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The Galactic Clusters Survey: the first two years of UKIDSS (revised)

Nigel Hambly, Simon Hodgkin, Richard Jameson, Giovanni Carraro,
Mark McCaughrean, Mark Casali, Phil Lucas, & Motohide Tamura
(on behalf of the GCS Working Group)

August 2003


The Galactic Clusters Survey component of UKIDSS (GCS) proposes JHK imaging of 11 open clusters/star formation associations to study the stellar mass function (MF) to masses $m\sim10$  ${\rm M}_{\rm Jupiter}$. A key science goal is the form of the MF in the substellar régime and thereby a measurement of the contribution to the total mass in stellar systems made by brown dwarfs (BDs). As illustrated in Figure 4.1 in the GCS section of the UKIDSS case, at $m=10$  ${\rm M}_{\rm Jupiter}$ the number of stars per unit mass is currently uncertain by more than an order of magnitude. Equally important is the question of the universality (or dependence on environment) of the IMF in the BD régime.

This report makes a case for measuring some of the open clusters in Table 1 of the GCS in the UKIDSS case as soon as possible within the first 2 year period of the survey. These observations will provide a quick science return: lists of candidate BDs in rich cluster environments and initial estimates of the form of the MF therein. This will stimulate much follow-up spectroscopy on 4m to 8m class facilities. Ultimately, towards the end of the UKIDSS programme, the clusters will be re-imaged in K to provide second epoch position measures and hence proper motion estimates. As a second priority within the first 2 years, we suggest that the best areas of Taurus-Auriga, Orion and Sco be imaged since these are currently major areas of research and promise good returns. The third priority is K band imaging of the Hyades, Coma-Ber and remaining areas of Taurus-Auriga and Sco to provide first epoch imaging for these clusters and associations.

GCS target list revisited

In the light of UK entry into ESO the GCS target list has been revisited to ensure the possibility of VLT follow-up. We trawled throught the Lynga Open Cluster Catalogue [3] looking for clusters that had measured ages and distances, $\vert b\vert > 5^{\circ}$, $-30 < {\rm Dec} < +30$, age $<1$ Gyr and distance $< 500$ pc. In addition to most of the existing targets, the trawl netted

Name Reject because
Blanco 1 Too far south for UKIRT really
NGC 1662 Too old/far away
NGC 2232 Could be nice, except Trumpler richness class is poor and $\mu$ small
Col 140 Too far south
IC 4665 The only possible compromise
NGC 6633 Too old/distant
IC 4756 Too old/distant
IC 2391 and NGC 2451 are additionally on the ASTRO-F list, but they are all too far south for UKIRT. As a reasonable compromise we have substituted IC 4665 for M39 in the original target list. The VLTs are alt-az designs and can see up to Dec=+30 quite comfortably (see, for example, the visibility plots at We see no great problem with the rest of our open cluster targets apart from Alpha Per, but are reluctant to lose that as it is such a nice rich target for UKIRT and Subaru. As far as SF regions go, Taurus/Orion/Sco are all VLT-accessible, so we propose that we do not alter those. The revised GCS target list is shown in Table 1. Note that the age of Alpha Per and the age and distance of Sco have been updated according to considerations in [1] and [2] and references therein. Co-ordinates have changed slightly to reflect the prioritisations of observations based on a more thorough literature survey (see later).

The importance of near-IR photometry

It is clear from our own experience and previous work by others that JHK photometry alone is insufficient to identify the cleanest samples of VLM stars/BDs in cluster environments. Ultimately, UKIDSS will provide second epoch observations for measurement of proper motions; however it will help a great deal if complimentary near-IR data are available. Predictions of near-IR colours range from $3.0 < ({\rm I})-{\rm K}) < 6.0$ and $6.0 < ({\rm R}-{\rm K}) < 9.0$ depending on mass and distance [4,5]. We suggest that I and Z bands are the best choice (eg. [6]) for GCS purposes.

