J. Setiawan

Basic physical parameters of a selected sample of evolved stars

We present the detailed spectroscopic analysis of 72 evolved stars, which were previously studied for accurate radial velocity variations. Using one Hyades giant and another well studied star as the reference abundance, we determine the [Fe/H] for the whole sample. These metallicities, together with the Teff values and the absolute V-band magnitude derived from Hipparcos parallaxes, are used to estimate basic stellar parameters (ages, masses, radii, (B−V)0 and log g) using theoretical isochrones and a Bayesian estimation method. The (B−V)0 values so estimated turn out to be in excellent agreement (to within ∼0.05 mag) with the observed (B−V), confirming the reliability of the Teff−(B−V)0 relation used in the isochrones. On the other hand, the estimated log g values are typically 0.2 dex lower than those derived from spectroscopy; this effect has a negligible impact on [Fe/H] determinations. The estimated diameters θ have been compared with limb darkening-corrected ones measured with independent methods, finding an agreement better than 0.3 mas within the 1 <θ< 10 mas interval (or, alternatively, finding mean differences of just 6%). We derive the age-metallicity relation for the solar neighborhood; for the first time to our knowledge, such a relation has been derived from observations of field giants rather than from open clusters and field dwarfs and subdwarfs. The age-metallicity relation is characterized by close-to-solar metallicities for stars younger than ∼4 Gyr, and by a large [Fe/H] spread with a trend towards lower metallicities for higher ages. In disagreement with other studies, we find that the [Fe/H] dispersion of young stars (less than 1 Gyr) is comparable to the observational errors, indicating that stars in the solar neighbourhood are formed from interstellar matter of quite homogeneous chemical composition. The three giants of our sample which have been proposed to host planets are not metal rich; this result is at odds with those for main sequence stars. However, two of these stars have masses much larger than a solar mass so we may be sampling a different stellar population from most radial velocity searches for extrasolar planets. We also confirm the previous indication that the radial velocity variability tends to increase along the RGB, and in particular with the stellar radius.

A young massive planet in a star–disk system

There is a general consensus that planets form within disks of dust and gas around newly born stars. Details of their formation process, however, are still a matter of ongoing debate. The timescale of planet formation remains unclear, so the detection of planets around young stars with protoplanetary disks is potentially of great interest. Hitherto, no such planet has been found. Here we report the detection of a planet of mass (9.8±3.3)MJupiter around TW Hydrae (TW Hya), a nearby young star with an age of only 8–10 Myr that is surrounded by a well-studied circumstellar disk. It orbits the star with a period of 3.56 days at 0.04 au, inside the inner rim of the disk. This demonstrates that planets can form within 10 Myr, before the disk has been dissipated by stellar winds and radiation.

Evolved stars suggest an external origin of the enhanced metallicity in planet-hosting stars

Aims. Exo-planets are preferentially found around high metallicity main sequence stars. We aim at investigating whether evolved stars share this property, and what this tells about planet formation. Methods. Statistical tools and the basic concepts of stellar evolution theory are applied to published results as well as our own radial velocity and chemical analyses of evolved stars. Results. We show that the metal distributions of planet-hosting (P-H) dwarfs and giants are different, and that the latter do not favor metal-rich systems. Rather, these stars follow the same age-metalicity relation as the giants without planets in our sample. The straightforward explanation is to attribute the difference between dwarfs and giants to the much larger masses of giants’ convective envelopes. If the metal excess on the main sequence is due to pollution, the effects of dilution naturally explains why it is not observed among evolved stars. Conclusions. Although we cannot exclude other explanations, the lack of any preference for metal-rich systems among P-H giants could be a strong indication of the accretion of metal-rich material. We discuss further tests, as well as some predictions and consequences of this hypothesis.

A substellar companion around the intermediate-mass giant star HD 11977

We report the discovery of a substellar companion to the intermediate-mass star HD 11977 (G5 III). Radial velocities of this star have been monitored for five years with FEROS at the 1.52-m ESO and later at the 2.2-m MPG/ESO telescope in of K 1 = Chile. Based on the collected data we calculated an orbital solution with a period of P = 711 days, a semi-amplitude of K 1 = 105 ms -1 , and an eccentricity of e = 0.4. The period of the radial-velocity variation is longer than that of the estimated stellar rotation, rendering it unlikely that rotational modulation is the source of the variation in the radial velocity. This hypothesis is supported by the absence of a correlation between stellar activity indicators and radial-velocity variation. By determining a primary stellar mass of M * = 1.91 M ○. , the best-fit minimum mass of the companion and semi-major axis of the orbit are m 2 sin i = 6.54 M Jup and a 2 = 1.93 AU, respectively. An upper limit for the mass of the companion of m 2 ? 65.5 M Jup has been calculated from HIPPARCOS astrometric measurements. Although the possibility of a brown-dwarf companion cannot be excluded, HD 11977 B is one of the few planet candidates detected around an intermediate-mass star. The progenitor main-sequence star of HD 11977 is probably an A-type star. This discovery gives an indirect evidence for planetary companions around early type main-sequence stars.

