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[ [ low - mass - stars - in - ob - associations ] ] low - mass stars in ob associations + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +    historically , ob associations were identified as loose groups of coeval o - and b - type stars .
however when extrapolating the mass function for ob stars to lower masses , it was found that most of the mass in such groups should actually be in low - mass stars ( @xmath2m@xmath3 ) ( e.g.  brown 2001 ) .
indeed , h@xmath4 objective prism surveys and x - ray surveys unveiled large populations of low - mass stars in these associations .
+ most stars may form in such associations ( walter et al .
2001 ) , with massive and low - mass stars originating in the same star forming regions and at approximately the same epoch .
if this is the case , then the presence of these luminous stars in the midst of lower mass siblings raises important questions : do ob stars influence the rate of lower - mass star formation and do they affect pre - main - sequence ( pms ) stellar evolution and , more crucially , circumstellar disk evolution ?    [ [ the - importance - of - circumstellar - disks ] ] the importance of circumstellar disks + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +    circumstellar disks are an important part of the star formation process and are ubiquitous around young stars . even though they seem to be relatively short lived , they impact strongly on stellar evolution .
the interaction between the disk and the stellar magnetic field plays a central role in angular momentum regulation and it is thereby likely to influence the spread of rotation rates in young stars .
young stars may also accrete a significant fraction of their final mass from their disks , so disk evolution could impact strongly on theoretical pms models and attempts to determine stellar masses and ages .
+ two very important questions are disk dissipation timescales and its possible mass dependence .
disk dissipation timescales might be the strongest factor in determining the timescales for planet formation or whether planets form at all in a particular stellar system
. this could be even more crucial in ob associations where there are exterior sources of photo - evaporating radiation .
mass dependence of disk dissipation could be important in the context of brown dwarf formation .
it has been proposed that brown dwarfs form by ejection from their parent systems ; numerical simulations ( bate et al .
2003 ) propose that the dynamical interactions that eject brown dwarfs also truncate their disks , making them relatively small and of low mass .
a testable prediction of such models is that brown dwarf disks dissipate faster .
[ [ the - sigmaorionis - cluster ] ] the @xmath0orionis cluster + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +    @xmath0orionis is a trapezium - like system with an o9.5v primary .
the population of low - mass stars spatially clustered around this system was discovered as bright sources in rosat x - ray images , and follow - up optical spectroscopy confirmed most sources as pms stars ( walter et al .
this association is young , nearby and affected by low reddening , making it an ideal target to analyse the pms population , even down to brown dwarfs and isolated planetary mass objects .
the main goal of the work we summarise here is to probe the properties of the pms population in the @xmath0ori cluster , establishing a representative sample of cluster members and analysing the frequency and properties of circumstellar disks .
[ [ photometric - surveys ] ] photometric surveys + + + + + + + + + + + + + + + + + + +    firstly , it is necessary to identify the low - mass pms content near @xmath0ori .
fig.1 ( left ) shows an @xmath1 colour - magnitude diagram , obtained with the wide field camera ( wfc ) at the isaac newton telescope ( int ) , where the pms stars can be seen separated from the bulk of the contamination .
it was based on this diagram that targets were identified for follow - up spectroscopy .
the figure also shows ( right ) the @xmath5 colour - magnitude diagram for cluster members  j - magnitude comes from the 2mass database .
it shows most of the ( over 100 ) cluster members identified so far , either by us ( see below ) or from the literature ( e.g.  bjar et al . 2001 ; zapatero osorio et al . 2002 ) .
overplotted are the model tracks and isochrones from baraffe et al .
we estimate that the median age of the cluster is approximately 4myr ( oliveira et al .
2002 ) and the sample covers the mass range 0.02@xmath61m@xmath3 .
it is worth pointing out that a large number of brown dwarfs have been identified ( 30@xmath640 depending on the adopted age ) .
several isolated planetary mass objects ( @xmath7m@xmath3 ) have been discovered in the cluster ( e.g. zapatero osorio et al .
2000 ) .
[ [ membership - analysis ] ] membership analysis + + + + + + + + + + + + + + + + + + +    one of the main drives of our spectroscopic observations was to investigate how efficient photometric surveys are in identifying pms stars and thus cluster members .
this is actually quite important for the disk frequency analysis as contamination leads to lower disk frequencies .
+ we have performed spectroscopic observations of approximately 70 photometric cluster candidates with wyffos at the william hershel telescope ( wht ) .
we use 3 means of diagnostic for cluster membership : radial velocity measurements ( compared with the cluster mean of @xmath832kms@xmath9 ) , the equivalent width of the lii 6708  feature ( a fragile element that remains unburned for stars younger than 10myr ) , and the equivalent width of the nai 8190  doublet ( a gravity indicator that is very weak in the spectra of giant stars , is strong for field dwarfs and weak for pms stars ) . using these indicators together we are able to firmly identify cluster members and binary candidates .
we found that 90% of the objects in the photometric sample are indeed cluster members .
this implies that photometric surveys are quite efficient in identifying cluster members .
this analysis is described in kenyon et al .
( 2003 in preparation ) .
we have an on - going program to survey the circumstellar disk population in the @xmath0 ori cluster .
the traditional first step is to look for k - band excesses in our sample of cluster members . using @xmath10 photometry from 2mass ,
it has been established ( oliveira et al .
