Dynamical evolution and spectral characteristics of the stellar group Mamajek 2

Abstract

The dynamical evolution of the recently detected stellar group Mamajek 2 is studied by means of its past 3D orbit. The past orbits of the open clusters NGC 2516 and $\alpha$ Persei, belonging to the so-called "Local Association", were also computed in order to check for a possible common past dynamical evolution of these systems. To complete the data of the Mamajek 2 small group, we have obtained high resolution FEROS spectra to measure the radial and also the projected rotational velocities of its members; an estimate of its metallicity was obtained as well. Two exceptionally low rotating A-type stars turned out to be a strong magnetic Ap star in one case, and a normal A0 star with near-solar metallicity in the other. The dynamical results showed that NGC 2516 and Mamajek 2 may have had a common origin at the age of 135 $\pm$ 5 Myr. This dynamical age confirms the individual ages of 140 Myr for NGC 2516 and 120 $\pm$ 25 Myr for Mamajek 2 obtained independently by photometric methods. Both these groups appear to have the same solar metallicity giving support to a common birth scenario. The dynamical approach is showing that some bound open clusters can form in a coeval fashion with unbound stellar groups or with associations.

Full text

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arXiv:0810.1198v1 [astro-ph] 7 Oct 2008
Dynamical evolution and spectral characteristics of the stellar
group Mamajek 2
E. Jilinski1,2, V. G. Ortega1, R. de la Reza1, N.A. Drake1,3, B. Bazzanella1
jilinski@on.br
Received
; accepted
To appear in Astrophysical J.
1Observat´ orio Nacional, Rua General Jos´ e Cristino 77, S˜ ao Cristov˜ ao, 20921-400, Rio de
Janeiro, Brazil.
2Pulkovo Observatory, Russian Academy of Sciences, 65, Pulkovo, 196140 St. Petersburg,
Russia.
3Sobolev Astronomical Institute, St. Petersburg State University, Universitetskii pr. 2,
Petrodvorets, 198504, St. Petersburg, Russia.

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ABSTRACT
The dynamical evolution of the recently detected stellar group Mamajek 2
is studied by means of its past 3D orbit. The past orbits of the open clusters
NGC 2516 and α Persei, belonging to the so-called “Local Association”, were
also computed in order to check for a possible common past dynamical evolution
of these systems. To complete the data of the Mamajek 2 small group, we
have obtained high resolution FEROS spectra to measure the radial and also
the projected rotational velocities of its members; an estimate of its metallicity
was obtained as well. Two exceptionally low rotating A-type stars turned out
to be a strong magnetic Ap star in one case, and a normal A0 star with near-
solar metallicity in the other. The dynamical results showed that NGC 2516
and Mamajek 2 may have had a common origin at the age of 135 ± 5 Myr.
This dynamical age confirms the individual ages of 140 Myr for NGC 2516 and
120 ± 25 Myr for Mamajek 2 obtained independently by photometric methods.
Both these groups appear to have the same solar metallicity giving support to
a common birth scenario. The dynamical approach is showing that some bound
open clusters can form in a coeval fashion with unbound stellar groups or with
associations.
Subject headings: (GALAXY:) open clusters and associations: individual NGC 2516,
Mamajek 2, α Persei, stars: individual HD 158450, HD 160142

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1. Introduction
The recent discoveries of young loose stellar groups or unbound associations in the
solar neighborhood, at distances up to about 100 pc (Zuckerman & Song 2004, Torres et
al. 2008) motivated a deeper research of what is known as the Local Association or the
Pleiades Supercluster (Eggen 1975, 1983). Historically, this supercluster has been conceived
as a very large structure with a radius of a few hundred parsecs containing stars with
space velocities similar to those of the Pleiades cluster. Considered like this, the Local
Association would contain different smaller structures, such as the Pleiades, NGC 2516,
α Persei, NGC 1039, IC 2602 open clusters and the whole OB Sco-Cen association.
In an attempt to visualize the structural/dynamics of the Pleiades Supercluster,
Skuljan et al. (1999), based on Hipparcos astrometry, made a 2D analysis and found among
others, the existence of a “Pleiades branch” in the U,V space of velocities containing the
separated Hyades and Pleiades groups. However, Skuljan et al. (1999) had difficulties in
concluding whether this extended branch was due either to some special feature of the
potential of the Galaxy or to the local spiral structure. Moreover, a complete mixture of
ages was in play in this branch covering some few hundred million years.
Some years ago we initiated a 3D Galactic dynamical approach to study in detail the
past evolution (ages and formation regions) of different young moving groups related to
the Sco-Cen association. First, we studied the low-mass, loose stellar group β Pic, the ǫ
and η Chamaeleontis young cluster and the TW Hya association (Ortega et al. 2002, 2004,
Jilinski et al. 2005 and de la Reza et al. 2006). All these groups were probably originated,
as well as the younger Sco-Cen component Upper Scorpius (US), in the mainstream of the
older Sco-Cen subgroups Lower Centaurus Crux (LCC) and Upper Centaurus Lupus (UCL)
during the last 5 to 11 Myrs.
In Ortega et al. (2007) we tackled the dynamical evolution of the Pleiades open cluster

