scholarly journals The Relationship Between the BÖöhm-Vitense Gap and Stellar Activity in Open Clusters

2000 ◽  
Vol 119 (3) ◽  
pp. 1296-1302 ◽  
Author(s):  
Brian L. Rachford ◽  
R. Canterna
Keyword(s):  
Open Clusters ◽  
Stellar Activity ◽  
The Relationship

Stellar Activity in Coeval Open Clusters: Praesepe and the Hyades

1998 ◽  
Vol 506 (1) ◽  
pp. 347-359 ◽  
Author(s):  
David Barrado y Navascués ◽  
John R. Stauffer ◽  
Sofia Randich
Keyword(s):  
Open Clusters ◽  
Stellar Activity

scholarly journals Stellar activity with LAMOST. III. Temporal variability pattern in Pleiades, Praesepe, and Hyades

2020 ◽  
Vol 495 (3) ◽  
pp. 2949-2965
Author(s):  
Xiang-Song Fang ◽  
Christian Moni Bidin ◽  
Gang Zhao ◽  
Li-Yun Zhang ◽  
Yerra Bharat Kumar
Keyword(s):  
Active Regions ◽  
Temporal Variations ◽  
Open Clusters ◽  
M Dwarfs ◽  
Stellar Activity ◽  
Cool Star ◽  
Level Of Activity ◽  
Variation Patterns ◽  
Wide Range ◽  
Chromospheric Emission

ABSTRACT We present the results from a systematic study of temporal variation of stellar activity in young late-type stars. We used multi-epoch LAMOST (Large sky Area Multi-Object fiber Spectroscopic Telescope) low-resolution spectra of over 300 member candidates in three young open clusters: Pleiades, Praesepe, and Hyades. The spectral measurements of TiO band strength near 7050 Å (TiO2) and equivalent width of H α line (EWH α) are used as the tracers of cool spot coverage and chromospheric emission strength, respectively. The analysis of time-variation patterns of these two tracers suggested that there exist detectable variabilities in TiO2 and EWH α, and their time-scales are in the wide range from days to years. Results showed that more active stars, younger and fast rotators, tend to have larger activity variations. There is a tendency of anticorrelation between temporal variations in TiO2 and EWH α. Also, appreciable anticorrelation in the rotational phase between H α emission and K2 brightness is detected in some M dwarfs, indicating spatial co-location of the plages with cool star-spots; however, cool stars do not always show such co-location features. Furthermore, spot coverage and H α emission were evident at all rotational phases of several M dwarfs, indicating a basal level of activity, perhaps due to many small and randomly located active regions in the atmosphere.

scholarly journals Sixteen overlooked open clusters in the fourth Galactic quadrant

2020 ◽  
Vol 637 ◽  
pp. A95 ◽  
Author(s):  
G. I. Perren ◽  
E. E. Giorgi ◽  
A. Moitinho ◽  
G. Carraro ◽  
M. S. Pera ◽  
...  
Keyword(s):  
Zero Point ◽  
Open Clusters ◽  
Solar Neighborhood ◽  
Intrinsic Properties ◽  
Distance Analysis ◽  
Variable Level ◽  
The Galaxy ◽  
Relationship Of ◽  
The Relationship ◽  
Photometric Distance

Aims. This paper has two main objectives: (1) To determine the intrinsic properties of 16 faint and mostly unstudied open clusters in the poorly known sector of the Galaxy at 270° −300° to probe the Milky Way structure in future investigations. (2) To address previously reported systematics in Gaia DR2 parallaxes by comparing the cluster distances derived from photometry with those derived from parallaxes. Methods. Deep UBVI photometry of 16 open clusters was carried out. Observations were reduced and analyzed in an automatic way using the ASteCA package to obtain individual distances, reddening, masses, ages, and metallicities. Photometric distances were compared to those obtained from a Bayesian analysis of Gaia DR2 parallaxes. Results. Ten out of the sixteen clusters are true or highly probable open clusters. Two of them are quite young and follow the trace of the Carina Arm and the already detected warp. The remaining clusters are placed in the interarm zone between the Perseus and Carina Arms, as expected for older objects. We found that the cluster van den Berg-Hagen 85 is 7.5 × 109 yr old, which means that it is one of the oldest open clusters detected in our Galaxy so far. The relationship of these ten clusters with the Galaxy structure in the solar neighborhood is discussed. The comparison of distances from photometry and parallaxes data in turn reveals a variable level of disagreement. Conclusions. Various zero-point corrections for Gaia DR2 parallax data recently reported were considered for a comparison between photometry- and parallax-based distances. The results tend to improve with some of these corrections. Photometric distance analysis suggests an average correction of ∼+0.026 mas (to be added to the parallaxes). The correction may have a more intricate dependence on distance, but addressing this level of detail will require a larger cluster sample.

