The advantages of using clusters/associations for determining the IMF are many, especially when compared to studies of the general field population (e.g. Liebert et al. 2000). A cluster presents a single coeval population of stars whose global properties (e.g. distance, age, metallicity) are known, in some cases very accurately, via independent means. This is in stark contrast to the field population, whose complex dynamical evolution and star formation history makes measurement of the MF very difficult and derivation of the IMF almost impossible.
The requirements for this programme are a selection of clusters/associations with: small distances (to allow investigation of the least luminous and therefore lowest mass objects); a good spread in ages (to allow investigation of temporal evolution of the MF and inference of the IMF); and a variety of Galactic environments (to allow investigation of the universality of the IMF). Such a sample is presented in Table 4.1, with explanation in the caption. In order to predict the mass limits achievable we have used the DUSTY models of Chabrier et al. (2000), interpolating/extrapolating where required. While some of these numbers are necessarily uncertain, they give an estimate of the mass limits achievable. The numbers of BDs expected given certain assumptions about the form of the mass function are also given. Here, we have assumed for the sake of argument a universal MF based on that measured in the Pleiades (Martin et al. 1998; Hambly et al. 1999b) with various extrapolations to low masses. The normalisation is to the total mass estimate for the cluster/association. In order to be able to disentangle age/metallicity/environment effects and to quantify evolutionary effects as the IMF translates into the present-day MF, we feel that 10 targets is close to a minimum requirement. Assuming that these targets are representative of the birthplace of most Galactic disk stars, this study will yield information applicable to the Galactic disk as a whole. Follow-up observations may include studies of the binary fraction and searches for eclipsing BDs that will confront theoretical models with real data.
Photometric identification of candidates requires JHK measurements: two-colour diagrams are needed to measure reddening and hence extinction before transformation of luminosity to mass; K photometry is useful for detecting very young sources via hot dust emission. Proper motion measurements are highly desirable to refine membership lists selected by photometry alone since some cluster non-members will inevitably contaminate regions of colour-magnitude and colour-colour diagrams where the cluster members are found. All the regions surveyed will be revisited in the K band for proper motion measurement over a 5 year baseline. The second epoch observations will also enable detection of YSOs from K-band variability.
| Cluster or association |
RA | Dec | µ | Ang. rad. |
AV | Total mass |
Dist | Age | Depth | Mass limit mL | Tot. exp. time |
No. of BDs in mL to mL+10M(Jup) | ||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| (J2000) | J | K | power law MF | log norm | ||||||||||||
| hh mm | sdd mm | mas/yr | arc min. | mag. | M(sun) | pc | Myr | mag. | mag. | M(Jup) | ksec | +1 | 0 | -1 | ||
| (1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) | (9) | (10) | (11) | (12) | (13) | (14) | (15) | (16) | (17) |
| Open clusters | ||||||||||||||||
| Alpha Per | 03 26 | +40 30 | 34 | 240 | 0.3 | 530 | 190 | 50 | 19.7 | 18.4 | 15 | 42.3 | 399 | 80 | 16 | 7 |
| Pleiades | 03 50 | +24 14 | 50 | 300 | 0.2 | 800 | 118 | 100 | 19.7 | 18.4 | 24 | 66.1 | 422 | 121 | 35 | 17 |
| Hyades | 04 19 | +15 34 | 100 | 600 | 0.0 | 670 | 45 | 600 | 19.7 | 18.4 | 41 | 264.6 | 221 | 101 | 47 | 24 |
| Praesepe | 08 39 | +19 26 | 38 | 180 | 0.0 | 670 | 180 | 400 | 19.7 | 18.4 | 46 | 23.8 | 199 | 101 | 52 | 26 |
| Coma-Ber | 12 25 | +26 08 | 15 | 300 | 0.0 | 270 | 87 | 500 | 19.7 | 18.4 | 43 | 66.1 | 85 | 41 | 20 | 10 |
| M39 | 21 29 | +48 20 | 21 | 60 | 0.2 | 130 | 310 | 100 | 19.7 | 18.4 | 30 | 2.6 | 56 | 20 | 7 | 4 |
| Star formation associations | ||||||||||||||||
| Perseus | 03 46 | +31 42 | 0 | 60 | 5.0 | 100 | 350 | 1 | 19.7 | 18.4 | 13 | 2.6 | 86 | 15 | 3 | 1 |
| Taurus | 04 25 | +23 01 | 23 | 600 | 5.0 | 150 | 185 | 1 | 19.7 | 18.4 | 11 | 264.6 | 146 | 23 | 4 | 1 |
| Auriga | 04 58 | +31 33 | 23 | 400 | 5.0 | 100 | 170 | 1 | 19.7 | 18.4 | 11 | 117.6 | 99 | 15 | 2 | 1 |
| Orion | 05 27 | +03 17 | 0 | 600 | 5.0 | 1000 | 440 | 1 | 19.7 | 18.4 | 14 | 264.6 | 816 | 151 | 29 | 11 |
| Sco | 16 19 | -21 41 | 0 | 540 | 5.0 | 200 | 220 | 10 | 19.7 | 18.4 | 12 | 214.3 | 179 | 30 | 5 | 2 |
| TOTAL: ~1330 | ||||||||||||||||
Table 4.1. Cluster/star formation association targets. Notes: for columns (1) to (9) the primary data source is Allen (1973) with updated information where available; (5) is the angular extent proposed to be surveyed; (6) is the average visual extinction and we have assumed AJ,H,K 0.27, 0.16, 0.10 AV; total masses (7) have been calculated assuming the clusters to be 'Pleiades-like' and have masses scaled by the number of known higher mass stars; where available in (8) Hipparcos distances from Robichon et al. (1999) have been used; the JK depths (10,11) are the proposed depths in each case to survey the cluster to obtain the mass limit (12) calculated using the DUSTY models of Chabrier et al. (2000) - luminosities depend on both distance and age; total exposure times are on source integration times and include H plus repeat K measurements for proper motions; columns (14) to (17) give model predictions of the number of BDs expected in the cluster given power-law or log-normal 'Pleiades-like' MFs normalised to give the total mass in (7) - these give an indication of the sampling errors in the lowest mass bin of the MF derived from these data for the different MF forms.