The Distance and Dynamical History of the Virgo Cluster Ultradiffuse Galaxy VCC 615

We use deep Hubble Space Telescope imaging to derive a distance to the Virgo Cluster ultradiffuse galaxy (UDG) VCC 615 using the tip of the red giant branch (TRGB) distance estimator. We detect 5023 stars within the galaxy, down to a 50% completeness limit of F814W ≈ 28.0, using counts in the surrounding field to correct for contamination due to background sources and Virgo intracluster stars. We derive an extinction-corrected F814W tip magnitude of m tip , 0 = 27.19 − 0.05 + 0.07 , yielding a distance of d = 17.7 − 0.4 + 0.6 Mpc. This places VCC 615 on the far side of the Virgo Cluster (d Virgo = 16.5 Mpc), at a Virgocentric distance of 1.3 Mpc and near the virial radius of the main body of Virgo. Coupling this distance with the galaxy’s observed radial velocity, we find that VCC 615 is on an outbound trajectory, having survived a recent passage through the inner parts of the cluster. Indeed, our orbit modeling gives a 50% chance the galaxy passed inside the Virgo core (r < 620 kpc) within the past gigayear, although very close passages directly through the cluster center (r < 200 kpc) are unlikely. Given VCC 615's undisturbed morphology, we argue that the galaxy has experienced no recent and sudden transformation into a UDG due to the cluster potential, but rather is a long-lived UDG whose relatively wide orbit and large dynamical mass protect it from stripping and destruction by the Virgo cluster tides. Finally, we also describe the serendipitous discovery of a nearby Virgo dwarf galaxy projected 90″ (7.2 kpc) away from VCC 615.

Stellar loci IV. red giant stars

In the fourth paper of this series, we present the metallicity-dependent Sloan Digital Sky Survey (SDSS) stellar color loci of red giant stars, using a spectroscopic sample of red giants in the SDSS Stripe 82 region. The stars span a range of 0.55 – 1.2 mag in color g – i, –0.3 – –2.5 in metallicity [Fe/H], and have values of surface gravity log g smaller than 3.5 dex. As in the case of main-sequence (MS) stars, the intrinsic widths of loci of red giants are also found to be quite narrow, a few mmag at maximum. There are however systematic differences between the metallicity-dependent stellar loci of red giants and MS stars. The colors of red giants are less sensitive to metallicity than those of MS stars. With good photometry, photometric metallicities of red giants can be reliably determined by fitting the u – g, g – r, r – i, and i – z colors simultaneously to an accuracy of 0.2 – 0.25 dex, comparable to the precision achievable with low-resolution spectroscopy for a signal-to-noise ratio of 10. By comparing fitting results to the stellar loci of red giants and MS stars, we propose a new technique to discriminate between red giants and MS stars based on the SDSS photometry. The technique achieves completeness of ∼70 per cent and efficiency of ∼80 per cent in selecting metal-poor red giant stars of [Fe/H] ≤ –1.2. It thus provides an important tool to probe the structure and assemblage history of the Galactic halo using red giant stars.

Expanding the Time Domain of Multiple Populations: Evidence of Nitrogen Variations in the ∼1.5 Gyr Old Star Cluster NGC 1783

We present the result of a detailed analysis of Hubble Space Telescope UV and optical deep images of the massive and young (∼1.5 Gyr) stellar cluster NGC 1783 in the Large Magellanic Cloud. This system does not show evidence of multiple populations (MPs) along the red giant branch (RGB) stars. However, we find that the cluster main sequence (MS) shows evidence of a significant broadening (50% larger than what is expected from photometric errors) along with hints of possible bimodality in the MP sensitive (m F343N − m F438W, m F438W) color–magnitude diagram (CMD). Such an effect is observed in all color combinations including the m F343N filter, while it is not found in the optical CMDs. This observational evidence suggests we might have found light-element chemical abundance variations along the MS of NGC 1783, which represents the first detection of MPs in a system younger than 2 Gyr. A comparison with isochrones including MP-like abundances shows that the observed broadening is compatible with a N abundance enhancement of Δ([N/Fe]) ∼ 0.3. Our analysis also confirms previous results about the lack of MPs along the cluster RGB. However, we find that the apparent disagreement between the results found on the MS and the RGB is compatible with the mixing effects linked to the first dredge up. This study provides new key information about the MP phenomenon and suggests that star clusters form in a similar way at any cosmic age.

