scholarly journals Spectroscopy of main-sequence and subgiant stars in globular star clusters

1984 ◽  
Vol 105 ◽  
pp. 139-142
Author(s):  
James E. Hesser ◽  
Gretchen L.H. Harris ◽  
R.A. Bell ◽  
R.D. Cannon
Keyword(s):  
Globular Cluster ◽  
Main Sequence ◽  
Star Clusters ◽  
Observational Constraints ◽  
Spectral Features ◽  
Small Temperature ◽  
Giant Stars ◽  
Magnitude Diagram ◽  
Subgiant Stars ◽  
Complex Subject

Since the mid-1970's it has been apparent that giant stars of similar V and B-V within a “normal” globular cluster [i.e., one with a narrow giant branch in its color-magnitude diagram (CMD)] exhibit a perplexing range of strengths for such spectral features as CH, CN and NH. This complex subject has been reviewed by Kraft (1979), McClure (1979) and Freeman and Norris (1981). DDO photometry first revealed star-to-star differences of CN strengths at MV>+1, where observational confusion between asymptotic and first-ascent giant stars is removed (Hesser, Hartwick and McClure 1976, 1977; Hesser 1978). Subsequently, we have sought to place observational constraints on possible mechanisms by studying such questions as: At what MV's do spectral differences first become observable? Do spectral features other than those from CNO-based molecules vary from star-to-star? Can small temperature or gravity differences produce the observed ranges?

scholarly journals A preliminary color-magnitude diagram for late-type stars in the solar neighborhood

1959 ◽  
Vol 10 ◽  
pp. 39-40
Author(s):  
O. C. Wilson
Keyword(s):  
Main Sequence ◽  
Star Clusters ◽  
Solar Neighborhood ◽  
Late Type ◽  
Main Sequence Stars ◽  
Individual Values ◽  
Magnitude Diagram ◽  
Trigonometric Parallaxes ◽  
Nearby Stars ◽  
The Mean

Modern photoelectric techniques yield magnitudes and colors of stars with accuracies of the order of a few thousandths and a few hundredths of a magnitude respectively. Hence for star clusters it is possible to derive highly accurate color-magnitude arrays since all of the members of a cluster may be considered to be at the same distance from the observer. It is much more difficult to do this for the nearby stars where all of the objects concerned are at different, and often poorly determined, distances. If one depends upon trigonometric parallaxes, the bulk of the reliable individual values will refer to main sequence stars, and while the mean luminosities of brighter stars are given reasonably well by this method, the scatter introduced into a color-magnitude array by using individual trigonometrically determined luminosities could obscure important features. Somewhat similar objections could be raised against the use of the usual spectroscopic parallaxes which also should be quite good for the main sequence but undoubtedly exhibit appreciable scatter for some, at least, of the brighter stars.

scholarly journals Globular cluster candidates in the Galactic bulge: Gaia and VVV view of the latest discoveries

2019 ◽  
Vol 628 ◽  
pp. A45 ◽  
Author(s):  
F. Gran ◽  
M. Zoccali ◽  
R. Contreras Ramos ◽  
E. Valenti ◽  
A. Rojas-Arriagada ◽  
...  
Keyword(s):  
Main Sequence ◽  
Star Clusters ◽  
Forthcoming Paper ◽  
Variable Stars ◽  
Star Cluster ◽  
Proper Motions ◽  
Magnitude Diagram ◽  
Spatial Coordinates ◽  
Red Giant ◽  
Moving Groups

