LMC Field Research Articles

Variable Star Classification with a Multiple-input Neural Network

In this experiment, we created a Multiple-Input Neural Network, consisting of convolutional and multilayer neural networks. With this setup the selected highest-performing neural network was able to distinguish variable stars based on the visual characteristics of their light curves, while taking also into account additional numerical information (e.g., period, reddening-free brightness) to differentiate visually similar light curves. The network was trained and tested on Optical Gravitational Lensing Experiment-III (OGLE-III) data using all OGLE-III observation fields, phase-folded light curves, and period data. The neural network yielded accuracies of 89%–99% for most of the main classes (Cepheids, δ Scutis, eclipsing binaries, RR Lyrae stars, Type-II Cepheids), only the first-overtone anomalous Cepheids had an accuracy of 45%. To counteract the large confusion between the first-overtone anomalous Cepheids and the RRab stars we added the reddening-free brightness as a new input and only stars from the LMC field were retained to have a fixed distance. With this change we improved the neural network’s result for the first-overtone anomalous Cepheids to almost 80%. Overall, the Multiple-input Neural Network method developed by our team is a promising alternative to existing classification methods.

A POPULATION OF ACCRETED SMALL MAGELLANIC CLOUD STARS IN THE LARGE MAGELLANIC CLOUD

We present an analysis of the stellar kinematics of the Large Magellanic Cloud based on ~5900 new and existing velocities of massive red supergiants, oxygen-rich and carbon-rich AGB stars, and other giants. After correcting the line-of-sight velocities for the LMC's space motion and accounting for asymmetric drift in the AGB population, we derive a rotation curve that is consistent with all of the tracers used, as well as that of published HI data. The amplitude of the rotation curve is v_0=87+/-5 km s^-1 beyond a radius R_0=2.4+/-0.1 kpc, and has a position angle of the kinematic line of nodes of theta=142 degrees +/-5 degrees. By examining the outliers from our fits, we identify a population of 376 stars, or >~5% of our sample, that have line-of-sight velocities that apparently oppose the sense of rotation of the LMC disk. We find that these kinematically distinct stars are either counter-rotating in a plane closely aligned with the LMC disk, or rotating in the same sense as the LMC disk, but in a plane that is inclined by 54 degrees +/- 2 degrees to the LMC. Their kinematics clearly link them to two known HI arms, which have previously been interpreted as being pulled out from the LMC. We measure metallicities from the Ca triplet lines of ~1000 LMC field stars and 30 stars in the kinematically distinct population. For the LMC field, we find a median [Fe/H]=-0.56 +/- 0.02 with dispersion of 0.5 dex, while for the kinematically distinct stars the median [Fe/H] is -1.25 +/- 0.13 with a dispersion of 0.7 dex. The metallicity differences provide strong evidence that the kinematically distinct population originated in the SMC. This interpretation has the consequence that the HI arms kinematically associated with the stars are likely falling into the LMC, instead of being pulled out.

The star formation history of the Large Magellanic Cloud star cluster NGC 1751★

The HST/ACS colour-magnitude diagrams (CMD) of the populous LMC star cluster NGC1751 present both a broad main sequence turn-off and a dual clump of red giants. We show that the latter feature is real and associate it to the first appearance of electron-degeneracy in the H-exhausted cores of the cluster stars. We then apply to the NGC1751 data the classical method of star formation history (SFH) recovery via CMD reconstruction, for different radii corresponding to the cluster centre, the cluster outskirts, and the underlying LMC field. The mean SFH derived from the LMC field is taken into account during the stage of SFH-recovery in the cluster regions, in a novel approach which is shown to significantly improve the quality of the SFH results. For the cluster centre, we find a best-fitting solution corresponding to prolonged star formation for a for a timespan of 460 Myr, instead of the two peaks separated by 200 Myr favoured by a previous work based on isochrone fitting. Remarkably, our global best-fitting solution provides an excellent fit to the data - with chi^2 and residuals close to the theoretical minimum - reproducing all the CMD features including the dual red clump. The results for a larger ring region around the centre indicate even longer star formation, but in this case the results are of lower quality, probably because of the differential extinction detected in the area. Therefore, the presence of age gradients in NGC1751 could not be probed. Together with our previous findings for the SMC cluster NGC419, the present results for the NGC1751 centre argue in favour of multiple star formation episodes (or continued star formation) being at the origin of the multiple main sequence turn-offs in Magellanic Cloud clusters with ages around 1.5 Gyr.