2002 ; barrado y navascus 2003 ) that at most 10% of the cluster members exhibit a k - band excess that could indicate the presence of a circumstellar disk .
however , a k - band excess is an unreliable disk indicator .
it depends strongly on the inner disk temperature ; in particular for the lower mass stars , there is evidence that their disks are not hot enough to produce significant k - band excess ( natta & testi 2001 ; comern et al .
2000 ) .      at longer wavelengths ,
infrared excesses grow rapidly thus l - band observations are the most reliable and efficient way to detect circumstellar disks , down to the substellar regime .
we have a program to observe all known @xmath0ori cluster members in the l - band ( and also in the k - band to avoid uncertainties related with variability , carpenter et al .
2002 ) ; we have obtained data in january 2003 with the newly installed uist at the uk infrared telescope ( ukirt ) .
due to adverse weather conditions we could only perform about 30% of the program , observing the targets in the mass range 0.2@xmath61.0m@xmath3 .
we have since then been allocated more observing time at ukirt to complete this program .
+ fig.2 shows colour - colour diagrams for the sample observed at ukirt . on the top left
is the 2mass @xmath10 diagram ; only one object has a significant k - band excess , indicative of a circumstellar disk .
the situation changes dramatically in the @xmath11 diagram ( top right ) where many stars seem to exhibit @xmath12 excess , evidence of circumstellar disks .
a very revealing diagram is @xmath13 ( bottom ) . both photometric variability in i- and j - band and reddening would move an object s position almost horizontally in this diagram , while a disk excess moves an object vertically .
this means that the diagnostic value of this diagram for the detection of disks is for all effects insensitive to variability and reddening .
at least 30% of the objects seem to have an excess that indicates the presence of a circumstellar disk , but this value might be as high as 50% . only the computation of the intrinsic colour for each object will reveal the true percentage of objects with an excess ( oliveira et al .
2003 in preparation ) .
l - band observations are adequate for disk surveys , but in order to characterise circumstellar disk properties , observations at even longer wavelengths are necessary . based on estimates from the original iras data for the possible mid - ir emission of pms stars around @xmath0ori
, we have targetted 12 such objects with timmi2 at the eso 3.6 m telescope to obtain n - band photometry , as well as q - band photometry and n - band spectroscopy for the brightest amongst them .
+ fig.3 shows the spectral energy distribution ( sed ) of these sources
. indicated in the upper right corner of each graph is the spectral index @xmath4  that is a measure of the mid - ir excess .
the three objects on the top row , @xmath0ori itself , @xmath0ori e and a rosat source are the more massive , early spectral type objects .
their seds are consistent with the slope of the rayleigh - jeans tail of a blackbody ( @xmath14 ) , so there is no excess emission .
the other objects all show an excess at these wavelengths , evidence of circumstellar disks .
for instance , tx ori has @xmath15 consistent with a classii classification ( classical t tauri star , ctts ) ; the spectrum in the n - band reveals a feature consistent with silicate dust emission at 10@xmath16 m .
based on the value of @xmath4 , most objects can be classified as classii objects .
two objects exhibit seds that suggest more complex circumstellar structures : v510ori ( associated with a jet ) has @xmath4 consistent with a flat - spectrum source and iras05358 - 0238 has @xmath4 consistent with a classi source ( an intermediate state between an embedded class0 source and a ctts ) .
this analysis is described in full in oliveira & van loon ( 2003 in preparation ) .
we have combined photometric and spectroscopic surveys and identified a very significant number of cluster members , that together with other surveys constitutes a unique sample of pms objects , covering the mass range 0.02@xmath61.0m@xmath3 .
+ we have begun a program to search for cluster members with circumstellar disks in the l - band .
preliminary results on a representative sample ( 0.2@xmath61.0m@xmath3 ) indicate that at least 30% of @xmath0ori cluster members have circumstellar disks .
when compared with two other clusters of similar ages ( ngc2264 at @xmath83.2myr and ngc2362 at @xmath85myr ) , a disk frequency of 30% for the @xmath0ori cluster would fit nicely between these two clusters ( disk frequencies respectively 52% and 12% ) , broadly agreeing with the 6myr timescale for all stars to lose their disk as proposed by haisch et al .
as our l - band survey does not yet reach low enough masses , we are not able to address the issue of mass dependence of disk frequency .
we intend to continue our program to search for and characterise circumstellar disks around fainter members of the @xmath0ori cluster .    99 baraffe i. , chabrier g. , allard f. , hauschildt p. , 1998
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the @xmath0orionis cluster is a young association evolving under the disruptive influence of its massive o - star namesake . 
 we are analysing this cluster as part of a program to characterise the influence of o - stars on the early stages of stellar evolution . at an age of approximately 4myr 
 , this cluster is at a crucial stage in terms of disk evolution and therefore it is a key case to better constrain disk dissipation timescales . 
 we have obtained @xmath1 photometry and optical spectroscopy of the @xmath0ori cluster ; we have analysed the lii and nai features to establish cluster membership . 
 we have thus gathered a unique sample of spectroscopically confirmed low - mass cluster members and brown dwarfs . 
 disk frequencies from k - band excesses from 2mass suggest that less than 7% of the very low - mass @xmath0ori members have disks ( oliveira et al . 
 2002 ) , in stark contrast with even younger clusters ( e.g. trapezium ) . however , near - infrared disk frequencies have to be taken with caution . 
 we are currently undertaking an l - band ( imaging ) and mid - infrared ( imaging and spectroscopy ) program to identify and probe the properties of circumstellar disks around young stars in this cluster . 
 preliminary results indicate that at least 30% of cluster members have circumstellar disks .