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Register a new userCensus of the Galactic Centre early-type stars using spectro-imagery
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Authors
T. Paumard
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The few central parsecs of the Galaxy are known to contain a surprising population of early-type stars, including at least 30 Wolf-Rayet stars and luminous blue variables (LBV), identified thanks to their strong emission lines. Despite the presence of emission from ionised interstellar material in the same lines, the latest advances in spectro-imaging have made it possible to use the absorption lines of the OB stars to characterise them as well. This stellar population is particularly intriguing in the deep potential well of the 4 million solar mass black hole Sgr A*. We will review the properties of these early-type stars known from spectro-imagery, and discuss possible formation scenarios.
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Integral field spectroscopy of the central parsec of the Galactic Center was obtained at 2.06 microns using BEAR, an imaging Fourier Transform Spectrometer, at a spectral resolution of 74 km/s. Sixteen stars were confirmed as helium stars by detecting the He I 2.058 microns line in emission, providing a homogeneous set of fully resolved line profiles. These observations allow us to discard some of the earlier detections of such stars in the central cluster and to add three new stars. The sources detected in the BEAR data were compared with adaptive optics images in the K band to determine whether the emission was due to single stars. Two sub-classes of almost equal number are clearly identified from the width of their line profiles, and from the brightness of their continuum. Most of the emission lines show a P Cygni profile. From these results, we propose that the latter group is formed of stars in or near the LBV phase, and the other one of stars at the WR stage. The division into two groups is also shown by their spatial distribution, with the narrow-line stars in a compact central cluster (IRS 16) and the other group distributed at the periphery of the central cluster of hot stars. In the same data cube, streamers of interstellar helium gas are also detected. The helium emission traces the densest parts of the SgrA West Mini-Spiral. Several helium stars have a radial velocity comparable to the velocity of the interstellar gas in which they are embedded. In the final discussion, all these findings are examined to present a possible scenario for the formation of very massive stars in the exceptional conditions of the vicinity of the central Black Hole.
Knowing the metallicity distribution of stars in the Galactic Centre has important implications for the formation history of the Milky Way nuclear star cluster. However, this distribution is not well known, and is currently based on a small sample of fewer than 100 stars. We obtained near-infrared K-band spectra of more than 700 late-type stars in the central 4 pc^2 of the Milky Way nuclear star cluster with the integral-field spectrograph KMOS (VLT). We analyse the medium-resolution spectra using a full-spectral fitting method employing the Gottingen Spectral library of synthetic PHOENIX spectra. The derived stellar metallicities range from metal-rich [M/H]>+0.3 dex to metal-poor [M/H]<-1.0 dex, with a fraction of 5.2(^{+6.0}+{-3.1}) per cent metal-poor ([M/H]<-0.5 dex) stars. The metal-poor stars are distributed over the entire observed field. The origin of metal-poor stars remains unclear. They could originate from infalling globular clusters. For the metal-rich stellar population ([M/H]>0 dex) a globular cluster origin can be ruled out. As there is only a very low fraction of metal-poor stars in the central 4 pc^2 of the Galactic Centre, we believe that our data can discard a scenario in which the Milky Way nuclear star cluster is purely formed from infalling globular clusters.
Integral field spectroscopy of the inner region of the Galactic Center, over a field of roughly 40x40 was obtained at 2.06 microns (He I) and 2.16 microns (Brackett-gamma) using BEAR, an imaging Fourier Transform Spectrometer, at spectral resolutions respectively of 52.9 km/s and 21.3 km/s, and a spatial resolution of ~0.5. The analysis of the data was focused on the kinematics of the gas flows, traditionally called the Minispiral, concentrated in the neighborhood of the central black hole, Sgr A*. From the decomposition into several velocity components of the line profile extracted at each point of the field, velocity features were identified. Nine distinguishable structures are described: the standard Northern Arm, Eastern Arm, Bar, Western Arc, and five additional, coherently-moving patches of gas. From this analysis, the Northern Arm appears not limited, as usually thought, to the bright, narrow North-South lane seen on intensity images, but it instead consists of a weak, continuous, triangular-shaped surface, drawn out into a narrow stream in the vicinity of Sgr A* where it shows a strong velocity gradient, and a bright western rim. The Eastern Arm is split into three components. We also report extinction of some interstellar structures by others, providing information on their relative position along the line of sight. A system of Keplerian orbits can be fitted to most of the Northern Arm, and the bright rim of this feature can be interpreted in terms of line-of-sight orbit crowding caused by the warping of the flowing surface at the western edge facing Sgr A*. The question of the origin of the ionized gas is addressed and a discussion of the lifetime of these features is presented.
Near-infrared high-angular resolution imaging observations of the Milky Ways nuclear star cluster have revealed all luminous members of the existing stellar population within the central parsec. Generally, these stars are either evolved late-type giants or massive young, early-type stars. We revisit the problem of stellar classification based on intermediate-band photometry in the K-band, with the primary aim of identifying faint early-type candidate stars in the extended vicinity of the central massive black hole. A random forest classifier, trained on a subsample of spectroscopically identified stars, performs similarly well as competitive methods (F1=0.85), without involving any model of stellar spectral energy distributions. Advantages of using such a machine-trained classifier are a minimum of required calibration effort, a predictive accuracy expected to improve as more training data becomes available, and the ease of application to future, larger data sets. By applying this classifier to archive data, we are also able to reproduce the results of previous studies of the spatial distribution and the K-band luminosity function of both the early- and late-type stars.
Two groups of astronomers used large telescopes Keck and VLT for decades to observe trajectories of bright stars near the Galactic Centre. Based on results of their observations astronomers concluded that trajectories of the stars are roughly elliptical and foci of the orbits are approximately coincide with the Galactic Centre position. It gives an opportunity to claim that the Newtonian potential of point like mass around $4.3times 10^6 M_odot$ is a good initial approximation for the gravitational potential near the Galactic Centre. In the last years, the astronomers found that gravitational redshift of S2 star near pericenter passage in May 2018 is in accordance with general relativity predictions. In 2020 the GRAVITY team found that the observed relativistic precession of S2 star orbit is also consistent with theoretical estimates calculated for a weak gravitational field approximation in a Schwarzschild black hole. In last years a a self-gravitating dark matter core--halo distribution suggested by Ruffini, Arguelles and Rueda (MNRAS, 2015) (RAR model) was proposed and recently Becerra-Vergara et al. (MNRAS, 2021) claimed that this model provides a better fit of trajectories of bright stars in comparison with the conventional model with the supermassive black hole. We confirm that in the case of this dark matter distribution model for a dense core trajectories of test bodies are elliptical but in this case centers (not foci) of these ellipses should coincide with the Galactic Centre and orbital periods do not depend on semi-major axis and it contradicts observational data and therefore, we concluded supermassive black hole is a preferable model in comparison with the a dense core--diluted halo density profile for the Galactic Centre.
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