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and the AB Dor group. Here, we investigate the common evolution of two further members
of the Local Association: the open cluster NGC 2516, and Mamajek 2, a recent detected
and studied stellar group (Mamajek 2006). This author suggests that the Mamajek 2 group
may have formed in the same star forming region as the clusters Pleiades, NGC 2516 and
α Persei and the AB Dor group. In our previous work (Ortega et al 2007), we found that
while the Pleiades cluster and the AB Dor group, in fact, could have had the same origin,
the α Persei cluster shows a completely different past dynamical evolution. In the present
work, we show that the group Mamajek 2 and the open cluster NGC 2516 may have had a
common origin, however, again quite distinct from that of the α Persei cluster.
This paper is organized as follows: In Section 2 we present the main properties of the
concerned stellar groups. Section 3 is devoted to the presentation of the observations of the
stars of the Mamajek 2 group together with the measured radial and rotational velocities.
Section 4 contains the dynamical aspects of the involved stellar groups. Section 5 presents
the spectral analysis for two low- rotation A-type stars belonging to the Mamajek 2 group.
Finally, Section 6 is devoted to the discussion and conclusions.
2. Main properties of the open cluster NGC 2516 and the Mamajek 2 stellar
group
2.1.The NGC 2516 open cluster
NGC 2516, also called the “southern Pleiades” by Eggen (1972, 1983), is a rich, nearby,
bright open cluster affected by small extinction. In studying possible large gravitational
tidal effects (its present tidal radius is about 9 pc (Piskunov et al. 2008) on this cluster,
Bergond et al. (2001) noted the regular circular geometry of its center from where two
tails emerge almost perpendicular to the Galactic plane. The total mass of NGC 2516 is

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presently not well known. Values as low as 170 solar masses have been proposed by Pandey
et al. (1987) and as high as about 1000 M⊙by Dachs & Kabus (1989). More recently a
mass of ∼ 250 M⊙for this cluster has been quoted by Piskunov et al. (2008). In contrast,
its age appears to be quite well established. Recent literature adopts the age determined by
Meynet et al. (1993) by fitting NGC 2516 with an isochrone at 140 Myr.
Concerning the metallicity of NGC 2516, the situation does not seem to be entirely
clear. This is mainly due to the fact that bright F stars in NGC 2516, which would
normally be used to derive the abundances, possess, in general, largely broadened spectral
lines. Terndrup et al. (2002) present a quite thorough discussion of past and more recent
metallicity determinations. Nevertheless, because of the importance of this parameter for
the present study, some insights will be furnished. After an initial period where several
authors (see Terndrup et al. 2002) found for NGC 2516 a metallicity of a few tenths dex
below the solar, more recent analysis (Irwin et al. 2007, Jeffries et al. 2001, Sciortino et al.
2001, Sung et al. 2002) places NGC 2516 with a metallicity close to solar. This is in fact
the conclusion of a careful spectroscopic analysis, albeit based on only two low rotating,
relatively hot stars, that gives [Fe/H] = +0.01 ± 0.17. Also, a photometric determination
yielded a value of [Fe/H] = −0.05 ± 0.14. As commented by these authors, an analysis of
faint G stars of NGC 2516 with larger telescopes will be necessary to settle the question.
In our discussion we shall adopt a near solar metallicity for NGC 2516 which then will
be compared with that estimated for Mamajek 2. In Section 4 we will see how a near solar
abundance of NGC 2516 will be indirectly compatible with its dynamical age.

