1,184 research outputs found
Young, Massive Star Candidates Detected throughout the Nuclear Star Cluster of the Milky Way
Aims. Young, massive stars have been found at projected distances R < 0.5 pc
from supermassive black hole, Sgr A* at the center of our Galay. In recent
years, increasing evidence has been found for the presence of young, massive
stars also at R > 0.5 pc. Our goal in this work is a systematic search for
young, massive star candidates throughout the entire region within R ~ 2.5 pc
of the black hole. Methods. The main criterion for the photometric
identification of young, massive early-type stars is the lack of CO-absorption
in the spectra. We used narrow-band imaging with VLT/ISAAC to search for young,
massive stars within ~2.5 pc of Sgr A*. Results. We have found 63 early-type
star candidates at R < 2.5 pc, with an estimated erroneous identification rate
of only about 20%. Considering their K-band magnitudes and interstellar
extinction, they are candidates for Wolf-Rayet stars, supergiants, or early
O-type stars. Of these, 31 stars are so far unknown young, massive star
candidates, all of which lie at R>0.5pc. The surface number density profile of
the young, massive star candidates can be well fit by a single power-law, with
Gamma = 1.6 +- 0.17 at R < 2.5 pc, which is significantly steeper than that of
the late-type giants that make up the bulk of the observable stars in the NSC.
Intriguingly, this power-law is consistent with the power-law that describes
the surface density of young, massive stars in the same brightness range at R <
0.5 pc. Conclusions. The finding of a significant number of newly identified
early-type star candidates at the Galactic center suggests that young, massive
stars can be found throughout the entire cluster which may require us to modify
existing theories for star formation at the Galactic center. Follow-up studies
are needed to improve the existing data and lay the foundations for a unified
theory of star formation in the Milky Way's NSC.Comment: 20 pages, 11 figures, Accepted for publication in Astronomy and
Astrophysic
Holographic Imaging of Crowded Fields: High Angular Resolution Imaging with Excellent Quality at Very Low Cost
We present a method for speckle holography that is optimised for crowded
fields. Its two key features are an iterativ improvement of the instantaneous
Point Spread Functions (PSFs) extracted from each speckle frame and the
(optional) simultaneous use of multiple reference stars. In this way, high
signal-to-noise and accuracy can be achieved on the PSF for each short
exposure, which results in sensitive, high-Strehl re- constructed images. We
have tested our method with different instruments, on a range of targets, and
from the N- to the I-band. In terms of PSF cosmetics, stability and Strehl
ratio, holographic imaging can be equal, and even superior, to the capabilities
of currently available Adaptive Optics (AO) systems, particularly at short
near-infrared to optical wavelengths. It outperforms lucky imaging because it
makes use of the entire PSF and reduces the need for frame selection, thus
leading to higher Strehl and improved sensitivity. Image reconstruction a
posteriori, the possibility to use multiple reference stars and the fact that
these reference stars can be rather faint means that holographic imaging offers
a simple way to image large, dense stellar fields near the diffraction limit of
large telescopes, similar to, but much less technologically demanding than, the
capabilities of a multi-conjugate adaptive optics system. The method can be
used with a large range of already existing imaging instruments and can also be
combined with AO imaging when the corrected PSF is unstable.Comment: Accepted for publication in MNRAS on 15 Nov 201
The distribution of stars around the Milky Way's black hole III: Comparison with simulations
The distribution of stars around a massive black hole (MBH) has been
addressed in stellar dynamics for the last four decades by a number of authors.
Because of its proximity, the centre of the Milky Way is the only observational
test case where the stellar distribution can be accurately tested. Past
observational work indicated that the brightest giants in the Galactic Centre
(GC) may show a density deficit around the central black hole, not a cusp-like
distribution, while we theoretically expect the presence of a stellar cusp. We
here present a solution to this long-standing problem. We performed
direct-summation body simulations of star clusters around massive black
holes and compared the results of our simulations with new observational data
of the GC's nuclear cluster. We find that after a Hubble time, the distribution
of bright stars as well as the diffuse light follow power-law distributions in
projection with slopes of in our simulations. This is in
excellent agreement with what is seen in star counts and in the distribution of
the diffuse stellar light extracted from adaptive-optics (AO) assisted
near-infrared observations of the GC. Our simulations also confirm that there
exists a missing giant star population within a projected radius of a few
arcsec around Sgr A*. Such a depletion of giant stars in the innermost 0.1 pc
could be explained by a previously present gaseous disc and collisions, which
means that a stellar cusp would also be present at the innermost radii, but in
the form of degenerate compact cores.Comment: Accepted for publication, few typos fixe
SOWAT: Speckle Observations With Alleviated Turbulence
Adaptive optics (AO) systems and image reconstruction algorithms are
indispensable tools when it comes to high-precision astrometry. In this paper,
we analyze the potential of combining both techniques, i.e. by applying image
reconstruction on partially AO corrected short exposures. Therefore we simulate
speckle clouds with and without AO corrections and create synthetic
observations. We apply holographic image reconstruction to the obtained
observations and find that (i) the residual wavefronts decorrelate slowlier and
to a lower limit when AO systems are used, (ii) the same reference stars yield
a better reconstruction, and (iii) using fainter reference stars we achieve a
similar image quality. These results suggest that holographic imaging of
speckle observations is feasible with 2-3 times longer integration times and
3mag fainter reference stars, to obtain diffraction-limited imaging from
low-order AO systems that are less restricted in sky-coverage than typical
high-order AO systems.Comment: 18 pages, 13 figures, and 3 table
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
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