1,722,612 research outputs found
CCD UBV photometry of the open cluster NGC 6819
No full textWe present the results of CCD UBV observations of the open cluster NGC 6819. We calculated the stellar density profile in the cluster's field to determine the structural parameters of NGC 6819. Using the existing astrometric data, we calculated the probabilities of the stars being physical members of the cluster, and used these objects in the determination of the astrophysical parameters of NGC 6819. We inferred the reddening and metallicity of the cluster as E(B-V) = 0.130 +/- 0.035 mag and [Fe/H] = +0.051 +/- 0.020 dex, respectively, using the U-B vs B-V two-colour diagram and UV excesses of the F-G type main-sequence stars. We fit the colour-magnitude diagrams of NGC 6819 with the PARSEC isochrones and derived the distance modula, distance and age of the cluster as mu(V) = 12.22 +/- 0.10 mag, d = 2309 +/- 106 pc and t = 2.4 +/- 0.2 Gyr, respectively. The parameters of the galactic orbit estimated for NGC 6819 indicate that the cluster is orbiting in a slightly eccentric orbit of e = 0.06 with a period of P-orb = 142 Myr. The slope of the mass function estimated for the cluster is close to the one found for the stars in the solar neighbourhood
HR 6819 is a binary system with no black hole
No full textContext. Two scenarios have been proposed to match the existing observational constraints of the object HR 6819. The system could consist of a close inner B-type giant plus a black hole (BH) binary with an additional Be companion in a wide orbit. Alternatively, it could be a binary composed of a stripped B star and a Be star in a close orbit. Either scenario makes HR 6819 a cornerstone object as the stellar BH closest to Earth, or as an example of an important transitional, non-equilibrium phase for Be stars with solid evidence for its nature.
Aims. We aim to distinguish between the two scenarios for HR 6819. Both models predict two luminous stars but with very different angular separations and orbital motions. Therefore, the presence of bright sources in the 1−100 milliarcsec (mas) regime is a key diagnostic for determining the nature of the HR 6819 system.
Methods. We obtained new high-angular resolution data with VLT/MUSE and VLTI/GRAVITY of HR 6819. The MUSE data are sensitive to bright companions at large scales, whilst the interferometric GRAVITY data are sensitive down to separations on mas scales and large magnitude differences.
Results. The MUSE observations reveal no bright companion at large separations and the GRAVITY observations indicate the presence of a stellar companion at an angular separation of ∼1.2 mas that moves on the plane of the sky over a timescale compatible with the known spectroscopic 40-day period.
Conclusions. We conclude that HR 6819 is a binary system and that no BH is present in the system. The unique nature of HR 6819, and its proximity to Earth make it an ideal system for quantitatively characterising the immediate outcome of binary interaction and probing how Be stars form
CCD UBV photometry of the open cluster NGC 6819
No full textWe present the results of CCD UBV observations of the open cluster NGC 6819. We calculated the stellar density profile in the cluster's field to determine the structural parameters of NGC 6819. Using the existing astrometric data, we calculated the probabilities of the stars being physical members of the cluster, and used these objects in the determination of the astrophysical parameters of NGC 6819. We inferred the reddening and metallicity of the cluster as E(B-V) = 0.130 +/- 0.035 mag and [Fe/H] = +0.051 +/- 0.020 dex, respectively, using the U-B vs B-V two-colour diagram and UV excesses of the F-G type main-sequence stars. We fit the colour-magnitude diagrams of NGC 6819 with the PARSEC isochrones and derived the distance modula, distance and age of the cluster as mu(V) = 12.22 +/- 0.10 mag, d = 2309 +/- 106 pc and t = 2.4 +/- 0.2 Gyr, respectively. The parameters of the galactic orbit estimated for NGC 6819 indicate that the cluster is orbiting in a slightly eccentric orbit of e = 0.06 with a period of P-orb = 142 Myr. The slope of the mass function estimated for the cluster is close to the one found for the stars in the solar neighbourhood
Testing the presence of a dormant black hole inside HR 6819
Get PDFHR 6819 was recently reported to be a triple system with a dormant black hole
(BH). The inner binary system was defined as a star estimated to be at the end
of its main sequence and a dormant BH. As the inner binary is not resolved, the
third component may actually just be spatially coinciding with the inner
binary. In this study we test whether the system inner binary can be
reconstructed using the isolated binary evolution in the Galactic field or
through the dynamical evolution within globular star clusters. Our goal is to
understand the formation of the HR 6819 inner binary. To simulate the inner
binary evolution we assumed that the influence of the third body on the inner
binary is negligible. We created synthetic populations of BH main-sequence
binaries for the Galactic disc and the Galactic globular clusters to compare to
the reported parameters of the HR 6819 inner binary. We have adopted very
optimistic input physics, in terms of common envelope evolution and BH
formation, for the formation of binaries similar to the reported inner HR 6819
binary. Despite our optimistic assumptions we cannot form systems like the
inner HR 6819 binary in globular clusters. Even with our extreme assumptions,
the formation of an HR 6819-like binary in the Galactic field population is not
expected. We argue that if a dormant BH actually exists in the reported
configuration inside HR 6819, its presence cannot easily be explained by our
models based on isolated and dynamical binary evolution.Comment: 11 page
Differential cross sections for rotational excitation of NH3 by collisions with Ar and He. Close coupling results and comparison with experiment
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6819.pdf (Publisher’s version ) (Open Access
Sounding Open Clusters: Asteroseismic Constraints From Kepler on the Properties of Ngc 6791 and Ngc 6819
Get PDFWe present initial results on some of the properties of open clusters NGC 6791 and NGC 6819 derived from asteroseismic data obtained by NASA's Kepler mission. In addition to estimating the mass, radius, and log g of stars on the red giant branch (RGB) of these clusters, we estimate the distance to the clusters and their ages. Our model-independent estimate of the distance modulus of NGC 6791 is (m - M)(0) = 13.11 +/- 0.06. We find (m - M)(0) = 11.85 +/- 0.05 for NGC 6819. The average mass of stars on the RGB of NGC 6791 is 1.20 +/- 0.01 M-circle dot, while that of NGC 6819 is 1.68 +/- 0.03 M-circle dot. It should be noted that we do not have data that cover the entire RGB and the actual mass will be somewhat lower. We have determined model-dependent estimates of ages of these clusters. We find ages between 6.8 and 8.6 Gyr for NGC 6791, however, most sets of models give ages around 7 Gyr. We obtain ages between 2 and 2.4 Gyr for NGC 6819
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
WOCS 40007: A detached eclipsing binary near the turnoff of the open cluster NGC 6819
No full textWe analyze extensive BVRcIc time-series photometry and radial-velocity measurements for WOCS 40007 (Auner 259; KIC 5113053), a double-lined detached eclipsing binary and a member of the open cluster NGC 6819. Utilizing photometric observations from the
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
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