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2.2.The Mamajek 2 stellar group
This new stellar aggregate was discovered by Mamajek (2006) based on the common
parallel proper motions and similar trigonometric parallaxes of the stars. It contains the
bright B8 giant µ Ophiuchus and eight further B and A type stars. This author proposes
the coevality of this group located at a present distance of 170 pc and with an age of
120 ± 25 Myr. The scatter distance of the group members is ± 5 pc and the total mass is
of the order of 24 M⊙. According to Mamajek (2006), the half-mass radius of this cluster is
0.4 pc while its tidal radius is of the order of 4 pc.
Adopting the canonical Initial Mass Function (IMF) of Kroupa (2001), Mamajek (2006)
proposes that the present existence of these nine stars would imply an initial population of
∼ 200 systems. So far the low mass members of this group have not been detected. In this
work we have measured the radial velocities of the proposed members in order to confirm
the reality of this stellar group. In fact, previously only two stars, (µ Oph (HD 159975)
and HD 158450) had their radial velocities measured (Mamajek 2006). No attempt is made
here to detect other cooler and low mass members.
3. Observations of the Mamajek 2 stellar group
3.1.Radial velocity determinations
High resolution spectra have been obtained for eight of the nine known members of
the Mamajek 2 group. Unfortunately, for only one star (HD 159874) this was not possible
because of bad weather conditions. There are no data concerning the radial velocity of this
star in the literature. For these observations we used the FEROS spectrograph attached to
the 2.2 m telescope of ESO at La Silla - Chile. The spectra were obtained using a resolution
of about 48000 and covering a spectral range from 3800 to 9200˚ A. The main objective of

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these observations was to measure the radial and rotational projected velocities and also
to determine metallic abundances for some representative stars of the group. Standard
FEROS pipeline resources for calibration purposes have been used.
Precise radial velocities measurements for B and A-type stars are intrinsically difficult.
This is partly due to effects of their high temperatures and large or very large rotational
velocities, especially of B stars. In addition, the few lines available for both types of stars
makes the measurements difficult. These difficulties are reflected in the fact that there
are no appropriate radial velocities standard stars (again especially for B-type stars).
Nevertheless, the high stability of FEROS and its large spectral dispersion, make this
instrument one of the best available for radial velocity measurements. To measure these
velocities we have followed the same methodology as previously used with hot stars of the
Sco-Cen OB association (Jilinski et al. 2006). The cross-correlation technique, which is used
for precise RV determinations in later type stars, when applied to the hotter BA stars can
be problematical as early type stars spectra show few absorption lines which are, in many
cases, intrinsically broad (up to a few hundreds kms−1) due to stellar rotation. Because of
this we determine the doppler shift for each measurable unblended spectral line of He I, C
II, N II, O II, Mg II, Si II and Si III, Fe I and Fe II, Ti I and Ti II relative to their rest
wavelengths. The measured radial velocities with their error bars are listed in Table 1.
3.2. Projected rotational velocities
Projected rotational velocities were determined by the synthetic spectra method,
the most accurate method for vsini determination consisting in computing the synthetic
spectrum and comparing with the observed one (de Medeiros et al. 2006). To select the
appropriate atmospheric models, we used the (B − V )0– Teff calibrations from Kenyon
& Hartmann (1995) to determine the effective temperature. Observed values of the color

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indexes B − V and color excesses EB−V were taken from Table 2 of Mamajek (2006). For
the star µ Oph we used Teff = 12020 K taken from Glagolevsky (1994). A value of the
surface gravity equals to logg = 4.0 was adopted for the synthetic spectra calculations.
In general, in order to determine vsini, we selected two spectral regions: 7770 - 7780˚ A
containing the Oi infrared triplet and 4470 - 4490˚ A containing the Mgii 4481.2 line. The
values of vsini obtained for the Mamajek 2 group are presented in Table 1. Six stars of the
group have high rotation compatible with their spectral types (Royer et al. 2002, 2004, Abt
& Morrell 1995) and two stars, HD 158450 and HD 160142, show low projected rotation
velocities. Star HD 158450 is a peculiar A-type star with an important magnetic field
(Kudryavtsev et al. 2006) believed to be responsible for its low rotation. Star HD 160142,
is a normal A0 star, probably seen pole-on. For HD 160142, besides the above mentioned
spectral lines, we consider also the spectral synthesis of the Feii lines at 4489.183, 4491.405,
and 6147.741˚ A as well as the Tiii lines at 4464.449, 4468.507, and 4488.325˚ A, which
allowed us to determine the projected rotation velocity of this star with an even higher
precision. In the case of the chemically peculiar star HD 158450 which has spectral lines
broadened by the magnetic field effects, we performed an analysis of the magnetically
splitted Feii line at 6149.258˚ A.
4. Dynamical evolution calculations
To study the dynamical evolution of stellar groups it is necessary to integrate back
in time the 3D orbits of the stars. Beginning with the present distance or initial XY Z
positions relative to the Sun and with the presently observed spatial velocities (UV W),
the 3D past orbits are calculated using a modeled Galactic potential. More details of the
adopted methodology can be found in previous works (Ortega et al. 2002, 2004, 2007,
Jilinski et al. 2005 and de la Reza et al. 2006).

