We report on the analysis of 2MASS near-infrared data of a sample of low-mass
stars and brown dwarfs in the sigma Orionis cluster. Youth and cluster
membership have been spectroscopically confirmed using the Li I spectral line.
We find little evidence in the JHKs colour-colour diagram for near-infrared
excess emission for these cluster members. By comparison with model
expectations, at most 2 out of 34 stars show (H-K) colour consistent with a
near-infrared excess. This scarcity of near-infrared signatures of
circumstellar disks in the lower-mass and substellar regimes of this cluster
contrasts with findings in younger clusters, hinting at an age dependence of
the disk frequency. Taking into account the apparent cluster age, our result
supports the idea of a relatively fast (few Myr) disk dissipation and extends
this conclusion to the substellar regime. We also find some evidence that, in
this cluster, the disk frequency as measured by the Ks-band excess may be mass
dependent.Comment: 4 pages, 2 eps figures, accepted by Astronomy and Astrophysics
  Letter

Bally

Baraffe

Beichman

Bessell

Béjar

Carpenter

Cohen

Comerón

de Geus

Dolan

Evans

Folha

Gullbring

Haisch

J. M. Oliveira

Johnstone

Kirkpatrick

Lada

Lucas

M. J. Kenyon

Magazzù

Meyer

Muench

Natta

Pickett

Preibisch

R. D. Jeffries

Reipurth

S. A. Thompson

Stassun

Stelzer

T. Naylor

Walter

Warren

English

arXiv

We report on the analysis of 2MASS near-infrared data of a sample of low-mass stars and brown dwarfs in the sigma Orionis cluster. Youth and cluster membership have been spectroscopically confirmed using the Li I spectral line. We find little evidence in the JHKs colour-colour diagram for near-infrared excess emission for these cluster members. By comparison with model expectations, at most 2 out of 34 stars show (H-K) colour consistent with a near-infrared excess. This scarcity of near-infrared signatures of circumstellar disks in the lower-mass and substellar regimes of this cluster contrasts with findings in younger clusters, hinting at an age dependence of the disk frequency. Taking into account the apparent cluster age, our result supports the idea of a relatively fast (few Myr) disk dissipation and extends this conclusion to the substellar regime. We also find some evidence that, in this cluster, the disk frequency as measured by the Ks-band excess may be mass dependent

Oliveira, JM

Jeffries, RD

Kenyon, MJ

Thompson, SA

Naylor, T

Name not available

No disks around low-mass stars and brown dwarfs in the young sigma Orionis cluster?

Crossref

No disks around low-mass stars and brown dwarfs in the young
                    σ

We report on the analysis of 2MASS near-infrared data of a sample of low-mass stars and brown dwarfs in the σ Orionis cluster. Youth and cluster membership have been spectroscopically confirmed using the Li 

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No disks around low-mass stars and brown dwarfs in the young

