-Haro object (HH 2) and derive the proper-motion of the X-ray emitting plasma and its cooling history. We argue that the most likely explanation for the constancy of the X-ray luminosity, the alignment with the optical emission and the proper-motion is that the cooling is dominated by radiative losses leading to cooling times exceeding a decade. We explain that a strong shock caused by fast material ramming into slower gas in front of it about ten years ago can explain the X-ray emission while being compatible with the available multi-wavelength data of HH 2.Comment: 5 pages with 4 figures; accepted for publication by Astronomy and Astrophysic

DN Tauri - coronal activity and accretion in a young low-mass CTTS

We present a deep XMM-Newton observation of DN Tau, a young M0-type accreting CTTS and investigate its X-ray properties and X-ray generating mechanisms. Specifically we examine the presence of X-ray emission from magnetic activity and accretion shocks and compare our new X-ray data with UV data taken simultaneously and with X-ray/UV observations performed before. We find that the X-ray emission from DN Tau is dominated by coronal plasma generated via magnetic activity, but also clearly detect a contribution of the accretion shocks to the cool plasma component at about 2 MK as consistently inferred from density and temperature analysis. Typical phenomena of active coronae like flaring, the presence of very hot plasma at 30 MK and an abundance pattern showing the inverse FIP effect are seen on DN Tau. Strong variations in the emission measure of the cooler plasma components between the 2005 and 2010 XMM data point to accretion related changes. The UV light curve taken simultaneously is in general not related to the X-ray brightness, but exhibits clear counterparts during the observed X-ray flares. The X-ray properties of DN Tau are similar to those of more massive CTTS, but its low mass and large radius shift the accretion shocks to lower temperatures, reducing their imprint in the X-ray regime.Comment: 11 pages, 8 figures, accepted for publication in A&

An algorithm for correcting CoRoT raw light curves

We introduce the CoRoT detrend algorithm (CDA) for detrending CoRoT stellar light curves. The algorithm CDA has the capability to remove random jumps and systematic trends encountered in typical CoRoT data in a fully automatic fashion. Since enormous jumps in flux can destroy the information content of a light curve, such an algorithm is essential. From a study of 1030 light curves in the CoRoT IRa01 field, we developed three simple assumptions which upon CDA is based. We describe the algorithm analytically and provide some examples of how it works. We demonstrate the functionality of the algorithm in the cases of CoRoT0102702789, CoRoT0102874481, CoRoT0102741994, and CoRoT0102729260. Using CDA in the specific case of CoRoT0102729260, we detect a candidate exoplanet around the host star of spectral type G5, which remains undetected in the raw light curve, and estimate the planetary parameters to be Rp=6.27Re and P=1.6986 days.Comment: 8 pages, 13 figure

HST FUV C IV observations of the hot DG Tauri jet

Protostellar jets are tightly connected to the accretion process and regulate the angular momentum balance of accreting star-disk systems. The DG Tau jet is one of the best-studied protostellar jets and contains plasma with temperatures ranging over three orders of magnitude within the innermost 50 AU of the jet. We present new Hubble Space Telescope (HST) far ultraviolet (FUV) long-slit spectra spatially resolving the C IV emission (T~1e5 K) from the jet for the first time, and quasi-simultaneous HST observations of optical forbidden emission lines ([O I], [N II], [S II] and [O III]) and fluorescent H2 lines. The C IV emission peaks at 42 AU from the stellar position and has a FWHM of 52 AU along the jet. Its deprojected velocity of around 200 km/s decreases monotonically away from the driving source. In addition, we compare our HST data with the X-ray emission from the DG Tau jet. We investigate the requirements to explain the data by an initially hot jet compared to local heating. Both scenarios indicate a mass loss by the T~1e5 K jet of ~1e-9 Msun/year, i.e., between the values for the lower temperature jet (T~1e4 K) and the hotter X-ray emitting part (T>1e6 K). However, a simple initially hot wind requires a large launching region (~1 AU), and we therefore favor local heating.Comment: 5 pages, 2 figures, accepted by A&A letter

Transit mapping of a starspot on CoRoT-2 - Probing a stellar surface by planetary transits

We analyze variations in the transit lightcurves of CoRoT-2b, a massive hot Jupiter orbiting a highly active G star. We use one transit lightcurve to eclipse-map a photospheric spot occulted by the planet. In this case study we determine the size and longitude of the eclipsed portion of the starspot and systematically study the corresponding uncertainties. We determine a spot radius between 4.5\degr and 10.5\degr on the stellar surface and the spot longitude with a precision of about $\pm 1$ degree. Given the well-known transit geometry of the CoRoT-2 system, this implies a reliable detection of spots on latitudes typically covered by sunspots; also regarding its size the modelled spot is comparable to large spot groups on the Sun. We discuss the future potential of eclipse mapping by planetary transits for the high-resolution analysis of stellar surface features.Comment: accepted by Astronomy and Astrophysic

The high-energy environment in the super-earth system CoRoT-7

High-energy irradiation of exoplanets has been identified to be a key influence on the stability of these planets' atmospheres. So far, irradiation-driven mass-loss has been observed only in two Hot Jupiters, and the observational data remain even more sparse in the super-earth regime. We present an investigation of the high-energy emission in the CoRoT-7 system, which hosts the first known transiting super-earth. To characterize the high-energy XUV radiation field into which the rocky planets CoRoT-7b and CoRoT-7c are immersed, we analyzed a 25 ks XMM-Newton observation of the host star. Our analysis yields the first clear (3.5 sigma) X-ray detection of CoRoT-7. We determine a coronal temperature of ca. 3 MK and an X-ray luminosity of 3*10^28 erg/s. The level of XUV irradiation on CoRoT-7b amounts to ca. 37000 erg/cm^2/s. Current theories for planetary evaporation can only provide an order-of-magnitude estimate for the planetary mass loss; assuming that CoRoT-7b has formed as a rocky planet, we estimate that CoRoT-7b evaporates at a rate of about 1.3*10^11 g/s and has lost ca. 4-10 earth masses in total.Comment: 5 pages, accepted for publication by Astronomy & Astrophysic