J/AJ/148/27 Photometry in globular clusters. I. NGC 1851 (Cummings+, 2014)
Uncovering multiple populations with Washington photometry.
I. The globular cluster NGC 1851.
Cummings J.D., Geisler D., Villanova S., Carraro G.
<Astron. J., 148, 27 (2014)>
=2014AJ....148...27C 2014AJ....148...27C
ADC_Keywords: Clusters, globular ; Photometry, CMT1T2V
Keywords: globular clusters: individual: NGC 1851 - stars: imaging -
Hertzsprung-Russell and C-M diagrams - stars: abundances
Abstract:
The analysis of multiple populations (MPs) in globular clusters (GCs)
has become a forefront area of research in astronomy. Multiple red
giant branches (RGBs), subgiant branches (SGBs), and even main
sequences (MSs) have now been observed photometrically in many GCs,
while broad abundance distributions of certain elements have been
detected spectroscopically in most, if not all, GCs. UV photometry has
been crucial in discovering and analyzing these MPs, but the Johnson U
and the Stromgren and Sloan u filters that have generally been used
are relatively inefficient and very sensitive to reddening and
atmospheric extinction. In contrast, the Washington C filter is much
broader and redder than these competing UV filters, making it far more
efficient at detecting MPs and much less sensitive to reddening and
extinction. Here, we investigate the use of the Washington system to
uncover MPs using only a 1 m telescope. Our analysis of the
well-studied GC NGC 1851 finds that the C filter is both very
efficient and effective at detecting its previously discovered MPs in
the RGB and SGB. Remarkably, we have also detected an intrinsically
broad MS best characterized by two distinct but heavily overlapping
populations that cannot be explained by binaries, field stars, or
photometric errors. The MS distribution is in very good agreement with
that seen on the RGB, with ∼30% of the stars belonging to the second
population. There is also evidence for two sequences in the red
horizontal branch, but this appears to be unrelated to the MPs in this
cluster. Neither of these latter phenomena have been observed
previously in this cluster. The redder MS stars are also more
centrally concentrated than the blue MS. This is the first time MPs in
an MS have been discovered from the ground, and using only a 1 m
telescope. The Washington system thus proves to be a very powerful
tool for investigating MPs, and holds particular promise for
extragalactic objects where photons are limited.
Description:
Our observations of NGC 1851 were performed at the SWOPE 1m telescope
at Las Campanas Observatory. Both the R and T2 observations were
performed during gray time on 2011-10-21 with one short C image
taken that night before the Moon rose, and the remainder of the C
observations were performed before the Moon rose on 2011-10-25.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 95 18460 The Washington photometric catalog of NGC 1851
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See also:
VII/202 : Globular Clusters in the Milky Way (Harris, 1997)
J/A+A/539/A19 : Horizontal branch stars in NGC 1851 (Gratton+, 2012)
J/A+A/533/A69 : Spectroscopy of 124 RGB stars in NGC 1851 (Carretta+, 2011)
J/AJ/122/2569 : VI photometry of NGC 288, 362 and 1851 (Bellazzini+, 2001)
J/PASP/104/1063 : BV Color-Magnitude Diagram for NGC 1851 (Walker 1992)
Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 5 I5 --- Name [2/18461] Source identifier
<[CGV2014] NNNNN> in Simbad
7- 13 F7.2 pix Xpos X pixel coordinate (scale=0.435arcsec/pix)
15- 21 F7.2 pix Ypos Y pixel coordinate (scale=0.435arcsec/pix)
23- 29 F7.3 mag Cmag ?=-99.999 Final Washington C band magnitude
31- 37 F7.3 mag e_Cmag ?=-99.999 Error in Cmag
39- 45 F7.3 mag s_Cmag ?=-99.999 Photometric dispersion in Cmag
47- 52 F6.3 mag T1mag Final Washington T1 band magnitude
54- 58 F5.3 mag e_T1mag Error in T1mag
60- 64 F5.3 mag s_T1mag ?=-99.999 Photometric dispersion in T2mag
66- 72 F7.3 mag T2mag ?=-99.999 Final Washington T2 band magnitude
74- 80 F7.3 mag e_T2mag ?=-99.999 Error in T2mag
82- 88 F7.3 mag s_T2mag ?=-99.999 Photometric dispersion in T2mag
90- 91 I2 --- o_Cmag [0/11] Number of independent observations for Cmag
93 I1 --- o_T1mag [1/4] Number of independent observations for T1mag
95 I1 --- o_T2mag [0/4] Number of independent observations for T2mag
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History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Sylvain Guehenneux [CDS] 13-Oct-2014