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Hot Populations in M87 Globular Clusters
Sohn, Sangmo T.,O'Connell, Robert W.,Kundu, Arunav,Landsman, Wayne B.,Burstein, David,Bohlin, Ralph C.,Frogel, Jay A.,Rose, James A. American Institute of Physics 2006 The Astronomical journal Vol.131 No.2
<P>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<SUB>FUV</SUB> ~ 25 we detect a total of 66 GCs in common with the deep HST optical-band study of Kundu et al. Despite strong overlap in V- and I-band properties, the M87 GCs have UV-optical properties that are distinct from clusters in the Milky Way and in M31. M87 clusters, especially metal-poor ones, produce larger hot horizontal-branch populations than do Milky Way analogs. In color plots including the near-UV band, the M87 clusters appear to represent an extension of the Milky Way sequence. Cluster mass is probably not a factor in these distinctions. The most metal-rich M87 GCs in our sample are near solar metallicity and overlap the local E galaxy sample in estimated Mg<SUB>2</SUB> line indices. Nonetheless, the clusters produce much more UV light at a given Mg<SUB>2</SUB>, being up to 1 mag bluer than any gE galaxy in (FUV - V) color. The M87 GCs do not appear to represent a transition between Milky Way-type clusters and E galaxies. The differences are in the correct sense if the clusters are significantly older than the E galaxies.</P><P>Comparisons with Galactic open clusters indicate that the hot stars lie on the extreme horizontal branch, rather than being blue stragglers, and that the extreme horizontal branch becomes well populated for ages ≳5 Gyr. Existing model grids for clusters do not match the observations well, due to poorly understood giant branch mass loss or perhaps high helium abundances. We find that 41 of our UV detections have no optical-band counterparts. Most appear to be UV-bright background galaxies seen through M87. Eleven near-UV variable sources detected at only one epoch in the central field are probably classical novae. Two recurrent variable sources have no obvious explanation but could be related to activity in the relativistic jet.</P>
UV bright globular clusters in M87: more evidence for super-He-rich stellar populations?
Kaviraj, S.,Sohn, S. T.,O'Connell, R. W.,Yoon, S.-J.,Lee, Y. W.,Yi, S. K. Blackwell Publishing Ltd 2007 Monthly notices of the Royal Astronomical Society Vol.377 No.3
<P>ABSTRACT</P><P>We study the ultraviolet (UV) and optical properties of 38 massive globular clusters (GCs) in the Virgo elliptical, M87, imaged using the STIS and WFPC2 instruments onboard the <I>Hubble Space Telescope</I>. The majority of these GCs appear extremely bright in the far-UV (FUV) – roughly a magnitude brighter than their Galactic counterparts with similar metallicities. The observed FUV flux is several times larger than predictions of canonical old stellar population models. These canonical models, which assume a fiducial helium enrichment parameter, Δ<I>Y</I>/Δ<I>Z</I>= 2, are able to reproduce the observed FUV fluxes only if ages ∼3–5 Gyr larger than the <I>Wilkinson Microwave Anisotropy Probe</I> (<I>WMAP</I>) age of the Universe are invoked, although the same models fit the UV photometry of Galactic and M31 GCs for ages less than the ‘<I>WMAP</I> age’. A similar discrepancy (∼3 Gyr) is found between the mass-weighted and UV-luminosity-weighted ages of the massive Galactic GC ω Cen, whose colour–magnitude diagram (including peculiar features on its well-populated horizontal branch) can be accurately reproduced by invoking a small super-He-rich (Δ<I>Y</I>/Δ<I>Z</I>≳ 90) stellar component. By comparison to ω Cen, we propose that the majority of M87 GCs in our sample contain strong signatures of similarly minor super-He-rich subcomponents. This hypothesis is supported by simulations which suggest that, based on the UV detection limit of this survey, the number of GCs detected is several times that predicted by canonical models. Although we cannot prove or disprove the extreme helium scenario at the moment, we show that the same phenomenon that causes the extended horizontal branch of ω Cen explains the UV brightness of our sample. If this is indeed due to the extreme helium, this study would be the first to find its signatures in extragalactic objects.</P>
ULTRAVIOLET PROPERTIES OF GALACTIC GLOBULAR CLUSTERS WITH GALEX. I. THE COLOR-MAGNITUDE DIAGRAMS
Schiavon, Ricardo P.,Dalessandro, Emanuele,Sohn, Sangmo T.,Rood, Robert T.,O'Connell, Robert W.,Ferraro, Francesco R.,Lanzoni, Barbara,Beccari, Giacomo,Rey, Soo-Chang,Rhee, Jaehyon,Rich, R. Michael,Yo American Institute of Physics 2012 The Astronomical journal Vol.143 No.5
<P>We present Galaxy Evolution Explorer (GALEX) data for 44 Galactic globular clusters (GCs) obtained during three GALEX observing cycles between 2004 and 2008. This is the largest homogeneous data set on the UV photometric properties of Galactic GCs ever collected. The sample selection and photometric analysis are discussed, and color-magnitude diagrams (CMDs) are presented. The blue and intermediate-blue horizontal branch is the dominant feature of the UV CMDs of old Galactic GCs. Our sample is large enough to display the remarkable variety of horizontal branch shapes found in old stellar populations. Other stellar types that are obviously detected are blue stragglers and post-core-He burning stars. The main features of UV CMDs of Galactic GCs are briefly discussed. We establish the locus of post-core-He burning stars in the UV CMD and present a catalog of candidate asymptotic giant branch (AGB), AGB-manqué, post early-AGB, and post-AGB stars within our cluster sample.</P>
Yang, Y.,He, S.,Wang, Q.,Li, F.,Kwak, M.J.,Chen, S.,O'Connell, D.,Zhang, T.,Pirooz, S.,Jeon, Y.,Chimge, N.O.,Frenkel, B.,Choi, Y.,Aldrovandi, Grace M.,Oh, B.H.,Yuan, Z.,Liang, C. Cell Press 2016 Molecular cell Vol.62 No.4
<P>UV-induced DNA damage, a major risk factor for skin cancers, is primarily repaired by nucleotide excision repair (NER). UV radiation resistance-associated gene (UVRAG) is a tumor suppressor involved in autophagy. It was initially isolated as a cDNA partially complementing UV sensitivity in xeroderma pigmentosum (XP), but this was not explored further. Here we show that UVRAG plays an integral role in UV-induced DNA damage repair. It localizes to photolesions and associates with DDB1 to promote the assembly and activity of the DDB2-DDB1-Cul4A-Roc1 (CRL4(DDB2)) ubiquitin ligase complex, leading to efficient XPC recruitment and global genomic NER. UVRAG depletion decreased substrate handover to XPC and conferred UV-damage hypersensitivity. We confirmed the importance of UVRAG for UV-damage tolerance using a Drosophila model. Furthermore, increased UV-signature mutations in melanoma correlate with reduced expression of UVRAG. Our results identify UVRAG as a regulator of CRL4(DDB2)-mediated NER and suggest that its expression levels may influence melanoma predisposition.</P>