Properties of bow-shock sources at the Galactic center

There are an enigmatic population of massive stars around the Galactic Center (GC) that were formed some Ma ago. A fraction of these stars has been found to orbit the supermassive black hole, SgrA*, in a projected clockwise disk, which suggests that they were formed in a formerly existing dense disk around SgrA*. We focus on the extended, near-infrared (NIR) sources IRS1W, IRS5, IRS10W, and IRS21 that have been suggested to be young, massive stars that form bow-shocks through their interaction with the ISM. Their nature has impeded accurate determination of their orbital parameters. We aim at establishing their nature and kinematics to test whether they form part of the clockwise disk. We performed NIR multi-wavelength imaging using adaptive optics (AO) and sparse aperture masking (SAM). We introduce a new method for self-calibration of the SAM PSF in dense stellar fields. The emission mechanism, morphology and kinematics of the targets were examined via 3D bow-shock models. We confirm previous findings that IRS21, IRS1W, and IRS5 are bow-shocks created by the interaction between mass-losing stars and the interstellar gas. The nature of IRS10W remains unclear. Our modeling shows that the bow-shock-emission is caused by thermal emission while the scattering of stellar light does not play any significant role. IRS 1W appears to be a bow-shock produced by an anisotropic stellar wind or by locally inhomogeneous ISM density. Our best-fit models provide an estimate of the local proper motion of the ISM in the NA in agreement with the published models. Assuming that all of the sources are tied to SgrA*, their orbital planes were obtained via a Monte-Carlo simulation. Our orbital analysis suggests that they are not part of any of the clockwise disk. We thus add more evidence to recent findings that a large part of the massive stars show apparently random orbital orientations.Comment: accepted for publication by A&A, 17 pages, 11 figures, 1 appendi

L- and M-band imaging observations of the Galactic Center region

We present near-infrared H-, K-, L- and M-band photometry of the Galactic Center from images obtained at the ESO VLT in May and August 2002, using the NAOS/CONICA (H and K) and the ISAAC (L and M) instruments. The large field of view (70" x 70") of the ISAAC instrument and the large number of sources identified (L-M data for 541 sources) allows us to investigate colors, infrared excesses and extended dust emission. Our new L-band magnitude calibration reveals an offset to the traditionally used calibrations, which we attribute to the use of the variable star IRS7 as a flux calibrator. Together with new results on the extinction towards the Galactic Center (Scoville et al. 2003; Raab 2000), our magnitude calibration results in stellar color properties expected from standard stars and removes any necessity to modify the K-band extinction. The large number of sources for which we have obtained L-M colors allows us to measure the M-band extinction to A_M=(0.056+-0.006)A_V (approximately =A_L), a considerably higher value than what has so far been assumed. L-M color data has not been investigated previously, due to lack of useful M-band data. We find that this color is a useful diagnostic tool for the preliminary identification of stellar types, since hot and cool stars show a fairly clear L-M color separation. This is especially important if visual colors are not available, as in the Galactic Center. For one of the most prominent dust embedded sources, IRS3, we find extended L- and M-band continuum emission with a characteristic bow-shock shape. An explanation for this appearance is that IRS3 consists of a massive, hot, young mass-losing star surrounded by an optically thick, extended dust shell, which is pushed northwest by wind from the direction of the IRS16 cluster and SgrA*.Comment: 24 pages, 7 figures, 2 tables, accepted for publication in Astronomy & Astrophysic

The enigma of GCIRS 3 - Constraining the properties of the mid-infrared reference star of the central parsec of the Milky Way with optical long baseline interferometry

GCIRS3 is the most prominent MIR source in the central pc of the Galaxy. NIR spectroscopy failed to solve the enigma of its nature. The properties of extreme individual objects of the central stellar cluster contribute to our knowledge of star and dust formation close to a supermassive black hole. We initiated an interferometric experiment to understand IRS3 and investigate its properties as spectroscopic and interferometric reference star at 10um. VISIR imaging separates a compact source from diffuse, surrounding emission. The VLTI/MIDI instrument was used to measure visibilities at 10mas resolution of that compact 10um source, still unresolved by a single VLT. Photometry data were added to enable simple SED- and full radiative transfer-models of the data. The luminosity and size estimates show that IRS3 is probably a cool carbon star enshrouded by a complex dust distribution. Dust temperatures were derived. The coinciding interpretation of multiple datasets confirm dust emission at several spatial scales. The IF data resolve the innermost area of dust formation. Despite observed deep silicate absorption towards IRS3 we favor a carbon rich chemistry of the circumstellar dust shell. The silicate absorption most probably takes place in the outer diffuse dust, which is mostly ignored by MIDI measurements. This indicates physically and chemically distinct conditions of the local dust, changing with the distance to IRS3. We have demonstrated that optical long baseline interferometry at infrared wavelengths is an indispensable tool to investigate sources at the Galactic Center. Our findings suggest further studies of the composition of interstellar dust and the shape of the 10um silicate feature at this outstanding region.Comment: accepted by A&A, now in press; 19 pages, 22 figures, 5 table

