Issue |
A&A
Volume 433, Number 3, April III 2005
|
|
---|---|---|
Page(s) | 917 - 929 | |
Section | Galactic structure, stellar clusters, and populations | |
DOI | https://doi.org/10.1051/0004-6361:20041113 | |
Published online | 29 March 2005 |
Spatial dependence of 2MASS luminosity and mass functions in the old open cluster NGC 188
1
Universidade Federal do Rio Grande do Sul, Instituto de Física, CP 15051, Porto Alegre 91501-970, RS, Brazil e-mail: charles@if.ufrgs.br
2
Universidade Federal de Minas Gerais, ICEx, Departamento de Física, CP 702, Belo Horizonte 30162-970, MG, Brazil
Received:
16
April
2004
Accepted:
10
December
2004
Luminosity and mass functions in the old open cluster NGC 188 are analysed
by means of J and H 2MASS photometry, which provides uniformity and spatial coverage
for a proper background subtraction. With an age of about 6–8 Gyr, NGC 188 is expected
to be suffering the effects of advanced dynamical evolution. Indeed, previous works in
optical bands have suggested the presence of mass segregation. Within the uncertainties,
the observed projected radial density profile of NGC 188 departs from the two-parameter
King model in two inner regions, which reflects the non-virialized dynamical state and
possibly, some degree of non-sphericity in the spatial shape of this old open cluster.
Fits with two and three-parameter King models to the radial distribution of stars resulted
in a core radius Rcore pc and a tidal radius Rtidal pc, about twice
as large as the visual limiting radius. The concentration parameter of
NGC 188 makes this open cluster structurally comparable to the loose globular clusters.
The present 2MASS analysis resulted in significant slope variations with distance in
the mass function
, being flat in the central parts
(
) and steep in the cluster outskirts (
). The overall
mass function has a slope
, slightly steeper than a standard Salpeter
mass function. In this context, NGC 188 is similar to the 3.2 Gyr, dynamically evolved
open cluster M 67. Solar metallicity Padova isochrone fits to the near-infrared colour-magnitude
diagram of NGC 188 resulted in an age of
Gyr. The best fit, obtained with the
7.1 Gyr isochrone, produced a distance modulus
,
, and a distance
to the Sun
kpc. The observed stellar mass (in the range
) in NGC 188 is mobs. A simple extrapolation of the observed
overall mass function to stars with
resulted in a total present mass of
mtot. On the other hand, for a more realistic initial
mass function which flattens in the low-mass range, the total mass in NGC 188 drops to
mtot. Since mass-loss processes such as evaporation
and tidal stripping have been occurring in this old open cluster for about 7 Gyr, the
primordial mass in NGC 188 must have been significantly larger than
.
We also examined the consequences of the presence of unresolved binaries and concluded that,
even if dominant in the CMD, binaries alone are not responsible for the flat central mass
function, which supports the mass-segregation scenario.
Key words: Galaxy: open clusters and associations: individual: NGC 188
© ESO, 2005
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.