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For small stellar groups and stellar associations, with ages less than ∼ 12 Myr, it was
possible to find the dynamical ages and the respective birthplaces by determining the first
maximum confinement of the individual orbits. This was the case for the β Pic and TW Hya
associations (Ortega et al. 2002, 2004 and de la Reza et al. 2006). For clusters, instead,
it is advantageous to work with the mean values of the velocities of the stars in order to
minimize input errors, especially in the case of a relatively large age of the system. In this
case, we evolve the group with unknown age together with another system, preferentially a
cluster, having a reasonably well determined age. If both stellar systems attain a maximum
approach at some time and if such approach occurs with low relative velocity, we consider
this time as the age of the first group. This technique was employed by us in a study of the
Pleiades open cluster and the AB Dor association (Ortega et al. 2007). We shall use this
methodology in the present work.
Dynamical calculations can give good results only if present positions and space
velocities of cluster members have the most precise values as possible. For this reason stars
with Hipparcos (ESA, 1997) astrometric data are preferentially used in this work. Likewise,
radial velocities of high quality are important. For the Mamajek 2 group we used the mean
value of the vradof the Hipparcos stellar members which were measured in this work. These
measurements are shown in Table 1. Concerning the mean radial velocities for NGC 2516,
we considered the following data: 23.8 ± 0.3 kms−1obtained for 24 members by Jeffries et
al. (1998); 22.0±0.2 kms−1for 22 members (Gonz´ alez & Lapasset 2000); 22.7±0.4 kms−1
for 14 members (Robichon et al. 1999) and finally 24.2 ± 0.2 kms−1for 57 members by
Terndrup et al. (2002). We also check the possibility of common evolution of the Mamajek 2
group with the α Persei cluster, as suggested by Mamajek (2006). For the α Persei cluster
we used the published UV W values of Robichon et al. (1999) obtained using 46 Hipparcos
star members. Spatial velocities given by Makarov (2007) were also considered for this
cluster, noting however, that in this case the distances were kinematically determined.

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For NGC 2516, the use of a set of different mean radial velocities resulted in different
past orbits, clearly indicating that the dynamical calculations are sensitive to this quantity,
especially for ages larger than 100 Myr. The sample of Terndrup et al (2002) contains a
largest number of stars. The mean radial velocity quoted by these authors were obtained
combining their own observed velocities (33 determinations) with those in Jefferies et al.
(1998) for six common members. This also allowed to have an estimate of the systematical
error. The best result indicating a very probable common origin of NGC 2516 and
Mamajek 2 groups is that obtained using the data of Terndrup et al. (2002). This can
be appreciated in Figure 1 where the past 3D distance between these two groups is shown
as a function of time in the past. A maximum approach or a minimum distance of nearly
20 pc is obtained at −135 ± 5 Myr. The uncertainty in the age was estimated through
Monte Carlo simulations using 1000 realizations. In Figure 1 (low panel) we show the past
evolution of their relative velocity and angle of approximation.
In Figure 2 we show a similar analysis for the past evolution of the α Persei cluster and
the Mamajek 2 stellar group. It shows that, for both cases Robichon and Makarov, these
systems do not achieve any approach in their evolution. Dynamically there is no relation
between them.
Figure 3 shows the mean 3D orbits of Mamajek 2 and the cluster NGC 2516 projected
on to the Galactic plane (XY) and on the plane (YZ)perpendicular to the Galactic plane.
5. The magnetic chemically peculiar star HD 158450
One of the stars of Mamajek 2 group, HD 158450, shows a highly peculiar spectrum.
This star was included in the list of “the brighter stars of astrophysical interest in the
southern sky” by Bidelman & MacConnell (1973) based on the Michigan blue objective-

