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Register a new userStellar and wind properties of massive stars in the central parsec of the Galaxy
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F. Martins
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We study the stellar and wind properties of massive stars in the central cluster of the Galaxy. We use non-LTE atmosphere models including winds and line-blanketing to fit their H and K band spectra obtained with the 3D spectrograph SINFONI on the VLT. We derive the main stellar (Teff, L, abundances, ionizing flux) and wind (mass loss rate, terminal velocity) properties. They are found to be similar to other galactic massive stars. We show that a direct evolutionary link between Ofpe/WN9, WN8 and WN/C stars exists. Using individual SEDs for each massive star, we construct the total spectral energy distribution of the cluster and use it to compute photoionization models. We show that the nebular properties of the central HII region are well reproduced. We conclude that, contrary to previous claims, standard stellar evolution and atmosphere models are well suited to explain the properties of the central cluster. Our results indicate that massive stars in the central cluster do not have a peculiar evolution as could be expected from their proximity to the supermassive black hole SgrA*.
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We report the definite spectroscopic identification of 41 OB supergiants, giants and main sequence stars in the central parsec of the Galaxy. Detection of their absorption lines have become possible with the high spatial and spectral resolution and sensitivity of the adaptive optics integral field spectrometer SPIFFI/SINFONI on the ESO VLT. Several of these OB stars appear to be helium and nitrogen rich. Almost all of the ~80 massive stars now known in the central parsec (central arcsecond excluded) reside in one of two somewhat thick (<|h|/R>~0.14) rotating disks. These stellar disks have fairly sharp inner edges (R~1) and surface density profiles that scale as R^{-2}. We do not detect any OB stars outside the central 0.5 pc. The majority of the stars in the clockwise system appear to be on almost circular orbits, whereas most of those in the `counter-clockwise disk appear to be on eccentric orbits. Based on its stellar surface density distribution and dynamics we propose that IRS 13E is an extremely dense cluster (core density > 3x10^8 sunmass/pc^3), which has formed in the counter-clockwise disk. The stellar contents of both systems are remarkably similar, indicating a common age of ~6+/-2 Myr. The K-band luminosity function of the massive stars suggests a top-heavy mass function and limits the total stellar mass contained in both disks to ~1.5x10^4 sunmass. Our data strongly favor in situ star formation from dense gas accretion disks for the two stellar disks. This conclusion is very clear for the clockwise disk and highly plausible for the counter-clockwise system.
Fast line-driven stellar winds play an important role in the evolution of planetary nebulae. We provide global hot star wind models of central stars of planetary nebulae. The models predict wind structure including the mass-loss rates, terminal velocities, and emergent fluxes from basic stellar parameters. We applied our wind code for parameters corresponding to evolutionary stages between the asymptotic giant branch and white dwarf phases. We study the influence of metallicity and wind inhomogeneities (clumping) on the wind properties. Line-driven winds appear very early after the star leaves the asymptotic giant branch (at the latest for $T_rm{eff}approx10,$kK) and fade away at the white dwarf cooling track (below $T_rm{eff}=105,$kK). Their mass-loss rate mostly scales with the stellar luminosity and, consequently, the mass-loss rate only varies slightly during the transition from the red to the blue part of the Hertzsprung-Russell diagram. There are the following two exceptions to the monotonic behavior: a bistability jump at around $20,$kK, where the mass-loss rate decreases by a factor of a few (during evolution) due to a change in iron ionization, and an additional maximum at about $T_rm{eff}=40-50,$kK. On the other hand, the terminal velocity increases from about a few hundreds of $rm{km},rm{s}^{-1}$ to a few thousands of $rm{km},rm{s}^{-1}$ during the transition as a result of stellar radius decrease. The wind terminal velocity also significantly increases at the bistability jump. Derived wind parameters reasonably agree with observations. The effect of clumping is stronger at the hot side of the bistability jump than at the cool side. Derived fits to wind parameters can be used in evolutionary models and in studies of planetary nebula formation. A predicted bistability jump in mass-loss rates can cause the appearance of an additional shell of planetary nebula.
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We report a study of the H30$alpha$ line emission at 1.3 mm from the region around Sgr A* made with the Submillimeter Array at a resolution of 2arcsec over a field of 60arcsec (2 parsec) and a velocity range of -360 to +345 kms. This field encompasses most of the Galactic centers minispiral. With an isothermal homogeneous HII model, we determined the physical conditions of the ionized gas at specific locations in the Northern and Eastern Arms from the H30$alpha$ line data along with Very Large Array data from the H92$alpha$ line at 3.6 cm and from the radio continuum emission at 1.3 cm. The typical electron density and kinetic temperature in the minispiral arms are 3-21$times10^4$ cm$^{-3}$ and 5,000-13,000 K, respectively. The H30$alpha$ and H92$alpha$ line profiles are broadened due to the large velocity shear within and along the beam produced by dynamical motions in the strong gravitational field near Sgr A*. We constructed a 3D model of the minispiral using the orbital parameters derived under the assumptions that the gas flows are in Keplerian motion. The gas in the Eastern Arm appears to collide with the Northern Arm flow in the Bar region, which is located 0.1-0.2 parsec south of and behind Sgr A*. Finally, a total Lyman continuum flux of $3times10^{50}$ photons s$^{-1}$ is inferred from the assumption that the gas is photoionized and the ionizing photons for the high-density gas in the minispiral arms are from external sources, which is equivalent to $sim250$ O9-type zero-age-main-sequence stars.
We present a metallicity analysis of 83 late-type giants within the central 1 pc of the Milky Way. K-band spectroscopy of these stars were obtained with the medium-spectral resolution integral-field spectrograph NIFS on Gemini North using laser-guide star adaptive optics. Using spectral template fitting with the MARCS synthetic spectral grid, we find that there is large variation in metallicity, with stars ranging from [M/H] $<$ -1.0 to above solar metallicity. About 6% of the stars have [M/H] $<$ -0.5. This result is in contrast to previous observations, with smaller samples, that show stars at the Galactic center have approximately solar metallicity with only small variations. Our current measurement uncertainties are dominated by systematics in the model, especially at [M/H] $>$ 0, where there are stellar lines not represented in the model. However, the conclusion that there are low metallicity stars, as well as large variations in metallicity is robust. The metallicity may be an indicator of the origin of these stars. The low-metallicity population is consistent with that of globular clusters in the Milky Way, but their small fraction likely means that globular cluster infall is not the dominant mechanism for forming the Milky Way nuclear star cluster. The majority of stars are at or above solar metallicity, which suggests they were formed closer to the Galactic center or from the disk. In addition, our results indicate that it will be important for star formation history analyses using red giants at the Galactic center to consider the effect of varying metallicity.
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