First VLTI infrared spectro-interferometry on GCIRS 7 - Characterizing the prime reference source for Galactic center observations at highest angular resolution

Investigating the environment of the massive black hole SgrA* at the center of the Galaxy requires the highest angular resolution available to avoid source confusion and to study the physical properties of the individual objects. GCIRS7 has been used as wavefront and astrometric reference. Our studies investigate, for the first time, its properties at 2&10um using VLTI/AMBER and MIDI. We aim at analyzing the suitability of IRS7 as an IF-phase-reference for the upcoming generation of dual-field facilities at optical interferometers. We observed with (R~30) and 50m (proj.) baseline, resulting in 9 and 45mas resolution for NIR and MIR, resp. The first K-band fringe detection of a GC star suggests that IRS7 could be marginally resolved at 2um, which would imply that the photosphere of the supergiant is enshrouded by a molecular and dusty envelope. At 10um, IRS7 is strongly resolved with a visibility of approximately 0.2. The MIR is dominated by moderately warm (200 K), extended dust, mostly distributed outside of a radius of about 120 AU (15 mas) around the star. A deep 9.8-silicate absorption in excess of the usual extinction law with respect to the NIR extinction has been found. This confirms recent findings of a relatively enhanced, interstellar 9.8-silicate absorption with respect to the NIR extinction towards another star in the central arcsec, suggesting an unusual dust composition in that region. Our VLTI observations show that interferometric NIR phase-referencing experiments with mas resolution using IRS7 as phase-reference appear to be feasible, but more such studies are required to definitely characterize the close environment around this star. We demonstrate that interferometry is required to resolve the innermost environment of stars at the Galactic center.Comment: 6 pages, 2 figures, accepted for publication in A&

Direct Detection of the Tertiary Component in the Massive Multiple HD 150 136 with VLTI

Massive stars are of fundamental importance for almost all aspects of astrophysics, but there still exist large gaps in our understanding of their properties and formation because they are rare and therefore distant. It has been found that most O-stars are multiples. HD 150 136 is the nearest system to Earth with >100 M_sol, and provides a unique opportunity to study an extremely massive system. Recently, evidence for the existence of a third component in HD 150 136, in addition to the tight spectroscopic binary that forms the main component, was found in spectroscopic observations. Our aim was to image and obtain astrometric and photometric measurements of this component using long baseline optical interferometry to further constrain the nature of this component. We observed HD150136 with the near-infrared instrument AMBER attached to the ESO VLT Interferometer. The recovered closure phases are robust to systematic errors and provide unique information on the source asymmetry. Therefore, they are of crucial relevance for both image reconstruction and model fitting of the source structure. The third component in HD 150 136 is clearly detected in the high-quality data from AMBER. It is located at a projected angular distance of 7.3 mas, or about 13 AU at the line-of-sight distance of HD 150 136, at a position angle of 209 degrees East of North, and has a flux ratio of 0.25 with respect to the inner binary. We resolved the third component of HD 150 136 in J, H and K filters. The luminosity and color of the tertiary agrees with the predictions and shows that it is also an O main-sequence star. The small measured angular separation indicates that the tertiary may be approaching the periastron of its orbit. These results, only achievable with long baseline near infrared interferometry, constitute the first step towards the understanding of the massive star formation mechanisms

