129 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
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
GALACTICNUCLEUS: A high-angular-resolution imaging survey of the Galactic centre. IV. Extinction maps and de-reddened photometry
The extreme extinction (\,mag) and its variation on arc-second
scales towards the Galactic centre hamper the study of its stars. Their
analysis is restricted to the near infrared (NIR) regime, where the extinction
curve can be approximated by a broken power law. Therefore, correcting for
extinction is fundamental to analyse the structure and stellar population of
the central regions of our Galaxy. We aim to, (1) discuss different strategies
to de-redden the photometry and check the usefulness of extinction; (2) build
extinction maps for the NIR bands and make them publicly available; (3)
create a de-reddened catalogue of the GALACTICNUCLEUS (GNS) survey, identifying
foreground stars; and (4) perform a preliminary analysis of the de-reddened
luminosity functions (KLFs). We used photometry from the GNS survey to
create extinction maps for the whole catalogue. We took red clump (RC) and red
giant stars of similar brightnesses as a reference to build the maps and
de-reddened the GNS photometry. We discussed the limitations of the process and
analysed non-linear effects of the de-reddening. We obtained high resolution
() extinction maps with low uncertainties (\,\%) and
computed average extinctions for each of the regions covered by the GNS. We
checked that our maps effectively correct the differential extinction reducing
the spread of the RC features by a factor of . We assessed the validity
of the broken power law approach computing two equivalent extinction maps
using either and photometry for the same reference stars and
obtained compatible average extinctions within the uncertainties. Finally, we
analysed de-reddened KLFs for different lines of sight and found that the
regions belonging to the NSD contain a homogeneous stellar population that is
significantly different from that in the innermost bulge regions.Comment: Updated to the final version accepted for publication in Astronomy &
Astrophysics. 17 pages, 11 figure
Black-hole activity feedback across vast scales
Both observational and theoretical studies of black-hole activity or active
galactic nucleus (AGN) feedback have been ongoing since the first indication of
supermassive black holes powering quasar activity in the 1960s. Although
several crucial astrophysical questions have been answered in the following
decades, a number of open problems remain, in particular how AGN feedback
operates over nearly eight orders of magnitude - from scales of to the galaxy-cluster scales of a few hundred kiloparsecs.
At the beginning of June 2022, about 50 junior as well as senior researchers
met in Brno for the post-lockdown edition of the Cologne-Prague-Brno meeting to
try to connect the dots.Comment: in print in Nature Astronomy as a meeting report; 5 pages, 1 figur
The low-mass content of the massive young star cluster RCW 38
KM acknowledges funding by the Joint Committee of ESO/Government of Chile, and by the Science and Technology Foundation of Portugal (FCT), grant no. IF/00194/2015. Part of the research leading to these results has received funding from the European Research Council under the European Unionâs Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. [614922]. RJ acknowledges support from NSERC grants. JA acknowledges funding by the Science and Technology Foundation of Portugal (FCT), grant no. SFRH/BPD/101562/2014.RCWâ38 is a deeply embedded young (âŒ1âMyr), massive star cluster located at a distance of 1.7âkpc. Twice as dense as the Orion nebula cluster, orders of magnitude denser than other nearby star-forming regions and rich in massive stars, RCWâ38 is an ideal place to look for potential differences in brown dwarf formation efficiency as a function of environment. We present deep, high-resolution adaptive optics data of the central âŒ0.5 Ă 0.5âpc2 obtained with NACO at the Very Large Telescope. Through comparison with evolutionary models, we determine masses and extinction for âŒ480 candidate members, and derive the first initial mass function (IMF) of the cluster extending into the substellar regime. Representing the IMF as a set of power laws in the form dN/dM â Mâα, we derive the slope α = 1.60 ± 0.13 for the mass range 0.5â20âMâ,which is shallower than the Salpeter slope, but in agreement with results in several other young massive clusters. At the low-mass side, we find α = 0.71 ± 0.11 for masses between 0.02 and 0.5âMâ, or α = 0.81 ± 0.08 for masses between 0.02 and 1âMâ. Our result is in agreement with the values found in other young star-forming regions, revealing no evidence that a combination of high stellar densities and the presence of numerous massive stars affects the formation efficiency of brown dwarfs and very-low-mass stars. We estimate that the Milky Way galaxy contains between 25 and 100 billion brown dwarfs (with masses >0.03âMâ).Publisher PDFPeer reviewe
Status and new operation modes of the versatile VLT/NACO
This paper aims at giving an update on the most versatile adaptive optics fed
instrument to date, the well known and successful NACO . Although NACO is only
scheduled for about two more years at the Very Large Telescope (VLT), it keeps
on evolving with additional operation modes bringing original astronomical
results. The high contrast imaging community uses it creatively as a test-bench
for SPHERE and other second generation planet imagers. A new visible wavefront
sensor (WFS) optimized for Laser Guide Star (LGS) operations has been installed
and tested, the cube mode is more and more requested for frame selection on
bright sources, a seeing enhancer mode (no tip/tilt correction) is now offered
to provide full sky coverage and welcome all kind of extragalactic
applications, etc. The Instrument Operations Team (IOT) and Paranal engineers
are currently working hard at maintaining the instrument overall performances
but also at improving them and offering new capabilities, providing the
community with a well tuned and original instrument for the remaining time it
is being used. The present contribution delivers a non-exhaustive overview of
the new modes and experiments that have been carried out in the past months.Comment: 10 pages, 7 figures, SPIE 2010 Astronomical Instrumentation
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