1,721,111 research outputs found
Toward detection of terrestrial planets in the habitable zone of our closest neighbor: proxima Centauri
Get PDFContext: The precision of radial velocity (RV) measurements to detect indirectly planetary companions of nearby stars has improved to enable the discovery of extrasolar planets in the Neptune and Super-Earth mass range. Detections of extremely low mass planets, even as small as 1 Earth mass or below, in short-period orbits now appears conceivable in ongoing RV planet searches. Discoveries of these Earth-like planets by means of ground-based RV programs will help to determine the parameter etaâŠâ¢, the frequency of potentially habitable planets around other stars. Aims: In search of low-mass planetary companions we monitored Proxima Centauri (M5V) as part of our M dwarf program. In the absence of a significant detection, we use these data to demonstrate the general capability of the RV method in finding terrestrial planets. For late M dwarfs the classic liquid surface water habitable zone (HZ) is located close to the star, in which circumstances the RV method is most effective. We want to demonstrate that late M dwarfs are ideal targets for the search of terrestrial planets with the RV technique. Methods: Using the iodine cell technique we obtained differential RV measurements of Proxima Cen over a time span of 7 years with the UVES spectrograph at the ESO VLT. We determine upper limits to the masses of companions in circular orbits by means of numerical simulations. Results: The RV data of Proxima Cen have a total rms scatter of 3.1~m s-1 and a period search does not reveal any significant signals. In contrast to our earlier results for Barnard's star, the RV results for the active M dwarf Proxima Cen are only weakly correlated with Halpha line index measurements. As a result of our companion limit calculations, we find that we successfully recover all test signals with RV amplitudes corresponding to planets with m sin i >= 2-3 M_âŠ⢠residing inside the HZ of Proxima Cen with a statistical significance of >99%. Over the same period range, we can recover 50% of the test planets with masses of m sin i >= 1.5-2.5~M_âŠâ¢. Based on our simulations, we exclude the presence of any planet in a circular orbit with m sin i >= 1~M_Neptune at separations of a <= 1 AU. Based on observations collected at the European Southern Observatory, Paranal, Chile, programmes 65.L-0428, 66.C-0446, 267.C-5700, 68.C-0415, 69.C-0722, 70.C-0044, 71.C-0498, 072.C-0495, 173.C-0606 and 078.C-0829. Table 1 is only available in electronic form at http://www.aanda.or
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The Discovery Of Stellar Oscillations In The K Giant Iota Draconis
No full textiota Dra (HIP 75458) is a well-known example of a K giant hosting a substellar companion. We present radial velocity measurements of this star from observations taken with three different instruments spanning nearly 8 years. They show more clearly that the RV period is long-lived and coherent thus supporting the companion hypothesis. The longer time baseline now allows for a more accurate determination of the orbit with a revised period of P = 511 d and an additional small linear trend, indicative of another companion in a wide orbit. Moreover we show that the star exhibits low amplitude, solar-like oscillations with frequencies around 3-4 d(-1) (34.7-46.3 mu Hz).Astronom
The M dwarf planet search programme at the ESO VLT + UVES. A search for terrestrial planets in the habitable zone of M dwarfs
Get PDFWe present radial velocity (RV) measurements of our sample of 40 M dwarfs from our planet search programme with VLT+UVES begun in 2000. Although with our RV precision down to 2-2.5 m/s and timebase line of up to 7 years, we are capable of finding planets of a few Earth masses in the close-in habitable zones of M dwarfs, there is no detection of a planetary companion. To demonstrate this we present mass detection limits allowing us to exclude Jupiter-mass planets up to 1 AU for most of our sample stars. We identified 6 M dwarfs that host a brown dwarf or low-mass stellar companion. With the exception of these, all other sample stars show low RV variability with an rms <20 m/s. Some high proper motion stars exhibit a linear RV trend consistent with their secular acceleration. Furthermore, we examine our data sets for a possible correlation between RVs and stellar activity as seen in variations of the Halpha line strength. For Barnard's star we found a significant anticorrelation, but most of the sample stars do not show such a correlation. Based on observations collected at the European Southern Observatory, Paranal Chile, ESO programmes 65.L-0428, 66.C-0446, 267.C-5700, 68.C-0415, 69.C-0722, 70.C-0044, 71.C-0498, 072.C-0495, 173.C-0606, 078.C-0829. Radial velocity data are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/505/85
A probable close brown dwarf companion to GJ 1046 (M 2.5V)
Get PDFContext: Brown dwarf companions to stars at separations of a few AU or less are rare objects, and none have been found so far around early-type M dwarfs (M 0V-M 5V). With GJ 1046 (M 2.5V), a strong candidate for such a system with a separation of 0.42 AU is presented. Aims: We aim at constraining the mass of the companion in order to decide whether it is a brown dwarf or a low-mass star. Methods: We employed precision RV measurements to determine the orbital parameters and the minimum companion mass. We then derived an upper limit to the companion mass from the lack of disturbances of the RV measurements by a secondary spectrum. An even tighter upper limit is subsequently established by combining the RV-derived orbital parameters with the recent new version of the Hipparcos Intermediate Astrometric Data. Results: For the mass of the companion, we derive m >= 26.9 {M}_Jup from the RV data. Based on the RV data alone, the probability that the companion exceeds the stellar mass threshold is just 6.2% . The absence of effects from the secondary spectrum lets us constrain the companion mass to m <= 229 {M}_Jup. The combination of RV and Hipparcos data yields a 3sigma upper mass limit to the companion mass of 112 {M}_Jup with a formal optimum value at m=47.2 {M}_Jup. From the combination of RV and astrometric data, the chance probability that the companion is a star is 2.9% . Conclusions: We have found a low-mass, close companion to an early-type M dwarf. While the most likely interpretation of this object is that it is a brown dwarf, a low-mass stellar companion is not fully excluded. Based on observations collected at the European Southern Observatory, Paranal, Chile, programmes 173.C-0606 and 078.C-0829
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
A giant planet in the triple system HD 132563
Get PDFAs part of our radial velocity planet-search survey performed with SARG at TNG, we monitored the components of HD 132563 for ten years. It is a binary system formed by two rather similar solar type stars with a projected separation of 4.1 arcsec, which corresponds to 400 AU at the distance of 96 pc. The two components are moderately metal-poor ([Fe/H] = -0.19), and the age of the system is about 5 Gyr. We detected RV variations of HD 132563B with period of 1544 days and semi-amplitude of 26 m/s. From the star characteristics and line profile measurements, we infer their Keplerian origin. Therefore HD 132563B turns out to host a planet with a projected mass m sin i = 1.49 M-J at 2.6 AU with a moderately eccentric orbit (e = 0.22). The planet around HD 132563B is one of the few that are known in triple stellar systems, as we found that the primary HD 132563A is itself a spectroscopic binary with a period longer than 15 years and an eccentricity higher than 0.65. The spectroscopic component was not detected in adaptive-optics images taken with the instrument AdOpt mounted at the TNG, since it expected at a projected separation that was smaller than 0.2 arcsec at the time of our observations. A small excess in K band difference between the components with respect to the difference in V band is compatible with a companion of about 0.55 M-circle dot. A preliminary statistical analysis of when planets occur in triple systems indicate a similar frequency of planets around the isolated component in a triple system, components of wide binaries and single stars. There is no significant iron abundance difference between the components. The lack of stars in binary systems and open clusters showing strong enhancements of iron abundance, which are comparable to the typical metallicity difference between stars with and without giant planets, agrees with the idea that accretion of planetary material producing iron abundance anomalies over 0.1 dex is rare.PRIN-INAF 2008Astronom
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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