432 research outputs found
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
Radio Continuum Observations of the Galactic Center: Photoevaporative Proplyd-like Objects near Sgr A*
We present radio images within 30 of Sgr A* based on recent VLA
observations at 34 GHz with 7.8 microJy sensitivity and resolution
milliarcseconds (mas). We report 44 partially resolved compact
sources clustered in two regions in the E arm of ionized gas that orbits Sgr
A*. These sources have size scales ranging between ~50 and 200 mas (400 to 1600
AUs), and a bow-shock appearance facing the direction of Sgr A*. Unlike the
bow-shock sources previously identified in the near-IR but associated with
massive stars, these 34 GHz sources do not appear to have near-IR counterparts
at 3.8 m. We interpret these sources as a candidate population of
photoevaporative protoplanetary disks (proplyds) that are associated with newly
formed low mass stars with mass loss rates ~10^{-7} - 10^{-6} solar mass per
year and are located at the edge of a molecular cloud outlined by ionized gas.
The disks are externally illuminated by strong Lyman continuum radiation from
the ~100 OB and WR massive stars distributed within 10'' of Sgr A*. The
presence of proplyds implies current in-situ star formation activity near Sgr
A* and opens a window for the first time to study low mass star, planetary and
brown dwarf formations near a supermassive black hole.Comment: 13 pages, 4 figures, ApJL (in press
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
First VLTI infrared spectro-interferometry on GCIRS 7 - Characterizing the prime reference source for Galactic center observations at highest angular resolution
Investigating the environment of the massive black hole SgrA* at the center
of the Galaxy requires the highest angular resolution available to avoid source
confusion and to study the physical properties of the individual objects.
GCIRS7 has been used as wavefront and astrometric reference. Our studies
investigate, for the first time, its properties at 2&10um using VLTI/AMBER and
MIDI. We aim at analyzing the suitability of IRS7 as an IF-phase-reference for
the upcoming generation of dual-field facilities at optical interferometers. We
observed with (R~30) and 50m (proj.) baseline, resulting in 9 and 45mas
resolution for NIR and MIR, resp. The first K-band fringe detection of a GC
star suggests that IRS7 could be marginally resolved at 2um, which would imply
that the photosphere of the supergiant is enshrouded by a molecular and dusty
envelope. At 10um, IRS7 is strongly resolved with a visibility of approximately
0.2. The MIR is dominated by moderately warm (200 K), extended dust, mostly
distributed outside of a radius of about 120 AU (15 mas) around the star. A
deep 9.8-silicate absorption in excess of the usual extinction law with respect
to the NIR extinction has been found. This confirms recent findings of a
relatively enhanced, interstellar 9.8-silicate absorption with respect to the
NIR extinction towards another star in the central arcsec, suggesting an
unusual dust composition in that region. Our VLTI observations show that
interferometric NIR phase-referencing experiments with mas resolution using
IRS7 as phase-reference appear to be feasible, but more such studies are
required to definitely characterize the close environment around this star. We
demonstrate that interferometry is required to resolve the innermost
environment of stars at the Galactic center.Comment: 6 pages, 2 figures, accepted for publication in A&
Cometary shaped sources at the Galactic Center - Evidence for a wind from the central 0.2 pc
In 2007 we reported two cometary shaped sources in the vicinity of Sgr A*
(0.8" and 3.4" projected distance), named X7 and X3. The symmetry axes of the
two sources are aligned to within 5 degrees in the plane of the sky and the
tips of their bow-shocks point towards Sgr A*. Our measurements show that the
proper motion vectors of both features are pointing in directions more than 45
deg away from the line that connects them with Sgr A*. This misalignment of the
bow-shock symmetry axes and their proper motion vectors, combined with the high
proper motion velocities of several 100 km/s, suggest that the bow-shocks must
be produced by an interaction with some external fast wind, possibly coming
from Sgr A*, or stars in its vicinity. We have developed a bow-shock model to
fit the observed morphology and constrain the source of the external wind. The
result of our modeling allows us to estimate the velocity of the external wind,
making sure that all likely stellar types of the bow-shock stars are
considered. We show that neither of the two bow-shocks (one of which is clearly
associated with a stellar source) can be produced by influence of a stellar
wind of a single mass-losing star in the central parsec. Instead, an outflow
carrying a momentum comparable to the one contributed by the ensemble of the
massive young stars, can drive shock velocities capable of producing the
observed cometary features. We argue that a collimated outflow arising
perpendicular to the plane of the clockwise rotating stars (CWS), can easily
account for the two features and the mini-cavity. However, the collective wind
from the CWS has a scale of >10''. The presence of a strong, mass-loaded
outbound wind at projected distances from Sgr A* of <1'' is in fact in
agreement with models that predict a highly inefficient accretion onto the
central black hole due to a strongly radius dependent accretion flow.Comment: to appear in A&
ALMA and VLA Observations: Evidence for Ongoing Low-mass Star Formation near Sgr A*
Using the VLA, we recently detected a large number of protoplanetary disk
(proplyd) candidates lying within a couple of light years of the massive black
hole Sgr A*. The bow-shock appearance of proplyd candidates point toward the
young massive stars located near Sgr A*. Similar to Orion proplyds, the strong
UV radiation from the cluster of massive stars at the Galactic center is
expected to photoevaporate and photoionize the circumstellar disks around
young, low mass stars, thus allowing detection of the ionized outflows from the
photoionized layer surrounding cool and dense gaseous disks. To confirm this
picture, ALMA observations detect millimeter emission at 226 GHz from five
proplyd candidates that had been detected at 44 and 34 GHz with the VLA. We
present the derived disk masses for four sources as a function of the assumed
dust temperature. The mass of protoplanetary disks from cool dust emission
ranges between 0.03 -- 0.05 solar mass. These estimates are consistent with the
disk masses found in star forming sites in the Galaxy. These measurements show
the presence of on-going star formation with the implication that gas clouds
can survive near Sgr A* and the relative importance of high vs low-mass star
formation in the strong tidal and radiation fields of the Galactic center.Comment: 13 pages, 3 figures, MNRAS (in press
On the orientation of the Sagittarius A* system
The near-infrared emission from the black hole at the Galactic center (Sgr
A*) has unique properties. The most striking feature is a suggestive periodic
sub-structure that has been observed in a couple of flares so far. Using
near-infrared polarimetric observations and modelling the quasi-periodicity in
terms of an orbiting blob, we try to constrain the three dimensional
orientation of the Sgr A* system. We report on so far unpublished polarimetric
data from 2003. They support the observations of a roughly constant mean
polarization angle of 60 degr \pm 20 degr from 2004-2006. Prior investigations
of the 2006 data are deepened. In particular, the blob model fits are evaluated
such that constraints on the position angle of Sgr A* can be derived.