NGC 1866: a milestone for understanding the chemical evolution of stellar populations in the Large Magellanic Cloud★

We present new FLAMES@VLT spectroscopic observations of 30 stars in the field of the LMC stellar cluster NGC 1866. NGC 1866 is one of the few young and massive globular cluster that is close enough so that its stars can be individually studied in detail. Radial velocities have been used to separate stars belonging to the cluster and to the LMC field and the same spectra have been used to derive chemical abundances for a variety of elements, from [Fe/H] to the light (i.e. Na, O, Mg...) to the heavy ones. The average iron abundance of NGC 1866 turns out to be [Fe/H]= -0.43+-0.01 dex (with a dispersion of 0.04 dex), from the analysis of 14 cluster-member stars. Within our uncertainties, the cluster stars are homogeneous, as far as chemical composition is concerned, independent of the evolutionary status. The observed cluster stars do not show any sign of the light elements 'anti-correlation' present in all the Galactic globular clusters so far studied, and also found in the old LMC stellar clusters. A similar lack of anti-correlations has been detected in the massive intermediate-age LMC clusters, indicating a different formation/evolution scenario for the LMC massive clusters younger than ~3 Gyr with respect to the old ones. Also opposite to the Galactic globulars, the chemical composition of the older RGB field stars and of the young post-MS cluster stars show robust homogeneity suggesting a quite similar process of chemical evolution. The field and cluster abundances are in agreement with recent chemical analysis of LMC stars, which show a distinctive chemical pattern for this galaxy with respect to the Milky Way. We discuss these findings in light of the theoretical scenario of chemical evolution of the LMC.

Clusters of PMS stars in the Magellanic Clouds: the case of LH95

AbstractWe present a study of the pre-main-sequence (PMS) population in the stellar association LH95 in the LMC, based on the deepest HST/ACS photometry ever taken in a region of the Magellanic Clouds. This association hosts ~ 2500 PMS stars, distributed in three main subclusters. We isolate this population by subtracting the LMC field, study the reddening distribution which peaks at AV ~ 0.5 mag, and assign masses, down to 0.2 M⊙, to each member using a newly derived conversion of evolutionary models for the LMC metallicity and the ACS photometric system. We derive the first IMF in the subsolar regime ever measured outside our Galaxy, complete down to 0.4 M⊙. It presents a flattening at 1 M⊙, changing its slope from x = 2.05 for intermediate-mass stars to x = 1.05 in the subsolar regime. Correcting for unresolved binarity, it is compatible with the Galactic IMF. We do not find evidence of spatial variations of the IMF in the region. We study the ages of the PMS members using maximum-likelihood methods based on the location of the stars in the color–magnitude diagram with respect to 2D isochrones obtained by applying intrinsic and observational biases (differential extinction, variability, confusion, unresolved binarity) to modeled simple stellar populations. We find a most likely global age of ~ 10 Myr, and demonstrate that neglecting the aforementioned biases leads to an underestimation of the cluster age of up to 50%. We find that the observed luminosity spread is more than twice larger than can be explained by such biases, confirming that the star formation in LH95 lasted a few Myr.

Period-luminosity relations for type II Cepheids and their application

JHKs magnitudes corrected to mean intensity are estimated for LMC type II Cepheids in the OGLE-III survey. Period-luminosity (PL) relations are derived in JHKs as well as in a reddening-free VI parameter. Within the uncertainties the BL Her stars (P < 4 d) and the W Vir stars (P = 4 to 20 d) are co-linear in these PL relations. The slopes of the infrared relations agree with those found previously for type II Cepheids in globular clusters within the uncertainties. Using the pulsation parallaxes of V553 Cen and SW Tau the data lead to an LMC modulus uncorrected for any metallicity effects of 18.46 +- 0.10 mag. The type II Cepheids in the second-parameter globular cluster, NGC 6441, show a PL(VI) relation of the same slope as that in the LMC and this leads to a cluster distance modulus of 15.46 +- 0.11 mag, confirming the hypothesis that the RR Lyrae variables in this cluster are overluminous for their metallicity. It is suggested that the Galactic variable kappa Pav is a member of the peculiar W Vir class found by the OGLE-III group in the LMC. Low-resolution spectra of OGLE-III type II Cepheids with P > 20 d (RV Tau stars) show that a high proportion have TiO bands; only one has been found showing C_2. The LMC RV Tau stars, as a group, are not co-linear with the shorter-period type II Cepheids in the infrared PL relations in marked contrast to such stars in globular clusters. Other differences between LMC, globular cluster and Galactic field type II Cepheids are noted in period distribution and infrared colours.

