We present structural and kinematic evidence pointing to a gravitational interaction between the two Magellanic Clouds. We discuss, in particular, the results of our kinematic survey of carbon stars in the periphery of the LMC. Stars with peculiar velocities, in the LMC southwest, are identified as part of debris left on the SMC orbit around the LMC.

Since gravity alone acts on them, and their evolutionary age restricts sampling to “mid-life”, carbon stars are examined as tracers of recent dynamical evolution of the Magellanic Clouds. Ablation processes modify the distribution of HI, and velocity outliers reflect recent tidal interactios.

Adopting observed proper motions of the MC's and cluster formation histories of the LMC as observational constraints, we found a new binary orbit for the LMC and SMC in which the geometry and kinematics of the Magellanic Stream are well reproduced. In this orbit, the LMC and SMC experienced very close encouters 0.2 and 3 Gyr ago.

HST ultra-violet and ground-based optical spectroscopy are presented for DI 1388, an early-type star in the Magellanic Bridge. These data have been supplemented with H I radio mapping of its environment. The spectroscopy shows interstellar components at LSR velocities of 0, 90 and 200 km s−1 – corresponding to the local ISM, a high velocity cloud and the Magellanic Bridge ISM, respectively. The last is the first absorption line detection of this material and shows the presence of both low (e.g., N I) and high (Si IV) ionization material. Further optical and HST UV observations of DI 1338 and another Bridge star, DGIK 975, are scheduled to investigate the chemical homogeneity and star formation history of the Bridge.

We use a modified TreeSPH code to simulate the MilkyWay-LMC-SMC system using the latest observational results from Hipparcos on the proper motion of the Clouds. More than 120,000 particles are used for the simulation. Gravitational and hydrodynamic interactions among gaseous and stellar components in the system have been considered. The Magellanic Stream, some high velocity clouds, and the barred structure of LMC are reproduced. A leading gas arm from the SMC is predicted. From the simulation, the long term binary status of LMC-SMC is questioned as frequent encounters between the LMC and the SMC will have difficulty in explaining the survival of the SMC.

We present evidence that the properties of the high-velocity cloud catalogued as #187 by Wakker & van Woerden (1991) (HVC287+22+240) can be explained if it is part of a leading arm counterpart of the Magellanic Stream, and we use this to argue that the HVC originated in the Magellanic Clouds after a tidal interaction.

An important discriminant between leading models for the origin of the Magellanic Stream is the presence of a stellar counterpart to the HI gas stream: ram pressure stripping of gas by a putative hot Galactic halo would act only on Magellanic gas while gravitational tidal stripping would act on both gas and stars. Several previous attempts to find tidal stellar debris have failed to find carbon stars, A stars, or other main sequence stars in the Magellanic Stream (Mathewson et al. 1979; Recillas-Cruz 1982; Brück & Hawkins 1983; Guhathakurta & Lin 1999). However, there has long been a suggestion (Kunkel 1979; Lynden-Bell 1982) of a possible Magellanic association of satellite galaxies and globular clusters that have similar orbits and may derive from the break up of a greater Magellanic galaxy (Lynden-Bell & Lynden-Bell 1995; Majewski et al. 1997). Recent models (Moore & Davis 1994; Johnston 1998) of the tidal disruption of Large Magellanic Cloud (LMC)-like systems indicate a wide dispersal of debris, much wider than the rather confined HI stream, so that the contrast of tidal debris against the Galactic fore/background would be low. If true, this could explain some of the previous negative results for tidal debris searches.

In this paper, we will review major new results regarding classical Cepheids, in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC). Specifically, we discuss recent work regarding multimode Cepheids and describe new observations of a W Vir star (HV 5756) and a Cepheid which are each in eclipsing binary systems. An additional interesting pulsating supergiant in an eclipsing system is also identified. Ephemerides for eclipses for the three systems are provided.

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.

Between October 1996 and February 1997 a specific campaign of the EROS experiment was devoted to Cepheids in central regions of the LMC and the SMC. The Period–Luminosity (PL) relations were determined in the EROS photometric system, and a significant slope change of the period–luminosity relation has been observed for the SMC fundamental mode Cepheids with periods shorter than 2 days.

