J/ApJ/766/136 Stellar encounter rates in Galactic GCs (Bahramian+, 2013)
Stellar encounter rate in Galactic globular clusters.
Bahramian A., Heinke C.O., Sivakoff G.R., Gladstone J.C.
<Astrophys. J., 766, 136 (2013)>
=2013ApJ...766..136B 2013ApJ...766..136B
ADC_Keywords: Clusters, globular ; Stars, distances ; Velocity dispersion
Keywords: globular clusters: general; pulsars: general; X-rays: binaries
Abstract:
The high stellar densities in the cores of globular clusters cause
significant stellar interactions. These stellar interactions can
produce close binary mass-transferring systems involving compact
objects and their progeny, such as X-ray binaries and radio
millisecond pulsars. Comparing the numbers of these systems and
interaction rates in different clusters drives our understanding of
how cluster parameters affect the production of close binaries. In
this paper we estimate stellar encounter rates (Γ) for 124
Galactic globular clusters based on observational data as opposed to
the methods previously employed, which assumed "King-model" profiles
for all clusters. By deprojecting cluster surface brightness profiles
to estimate luminosity density profiles, we treat "King-model" and
"core-collapsed" clusters in the same way. In addition, we use Monte
Carlo simulations to investigate the effects of uncertainties in
various observational parameters (distance, reddening, surface
brightness) on Γ, producing the first catalog of globular
cluster stellar encounter rates with estimated errors. Comparing our
results with published observations of likely products of stellar
interactions (numbers of X-ray binaries, numbers of radio millisecond
pulsars, and γ-ray luminosity) we find both clear correlations
and some differences with published results.
Description:
Our sample includes 124 Galactic GCs for which found published surface
brightness (SB) profiles. For 85 GCs we used the SB profiles compiled
by Trager et al. (1995, J/AJ/109/218). These data sets were obtained
from various ground-based observations, mostly from the Berkeley
Globular Cluster survey by Djorgovski & King (1986ApJ...305L..61D 1986ApJ...305L..61D).
Noyola & Gebhardt (2006, J/AJ/132/447) provide SB profiles (from HST
observations) for 38 GCs, some of which are also listed in Harris
Catalog (VII/202).
We have full velocity dispersion (σ) profiles for only 14
clusters (from Zocchi et al. 2012A&A...539A..65Z 2012A&A...539A..65Z (using their
King-model fits to the profiles), Sollima et al. 2012ApJ...755..156S 2012ApJ...755..156S
and Murphy et al. 2011ApJ...732...67M 2011ApJ...732...67M). For the remaining clusters, we
only consider the central σ value. See section 2.2.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table2.dat 89 124 The Γ values of the globular clusters (GCs)
table4.dat 92 143 The Γ calculations and 1σ error
estimations based on different equations
--------------------------------------------------------------------------------
See also:
VII/202 : Globular Clusters in the Milky Way (Harris, 1997)
J/MNRAS/392/L55 : GCs in M31 from K-band photometry (Peacock+, 2009)
J/AJ/133/1287 : JHK phot. of 24 Galactic globular clusters (Valenti+, 2007)
J/ApJ/657/286 : Chandra X-ray sources in M30 (NGC 7099) (Lugger+, 2007)
J/ApJ/651/1098 : Faint X-ray sources in Terzan 5 (Heinke+, 2006)
J/AJ/132/447 : Surface Brightness of 38 Gal. GCs (Noyola+, 2006)
J/ApJS/161/304 : Star clusters in Milky Way & satellites (McLaughlin+, 2005)
J/ApJ/613/279 : ACS Virgo Cluster Survey. III. M87 (Jordan+, 2004)
