The eruptive variable V838 Monocerotis gained notoriety in 2002 when it brightened nine magnitudes in a series of three outbursts and then rapidly evolved into an extremely cool supergiant. We present optical, near-IR, and mid-IR spectroscopic and ph
otometric observations of V838 Monocerotis obtained between 2008 and 2012 at the Apache Point Observatory 3.5m, NASA IRTF 3m, and Gemini South 8m telescopes. We contemporaneously analyze the optical & IR spectroscopic properties of V838 Monocerotis to arrive at a revised spectral type L3 supergiant and effective temperature Teff~2000--2200 K. Because there are no existing optical observational data for L supergiants in the optical, we speculate that V838 Monocerotis may represent the prototype for L supergiants in this wavelength regime. We find a low level of Halpha emission present in the system, consistent with interaction between V838 Monocerotis and its B3V binary; however, we cannot rule out a stellar collision as the genesis event, which could result in the observed Halpha activity. Based upon a two-component blackbody fit to all wavelengths of our data, we conclude that, as of 2009, a shell of ejecta surrounded V838 Monocerotis at a radius of R=263+/-10 AU with a temperature of T=285+/-2 K. This result is consistent with IR interferometric observations from the same era and predictions from the Lynch et al. model of the expanding system, which provides a simple framework for understanding this complicated system.
The separation of the Milky Way disk into a thin and thick component is supported by differences in the spatial, kinematic and metallicity distributions of their stars. These differences have led to the view that the thick disk formed early via a cat
aclysmic event and constitutes fossil evidence of the hierarchical growth of the Milky Way. We show here, using N-body simulations, how a double-exponential vertical structure, with stellar populations displaying similar dichotomies can arise purely through internal evolution. In this picture, stars migrate radially, while retaining nearly circular orbits, as described by Sellwood & Binney (2002). As stars move outwards they populate a thickened component. Such stars found at the present time in the solar neighborhood formed early in the disks history at smaller radii where stars are more metal-poor and alpha-enhanced, leading to the properties observed for thick disk stars. Classifying stars as members of the thin or thick disk by either velocity or metallicity leads to an apparent separation in the other property as observed. This scenario is supported by the SDSS observation that stars in the transition region do not show any correlation between rotational velocity and metallicity. The good qualitative agreement between our simulation and observations in the Milky Way hint that the thick disk may be a ubiquitous galaxy feature generated by stellar migration. Nonetheless, we cannot exclude that some fraction of the thick disk is a fossil of a past more violent history, nor can this scenario explain thick disks in all galaxies.