scholarly journals Li Abundance in Pop I Stars

2000 ◽  
Vol 198 ◽  
pp. 269-278
Author(s):  
Luca Pasquini
Keyword(s):  
Large Range ◽  
Large Fraction ◽  
Open Clusters ◽  
Different Ages ◽  
Primordial Abundance ◽  
Total Lack ◽  
The Relationship

The study of Li in Pop I stars has focussed in the last years on observations of open clusters, spanning a large range of ages and metallicities. So far the observational picture is quite complex: the data indicate several phenomena which are not well understood, from the 'Li dip', to the scatter in Li abundances found among (otherwise similar) stars both in young and old clusters. Models fail in reproducing most of the observed features; in particular the almost total lack of dependence of Li depletion with metallicity. The comparison between clusters of different ages show that no PMS depletion occurs, and that for a large fraction of old star no additional depletion occurs after the first ˜1.5 Gyrs, while the depletion is strong at earlier ages. These observations are crucial for our understanding of the relationship between the Pop II Li Plateau and the Li primordial abundance.

scholarly journals Stellar activity with LAMOST – II. Chromospheric activity in open clusters

2018 ◽  
Vol 476 (1) ◽  
pp. 908-926 ◽  
Author(s):  
Xiang-Song Fang ◽  
Gang Zhao ◽  
Jing-Kun Zhao ◽  
Yerra Bharat Kumar
Keyword(s):  
Open Clusters ◽  
Stellar Activity ◽  
Chromospheric Activity

scholarly journals Planets around evolved intermediate-mass stars

2018 ◽  
Vol 619 ◽  
pp. A2 ◽  
Author(s):  
E. Delgado Mena ◽  
C. Lovis ◽  
N. C. Santos ◽  
J. Gomes da Silva ◽  
A. Mortier ◽  
...  
Keyword(s):  
Open Cluster ◽  
Open Clusters ◽  
Periodic Signal ◽  
Red Giants ◽  
Intermediate Mass ◽  
Inverse Slope ◽  
Stellar Activity ◽  
Intermediate Mass Stars ◽  
Red Giant

Aims. The aim of this work is to search for planets around intermediate-mass stars in open clusters using data from an extensive survey with more than 15 yr of observations. Methods. We obtain high-precision radial velocities (RV) with the HARPS spectrograph for a sample of 142 giant stars in 17 open clusters. We fit Keplerian orbits when a significant periodic signal is detected. We also study the variation of stellar activity indicators and line-profile variations to discard stellar-induced signals. Results. We present the discovery of a periodic RV signal compatible with the presence of a planet candidate in the 1.15 Gyr open cluster IC 4651 orbiting the 2.06 M⊙ star No. 9122. If confirmed, the planet candidate would have a minimum mass of 7.2 MJ and a period of 747 days. However, we also find that the full width at half maximum (FWHM) of the cross-correlation function (CCF) varies with a period close to the RV, casting doubts on the planetary nature of the signal. We also provide refined parameters for the previously discovered planet around NGC 2423 No. 3, but show evidence that the bisector inverse slope (BIS) of the CCF is correlated with the RV during some of the observing periods. We consider this fact as a warning that this might not be a real planet and that the RV variations could be caused by stellar activity and/or pulsations. Finally, we show that the previously reported signal by a brown dwarf around NGC 4349 No. 127 is presumably produced by stellar activity modulation. Conclusions. The long-term monitoring of several red giants in open clusters has allowed us to find periodic RV variations in several stars. However, we also show that the follow-up of this kind of stars should last more than one orbital period to detect long-term signals of stellar origin. This work highlights the fact that although it is possible to detect planets around red giants, large-amplitude, long-period RV modulations do exist in such stars that can mimic the presence of an orbiting planetary body. Therefore, we need to better understand how such RV modulations behave as stars evolve along the red giant branch and perform a detailed study of all the possible stellar-induced signals (e.g., spots, pulsations, granulation) to comprehend the origin of RV variations.