In order to compliment the UKIDSS GCS, we ideally require complimentary near-IR imaging to I $\sim24$ and Z $\sim22.5$, assuming (I-Z)=1.5. This can be achieved with the VST for fields having Dec $<+25^{\circ}$; the possibility of using CFHT data for the most northerly targets is being actively pursued. Our approach is to aim initially for full five filter coverage (ZYJHK) in our targets, and to refine this selection in the light of first results and on a target-by-target basis taking into account reddening and age. Note that for the oldest clusters it will not be possible to detect objects at the mass limits in Table 1 in IZ; however non-detection in IZ of a JHK-detected candidates indicates very strongly that an extremely red, and therefore highly probable, BD has been found.

Current state of research in the target clusters/associations

There is currently much research activity in the cluster/association area (see, for example, [7]); in May 2002 IAU Symposium 211, ``Brown Dwarfs'', was held. All members of the GCS working group attended this meeting so we are up-to-date on the latest research in progress and can refine/revise our target list as appropriate. There follows a brief summary of the current state of play in each of our targets:
Alpha Per: The most recent study is the optical imaging survey reported in [1]. These authors made a 10 square degree CCD survey reaching masses $m\sim0.08{\rm M}_{\odot}$. John Stauffer is also presenting their latest work at IAU 211.
Pleiades: Moraux et al. [8] report a steadily rising MF ($\alpha=+0.5$) in the mass range $0.4 > m/{\rm M}_{\odot} > 0.04$ based on a 2.5 square degree optical survey; Pinfield et al. [9] surveyed 6 square degrees in IZ to a mass limit of $m\sim0.05{\rm M}_{\odot}$ and found 30 BD candidates.
Hyades: Dobbie et al. [6] made an IZ survey of $\sim10$ square degrees to $m\sim0.06{\rm M}_{\odot}$ but found very few BD candidates.
Praesepe: Adams et al. [10] describe a 100 square degree survey employing photographic plates and 2MASS to $m\sim0.1{\rm M}_{\odot}$; conclusions as to the form of the MF are unknown. The deepest survey to date is as reported in Magazzù et al. [11] who found a single high mass BD candidate in 800 square arcmin surveyed to $m\sim0.07{\rm M}_{\odot}$.
Coma-Ber: Garcia-Lopez et al. [7] have searched for new late-type stars in this cluster. It appears to be a loose association with a possible deficit of low mass stars. The only MF determination seems to be that of Bounatiro & Arimoto [12], who quote $\alpha=1.9$ in the range $2.5 > m/{\rm M}_{\odot} > 1.0$.
IC 4665: No deep imaging of this cluster appears to have been done to date, although it is targeted in the CFHT Open Cluster Survey [13] and does appear to have good M-dwarf membership [14]
In summary, we emphasise that there have been no deep, wide-field IR imaging surveys in any of the above clusters. The potential for major new results is clear.

In the star formation associations:
Perseus: the deepest survey so far is the small-scale search in IC 348 by Preibisch et al. [15] who covered an area of $13\times13$ arcmin$^2$ to a depth of J=19 (equivalent to a single WFCAM device FOV to a less sensitive magnitude limit than that in the GCS!). They conclude that the low mass population of IC 348 at least seems to be suppressed. Najita et al. [16] also find a slowly declining MF in the BD régime using an even smaller scale HST imaging survey for IC 348.
Taurus-Auriga: the most recent study is that of Martín et al. [17] who surveyed 2.3 square degrees to I=17 with comparison against 2MASS. They found 4 BD candidates to $m\sim0.04{\rm m}_{\odot}$. Luhman [18] argues for a deficit of BDs in Taurus, but based on a very limited search of $\sim1000$ square arcmin.
Orion: Lucas & Roche [19] and Zapatero-Osorio et al. [20] report major new results in the Trapezium and $\sigma$-Orionis areas respectively of the Orion complex. Expanding these surveys is a clear priority in this region.
Sco: Ardila et al. [2] describe the most recent and deepest survey of the lowest mass members. Using RIZ imaging to I=18.5, they find an MF in excess if the Miller-Scalo into the BD régime; their lowest mass object in the 14 square degrees surveyed is $m\sim0.02{\rm m}_{\odot}$ (this survey constitutes about 10% of the area of the association).
Again, clearly there are great gains to be had over existing work - wide angle surveys have been limited to optical/near infrared, while any work in the infrared has been limited in areal coverage. There is as yet no clear consensus as to the form and universality of the IMF.