Context. Thanks to the recent wide-area photometric surveys, the number of star cluster candidates have risen exponentially in the last few years. Most detections, however, are based only on the presence of an overdensity of stars in a given region or an overdensity of variable stars, regardless of their distance. As candidates, their detection has not been dynamically confirmed. Therefore, it is currently unknown how many and which of the published candidates are true clusters and which are chance alignments. Aims. We present a method to detect and confirm star clusters based on the spatial distribution, coherence in motion, and appearance on the color-magnitude diagram. We explain and apply this approach to one new star cluster and several candidate star clusters published in the literature. Methods. The presented method is based on data from the second data release of Gaia complemented with data from the VISTA Variables in the Vía Láctea survey for the innermost bulge regions. This method consists of a nearest neighbors algorithm applied simultaneously over spatial coordinates, star color, and proper motions to detect groups of stars that are close in the sky, move coherently, and define narrow sequences in the color-magnitude diagram, such as a young main sequence or a red giant branch. Results. When tested in the bulge area (−10 <  ℓ (deg) <  +10; −10 <  b (deg) <  +10) the method successfully recovered several known young and old star clusters. We report in this work the detection of one new, likely old star cluster, while deferring the others to a forthcoming paper. Additionally, the code has been applied to the position of 93 candidate star clusters published in the literature. As a result, only two of these clusters are confirmed as coherently moving groups of stars at their nominal positions.

scholarly journals CCD Photometry in the Globular Cluster M4

1995 ◽  
Vol 164 ◽  
pp. 411-411
Author(s):  
W.K. Griffiths ◽  
I.N. Kanatas ◽  
R.J. Dickens ◽  
A.J. Penny
Keyword(s):  
Globular Cluster ◽  
Main Sequence ◽  
Ccd Photometry ◽  
Magnitude Diagram ◽  
Red Giant

A V, B- V composite colour-magnitude diagram, based upon CCD photometry from V~ 12 on the red giant branch to V~ 25 on the main-sequence has been derived for the globular cluster M4. A distance to the cluster of (m – M)v = 12.84 ± 0.19 is determined and the best match to theoretical isochrones is for the case [Fe/H]=−1.27 and an age of 16±1 Gyr. A differential age comparison with NGC 362 shows that M4 is approximately 1.7 Gyr older.

scholarly journals Main-sequence and sub-giant stars in the globular cluster NGC 6397: The complex evolution of the lithium abundance

2009 ◽  
Vol 5 (S268) ◽  
pp. 257-261
Author(s):  
J. I. González Hernández ◽  
P. Bonifacio ◽  
E. Caffau ◽  
M. Steffen ◽  
H.-G. Ludwig ◽  
...  
Keyword(s):  
Globular Cluster ◽  
Main Sequence ◽  
3D Models ◽  
Stellar Atmospheres ◽  
Evolutionary Status ◽  
Thermodynamical Equilibrium ◽  
Lithium Abundance ◽  
Giant Stars ◽  
Effective Temperatures ◽  
Hydrodynamical Simulations

AbstractThanks to the high multiplex and efficiency of Giraffe at the VLT we have been able for the first time to observe the Li I doublet in the Main Sequence stars of a globular cluster. At the same time we observed Li in a sample of Sub-Giant stars of the same B-V colour.Our final sample is composed of 84 SG stars and 79 MS stars. In spite of the fact that SG and MS span the same temperature range we find that the equivalent widths of the Li I doublet in SG stars are systematically larger than those in MS stars, suggesting a higher Li content among SG stars. This is confirmed by our quantitative analysis carried out making use of 1D hydrostatic plane-parallel models and 3D hydrodynamical simulations of the stellar atmospheres.We derived the effective temperatures of stars in our the sample from Hα fitting. Theoretical profiles were computed using 3D hydrodynamical simulations and 1D ATLAS models. Therefore, we are able to determined 1D and 3D-based effective temperatures. We then infer Li abundances taking into account non-local thermodynamical equilibrium effects when using both 1D and 3D models.We find that SG stars have a mean Li abundance higher by 0.1 dex than MS stars. This result is obtained using both 1D and 3D models. We also detect a positive slope of Li abundance with effective temperature, the higher the temperature the higher the Li abundance, both for SG and MS stars, although the slope is slightly steeper for MS stars. These results provide an unambiguous evidence that the Li abundance changes with evolutionary status.The physical mechanisms responsible for this behaviour are not yet clear, and none of the existing models seems to describe accurately these observations. Based on these conclusions, we believe that the cosmological lithium problem still remains an open question.

scholarly journals Similarities of the intermediate-age SMC star cluster NGC 152 with inner disk clusters in the Milky Way

