215 research outputs found
Mid-infrared interferometry of the massive young stellar object NGC3603 - IRS 9A
We present observations and models for one of these MYSO candidates, NGC3603
IRS 9A. Our goal is to investigate with infrared interferometry the structure
of IRS 9A on scales as small as 200AU, exploiting the fact that a cluster of O
and B stars has blown away much of the obscuring foreground dust and gas.
Observations in the N-band were carried out with the MIDI beam combiner
attached to the VLTI. Additional interferometric observations which probe the
structure of IRS 9A on larger scales were performed with an aperture mask
installed in the T-ReCS instrument of Gemini South. The spectral energy
distribution (SED) is constrained by the MIDI N-band spectrum and by data from
the Spitzer Space Telescope. Our efforts to model the structure and SED of IRS
9A range from simple geometrical models of the brightness distribution to one-
and two-dimensional radiative transfer computations. The target is resolved by
T-ReCS, with an equivalent (elliptical) Gaussian width of 330mas by 280mas
(2300 AU by 2000 AU). Despite this fact, a warm compact unresolved component
was detected by MIDI which is possibly associated with the inner regions of a
flattened dust distribution. Based on our interferometric data, no sign of
multiplicity was found on scales between about 200AU and 700AU projected
separation. A geometric model consisting of a warm (1000 K) ring (400 AU
diameter) and a cool (140 K) large envelope provides a good fit to the data. No
single model fitting all visibility and photometric data could be found, with
disk models performing better than spherical models. While the data are clearly
inconsistent with a spherical dust distribution they are insufficient to prove
the existence of a disk but rather hint at a more complex dust distribution.Comment: 8 pages, 11 figures. Accepted for publication in A&
Overlap of QRPA states based on ground states of different nuclei --mathematical properties and test calculations--
The overlap of the excited states in quasiparticle random-phase approximation
(QRPA) is calculated in order to simulate the overlap of the intermediate
nuclear states of the double-beta decay. Our basic idea is to use the
like-particle QRPA with the aid of the closure approximation and calculate the
overlap as rigorously as possible by making use of the explicit equation of the
QRPA ground state. The formulation is shown in detail, and the mathematical
properties of the overlap matrix are investigated. Two test calculations are
performed for relatively light nuclei with the Skyrme and volume delta-pairing
energy functionals. The validity of the truncations used in the calculation is
examined and confirmed.Comment: 17 pages, 15 figures, full paper following arXiv:1205.5354 and Phys.
Rev. C 86 (2012) 021301(R
The cool atmospheres of the binary brown dwarf eps Indi B
We have imaged Indi B, the closest brown dwarf binary known, with
VISIR at the VLT in three narrow-band mid-infrared bandpasses located around
8.6m, 10.5m and 11.3m. We are able to spatially resolve both
components, and determine accurate mid-infrared photometry for both components
independently. In particular, our VISIR observations probe the NH feature
in the atmospheres of the cooler and warmer brown dwarfs. For the first time,
we can disentangle the contributions of the two components, and find that % our
photometry of IndiBb is in good agreement with recent ``cloud-free''
atmosphere models having an effective temperature of K.
With an assumed age of 1 Gyr for the Indi system, component Ba
agrees more with K rather than with
K, as suggested by SPITZER spectroscopic observations of
the combined Indi B system (Roellig et al., 2004). Even higher
effective temperatures appear inconsistent with our absolute photometry, as
they would imply an unphysical small size of the brown dwarf IndiBa.Comment: 4 pages, 2 figure
Molecular Outflows and a Mid-Infrared Census of the Massive Star Formation Region Associated with IRAS 18507+0121
We have observed the central region of the infrared-dark cloud filament associated with IRAS 18507+0121 at millimeter wavelengths in CO(J = 1-0), ^(13)CO(J = 1-0), and C^(18)O(J = 1-0) line emission and with Spitzer at mid-infrared wavelengths. Five massive outflows from two cloud cores were discovered. Three outflows are centered on or near an ultracompact (UC) H II region (G34.4+0.23), while the remaining two outflows originate from the millimeter core G34.4+0.23 MM. Modeling of the spectral energy distributions of the mid-infrared sources identified 31 young stellar objects in the filament with a combined stellar mass of ~127 ± 27 M_☉. An additional 22 sources were identified as probable cluster members based on the presence of strong 24 μm emission. The total star formation efficiency in the G34.4 cloud filament is estimated to be ~7%, while the massive and intermediate-mass star formation efficiency in the entire cloud filament is estimated to be roughly 2%. A comparison of the gravitational binding energy with the outflow kinetic energy suggests that the compact core containing G34.4+0.23 MM is being destroyed by its molecular outflows, whereas the outflows associated with the more massive core surrounding the G34.4 UC H II region are not likely to totally disrupt the cloud. In addition, a qualitative evaluation of the region appears to suggest that stars in this region may have formed in two stages: first lower mass stars formed and then, a few Myr later, the more massive stars began to form
Probing the centre of the large circumstellar disc in M17
We investigated the nature of the hitherto unresolved elliptical infrared
emission in the centre of the ~20000 AU disc silhouette in M 17. We combined
high-resolution JHKsL'M' band imaging carried out with NAOS/CONICA at the VLT
with [Fe II] narrow band imaging using SOFI at the NTT. The analysis is
supported by Spitzer/GLIMPSE archival data and by already published SINFONI/VLT
Integral Field Spectroscopy data. For the first time, we resolve the elongated
central infrared emission into a point-source and a jet-like feature that
extends to the northeast in the opposite direction of the recently discovered
collimated H2 jet. They are both orientated almost perpendicular to the disc
plane. In addition, our images reveal a curved southwestern emission nebula
whose morphology resembles that of the previously detected northeastern one.
