J/A+A/651/A102ivo://CDS.VizieR/J/A+A/651/A102doi:10.26093/cds/vizier.36510102CDSLarsen S.S.Romanowsky A.J.Brodie J.P.2024-05-29T12:37:01Z2021-07-28T06:59:42ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frglobular-star-clusterstwo-color-diagramshertzsprung-russell-diagramhst-photometrygiant-starshorizontal-branch-starschemically-peculiar-starsultraviolet-astronomyWe recently found the globular cluster (GC) EXT8 in M31 to have an extremely low metallicity of [Fe/H]=-2.91+/-0.04 using high-resolution spectroscopy. Here we present a colour-magnitude diagram (CMD) for EXT8, obtained with the Wide Field Camera 3 on board the Hubble Space Telescope. Compared with the CMDs of metal-poor Galactic GCs, we find that the upper red giant branch (RGB) of EXT8 is about 0.03mag bluer in F606W-F814W and slightly steeper, as expected from the low spectroscopic metallicity. The observed colour spread on the upper RGB is consistent with being caused entirely by the measurement uncertainties, and we place an upper limit of sigma(F606W-F814W)=0.015mag on any intrinsic colour spread. The corresponding metallicity spread can be up to sigma([Fe/H])=0.2dex or >0.7dex, depending on the isochrone library adopted. The horizontal branch (HB) is located mostly on the blue side of the instability strip and has a tail extending to at least M(F606W)=+3, as in the Galactic GC M15. We identify two candidate RR Lyrae variables and several UV-luminous post-HB/post-AGB star candidates, including one very bright (M(F300X)=-3.2) source near the centre of EXT8. The surface brightness of EXT8 out to a radius of 25 arcsec is well fitted by a Wilson-type profile with an ellipticity of epsilon=0.20, a semi-major axis core radius of 0.25", and a central surface brightness of 15.2mag per square arcsec in the F606W band, with no evidence of extra-tidal structure. Overall, EXT8 has properties consistent with it being a "normal", but very metal-poor GC, and its combination of relatively high mass and very low metallicity thus remains challenging to explain in the context of GC formation theories operating within the hierarchical galaxy assembly paradigm.2021A&A...651A.102Lhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/651/A102CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicehttps://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/651/A102https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/651/A102http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/651/A102https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/651/A102https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/651/A102http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/651/A102GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/A+A/651/A102/tablea1 (F606W, F814W photometry for EXT8)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/651/A102/tablea1?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/651/A102/tablea1?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/651/A102/tablea1?GETtext/xml+votable180.050000true13.295748341.54528890.005555555555555556Cone search capability for table J/A+A/651/A102/tablea2 (F300X, F606W, F814W photometry for EXT8)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/651/A102/tablea2?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/651/A102/tablea2?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/651/A102/tablea2?GETtext/xml+votable180.050000true13.295296741.54473530.0055555555555555566/26652.0401237512456292e-19 3.9057996558789745e-19https://cdsarc.cds.unistra.fr/viz-bin/moc/J/A+A/651/A102?format=asciiUVdefaultJ/A+A/651/A102/tablea1F606W, F814W photometry for EXT8recnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintIDRunning IDmeta.id;meta.mainintX606F606W CCD X coordinate (1)pixpos.cartesian.x;instr.detdoubleY606F606W CCD Y coordinate (1)pixpos.cartesian.y;instr.detdoublemF606WF606W STMAG magnitudemagphot.mag;em.opt.Rfloate_mF606WError on F606W magnitudemagstat.errorfloatrF606WR.M.S on F606W magnitude measurementsmagstat.errorfloatX814F814W CCD X coordinatepixpos.cartesian.x;instr.detdoubleY814F814W CCD Y coordinatepixpos.cartesian.y;instr.detdoublemF814WF814W STMAG magnitudemagphot.mag;em.opt.Ifloate_mF814WError on F814W magnitudemagstat.errorfloatrF814WR.M.S on F814W magnitude measurementsmagstat.errorfloat_RAPositions from X,Y ones (right ascension part)degpos.eq.ra;meta.main_DEPositions from X,Y ones (declination part)degpos.eq.dec;meta.mainJ/A+A/651/A102/tablea2F300X, F606W, F814W photometry for EXT8IDRunning IDmeta.id;meta.mainintrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintX300F300X X coordinate on drizzled image (1)pixpos.cartesian.x;instr.detdoubleY300F300X Y coordinate on drizzled image (1)pixpos.cartesian.y;instr.detdoublemF300XF300X STMAG magnitudemagphot.mag;em.UVfloate_mF300XError on F300X magnitudemagstat.errorfloatcF300XALLFRAME chi-square for F300X fitmagstat.fit.chi2floatsF300XALLFRAME sharpness for F300X fitmagstat.fit.paramfloatX606F606W X coordinate on drizzled imagepixpos.cartesian.x;instr.detdoubleY606F606W Y coordinate on drizzled imagepixpos.cartesian.y;instr.detdoublemF606WF606W STMAG magnitudemagphot.mag;em.opt.Rfloate_mF606WError on F606W magnitudemagstat.errorfloatcF606WALLFRAME chi-square for F606W fitmagstat.fit.chi2floatsF606W? ALLFRAME sharpness for F606W fitmagstat.fit.paramfloatnullableX814F814W X coordinate on drizzled imagepixpos.cartesian.x;instr.detdoubleY814F814W Y coordinate on drizzled imagepixpos.cartesian.y;instr.detdoublemF814WF814W STMAG magnitudemagphot.mag;em.opt.Ifloate_mF814WError on F814W magnitudemagstat.errorfloatcF814WALLFRAME chi-square for F814W fitmagstat.fit.chi2floatsF814WALLFRAME sharpness for F814W fitmagstat.fit.paramfloat_RAPositions from X,Y ones (right ascension part)degpos.eq.ra;meta.main_DEPositions from X,Y ones (declination part)degpos.eq.dec;meta.main