1991 ◽  
Vol 148 ◽  
pp. 198-199
Author(s):  
T. Richtler ◽  
Klaas S. De Boer ◽  
A. Vallenari ◽  
W. Seggewiss
Keyword(s):  
Globular Cluster ◽  
Milky Way ◽  
Star Clusters ◽  
Star Cluster ◽  
Horizontal Branch ◽  
Vertical Extension ◽  
Magnitude Diagram ◽  
The Mean ◽  
Normal Feature ◽  
Selection Of

A colour-magnitude diagram (CMD) of the region containing the intermediate-age SMC globular cluster NGC 152 was published recently (Melcher & Richtler 1989). A particularly interesting feature of this CMD is the “clump” of He-core burning stars, which are predominantly field stars. A selection of stars near the cluster centre leads to the CMD shown in Figure 1. The vertical extension of the clump (explainable by the evolution of stars younger than 1 Gyr) is replaced by a “tilted horizontal branch” (we use this expression for lack of a better one). The age of NGC 152 is about 1.3 Gyr and the reddening is small; the metallicity is unknown but less than −0.6 dex, which is the mean metallicity of the young SMC population. The tilted HB can be reproduced in CMD simulations using the method developed by Vallenari et al. (1990), and thus can be considered as a normal feature of star clusters like NGC 152. It is evident also in other intermediate-age MC clusters like Kron 3 (Rich et al. 1984).

scholarly journals Signatures of tidal disruption in the Milky Way globular cluster NGC 6981 (M72)

2021 ◽  
Vol 646 ◽  
pp. A176
Author(s):  
Andrés E. Piatti ◽  
Martín F. Mestre ◽  
Julio A. Carballo-Bello ◽  
Daniel D. Carpintero ◽  
Camila Navarrete ◽  
...  
Keyword(s):  
Globular Cluster ◽  
Main Sequence ◽  
Milky Way ◽  
Structural Features ◽  
Field Of View ◽  
Data Sets ◽  
Tidal Disruption ◽  
Density Maps ◽  
Magnitude Diagram ◽  
Stellar Density

We study the outer regions of the Milky Way globular cluster NGC 6981 based on publicly available BV photometry and new Dark Energy Camera (DECam) observations, both of which reach nearly 4 mag below the cluster main sequence (MS) turnoff. While the BV data sets reveal the present of extra-tidal features around the cluster, the much larger field of view of the DECam observations allowed us to identify some other tidal features, which extend from the cluster toward the opposite direction to the Milky Way center. Such structural features of clusters arise from stellar density maps built using MS stars, following a cleaning of the cluster color-magnitude diagram to remove the contamination of field stars. We also performed N-body simulations in order to help us to understand the spatial distribution of the extra-tidal debris. The outcomes reveal the presence of long trailing and leading tails that are mostly parallel to the direction of the cluster velocity vector. We find that the cluster loses most of its mass by tidal disruption during its perigalactic passages, each of which lasted nearly 20 Myr. Hence, a decrease in the density of escaping stars near the cluster is expected from our N-body simulations, which, in turn, means that stronger extra-tidal features could be found by exploring much larger areas around NGC 6891.

Spectroscopy of Main-Sequence and Subgiant Stars in Globular Star Clusters

1984 ◽  
pp. 139-142 ◽  
Author(s):  
James E. Hesser ◽  
Gretchen L. H. Harris ◽  
R. A. Bell ◽  
R. D. Cannon
Keyword(s):  
Main Sequence ◽  
Star Clusters ◽  
Subgiant Stars

scholarly journals Lithium abundances of main-sequence and subgiant stars in the globular cluster NGC 6397

2009 ◽  
Vol 5 (S266) ◽  
pp. 407-410
Author(s):  
J. I. González Hernández ◽  
P. Bonifacio ◽  
E. Caffau ◽  
M. Steffen ◽  
H.-G. Ludwig ◽  
...  
Keyword(s):  
Globular Cluster ◽  
Main Sequence ◽  
Positive Slope ◽  
Physical Mechanisms ◽  
Very Large Telescope ◽  
Final Sample ◽  
Subgiant Stars ◽  
Open Question ◽  
First Time ◽  
Li Content

