Stellar and substellar mass function of the young open cluster candidates Alessi 5 and beta Monocerotis
aa r X i v : . [ a s t r o - ph . S R ] N ov **Volume Title**ASP Conference Series, Vol. **Volume Number****Author** c (cid:13) **Copyright Year** Astronomical Society of the Pacific Stellar and substellar mass function of the young open clustercandidates Alessi 5 and β Monocerotis
S. Boudreault , , and J. A. Caballero Mullard Space Science Laboratory, University College London, Holmbury StMary, Dorking, Surrey, RH5 6NT, United Kingdom Visiting Astronomer at the Department of Physics and Astronomy, StateUniversity of New York, Stony Brook, NY 11794-3800, USA Max-Planck-Institut f¨ur Astronomie, K ¨onigstuhl 17, D-69117 Heidelberg,Germany Centro de Astrobiolog´ıa (CSIC-INTA), Departamento de Astrof´ısica,PO Box 78, E-28691 Villanueva de la Ca˜nada, Madrid, Spain
Abstract.
Although the stellar and substellar populations have been studied in var-ious young and old open clusters, additional studies in clusters in the age range from5 to 100 Myr is crucial (e.g. to give more constrains on initial mass function varia-tion with improved statistics). Among the open cluster candidates from recent studies,two clusters are best suited for photometric survey of very-low mass stars and browndwarfs, considering their youth and relative proximity: Alessi 5 ( τ ∼
40 Myr, d ∼
400 pc)and β Monocerotis ( τ ∼ d ∼
400 pc). For both clusters, we performed an opticaland near-infrared photometric survey, and a virtual observatory survey. Our survey ispredicted to be sensitive from the massive B main sequence stars down to brown dwarfsof 30 M
Jup . Here, we present and discuss preliminary results, including the mass func-tion obtained for Alessi 5, which is surprisingly very similar to the mass function of theHyades ( τ ∼
600 Myr), although they are of very di ff erent ages.
1. Introduction
Several studies over the past ten years have presented surveys of open clusters inorder to study the mass function (MF) of stellar and substellar populations, includ-ing the Orion Nebula Cluster, σ Orionis, IC 2391, the Pleiades and the Praesepe, tolist just a few examples. These studies are important since stars and brown dwarfs(BD)s in open clusters possess modest age and metalicity spreads and share a com-mon distance, in comparisons with large uncertainties for the field stellar and sub-stellar objects (Bastian et al. 2010). In addition, determination of the MF in clusterswith di ff erent properties (e.g. di ff erent density and ages) has led some investigatorsto draw conclusions about the relative e ffi ciency of possible BD formation mechanisms(Brice˜no et al. 2002; Chabrier 2003; Kroupa & Bouvier 2003; Kumar & Schmeja 2007;Boudreault & Bailer-Jones 2009).Many earlier studies of the substellar MF have focused on young open clusterswith ages less than ∼ . ffi culties: intra-cluster extinction plagues thedetermination of the intrinsic luminosity function from the measured photometry, andat these ages the BD models have large(r) uncertainties (Bara ff e et al. 2002). Stud-ies in older clusters ( &
100 Myr) present di ffi culties too: lacking a significant nuclearenergy source, BDs cool and get faint as they age, so deeper surveys are required todetect them, and low-mass objects evaporate from clusters by dynamical evaporation(de La Fuente Marcos & de La Fuente Marcos 2000; Adams et al. 2002; Bouvier et al.2008). Clusters in the range of age 5–100 Ma are perfect tools for MF studies on theBDs and very low-mass star populations since (1) these objects are bright, (2) have notlost trace of initial condition due to dynamical mass segregation or dynamical evapora-tion and (3) low extinction is expected towards these clusters. Despite these advantages,only a few open clusters are known in this age range.Some works have been performed to search for previously unknown open clus-ters. Among the open cluster candidates from Alessi et al. (2003) and Kharchenko et al.(2005), two clusters are best suited for photometric survey of very low-mass star andBDs considering their youth and relative proximity : Alessi 5 ( τ ∼
40 Myr, d ∼
400 pc;Alessi et al. 2003) and β Monocerotis ( τ ∼ d ∼
400 pc; Kharchenko et al. 2005).(This cluster is presented as “ASCC 24” in Kharchenko et al. (2005).) So far, no accu-rate studies of these two clusters have been performed.