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prism survey of the southern sky as a peculiar A star of the Sr-Cr-Eu type. The presence
of a magnetic field on the surface of this star was recently discovered by Kudryavtsev et al.
(2006) from spectropolarimetric observations of a sample of chemically peculiar stars at the
6-m telescope of the SAO RAS, Russia.
We have only one high resolution spectrum of HD 158450 obtained at June 2,
2007 (MJD = 54253.31316). Analysis of this spectrum indicated the presence of a strong
magnetic field resulting in the magnetic splitting of some spectral lines. The most prominent
spectral feature is the Feii 6149.258˚ A line, commonly used for magnetic field strength
determination, due to the specific Zeeman pattern of this line consisting of two π- and two
σ-components with the same wavelength shift (Mathys et al. 1997). In unpolarized light
the profile of this line in the presence of a magnetic field is a simple doublet (see Figure
4). The wavelength shift between the red and blue components of the Feii 6149.258˚ A line
in our spectrum of HD 158450 is ∆λZ= λr− λb= 0.530 ± 0.010˚ A.The mean magnetic
field modulus (the line-intensity weighted average over the visible stellar hemisphere of the
modulus of the magnetic vector) can be estimated by the equation (St¨ utz et al. 2003):
∆λZ
2
= 4.67 · 10−13· geff· λ2· ?H?,
where ∆λZ- measured Zeeman splitting in˚ A, geff= 1.35 - effective Land´ e factor, λ - central
wavelength of the unshifted line in˚ A, and ?H? - mean magnetic field modulus in Gauss.
Using this formula we find the mean magnetic field modulus ?H? = 11100 ± 200 G.
In Kudryavtsev et al. (2006) the mean longitudinal component of the magnetic
field was determined to be ?Bl2?
1
2= 1570 ± 180 G, whereas the individual values of the
longitudinal magnetic field vary for different dates from −2920 ± 200 G to +810 ± 240 G,
indicating strong variation of the magnetic field strength with stellar rotation.

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The high resolution spectrum obtained by us permitted to measure the projected
rotational velocity of HD 158450 with high precision. Kudryavtsev et al. (2007) determined
vsini = 20 ± 2 kms−1, a value close to the lower limit for a projected rotational velocity
of 18 kms−1which can be achieved with moderate-resolution spectra (R = 15000) used in
the above mentioned paper. Kudryavtsev et al. (2007) explicitly say in their article that
rotational velocities of magnetic stars have to be determined using high resolution spectra
and by the comparison of observed spectra with the synthetic ones. We note that the
rotational velocities in Kudryavtsev et al. were estimated by FWHM measurements of two
Feii lines (4508 and 4491˚ A) having low Lande factors.
The approximation of the Feii 6149.258˚ A magnetically split line by a synthetic
spectrum showed that this star has the significantly lower projected rotational velocity of
vsini = 9 ± 1 kms−1.
Our determination of the radial velocity of this star (vrad= −17.3 kms−1) is in perfect
agreement with the value of vrad= −17.2 ± 1.4 kms−1obtained by Kudryavtsev et al.
(2007). Previously, Grenier et al. (1999) found the value of −22.0±4.2 kms−1for the radial
velocity of this star. Although the difference in radial velocity is rather large, we note that
Grenier at al. (1999) measured the radial velocity of this star by correlation with template
of the same spectral class. The peculiar nature of the spectrum of HD 158450 could lead to
some discrepancy. That is why the double, or multiple nature of this star, as pointed out in
the SIMBAD database, needs further investigation by monitoring its radial velocity.
We detected resolved magnetically split lines in this this chemically peculiar star.
However, a careful determination of the atmospheric parameters and a detailed abundance
analysis of this star is beyond the scope of this paper. We emphasize that HD 158450 is a
member of the Mamajek 2 stellar group which has a quite well determined age. A detailed
study of this star would then be very important for the understanding of the origin and

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evolution of stellar magnetic fields, a problem not clarified until now.
6. A chemical analysis of HD 160142
The star HD 158450 being an Ap star, is not appropriate for the metallicity estimation
of the Mamajek 2 group. We invested then in an analysis of the normal low rotating A0 star
HD 160142 using the last version of the well-known MOOG program (Sneden 1973). In order
to determine the atmospheric parameters of HD 160142, we measured the equivalent widths
of neutral and ionized iron lines whose oscillator strength values (loggf) were analyzed
by Lambert et al. (1996). Our analysis was somewhat hampered by the relatively noisy
observed spectrum of this star what makes difficult the measurement of faint Fei lines with
equivalent widths lower than 10 - 15 m˚ A. After eliminating suspected blends and very weak
lines, whose equivalent widths could not be measured in our spectrum with high precision,
we used 25 Fei and 17 Feii lines. Following the usual iterative procedure, we derived the
effective temperature and microturbulent velocity by requiring the iron abundance to be
independent of the excitation potential and of the equivalent width (Fig. 5). The surface
gravity was derived from the ionization equilibrium by finding the value for which the iron
abundances from Fei and Feii coincide. The Kurucz’s (1993) grid of atmospheric models
was used in the calculations. The following atmospheric parameters (effective temperature,
surface gravity, and microturbulent velocity) were derived: Teff= 9320 K, logg = 3.8, and
ξm= 2.07 kms−1. In this case, the metallicity of HD 160142 is logε(Fe) = 7.62 ± 0.05,
i.e. [Fe/H]= +0.10. We must note however, that even if the Teffvalue coincides with the
photometric temperature based on the Teffversus (B −V )0calibration (Kenyon & Hartman
1995), the obtained value of the surface gravity (logg = 3.8) is too low for a star of such a
temperature and age.
A different method was then explored to estimate the surface gravity of HD 160142 by