The Scale‐Free Character of the Cluster Mass Function and the Universality of the Stellar Initial Mass Function

Our recent determination of a Salpeter slope for the initial mass function (IMF) in the field of 30 Doradus appears to be in conflict with simple probabilistic counting arguments advanced in the past to support observational claims of a steeper IMF in the LMC field. In this paper we reexamine these arguments and show by explicit construction that, contrary to these claims, the field IMF is expected to be exactly the same as the stellar IMF of the clusters out of which the field was presumably formed. We show that the current data on the mass distribution of clusters themselves is in excellent agreement with our model, and is consistent with a single spectrum by the number of stars of the type nβ, with β between –1.8 and –2.2 down to the smallest clusters, without any preferred mass scale for cluster formation. We also use the random sampling model to estimate the statistics of the maximal mass star in clusters, and confirm the discrepancy with observations found by Weidner & Kroupa. We argue that rather than signaling the violation of the random sampling model, these observations reflect the gravitationally unstable nature of systems with one very large mass star. We stress the importance of the random sampling model as a null hypothesis, whose violation would signal the presence of interesting physics.

Ultralow-Amplitude Cepheids in the Large Magellanic Cloud

The MACHO variables of LMC field 77 that lie in the vicinity of the Cepheid instability strip are reexamined. Among the 144 variables that we identify as Cepheids, we find 14 that have Fourier amplitudes <0.05 mag in the MACHO red band, of which seven have an amplitude <0.006 mag: we dub the latter group of stars ultralow-amplitude (ULA) Cepheids. The variability of these objects is verified by a comparison of the MACHO red with the MACHO blue light curves and with those of the corresponding OGLE LMC stars. The occurrence of ULA Cepheids is in agreement with theory. We have also discovered two low-amplitude variables whose periods are about a factor of 5-6 smaller than those of F Cepheids of equal apparent magnitude. We suggest that these objects are Cepheids undergoing pulsations in a surface mode and that they belong to a novel class of Strange Cepheids (or surface mode Cepheids) whose existence was predicted by Buchler et al. (1997).

The Initial Mass Function toward the Low‐Mass End in the Large Magellanic Cloud withHubble Space TelescopeWFPC2 Observations

We present V- and I-equivalent Hubble Space Telescope (HST) WFPC2 photometry of two areas in the Large Magellanic Cloud: the southern part of the stellar association LH 52, located on the western edge of the supershell LMC 4, and a field between two associations, which is located on the southwestern edge of the shell and accounts for the general background field of the galaxy. The HST WFPC2 observations reach magnitudes as faint as V = 25 mag, much deeper than have been observed earlier in stellar associations in the LMC. We determine the mass function (MF) for main-sequence stars in the areas. Its slope in both areas is steeper for stars with masses M ≲ 2 M☉ (-4 ≲ Γ ≲ -6) than for stars of M ≳ 2 M☉ (-1 ≲ Γ ≲ -2). Thus, as far as the field of the LMC is concerned, the MF does not have a uniform slope throughout its observed mass range. The MF of the general field of the LMC was found previously to be steeper than the MF of a stellar association for massive stars with M ≳ 5 M☉. We conclude that this seems to also be the case toward lower masses down to M ~ 1 M☉. Our data allow us to construct the field-subtracted, incompleteness-corrected, main-sequence MF of the southwestern part of the young stellar association LH 52, which accounts for the initial mass function (IMF) of the system. Its mean slope is found to be comparable to, but more shallow than, a typical Salpeter IMF (Γ ≃ -1.12 ± 0.24) for masses down to ~1 M☉. We found indications that the IMF of the association probably is "top heavy," owing to the large number of intermediate-mass stars in the field of the system, while the general LMC field is found to be responsible for the low-mass population, with M ≲ 2 M☉, observed in both fields. This finding suggests that the local conditions seem to favor the formation of higher mass stars in associations, and not in the background field. No evidence for flattening of the IMF toward the low-mass regime or for a lower mass cutoff in the IMF was detected in our data.

The X-ray emission of the Crab-like pulsar PSR J0537–6910

In this paper we present some preliminary result on the spectral and timing analysis of the X-ray pulsed emission from the 16 ms pulsar PSR J0537-6910 in the energy range 0.1--30 keV, based on archival BeppoSAX and RossiXTE observations. This pulsar, discovered by Marshall et al.(1998) in the LMC field with RXTE, is the fastest spinning pulsar associated with a supernova remnant. It is characterized by strong glitch activity with the highest rate of all known Crab-like system.