Kinematics of the Milky Way Thick disk in solar neighborhood

Using GAIA EDR3 catalog, we present the detailed analysis of the two-component Milky Way stellar disk in the solar neighborhood. To determine the kinematical properties of the thin and of the Thick disks, we select the complete sample of about 278,000 evolved red giant branch (RGB) stars distributed in the cylinder of 1 kpc radius and 0.5 kpc height centered at the Sun. We measured the following mean velocities and dispersions for the thin and the Thick disks, respectively: [Formula: see text][Formula: see text]km s[Formula: see text] with [Formula: see text][Formula: see text]km s[Formula: see text], and [Formula: see text][Formula: see text]km s[Formula: see text] with [Formula: see text][Formula: see text]km s[Formula: see text]. Errors in mean velocities and dispersions are all less than 1[Formula: see text]km s[Formula: see text]. Same values were computed on much smaller subsamples of our Gaia data with RAVE DR5 [Fe/H] values, from which a metallicity selection was added. Results are basically the same. We find that up to 500 pc height above/below the galactic plane, Thick disk stars comprise about half the stars of the disk. We also find evidence of a substructure in [Formula: see text] versus [Formula: see text] in the thick disk population mostly that would give support to the accretion scenario for the formation of the thick disk.

Evaluating Helium Varations By Modeling Red Giant Branch Bump of Large Magellanic Cluster NGC 1978

Many evidence show that the Multiple Population (MP) features ex- ist not only in the old Galactic globular clusters but also in the intermediate-age clusters in the Megallanic Clouds (MCs), which are characterized by star-to-star abundance scatter of several elements, including Helium (He). The photometric properties of the red giant branch bump (RGBB) are proved to be related to the variation in helium abundances of the member stars of the star clusters. We use the “Modules for Experiments in Stellar Astrophysics” (MESA) stellar evolution code to calculate the evolution sequences of stars along the red giant branch with changing helium content. Following the RGB sequences, we then generate a lu- minosity function of the RGB stars within the grid of input helium abundances, which are compared with the observational data of an intermediate-age MC cluster NGC 1978. The result of the current study reveals that the star to star helium abundance variation is 0.03.

ACTINIUM ABUNDANCES IN STELLAR ATMOSPHERES

This paper presents a study of radioactive actinium in the atmospheres of stars located in galaxies with different chemical evolution history – namely, Przybylski's Star (HD 101065) in the Milky Way and the red supergiant PMMR27 in the Small Magellanic Cloud; it also reports the findings of the previous research of the red supergiant RM 1-667 in the Large Magellanic Cloud and the red giant BL138 in the Fornax dwarf spheroidal galaxy. The actinium abundance is close to that of uranium in the atmospheres of certain stars in the Milky Way’s halo and in the atmosphere of Arcturus. The following actinium abundances have been obtained (in a scale of lg N(H) = 12): for the red supergiants PMMR27 and RM 1- 667 lg N(Ac) = -1.7 and lg N(Ac) = -1.3, respectively, and for the red giant BL138 lg N(Ac) = -1.6. The actinium abundance in the atmosphere of Przybylski's Star (HD 101065) is lg N(Ac) = `0.94±0.09, which is more than two orders of magnitude higher than those in the atmospheres of the other studied stars.

