arXiv:astro-ph/0307007v1 1 Jul 2003 a lmn n ees Brainerd PUBLICA Tereasa OF Our and **YEAR of Clemens **VOLUME***, Evolution Dan Vol. and Series, Structure Conference The ASP Surveys: Way Milky scrn rmtePdv ru Grrie l 00.Fo t From abund 2000). solar al. a clu et by The (Girardi fit Magn group panel). nicely Padova Color (left are the their 3 which from Fig. up (MS), isochrone in Sequence build presented Main we is clear selected, which (CMD), been gram have members Once km/sec. 2.4 of sion oehrwt h etrpitdarm-t iciiaebet distribut discriminate velocity that to find radial - We the diagram members. point show non vector we the resolut panel) with high (right of together example 2 bri few a Fig. 16 present we in the panel) (left of parameters. memb 2 Fig. fundamental cluster survey In the its velocity constrain of better radial estimates to updated a us allow present which pane mot 1901, we (right tangential NGC 1 study same cluster Fig the this not in having open In are diagram an of point features concentration vector describe whose clear the (2001). better from aggregate, panel) but clear al. physical quite a left cluster, et a motions Pavani 1, open is proper by Fig. classical with paper 1901 combine (see recent NGC they a stars that which of photometry, of subject CCD aggregate the new loose is it a and is 1901 NGC 1901 NGC 1. ido beti eyipratt osri dynamical Bica evolut constrain dynamical to POCRs, of important ( very stage clusters advanced is open an object in of of kind be sample to a believed of 2001) nature physical the Abstract. Vi C/ Campus, Madrid Spain University E-28003,Madrid, Suffolk Physics, of Department Marcos Fuente la Ra´ul de Osserv Italy Vicolo Padova, University, I-35122, Padova Astronomy, of Department Carraro Giovanni Villanova, Sandro the in Remnants Cluster Open Disk Probable Galactic of survey A opoeteerysae fteGlci ikeouin nt In evolution. 1901. disk NGC Galactic i for the paramount results of of our stages are present early which the rep clusters, probe might old to they larger Moreover much of th population. remains understand stars to field and Galactic dissolution, the and evolution clusters of epeetpeiiayrslso nogigsre oprobe to survey ongoing an of results preliminary present We v r -3.5 = ± 1 disper- velocity radial a and km/sec, 1.0 o pcr,whereas spectra, ion ion. TION** enmmesand members ween o hc los- allows which ion enn.Ti is This remnant. )wihsosa shows which l) rhp nderive an ership, i tw derive we fit his N hetsasin stars ghtest trehbt a exhibits ster omto of formation e ne50Myr 500 ance bd models -body hypresent They i ae we paper his dconclude nd Doradus, in yia fa of typical eetwhat resent tro2, atorio mportance td Dia- itude o.This ion. ˜na 3, tal. et 2 Villanova et al.

Figure 1. Left: a DSS map of NGC 1901 field. Right: Vector point diagram for the stars in the field of NGC 1901.

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Figure 2. Left: a sample of Echelle spectra. Right: distribution of the stars in NGC 1901. APS Conf. Ser. Style 3

Figure 3. Left: CMD for cluster members and age determination. The filled square is a giant which actually turns out to be a non member. Right: NGC 1901 Present Day Mass Function. The dashed line is the classical Salpeter IMF, whereas the solid line is the best fit to our data. an absolute distance modulus (m − M)0 = 8.25±0.16 and a reddening E(B − V ) = 0.06±0.03, in agreement with previous study. Our spectra are single spectra, and therefore we are not able to derive a binary fraction estimate, which would be quite an important constrain for N-body models. Second epoch ob- servations are however planned. Nonetheless, by inspecting the CMD, we can argue that two stars off the Main Sequence at V ≈ 11 might be actually binaries. The Present Day Mass Function (PDMF) is not the Salpeter anymore, but (see Fig. 3, right panel) it is a power law with a slope amounting at -1.6 (solid line). Noticeably, there is a significant lack of low mass stars with respect to the Salpeter MF (dashed line). The comparison with available N-body models of clusters evolution and dissolu- tion (de la Fuente Marcos 1997) leads us to conclude that the initial population of the clusters was around 500 stars. In fact such an initial models after half a billion years should have 15 members, exactly like NGC 1901.

Work is in progress to secure membership definition for a number of POCRs on a radial velocity basis (Villanova 2003, Villanova et al. 2003).

References

Bica E., et al., 2001, A&A 366, 827 de la Fuente Marcos R., 1997, A&A 332, 764 Girardi L., et al. , 2000, A&AS 141, 371 Pavani D.B., et al., 2001, A&A 374, 554 Villanova S., 2003, Master Thesis, Padova University Villanova S., Carraro G., Skiff B., de la Fuente Marcos R., 2003, in preparation