2. Observations2.1. Optical photometry: WFI observations, data reduction, astrometry andphotometric calibration
Our survey consists of one single Wide Field Imager (WFI) field of size 34 ×
33 arcmin ,observed in wide band R c , and medium band 770 /
19, 815 /
20, 856 /
14 and 914 /
27 (wherethe filter name notation is central wavelength on the full width at half maximum,FWHM, in nm). This gives a total coverage of 0.26 deg observed in all five bands, cen-tred on the brightest stars of each cluster candidate. The data reduction and photometriccalibration was performed in a similar way as presented in Boudreault & Bailer-Jones(2009). To correct for Earth-atmospheric absorption on the photometry, we observedthe spectrophotometric standard stars observed were LTT 3864 and 4364.For our β Mon and Alessi 5 observations, the 5 σ detection limits of our survey are R c = ∼
30 M
Jup according to our dust-free isochrone. Theroot mean square accuracy of our astrometric solution was 0.15–0.20. Ω
2k and and 2MASS
The near infrared observations were performed only for β Mon. There were made usingfour Omega 2000 ( Ω Ω
2k was performed in a similar way as presented in Boudreault et al.(2010). The 5 σ detection limit at J = ∼
30 M
Jup in β Mon. For our survey on Alessi 5, we used the J and K s photometry from 2MASS.oudreault &Caballero 3
3. Candidate Selection Procedure
The procedure to compute the masses and e ff ective temperature based on photome-try is done in a similar way as presented by Boudreault & Bailer-Jones (2009) andBoudreault et al. (2010).In order to perform the selection of candidates, we compute an isochrones for bothAlessi 5 and β Mon. We used the spectral energy distribution to derive the mass ande ff ective temperature, T e ff , assuming that all our photometric candidates belong to theclusters studied. We used evolutionary tracks from Bara ff e et al. (1998) and atmospheremodels from Hauschildt et al. (1999) (assuming a dust-free atmosphere; the NextGenmodel) to compute an isochrone for Alessi 5 using an age of 40 Myr, distance of 400 pc,a solar metalicity and neglecting the reddening ( E ( B − V ) = β Mon usingan age of 9.1 Myr, distance of 210 pc, a solar metalicity and neglecting the reddening.Candidates were first selected from the CMD involving the wide band R c and913 /
27 for Alessi 5, and the wide bands R c and J . The candidates are only objectswithin a selection area defines to include (1) error on the distance, (2) error on the age,(3) error on the photometry and (4) objects brighter than the isochrones by 0.753 mag inorder to include unresolved binaries. In Fig. 1 we present the CMDs where candidateswere selected. The Fig. 1 also show cluster member of Kharchenko et al. (2005) with amembership probability higer than 10%, based on proper motion.The second stage of candidate selection was achieved using colour-colour dia-grams using the R c , 815 /
20 and K s bands for Alessi 5, and using the R c , 815 /
20 and J bands for β Mon (Fig. 2). Since colours are used here, the selection area is defined bythe error on the age on the isochrones and by the error on the photometry. For clarity,we present a contour plot of the colour–colour diagram for both clusters.As indicated previously, our determination of T e ff is based on the spectral energydistribution of each object and is independent of the assumed distance. The membershipstatus of an object can therefore be assessed by comparing its observed magnitude ina band with its magnitude predicted from its T e ff and β Mon and Alessi 5’s isochrone(which assumes a distance and an age). This selection step is only a verification ofthe consistency between the physical parameters obtained of the photometric clustercandidates with the physical properties assumed for the cluster itself when computingthe isochrones. To avoid removing unresolved binaries that are real members of thecluster, we keep all objects with a computed magnitude of up to 0.753 mag brighterthan the observed magnitude.