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analyzing its position on the HR diagram. Knowing the effective temperature Teff= 9320 K
from the photometric data, the surface gravity can be inferred from internal structure
models. Using isochrones from the models of Lejeune & Schaerer (2001) for solar metallicity
(Z = 0.02) and assuming for HD 160142 an age of logt = 8.10 or 126 Myr (similar to the
age of the Mamajek 2 group) for HD 160142 we obtained for the surface gravity the value
logg = 4.27 resulting in a solar iron abundance for the Fei lines (logε(Fe) = 7.51 ± 0.05)
and a higher abundance for the Feii lines (logε(Fe) = 7.69 ± 0.08).
This discrepancy between the iron abundances derived from the Fei and Feii lines
may be due to different reasons. Problems with the Fei/Feii ionization balance have
been reported for a wide range of stars (e.g. Allende Prieto et al. 1999). Recently, Yoon
et al. (2008) analyzing a high-resolution spectrum of the known A-type star Vega have
demonstrated the effects of rotation on the derived abundances. They found that, in the
case of Vega, the rotation induces an iron ionization imbalance amounting to ∼ 0.35 dex,
but in the opposite sense of that induced by departures from LTE. Thus, if HD 160142
is a high rotating star seen nearly pole-on, we have to use the model with a lower value
of the surface gravity forcing to achieve the ionization balance, i.e. equality between the
iron abundance derived from Fei lines and that derived from Feii lines. We also note, that
in the case of HD 160142 the microturbulent velocity (ξ = 2.07 kms−1) was determined
using the relatively weak Fei lines, and may be different for the more intense Feii lines.
On the other hand, noise can perturb somehow a proper determination of the continuum
introducing an error in the equivalent width measurements of the weak Fei lines.
Error estimates of the derived atmospheric parameters are not straightforward. An
uncertainty of ±0.02 mag in the correction for interstellar reddening of the observed
(B −V ) index results in an uncertainty of about ±200 K in the “photometric” temperature.
The typical uncertainty in the microturbulent velocity, obtained in the usual way by

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finding the value which provides iron abundance independent of the equivalent width of
the Fei lines, is about ±0.2 kms−1. Taking into account uncertainties in the atmospheric
parameters, equivalents widths determinations, non-LTE effects, and possible effects of
rotation (if HD 160142 is indeed highly rotating A star seen nearly pole-on) we estimate
that HD 160142 has near solar metallicity logε(Fe) = 7.62 ± 0.10. Nevertheless, taking into
consideration all possible sources of uncertainties, we have to note that the real value of the
error can be somewhat greater than 0.10 dex.
It is then fair to conclude that until other cooler stars of Mamajek 2 will be discovered
and analyzed, a solar abundance for the A0-type star HD 160142 may be adopted as
representative of the metallicity of this stellar group.
7. Discussion and conclusions
Calculations of the dynamical history of young stellar groups is a new source of
astrophysical knowledge. In the present work we have investigated the past evolution of
three stellar groups, belonging to what historically has been known as the Local Association
or the Pleiades Supercluster, with the aim to disentangle the dynamical evolution of their
components and make evident possible relations existing between them. These three groups
are the open clusters NGC 2516 and α Persei and the recent detected small group called
Mamajek 2 (Mamajek 2006).
We have obtained new data new data for eight of the nine known members of the
Mamajek 2 group by means of high resolution FEROS spectra. This allows to measure the
radial velocities with a method especially devised for hot stars of spectral types B and A
(Jilinski et al. 2006)(see section 3.1). Taking advantage of the high quality of the data,
rotational, projected velocities as well as information concerning the nature and chemical