J/ApJ/804/71 HST/WFC3 observations of stars in M15 (Larsen+, 2015)
Radial distributions of sub-populations in the globular cluster M15:
a more centrally concentrated primordial population.
Larsen S.S., Baumgardt H., Bastian N., Brodie J.P., Grundahl F., Strader J.
<Astrophys. J., 804, 71 (2015)>
=2015ApJ...804...71L 2015ApJ...804...71L
ADC_Keywords: Clusters, globular ; Photometry, HST
Keywords: globular clusters: individual: M15;
Hertzsprung-Russell and C-M diagrams; stars: abundances
Abstract:
We examine the radial distributions of stellar populations in the
globular cluster (GC) M15, using Hubble Space Telescope/Wide Field
Camera 3 (WFC3) photometry of red giants in the nitrogen-sensitive
F343N-F555W color. Surprisingly, we find that giants with "primordial"
composition (i.e., N abundances similar to those in field stars) are
the most centrally concentrated within the WFC3 field. We then combine
our WFC3 data with Sloan Digital Sky Survey u,g photometry and find
that the trend reverses for radii ≳1' (3pc) where the ratio of
primordial to N-enhanced giants increases outward, as already found by
Lardo et al. (2011A&A...525A.114L 2011A&A...525A.114L). The ratio of primordial to
enriched stars thus has a U-shaped dependency on radius with a minimum
near the half-light radius. N-body simulations show that mass
segregation might produce a trend resembling the observed one, but
only if the N-enhanced giants are ∼0.25M☉ less massive than the
primordial giants, which requires extreme He enhancement (Y≳0.40).
However, such a large difference in Y is incompatible with the
negligible optical color differences between primordial and enriched
giants, which suggest ΔY≲0.03 and thus a difference in turn-off
mass of ΔM≲0.04M☉ between the different populations. The
radial trends in M15 are thus unlikely to be of dynamical origin and
presumably reflect initial conditions, a result that challenges all
current GC formation scenarios. We note that population gradients in
the central regions of GCs remain poorly investigated and may show a
more diverse behavior than hitherto thought.
Description:
As part of our HST program (Program ID: GO-13295, P.I.: S. S. Larsen)
to study the GCs in the Fornax dwarf spheroidal galaxy (Larsen et al.
2014, J/ApJ/797/15, Paper I), we obtained short exposures of M15 in
the same filters as those used for our main program: F343N, F555W, and
F814W.
In addition to our own WFC3 data, we use imaging of M15 in the F606W
and F814W filters obtained with the Advanced Camera for Surveys (ACS)
on HST as part of the ACS Galactic GC Survey (ACSGCS; Sarajedini et
al. 2007AJ....133.1658S 2007AJ....133.1658S).
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 89 4373 *ALLFRAME photometry of M15
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Note on table1.dat: Point-spread function (PSF) fitting photometry was carried
out with ALLFRAME (Stetson 1994PASP..106..250S 1994PASP..106..250S) and calibrated to standard
STMAG magnitudes as described in Paper I (Larsen et al., 2014, J/ApJ/797/15).
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See also:
B/hst : HST Archived Exposures Catalog (STScI, 2007)
VII/202 : Globular Clusters in the Milky Way (Harris, 1997)
J/ApJ/797/15 : HST/WFC3 obs. in 4 metal-poor GCs of Fornax (Larsen+, 2014)
J/PASP/126/733 : BVI photometry of M15 stars (Feuillet+, 2014)
J/MNRAS/423/2260 : VI photometry of M15 core (Diaz-Sanchez+, 2012)
J/A+A/534/A9 : Synthetic spectra of galactic GCs (Sbordone+, 2011)
J/AJ/141/175 : Abundances in M15 RGB/RHB stars (Sobeck+, 2011)
J/A+A/524/A44 : CN and CH line strengths in 12 globulars (Pancino+, 2010)
J/A+A/505/117 : Abundances of red giants in 15 GCs (Carretta+, 2009)
J/A+A/486/437 : CN and CH line strengths in 8 Galactic GlCl (Kayser+, 2008)
J/AJ/130/1177 : C and N abundances for M15 stars (Cohen+, 2005)
J/A+A/354/169 : Metal-poor field stars abundances (Gratton+, 2000)
J/A+AS/139/393 : Young Massive Star Clusters. II. (Larsen, 1999)
J/AJ/107/1745 : HST UVI photometry of M15. II. (Yanny+, 1994)
J/AJ/106/154 : HST Observations in inner region of M15 (Ferraro+ 1993)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 6 I6 --- ID [102/569880] Source identifier
8- 14 F7.2 pix Xpix X pixel coordinate on drizzled CCD frame
16- 22 F7.2 pix Ypix Y pixel coordinate on drizzled CCD frame
24- 33 F10.6 deg RAdeg Right Ascension; decimal degrees (J2000)
35- 43 F9.6 deg DEdeg Declination; decimal degrees (J2000)
45- 50 F6.2 arcsec R [4/130] Projected distance from cluster center
52- 57 F6.3 mag F343N [15.9/20.8] HST/WFC3 F343N filter ALLFRAME
STMAG magnitude
59- 63 F5.3 mag e_F343N [0.005/0.3] Error in F343N
65- 70 F6.3 mag F555W [14.9/19] HST/WFC3 F555W filter ALLFRAME
STMAG magnitude
72- 76 F5.3 mag e_F555W [0.004/0.2] Error in F555W
78- 83 F6.3 mag F814W [15/20.5] HST/WFC3 F814W filter ALLFRAME
STMAG magnitude
85- 89 F5.3 mag e_F814W [0.006/0.5] Error in F814W
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History:
From electronic version of the journal
Reference:
Larsen et al. Paper I. 2014ApJ...797...15L 2014ApJ...797...15L Cat. J/ApJ/797/15
(End) Greg Schwarz [AAS], Emmanuelle Perret [CDS] 04-Aug-2015