4. Results and discussions4.1. Alessi 5
We obtain a total of 234 cluster candidates based on our deep photometric survey. The J and K s photometry of 2MASS is shallower in terms of mass in Alessi 5 compared toour optical photometry. To compute the MF of Alessi 5 to the lowest mass bin reachedwithout optical data, we have computed a MF using only the optical photometry withWFI. We present this MF on Fig. 3 (lower left panel) as crosses. We computed a secondMF from the list of candidates that passes all selections criteria with near infrared J and K s photometry from 2MASS (presented on Fig. 3, lower left panel, as filled triangles).For each mass bin, we computed the number of object removed by adding the J and K s S.Boudreault and J.A.Caballero
Figure 1. Colour–magnitude diagram for Alessi 5 ( top two panels ) with the R c and 913 /
27 bands used in the selection procedure, and for β Mon ( top two pan-els ) with the R c and J bands. We present the colour–magnitude diagrams from ourshallow images ( left panels ) and deep images ( right panels ) we have taken. Assolid lines we show the isochrone computed from an evolutionary model with a dust-free atmosphere (NextGen model). The numbers indicate the masses (in M ⊙ ) on themodel sequence for various R c magnitudes. We also show candidate cluster mem-bers that we detected in our survey from Kharchenko et al. (2005) ( circles ). Thedashed lines delimit our selection band. photometry of 2MASS to our selection process and mass determination (this is plottedas a function of mass in Fig. 3, top left panel). We fitted a power-law function toestimate the number of object that would be removed if we would had additional J and K s photometry added to our optical photometry. The corresponding extension of theMF is given as large triangles (Fig. 3, lower left panel).oudreault &Caballero 5 Figure 2. Colour–colour diagram for Alessi 5 ( left panel ) with the R c , 815 / K s bands used in the selection procedure, and for β Mon ( right panel ) with the R c , 815 /
20 and J bands. As for Fig. 1, as solid lines we show the isochrone computedfrom an evolutionary model with a dust-free atmosphere and the dashed lines delimitour selection band. For Alessi 5, we clearly see a structure overlaping the isochronesfor masses lover than ∼ ⊙ . On the other hand, we don’t observe any structure inthe colour–colour diagram of β Mon that overlap with the isochrone.
In Fig. 3 (right panel) we compare the MF of Alessi 5 with the MF of the opencluster NGC 2546 and the Hyades. The MF of Alessi 5 is surprisingly similar to theMF of the Hyades (Bouvier et al. 2008) with a decrease in the MF below ∼ ⊙ ,although they are of very di ff erent ages (with about 40 and 600 Myr respectively). Thissupport the conclusion that initial conditions are more likely to influence the shape ofthe MF more significantly than dynamical evolution. In addition, the MF of Alessi 5shows some similarities with the MF of NGC 2547 (Je ff ries et al. 2004): both showa decrease in the MF below ∼ ⊙ and both open clusters present a peak at 0.7–1.0 M ⊙ . It was shown by Boudreault & Bailer-Jones (2009) that this peak is due to redgiant background contaminants. β Mon
We obtain a total number of object of 19 candidates from our deep photometric surveyin β Mon. Considering this and the absence of any structure in the colour-colour dia-gram of β Mon (see Fig. 2), we conclude that were is no cluster towards β Mon. This isfurther confirmed with our virtual observatory study in the following section.
5. Virtual Observatory study
We performed a 6.3 deg Aladin-based virtual observatory analysis of the high-mass(about 8 to 1 M ⊙ ) population of stars in the Tycho-2 catalogue over both clusters. Weuse near infrared-optical CMDs (presented in Fig. 4, top two panels), proper-motions,spatial location diagrams of the cross-matched Tycho-2 and 2MASS sources in the1 deg-radius circular areas centred on HD 93010 A for Alessi 5 and on β Mon ABC for S.Boudreault and J.A.Caballero
Figure 3.
