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Start Over Subject stellar associations Publisher CDS Content Level Research
1. Abundance ratio for 5 local stellar associations
ID:
ivo://CDS.VizieR/J/MNRAS/454/1976
Title:
Abundance ratio for 5 local stellar associations
Short Name:
J/MNRAS/454/1976
Date:
21 Oct 2021
Publisher:
CDS
Description:
We have observed high-dispersion echelle spectra of main-sequence stars in five nearby young associations - Argus, Carina-Near, Hercules-Lyra, Orion and Subgroup B4 - and derived abundances for elements ranging from Na to Eu. These are the first chemical abundance measurements for two of the five associations, while the remaining three associations are analysed more extensively in our study. Our results support the presence of chemical homogeneity among association members with a typical star-to-star abundance scatter of about 0.06dex or less over many elements. The five associations show log {epsilon}(Li) consistent with their age and share a solar chemical composition for all elements with the exception of Ba. We find that all the heavy elements (Y, Zr, La, Ce, Nd, Sm and Eu) exhibit solar ratios, i.e. [X/Fe]=~0, while Ba is overabundant by about 0.2-0.3dex. The origin of the overabundance of Ba is a puzzle. Within the formulation of the s-process, it is difficult to create a higher Ba abundance without a similar increase in the s-process contributions to other heavy elements (La-Sm). Given that Ba is represented by strong lines of Ba II and La-Sm are represented by rather weak ionized lines, the suggestion, as previously made by other studies, is that the Ba abundance may be systematically overestimated by standard methods of abundance analysis perhaps because the upper reaches of the stellar atmospheres are poorly represented by standard model atmospheres. A novel attempt to analyse the Ba I line at 5535{AA} gives a solar Ba abundance for stars with effective temperatures hotter than about 5800K but increasingly subsolar Ba abundances for cooler stars with apparent Ba deficiencies of 0.5dex at 5100K. This trend with temperature may signal a serious non-local thermodynamical equilibrium effect on the Ba I line.
2. Abundances for all seven stars in Latham 1
ID:
ivo://CDS.VizieR/J/AJ/152/176
Title:
Abundances for all seven stars in Latham 1
Short Name:
J/AJ/152/176
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present elemental abundances for all seven stars in Moving Group W11450 (Latham 1) to determine if they may be chemically related. These stars appear to be both spatially and kinematically related, but no spectroscopic abundance analysis exists in literature. Abundances for eight elements were derived via equivalent width analyses of high-resolution (R~60000), high-signal-to-noise ratio (<S/N> ~100) spectra obtained with the Otto Struve 2.1m telescope and the Sandiford Echelle Spectrograph at McDonald Observatory. The large star-to-star scatter in metallicity, -0.55{<=}[Fe/H]{<=}0.06dex ({sigma}=0.25), implies these stars were not produced from the same chemically homogeneous molecular cloud, and are therefore not part of a remnant or open cluster as previously proposed. Prior to this analysis, it was suggested that two stars in the group, W11449 and W11450, are possible wide binaries. The candidate wide binary pair show similar chemical abundance patterns with not only iron but with other elements analyzed in this study, suggesting the proposed connection between these two stars may be real.
3. Activity cycles in young solar-like stars
ID:
ivo://CDS.VizieR/J/A+A/606/A58
Title:
Activity cycles in young solar-like stars
Short Name:
J/A+A/606/A58
Date:
21 Oct 2021
Publisher:
CDS
Description:
Magnetic cycles analogous to the solar cycle have been detected in tens of solar-like stars by analyzing long-term time series of different magnetic activity indexes. The relationship between the cycle properties and global stellar parameters is not fully understood yet. One reason for this is the lack of long-term time series for stars covering a wide range of stellar parameters. We searched for activity cycles in a sample of 90 young solar-like stars with ages between 4 and 95 Myr with the aim to investigate the properties of activity cycles in this age range. We measured the length P_cyc_ of a given cycle by analyzing the long-term time series of three different activity indexes: the period of rotational modulation, the amplitude of the rotational modulation and the median magnitude in the V band. For each star, we also computed the global magnetic activity index <IQR> that is proportional to the amplitude of the rotational modulation and can be regarded as a proxy of the mean level of the surface magnetic activity. We detected activity cycles in 67 stars. Secondary cycles were also detected in 32 stars of the sample. The lack of correlation between P_cyc_ and P_rot_ and the position of our targets in the P_cyc_/P_rot_-Ro^-1^ diagram suggest that these stars belong to the so-called transitional branch and that the dynamo acting in these stars is different from the solar dynamo and from that acting in the older Mt. Wilson stars. This statement is also supported by the analysis of the butterfly diagrams whose patterns are very different from those seen in the solar case. We computed the Spearman correlation coefficient r_S_ between P_cyc_, <IQR> and various stellar parameters. We found that P_cyc_ in our sample is uncorrelated with all the investigated parameters. The <IQR> index is positively correlated with the convective turnover timescale, the magnetic diffusivity timescale {tau}_diff_, and the dynamo number D_N_, whereas it is anti-correlated with the effective temperature Teff, the photometric shear {Delta}{Omega}_phot_ and the radius R_C_ at which the convective zone is located. We investigated how P_cyc_ and <IQR> evolve with the stellar age. We found that P_cyc_ is about constant and that <IQR> decreases with the stellar age in the range 4-95Myr. Finally we investigated the magnetic activity of the star AB Dor A by merging All Sky Automatic Survey (ASAS) time series with previous long-term photometric data. We estimated the length of the AB Dor A primary cycle as P_cyc_=16.78+/-2yr and we also found shorter secondary cycles with lengths of 400d, 190d, and 90d, respectively.
