Issue |
A&A
Volume 415, Number 3, March I 2004
|
|
---|---|---|
Page(s) | 971 - 985 | |
Section | Galactic structure, stellar clusters, and populations | |
DOI | https://doi.org/10.1051/0004-6361:20031588 | |
Published online | 13 February 2004 |
On the age and mass function of the globular cluster M 4: A different interpretation of recent deep HST observations*
1
European Space Agency, Space Telescope Operations Division, 3700 San Martin Drive, Baltimore MD 21218, USA
2
European Southern Observatory, Karl–Schwarzschild-Str. 2, 85748 Garching, Germany e-mail: fparesce@eso.org
3
Osservatorio Astronomico di Teramo, Via Maggini, 64100 Teramo, Italy e-mail: straniero@te.astro.it
4
Dipartimento di Fisica, Università di Pisa, Via Buonarroti 2, 56127 Pisa, Italy e-mail: prada@df.unipi.it
5
Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, 56010 Pisa, Italy
6
Osservatorio Astronomico di Teramo, Via Maggini, 64100 Teramo, Italy
Corresponding author: G. De Marchi, gdemarchi@rssd.esa.int
Received:
2
June
2003
Accepted:
30
September
2003
Very deep images of the Galactic globular cluster M 4 (NGC 6121)
through the F606W and F814W filters were taken in 2001 with the WFPC2
on board the HST. A first published analysis of this data set (Richer
et al. [CITE]) produced the result that the age of M 4 is Gyr (Hansen et al. [CITE]), thus setting a robust lower limit to the age
of the universe. In view of the great astronomical importance of
getting this number right, we have subjected the same data set to the
simplest possible photometric analysis that completely avoids uncertain
assumptions about the origin of the detected sources. This analysis
clearly reveals both a thin main sequence, from which can be deduced
the deepest statistically complete mass function yet determined for a
globular cluster, and a white dwarf (WD) sequence extending all the way
down to the
detection limit at
. The WD sequence
is abruptly terminated at exactly this limit as expected by detection
statistics. Using our most recent theoretical WD models (Prada Moroni
& Straniero [CITE]) to obtain the expected WD sequence for different
ages in the observed bandpasses, we find that the data so far obtained
do not reach the peak of the WD luminosity function, thus only
allowing one to set a lower limit to the age of M 4 of ~9 Gyr.
Thus, the problem of determining the absolute age of a globular cluster
and, therefore, the onset of GC formation with cosmologically
significant accuracy remains completely open. Only observations several
magnitudes deeper than the limit obtained so far would allow one to
approach this objective.
Key words: globular clusters: individual: M4 / stars: white dwarfs / stars: low-mass, brown dwarfs / stars: luminosity function, mass function / cosmology: cosmological parameters
© ESO, 2004
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.