Globular Cluster M15 Research Articles

Zwicky Transient Facility and Globular Clusters: The Period–Luminosity and Period–Wesenheit Relations for Anomalous Cepheids Supplemented with Large Magellanic Cloud Sample

We present the first gri-band period–luminosity (PL) and period–Wesenheit (PW) relations for the fundamental mode anomalous Cepheids. These PL and PW relations were derived from a combined sample of five anomalous Cepheids in globular cluster M92 and the Large Magellanic Cloud, both of which have distance accurate to ∼1% available from literature. Our g-band PL relation is similar to the B-band PL relation as reported in previous study. We applied our PL and PW relations to anomalous Cepheids discovered in dwarf galaxy Crater II, and found a larger but consistent distance modulus than the recent measurements based on RR Lyrae. Our calibrations of gri-band PL and PW relations, even though less precise due to small number of anomalous Cepheids, will be useful for distance measurements to dwarf galaxies.

Investigating the UV-excess in Star Clusters with N-body Simulations: Predictions for Future CSST Observations* *Supported by the research grants from the China Manned Space Project with No. CMS-CSST-2021-A08.

We study the origin of the UV-excess in star clusters by performing N-body simulations of six clusters with N = 10 k and N = 100 k (single stars & binary systems) and metallicities of Z = 0.01, 0.001 and 0.0001, using petar. All models initially have a 50% primordial binary fraction. Using GalevNB we convert the simulated data into synthetic spectra and photometry for the China Space Station Telescope (CSST) and Hubble Space Telescope (HST). From the spectral energy distributions we identify three stellar populations that contribute to the UV-excess: (1) second asymptotic giant branch stars, which contribute to the UV flux at early times; (2) naked helium stars and (3) white dwarfs, which are long-term contributors to the FUV spectra. Binary stars consisting of a white dwarf and a main sequence star are cataclysmic variable (CV) candidates. The magnitude distribution of CV candidates is bimodal up to 2 Gyr. The bright CV population is particularly bright in FUV − NUV. The FUV − NUV color of our model clusters is 1–2 mag redder than the UV-excess globular clusters in M87 and in the Milky Way. This discrepancy may be induced by helium enrichment in observed clusters. Our simulations are based on simple stellar evolution; we do not include the effects of variations in helium and light elements or multiple stellar populations. A positive radial color gradient is present in CSST NUV − y for main sequence stars in all models with a color difference of 0.2–0.5 mag, up to 4 half-mass radii. The CSST NUV − g color correlates strongly with HST FUV − NUV for NUV − g > 1 mag, with the linear relation FUV − NUV =(1.09 ± 0.12) × (NUV − g) + (−1.01 ± 0.22). This allows for conversion of future CSST NUV − g colors into HST FUV − NUV colors, which are sensitive to UV-excess features. We find that CSST will be able to detect UV-excess in Galactic/extragalactic star clusters with ages >200 Myr.

Slowly Cooling White Dwarfs in NGC 6752

Recently, a new class of white dwarfs (“slowly cooling WDs”) has been identified in the globular cluster M13. The cooling time of these stars is increased by stable thermonuclear hydrogen burning in their residual envelope. These WDs are thought to be originated by horizontal branch (HB) stars populating the HB blue tail that skipped the asymptotic giant branch phase. To further explore this phenomenon, we took advantage of deep photometric data acquired with the Hubble Space Telescope in the near-ultraviolet and investigate the bright portion of the WD cooling sequence in NGC 6752, another Galactic globular cluster with a metallicity, age, and HB morphology similar to M13. The normalized WD luminosity function derived in NGC 6752 turns out to be impressively similar to that observed in M13, in agreement with the fact that the stellar mass distribution along the HB of these two systems is almost identical. As in the case of M13, the comparison with theoretical predictions is consistent with ∼70% of the investigated WDs evolving at slower rates than standard, purely cooling WDs. Thanks to its relatively short distance from Earth, NGC 6752 photometry reaches a luminosity 1 order of a magnitude fainter than the case of M13, allowing us to sample a regime where the cooling time delay, with respect to standard WD models, reaches ∼300 Myr. The results presented in this paper provide new evidence for the existence of slowly cooling WDs and further support to the scenario proposing a direct causal connection between this phenomenon and the HB morphology of the host stellar cluster.

