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Register a new userIntrinsic Metallicity Variation in the Intermediate Mass Type II Globular Cluster NGC 1261
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Globular Clusters (GCs) are now well known to almost universally show multiple popu-lations (MPs). The HST UV Legacy Survey of a large number of Galactic GCs in UV filters optimized to explore MPs finds that a small fraction of GCs, termed Type II, also display more complex, anomalous behavior. Several well-studied Type II GCs show intrinsic Fe abundance variations, suggesting that the other, less well-studied, Type II GCs should also exhibit similar behavior. Our aim is to perform the first detailed metallicity analysis of NGC 1261, an intermediate mass Type II GC, in order to determine if this object shows an intrinsic Fe variation. We determined the Fe abundance in eight red giant members using Magellan-MIKE and UVES-FLAMES high-resolution, high S/N spectroscopy. The full range of [Fe/H] for the entire sample from the spectra is from -1.05 to -1.43 dexwith an observed spread sigma_obs=0.133 dex. Compared with the total internal error of Sigma_tot=0.06,this indicates a significant intrinsic metallicity spread of Sigma_int=0.119 dex. We found a very similar variation in [Fe/H] using an independent method to derive the atmospheric parameters based on near-IR photometry. More importantly, the mean metallicity of the five presumed normal metallicity stars is -1.37+/-0.02, while that of the three presumed anomalous/highmetallicity stars is -1.18+/-0.09. This difference is significant at the $pm$2.4Sigma level. We find indications from existing data of other Type II GCs that several of them presumedto have real metallicity spreads may in fact posses none. The minimum mass required for a GC to acquire an intrinsic Fe spread appears to be $pm$10^5 Msun. We find no strong correlation betwee nmass and metallicity variation for Type II GCs. The metallicity spread is also independent of the fraction of anomalous stars within the Type II GCs and of GC origin.
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(Abriged)This work studies in more detail the stellar population, including its photometric properties and characteristics, in the rarely studied southern Galactic globular cluster NGC 1261. We focus on the brighter sequences of the clusters color-magnitude diagram (CMD). Like in our previous works, we rely upon photometry in several passbands to achieve more reliable results and conclusions. We carried out and analyzed new multi-color photometry of NGC 1261 in UBVI reaching below the turnoff point in all passbands in a fairly extended cluster field, about 14x14. We found several signs of the inhomogeneity (multiplicity) in the stellar population. The most prominent of them are: (1) the dependence of the radial distribution of sub-giant branch (SGB) stars in the cluster on their U magnitude, with brighter stars less centrally concentrated at the 99.9 % level than their fainter counterparts; (2) the dependence of the location of red giant branch (RGB) stars in the U-(U-B) CMD on their radial distance from the cluster center, with the portion of stars bluer in the (U-B) color increasing towards the cluster outskirts. Additionally, the radial variation of the RGB luminosity function in the bump region is suspected. We assume that both the SGB stars brighter in the U and the RGB stars bluer in the (U-B) color are probably associated with blue horizontal branch stars, because of a similarity in their radial distribution in the cluster. We estimated the metalicity of NGC 1261 from the slope of the RGB in U-based CMDs and the location of the RGB bump on the branch. These metallicity indicators give [Fe/H]zw = -1.34 +/- 0.16 dex and [Fe/H]zw = -1.41 +/- 0.10 dex, respectively. We isolated 18 probable blue straggler candidates. They are more centrally concentrated than the lower red giants of comparable brightness at the 97.9 % level.
We used a combination of Hubble Space Telescope and ground based data to probe the dynamical state of the low mass Galactic globular cluster NGC 6101. We have re-derived the structural parameters of the cluster by using star counts and we find that it is about three times more extended than thought before. By using three different indicators, namely the radial distribution of Blue Straggler Stars, that of Main Sequence binaries and the luminosity (mass) function, we demonstrated that NGC 6101 shows no evidence of mass segregation, even in the innermost regions. Indeed, both the BSS and the binary radial distributions fully resemble that of any other cluster population. In addition the slope of the luminosity (mass) functions does not change with the distance, as expected for non relaxed stellar systems. NGC 6101 is one of the few globulars where the absence of mass segregation has been observed so far. This result provides additional support to the use of the dynamical clock calibrated on the radial distribution of the Blue Stragglers as a powerful indicator of the cluster dynamical age.
Using the Next Generation Very Large Array (ngVLA), we will make a comprehensive inventory of intermediate-mass black holes (IMBHs) in hundreds of globular cluster systems out to a distance of 25 Mpc. IMBHs have masses of about 100 to 100,000 solar masses. Finding them in globular clusters would validate a formation channel for seed black holes in the early universe and inform event predictions for gravitational wave facilities. Reaching a large number of globular clusters is key, as Fragione et al. (2018) predict that only a few percent will have retained their gravitational-wave fostering IMBHs.
Intermediate-mass black holes (IMBHs) are of interest in a wide range of astrophysical fields. In particular, the possibility of finding them at the centers of globular clusters has recently drawn attention. IMBHs became detectable since the quality of observational data sets, particularly those obtained with HST and with high resolution ground based spectrographs, advanced to the point where it is possible to measure velocity dispersions at a spatial resolution comparable to the size of the gravitational sphere of influence for plausible IMBH masses. We present results from ground based VLT/FLAMES spectroscopy in combination with HST data for the globular cluster NGC 6388. The aim of this work is to probe whether this massive cluster hosts an intermediate-mass black hole at its center and to compare the results with the expected value predicted by the $M_{bullet} - sigma$ scaling relation. The spectroscopic data, containing integral field unit measurements, provide kinematic signatures in the center of the cluster while the photometric data give information of the stellar density. Together, these data sets are compared to dynamical models and present evidence of an additional compact dark mass at the center: a black hole. Using analytical Jeans models in combination with various Monte Carlo simulations to estimate the errors, we derive (with 68% confidence limits) a best fit black-hole mass of $ (17 pm 9) times 10^3 M_{odot}$ and a global mass-to-light ratio of $M/L_V = (1.6 pm 0.3) M_{odot}/L_{odot}$.
We present the UV photometry of the globular cluster NGC 1261 using images acquired with the Ultraviolet Imaging Telescope (UVIT) on-board ASTROSAT. We performed PSF photometry on four near-UV (NUV) and two far-UV (FUV) images and constructed UV colour-magnitude diagrams (CMDs), in combination with HST, Gaia, and ground-based optical photometry for member stars. We detected the full horizontal branch (HB) in NUV, blue HB in the FUV and identified two extreme HB (EHB) stars. HB stars have a tight sequence in UV-optical CMDs well-fitted with isochrones generated (12.6 Gyr age, [Fe/H] = -1.27 metallicity) using updated BaSTI-IAC models. Effective temperatures (Teff), luminosities and radii of bright HB stars were estimated using spectral energy distribution. As we detect the complete sample of UV bright HB stars, the hot end of the HB distribution is found to terminate at the G-jump (Teff ~ 11500 K). The two EHB stars, fitted well with single spectra, have Teff= 31,000 K and a mass = 0.495Msun and follow the same Teff-Radius relation of the blue HB stars. We constrain the formation pathways of these EHB stars to extreme mass loss in the RGB phase (either due to rotation or enhanced Helium), OR early hot-flash scenario.
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