- ID:
- ivo://CDS.VizieR/J/ApJ/735/55
- Title:
- High-resolution GCs abundances. III. LMC
- Short Name:
- J/ApJ/735/55
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In this paper, we refine our method for the abundance analysis of high-resolution spectroscopy of the integrated light of unresolved globular clusters (GCs). This method was previously demonstrated for the analysis of old (>10Gyr) Milky Way (MW) GCs. Here, we extend the technique to young clusters using a training set of nine GCs in the Large Magellanic Cloud. Depending on the signal-to-noise ratio of the data, we use 20-100 Fe lines per cluster to successfully constrain the ages of old clusters to within a ~5Gyr range, the ages of ~2Gyr clusters to a 1-2Gyr range, and the ages of the youngest clusters (0.05-1Gyr) to a ~200Myr range. We also demonstrate that we can measure [Fe/H] in clusters with any age less than 12Gyr with similar or only slightly larger uncertainties (0.1-0.25dex) than those obtained for old MW GCs (0.1dex); the slightly larger uncertainties are due to the rapid evolution in stellar populations at these ages. In this paper, we present only Fe abundances and ages. For several of the clusters in this sample, there are no high-resolution abundances in the literature from individual member stars; our results are the first detailed chemical abundances available.
Number of results to display per page
Search Results
- ID:
- ivo://CDS.VizieR/J/ApJ/816/70
- Title:
- High resolution spectra of 3 NGC104 member stars
- Short Name:
- J/ApJ/816/70
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Globular clusters are known to host peculiar objects named blue straggler stars (BSSs), significantly heavier than the normal stellar population. While these stars can be easily identified during their core hydrogen-burning phase, they are photometrically indistinguishable from their low-mass sisters in advanced stages of the subsequent evolution. A clear-cut identification of these objects would require the direct measurement of the stellar mass. We used the detailed comparison between chemical abundances derived from neutral and from ionized spectral lines as a powerful stellar "weighing device" to measure stellar mass and to identify an evolved BSS in 47Tucanae. In particular, high-resolution spectra of three bright stars, located slightly above the level of the "canonical" horizontal branch (HB) sequence in the color-magnitude diagram of 47 Tucanae, have been obtained with the UVES spectrograph. The measurements of iron and titanium abundances performed separately from neutral and ionized lines reveal that two targets have stellar parameters fully consistent with those expected for low-mass post-HB objects, while for the other target the elemental ionization balance is obtained only by assuming a mass of ~1.4M_{sun}_, which is significantly larger than the main sequence turn-off mass of the cluster (~0.85M_{sun}_). The comparison with theoretical stellar tracks suggests that this is a BSS descendant possibly experiencing its core helium-burning phase. The large applicability of the proposed method to most of the globular clusters in our Galaxy opens the possibility to initiate systematic searches for evolved BSSs, thus giving access to still unexplored phases of their evolution.
- ID:
- ivo://CDS.VizieR/J/ApJ/832/99
- Title:
- HK', CN & CH chemical indexes in NGC 362 & NGC 6723
- Short Name:
- J/ApJ/832/99
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Most globular clusters (GCs) are now known to host multiple stellar populations with different abundances of light elements. Here we use narrow-band photometry and low-resolution spectroscopy for NGC 362 and NGC 6723 to investigate their chemical properties and radial distributions of subpopulations. We confirm that NGC 362 and NGC 6723 are among the GCs with multiple populations showing bimodal CN distribution and CN-CH anticorrelation without a significant spread in calcium abundance. These two GCs show more centrally concentrated CN-weak, earlier generation stars compared to the CN-strong, later generation stars. These trends are reversed with respect to those found in previous studies for many other GCs. Our findings, therefore, seem contradictory to the current scenario for the formation of multiple stellar populations, but mass segregation acting on the two subpopulations might be a possible solution to explain this reversed radial trend.