Asteroseismology of the multiple stellar populations in the globular cluster M4

We present a new asteroseismic analysis of the stars in the globular cluster (GC) M4 based on the data collected by the K2 mission. We report the detection of solar-like oscillation in 37 stars, 32 red giant branch (RGB) and six red horizontal branch (rHB) stars, which is the largest sample for this kind of study in GCs up to date. Combining information from asteroseismology and multi-band photometry, we estimate both the masses and the radii of our targets. Our estimates are in agreement with independent sources, serving as a crucial verification of asteroseismology in the low metallicity regime. As M4 is an old GC, it hosts multiple stellar populations differing in light element abundances and in helium mass fraction. This generates a mass difference between the populations along the RGB, which in the case of M4 is estimated to be 0.017 M⊙. With this wealth of information, we can assign population membership and estimate the average mass of the stellar populations; however, the current uncertainties do not allow us to resolve this mass difference. The population membership and the seismic data of RGB and HB stars allow us, however, to assess the integrated mass loss along the RGB of the first generation stars in the cluster. We obtain ΔM = 0.227 ± 0.028 M⊙, which is in good agreement with independent estimates. Finally, we observe the presence of a statistically significant mass-temperature gradient in the rHB stars. This represents the first direct, model-independent observation of the colour-temperature-mass correlation predicted by the theory.

Far-ultraviolet investigation into the galactic globular cluster M30 (NGC 7099) – II. Potential X-ray counterparts and variable sources

ABSTRACT We present a far-ultraviolet (FUV) study of the globular cluster M30 (NGC 7099). The images were obtained using the Advanced Camera for Surveys (ACS/SBC, F150LP, FUV) and the Wide Field Planetary Camera 2 (WFPC2; F300W, UV) on board the Hubble Space Telescope (HST). We compare the catalogue of FUV objects to 10 known X-ray sources and find six confident matches of two cataclysmic variables (CVs), one RS CVn, one red giant with strong FUV emission, and two sources only detected in the FUV. We also searched for variable sources in our data set and found a total of seven blue stragglers (BSs), four horizontal branch (HB) stars, five red giant branch stars, 28 main-sequence stars, and four gap objects that demonstrated variability. One BS star is a known W-UMa contact binary, one of the gap objects is a known CV identified in this work to be a dwarf nova, and the three other gap sources are weak variables. The periods and positions of two of the variable HB stars match them to two previously known RR Lyrae variables of types RRab and RRc.

Chemical composition and ages of four globular clusters in M31 from the analysis of their integrated-light spectra

Abstract We compare the results on the chemical composition of four globular clusters (GCs) in M31 (Bol 6, Bol 20, Bol 45, and Bol 50) (Maricheva M. 2021. Study of integrated spectra of four globular clusters in M 31. Astrophys. Bull. 76:389–404. doi: https://doi.org/10.1134/S199034132104009X) to the available literature data on integrated-light spectroscopy of GCs with similar ages and chemical abundances in our Galaxy and M31 and on the chemical abundances of stars in two galaxies. The clusters and their literature analogues are of moderate metallicity − 1.1 < [ Fe/H ] < − 0.75 -1.1\lt \left[\hspace{0.1em}\text{Fe/H}\hspace{0.1em}]\lt -0.75 dex and old (10 Gyr). Mg, Ca, and C abundances of four GCs are higher than literature estimates for the GCs in M31 with [ Fe/H ] ∼ − 1 \left[\hspace{0.1em}\text{Fe/H}\hspace{0.1em}] \sim -1 dex obtained using high-resolution integrated-light spectroscopy methods. Using literature data, we did not find complete analogues for the studied clusters in our Galaxy and M31 in terms of age, helium mass fraction ( Y Y ), and chemical composition. The alpha-element abundances in four clusters are about 0.2 dex higher than the average for stars in the Galactic field at [ Fe/H ] ∼ − 1 \left[\hspace{0.1em}\text{Fe/H}\hspace{0.1em}] \sim -1 dex. We suggest that these and M. Maricheva’s (Maricheva M. 2021) findings about lower metallicities of the studied objects than the average metallicity of red giants in the M31 halo and about the abundances of alpha-process elements in them corresponding to the average value for stars in the M31 inner halo likely indicate that the star formation process in the vicinity of M31 at the time of our sample cluster formation was complex with the inflow of fresh gas from the intergalactic medium and violent star forming events associated with SNe II bursts.