The distance to the LMC cluster NGC 1866; clues from the cluster Cepheid population

Recent investigations aimed at estimating the distance to t he young LMC cluster NGC 1866 have made use of Red Clump stars in the surrounding LMC field, together with em pirical and theoretical Main-Sequence fitting methods, and have found significantly different distances for the field an d the cluster, the latter being closer by in distance modulus by �(DM)�0.20 mag. In this paper we (re-)consider the Cepheid star population of NGC 1866, to try to shed some light on this discrepancy. By combining various extensive photometric datasets in B, V,I and single-epoch 2MASS JHK photometry, PL relationships for the cluster Cepheids are obtained. A comparison between the field LMC and cluster PL relationships for the reddening free Wesenheit index gives a firm determina tion of the distance between the cluster and the LMC main body (0.04 mag in distance modulus, the cluster being more distant) which, coupled to a model for the geometry of the LMC disk, provides �(DM) ranging between 0.0 and 0.11 mag. The simultaneous comparison of the PL relationships in B, V and I for the cluster and LMC field gives an estimate of the cluster r eddening, which results to be E(B V ) = 0.12 ± 0.02. This determination is higher than the canonical value of 0.06 mag used in all previous studies, but we show that it is not in contradiction with a re-analysis of independent esti mates. The adoption of the LMC extinction law recently presented by Gordon et al. (2003) does not change these results. The cluster Main Sequence fitting distance obtained with this new redd ening is DM = 18.58 ± 0.08, fully compatible with the Red Clump value of DM = 18.53 ±0.07(random) +0.02 −0.05 (systematic) and the Cepheid constraint on �(DM). Finally, we determined the distance to the cluster by using a Cepheid Wesenheit PL relationship with slope coming from LMC observations, and absolute magnitude zero point calibrated on Hipparcos parallaxes of Galactic Cepheids, in the assumption that the relationship is independent of metallicity; the resulting DM = 18.65 ± 0.10 is not an accurate estimate of the LMC distance because of possible metallicity effects but, when compared to the revised Main Sequence fitting value, it points out to a possibly weak depen dence of the Wesenheit PL relationship on the Cepheid chemical composition, at least in the period range between 2.5 and 3.5 days.

Using Optical and Near‐Infrared Photometry to Test MACHO Lens Candidates

We obtained new VLT/ISAAC H-band observations for five MACHO LMC source stars and adjacent LMC field regions. After combining our near-IR photometry with Hubble Space Telescope/PC BVRI optical photometry, we compared the MACHO objects to the adjacent field stars in a variety of color-magnitude and color-color diagrams. These diagnostic diagrams were chosen to be sensitive to our hypothesis that at least some of the MACHO lenses were foreground Galactic disk or thick-disk M dwarfs. For the five lensed objects we studied, our hypothesis could be ruled out for main-sequence lens masses 0.1 M? for distances out to 4 kpc. On the other hand, the fact that LMC MACHO 5, an object not in our study, has been recently found to have just such a foreground lens highlights that the remainder of the LMC MACHO objects should be searched for the signature of their lenses using our photometric technique or via near-IR spectroscopy. We also constructed diagnostic color-color diagrams sensitive to determining reddening for the individual MACHO source stars and found that these five objects did not show evidence for significant additional reddening. At least these five MACHO objects are thus also inconsistent with the LMC self-lensing hypothesis.

The Distance to the Large Magellanic Cloud Cluster NGC 1866 and the Surrounding Field

We use the main-sequence stars in the LMC cluster NGC 1866 and red clump stars in the local field to obtain two independent estimates of the LMC distance. We apply an empirical main-sequence-fitting technique based on a large sample of subdwarfs with accurate Hipparcos parallaxes in order to estimate the cluster distance modulus, and the multicolor red clump method to derive distance and reddening of the LMC field. We find that the main-sequence fitting and the red clump distance moduli are in significant disagreement; the NGC 1866 distance modulus is equal to (m - M)0,NGC 1866 = 18.33 ± 0.08 (consistent with a previous estimate using the same data and theoretical main-sequence isochrones), while the field stars provide (m - M)0,field = 18.53 ± 0.07. This difference reflects the more general dichotomy in the LMC distance estimates found in the literature. Various possible causes for this disagreement are explored, with particular attention paid to the still uncertain metallicity of the cluster and the star formation history of the field stars.