- ID:
- ivo://CDS.VizieR/J/A+A/563/A13
- Title:
- Horizontal branch stars in M22
- Short Name:
- J/A+A/563/A13
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We obtained high-resolution spectra for 94 candidate stars belonging to the HB of M 22 with FLAMES. Previous works have indicated that this cluster has split subgiant (SGB) and red giant branches (RGB) and hosts two different stellar populations, differing in overall metal abundance and both exhibiting a Na-O anti-correlation. The HB stars we observed span a restricted temperature range (7800<Teff<11000K), where about 60% of the HB stars of M 22 are. Within our sample, we can distinguish three groups of stars segregated (though contiguous) in colours: Group 1 (49 stars) is metal-poor, N-normal, Na-poor and O-rich: our abundances for this (cooler) group match very well those determined for the primordial group of RGB stars (a third of the total) from previous studies. Group 2 (23 stars) is still metal-poor, but it is N- and Na-rich, though only very mildly depleted in O. We can identify this intermediate group as the progeny of the metal-poor RGB stars that occupy an intermediate location along the Na-O anti-correlation and include about 10% of the RGB stars. The third group (20 stars) is metal-rich, Na-rich, and O-rich. This hotter group likely corresponds to the most O-rich component of the previously found metal-rich RGB population (a quarter of the total). We did not observe any severely O-depleted stars and we think that the progeny of these stars falls on the hotter part of the HB. Furthermore, we found that the metal-rich population is also over-abundant in Sr, in agreement with results for corresponding RGB and SGB stars. However, we do not find any significant variation in the ratio between the sum of N and O abundances to Fe. We do not have C abundances for our stars. There is some evidence of an enhancement of He content for Groups 2 and 3 stars (Y=0.338+/-0.014+/-0.05); the error bar due to systematics is large, but a consistent analysis of data for several GCs confirms that stars in these groups within M22 are likely overabundant in He. We conclude that on the whole, our results agree with the proposition that chemical composition drives the location of stars along the HB of a GC. Furthermore, we found a number of fast rotators. They are concentrated in a restricted temperature range along the HB of M22. Fast rotating stars might be slightly less massive and bluer than slowly rotating ones, but other interpretations are possible.
- ID:
- ivo://CDS.VizieR/J/A+A/539/A19
- Title:
- Horizontal branch stars in NGC 1851
- Short Name:
- J/A+A/539/A19
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The chemical composition of horizontal branch (HB) stars might help to clarify the formation history of individual globular clusters (GCs). We studied the Na-O anti-correlation from moderately high resolution spectra for 91 stars on the bimodal HB of NGC 1851; in addition we observed 13 stars on the lower red giant branch (RGB). In our HB sample, 35 stars are on the blue HB (BHB), one is an RR Lyrae, and 55 stars are on the red HB (RHB). The ratio of BHB to RHB stars is close to the total in the cluster (35 and 54%, respectively), while RR Lyrae variables are under-represented, (they are ~12% of the NGC 1851 stars). We also derived abundances for He and N in BHB stars.
- ID:
- ivo://CDS.VizieR/J/A+A/573/A92
- Title:
- Horizontal branch stars in NGC 6723
- Short Name:
- J/A+A/573/A92
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We used FLAMES+GIRAFFE (Medusa mode) at the VLT to obtain moderately high resolution spectra for 30 red horizontal branch (RHB) stars, 4 RR Lyrae variables, and 17 blue horizontal branch (BHB) stars in the low-concentration, moderately metal-rich globular cluster NGC 6723 ([Fe/H]=-1.22+/-0.08 from our present sample). The spectra were optimized to derive O and Na abundances. In addition, we obtained abundances for other elements, including N, Fe, Mg, Ca, Ni, and Ba. We used these data to discuss the evidence of a connection between the distribution of stars along the horizontal branch (HB) and the multiple populations that are typically present in globular clusters. We found that all RHB and most (13 out of 17) BHB stars are O-rich, Na-poor, and N-poor; these stars probably belong to the first stellar generation in this cluster. Only the four warmest observed stars are (moderately) O-poor, Na-rich, and N-rich, and they probably belong to the second generation. While our sample is not fully representative of the whole HB population in NGC 6723, our data suggest that in this cluster only HB stars warmer than ~9000K, that is one fourth of the total, belong to the second generation, if at all. Since in many other clusters this fraction is about two thirds, we conclude that the fraction of first/second generation in globular clusters may be strongly variable. In addition, the wide range in colour of chemically homogeneous first-generation HB stars requires a considerable spread in mass loss (>0.10M_{sun}_). The reason for this spread is yet to be understood. Finally, we found a high Ba abundance, with a statistically significant radial abundance gradient.