AbstractWe present FLAMES/GIRAFFE spectroscopy obtained with the Very Large Telescope (VLT). Using these observations, we have been able (for the first time) to observe the Lii doublet in the main-sequence (MS) stars of a globular cluster. We also observed Li in a sample of subgiant (SG) stars of the same B − V colour. Our final sample is composed of 84 SG and 79 MS stars. In spite of the fact that SG and MS stars span the same temperature range, we find that the equivalent widths of the Lii doublet in SG stars are systematically greater than in MS stars, suggesting a higher Li content among SG stars. This is confirmed by our quantitative analysis, which makes use of both 1D and 3D model atmospheres. We find that SG stars show, on average, a higher Li abundance, by 0.1 dex, than MS stars. We also detect a positive slope of Li abundance with effective temperature: the higher the temperature the higher the Li abundance, both for SG and MS stars, although the slope is slightly steeper for MS stars. These results provide unambiguous evidence that the Li abundance changes with evolutionary state. The physical mechanisms that contribute to this are not yet clear, since none of the proposed models seem to describe accurately the observations. Whether such a mechanism can explain the cosmological lithium problem is still an open question.

scholarly journals Pal 13: its moderately extended low-density halo and its accretion history

2020 ◽  
Vol 635 ◽  
pp. A93 ◽  
Author(s):  
Andrés E. Piatti ◽  
José G. Fernández-Trincado
Keyword(s):  
Globular Cluster ◽  
Globular Clusters ◽  
Main Sequence ◽  
Milky Way ◽  
Orbital Motion ◽  
Data Sets ◽  
Recent Estimate ◽  
Magnitude Diagram ◽  
Density Map ◽  
Stellar Density

We present results based on Dark Energy Camera Legacy Survey (DECaLS) DR8 astrometric and photometric data sets of the Milky Way globular cluster Pal 13. Because of its relatively small size and mass, there is not yet a general consensus on the existence of extra-tidal structures surrounding it. While some previous results suggest the absence of such features, others show that the cluster is under the effects of tidal stripping. We have built a cluster stellar density map from DECaLS g, r magnitudes – previously corrected for interstellar reddening – of stars placed along the cluster main sequence in the color-magnitude diagram. The resulting density map shows nearly smooth contours around Pal 13 out to approximately 1.6 t the most recent estimate of its Jacobi radius, which was derived whilst taking into account the variation along its orbital motion. This outcome favors the presence of stars escaping the cluster, a phenomenon frequently seen in globular clusters that have crossed the Milky Way disk a comparably large number of times. Particularly, the orbital high eccentricity and large inclination angle of this accreted globular cluster could have been responsible for the relatively large amount of lost cluster mass.

scholarly journals On the Derivation of Abundances by Means of Features in the Sequences of the old Open Star Clusters

1976 ◽  
Vol 72 ◽  
pp. 97-99
Author(s):  
A. Maeder
Keyword(s):  
Metal Content ◽  
Main Sequence ◽  
Star Clusters ◽  
Open Star Clusters ◽  
Open Star ◽  
High Metal Content ◽  
High Metal ◽  
Magnitude Diagram ◽  
Homogeneous Models ◽  
Systematic Effects

Stellar evolution near the main-sequence is still subject to many discussions. In addition to the case of the cluster M 67 (Racine, 1971), quite systematic differences have been encountered during the comparison of theoretical isochrones and observed sequences in the colour-magnitude diagram of the old open star clusters (Maeder, 1974). In the case of M 67 and NGC 188, many attempts to discuss the characteristics of their sequences (such as the gap corresponding to the hydrogen-exhaustion phase) in terms of chemical composition have been made (e.g. Aizenman et al., 1969; Demarque and Schlesinger, 1969; Demarque and Heasley, 1971; Torres-Peimbert, 1971; Hejlesen et al., 1972; Caloi et al., 1974); some of these works have suggested to explain the gap parameters by a high metal content. Attemps to explain the differences between models and observations by means of simplified models with overshooting from convective cores have also been made (Maeder, 1973; Prather and Demarque, 1974). Let us also note that it has been shown that it is unlikely that the anomalies found are due to systematic effects, like the interstellar reddening, binarity, rotation or effects in calibrations or composition of the initial homogeneous models.

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