4. Age estimates of SMC clusters
ID:
ivo://CDS.VizieR/J/MNRAS/478/784
Title:
Age estimates of SMC clusters
Short Name:
J/MNRAS/478/784
Date:
21 Oct 2021
Publisher:
CDS
Description:
The Small Magellanic Cloud (SMC) has recently been found to harbour an increase of more than 200 per cent in its known cluster population. Here, we provide solid evidence that this unprecedented number of clusters could be greatly overestimated. On the one hand, the fully automatic procedure used to identify such an enormous cluster candidate sample did not recover ~50 per cent, on average, of the known relatively bright clusters located in the SMC main body. On the other hand, the number of new cluster candidates per time unit as a function of time is noticeably different from the intrinsic SMC cluster frequency (CF), which should not be the case if these new detections were genuine physical systems. We found additionally that the SMC CF varies spatially, in such a way that it resembles an outside-in process coupled with the effects of a relatively recent interaction with the Large Magellanic Cloud. By assuming that clusters and field stars share the same formation history, we showed for the first time that the cluster dissolution rate also depends on position in the galaxy. The cluster dissolution becomes higher as the concentration of galaxy mass increases or if external tidal forces are present.
5. Ages and masses of LMC clusters
ID:
ivo://CDS.VizieR/J/MNRAS/436/136
Title:
Ages and masses of LMC clusters
Short Name:
J/MNRAS/436/136
Date:
21 Oct 2021
Publisher:
CDS
Description:
Whether or not the rich star cluster population in the Large Magellanic Cloud (LMC) is affected by significant disruption during the first few x10^8^yr of its evolution is an open question and the subject of significant current debate. Here, we revisit the problem, adopting a homogeneous data set of broad-band imaging observations. We base our analysis mainly on two sets of self-consistently determined LMC cluster ages and masses, one using standard modelling and one which takes into account the effects of stochasticity in the clusters' stellar mass functions. On their own, the results based on any of the three complementary analysis approaches applied here are merely indicative of the physical conditions governing the cluster population. However, the combination of our results from all three different diagnostics leaves little room for any conclusion other than that the optically selected LMC star cluster population exhibits no compelling evidence of significant disruption - for clusters with masses, M_cl_, of log(M_cl_/M_{sun}_)>~3.0-3.5 - between the age ranges of [3-10 and 30-100]Myr, either 'infant mortality' or otherwise. In fact, there is no evidence of any destruction beyond that expected from simple models just including stellar dynamics and stellar evolution for ages up to 1Gyr. It seems, therefore, that the difference in environmental conditions in the Magellanic Clouds on the one hand and significantly more massive galaxies on the other may be the key to understanding the apparent variations in cluster disruption behaviour at early times.
6. Ages & luminosities of young SMC/LMC star clusters
ID:
ivo://CDS.VizieR/J/A+A/517/A50
Title:
Ages & luminosities of young SMC/LMC star clusters
Short Name:
J/A+A/517/A50
Date:
21 Oct 2021
Publisher:
CDS
Description:
In this paper we discuss the age and spatial distribution of young (age<1Gyr) SMC and LMC clusters using data from the Magellanic Cloud Photometric Surveys. Luminosities are calculated for all age-dated clusters. Ages of 324 and 1193 populous star clusters in the Small and the Large Magellanic Cloud have been determined fitting Padova and Geneva isochrone models to their resolved color-magnitude diagrams. The clusters cover an age range between 10Myr and 1Gyr in each galaxy. For the SMC a constant distance modulus of (m-M)_0_=18.90 and a metallicity of Z=0.004 were adopted. For the LMC, we used a constant distance modulus of (m-M)_0_=18.50 and a metallicity of Z=0.008. For both galaxies, we used a variable color excess to derive the cluster ages. We find two periods of enhanced cluster formation in both galaxies at 160Myr and 630Myr (SMC) and at 125Myr and 800Myr (LMC). We present the spatially resolved recent star formation history of both Clouds based on young star clusters. The first peak may have been triggered by a close encounter between the SMC and the LMC. In both galaxies the youngest clusters reside in the supergiant shells, giant shells, the inter-shell regions, and toward regions with a high H\alpha content, suggesting that their formation is related to expansion and shell-shell interaction. Most of the clusters are older than the dynamical age of the supergiant shells. No evidence for cluster dissolution was found. Computed V band luminosities show a trend for fainter magnitudes with increasing age as well as a trend for brighter magnitudes with increasing apparent cluster radii.
