Aims. We demonstrate the high multiplex advantage of crowded field 3D spectroscopy using the new integral field spectrograph MUSE by means of a spectroscopic analysis of more than 12,000 individual ...stars in the globular cluster NGC 6397. Methods. The stars are deblended with a PSF fitting technique, using a photometric reference catalogue from HST as prior, including relative positions and brightnesses. This catalogue is also used for a first analysis of the extracted spectra, followed by an automatic in-depth analysis using a full-spectrum fitting method based on a large grid of PHOENIX spectra. Results. With 18,932 spectra from 12,307 stars in NGC 6397 we have analysed the largest sample so far available for a single globular cluster. We derived a mean radial velocity of vrad=17.84+-0.07 km/s and a mean metallicity of Fe/H=-2.120+-0.002, with the latter seemingly varying with temperature for stars on the RGB. We determine T_eff and Fe/H from the spectra, and log g from HST photometry. This is the first very comprehensive HRD for a globular cluster based on the analysis of several thousands of stellar spectra, ranging from the main sequence to the tip of the RGB. Furthermore, two interesting objects were identified with one being a post-AGB star and the other a possible millisecond-pulsar companion.
We present a new library of high-resolution synthetic spectra based on the stellar atmosphere code PHOENIX that can be used for a wide range of applications of spectral analysis and stellar parameter ...synthesis. The spherical mode of PHOENIX was used to create model atmospheres and to derive detailed synthetic stellar spectra from them. We present a new self-consistent way of describing micro-turbulence for our model atmospheres. The synthetic spectra cover the wavelength range from 500AA to 50.000AA with resolutions of R=500.000 in the optical and near IR, R=100.000 in the IR and a step size of 0.1AA in the UV. The parameter space covers 2.300K<=Teff<=12.000K, 0.0<=log(g)<=+6.0, -4.0<=Fe/H<=+1.0, and -0.2<=alpha/Fe<=+1.2. The library is a work in progress and we expect to extend it up to Teff=25.000 K.
We report the first unambiguous detection and mass measurement of an isolated stellar-mass black hole (BH). We used the Hubble Space Telescope (HST) to carry out precise astrometry of the source star ...of the long-duration (t_E~270 days), high-magnification microlensing event MOA-2011-BLG-191/OGLE-2011-BLG-0462 (hereafter designated as MOA-11-191/OGLE-11-462), in the direction of the Galactic bulge. HST imaging, conducted at eight epochs over an interval of six years, reveals a clear relativistic astrometric deflection of the background star's apparent position. Ground-based photometry of MOA-11-191/OGLE-11-462 shows a parallactic signature of the effect of the Earth's motion on the microlensing light curve. Combining the HST astrometry with the ground-based light curve and the derived parallax, we obtain a lens mass of 7.1 +/- 1.3 Msun and a distance of 1.58 +/- 0.18 kpc. We show that the lens emits no detectable light, which, along with having a mass higher than is possible for a white dwarf or neutron star, confirms its BH nature. Our analysis also provides an absolute proper motion for the BH. The proper motion is offset from the mean motion of Galactic-disk stars at similar distances by an amount corresponding to a transverse space velocity of ~45 km/s, suggesting that the BH received a 'natal kick' from its supernova explosion. Previous mass determinations for stellar-mass BHs have come from radial-velocity measurements of Galactic X-ray binaries, and from gravitational radiation emitted by merging BHs in binary systems in external galaxies. Our mass measurement is the first for an isolated stellar-mass BH using any technique.
We compare the results of a large grid of N-body simulations with the surface
brightness and velocity dispersion profiles of the globular clusters $\omega$
Cen and NGC 6624. Our models include ...clusters with varying stellar-mass black
hole retention fractions and varying masses of a central intermediate-mass
black hole (IMBH). We find that an $\sim 45,000$ M$_\odot$ IMBH, whose presence
has been suggested based on the measured velocity dispersion profile of
$\omega$ Cen, predicts the existence of about 20 fast-moving, $m>0.5$ M$_\odot$
main-sequence stars with a (1D) velocity $v>60$ km/sec in the central 20 arcsec
of $\omega$ Cen. However no such star is present in the HST/ACS proper motion
catalogue of Bellini et al. (2017), strongly ruling out the presence of a
massive IMBH in the core of $\omega$ Cen. Instead, we find that all available
data can be fitted by a model that contains 4.6% of the mass of $\omega$ Cen in
a centrally concentrated cluster of stellar-mass black holes. We show that this
mass fraction in stellar-mass BHs is compatible with the predictions of stellar
evolution models of massive stars.
We also compare our grid of $N$-body simulations with NGC 6624, a cluster
recently claimed to harbor a 20,000 M$_\odot$ black hole based on timing
observations of millisecond pulsars. However, we find that models with
$M_{IMBH}>1,000$ M$_\odot$ IMBHs are incompatible with the observed velocity
dispersion and surface brightness profile of NGC 6624,ruling out the presence
of a massive IMBH in this cluster. Models without an IMBH provide again an
excellent fit to NGC 6624.
We compare the results of a large grid of N-body simulations with the surface brightness and velocity dispersion profiles of the globular clusters \(\omega\) Cen and NGC 6624. Our models include ...clusters with varying stellar-mass black hole retention fractions and varying masses of a central intermediate-mass black hole (IMBH). We find that an \(\sim 45,000\) M\(_\odot\) IMBH, whose presence has been suggested based on the measured velocity dispersion profile of \(\omega\) Cen, predicts the existence of about 20 fast-moving, \(m>0.5\) M\(_\odot\) main-sequence stars with a (1D) velocity \(v>60\) km/sec in the central 20 arcsec of \(\omega\) Cen. However no such star is present in the HST/ACS proper motion catalogue of Bellini et al. (2017), strongly ruling out the presence of a massive IMBH in the core of \(\omega\) Cen. Instead, we find that all available data can be fitted by a model that contains 4.6% of the mass of \(\omega\) Cen in a centrally concentrated cluster of stellar-mass black holes. We show that this mass fraction in stellar-mass BHs is compatible with the predictions of stellar evolution models of massive stars. We also compare our grid of \(N\)-body simulations with NGC 6624, a cluster recently claimed to harbor a 20,000 M\(_\odot\) black hole based on timing observations of millisecond pulsars. However, we find that models with \(M_{IMBH}>1,000\) M\(_\odot\) IMBHs are incompatible with the observed velocity dispersion and surface brightness profile of NGC 6624,ruling out the presence of a massive IMBH in this cluster. Models without an IMBH provide again an excellent fit to NGC 6624.
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