The IMF of the open star cluster NGC 581 (M103): A deviation from Salpeter's law?


Jörg Sanner, Michael Geffert, Bonn, Jürgen Schmoll, Potsdam


Complete text (abstract only):

Membership determination is an essential tool to derive the initial mass function (IMF, for comprehensive reviews see Scalo 1986, 1998) of open star clusters, as the contamination of the data with field stars is a major problem. Two methods are in use nowadays and each of them has its advantages and disadvantages:

The classical method is to distinguish between cluster and field stars by their proper motions: We can expect that all the cluster stars move in the same way, whereas the field stars show different proper motions. Therefore, we can derive a membership probability for each detected star. As both old and recent photographic plates are needed to measure the proper motions, this method is limited by the poor sensitivity (compared with CCD) of the old plates.

With the introduction of CCD imaging to astronomy, the method of statistical field star subtraction became more popular. Assuming that the field stars within the cluster area are distributed in a similar way compared to those nearby, the distribution of the field stars can be subtracted from that of the (contaminated) cluster area. This makes sense for fainter stars only because of small number statistics for the bright stars.

Our project aims at a combination of these two methods for the membership determination: For the bright stars in a field of a cluster, a proper motion study is made while the faint members are selected by a statistical field star subtraction. From the cleaned data we will derive the luminosity and mass functions of the clusters. The shape of the IMF is to be studied as well as a possible inhomogenity of the mass distribution within the clusters. Finally, the universality of the IMF is to be tested.

We presented proper motions and CCD photometry of stars in the region of the open star cluster NGC 581 (M 103). Fitting isochrones of the Padua group (Bertelli et al. 1994) to the colour-magnitude diagram, we derived the following parameters for the object: log t = 7.3 (i.e. t = 2107 years), Z = 0.02, EB-V = 0.40 mag, m - M = 12.3 mag (which leads to a distance of r = 1.6 kpc).

We found a very steep slope of {Gamma} = -2.05 ± 0.09 for the initial mass function of NGC 581. This result is in good agreement with previous studies of Phelps & Janes (1993, 1994) who found {Gamma} = -1.78 for NGC 581, but does not fit to the value of {Gamma} = -1.35 for the solar neigbourhood (Salpeter 1955).



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Phelps R.L., Janes K.A., 1993, AJ 106, 1870.
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Scalo J.M., 1986, Fund. Cosm. Phys. 11, 1.
Scalo J.M., 1998, ASP Conference Series 142, 201; eds.: Gilmore, G., Howell, D.

Bibliographical details:

Jörg Sanner, Michael Geffert, Jürgen Schmoll: The IMF of the open star cluster NGC 581 (M103): A deviation from Salpeter's law? In: Peter Brosche, Wolfgang R. Dick, Oliver Schwarz, Roland Wielen (Eds.): The Message of the Angles - Astrometry from 1798 to 1998. Proceedings of the International Spring Meeting of the Astronomische Gesellschaft, Gotha, May 11-15, 1998. (Acta Historica Astronomiae; 3). Thun ; Frankfurt am Main : Deutsch, 1998, p. 203-204.