Deep colour-magnitude diagrams of LMC field stars imaged withHST

We present deep photometry (V≲26) in V and I bands obtained with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope for 7 fields ∼5° away from the Large Magellanic Cloud centre. The fields contain, typically, 2000 stars each. Isochrones were fitted to the colour–magnitude diagrams in order to identify different star populations in these fields. An old population (τ>10 Gyr) has been found in all fields. Some events of enhanced star formation, with ages between 2 and 4 Gyr, were identified in the fields localized in the north to north-west regions. Luminosity functions of low-mass stars were also obtained for all fields. Kolmogorov Smirnov test results suggest differences smaller than 30 per cent in the mixture of stellar populations contributing to the fields. Finally, density profiles were derived for old and intermediate-age stars. The former shows a slightly steeper decline than the latter.

On the detection of the first extragalactic classical chemically peculiar stars

We report on the detection of the first extragalactic, classical chemically peculiar (CP) stars more than 100 years after the discovery of this group. Using the tool of CCD -photometry, eight definite positive detections in the globular-like stellar cluster NGC 1866 and its surrounding in the Large Magellanic Cloud were made. More than 1950 objects within a radius of 9.5′ of the cluster center down to 20.5 magnitude were observed in this three filter, narrow band photometric system. Averaging more than 70 individual frames for all three filters yielded a 3σ detection limit of 0.015 mag for stars with mag. Furthermore we found three objects with significantly negative -values indicating either an Ae/shell or λ Bootis nature. The overall percentage of chemically peculiar stars for NGC 1866 (1.5% ) seems lower than for the majority of open clusters in our Milky Way whereas the incidence within the LMC field is decisively less (0.3% ).

Stellar Populations in the Large Magellanic Cloud from 2MASS

We present a morphological analysis of the feature-rich 2MASS LMC color-magnitude diagram, identifying Galactic and LMC populations and estimating the density of LMC populations alone. We also present the projected spatial distributions of various stellar populations. The conditions that prevailed when 2MASS observed the LMC provided 10 σ limiting sensitivity for J 16.3, H 15.3, and Ks 14.7. Major populations are identified based on matching morphological features of the color-magnitude diagram with expected positions of known populations, isochrone fits, and analysis of the projected spatial distributions. 2MASS has detected a significant population of asymptotic giant branch (AGB) stars (104 sources) and obscured AGB stars (2 × 103 sources). The LMC populations along the first-ascent red giant branch (RGB) and AGB are quantified. Comparison of the giant luminosity functions in the bar and the outer regions of the LMC shows that both luminosity functions appear consistent with each other. The luminosity function in the central (bar) field has well-defined drop-off near Ks = 12.3 mag corresponding to the location of the RGB tip; the same feature is seen in the luminosity function of the entire LMC field. Isochrone fits for the corresponding giant branches reveal no significant differences in metallicities and ages between central and outer regions of the LMC. This may be evidence for strong dynamical evolution in the last several gigayears. In particular, the observed LMC giant branch is well fit by published tracks in the CIT/CTIO system with a distance modulus of μ = 18.5 ± 0.1, reddening EB-V = 0.15-0.20, metallicity Z = 0.004, and age 3-13 Gyr. Analysis of deep 2MASS engineering data with 6 times the standard exposure produces similar estimates.

The MACHO Project LMC Variable Star Inventory: Classical Cepheids, AGB Variables, and the Nine Million Star Color-Magnitude Diagram

We measure the ratio of ~ 5M⊙ blue and red LMC supergiants (representative of classical Cepheids) using the MACHO Project's 9 million star color-magnitude diagram (9M CMD) of the LMC bar. We find b/r = 0.39, which favors the Z=0.008, 5M⊙ theoretical model of Schaerer et al. (1993) over that of Fagatto et al. (1994). Next, we examine the low mass (old) and low metallicity LMC field population (Pop. II). Features in the 9M CMD and properties of LMC field RRab variables are consistent with a mean iron abundance of [Fe/H] ≈ −1.5 dex for this population. Newly discovered post-HB/early-AGB Pop. II variables are identified in order to delineate the instability strip (IS). Good agreement with the theoretical IS of Bono et al. (1997) is found. We then compare the field RRab with newly identified RRab variables in the LMC clusters NGC 1898 and NGC 1835. We find the mean colors of these cluster RRab lie near the red and blue edges of the IS, respectively, which is similar to their respective (overall) red and blue HB morphologies. Since the field RRab lie on the red side of the IS, we infer the LMC field Pop. II is likely to have a red HB morphology.

