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Register a new userLuminosity and Mass Function of the Galactic open cluster NGC 2422
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Loredana Prisinzano
Publication date
2003
fields
Physics
and research's language is
English
Authors
L. Prisinzano
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We present UBVRI photometry of the open cluster NGC 2422 (age $sim 10^8$ yr) down to a limiting magnitude $Vsimeq19$. These data are used to derive the Luminosity and Mass Functions and to study the cluster spatial distribution. By considering the color-magnitude diagram data and adopting a representative cluster main sequence, we obtained a list of candidate cluster members based on a photometric criterion. Using a reference field region and an iterative procedure, a correction for contaminating field stars has been derived in order to obtain the Luminosity and the Mass Functions in the $M=0.4-3.5 M_odot$ range. By fitting the spatial distribution, we infer that a non-negligible number of cluster stars lies outside our investigated region. We have estimated a correction to the Mass Function of the cluster in order to take into account the missing cluster stars. The Present Day Mass Function of gc2422 can be represented by a power-law of index $alpha = 3.07 pm0.08 $ (rms) -- the Salpeter Mass Function in this notation has index $alpha = 2.35$ -- in the mass range $ 0.9 leq M/M_odotleq 2.5 $. The index $alpha$ and the total mass of the cluster are very similar to those of the Pleiades.
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(... abridged) The observed luminosity function can be constructed in a range of absolute integrated magnitudes $I_{M_V}= [-10, -0.5]$ mag, i.e. about 5 magnitudes deeper than in the most nearby galaxies. It increases linearly from the brightest limit to a turnover at about $I_{M_V}approx-2.5$. The slope of this linear portion is $a=0.41pm0.01$, which agrees perfectly with the slope deduced for star cluster observations in nearby galaxies. (...) We find that the initial mass function of open clusters (CIMF) has a two-segment structure with the slopes $alpha=1.66pm0.14$ in the range $log M_c/M_odot=3.37...4.93$ and $alpha=0.82pm0.14$ in the range $log M_c/M_odot=1.7...3.37$. The average mass of open clusters at birth is $4.5cdot 10^3 M_odot$, which should be compared to the average observed mass of about $700 M_odot$. The average cluster formation rate derived from the comparison of initial and observed mass functions is $bar{upsilon}=0.4 mathrm{kpc}^{-2}mathrm{Myr}^{-1}$. Multiplying by the age of the Galactic disc (T = 13 Gyr) the predicted surface density of Galactic disc field stars originating from dissolved open clusters amounts to $22 M_odot mathrm{pc}^{-2}$ which is about 40% of the total surface density of the Galactic disc in the solar neighbourhood. Thus, we conclude that almost half of all field stars were born in open clusters, a much higher fraction than previously thought.
AIMS. While observational evidence shows that most of the decline in a stars X-ray activity occurs between the age of the Hyades (~8 x 10^8 yrs) and that of the Sun, very little is known about the evolution of stellar activity between these ages. To gain information on the typical level of coronal activity at a stars intermediate age, we studied the X-ray emission from stars in the 1.9 Gyr old open cluster NGC 752. METHODS. We analysed a ~140 ks Chandra observation of NGC 752 and a ~50 ks XMM-Newton observation of the same cluster. We detected 262 X-ray sources in the Chandra data and 145 sources in the XMM-Newton observation. Around 90% of the catalogued cluster members within Chandras field-of-view are detected in the X-ray. The X-ray luminosity of all observed cluster members (28 stars) and of 11 cluster member candidates was derived. RESULTS. Our data indicate that, at an age of 1.9 Gyr, the typical X-ray luminosity of the cluster members with M=0.8-1.2 Msun is Lx = 1.3 x 10^28 erg s^-1, so approximately a factor of 6 less intense than that observed in the younger Hyades. Given that Lx is proportional to the square of a stars rotational rate, the median Lx of NGC 752 is consistent, for t > 1 Gyr, to a decaying rate in rotational velocities v_rot ~ t^-alpha with alpha ~ 0.75, steeper than the Skumanich relation (alpha ~ 0.5) and significantly steeper than observed between the Pleiades and the Hyades (where alpha < 0.3), suggesting that a change in the rotational regimes of the stellar interiors is taking place at t ~ 1 Gyr.
Galactic open clusters (OCs) that survive the early gas-expulsion phase are gradually destroyed over time by the action of disruptive dynamical processes. Their final evolutionary stages are characterized by a poorly populated concentration of stars called open cluster remnant (OCR). This study is devoted to assess the real physical nature of the OCR candidate NGC 7193. GMOS/Gemini spectroscopy of 53 stars in the inner target region were obtained to derive radial velocities and atmospheric parameters. We also employed photometric and proper motion data. The analysis method consists of the following steps: (i) analysis of the statistical resemblance between the cluster and a set of field samples with respect to the sequences defined in colour-magnitude diagrams (CMDs); (ii) a 5-dimensional iteractive exclusion routine was employed to identify outliers from kinematical and positional data; (iii) isochrone fitting to the $K_{s}times(J-K_{s})$ CMD of the remaining stars and the dispersion of spectral types along empirical sequences in the $(J-H)times(H-K_{s})$ diagram was checked. A group of stars was identified for which the mean heliocentric distance is compatible with that obtained via isochrone fitting and whose metallicities are compatible with each other. Fifteen member stars observed spectroscopically were identified together with other 19 probable members. Our results indicate that NGC 7193 is a genuine OCR, of an once very populous OC, for which the following parameters were derived: $d=501,pm,46,$pc, $t=2.5,pm,1.2,$Gyr, $langle,[Fe/H],rangle=-0.17,pm,0.23$ and $E(B-V)=0.05,pm,0.05$. Its luminosity and mass functions show depletion of low mass stars, confirming the OCR dynamically evolved state.
We present {it Hubble Space Telescope} {it V,I} photometry of the central region of the LMC cluster NGC 1866, reaching magnitudes as faint as V=27 mag. We find evidence that the cluster luminosity function shows a strong dependence on the distance from the cluster center, with a clear deficiency of low luminosity stars in the inner region. We discuss a {it global} cluster luminosity function as obtained from stars in all the investigated region, which appears in impressive agreement with the prediction from a Salpeter mass distribution. We also revisit the use of NGC 1866 as a probe for determining the efficiency of core overshooting, and conclude that a definitive answer to this question is not possible from this cluster.
NGC 6611 is the massive young cluster (2-3 Myr) that ionises the Eagle Nebula. We present very deep photometric observations of the central region of NGC 6611 obtained with the Hubble Space Telescope and the following filters: ACS/WFC F775W and F850LP and NIC2 F110W and F160W, loosely equivalent to ground-based IZJH filters. This survey reaches down to I ~ 26 mag. We construct the Initial Mass Function (IMF) from ~ 1.5 Msun well into the brown dwarf regime (down to ~ 0.02 Msun). We have detected 30-35 brown dwarf candidates in this sample. The low-mass IMF is combined with a higher-mass IMF constructed from the groundbased catalogue from Oliveira et al. (2005). We compare the final IMF with those of well studied star forming regions: we find that the IMF of NGC 6611 more closely resembles that of the low-mass star forming region in Taurus than that of the more massive Orion Nebula Cluster (ONC). We conclude that there seems to be no severe environmental effect in the IMF due to the proximity of the massive stars in NGC 6611.
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