Calibration of Surface Brightness Fluctuations for Dwarf Galaxies in the Hyper Suprime-Cam gi Filter System

Surface brightness fluctuation (SBF) magnitudes are a powerful standard candle to measure distances to semiresolved galaxies in the local universe, a majority of which are dwarf galaxies that often have bluer colors than bright early-type galaxies. We present an empirical i-band SBF calibration in a blue regime, 0.2 ≲ (g − i)0 ≲ 0.8 in the Hyper Suprime-Cam (HSC) magnitude system. We measure SBF magnitudes for 12 nearby dwarf galaxies of various morphological types with archival HSC imaging data, and use their tip of the red giant branch distances to derive fluctuation–color relations. In order to subtract contributions of fluctuations due to young stellar populations, we use five different g-band magnitude masking thresholds, M g,thres = −3.5, −4.0, −4.5, −5.0, and −5.5 mag. We find that the rms scatter of the linear fit to the relation is the smallest (rms = 0.16 mag) in the case of M g,thres = −4.0 mag, M ¯ i = (−2.65 ± 0.13) + (1.28 ± 0.24) × (g − i)0. This scatter is much smaller than those in the previous studies (rms = 0.26 mag), and is closer to the value for bright red galaxies (rms = 0.12 mag). This calibration is consistent with predictions from metal-poor simple stellar population models.

APOGEE Chemical Abundance Patterns of the Massive Milky Way Satellites

The SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey has obtained high-resolution spectra for thousands of red giant stars distributed among the massive satellite galaxies of the Milky Way (MW): the Large and Small Magellanic Clouds (LMC/SMC), the Sagittarius Dwarf Galaxy (Sgr), Fornax (Fnx), and the now fully disrupted Gaia Sausage/Enceladus (GSE) system. We present and analyze the APOGEE chemical abundance patterns of each galaxy to draw robust conclusions about their star formation histories, by quantifying the relative abundance trends of multiple elements (C, N, O, Mg, Al, Si, Ca, Fe, Ni, and Ce), as well as by fitting chemical evolution models to the [α/Fe]–[Fe/H] abundance plane for each galaxy. Results show that the chemical signatures of the starburst in the Magellanic Clouds (MCs) observed by Nidever et al. in the α-element abundances extend to C+N, Al, and Ni, with the major burst in the SMC occurring some 3–4 Gyr before the burst in the LMC. We find that Sgr and Fnx also exhibit chemical abundance patterns suggestive of secondary star formation epochs, but these events were weaker and earlier (∼5–7 Gyr ago) than those observed in the MCs. There is no chemical evidence of a second starburst in GSE, but this galaxy shows the strongest initial star formation as compared to the other four galaxies. All dwarf galaxies had greater relative contributions of AGB stars to their enrichment than the MW. Comparing and contrasting these chemical patterns highlight the importance of galaxy environment on its chemical evolution.

Spectroscopy and Photometry of the Least Massive Type II Globular Clusters: NGC 1261 and NGC 6934*

Recent work has revealed two classes of globular clusters (GCs), dubbed Type I and Type II. Type II GCs are characterized by both a blue and a red red giant branch composed of stars with different metallicities, often coupled with distinct abundances in the slow neutron-capture elements (s-elements). Here we continue the chemical tagging of Type II GCs by adding the two least massive clusters of this class, NGC 1261 and NGC 6934. Based on both spectroscopy and photometry, we find red stars in NGC 1261 to be slightly enhanced in [Fe/H] by ∼0.1 dex and confirm that red stars of NGC 6934 are enhanced in iron by ∼0.2 dex. Neither NGC 1261 nor NGC 6934 show internal variations in the s-elements, which suggests a GC mass threshold for the occurrence of s-process enrichment. We found a significant correlation between the additional Fe locked in the red stars of Type II GCs and the present-day mass of the cluster. Nevertheless, most Type II GCs retained a small fraction of Fe produced by SNe II, lower than the 2%; NGC 6273, M54, and ω Centauri are remarkable exceptions. In the Appendix, we infer for the first time chemical abundances of lanthanum, assumed as representative of the s-elements, in M54, the GC located in the nucleus of the Sagittarius dwarf galaxy. Red-sequence stars are marginally enhanced in [La/Fe] by 0.10 ± 0.06 dex, in contrast with the large [La/Fe] spread of most Type II GCs. We suggest that different processes are responsible for the enrichment in iron and s-elements in Type II GCs.