435 research outputs found
Radial Velocities of Stars in the Galactic Center
We present results from K band slit scan observations of a ~20''x20'' region
of the Galactic center (GC) in two separate epochs more than five years apart.
The high resolution (R>=14,000) observations allow the most accurate radial
velocity and acceleration measurements of the stars in the central parsec of
the Galaxy. Detected stars can be divided into three groups based on the CO
absorption band heads at ~2.2935 microns and the He I lines at ~2.0581 microns
and ~2.112, 2.113 microns: cool, narrow-line hot and broad-line hot. The radial
velocities of the cool, late-type stars have approximately a symmetrical
distribution with its center at ~-7.8(+/-10.3) km/s and a standard deviation
~113.7(+/-10.3) km/s. Although our statistics are dominated by the brightest
stars, we estimate a central black hole mass of 3.9(+/-1.1) million solar
masses, consistent with current estimates from complete orbits of individual
stars. Our surface density profile and the velocity dispersion of the late type
stars support the existence of a low density region at the Galactic center
suggested by earlier observations. Many hot, early-type stars show radial
velocity changes higher than maximum values allowed by pure circular orbital
motions around a central massive object, suggesting that the motions of these
stars greatly deviate from circular orbital motions around the Galactic center.
The correlation between the radial velocities of the early type He I stars and
their declination offsets from Sagittarius A* suggests that a systematic
rotation is present for the early-type population. No figure rotation around
the Galactic center for the late type stars is supported by the new
observations.Comment: 61 pages, 18 figures, 7 tables; accepted for publication in
Astrophysical Journa
A Second Luminous Blue Variable in the Quintuplet Cluster
H and K band moderate resolution and 4 m high resolution spectra have
been obtained for FMM#362, a bright star in the Quintuplet Cluster near the
Galactic Center. The spectral features in these bands closely match those of
the Pistol Star, a luminous blue variable and one of the most luminous stars
known. The new spectra and previously-obtained photometry imply a very high
luminosity for FMM#362, L \Lsun, and a temperature of 10,000 -
13,000 K. Based on its luminosity, temperature, photometric variability, and
similarities to the Pistol Star, we conclude that FMM#362 is a luminous blue
variable.Comment: Accepted for publication in The Astrophysical Journal Letters, 4
PostScript figures, 2 table
Peering through the veil: near-infrared photometry and extinction for the Galactic nuclear star cluster
The aims of this work are to provide accurate photometry in multiple
near-infrared broadband filters, to determine the power-law index of the
extinction-law toward the central parsec of the Galaxy, to provide measurements
of the absolute extinction toward the Galactic center, and finally to measure
the spatial variability of extinction on arcsecond scales.We use adaptive
optics observations of the central parsec of the Milky Way. Absolute values for
the extinction in the H, Ks, and L'-bands as well as of the power-law indices
of the H to Ks and Ks to L' extinction-laws are measured based on the
well-known properties of red clump stars. Extinction maps are derived based on
H-Ks and Ks-L' colors. We present Ks-band photometry for ~7700 stars (H and L'
photometry for a subset). From a number of recently published values we compute
a mean distance of the Galactic center of R_0=8.03+-0.15 kpc, which has an
uncertainty of just 2%. Based on this R_0 and on the RC method, we derive
absolute mean extinction values toward the central parsec of the Galaxy of
A_H=4.48+-0.13 mag, A_Ks=2.54+-0.12$ mag, and A_L'=1.27+-0.18 mag. We estimate
values of the power-law indices of the extinction-law of
alpha_{H-Ks}=2.21+-0.24 and alpha_{Ks-L'}=1.34+-0.29. A Ks-band extinction map
for the Galactic center is computed based on this extinction law and on stellar
H-Ks colors. Mean extinction values in a circular region with 0.5" radius
centered on Sagittarius A* are A_{H, SgrA*}=4.35+-0.12, A_{Ks,
SgrA*}=2.46+-0.03, and A_{L', SgrA*}=1.23+-0.08.Comment: accepted for publication by Astronomy & Astrophysics; please contact
RS for higher quality figure
The Arches cluster revisited: I. Data presentation and stellar census
Context. Located within the central region of the Galaxy, the Arches cluster appears to be one of the youngest, densest and most massive stellar aggregates within the Milky Way. As such it has the potential to be a uniquely instructive laboratory for the study of star formation in extreme environments and the physics of very massive stars.
Aims. To realise this possibility, the fundamental physical properties of both cluster and constituent stars need to be robustly determined; tasks we attempt here.
Methods. In order to accomplish these goals we provide and analyse new multi-epoch near-IR spectroscopic data obtained with the VLT/SINFONI and photometry from the HST/WFC3. We are able to stack multiple epochs of spectroscopy for individual stars in order to obtain the deepest view of the cluster members ever obtained.
