Eike W. Guenther

Exploring the Full Stellar Population of the Upper Scorpius OB Association

We investigate the stellar population and star formation history of the Upper Scorpius OB association, the most nearby region of recent massive star formation, over the full stellar mass range from 0.1 to 20 M?. The first part of this paper describes an extension of our large spectroscopic survey (Preibisch et al., published in 2001) for low-mass pre?main-sequence (PMS) stars in Upper Scorpius. Using the multiobject spectrograph 2dF at the Anglo-Australian Telescope, we obtained spectra of 469 stars with magnitudes R = 12.5?18.0 in a 6 deg2 area. Among these, we find 68 new PMS stars, nearly all of them M-type stars, by their strong lithium absorption lines. The total area covered by our 2dF survey is now 9 deg2 and contains 166 new PMS stars. Combining these results with our earlier investigation (Preibisch & Zinnecker) yields a sample of 250 PMS stars in the mass range ~0.1 to ~2 M?. The location of these stars in the HR diagram suggests a mean age of 5 Myr without a significant age spread. In the second part of this paper, we also consider the population of 114 high-mass members identified in detailed Hipparcos studies. We construct a combined HR diagram for the 364 high- and low-mass members and find that the whole stellar population is very well characterized by a very narrow age distribution around 5 Myr. We estimate individual masses for all members and construct an empirical mass function covering the mass range from 0.1 up to 20 M?. A power-law fit to the mass function gives a slope of ? ~ -2.6 above ~2 M? and a much flatter slope (? ~ -0.9) below ~0.6 M?. The initial mass function of Upper Sco is not identical, but within the errors consistent with recent determinations of the field initial mass function. There is certainly no deficit of low-mass stars in the Upper Sco OB association, but rather a small excess of low-mass stars. Our results on the stellar age distribution confirm earlier indications that the star formation process in Upper Sco was triggered and support previous conjectures that the triggering event was a supernova shock wave originating from the nearby Upper Centaurus?Lupus association.

Radial Velocity Survey of Members and Candidate Members of the TW Hydrae Association

We report spectroscopic observations of stars belonging to the young nearby group known as the TW Hydrae association, as well as of a number of potential members of the association identified in kinematic and X-ray surveys. Multiple radial velocity measurements were obtained for each object, several of which turn out to be multiple systems. Orbital solutions are presented for three double-lined binaries, one singlelined binary, and a double-lined triple system, all with short periods. Effective temperatures and projected rotational velocities are presented for each visible object. None of the candidate members of the association in our sample are confirmed as a true member. The large fraction of close binaries among the candidate members has to do with their selection based on X-ray emission from ROSAT, which tends to favor the inclusion of tidally locked systems that are active but not necessarily young.

A Large Spectroscopic Survey for Young Low-Mass Members of the Upper Scorpius OB Association

We present the results of a large intermediate-resolution spectroscopic survey for pre–main-sequence (PMS) stars in the Upper Scorpius OB association, the nearest region of recent massive star formation. Utilizing the 2dF multiobject spectrograph at the Anglo-Australian Telescope, we obtained spectra of 576 stars with magnitudes 12.5 ≤ R ≤ 16.5 in a 6 deg2 area in Upper Scorpius. Among these objects we were able to identify 98 new PMS stars, nearly all of them M-type stars, by their strong lithium absorption lines. We place the new PMS stars into the H-R diagram and find that their ages agree well with the mean age of 5 Myr determined in our earlier investigation based on a smaller sample of stars. The number of low-mass (0.1 M⊙ M* 0.5 M⊙) members is at least as high as expected from the known number of high-mass (M* 3 M⊙) association members and from recent determinations of the field star initial mass function. Thus, there is no deficit of low-mass stars in Upper Scorpius.

Spectroscopic Binaries in a Sample of ROSAT X-Ray Sources South of the Taurus Molecular Clouds*

We report the results of our radial velocity monitoring of spectroscopic binary systems in a sample of X-ray sources from the ROSAT All Sky Survey south of the Taurus-Auriga star-forming region. The original sample of ~120 sources by Neuhauser et al. was selected on the basis of their X-ray properties and the visual magnitude of the nearest optical counterpart, in such a way as to promote the inclusion of young objects. Roughly 20% of those sources have previously been confirmed to be very young. We focus here on the subset of the original sample that shows variable radial velocities (43 objects), a few of which have also been flagged previously as being young. New spectroscopic orbits are presented for 42 of those systems. Two of the binaries, RX J0528.9+1046 and RX J0529.3+1210, are indeed weak-lined T Tauri stars likely to be associated with the λ Ori region. Most of the other binaries are active objects of the RS Canum Venaticorum type, including several W Ursae Majoris and Algol systems. We detect a strong excess of short-period binaries compared with the field and an unusually large fraction of double-lined systems. This, along with the overall high frequency of binaries out of the original sample of ~120 sources, can be understood as a selection effect, since all these properties tend to favor the inclusion of the objects in a flux-limited X-ray survey such as this by making them brighter in X-rays. A short description of the physical properties of each binary is provided, and a comparison with evolutionary tracks is made using the stellar density as a distance-independent measure of evolution. We rely for this on our new determinations of the effective temperature and projected rotational velocities of all visible components of the binaries. A number of the systems merit follow-up observations, including at least four confirmed or probable eclipsing binaries. One of these, RX J0239.1-1028, consists of a pair of detached K dwarfs and may provide for a potentially important test of stellar evolution models once the absolute dimensions of the components are determined.