Naylor, Tim

Keele Research Repository

arXiv:astro-ph/0112319v1  13 Dec 20011Abstract. We report on the analysis of 2MASS near-infrared data of a sample of low-mass stars and browndwarfs in the σ Orionis cluster. Youth and cluster member-ship have been spectroscopically confirmed using the Li ispectral line. We find little evidence in the JHKs colour-colour diagram for near-infrared excess emission for thesecluster members. By comparison with model expectations,at most 2 out of 34 stars show (H −Ks) colour consistentwith a near-infrared excess. This scarcity of near-infraredsignatures of circumstellar disks in the lower-mass andsubstellar regimes of this cluster contrasts with findingsin younger clusters, hinting at an age dependence of thedisk frequency. Taking into account the apparent clusterage, our result supports the idea of a relatively fast (fewMyr) disk dissipation and extends this conclusion to thesubstellar regime. We also find some evidence that, in thiscluster, the disk frequency as measured by the Ks-bandexcess may be mass dependent.Key words: circumstellar matter — infrared: stars —stars: pre-main sequence — stars: low mass, brown dwarfs— open clusters and associations: individual (σ Orionis)A&A manuscript no.(will be inserted by hand later)Your thesaurus codes are:missing; you have not inserted themASTRONOMYANDASTROPHYSICSNo disks around low-mass stars and brown dwarfs in theyoung σ Orionis cluster?J.M. Oliveira1, R.D. Jeffries1, M.J. Kenyon1, S.A. Thompson1 and Tim Naylor1,21 Department of Physics, Keele University, Staffordshire ST5 5BG, United Kingdom2 School of Physics, University of Exeter, Exeter EX4 4QL, United KingdomReceived ; accepted1. IntroductionStar formation is believed to start with the fragmentationof rotating and magnetized clouds of interstellar gas, andfurther gravitational collapse forms stars across the wholemass range. Flattened disk-like structures arise during thisstar formation process (see review by Evans 1999), andcircumstellar disks are common around young stars. Eventhough it is thought that brown dwarfs are just the sub-stellar counterparts of stars, forming as outlined above,it has also been proposed that brown dwarfs form eitherthrough instabilities in circumstellar disks (e.g. Pickett etal. 2000) or as stellar embryos ejected from multiple proto-stellar systems (Reipurth & Clarke 2001).Disk dissipation timescales, together with the environ-ment of the young stellar population, are of particularimportance, in the context of pre-main-sequence stellarevolution and of planet formation processes. Haisch et al.(2001) have found evidence for an age dependence of thedisk frequency from the analysis of low-mass stars in sev-eral young clusters in OB associations and star formingregions. Even though the importance of the cluster envi-ronment is not yet clear, it has been proposed that photo-evaporation by the massive OB stars plays a role in diskdissipation (e.g. Johnstone et al. 1998; Bally et al. 2000).It is also relevant to probe how the observed disk proper-ties extend into the substellar regime.σ Orionis is a member of the OrionOB1b association,with an estimated age of 1.7−7Myr and a distance mod-ulus of 7.8−8mag (e.g. Warren & Hesser 1978; Brownet al. 1994) — the Hipparcos distance to this star is352+166−85 pc. ROSAT observations and follow-up photome-try (Wolk 1996) revealed a cluster of low-mass young stel-lar objects near σ Ori. The reddening towards this massiveO-type star is low (E(B−V )=0.05mag, e.g. Brown et al.1994), suggesting that the young stellar population is af-fected by low extinction.Kenyon et al. (2001) have confirmed both youth andcluster membership for a sample of low-mass and substel-lar objects within 30′ of σ Ori. We analyse 2MASS near-infrared photometric data for their sample of stars. TheSend offprint requests to: joana@astro.keele.ac.ukJHKs colour-colour diagram of these cluster membersshows little evidence for near-infrared excess. By compar-ing observed (H − Ks) with model expectations, we findthat only 2 objects out of 34 show an infrared excess sug-gestive of circumstellar disks. This is in striking contrastto high disk frequencies determined for the low-mass pop-ulations in younger clusters (Trapezium, NGC 2024 andNGC 2264, Haisch et al. (2001) and references therein).Our result provides valuable support for the age depen-dence of disk frequency in low-mass stars and may indicatethat this dependence continues into the substellar regime.2. Target selection and near-infrared dataAn R- and I-band survey, obtained with the Wide-FieldCamera (WFC) at the 2.5m Isaac Newton Telescope(INT) and covering approximately 1 square degree cen-tered on σ Ori, was used by Kenyon et al. (2001) to selectlow-mass and brown dwarf cluster candidates for follow-upoptical spectroscopy. These photometric candidates wereselected as stars younger than about 100My, accordingto Baraffe et al. (1998) isochrones. High-resolution spec-troscopy in the 6400−6800Å range was obtained at the4.2m William Herschel Telescope (WHT), with the Wide-Field Fiber Optic Spectrograph (WYFFOS). From the83 spectroscopically observed candidates: 39 are lithiumrich, based on their Li i (λ 6707 Å) equivalent widths, con-firming their status as very young low-mass stars andbrown dwarfs and almost certainly cluster members; 44objects show no evidence for lithium and are thereforelikely (though see below) to be non-members. This sam-ple of stars is described in detail elsewhere (Kenyon et al.2001; Kenyon et al. in preparation).We have searched the Point Source Catalog of the2MASS Second Incremental Data Release for infraredcounterparts of this sample of cluster members. Informa-tion on the 2MASS survey and catalogue products can befound in Beichman et al. (1998) and Cutri et al. (2001),respectively. Of the 39 sample objects, 34 have 2MASScounterparts within a radius of 0.4′′ of the optical sources(of the 44 lithium-poor objects, 43 have 2MASS counter-parts).These stars were detected in the J-, H- and Ks-Oliveira et al.: No disks around low-mass stars and brown dwarfs in the young σ Orionis cluster? 