Metallicities of outer halo M31 globular clusters from integrated light calcium-II triplet spectroscopy

ABSTRACT This paper presents [Fe/H] ratios for globular clusters (GCs) in the outer halo of the Andromeda Galaxy, M31, based on moderate-resolution, integrated light (IL) spectroscopy of the calcium-II triplet (CaT) lines. The CaT strengths are measured by fitting Voigt profiles to the lines and integrating those profiles; integrations of defined bandpasses are also considered. The [Fe/H] ratios are determined using an empirical calibration with CaT line strength, as derived from another sample of M31 GCs that were previously studied at high-resolution. The [Fe/H] ratios for the new GCs reveal that the outer halo GCs are indeed generally more metal-poor than typical inner halo GCs, though there are several more metal-rich GCs that look to have been accreted from dwarf satellites. The metallicities of these GCs also place important constraints on the nature of the substructure in the outer halo and the dwarf satellites that created this substructure.

Two New Black Widow Millisecond Pulsars in M28

Abstract We report the discovery of two Black Widow millisecond pulsars in the globular cluster (GC) M28 with the MeerKAT telescope. PSR J1824−2452M (M28M) is a 4.78 ms pulsar in a 5.82 hr orbit, and PSR J1824−2452N (M28N) is a 3.35 ms pulsar in a 4.76 hr orbit. Both pulsars have dispersion measures near 119.30 pc cm−3 and have low-mass companion stars (∼0.01–0.03 M ⊙) that do not cause strong radio eclipses or orbital variations. Including these systems, there are now five known black widow pulsars in M28. The pulsar searches were conducted as a part of an initial phase of MeerKAT’s GC census (within the TRAPUM Large Survey Project). These faint discoveries demonstrate the advantages of MeerKAT’s survey sensitivity over previous searches, and we expect to find additional pulsars in continued searches of this cluster.

Far-ultraviolet investigation into the galactic globular cluster M30 (NGC 7099): I. Photometry and radial distributions

ABSTRACT We present a far-ultraviolet (FUV) study of the globular cluster M30 (NGC 7099). The images were obtained using the Advanced Camera for Surveys (ACS/SBC, F150LP, FUV) and the Wide Field Planetary Camera 2 (WFPC2, F300W, UV) which were both onboard the Hubble Space Telescope. The FUV − UV colour–magnitude diagram (CMD) shows a main sequence (MS) turnoff at FUV ≈ 22 mag and $FUV-UV\, \approx 3$ mag. The MS extends 4 mag below the turnoff, and a prominent horizontal branch (HB) and blue straggler (BS) sequence can be seen. A total of 1218 MS stars, 185 red giant branch stars, 47 BS stars, and 41 HB stars are identified, along with 78 sources blueward of the MS which consist of white dwarfs (WDs) and objects in the gap between the WDs and the MS that include potential cataclysmic variable (CV) candidates. The radial distribution of the BS population is concentrated towards the cluster centre, indicating that mass segregation has occurred. The blue and red sub-populations of the double BS sequence appear mixed in the ultraviolet CMD, and no significant central concentration of CV candidates is seen in this cluster.