A Photometric Survey of the LMC Field near NGC 2257

Star formation in the outermost regions of the LMC, several kpc from the center, has been dormant for some Gyr. With deep HST and wide-field ground-based imaging surveys, sufficient stars can be measured to accurately delineate the critical turn-off and subgiant branch regions of the CMD, and thus provide a picture of the first 10 Gyr of star formation in the LMC.

HST colour-magnitude diagrams of six old globular clusters in the LMC

We report on HST observations of six candidate old globular clusters in the Large Magellanic Cloud: NGC 1754, NGC 1835, NGC 1898, NGC 1916, NGC 2005 and NGC 2019. Deep exposures with the F555W and F814W filters provide us with colourmagnitude diagrams that reach to an apparent magnitude in V of �25, well below the main sequence turnoff. These particular clusters are involved with significantly high LMC field star densities and care was taken to subtract the field stars from the cluster colour-magnitude diagrams accurately. In two cases there is significant variable reddening across at least part of the image, but only for NGC 1916 does the differential reddening preclude accurate measurements of the CMD characteristics. The morphologies of the colour-magnitude diagrams match well those of Galactic globular clusters of similar metallicity. All six have well-developed horizontal branches, while four clearly have stars on both sides of the RR Lyrae gap. The abundances obtained from measurements of the height of the red giant branch above the level of the horizontal branch are 0.3 dex higher, on average, than previously measured spectroscopic abundances. Detailed comparisons with Galactic globular cluster fiducials show that all six clusters are old objects, very similar in age to classical Galactic globulars such as M5, with little age spread among the clusters. This result is consistent with ages derived by measuring the magnitude difference between the horizontal branch and main sequence turnoff. We also find a similar chronology by comparing the horizontal branch morphologies and abundances with the horizontal branch evolutionary tracks of Lee, Demarque, & Zinn (1994). Our results imply that the LMC formed at the same time as the Milky Way Galaxy.

Ultraviolet Imaging Telescope Observations of the Magellanic Clouds

We present wide-field far-ultraviolet (FUV; 1300–1800 A) images of the Large and Small Magellanic Clouds (LMC, SMC). These data were obtained by the Ultraviolet Imaging Telescope (UIT) during the Astro-1 (1990 December 1–10) and Astro-2 (1995 March 2–18) missions; the images provide an extensive FUV mosaic of the SMC and contain numerous regions in the LMC, covering a wide range of stellar densities and current star formation activity. A total of 47 LMC/Lucke-Hodge and 37 SMC/Hodge OB associations are completely or partially included in the observed fields. FUV data can identify the hottest OB stars more easily than can optical photometry, and these stars dominate the ionizing flux, which is correlated to the observed Hα flux of the associated H II regions. Of the H II regions in the catalog of Davies, Elliott, & Meaburn (DEM), the UIT fields completely or partially include 102 DEM regions in the LMC and 74 DEM regions in the SMC. We present a catalog of FUV magnitudes derived from point-spread function photometry for 37,333 stars in the LMC (the UIT FUV magnitudes for 11,306 stars in the SMC were presented recently by Cornett et al.), with a completeness limit of mUV ≈ 15 mag and a detection limit of mUV ≈ 17.5. The average uncertainty in the photometry is ~0.1 mag. The full catalog with astrometric positions, photometry, and other information is also available from publicly accessible astronomical data archives. We divide the catalog into field stars and stars that are in DEM regions. We analyze each of these two sets of stars independently, comparing the composite UV luminosity function of our data with UV magnitudes derived from stellar evolution and atmosphere models in order to derive the underlying stellar formation parameters. We find a most probable initial mass function (IMF) slope for the LMC field stars of Γ = -1.80 ± 0.09. The statistical significance of this single slope for the LMC field stars is extremely high, though we also find some evidence for a field star IMF slope of Γ ~ -1.4, roughly equal to the Salpeter slope. However, in the case of the stars in the DEM regions (the stars in all the regions were analyzed together as a single group), we find three IMF slopes of roughly equal likelihood: Γ = -1.0, -1.6, and -2.0. No typical age for the field stars is found in our data for time periods up to a continuous star formation age of 500 Myr, which is the maximum age consistent with the completeness limit magnitude of the catalog's luminosity function. The best age for the collection of cluster stars was found to be t0 = 3.4 ± 1.9 Myr; this is consistent with the age expected for a collection of OB stars from many different clusters.