3Fig. 1. I/(I − J) colour-magnitude diagram for the clus-ter members and the control sample (see text). TheBaraffe et al. (1998) evolutionary tracks (dotted line) andisochrones (full line) were used to estimate the masses andages of these objects. A Hipparcos distance to the clus-ter d = 352pc and optical extinction AV =0.15mag areadopted.bands and the observations are not contaminated by arti-facts or confusion with bright or extended sources.We want to infer the disk fraction amongst the 34 clus-ter members. We analyse the JHKs colour-colour diagramof the cluster members, together with a control sample ofphotometric cluster candidates: 43 lithium-poor sources(see above) and 25 photometric candidates from Béjar etal. (2001 and references therein). The role of the controlsample is to assess whether or not the spectroscopic se-lection method is biased against stars with disks. Pho-tospheric spectral lines in classical T Tauri stars can beheavily veiled at optical (e.g. Gullbring et al. 1998) andinfrared (Folha & Emerson 1999) wavelengths. In partic-ular, the hot continuum attributed to disk accretion canfill-in the Li i 6707 Å line (e.g. Magazzù et al. 1992). If thecolour-colour diagram of the control sample revealed a sig-nificant fraction of objects with near-infrared excess, thenthe spectroscopically confirmed sample could be biased.3. ResultsWe have determined masses and ages for the cluster mem-bers using the I/(I − J) colour-magnitude diagram andBaraffe et al. (1998) evolutionary models (Fig. 1). Thesample populates the mass range of 0.04−0.45M⊙ and theage range of 1−50Myr. The median age of the sample is4.2+2.7−1.5 Myr, where the uncertainty is a very conservativeestimate based on the error in the Hipparcos distance to σOri. If the objects in the control sample are cluster mem-bers, then they have similar masses and ages (Fig. 1). Ofthe 34 cluster members, 7 have masses below the hydrogenburning limit (M ∼ 0.08M⊙) and are candidate browndwarfs (5−9 brown dwarf candidates if the extreme dis-tance uncertainties are taken into account). From Fig. 1, 4objects seem to be older than about 10Myr and thereforethere are some doubts about their cluster membership.The observed age spread could either be real, in the sensethat star formation in this cluster lasted for a few millionyears, or it could just be a combination of binarity, pho-tometric uncertainties and source variability (the I- andJ-band observations are non-simultaneous).In Fig. 2 we show the JHKs colour-colour diagram forthe sample of cluster members (a) and for our control sam-ple (b). We plot the empirical loci of main-sequence and gi-ant stars (Bessell & Brett 1988), M8−M9.5 dwarfs (Kirk-patrick et al. 2000) and classical T Tauri stars (Meyer etal. 1997). All these loci were converted to the 2MASS pho-tometric system using the colour-colour transformationsfrom Carpenter (2001). Also in this figure are relevantreddening bands (Cohen et al. 1981).From Fig. 2a no cluster member shows unequivocalnear-infrared excess, as none of these stars appear signif-icantly to the right of the reddening band for M dwarfs.From the 68 objects in the control sample, at most onelithium-poor star and three photometric candidates couldbe said to possess near-infrared excess (Fig. 2b), albeitwith little confidence (less than a 2σ (H−Ks) excess detec-tion). This demonstrates that there is no significant pop-ulation of heavily veiled, low-mass T Tauri objects withweak Li i 6707 Å lines, but large near-infrared excesses.From Fig. 2a alone, it is not possible to ascer-tain whether a low-reddening object with (H − Ks) ∼0.4−0.5mag is a low-mass star with purely photosphericH- and Ks-band magnitudes or a slightly more massivestar with considerable (H−Ks) excess. To do this we haveparameterised possible excesses by using ∆(H − Ks) =(H −Ks)− (H −Ks)0, where (H −Ks)0 are the Baraffe etal. (1998) predictions for photospheric (H − Ks) coloursaccording to the ages and masses derived from Fig. 1.When compared with the photometric uncertainties, wefind that only one cluster member exhibits significant(H − Ks) excess (∼ 3σ detection) and another objectjust a 1σ detection. Both objects have masses in therange 0.15−0.175M⊙. The near-infrared colours from theBaraffe models seem to be slightly redder than the ob-served colours (as also noted by Lucas et al. 2001). Eventaking this into account, at most two stars show a signifi-cant (H − Ks) excess.4 Oliveira et al.: No disks around low-mass stars and brown dwarfs in the young σ Orionis cluster?Fig. 2. JHKs colour-colour diagram for a) the cluster members and b) the control sample (see text). The loci ofmain-sequence and giants stars (Bessell & Brett 1988) and of M8.5−M9.5 dwarfs (Kirkpatrick et al. 2000) are plotted(full line), as well as the locus of classical T Tauri stars (dashed line, Meyer et al. 1997). Reddening bands for thegiant stars and for late-M dwarfs are also plotted (dotted lines). The cluster members show no significant evidence fornear-infrared excess.4. Discussion and conclusionsOur sample of low-mass σ Ori cluster members shows anextremely low fraction of objects with near-infrared