|Annu. Rev. Astron. Astrophys. 1996. 34:
Copyright © 1996 by . All rights reserved
3.3. Main-Sequence Stars
Butcher (1977) identified a break in the slope of the main-sequence luminosity function of field stars in the LMC. This change in slope occurs about 1 mag brighter than the turnoff in globular clusters, leading him to conclude that the field was predominately composed of stars with ages 3-4 Gyr or younger. Later studies by Stryker (1983), Hardy et al (1984) reached nearly identical conclusions for other fields in the LMC. In the SMC, Brück and collaborators (e.g. Brück 1980) and others (Hardy & Durand 1984, Bolte 1987) have also identified the presence of an intermediate-age population, but in this case with a mean age that seems to be significantly older than in the LMC.
More recently, studies using CCDs have begun to map out the detailed composition of the intermediate-age populations in the MCs. In order to adequately sample the old and intermediate-age stars, such data must reach to V ~ 24 to obtain good photometric precision at the level of the ancient turnoff, at V ~ 22.5 in the LMC and ~ 22.9 in the SMC. They should also cover a large area on the sky to adequately sample the rarest population present in a given field. Table 2 lists all MC fields for which deep photometry has been obtained that can constrain the frequency of populations as old as 10-15 Gyr using main-sequence stars. Linde et al (1995) provide a chart showing the locations of some of these fields in the LMC.
|Field||2000||2000||Galaxy||Vmaxa||Area (arcmin2)||Nfb||N*c||Age range (Gyr)|
|B77||05 14||-65 46||LMC||23.0||0.6 (2.5)d||1||120||< 3-5|
|H84||05 09||-68 53||LMC||21.3||72||1||18000||< 3|
|DH84||01 08||-72 34||SMC||21.4||33||3||~ 2000||0.13|
|S84||06 30||-64 04||LMC||22.8||540||11||3700||1-6|
|A85||05 19||-70 57||LMC||23.0||3.2||2||1150||0.11|
|B87||01 03||-70 51||SMC||23.8||15||1||~ 800||0.2-8|
|H87||04 33||-72 14||LMC||22.0||400||1||350||2-3|
|B92-N1783||04 58||-65 58||LMC||23.5||15||1||2030||0.5-4.5|
|B92-N1866||05 13||-65 26||LMC||23.5||15||1||2370||0.5-3.8|
|B92-N2115||05 58||-65 28||LMC||23.8||15||1||1780||1.3-4|
|E94-F1||05 39||-68 54||LMC||24.1||7.3||1||3000||0.7-5 + anc?e|
|E94-F2||05 35||-69 10||LMC||24.1||7.3||1||4200||0.7-5 + anc?|
|W95-NW||05 03||-65 52||LMC||22.9||15.5||4||2190||0.2-3|
|W95-SW||05 48||-73 32||LMC||23.0||11.6||3||940||1-3|
|a Vmax is the limiting V-band magnitude of the survey.|
|b Nf is the number of fields studied; in some cases different fields come from the same imaging data, but were measured and analyzed separately (e.g. S84, A85).|
|c N* is the approximate number of stars observed; this could only be very roughly estimated in the case of HD84.|
|d Two field sizes are listed for B77; the smaller value refers to the field size in which stars as faint as V ~ 23 were measured.|
|e E94 claim detection of a possibly ancient field-star population, denoted "anc."|
|This table lists only studies aimed principally to study intermediate-age MC field-star populations. Other field-star results can be found in the references listed in Section 3.3 of the text. Projects in progress are not listed here. Stryker (1984b) Stryker (1984b) gives a summary of additional MC field-star studies. References: B77 = Butcher 1977 Butcher 1977; H84 = Hardy et al 1984 Hardy et al 1984; HD84 = Hardy & Durand 1984 Hardy & Durand 1984; S84 = Stryker 1984a Stryker 1984a; A85 = Ardeberg et al 1985 Ardeberg et al 1985; B87 = Bolte 1987 Bolte 1987; H87 = Hodge 1987 Hodge 1987; B92 = Bertelli et al 1992 Bertelli et al 1992; E94 = Elson et al 1994 Elson et al 1994; W95 = Westerlund et al 1995 Westerlund et al 1995 (data for this study are described in Linde et al 1995 Linde et al 1995).|
Bertelli et al (1992) analyzed deep CCD data in three LMC fields with the aim of deriving the star-formation histories at each location. They calculated a number of well-defined parameters - such as the ratio of red subgiants to main-sequence stars in a specified magnitude interval - and compared these with predictions from synthesized color-magnitude diagrams generated from the Padova stellar evolutionary models (e.g. Bertelli et al 1994). The indices were designed to be easy and robust to measure, to be relatively insensitive to photometric incompleteness, and yet to remain sensitive to various important physical parameters, such as the slope of the mass function (assumed to be constant with time), the mean metallicity, and the relative numbers of stars formed at different epochs. Remarkably, only a limited range of star-formation histories and metallicities could simultaneously account for the observed values of these parameters. Bertelli et al (1992) suggest that all three fields underwent a significant enhancement in the star-formation rate - by at least a factor of 10 - some 3-5 Gyr ago. Though the precise age of this "burst" epoch depends on the models, the conclusion that the bulk of star formation commenced contemporaneously in all three fields is practically model independent.
Subsequent studies have broadly confirmed the Bertelli et al (1992) results, with some interesting differences. Westerlund et al (1995; see Linde et al 1995 for the input data) have studied two LMC fields, one each in the NW and SW regions of the galaxy. They find in both fields that the oldest major population corresponds to stars with ages of 1-3 Gyr. The ages of the youngest stars in the two fields appear to differ significantly. In the NW field, stars as young as 0.3-0.8 Gyr are present, whereas the youngest population in the SW field appears to be slightly older than 1 Gyr. This is surprising because both are located beyond 5° from the LMC center, far from the well-known active star-forming sites in the LMC. Westerlund et al (1995) claim also to see evidence for a 7-10 Gyr population in their SW field, but the depth of their photometry and their sample size is probably only marginally adequate to constrain the presence of stars this old. This interesting result demands confirmation with deeper photometry. Vallenari et al (1994a, b) report preliminary results of a study patterned after that of Bertelli et al (1992) for several fields scattered throughout the LMC. They report that the age of the oldest significant population in one of the fields is 7-8 Gyr. Some of their data also seem to show evidence of distinct intermediate-age bursts in one of their fields, not seen in other fields.
No comparably deep photometric studies have been carried out in the SMC. The most comprehensive effort (Gardiner & Hatzidimitrou 1992) only probes main-sequence stars younger than about 2 Gyr. Gardiner & Hatzidimitrou conclude from these data that the SMC star-formation rate seems to be globally declining over the past 2 Gyr, with the obvious exceptions of active star-forming regions in the inner SMC and in the SMC "wing," which contain stars as young as 200 Myr. Deep CCD studies of the distribution of ages of field stars in the SMC are badly needed. Some care should be taken in selecting SMC fields. The very large line-of-sight depth of the galaxy (particularly on its eastern side; Hatzidimitrou et al 1989, Gardiner & Hawkins 1991) is troublesome. It would be very difficult to disentangle depth and age effects without independent kinematic data (Hatzidimitrou et al 1993). To avoid this problem, deep population studies should focus on the western portions of the SMC at present.