J/A+A/651/A102 Photometry of M31 globular cluster EXT8 (Larsen+, 2021)
Hubble Space Telescope imaging of the extremely metal-poor globular cluster
EXT8 in Messier 31.
Larsen S.S., Romanowsky A.J., Brodie J.P.
<Astron. Astrophys. 651, A102 (2021)>
=2021A&A...651A.102L 2021A&A...651A.102L (SIMBAD/NED BibCode)
ADC_Keywords: Clusters, globular ; Colors ; HR diagrams ; Photometry, HST ;
Stars, giant ; Stars, horizontal branch ; Stars, metal-deficient ;
Ultraviolet
Keywords: globular clusters: individual: RBC EXT8 -
stars: Hertzsprung-Russell and C-M diagrams -
stars: horizontal-branch - stars: AGB and post-AGB
Abstract:
We 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.
Description:
EXT8 was observed with the Wide Field Camera 3 (WFC3) on board HST on
13-15 Jan. 2021 (programme ID 16459, PI S. S. Larsen). Exposures
were obtained in the filters F300X, F606W, and F814W, using two orbits
each for the F300X and F606W observations and three orbits for F814W.
This tables list ALLFRAME PSF photometry for stars in the extremely
metal-poor globular EXT8 in Messier 31.
tablea1.dat contains HST WFC3 PSF-fitting photometry in F606W and
F814W on the _flc images.
tablea2.dat contains HST WFC3 PSF-fitting photometry in F300X, F606W,
and F814W on the _drc images.
Objects:
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RA (2000) DE Designation(s)
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00 53 14.52 +41 33 24.4 EXT8 = M31GC J005314+413324
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 80 3712 F606W, F814W photometry for EXT8
tablea2.dat 147 5906 F300X, F606W, F814W photometry for EXT8
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Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 6 I6 --- ID Running ID
8- 15 F8.3 pix X606 F606W CCD X coordinate (1)
17- 24 F8.3 pix Y606 F606W CCD Y coordinate (1)
26- 31 F6.3 mag mF606W F606W STMAG magnitude
33- 37 F5.3 mag e_mF606W Error on F606W magnitude
39- 43 F5.3 mag rF606W R.M.S on F606W magnitude measurements
45- 52 F8.3 pix X814 F814W CCD X coordinate
54- 61 F8.3 pix Y814 F814W CCD Y coordinate
63- 68 F6.3 mag mF814W F814W STMAG magnitude
70- 74 F5.3 mag e_mF814W Error on F814W magnitude
76- 80 F5.3 mag rF814W R.M.S on F814W magnitude measurements
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Note (1): The centre of EXT8 is at coordinates (X606, Y606) = (1041, 1089).
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Byte-by-byte Description of file: tablea2.dat
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Bytes Format Units Label Explanations
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1- 6 I6 --- ID Running ID
8- 15 F8.3 pix X300 F300X X coordinate on drizzled image (1)
17- 24 F8.3 pix Y300 F300X Y coordinate on drizzled image (1)
26- 31 F6.3 mag mF300X F300X STMAG magnitude
33- 38 F6.3 mag e_mF300X Error on F300X magnitude
40- 45 F6.3 mag cF300X ALLFRAME chi-square for F300X fit
47- 53 F7.3 mag sF300X ALLFRAME sharpness for F300X fit
55- 62 F8.3 pix X606 F606W X coordinate on drizzled image
64- 71 F8.3 pix Y606 F606W Y coordinate on drizzled image
73- 78 F6.3 mag mF606W F606W STMAG magnitude
80- 85 F6.3 mag e_mF606W Error on F606W magnitude
87- 92 F6.3 mag cF606W ALLFRAME chi-square for F606W fit
94-100 F7.3 mag sF606W ?=99.999 ALLFRAME sharpness for F606W fit
102-109 F8.3 pix X814 F814W X coordinate on drizzled image
111-118 F8.3 pix Y814 F814W Y coordinate on drizzled image
120-125 F6.3 mag mF814W F814W STMAG magnitude
127-132 F6.3 mag e_mF814W Error on F814W magnitude
134-139 F6.3 mag cF814W ALLFRAME chi-square for F814W fit
141-147 F7.3 mag sF814W ALLFRAME sharpness for F814W fit
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Note (1): The centre of the cluster is at coordinates
(X300, Y300) = (1060.2, 1166.7)
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Acknowledgements:
Soeren Larsen, s.larsen(at)astro.ru.nl
(End) Soeren Larsen [Radboud University], Patricia Vannier [CDS] 17-May-2021