Results. We present spectral classifications for 88 cluster members, all of which are WNLh or O stars: a factor of three increase over previous studies. We find no further examples of Wolf-Rayet stars within the cluster; importantly no H-free examples were identified. The smooth and continuous progression in spectral morphologies from O super-/hypergiants through to the WNLh cohort implies a direct evolutionary connection. We identify candidate giant and main sequence O stars spectroscopically for the first time. No products of binary evolution may be unambiguously identified despite the presence of massive binaries within the Arches.
Conclusions. Notwithstanding difficulties imposed by the highly uncertain (differential) reddening to the Arches, we infer a main sequence/luminosity class V turn-off mass of ⌠30 â 38Mâ via the distribution of spectral types. Analysis of the eclipsing binary F2 suggests current masses of ⌠80Mâ and ⌠60Mâ for the WNLh and O hypergiant cohorts, respectively; we conclude that all classified stars have masses > 20Mâ. An age of ⌠2.0 â 3.3Myr is suggested by the turn-off between âŒO4-5 V; constraints imposed by the supergiant population and the lack of H-free WRs are consistent with this estimate. While the absence of highly evolved WC stars strongly argues against the prior occurrence of SNe within the Arches, the derived age does accommodate such events for exceptionally massive stars. Further progress will require quantitative analysis of multiple individual cluster members in addition to further spectroscopic observations to better constrain the binary and main sequence populations; nevertheless it is abundantly clear that the Arches offers an unprecedented insight into the formation, evolution and death of the most massive stars Nature allows to form
The Arches Cluster Mass Function
We have analyzed H and K_s-band images of the Arches cluster obtained using
the NIRC2 instrument on Keck with the laser guide star adaptive optics (LGS AO)
system. With the help of the LGS AO system, we were able to obtain the deepest
ever photometry for this cluster and its neighborhood, and derive the
background-subtracted present-day mass function (PDMF) down to 1.3 Msun for the
5 arcsec-9 arcsec annulus of the cluster. We find that the previously reported
turnover at 6 Msun is simply due to a local bump in the mass function (MF), and
that the MF continues to increase down to our 50 % completeness limit (1.3
Msun) with a power-law exponent of Gamma = -0.91 for the mass range of 1.3 <
M/Msun < 50. Our numerical calculations for the evolution of the Arches cluster
show that the Gamma values for our annulus increase by 0.1-0.2 during the
lifetime of the cluster, and thus suggest that the Arches cluster initially had
Gamma of -1.0 ~ -1.1, which is only slightly shallower than the Salpeter value.Comment: Accepted for publication in ApJ
Nonthermal THz to TeV Emission from Stellar Wind Shocks in the Galactic Center
The central parsec of the Galaxy contains dozens of massive stars with a
cumulative mass loss rate of ~ 10^{-3} solar masses per year. Shocks among
these stellar winds produce the hot plasma that pervades the central part of
the galaxy. We argue that these stellar wind shocks also efficiently accelerate
electrons and protons to relativistic energies. The relativistic electrons
inverse Compton scatter the ambient ultraviolet and far infrared radiation
field, producing high energy gamma-rays with a roughly constant luminosity from
\~ GeV to ~ 10 TeV. This can account for the TeV source seen by HESS in the
Galactic Center. Our model predicts a GLAST counterpart to the HESS source with
a luminosity of ~ 10^{35} ergs/s and cooling break at ~ 4 GeV. Synchrotron
radiation from the same relativistic electrons should produce detectable
emission at lower energies, with a surface brightness ~ 10^{32} B^2_{-3}
ergs/s/arcsec^2 from ~ THz to ~ keV, where B_{-3} is the magnetic field
strength in units of mG. The observed level of diffuse thermal X-ray emission
in the central parsec requires B < 300 micro-G in our models. Future detection
of the diffuse synchrotron background in the central parsec can directly
constrain the magnetic field strength, providing an important boundary
condition for models of accretion onto Sgr A*.Comment: submitted to ApJ Letter
A Medium Resolution Near-Infrared Spectral Atlas of O and Early B Stars
We present intermediate resolution (R ~ 8,000 - 12,000) high signal-to-noise
H- and K-band spectroscopy of a sample of 37 optically visible stars, ranging
in spectral type from O3 to B3 and representing most luminosity classes.
Spectra of this quality can be used to constrain the temperature, luminosity
and general wind properties of OB stars, when used in conjunction with
sophisticated atmospheric model codes. Most important is the need for
moderately high resolutions (R > 5000) and very high signal-to-noise (S/N >
150) spectra for a meaningful profile analysis. When using near-infrared
spectra for a classification system, moderately high signal-to-noise (S/N ~
100) is still required, though the resolution can be relaxed to just a thousand
or two. In the appendix we provide a set of very high quality near-infrared
spectra of Brackett lines in six early-A dwarfs. These can be used to aid in
the modeling and removal of such lines when early-A dwarfs are used for
telluric spectroscopic standards.Comment: 12 pages, 3 tables, 14 figures. AASTex preprint style. To appear in
ApJS, November 2005. All spectra are available by contacting M.M. Hanso
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