NACO/SAM observations of sources at the Galactic Center

Sparse aperture masking (SAM) interferometry combined with Adaptive Optics (AO) is a technique that is uniquely suited to investigate structures near the diffraction limit of large telescopes. The strengths of the technique are a robust calibration of the Point Spread Function (PSF) while maintaining a relatively high dynamic range. We used SAM+AO observations to investigate the circumstellar environment of several bright sources with infrared excess in the central parsec of the Galaxy. For our observations, unstable atmospheric conditions as well as significant residuals after the background subtraction presented serious problems for the standard approach of calibrating SAM data via interspersed observations of reference stars. We circumvented these difficulties by constructing a synthesized calibrator directly from sources within the field-of-view. When observing crowded fields, this novel method can boost the efficiency of SAM observations because it renders interspersed calibrator observations unnecessary. Here, we presented the first NaCo/SAM images reconstructed using this method.Comment: 8 pages, 10 figures, proceedings of the conference "Astrophysics at High Angular Resolution" (AHAR-2011

VLTI observations of IRS~3: The brightest compact MIR source at the Galactic Centre

The dust enshrouded star IRS~3 in the central light year of our galaxy was partially resolved in a recent VLTI experiment. The presented observation is the first step in investigating both IRS~3 in particular and the stellar population of the Galactic Centre in general with the VLTI at highest angular resolution. We will outline which scientific issues can be addressed by a complete MIDI dataset on IRS~3 in the mid infrared.Comment: 4 pages, 3 figures, published in: The ESO Messenge

Influence of a stellar cusp on the dynamics of young stellar discs and the origin of the S-stars in the Galactic Centre

Observations of the Galactic Centre show evidence of one or two disc-like structures of very young stars orbiting the central super-massive black hole within a distance of a few 0.1 pc. A number of analyses have been carried out to investigate the dynamical behaviour and consequences of these discs, including disc thickness and eccentricity growth as well as mutual interaction and warping. However, most of these studies have neglected the influence of the stellar cusp surrounding the black hole, which is believed to be 1-2 orders of magnitude more massive than the disc(s). By means of N-body integrations using our bhint code, we study the impact of stellar cusps of different compositions. We find that although the presence of a cusp does have an important effect on the evolution of an otherwise isolated flat disc, its influence on the evolution of disc thickness and warping is rather mild in a two-disc configuration. However, we show that the creation of highly eccentric orbits strongly depends on the graininess of the cusp (i.e. the mean and maximum stellar masses): While Chang (2009) recently found that full cycles of Kozai resonance are prevented by the presence of an analytic cusp, we show that relaxation processes play an important role in such highly dense regions and support short-term resonances. We thus find that young disc stars on initially circular orbits can achieve high eccentricities by resonant effects also in the presence of a cusp of stellar remnants, yielding a mechanism to create S-stars and hyper-velocity stars. Furthermore, we discuss the underlying initial mass function (IMF) of the young stellar discs and find no definite evidence for a non-canonical IMF.Comment: 10 pages, 7 figures, 1 table, accepted for publication in MNRA

Constraining the initial mass function of stars in the Galactic Centre

(abridged) Here we discuss the question whether the extreme circumstances in the centre of the Milky Way may be the reason for a significant variation of the IMF. By means of stellar evolution models using different codes we show that the observed luminosity in the central parsec is too high to be explained by a long-standing top-heavy IMF, considering the limited amount of mass inferred from stellar kinematics in this region. In contrast, continuous star formation over the Galaxy's lifetime following a canonical IMF results in a mass-to-light ratio and a total mass of stellar black holes (SBHs) consistent with the observations. Furthermore, these SBHs migrate towards the centre due to dynamical friction, turning the cusp of visible stars into a core as observed in the Galactic Centre. For the first time here we explain the luminosity and dynamical mass of the central cluster and both the presence and extent of the observed core, since the number of SBHs expected from a canonical IMF is just enough to make up for the missing luminous mass. We conclude that the Galactic Centre is consistent with the canonical IMF and do not suggest a systematic variation as a result of the region's properties such as high density, metallicity, strong tidal field etc.Comment: MNRAS, accepted, 8 pages, 4 figure

Coordinated NIR/mm observations of flare emission from Sagittarius A*

We report on a successful, simultaneous observation and modelling of the millimeter (mm) to near-infrared (NIR) flare emission of the Sgr A* counterpart associated with the supermassive black hole at the Galactic centre (GC). We present a mm/sub-mm light curve of Sgr A* with one of the highest quality continuous time coverages and study and model the physical processes giving rise to the variable emission of Sgr A*.Comment: 14 pages, 16 figure