Internal R-process Abundance Spread of M15 and a Single Stellar Population Model

Abstract The member stars in globular cluster M15 show a substantial spread in the abundances of r-process elements. We argue that a rare and prolific r-process event enriched the natal cloud of M15 in an inhomogeneous manner. To critically examine the possibility, we perform cosmological galaxy formation simulations and study the physical conditions for the inhomogeneous enrichment. We explore a large parameter space of the merger event time and the site. Our simulations reproduce the large r-process abundance spread if a neutron-star merger occurs at ∼100 pc away from the formation site of the cluster and in a limited time range of a few tens of millions of years before the formation. Interestingly, a bimodal feature is found in the Eu abundance distribution in some cases, similarly to that inferred from recent observations. M15 member stars do not show the clear correlation between the abundances of Eu and light elements such as Na that is expected in models with two stellar populations. We thus argue that a majority of the stars in M15 are formed in a single burst. The ratio of heavy to light r-process element abundance [Eu/Y] ∼ 1.0 is consistent with that of the so-called r-II stars, suggesting that a lanthanide-rich r-process event dominantly enriched M15.

Spatial modulation search applied to the search and confirmation of highly scintillated pulsars at FAST with a pulsar discovered in M3

We present a pulsar candidate identification and confirmation procedure based on a position-switch mode during the pulsar search observations. This method enables the simultaneous search and confirmation of a pulsar in a single observation, by utilizing the different spatial features of a pulsar signal and radio frequency interference (RFI). Based on this method, we performed test pulsar search observations in globular clusters M3, M15 and M92. We discovered and confirmed a new pulsar, M3F, and detected the known pulsars M3B, M15A to G (except C) and M92A.

Nonlinear Color–Metallicity Relations of Globular Clusters. X. Subaru/FOCAS Multiobject Spectroscopy of M87 Globular Clusters

Abstract We obtained spectra of some 140 globular clusters (GCs) associated with the Virgo central cD galaxy M87 with the Subaru/FOCAS MOS mode. The fundamental properties of GCs such as age, metallicity, and α-element abundance are investigated by using simple stellar population models. It is confirmed that the majority of M87 GCs are as old as, more metal-rich than, and more enhanced in α-elements than the Milky Way GCs. Our high-quality, homogeneous data set enables us to test the theoretical prediction of inflected color–metallicity relations (CMRs). The nonlinear-CMR hypothesis entails an alternative explanation for the widely observed GC color bimodality, in which even a unimodal metallicity spread yields a bimodal color distribution by virtue of nonlinear metallicity-to-color conversion. The newly derived CMRs of old, high-signal-to-noise-ratio GCs in M87 (the V − I CMR of 83 GCs and the M−T2 CMR of 78 GCs) corroborate the presence of the significant inflection. Furthermore, from a combined catalog with the previous study on M87 GC spectroscopy, we find that a total of 185 old GCs exhibit a broad, unimodal metallicity distribution. The results corroborate the nonlinear-CMR interpretation of the GC color bimodality, shedding further light on theories of galaxy formation.

Formation of Multiple Populations of M5 (NGC 5904)

Abstract With our new Ca-CN-CH-NH photometry, we revisit the globular cluster (GC) M5. We find that M5 is a mono-metallic GC with a small metallicity dispersion. Our carbon abundances show that the σ[C/Fe] of the M5 CN-s population, with depleted carbon and enhanced nitrogen abundances, is significantly large for a single stellar population. Our new analysis reveals that the M5 CN-s population is well described by the two stellar populations: the CN-sI, being the major CN-s component, with the intermediate carbon and nitrogen abundance and the CN-sE with the most carbon-poor and nitrogen-rich abundance. We find that the CN-sE is significantly more centrally concentrated than the others, while CN-w and CN-sI have similar cumulative radial distributions. The red giant branch bump V magnitude, the helium abundance barometer in mono-metallic populations, of individual populations appears to be correlated with their mean carbon abundance, indicating that carbon abundances are anticorrelated with helium abundances. We propose that the CN-sE formed out of gas that experienced proton-capture processes at high temperatures in the innermost region of the proto-GC of M5 that resided in a dense ambient density environment. Shortly after, the CN-sI formed out of gas diluted from the pristine gas in the more spatially extended region, consistent with the current development of numerical simulations by others.