A Giant Planet Around a Metal-Poor Star of Extragalactic Origin

Unlikely Planet Most known extrasolar planets orbit stars similar to the Sun. Very few planets have been detected around metal-poor stars whose abundances of elements other than hydrogen and helium are much lower than those of the Sun, or around stars that are at a late stage in their evolution. Setiawan et al. (p. 1642, published online 18 November) report the detection of a close-in giant planet around a metal-poor star belonging to a group of stars that formed in a satellite galaxy of the Milky Way. The star has gone past the red giant phase of stellar evolution, when stars like the Sun expand up to many times their original size, and so it is unclear why the planet was not engulfed by the star as it expanded. A planet is observed to orbit a star whose properties are different from those of all other known planet-hosting stars. Stars in their late stage of evolution, such as horizontal branch stars, are still largely unexplored for planets. We detected a planetary companion around HIP 13044, a very metal-poor star on the red horizontal branch, on the basis of radial velocity observations with a high-resolution spectrograph at the 2.2-meter Max-Planck Gesellschaft–European Southern Observatory telescope. The star’s periodic radial velocity variation of P = 16.2 days caused by the planet can be distinguished from the periods of the stellar activity indicators. The minimum mass of the planet is 1.25 times the mass of Jupiter and its orbital semimajor axis is 0.116 astronomical units. Because HIP 13044 belongs to a group of stars that have been accreted from a disrupted satellite galaxy of the Milky Way, the planet most likely has an extragalactic origin.

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.

Precise radial velocity measurements of G and K giants Multiple systems and variability trend along the Red Giant Branch

We present the results of our radial velocity (RV) measurements of G and K giants, concentrating on the presence of multiple systems in our sample. Eighty-three giants have been observed for 2.5 years with the fiber-fed echelle spectrograph FEROS at the 1.52 m ESO telescope in La Silla, Chile. Seventy-seven stars (93%) of the targets have been analyzed for RV variability using simultaneous Th-Ar calibration and a cross-correlation technique. We estimate the long-term precision of our measurement as better than 25 m s −1 . Projected rotational velocities have been measured for most stars of the sample. Within our time-base only 21 stars (or 27%) show variability below 2σ, while the others show RV variability with amplitudes up to several km s −1 . The large amplitude (several km s −1 ) and shape (high eccentricity) of the RV variations for 11 of the program stars are consistent with stellar companions, and possibly brown dwarf companions for two of the program stars. In those systems for which a full orbit could be derived, the companions have minimum masses from ∼0.6 Mdown to 0.1 M� .T o these multiple systems we add the two candidates of giant planets already discovered in the sample. This analysis shows that multiple systems contribute substantially to the long-term RV variability of giant stars, with about 20% of the sample being composed of multiple systems despite screening our sample for known binary stars. After removing binaries, the range of RV variability in the whole sample clearly decreases, but the remaining stars retain a statistical trend of RV variability with luminosity: luminous cool giants with B − V ≥ 1.2 show RV variations with σRV > 60 m s −1 , while giants with B − V < 1.2 including those in the clump region exhibit less variability or they are constant within our accuracy. The same trend is observed with respect to absolute visual magnitudes: brighter stars show a larger degree of variability and, when plotted in the RV variability vs. magnitude diagram a trend of increasing RV scatter with luminosity is seen. The amplitude of RV variability does not increase dramatically, as predicted, for instance, by simple scaling laws. At least two luminous and cooler stars of the sample show a correlation between RV and chromospheric activity and bisector asymmetry, indicating that in these two objects RV variability is likely induced by the presence of (chromospheric) surface structures.

