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
. 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
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.
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,
,
, age
Gyr and distance
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 ![]() |
Col 140 | Too far south |
IC 4665 | The only possible compromise |
NGC 6633 | Too old/distant |
IC 4756 | Too old/distant |
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
and
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 and Z
, assuming (I-Z)=1.5.
This can be achieved with the VST for fields
having Dec
; 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.
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
. John Stauffer is also
presenting their latest work at IAU 211.
Pleiades: Moraux et al. [8]
report a steadily rising MF ()
in the mass range
based on a 2.5 square
degree optical survey; Pinfield et al. [9] surveyed 6 square degrees
in IZ
to a mass limit of
and found 30 BD candidates.
Hyades: Dobbie et al. [6] made an IZ survey of
square degrees to
but found
very few BD candidates.
Praesepe: Adams et al. [10] describe a 100 square degree survey employing
photographic plates and 2MASS to
; 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
.
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 in the range
.
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 arcmin
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
. Luhman [18]
argues for a deficit of BDs in Taurus, but based on a very limited search
of
square arcmin.
Orion: Lucas & Roche [19] and
Zapatero-Osorio et al. [20] report major
new results in the Trapezium and -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
(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.
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.
(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 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.
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 yr UKIDSS GCS programme)
against associations where the IMF may be different.
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 -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.
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].
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
in the
-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
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.
Theoretical predictions (eg. [36,37])
show that centroiding precision is simply
related to the relative error in the flux detection as follows:
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