7. Ages of SMC young clusters and field stars
ID:
ivo://CDS.VizieR/J/A+A/452/179
Title:
Ages of SMC young clusters and field stars
Short Name:
J/A+A/452/179
Date:
21 Oct 2021
Publisher:
CDS
Description:
In this paper we discuss the cluster and field star formation in the central part of the Small Magellanic Cloud. The main goal is to study the correlation between young objects and their interstellar environment. The ages of about 164 associations and 311 clusters younger than 1Gyr are determined using isochrone fitting. The spatial distribution of the clusters is compared with the HI maps, with the HI velocity dispersion field, with the location of the CO clouds and with the distribution of young field stars.
8. Ages of star clusters in M33
ID:
ivo://CDS.VizieR/J/ApJ/700/103
Title:
Ages of star clusters in M33
Short Name:
J/ApJ/700/103
Date:
21 Oct 2021
Publisher:
CDS
Description:
We present a result of age estimation for star clusters in M33. We obtain color-magnitude diagrams (CMDs) of resolved stars in 242 star clusters from the Hubble Space Telescope/Wide Field Planetary Camera 2 images. We estimate ages of 100 star clusters among these, by fitting the Padova theoretical isochrones to the observational CMDs. Age distribution of the star clusters shows a dominant peak at log(t)~7.8. Majority of star clusters are younger than log(t)=9.0, while 10 star clusters are older than log(t)~9.0. There is only one cluster younger than log(t)=7 in this study, which is in contrast with the results based on the integrated photometry of star clusters in the previous studies. Radial distribution of the cluster ages shows that young- to intermediate-age clusters are found from the center to the outer region, while old clusters are distributed farther from the M33 center. We briefly discuss the implication of the results with regard to the formation of the M33 cluster system.
9. Agglomerate of early-type Hipparcos stars
ID:
ivo://CDS.VizieR/J/AN/329/801
Title:
Agglomerate of early-type Hipparcos stars
Short Name:
J/AN/329/801
Date:
21 Oct 2021
Publisher:
CDS
Description:
We study the spatial structure and sub-structure of regions rich in Hipparcos stars with blue BT-VT colours. These regions, which comprise large stellar complexes, OB associations, and young open clusters, are tracers of on-going star formation in the Galaxy. The DBSCAN (Density-Based Spatial Clustering of Applications with Noise) data clustering algorithm is used to look for spatial overdensities of early-type stars. Once an overdensity, "agglomerate", is identified, we carry out a data and bibliographic compilation of their star member candidates. The actual membership in agglomerate of each early-type star is studied based on its heliocentric distance, proper motion, and previous spectro-photometric information. We identify 35 agglomerates of early-type Hipparcos stars. Most of them are associated to previously known clusters and OB associations. The previously unknown P Puppis agglomerate is subject of a dedicated study with Virtual Observatory tools.
10. A large moving group within the LCC association
ID:
ivo://CDS.VizieR/J/ApJ/868/32
Title:
A large moving group within the LCC association
Short Name:
J/ApJ/868/32
Date:
21 Oct 2021
Publisher:
CDS
Description:
Scorpius-Centaurus is the nearest OB association, and its hundreds of members are divided into subgroups, including the Lower Centaurus Crux (LCC). Here we study the dynamics of the LCC area. We report the revelation of a large moving group containing more than 1800 intermediate- and low-mass young stellar objects and brown dwarfs that escaped identification until Gaia DR2 allowed a kinematic and photometric selection to be performed. We investigate the stellar and substellar content of this moving group using the Gaia DR2 astrometric and photometric measurements. The median distance of the members is 114.5pc, and 80% lie between 102 and 135pc from the Sun. Our new members cover a mass range of 0.02-5M_{sun}_ and add up to a total mass of about 700M_{sun}_. The present-day mass function follows a log-normal law with m_c_=0.22M_{sun}_ and {sigma}=0.64. We find more than 200 brown dwarfs in our sample. The star formation rate had its maximum of 8x10^-5^M_{sun}_/yr about 9Myr ago. We grouped the new members into four denser subgroups, which have increasing age from 7 to 10Myr, surrounded by "free-floating" young stars with mixed ages. Our isochronal ages, now based on accurate parallaxes, are compatible with several earlier studies of the region. The whole complex is presently expanding, and the expansion started between 8 and 10Myr ago. Two hundred members show infrared excess compatible with circumstellar disks from full to debris disks. This discovery provides a large sample of nearby young stellar and substellar objects for disk and exoplanet studies.

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