ex-cess: at most 2 out of 34. As near-infrared excesses inpre-main-sequence stars are normally attributed to cir-cumstellar disks, we interpret this result as a 6% disk fre-quency in the σ Ori cluster (7% if the 4 apparently olderstars are removed from the sample as possible contam-inants). Using an X-ray selected sample (M>∼ 0.5M⊙),Wolk (1996) proposed a 12% (6/49) disk frequency, usingHα emission as an indirect indicator of circumstellar ma-terial (emission equivalent width > 10 Å). X-ray selectionmight bias the sample against stars with disks (Stelzer& Neuhäuser 2001). However, Hα samples are biased to-wards stars with high accretion rates. Stassun et al. (1999)show that some low-mass stars (in Orion) with K-band ex-cess do not show strong Hα emission. For both these rea-sons, Wolk’s estimated disk fraction must be a lower limitto the fraction one would estimate from near-infrared pho-tometry. Therefore, there is some evidence, tempered bysmall number statistics, that the disk frequency (as mea-sured by K-band excess) decreases towards lower massesin the σ Ori association.As the near-infrared colours trace the warmer dust, themagnitude of the near-infrared excess depends on the posi-tion and temperature of the inner disk boundary. Towardsvery low masses, the stellar effective temperature is criti-cal for the disk detection efficiency at these wavelengths.Natta & Testi (2001) show that classical T Tauri disk mod-els describe well the observed spectral energy distribution(SED) for brown dwarf candidates. They suggest that, inthe substellar regime, the inner parts of such disks mightnot be hot enough to produce detectable near-infrared ex-cess. Indeed, Comerón et al. (2000) find that several low-mass stars and brown dwarfs (0.03M⊙ ≤ M ≤ 0.2M⊙)in Chamaeleon I exhibit 6.7µm excess emission but no K-band excess. It thus seems possible that towards less mas-sive objects one would expect to find comparatively fewerobjects with near-infrared excess and perhaps none amongbrown dwarfs. However, Muench et al. (2001) find a Ks-band excess in 65% of Trapezium cluster brown dwarfs,slightly more than the 50% K-band excess fraction thatLada et al. (2000) find in more massive stars (0.1−1M⊙).Clearly, the K-band excess frequency and presumablythe fraction of low-mass stars and brown dwarfs possess-ing warm circumstellar material is smaller around σ Orithan in the 1Myr old Trapezium cluster. This is true forboth the low-mass stars and also for the brown dwarfs.Even though our sample of brown dwarfs is small, thelack of even a single Ks-band excess detection (in theJHKs colour-colour diagram) is significant compared withthe 65% fraction seen by Muench et al. (2001) in theOliveira et al.: No disks around low-mass stars and brown dwarfs in the young σ Orionis cluster? 5Trapezium. Our results are consistent with the low diskfractions of 5−12%, determined from K- and L-band ex-cesses and Hα emission in the similarly aged Upper Scor-pius, NGC 2362 and λOri associations (Walter et al. 1994;Preibisch & Zinnecker 1999; Haisch et al. 2001; Dolan &Mathieu 1999, 2001). We could interpret our results withinthe framework of the decreasing disk fraction with age thathas been clearly established by Haisch et al. (2001 and ref-erences therein), using the (K −L) excesses found amongthe low-mass stars of several young clusters and associa-tions. Our results would then support their conclusions,but also extend the range of masses over which the diskdecay has been observed to the substellar regime.In summary, we have found no evidence for a sig-nificant population of low-mass stars and brown dwarfsaround σ Ori which exhibit Ks-band excesses. This pointsstrongly towards a low disk frequency, consistent with ob-servations of other similarly aged clusters. Thus this newresult adds support to the idea that disk evolution in clus-ters takes place on short (few Myr) timescales and extendsthis conclusion to very low masses. We find some evidenceto suggest that disk frequencies in the σ Ori association, asmeasured by the K-band excess, may be mass dependentin the opposite sense to those found in the Trapezium clus-ter. We do not attach great weight to this result at presentbecause of difficulties in comparing disk fractions mea-sured by different authors using differing techniques but, iftrue, it might suggest that the disk dissipation timescaleswere shorter for lower mass objects. More answers may beobtained on the mass dependence of the disk frequencyby extending infrared surveys into the brown dwarf do-main for more clusters with different ages, and preferablyat longer wavelengths, where disk presence can be morereliably diagnosed.Acknowledgements. This publication makes use of data prod-ucts from the Two Micron All Sky Survey, a joint project of theUniversity of Massachusetts and the IPAC, funded by NASAand the National Science Foundation. The 2MASS science dataand information services were provided by the InfraRed ScienceArchive (IRSA) at IPAC, which is operated by JPL. JMO is aPPARC postdoctoral research associate.ReferencesBally J., O’Dell C.R., McCaughrean M.J., 2000, AJ 119, 2919Baraffe I., Chabrier G., Allard F., Hauschildt P., 1998, A&A337, 403Beichman C.A., Chester T.J., Skrutskie M., Low F.J., GillettF., 1998, PASP 110, 480Béjar V.J.S., Mart́ın E.L., Zapatero Osorio M. 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No disks around low-mass stars and brown dwarfs in the young sigma
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No disks around low-mass stars and brown dwarfs in the young sigma Orionis cluster?

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