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Register a new userAstrometric and photometric initial mass functions from the UKIDSS Galactic Clusters Survey: III Praesepe
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Steve Boudreault Dr.
Publication date
2012
fields
Physics
and research's language is
English
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Over the past decades open clusters have been the subject of many studies. Such studies are crucial considering that the universality of the Initial Mass Function is still a subject of current investigations. Praesepe is an interesting open cluster for the study of the stellar and substellar mass function (MF), considering its intermediate age and its nearby distance. Here we present the results of a wide field, near-infrared study of Praesepe using the Data Release 9 (DR9) of the UKIRT Infrared Deep Sky Survey (UKIDSS) Galactic Clusters Survey (GCS). We obtained cluster candidates of Praesepe based on a 3sigma astrometric and 5 band photometric selection. We derived a binary frequency for Praesepe of 25.6+/-3.0% in the 0.2-0.45Msol mass range, 19.6+/-3.0% for 0.1-0.2Msol, and 23.2+/-5.6% for 0.07-0.1Msol. We also studied the variability of the cluster candidates of Praesepe and we conclude that seven objects could be variable. We inferred the luminosity function of Praesepe in the Z- and J- bands and derived its MF. We observe that our determination of the MF of Praesepe differs from previous studies: while previous MFs present an increase from 0.6 to 0.1Msol, our MF shows a decrease. We looked at the MF of Praesepe in two different regions of the cluster, i.e. within and beyond 1.25deg, and we observed that both regions present a MF which decrease to lower masses. We compared our results with the Hyades, the Pleiades and alpha Per MF in the mass range of 0.072-0.6Msol and showed that the Praesepe MF is more similar to alpha Per although they are respectively aged ~85 and ~600Myr. Even though of similar age, the Praesepe remains different than the Hyades, with a decrease in the MF of only ~0.2 dex from 0.6 down to 0.1Msol, compared to ~1 dex for the Hyades.
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Here we present the results of a wide-field (~36 sq. deg.) near-infrared (ZYJHK) survey of the Praesepe cluster using the Data Release 9 of the UKIRT Infrared Deep Sky Survey Galactic Clusters Survey. We selected cluster candidates of Praesepe based on astrometry and photometry. With our candidate list, we have obtained the luminosity function of Praesepe in the Z and J bands, and we have derived the mass function of Praesepe from 0.6 down to 0.072 Msol. Moreover, we have estimated the binarity of the Praesepe members in the 0.45-0.07 Msol mass range and as well as their variability.
We analysed the wide-field near-infrared survey of the Praesepe cluster carried out by the UKIRT Infrared Deep Sky Survey (UKIDSS) Galactic Clusters Survey (GCS) and released by the Data Release 9 (DR9). We compare our Praesepe mass function (MF) with the ones of the Pleiades, alpha Per, and the Hyades. We also present preliminary results of a spectroscopic follow-up for the low mass members (M=<0.1Msol) in Praesepe, alpha Per and Pleiades using the Optical System for Imaging and low Resolution Integrated Spectroscopy (OSIRIS) mounted on the 10.4m Gran Telescopio Canarias (GTC). We also present the optical spectrum of the first L dwarf in Praesepe.
Knowledge of the mass function in open clusters constitutes one way to constrain the formation of low-mass stars and brown dwarfs as does the knowledge of the frequency of multiple systems and the properties of disks. The aim of the project is to determine the shape of the mass function in the low-mass and substellar regimes in the pre-main sequence (27 Myr) cluster IC4665, which is located at 350 pc from the Sun. We have cross-matched the near-infrared photometric data from the Eighth Data Release of the UKIRT Infrared Deep Sky Survey (UKIDSS) Galactic Clusters Survey with previous optical data obtained with the Canada-France-Hawaii wide-field camera to improve the determination of the luminosity and mass functions in the low-mass and substellar regimes. The availability of i and z photometry taken with the CFH12K camera on the Canada France Hawaii Telescope added strong constraints to the UKIDSS photometric selection in this cluster, which is located in a dense region of our Galaxy. We have derived the luminosity and mass functions of the cluster down to J=18.5 mag, corresponding to masses of ~0.025 Msun at the distance and age of IC4665 according to theoretical models. In addition, we have extracted new candidate members down to ~20 Jupiter masses in a previously unstudied region of the cluster. We have derived the mass function over the 0.6-0.04 Msun mass range and found that it is best represented by a log-normal function with a peak at 0.25-0.16 Msun, consistent with the determination in the Pleiades.
We carried out a search for new infrared star clusters, stellar groups and candidates using WISE images, which are very sensitive to dust emission nebulae. We report the discovery of 437 embedded clusters and stellar groups that show a variety of structures, both in the stellar and nebular components. Pairs or small groupings of clusters are observed, suggesting multiple generations at the early formation stages. The resulting catalogue provides Galactic and equatorial coordinates, together with angular sizes for all objects. The nature of a representative test sub-sample of 14 clusters is investigated in detail by means of 2MASS photometry. The colour magnitude diagrams and radial density distributions characterize them as stellar clusters. The 437 new objects were found in the ranges $145^circ,leq,ell,leq 290^circ$ and $-25^circ,leq,b,leq 20^circ$, and they appear to be a major object source for future studies of star cluster formation and their early evolution. WISE is a powerful tool to further probe for very young clusters throughout the disk.
(... 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.
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