CARMENES: Calar Alto high-resolution search for M dwarfs with exo-earths with a near-infrared Echelle spectrograph

CARMENES (Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs) is a next-generation instrument to be built for the 3.5m telescope at the Calar Alto Observatory by a consortium of Spanish and German institutions. Conducting a five-year exoplanet survey targeting ~ 300 M stars with the completed instrument is an integral part of the project. The CARMENES instrument consists of two separate spectrographs covering the wavelength range from 0.52 to 1.7 μm at a spectral resolution of R = 85, 000, fed by fibers from the Cassegrain focus of the telescope. The spectrographs are housed in a temperature-stabilized environment in vacuum tanks, to enable a 1m/s radial velocity precision employing a simultaneous ThAr calibration.

ANGULAR DIAMETERS AND EFFECTIVE TEMPERATURES OF 25 K GIANT STARS FROM THE CHARA ARRAY

Using Georgia State Universitys Center for High Angular Resolution Astronomy Array interferometer, we measured angular diameters for 25 giant stars, six of which host exoplanets. The combination of these measurements and Hipparcos parallaxes produces physical linear radii for the sample. Except for two outliers, our values match angular diameters and physical radii estimated using photometric methods to within the associated errors with the advantage that our uncertainties are significantly lower. We also calculated the effective temperatures for the stars using the newly measured diameters. Our values do not match those derived from spectroscopic observations as well, perhaps due to the inherent properties of the methods used or because of a missing source of extinction in the stellar models that would affect the spectroscopic temperatures.

Companions of old brown dwarfs, and very low mass stars

Up to now, most planet search projects have concentrated on F to K stars. In order to considerably widen the view, we have started a survey for planets of old, nearby brown dwarfs and very low mass stars. Using UVES, we have observed 26 brown dwarfs and very low mass stars. These objects are quite inactive and are thus highly suitable for such a project. Two objects were found to be spectroscopic binaries. Another object shows significant radial velocity variations. From our measurements, we conclude that this object either has a planetary-mass companion, or the variations are caused by surface features. Within the errors of the measurements, the remaining objects are constant in radial velocity. While it is impossible to strictly exclude an orbiting planet from sparsely sampled RV data, we conclude that it is unlikely that these objects are orbited by massive planets with periods of 40 days or less.

Kepler-77b: a very low albedo, Saturn-mass transiting planet around a metal-rich solar-like star

We report the discovery of Kepler-77b (alias KOI-127.01), a Saturn-mass transiting planet in a 3.6-day orbit around a metal-rich solarlike star.Wecombined the publicly availableKepler photometry (quarters 1−13) withhigh-resolution spectroscopy from the Sandiford at McDonald and FIES at NOT spectrographs. We derived the system parameters via a simultaneous joint fit to the photometric and radial velocity measurements. Our analysis is based on the Bayesian approach and is carried out by sampling the parameter posterior distributions using a Markov chain Monte Carlo simulation. Kepler-77b is a moderately inflated planet with a mass of Mp = 0.430 ± 0.032 MJup, a radius of Rp = 0.960 ± 0.016 RJup, and a bulk density of ρp = 0.603 ± 0.055 gcm −3 . It orbits a slowly rotating (Prot = 36 ±6 days) G5V star with M� = 0.95 ±0.04 M� , R� = 0.99 ±0.02 R� , Teff = 5520 ±60 K, [M/H] = 0.20 ±0.05dex, that has an age of 7.5 ± 2.0 Gyr. The lack of detectable planetary occultation with a depth higher than ∼10ppm implies a planet geometric and Bond albedo of Ag ≤ 0.087 ± 0.008 and AB ≤ 0.058 ± 0.006, respectively, placing Kepler-77b among the gas-giant planets with the lowest albedo known so far. Wefound neither additional planetary transit signals nor transit-timing variations at a level of ∼0.5 min, in accordance with the trend that close-in gas giant planets seem to belong to single-planet systems. The 106 transits observed in short-cadence mode by Kepler for nearly 1.2 years show no detectable signatures of the planet’s passage in front of starspots. We explored the implications of the absence of detectable spot-crossing events for the inclination of the stellar spin-axis, the sky-projected spin-orbit obliquity, and the latitude of magnetically active regions.

Transit mapping of a starspot on CoRoT-2 - Probing a stellar surface with 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° and 10.5° on the stellar surface and the spot longitude with a precision of about ±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; the size of 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.

High Resolution Spectroscopy of the Planetary Host HD 13189: Highly Evolved and Metal-Poor

We report on the abundances of 13 elements in the planetary host HD 13189, a massive giant star. Abundances are found to be subsolar, with [Fe/H] = -0.58 ± 0.04; HD 13189 is one of the most metal-poor planetary hosts yet discovered. Abundance ratios relative to Fe show no peculiarities with respect to random field stars. A census of metallicities of the seven currently known planet-harboring giants results in a distribution that is more metal-poor than the well-known metal-rich distribution of main-sequence (MS) planetary hosts. This finding is discussed in terms of accretion of H-depleted material, one of the possible mechanisms responsible for the high-metallicity distribution of MS stars with planets. We estimate the mass of the HD 13189 progenitor to be 3.5 M☉ but cannot constrain this value to better than 2-6 M☉. A stellar mass of 3.5 M☉ implies a planetary mass of m sin i = 14.0MJ ± 0.8MJ, placing the companion at the planet/brown dwarf boundary. Given its physical characteristics, the HD 13189 system is potentially unique among planetary systems, and its continued investigation should provide invaluable data to extrasolar planetary research.