- 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.
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- 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/ApJS/231/1
- Title:
- Hot DA white dwarfs grid of synthetic spectra
- Short Name:
- J/ApJS/231/1
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In this work we present a grid of LTE and non-LTE synthetic spectra of hot DA white dwarfs (WDs). In addition to its usefulness for the determination of fundamental stellar parameters of isolated WDs and in binaries, this grid will be of interest for the construction of theoretical libraries for stellar studies from integrated light. The spectral grid covers both a wide temperature and gravity range, with 17000K<=Teff<=100000K and 7.0<=log(g)<=9.5. The stellar models are built for pure hydrogen and the spectra cover a wavelength range from 900{AA} to 2.5um. Additionally, we derive synthetic HST/ACS, HST/WFC3, Bessel UBVRI, and SDSS magnitudes. The grid was also used to model integrated spectral energy distributions of simple stellar populations and our modeling suggests that DAs might be detectable in ultraviolet bands for populations older than ~8Gyr.
- 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>).
- ID:
- ivo://CDS.VizieR/J/AJ/130/1693
- Title:
- HST-ACS observations of 5 globular clusters
- Short Name:
- J/AJ/130/1693
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present color-magnitude diagrams of five Galactic globular clusters and one Galactic open cluster spanning a wide range of metallicity (-2.1<~[Fe/H]<~+0.3), as observed in the F606W (broad V) and F814W (I) bands with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. These clusters are part of two large ACS programs measuring the star formation history in the Andromeda halo, tidal stream, and outer disk. In these programs, the clusters serve as empirical isochrones and as calibrators for the transformation of theoretical isochrones to the ACS bandpasses. To make these data more accessible to the community, for each cluster we provide a ridgeline tracing the stars on the main sequence, subgiant branch, and red giant branch, plus the locus of stars on the horizontal branch. In addition, we provide the transformation of the Victoria-Regina isochrones to the ACS bandpasses.
- ID:
- ivo://CDS.VizieR/J/A+A/507/1375
- Title:
- HST/ACS VI data of M31 globular clusters
- Short Name:
- J/A+A/507/1375
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- With the aim of increasing the sample of M31 clusters for which a colour-magnitude diagram is available, we searched the HST archive for ACS images containing objects included in the Revised Bologna Catalogue of M31 globular clusters (RBC Version 3.5 available at: http://www.bo.astro.it/M31). Sixty-three such objects were found. We used the ACS images to confirm or revise their classification and were able to obtain useful CMDs for 11 old globular clusters and 6 luminous young clusters. We obtained simultaneous estimates of the distance, reddening, and metallicity of old clusters by comparing their observed field-decontaminated CMDs with a grid of template clusters of the Milky Way. We estimated the age of the young clusters by fitting with theoretical isochrones.
- ID:
- ivo://CDS.VizieR/J/ApJ/865/33
- Title:
- HST and Chandra obs. of the GLIMPSE-C01 star cluster
- Short Name:
- J/ApJ/865/33
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report the results of Hubble Space Telescope and Chandra X-Ray Observatory observations of the GLIMPSE-C01 (hereafter GC01) star cluster. Color-magnitude and color-color diagrams suggest a cluster age of >~2Gyr up to ~10Gyr (dependent on GC01's metallicity), a distance of 3.3-3.5kpc, and strong differential reddening with A_V_=14-22. After performing astrometric corrections, we find that nine of the 15 X-ray sources have at least one near-infrared (NIR) counterpart within the 2{sigma} Chandra positional error circles. However, given the very high density of NIR sources in the cluster, most of these counterparts are likely due to chance coincidence. We jointly analyze the X-ray and NIR properties to assess the likelihood of true associations. Based primarily on their X-ray properties, we identify a low-mass X-ray binary candidate (source X2), a cataclysmic variable (CV) candidate (source X1), and an active binary (AB) candidate (source X9). Source X11 is detected during an X-ray flaring episode with a flare luminosity (L_X_=2.1x10^33^erg/s) and has a quiescent luminosity L_X_<8.0x10^30^erg/s, in 0.5-8keV at the distance of GC01, suggesting that the source is either an AB or CV. We also discuss the limits on an intermediate-mass black hole at the center of GC01 and the challenges of X-ray source classification imposed by the limitations of the existing data and instrumentation along with future prospects in the James Webb Space Telescope era.
- ID:
- ivo://CDS.VizieR/J/ApJ/738/186
- Title:
- HST and DEIMOS measurements of NGC 2419 stars
- Short Name:
- J/ApJ/738/186
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the analysis of a kinematic data set of stars in the globular cluster NGC 2419, taken with the DEep Imaging Multi-Object Spectrograph at the Keck II telescope. Combined with a reanalysis of deep Hubble Space Telescope and Subaru Telescope imaging data, which provide an accurate luminosity profile of the cluster, we investigate the validity of a large set of dynamical models of the system, which are checked for stability via N-body simulations.