We compare the theoretical and empirical PL relations in V and I bands for Cepheids in the LMC. We found that, within the current intrinsic dispersions, the theoretical predictions are in remarkable agreement with observational data. We also discuss the PLC relations in (V,B-V) and (V,V-I) as well as the dependence of distance determinations on uncertainties in colors, in reddening corrections and in metal content.

We review the process of determining the distances to Cepheid variables with the infrared surface brightness technique. We show that both versions of this technique (working with the V, V – K and the pure infrared K, J – K magnitude/color combinations) yield consistent distances to Cepheid variables. For a Cepheid with average-quality photometric and radial velocity data, the typical total uncertainty in the distance is ±5%. We use the distances of 34 Galactic variables to calibrate period-luminosity relations in the V, I, J, H, K bands and demonstrate that these relations lead to distance moduli of the LMC which agree to ±0.02 mag. The infrared surface brightness technique yields a best LMC distance modulus of 18.46 ±0.02. We discuss possible metallicity corrections, such as the ones determined by Sasselov et al. from EROS Cepheids comparison in the LMC and SMC, and conclude that our distance modulus is negligibly affected by these corrections. Finally, we present a program to obtain reddening- and metallicity-independent infrared surface brightness distances of Cepheid variables in LMC clusters which will yield an even more accurate mean LMC distance independent of any metallicity correction to Cepheid absolute magnitudes.

Five independent methods of deriving the distance modulus of the Large Magellanic Cloud are reviewed and compared. A value in the range 18.60 to 18.80 is presently preferred.

Using the new reductions of the IUE light curves by Sonneborn et al. (1997) and an extensive set of HST images of SN 1987A we have repeated and improved Panagia et al. (1991) analysis to obtain a better determination of the distance to the supernova. In this way we have derived an absolute size of the ring Rabs = (6.23 ± 0.08) x 1017 cm and an angular size R″ = 808 ± 17 mas, which give a distance to the supernova d(SN1987A) = 51.4 ± 1.2 kpc and a distance modulus m–M(SN1987A) = 18.55 ± 0.05. Allowing for a displacement of SN 1987A position relative to the LMC center, the distance to the barycenter of the Large Magellanic Cloud is also estimated to be d(LMC) = 52.0±1.3 kpc, which corresponds to a distance modulus of m–M(LMC) = 18.58±0.05.

On the basis of an extensive grid of Cepheid, nonlinear, nonlocal, time-dependent convective models by Bono et al., analytical Period-Color-Luminosity (PLC) relations as a function of chemical composition were evaluated and compared with observations in LMC and SMC. I provide an analysis of PL and PLC residuals and suggestions for disentangling the effects of reddening and metallicity on the Cepheid distance scale.

It is well known that Cepheid variables are very important distance indicators because of their PL and PLC relations. The distances derived through the quoted relations are affected by well known systematic effects, since, in general, we have a poor knowledge of both reddening and metallicity of the Cepheid sample. Good estimates of the quoted parameters can be achieved if the observed Cepheids are cluster members since, by comparison between cluster CMD and theoretical isochrones we can estimate the cluster reddening, metallicity and age, i.e., the evolving masses for each evolutionary phase. The ideal objects to be searched for Cepheid membership are Magellanic Cloud clusters since they are well populated in the relevant evolutionary phases.

An accurate calibration of the surface brightness scale SV as a function of the near-IR color (V – K) has been recently measured for non-variable Galactic dwarf and giant stars. It can be shown that this correlation can be applied to the SV scale of Galactic Cepheid variable stars, which are of major cosmological interest.

Our collaboration involves groups in Denmark, the U.S.A. Spain and of course New Zealand. Combining ground-based and satellite (IUE and HST) observations we aim to determine accurate and precise stellar fundamental parameters for the components of Magellanic Cloud Eclipsing Binaries as well as the distances to these systems and hence the parent galaxies themselves. This poster presents our latest progress.

Important problems to which we would like to find answers are:

1. • What are the distances to the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC)?

2. • What is the present distribution of stars, gas and dark matter in the Clouds, and how did it evolve?

3. • How, and where, did the Magellanic Clouds form, and how have their orbits evolved?

4. • Finally the recent discovery of numerous microlensing events in the Clouds provides answers to questions that we have only recently started to ask.