Central kinematics of the Galactic globular cluster M80

ABSTRACT We use spectra observed with the integral-field spectrograph Multi Unit Spectroscopic Explorer (MUSE) to reveal the central kinematics of the Galactic globular cluster Messier 80 (M80, NGC 6093). Using observations obtained with the recently commissioned narrow-field mode of MUSE, we are able to analyse 932 stars in the central 7.5 arcsec by 7.5 arcsec of the cluster for which no useful spectra previously existed. Mean radial velocities of individual stars derived from the spectra are compared to predictions from axisymmetric Jeans models, resulting in radial profiles of the velocity dispersion, the rotation amplitude, and the mass-to-light ratio. The new data allow us to search for an intermediate-mass black hole (IMBH) in the centre of the cluster. Our Jeans model finds two similarly probable solutions around different dynamical cluster centres. The first solution has a centre close to the photometric estimates available in the literature and does not need an IMBH to fit the observed kinematics. The second solution contains a location of the cluster centre that is offset by about 2.4 arcsec from the first one and it needs an IMBH mass of $4600^{+1700}_{-1400}~\text{M}_\odot {}$. N-body models support the existence of an IMBH in this cluster with a mass of up to 6000 M⊙ in this cluster, although models without an IMBH provide a better fit to the observed surface brightness profile. They further indicate that the cluster has lost nearly all stellar-mass black holes. We further discuss the detection of two potential high-velocity stars with radial velocities of 80–90 $\text{km}\, \text{s}^{-1}$ relative to the cluster mean.

Absence of Bursts between 4 and 8 GHz from FRB 20200120E Located in an M81 Globular Cluster

Abstract We report the non-detection of dispersed bursts between 4 and 8 GHz from 2.5 hr of observations of FRB 20200120E at 6 GHz using the Robert C. Byrd Green Bank Telescope. Our fluence limits are several times lower than the average burst fluences reported at 600 and 1400 MHz. We conclude that these non-detections are either due to high-frequency bursts being weaker and/or scintillation-induced modulated. It is also likely that our observations were non-concurrent with any activity window of FRB 20200120E.

Probing M87 Globular Clusters for Flaring Ultraluminous X-Ray Sources

Abstract We present X-ray analysis of three short Chandra observations of M87. The basis of this analysis is the search for possible new ultraluminous X-ray sources (ULXs) in M87's globular clusters (GCs) and attempt to quantify possible variability within the observations. We searched Chandra ObsIDs 1808 (2000 July 30, 14 ks) Wilson & Yang, 3975 (2002 November 17, 5 ks) and 3977 (2003 February 4, 5 ks) Harris et al. and identified one previously discovered GC ULX, and two new GC ULX candidates. Analysis of the light-curves revealed no new evidence of variability in these sources for the duration of the observations.

Light Element Abundances and Multiple Populations in M53

Abstract We present results from a study of 94 red giant stars in the globular cluster M53. We use low-resolution spectra to measure the strength of CN and CH features at ∼3800 and 4300 Å, respectively. The strengths of these features are used to classify stars into a CN-enhanced and CN-normal population and to measure C and N abundances in all 94 stars. We find the red giant branch stars to be evenly split between the two populations identified, and observe the presence of CN-enhanced stars on the asymptotic giant branch. In addition, we identify five CH star candidates based on the strength of their CN and CH band features, and the presence of a P-branch in their CH band. We compare our identification of multiple populations to those based on the Na–O anticorrelation and pseudo-color indices in Hubble Space Telescope UV photometry, and find general agreement between all three methods. Our large sample size also allows us to study the radial distribution of each population, and we find that the CN-enhanced population is more centrally concentrated. We use our C and N measurements to compare the evolutionary changes in these elements as a function of magnitude between the two populations, and show that both populations experience similar evolutionary changes to the surface abundances of C and N. Finally, we calculate C+N+O abundances for each population and compare them to similar measurements made in M10; we find that in both clusters, CN-enhanced stars have a slightly enhanced C+N+O (Δ(C+N+O) ∼ 0.2 dex).