Lower left panel.
MF of Alessi 5 using only our optical photometry( crosses ) and combined with the near infrared photometry of 2MASS ( triangles ).The vertical dotted line represents the saturation limit of our optical survey, the ver-tical dashed line represent the 5 sigma detection limit of our optical survey, and thevertical dash-dotted line represents the 10 σ detection limit of 2MASS. Error barsare Poissonian arising from the number of objects observed in each bin. Top leftpanel. Di ff erence of the number of object, in each mass bin, between the MF com-puted using our optical photometry and the MF computed using the combination ofthe optical data from WFI and the near infrared JK s data from 2MASS. Lower leftpanel.
MF of Alessi 5 and of the Hyades and NGC 2547. We also show the galacticfield star MF fit from Chabrier (2003) as a thin dashed line and the substellar limit asa thick dashed line. We have normalized all the MFs to the log-normal fit of Chabrier(2003) at ∼ ⊙ (log M = -0.5). β Mon (presented in Fig. 4, bottom two panels), and normalized cumulative distribu-tions. At a quick glance to the spatial distribution diagrams in Fig. 4, the “clustering”of Alessi 5 is obvious, with ten cluster members in the innermost 20 arcmin-radius cir-cular area and only five early-type stars possibly not associated to the cluster. However, β Mon has only four cluster member candidates in the same 20 arcmin-radius circulararea and up to twelve early-type stars homogeneously distributed in the corona between40 and 60 arcmin to β Mon ABC.From our virtual observatory studies, including our above analysis of the spatialdistribution of the cluster candidates, we conclude that there is no real clustering around β Mon (i.e. [KPR2004] 24 does not exist ), but confirm the existence of Alessi 5 aroundthe early-type giant binary HD 93010 AB.oudreault &Caballero 7
Figure 4. Near infrared-optical CMD of Alessi 5 ( top left panel ) and of β Mon( top right panel ) using V T and K S band from our virtual observatory study. Spatiallocation diagrams of the cross-matched Tycho-2 and 2MASS sources in the 1 deg-radius circular areas centred on HD 93010 A (in the centre of Alessi 5, lower leftpanel ), and β Mon ABC ( lower right panel ). In all the panels, (red) filled circles arecluster member candidates, (red) open circles are other early-type stars in the region,(blue) crosses are non-members based on abnormal proper motions and / or colours,and (black) small dots are the remaining cross-matched sources. For the CMDs, thehorizontal dotted lines indicate the size of the virtual observatory analysis.
6. Conclusions
In this proceeding we presented the results of a survey to identify high- to low-massstars and brown dwarf members of the recently discovered open cluster candidatesAlessi 5 and β Mon. Our survey consisted of an optical and near infrared photometricsurvey covering 0.26 deg and a virtual observatory survey of 6.3 deg for both Alessi 5and β Mon. With a 5 σ detection limits of R c = Jup inAlessi 5 and in β Mon.From our optical observations of Alessi 5 we identify 234 low-mass cluster mem-ber candidates from our WFI + ff erent ages (with about 40 and 600 Myr respectively). This support the con- S.Boudreault and J.A.Caballeroclusion that initial conditions are more likely to influence the shape of the MF moresignificantly than dynamical evolution. In addition, the MF of Alessi 5 shows somesimilarities with the one of of NGC 2547.As for the open cluster β Mon, we report a non detection of any clustering at thedistance surveyed.The results reported here will be presented with further details in an future publi-cation (Boudreault & Caballero 2010, in prep).
Acknowledgments.
S.B. acknowledge support from the Deutsche Forschungsge-meinschaft (DFG) grant BA2163 (Emmy-Noether Program) to Coryn A. L. Bailer-Jones. Partial financial support was provided by the Spanish Ministry of Science undergrant AyA2008-06423-C03-03. Some of the observations on which this work is basedwere obtained during ESO programmes 081.A-9001(A).
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