Evidence for a Planetary Companion around a Nearby Young Star

We report evidence for a planetary companion around the nearby young star HD 70573. The star is a G-type dwarf located at a distance of 46 pc. We carried out spectroscopic observations of this star with FEROS at the 2.2 m MPG/ESO telescope at La Silla. Our spectroscopic analysis yields a spectral type of G1-1.5 V and an age of about 100 Myr. Variations in stellar radial velocity (RV) of HD 70573 were monitored from 2003 December until 2007 January. HD 70573 shows an RV variation with a period of 852(±12) days and a semiamplitude of 149(±6) m s-1. The period of this variation is significantly longer than its rotational period. Based on the analysis of the Ca II K emission line, Hα, and Teff variation as stellar-activity indicators, as well as the lack of a correlation between the bisector velocity span and the RV, we can exclude rotational modulation and nonradial pulsations as the source of the long-period RV variation. Thus, the presence of a low-mass companion provides the best explanation for the observed RV variation. Assuming a primary mass m1 = 1.0 ± 0.1 M☉, we calculated a minimum mass of the companion m2 sin i of 6.1 MJup, which lies in the planetary-mass regime, and an orbital semimajor axis of 1.76 AU. The orbits eccentricity is e = 0.4. The planet discovery around HD 70573 gives an important input for the study of debris disks around young stars and their relation to the presence of planets.

EX Lupi in quiescence

Aims. EX Lupi is the prototype of EXors, a subclass of low-mass pre-main sequence stars whose episodic eruptions are attributed to temporarily increased accretion. In quiescence the optical and near-infrared properties of EX Lup cannot be distinguished from those of normal T Tau stars. Here we investigate whether it is the circumstellar disk structure that makes EX Lup an atypical Class II object. During outburst the disk might undergo structural changes. Our characterization of the quiescent disk is intended to serve as a reference for studying the physical changes related to one of EX Lupi’s strongest known eruptions in 2008 Jan–Sep. Methods. We searched the literature for photometric and spectroscopic observations including ground-based, IRAS, ISO, and Spitzer data. After constructing the optical–infrared spectral energy distribution (SED), we compared it with the typical SEDs of other young stellar objects and modeled it using the Monte Carlo radiative transfer code RADMC. We determined the mineralogical composition of the 10 μm silicate emission feature and also gave a description of the optical and near-infrared spectra. Results. The SED is similar to that of a typical T Tauri star in most aspects, though EX Lup emits higher flux above 7 μm. The quiescent phase data suggest low-level variability in the optical–mid-infrared domain. By integrating the optical and infrared fluxes, we derived a bolometric luminosity of 0.7 L� . The 10 μm silicate profile could be fitted by a mixture consisting of amorphous silicates,

Rotational velocities of nearby young stars

Context. Stellar rotation is a crucial parameter driving stellar magnetism, activity and mixing of chemical elements. Measuring rotational velocities of young stars can give additional insight in the initial conditions of the star formation process. Furthermore, the evolution of stellar rotation is coupled to the evolution of circumstellar disks. Disk-braking mechanisms are believed to be responsible for rotational deceleration during the accretion phase, and rotational spin-up during the contraction phase after decoupling from the disk for fast rotators arriving at the ZAMS. On the ZAMS, stars get rotationally braked by solar-type winds. Aims. We investigate the projected rotational velocities υ sin i of a sample of young stars with respect to the stellar mass and disk evolutionary state to search for possible indications of disk-braking mechanisms. Furthermore, we search for signs of rotational spin-up of stars that have already decoupled from their circumstellar disks. Methods. We analyse the stellar spectra of 220 nearby (mostly < 100 pc) young (2-600 Myr) stars for their ν sin i, stellar age, Hα emission, and accretion rates. The stars have been observed with FEROS at the 2.2 m MPG/ESO telescope and HARPS at the 3.6 m telescope in La Silla, Chile. The spectra have been cross-correlated with appropriate theoretical templates. We build a new calibration to be able to derive ν sin i values from the cross-correlated spectra. Stellar ages are estimated from the Li I equivalent width at 6708 A. The equivalent width and width at 10% height of the Hα emission are measured to identify accretors and used to estimate accretion rates M acc . The ν sin i is then analysed with respect to the evolutionary state of the circumstellar disks to search for indications of disk-braking mechanisms in accretors. Results. We find that the broad ν sin i distribution of our targets extends to rotation velocities of up to more than 100 km s ―1 and peaks at a value of 7.8 ± 1.2 km s ―1 , and that ∼70% of our stars show υ sin i < 30 km s ―1 . Furthermore, we can find indications for disk-braking in accretors and rotational spin-up of stars which are decoupled from their disks. In addition, we show that a number of young stars are suitable for precise radial-velocity measurements for planet-search surveys.