The importance of proper motions

Ultimately, the availability of proper motion measurements will help a great deal in refining membership lists for many of the GCS targets. It has been shown many times in the past that photometry alone is insufficient to obtain a final, clean list of cluster members. In order to facilitate accurate proper motion measurements, we require as long a time baseline as possible. This necessitates obtaining K band imaging as early as possible for any target where member proper motions are expected to be significant. This is a primary justification for imaging all targets except Orion as early as possible in the first two years of UKIDSS. Appendix B considers the feasibility of proper motion measures.

Bid for GCS observations in the first two years of UKIDSS

(We note that the UKIRT Board ``would prefer to see progress being made on the old cluster survey in the first instance, as a specific science item that might be largely completed within the first review period.'' and also that the Board ranks the GCS as second in priority only to the LAS)

The GCS science requirements necessitate early observations (in K band at least) for the targets in order to provide first epoch positions for eventual proper motion measurement. However, to enable quick exploitation, early follow-up and preliminary science results, we suggest three colour imaging in as large an area as possible in all targets. Assuming a baseline that UKIDSS receives 50% of all UK UKIRT time (which in turn is 80% of all nights each year), UKIDSS receives 145 n/yr for 2 yr, or 290 nights. The revised GCS target list (Table 1) comprises 7% of all UKIDSS time in the revised submission (71n out of 946n). Proportionately, the GCS should therefore use around 20 nights (we assume an average of 70% usable and efficiency 0.65). Hence, we may reasonably expect $\sim330$ ks of on-source exposure time. Our plan for the first two years is therefore to image as many targets as possible in the K band; all open clusters excluding the Hyades and Coma-Ber will additionally be observed in ZYJH. The remaining time in the first 2 years should be spent surveying Per-OB2 in ZYJH and then the same for the highest priority areas in the associations Orion, Sco and Taurus-Auriga (note that on source integration requires 0.21 ks per waveband per square degree, and also that Orion requires ZYJHK). Table 2 summarises the priorities.

Table: Suggested 2 year programme for the GCS. Note that the times listed here differ to those in Table 1 since the latter are total times for JHKK whereas the above are for ZYJHK or K only. In practice, any given pointing for the GCS will be observed at ZYJHK together or K only at the two epochs (except for Orion where two epochs are not required). The lowest priority targets 11 and 12 are currently outwith the two year plan but may be reinstated if fewer filters are needed for any of the higher priority targets in the final optimised programme.
Relative Target Bands/areas Time
priority   (filters/sq.deg) (ks)
1 IC 4665 ZYJHK/0.8 0.83
2 Pleiades ZYJHK/79 83.3
3 Alpha Per ZYJHK/50 53.3
4 Praesepe ZYJHK/28 30.0
5 Taurus-Auriga ZYJHK/central 24 25.2
6 Orion ZYJHK/central 16 16.8
7 Sco ZYJHK/central 24 25.2
8 Per-OB2 ZYJHK/12.6 13.2
9 Hyades K only/291 61.0
10 Coma-Ber K only/78.5 16.5
    2yr total: 325.3
11 Taurus-Auriga K only/remaining 362 76.0
12 Sco K only/remaining 130 27.3

Priority for the clusters is set by the minimum mass achievable modulo the cluster richness; clusters have priority over the star formation regions as requested by the Board. For immediate positive results, we lower the priority of the associations for which there is evidence of a paucity of low mass/substellar members (Hyades, Coma-Ber, Perseus) but of course one of the ultimate GCS goals is an examination of the universality (or otherwise) of the stellar IMF, so it is important not to bias the survey as a whole (ie. the full $\sim6$yr UKIDSS GCS programme) against associations where the IMF may be different.