Confidence contours in the position-inclination angle plane are derived. On a
3sigma level the position angle of the equatorial plane normal is in the range
60 degr - 108 degr (east of north) in combination with a large inclination
angle. This agrees well with recent independent work in which radio
spectral/morphological properties of Sgr A* and X-ray observations,
respectively, have been used. However, the quality of the presently available
data and the uncertainties in our model bring some ambiguity to our
conclusions.Comment: 10 pages, 4 figures; Research Note accepted by A&A for publicatio
Adaptive optics assisted near-infrared polarization measurements of sources in the Galactic Center
The goals of this work are to provide NIR polarimetry of the stellar sources
in the central pc at the resolution of an 8m telescope for the first time,
along with new insights into the nature of the known bright bow-shock sources.
We use AO assisted observations with the ESO VLT in the H- and Ks-band,
applying high precision photometric methods developed for crowded fields and a
new polarimetric calibration for NACO to produce polarization maps of the
central 3"x19", in addition to spatially resolved polarimetry and a variability
analysis on the extended sources. We find foreground polarization parallel to
the Galactic plane, with averages of (4.6+/-0.6)% at 26{\deg}+/-6{\deg} (Ks)
and (9.3+/-1.3)% at 20{\deg}+/-6{\deg} (H) in the center. At larger distances
from the center, we find different polarization parameters: (7.5+/-1.0)% at
11{\deg}+/-6{\deg} (Ks) and (12.1+/-2.1)% at 13{\deg}+/-6{\deg} (H). p_H/p_Ks
peaks at 1.9+/-0.4, with a power law index for the wavelength dependency of
alpha = 2.4+/-0.7. This also varies over the FOV, with higher values in the
center, indicating local effects on the total polarization, possibly dichroic
extinction by Northern Arm dust. The two extended sources IRS 21 and 1W show
similar intrinsic polarization degrees of 6.1% resp. 7.8% (Ks) and 6.9% (H,
only 1W) at polarization angles coincident with previous NIR and MIR findings,
both in total and resolved. The spatial polarization pattern of both sources
points to scattering on aligned elongated dust grains as the major source of
intrinsic polarization, and matches the known orientation of the magnetic
field. Our data also allows us to separate the bow shock of IRS 21 from the
central source for the first time in the Ks-band, with the apex north of the
center and a standoff distance of ~400 AU, matching previous estimates. This
source also shows a ~50% increase in flux in the NIR over several years.Comment: 26 pages, 33 figures, accepted for publication in A&
VLTI observations of IRS~3: The brightest compact MIR source at the Galactic Centre
The dust enshrouded star IRS~3 in the central light year of our galaxy was
partially resolved in a recent VLTI experiment. The presented observation is
the first step in investigating both IRS~3 in particular and the stellar
population of the Galactic Centre in general with the VLTI at highest angular
resolution. We will outline which scientific issues can be addressed by a
complete MIDI dataset on IRS~3 in the mid infrared.Comment: 4 pages, 3 figures, published in: The ESO Messenge
Detecting hot stars in the Galactic centre with combined near- and mid-infrared photometry
{The Galactic centre (GC) is a unique astrophysical laboratory to study the
stellar population of galactic nuclei because it is the only galactic nucleus
whose stars can be resolved down to milliparsec scales. However, the extreme
and spatially highly variable interstellar extinction towards the GC poses a
serious obstacle to photometric stellar classification.} {Our goal is to
identify hot, massive stars in the nuclear stellar disc (NSD) region through
combining near-infrared (NIR) and mid-infrared (MIR) photometry, and thus to
demonstrate the feasibility of this technique, which may gain great importance
with the arrival of the James Webb Space Telescope (JWST).} {We combined the
GALACTICNUCLEUS NIR survey with the IRAC/Spitzer MIR survey of the GC. We
applied the so-called Rayleigh-Jeans colour excess (RJCE) de-reddening method
to our combined NIR-MIR data to identify potential hot stars in
colour-magnitude diagrams (CMDs).} {Despite the very low angular resolution of
IRAC we find 12 clear candidates for young massive stars among the
sources that meet our selection criteria. Seven out of these 12 stars are
previously known hot, massive stars belonging to the Arches and Quintuplet
clusters, as well as sources detected by the Hubble Space Telescope/NICMOS
Paschen- survey. Five of our massive star candidates have not been
previously reported in the literature.} {We show that the RJCE method is a
valuable tool to identify hot stars in the GC using photometry alone. Upcoming
instruments with high angular resolution MIR imaging capabilities such as the
JWST could surely make more substantial use of this de-reddening method and
help establish a far more complete census of hot, young stars in the GC area
than what is possible at the moment.}Comment: 6 pages, 6 figures, accepted for publication in A&
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