J/AJ/109/218 : Catalogue of globular cluster profiles (Trager+ 1995)
Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 8 A8 --- Name Globular cluster identification
10- 12 A3 --- f_Name Flag(s) on cluster (1)
14- 17 F4.2 mag E(B-V) [0.01/2.7] Foreground reddening
19- 22 F4.2 mag e_E(B-V) Uncertainty in E(B-V)
24- 28 F5.2 mag m-M [12.3/21.5] Visual distance modulus
30- 32 F3.1 mag e_m-M Uncertainty in m-M
34- 39 F6.2 kpc Dist [2.2/119.6] Distance
41- 45 F5.2 km/s sigma [0.4/19] Velocity dispersion σ
47- 50 F4.2 km/s e_sigma Uncertainty in sigma
52 A1 --- r_sigma [gh] Reference for sigma (2)
54 A1 --- Ref Reference for surface brightness profile (3)
56- 57 A2 --- CC [c: ] c=core collapsed cluster flag, from
Harris Catalog (VII/202)
59- 62 F4.2 mag/arcsec2 dSB [0.03/0.5] Assumed uncertainty in surface
brightness profile
64- 71 E8.3 --- Gamma [0.001/6800] Calculated stellar encounter
rate Γ, normalized to NGC104-1000 (4)
73- 80 E8.3 --- e_Gamma Lower bound on Gamma
82- 89 E8.3 --- E_Gamma Upper bound on Gamma
--------------------------------------------------------------------------------
Note (1): Flag as follows:
a = Extinction calculation based on Massari et al 2012ApJ...755L..32M 2012ApJ...755L..32M. To
calculate Av based on this value, we used Rv=2.83; although for the
rest we assumed Rv=3.1
b = we ignored data points in the region log(r")>1.9 as there were
non-physical fluctuations probably due to high order of the Chebyshev fit.
c = Profiles from T95 are uncalibrated. Following McLaughlin & van der Marel
2005 (J/ApJS/161/304), we calibrated those by assuming that their values
of surface brightness for the center is equal to the central surface
brightness values from Harris Catalog (VII/202).
d = The reported uncertainties from Harris Catalog (VII/202) on sigma are more
than 50%. So for these two, we truncated the sigma distribution at
1 delta instead.
Note (2): Reference as follows:
g = Gnedin et al. (2002ApJ...568L..23G 2002ApJ...568L..23G);
h = Harris Catalog (VII/202).
Note (3): Reference as follows:
l = Lanzoni et al (2010ApJ...717..653L 2010ApJ...717..653L);
n = Noyola & Gebhardt (2006, Cat. J/AJ/132/447);
t = Trager et al. (1995, Cat. J/AJ/109/218).
Note (4): Normalization: Assumed 1000 for ngc104. Table sorted from highest
Gamma to lowest.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table4.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 8 A8 --- Name Globular cluster identification (6)
10 A1 --- n_Gamma3 [ab] Note on Gamma values (7)
12- 20 E9.3 --- Gamma1 ? Integrated stellar encounter rate
(4πσ-1∫ρ2r2dr)
22- 29 E8.3 --- e_Gamma1 ? Lower uncertainty in Gamma1
31- 38 E8.3 --- E_Gamma1 ? Upper uncertainty in Gamma1
40- 47 E8.3 --- Gamma2 ? Estimated stellar encounter rate based
on ρ2r3/σ
49- 56 E8.3 --- e_Gamma2 ? Lower uncertainty in Gamma2
58- 65 E8.3 --- E_Gamma2 ? Upper uncertainty in Gamma2
67- 74 E8.3 --- Gamma3 Estimated stellar encounter rate based
on ρ1.5r2
76- 83 E8.3 --- e_Gamma3 Lower uncertainty in Gamma3
85- 92 E8.3 --- E_Gamma3 Upper uncertainty in Gamma3
--------------------------------------------------------------------------------
Note (6): Sorted first by Gamma1, then by Gamma2 and finally by Gamma3.
Note (7): Note as follows:
a = Normalization: assumed 1000 for ngc104 in each case;
b = For Terzan10, the high value of stellar encounter rate (Gamma3) is not
trustworthy (see text).
--------------------------------------------------------------------------------
History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Emmanuelle Perret [CDS] 13-Nov-2014