The APOGEE Library of Infrared SSP Templates (A-LIST): High-resolution Simple Stellar Population Spectral Models in the H Band

Abstract Integrated light spectroscopy from galaxies can be used to study the stellar populations that cannot be resolved into individual stars. This analysis relies on stellar population synthesis (SPS) techniques to study the formation history and structure of galaxies. However, the spectral templates available for SPS are limited, especially in the near-infrared (near-IR). We present A-LIST (APOGEE Library of Infrared SSP Templates), a new set of high-resolution, near-IR SSP spectral templates spanning a wide range of ages (2–12 Gyr), metallicities ( − 2.2 < [M/H] < + 0.4) and α abundances ( − 0.2 < [α/M] < + 0.4). This set of SSP templates is the highest resolution (R ∼ 22, 500) available in the near-IR, and the first such based on an empirical stellar library. Our models are generated using spectra of ∼300,000 stars spread across the Milky Way, with a wide range of metallicities and abundances, from the APOGEE survey. We show that our model spectra provide accurate fits to M31 globular cluster spectra taken with APOGEE, with best-fit metallicities agreeing with those of previous estimates to within ∼0.1 dex. We also compare these model spectra to lower-resolution E-MILES models and demonstrate that we recover the ages of these models to within ∼1.5 Gyr. This library is available in https://github.com/aishashok/ALIST-library.

Multiple Stellar Populations of Globular Clusters from Homogeneous Ca–CN–CH–NH Photometry. VI. M3 (NGC 5272) Is Not a Prototypical Normal Globular Cluster* †

Abstract We present Ca–CN–CH–NH photometry for the well-known globular cluster (GC) M3 (NGC 5272). We show new evidence for two M3 populations with distinctly different carbon and nitrogen abundances, seen in a sharp division between CN-weak and CN-strong red-giant branches (RGBs) in M3. The CN-strong population shows a C–N anticorrelation that is a natural consequence of the CN cycle, while the CN-weak population shows at most a very weak C–N anticorrelation. Additionally, the CN-weak population exhibits an elongated spatial distribution that is likely linked to its fast rotation. Our derived metallicities reveal bimodal distributions in both populations, with 〈[Fe/H]〉 ≈ −1.60 and −1.45, which appear to be responsible for the discrete double RGB bumps in the CN-weak population and the large range. From this discovery, we propose that M3 consists of two GCs, namely C1 (23%, 〈[Fe/H]〉 ≈ −1.60) and C2 (77%, 〈[Fe/H]〉 ≈ −1.45), each of which has its own C–N anticorrelation and structural and kinematical properties, which are strong indications of independent systems in M3. The fractions of the CN-weak population for both C1 and C2 are high compared to Galactic GCs but they are in good agreement with GCs in the Magellanic Clouds. We suggest that M3 is a merger remnant of two GCs, most likely in a dwarf galaxy environment, and accreted to our Galaxy later in time. This is consistent with recent proposals of an ex situ origin for M3.

The Star Formation History of Eridanus II: On the Role of Supernova Feedback in the Quenching of Ultrafaint Dwarf Galaxies*

Abstract Eridanus II (Eri II) is an ultrafaint dwarf (UFD) galaxy (M V = −7.1) located at a distance close to the Milky Way virial radius. Early shallow color–magnitude diagrams (CMDs) indicated that it possibly hosted an intermediate-age or even young stellar population, which is unusual for a galaxy of this mass. In this paper, we present new Hubble Space Telescope/Advanced Camera for Surveys CMDs reaching the oldest main-sequence turnoff with excellent photometric precision and derive a precise star formation history (SFH) for this galaxy through CMD fitting. This SFH shows that the bulk of the stellar mass in Eri II formed in an extremely short star formation burst at the earliest possible time. The derived star formation rate profile has a width at half maximum of 500 Myr and reaches a value compatible with null star formation 13 Gyr ago. However, tests with mock stellar populations and with the CMD of the globular cluster M92 indicate that the star formation period could be shorter than 100 Myr. From the quantitative determination of the amount of mass turned into stars in this early star formation burst ( ∼2 × 105 M ⊙) we infer the number of supernova (SN) events and the corresponding energy injected into the interstellar medium. For reasonable estimates of the Eri II virial mass and values of the coupling efficiency of the SN energy, we conclude that Eri II could be quenched by SN feedback alone, thus casting doubts on the need to invoke cosmic reionization as the preferred explanation for the early quenching of old UFD galaxies.