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 $1<\theta<10$ 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.
The 1.5 meter solar telescope GREGOR Schmidt, W.; von der Lühe, O.; Volkmer, R. ...
Astronomische Nachrichten,
11/2012, Letnik:
333, Številka:
9
Journal Article
This paper describes the wave-front correction system developed for the
Sunrise
balloon telescope, and it provides information about its in-flight performance. For the correction of low-order ...aberrations, a Correlating Wave-Front Sensor (CWS) was used. It consisted of a six-element Shack – Hartmann wave-front sensor (WFS), a fast tip-tilt mirror for the compensation of image motion, and an active telescope secondary mirror for focus correction. The CWS delivered a stabilized image with a precision of 0.04 arcsec (rms), whenever the coarse pointing was better than ± 45 arcsec peak-to-peak. The automatic focus adjustment maintained a focus stability of 0.01 waves in the focal plane of the CWS. During the 5.5 day flight, good image quality and stability were achieved during 33 hours, containing 45 sequences, which lasted between 10 and 45 min.
Context. The various mechanisms of magneto-convective energy transport determine the structure of sunspots and active regions. Aims. We characterise the appearance of light bridges and other ...fine-structure details and elaborate on their magneto-convective nature. Methods. We present speckle-reconstructed images taken with the broad-band imager (BBI) at the 1.5 m GREGOR telescope in the 486 nm and 589 nm bands. We estimate the spatial resolution from the noise characteristics of the image bursts and obtain 0.08″ at 589 nm. We describe structure details in individual best images as well as the temporal evolution of selected features. Results. We find branched dark lanes extending along thin (≈1″) light bridges in sunspots at various heliocentric angles. In thick (≳ 2″) light bridges the branches are disconnected from the central lane and have a Y shape with a bright grain toward the umbra. The images reveal that light bridges exist on varying intensity levels and that their small-scale features evolve on timescales of minutes. Faint light bridges show dark lanes outlined by the surrounding bright features. Dark lanes are very common and are also found in the boundary of pores. They have a characteristic width of 0.1″ or smaller. Intergranular dark lanes of that width are seen in active region granulation. Conclusions. We interpret our images in the context of magneto-convective simulations and findings: while central dark lanes in thin light bridges are elevated and associated with a density increase above upflows, the dark lane branches correspond to locations of downflows and are depressed relative to the adjacent bright plasma. Thick light bridges with central dark lanes show no projection effect. They have a flat elevated plateau that falls off steeply at the umbral boundary. There, Y-shaped filaments form as they do in the inner penumbra. This indicates the presence of inclined magnetic fields, meaning that the umbral magnetic field is wrapped around the convective light bridge.
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 La Silla, Chile. Based on the collected data we calculated an orbital solution with a period of $P=711~{\rm days}$, a semi-amplitude of $K_{1}=105~{\rm m\,s}^{-1}$, 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 $M_{\star}=1.91~M_{\odot}$, the best-fit minimum mass of the companion and semi-major axis of the orbit are $m_{2}\sin{i}=6.54~M_{{\rm Jup}}$ and $a_{2}=1.93~{\rm AU}$, respectively. An upper limit for the mass of the companion of $m_{2} \la 65.5~M_{{\rm Jup}}$ 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.
Aims. The fine-structure of the magnetic field in a sunspot penumbra in the upper chromosphere is to be explored and compared to that in the photosphere. Methods. Spectropolarimetric observations ...with high spatial resolution were recorded with the 1.5-m GREGOR telescope using the GREGOR Infrared Spectrograph (GRIS). The observed spectral domain includes the upper chromospheric Hei triplet at 10 830 Å and the photospheric Sii 10 827.1 Å and Cai 10 833.4 Å spectral lines. The upper chromospheric magnetic field is obtained by inverting the Hei triplet assuming a Milne-Eddington-type model atmosphere. A height-dependent inversion was applied to the Sii 10 827.1 Å and Cai 10 833.4 Å lines to obtain the photospheric magnetic field. Results. We find that the inclination of the magnetic field varies in the azimuthal direction in the photosphere and in the upper chromosphere. The chromospheric variations coincide remarkably well with the variations in the inclination of the photospheric field and resemble the well-known spine and interspine structure in the photospheric layers of penumbrae. The typical peak-to-peak variations in the inclination of the magnetic field in the upper chromosphere are found to be 10°–15°, which is roughly half the variation in the photosphere. In contrast, the magnetic field strength of the observed penumbra does not vary on small spatial scales in the upper chromosphere. Conclusions. Thanks to the high spatial resolution of the observations that is possible with the GREGOR telescope at 1.08 microns, we find that the prominent small-scale fluctuations in the magnetic field inclination, which are a salient part of the property of sunspot penumbral photospheres, also persist in the chromosphere, although at somewhat reduced amplitudes. Such a complex magnetic configuration may facilitate penumbral chromospheric dynamic phenomena, such as penumbral micro-jets or transient bright dots.
We present a method for retrieving the wavefront phase and estimating the point spread function using data from a single imaging channel. The method involves a set of short exposures and its speckle ...reconstruction of the observed object, and a parameterized point spread function model which is used in order to estimate the wavefront phase and the point spread function from the data. Estimates of the wavefront phase and the point spread function are computed by minimizing an error function with the method of simulated annealing. Results of tests with simulated data and with real data are shown. We conclude that the proposed technique provides reliable estimates of the instantaneous point spread function and is robust against noise.
PDF