Interstellar gas in the central region of the Galaxy

1967 ◽  
Vol 31 ◽  
pp. 239-251 ◽  
Author(s):  
F. J. Kerr
Keyword(s):  
Central Region ◽  
Galactic Plane ◽  
Neutral Hydrogen ◽  
Continuum Emission ◽  
Galactic Centre ◽  
Interstellar Gas ◽  
Hydrogen Recombination ◽  
The Galaxy ◽  
Centre Region ◽  
The Relationship

A review is given of information on the galactic-centre region obtained from recent observations of the 21-cm line from neutral hydrogen, the 18-cm group of OH lines, a hydrogen recombination line at 6 cm wavelength, and the continuum emission from ionized hydrogen.Both inward and outward motions are important in this region, in addition to rotation. Several types of observation indicate the presence of material in features inclined to the galactic plane. The relationship between the H and OH concentrations is not yet clear, but a rough picture of the central region can be proposed.

Observations of neutral hydrogen in open clusters

1967 ◽  
Vol 31 ◽  
pp. 41-43
Author(s):  
Th. Schmidt-Kaler ◽  
R. Schwartz
Keyword(s):  
Neutral Hydrogen ◽  
Open Clusters

Neutral hydrogen is found in every young cluster observed, usually extending beyond the optical diameter, and in some cases showing expanding motions.

The relationship of the scleractinian corals to the rugose corals

Paleobiology ◽  
1980 ◽  
Vol 6 (02) ◽  
pp. 146-160 ◽  
Author(s):  
William A. Oliver
Keyword(s):  
Critical Points ◽  
Scleractinian Corals ◽  
Convincing Evidence ◽  
Early Triassic ◽  
Rugose Corals ◽  
History Of ◽  
The Subject ◽  
Relationship Of ◽  
The Relationship ◽  
Biologic Factors

The Mesozoic-Cenozoic coral Order Scleractinia has been suggested to have originated or evolved (1) by direct descent from the Paleozoic Order Rugosa or (2) by the development of a skeleton in members of one of the anemone groups that probably have existed throughout Phanerozoic time. In spite of much work on the subject, advocates of the direct descent hypothesis have failed to find convincing evidence of this relationship. Critical points are:(1) Rugosan septal insertion is serial; Scleractinian insertion is cyclic; no intermediate stages have been demonstrated. Apparent intermediates are Scleractinia having bilateral cyclic insertion or teratological Rugosa.(2) There is convincing evidence that the skeletons of many Rugosa were calcitic and none are known to be or to have been aragonitic. In contrast, the skeletons of all living Scleractinia are aragonitic and there is evidence that fossil Scleractinia were aragonitic also. The mineralogic difference is almost certainly due to intrinsic biologic factors.(3) No early Triassic corals of either group are known. This fact is not compelling (by itself) but is important in connection with points 1 and 2, because, given direct descent, both changes took place during this only stage in the history of the two groups in which there are no known corals.

Choosing a Markov blanket

2020 ◽  
Vol 43 ◽  
Author(s):  
Thomas Parr
Keyword(s):  
Markov Blanket ◽  
Target Article ◽  
The Relationship

Abstract This commentary focuses upon the relationship between two themes in the target article: the ways in which a Markov blanket may be defined and the role of precision and salience in mediating the interactions between what is internal and external to a system. These each rest upon the different perspectives we might take while “choosing” a Markov blanket.

Sign in / Sign up

Export Citation Format

Share Document