- ID:
- ivo://CDS.VizieR/J/A+A/534/A123
- Title:
- Horizontal branch stars in NGC 2808
- Short Name:
- J/A+A/534/A123
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Globular clusters have been recognized to host multiple stellar populations. A spectacular example of this is the massive cluster NGC 2808, where multiple populations have been found along the horizontal branch (HB) and the main sequence (MS). Studies of red giants showed that this cluster appears homogeneous insofar Fe abundance is concerned, but it shows an extended anticorrelation between Na and O abundances. The Na-poor, O-rich population can be identified with the red MS, and the Na-rich, O-poor one with the blue one. This may be understood in terms of different He content, He being correlated with Na. A prediction of this scenario is that He-rich, Na-rich He-core burning stars, because they are less massive, will end up on the bluer part of the HB, while He-poor, Na-poor stars will reside on the red HB. The aim of this paper is to verify this prediction. To this purpose, we acquired high-resolution spectra of regions including strong O and Na lines in several tens of HB stars of NGC 2808, sampling both the red and blue parts of the HB.
- ID:
- ivo://CDS.VizieR/J/A+A/549/A41
- Title:
- Horizontal branch stars in 47 Tuc and M5
- Short Name:
- J/A+A/549/A41
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- To check the impact of the multiple population scenario for globular clusters on their horizontal branch (HB), we present an analysis of the composition of 110 red HB (RHB) stars in 47 Tucanae and of 61 blue HB (BHB) and 30 RHB stars in M5. In 47 Tuc we found tight relations between the colours of the stars and their abundances of p-capture elements. This strongly supports the idea that the He content - which is expected to be closely correlated with the abundances of p-capture elements - is the third parameter (after overall metallicity and age) that determines the colour of HB stars. However, the range in He abundance must be small ({Delta}Y<0.03) in 47 Tuc to reproduce our observations; this agrees with previous analyses. There is possibly a correlation between the abundances of p- and n-capture elements in 47 Tuc. If confirmed, this might suggest that asymptotic giant branch stars of moderate mass contributed to the gas from which second-generation stars formed. Considering the selection effects in our sample (which does not include stars warmer than 11000K and RR Lyrae variables, which were excluded because we could not obtain accurate abundances with the adopted observing procedure) is important to understand our results for M5. In this case, we find that, as expected, RHB stars are Na-poor and O-rich, and likely belong to the primordial population. There is a clear correlation of the [Na/O] ratio and N abundance with colour along the BHB. A derivation of the He abundance for these stars yields a low value of Y=0.22+/-0.03. This is expected because HB stars of a putative He-rich population in this cluster should be warmer than 11000K, and would accordingly not have been sampled by our analysis. However, we need some additional source of scatter in the total mass loss of stars climbing up the red giant branch to reproduce our results for M5. Finally, we found a C-star on the HB of 47Tuc and a Ba-rich, fast-rotating, likely binary star on the HB of M5. These stars are among the brightest and coolest HB stars.
- ID:
- ivo://CDS.VizieR/J/A+A/526/A136
- Title:
- Hot HB stars in {omega} Cen
- Short Name:
- J/A+A/526/A136
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- UV observations of some massive globular clusters have revealed a significant population of stars hotter and fainter than the hot end of the horizontal branch (HB), the so-called blue hook stars. This feature might be explained either by the late hot flasher scenario where stars experience the helium flash while on the white dwarf cooling curve or by the progeny of the helium-enriched sub-population postulated to exist in some clusters. Previous spectroscopic analyses of blue hook stars in omega Cen and NGC 2808 support the late hot flasher scenario, but the stars contain much less helium than expected and the predicted C and N enrichment cannot be verified. We compare the observed effective temperatures, surface gravities, helium abundances, and carbon line strengths (where detectable) of our targets stars with the predictions of the two aforementioned scenarios.
- ID:
- ivo://CDS.VizieR/J/AJ/131/866
- Title:
- Hot populations in M87 globular clusters
- Short Name:
- J/AJ/131/866
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- To explore the production of UV-bright stars in old, metal-rich populations like those in elliptical galaxies, we have obtained Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph far- and near-UV photometry of globular clusters (GCs) in four fields in the giant elliptical (gE) galaxy M87. To a limit of m_FUV_~25 we detect a total of 66 GCs in common with the deep HST optical-band study of Kundu et al. (1999, Cat. <J/ApJ/513/733>).
