Near-Infrared Polarimetry and Stellar Classification in the Galactic Center

The Galactic center (GC) is the closest galactic nucleus, offering the unique possibility of studying the population of a dense stellar cluster surrounding a Supermassive Black Hole (SMBH), as well as stellar and bow-shock polarization effects in a dusty environment. The goals of the first part of this work are to develop a new method of separating early and late type stellar components of a dense stellar cluster based on intermediate band filters, applying it to the central parsec of the GC, and conducting a population analysis of this area. Adaptive optics assisted observations obtained at the Very Large Telescope (VLT, run by the European Southern Observatory, ESO) in the Near-Infrared (NIR) H-band (1.6 mum) and 7 intermediate bands covering the NIR K-band (2.18 mum) were used for this. A comparison of the resulting Spectral Energy Distributions (SEDs) with a blackbody of variable extinction then allowed to determine the presence and strength of a CO absorption feature to distinguish between early and late type stars. This new method is suitable for classifying K giants (and later), as well as B2 main sequence (and earlier) stars that are brighter than 15.5 mag in the K band in the central parsec. Compared to previous spectroscopic investigations that were limited to 13-14 mag, this represents a major improvement in the depth of the observations and reduces the needed observation time. Extremely red objects and foreground sources could also be reliably removed from the sample. Comparison to sources of known classification indicates that the method has an accuracy of better than 87%. 312 stars have been classified as early type candidates out of a sample of 5914 sources. Several results, such as the shape of the K-band Luminosity Function (KLF) and the spatial distribution of both early and late type stars, confirm and extend previous works. The distribution of the early type stars can be fitted with a steep power law (beta_1'' = -1.49 +/- 0.12), alternatively with a broken power law, beta_1-10'' = -1.08 +/- 0.12, beta_10-20'' = -3.46 +/- 0.58, since a drop in the early type density seems to occur at ~10''. In addition, early type candidates outside of 0.5 pc have been detected in significant numbers for the first time. The late type density function shows an inversion in the inner 6'', with a power-law slope of beta_R<6'' = 0.17 +/- 0.09. The late type KLF has a power-law slope of 0.30 +/- 0.01, closely resembling the KLF obtained for the bulge of the Milky Way. The early type KLF has a much flatter slope of (0.14 +/- 0.02). These results seem to agree best with an in-situ star formation scenario, although alternatives like the inspiraling cluster scenario cannot be ruled out yet. The second part of this work aims at providing NIR (H-, Ks-, and Lp-band) polarimetry of the stellar sources in the central parsec at the resolution of an 8m telescope for the first time, along with new insight into the nature of the known bright bow-shock sources. Using the NAOS-CONICA (NACO) intrument at the ESO VLT in its polarimetric mode and applying both high-precision photometric methods specifically developed for crowded fields along with a newly established polarimetric calibration for NACO, polarization maps covering parts of the central 30''*30'' (with different coverage and depth in the three wavelength bands) were produced. In addition, spatially resolved polarimetry and a flux variability analysis on the extended sources in this region were conducted. It has been confirmed for this larger sample that the foreground polarization mostly follows the orientation of the Galactic plane, with average values of 4.5 - 6.1% at ~26° (in the Ks-band, depending on the Field-of-View, FOV), (9.3 +/- 1.3)% at 20° +/- 6° (H-band), and (4.5 +/- 1.4)% at 20° +/- 5° (Lp-band, 3.8 mum). In the east of the FOV, higher polarization degrees and steeper polarization angles have been found: (7.5 +/- 1.0)% at 11° +/- 6° (Ks-band) and (12.1 +/- 2.1)% at 13° +/- 6° (H-band). p_H / p_Ks peaks at 1.9 +/- 0.4, corresponding to a power law index for the wavelength dependency of alpha = 2.4 +/- 0.7. These values also vary over the FOV, with higher values in the center. This may indicate the influence of local effects on the total polarization, possibly dichroic extinction by Northern Arm dust. The relation between the Lp- and Ks-band polarization degrees has an average of 0.7-0.8, consistent with previous measurements on a much smaller number of sources. The polarization efficiency in the H- and Ks-band shows the expected power-law dependency on the local extinction. Several of the extended sources, namely IRS 1W, IRS 5, IRS 10W, and IRS 21 show significant intrinsic polarization in all wavelength bands, as well as spatial polarization patterns that are consistent with emission and/or scattering on aligned grains as a polarization mechanism. The bow-shock structure around IRS 21 could be separated from the central source for the first time in the Ks-band, finding the apex north of the central source and determining a standoff distance of ~400 AU, which matches previous estimates. This source also shows a ~50% increase in flux in the NIR over several years. In addition, the Mid-Infrared (MIR) excess sources IRS 5NE, IRS 2L, and IRS 2S have been found to show a significant Lp-band polarization that agrees well with the scenario that these sources are lower luminosity versions of the bright bow-shock sources.