Field centres, priorities and observing strategy

For the cluster targets, field centres have been computed using 5% overlaps and priorities set on the basis of proximity to the nominal cluster centre. Figure 1 shows an example for IC 4665, where the crosses indicate pointings and star symbols are Tycho catalogue bright stars. Similar plots are available for all targets (references [21] to [29]). Note that the Hyades overlaps with the Pleiades and the Taurus-Auriga area is non-circular with overlap areas in the Hyades, Pleiades and Per-OB2 excluded. Overlap areas between Coma-Ber and the LAS and Alpha-Per and the GPS will be taken into account once those surveys are finally specified.

Cluster `centres' and `extents' are less well defined for the star formation regions. We have specified areas and prioritised on proximity to the co-ordinates listed in Table 1 based on the following considerations: Per-OB2 - de Zeeuw et al. [30], esp. Fig. 17; Taurus-Auriga - Ungerecht & Thaddeus [31] esp. Fig. 4, Gomez et al. [32] esp. Fig. 8 plus some current work of our own in this region; Orion - priority is based on proximity to the $\sigma$-Orionis and Trapezium clusters; Sco - we take as priority the Upper Scorpius part of Sco-OB2 ([30] Figs. 6 & 9; see also [2] Fig. 1). Prioritised lists of field pointings for all targets are available at [33]. The columns in these files are: relative priority, RA & Dec (J2000.0), distance from nominal centre.

Figure 1: Example GCS pointings: IC 4665 (see text).

Observing strategy considerations: photometric weather is preferred, but observations taken under non-photometric conditions may be possibly cross-calibrated using overlap regions. K observations will be used for astrometry and therefore have the tightest seeing limits: seeing worse than 1 arcsec would degrade astrometric accuracy by a factor 2 for the same depth. Lower priority targets can be substituted for poor conditions. Further implementation considerations can be found in an implementation discussion document [34].


Requirements for follow-up observations

Ultimately, spectroscopic confirmation of at least some of the candidate BDs in the clusters/associations will be required. Comprehensive spectroscopic surveying of all possible candidates is likely to be impractical. For example, Zapatero-Osorio `citezapa00 report candidate densities, to similar magnitude limits as those envisioned here, of 18 per 847 arcmin$^2$ in the $\sigma$-Orionis cluster, or 100s per square degree. Lucas & Roche [19] report higher candidate densities in Trapezium. In the Pleiades, Simons & Becklin [35] found candidate densities at K $\sim17$ of 100s per square degree. These numbers are not well matched to multi-object fibre spectroscopy. The faintest targets will of course require 8m facilities; brighter targets will be observable on 4m-class facilities.

Feasibility of proper motion measurements

Theoretical predictions (eg. [36,37]) show that centroiding precision is simply related to the relative error in the flux detection as follows:

\end{displaymath} (1)

where $\sigma_X$ is the error in centroiding (either co-ordinate) and $a$ is the scale size of the image (ie. FWHM/2). If we assume at the limits of the survey $\sigma_I/I=0.2$ (ie. $5\sigma$ detections) and well-sampled images (after micro-stepping) with image size of 0.47 arcsec (median image FWHM delivered by UKIRT with tip/tilt) then we have $\sigma_X=47$ mas. Hence, over a five year baseline relative proper motion measurements in either co-ordinate with precision of the order of $\surd2\times0.047/5\sim13$ mas yr$^{-1}$ will be possible. So, the faintest images in any cluster with $\mu_{\rm TOT}>26,39$ mas yr$^{-1}$ will have better than $2,3\sigma$ detection respectively of proper motion over a five year baseline (brighter images will have correspondingly more accurate pms up to a limit imposed by other factors, eg. position of incident photons on pixels and errors in pixel placement over the detector from the assumed regular ideal). Proper motion measurements should be made in the K band, since detections (and hence centroiding) will be most significant in K.


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The Galactic Clusters Survey: the first two years of UKIDSS (revised)

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