Both nebulae are located at a distance of 1500 AU from the disc centre. We
describe the infrared point-source in terms of a protostar that is embedded in
circumstellar material producing a visual extinction of 60 <= Av <= 82. The
observed Ks band magnitude is equivalent to a stellar mass range of 2.8 Msun <=
Mstar <= 8 Msun adopting conversions for a main-sequence star. Altogether, we
suggest that the large M 17 accretion disc is forming an intermediate to
high-mass protostar. Part of the accreted material is expelled through a
symmetric bipolar jet/outflow.Comment: 6 pages, 3 figures, accepted by MNRAS (16 May 2008
Near-infrared follow-up to the May 2008 activation of SGR 1627-41
On 28 May 2008, the Swift satellite detected the first reactivation of SGR
1627-41 since its discovery in 1998.
Following this event we began an observing campaign in near infrared
wavelengths to search for a possible counterpart inside the error circle of
this SGR, which is expected to show flaring activity simultaneous to the high
energy flares or at least some variability as compared to the quiescent state.
For the follow-up we used the 0.6m REM robotic telescope at La Silla
Observatory, which allowed a fast response within 24 hours and, through
director discretionary time, the 8.2m Very Large Telescope at Paranal
Observatory. There, we observed with NACO to produce high angular resolution
imaging with the aid of adaptive optics.
These observations represent the fastest near infrared observations after an
activation of this SGR and the deepest and highest spatial resolution
observations of the Chandra error circle.
5 sources are detected in the immediate vicinity of the most precise X-ray
localisation of this source. For 4 of them we do not detect variability,
although the X-ray counterpart experimented a significant decay during our
observation period. The 5th source is only detected in one epoch, where we have
the best image quality, so no variability constrains can be imposed and remains
as the only plausible counterpart. We can impose a limit of Ks > 21.6
magnitudes to any other counterpart candidate one week after the onset of the
activity. Our adaptive optics imaging, with a resolution of 0.2" provides a
reference frame for subsequent studies of future periods of activity.Comment: Accepted for publication in A&
A frozen super-Earth orbiting a star at the bottom of the Main Sequence
We observed the microlensing event MOA-2007-BLG-192 at high angular
resolution in JHKs with the NACO adaptive optics system on the VLT while the
object was still amplified by a factor 1.23 and then at baseline 18 months
later. We analyzed and calibrated the NACO photometry in the standard 2MASS
system in order to accurately constrain the source and the lens star fluxes. We
detect light from the host star of MOA-2007-BLG-192Lb, which significantly
reduces the uncertainties in its char- acteristics as compared to earlier
analyses. We find that MOA-2007-BLG-192L is most likely a very low mass late
type M-dwarf (0.084 [+0.015] [-0.012] M\odot) at a distance of 660 [+100] [-70]
pc orbited by a 3.2 [+5.2] [-1.8] M\oplus super-Earth at 0.66 [+0.51] [-0.22]
AU. We then discuss the properties of this cold planetary system.Comment: published version A&A 540, A78 (2012) A&A, 10 pages, 7 Figure
Investigating grain growth in disks around southern T Tauri stars at millimetre wavelengths
CONTEXT - Low-mass stars form with disks in which the coagulation of grains
may eventually lead to the formation of planets. It is not known when and where
grain growth occurs, as models that explain the observations are often
degenerate. A way to break this degeneracy is to resolve the sources under
study. AIMS - To find evidence for the existence of grains of millimetre sizes
in disks around in T Tauri stars, implying grain growth. METHODS - The
Australia Telescope Compact Array (ATCA) was used to observe 15 southern T
Tauri stars, five in the constellation Lupus and ten in Chamaeleon, at 3.3
millimetre. The five Lupus sources were also observed with the Submillimeter
Array (SMA) at 1.4 millimetre. Our new data are complemented with data from the
literature to determine the slopes of the spectral energy distributions in the
millimetre regime. RESULTS - Ten sources were detected at better than 3sigma
with the ATCA, with sigma ~1-2 mJy, and all sources that were observed with the
SMA were detected at better than 15sigma, with sigma ~4 mJy. Six of the sources
in our sample are resolved to physical radii of ~100 AU. Assuming that the
emission from such large disks is predominantly optically thin, the millimetre
slope can be related directly to the opacity index. For the other sources, the
opacity indices are lower limits. Four out of six resolved sources have opacity
indices <~1, indicating grain growth to millimetre sizes and larger. The masses
of the disks range from < 0.01 to 0.08 MSun, which is comparable to the minimum
mass solar nebula. A tentative correlation is found between the millimetre
slope and the strength and shape of the 10-micron silicate feature, indicating
that grain growth occurs on similar (short) timescales in both the inner and
outer disk.Comment: 13 pages, 7 figures, 5 tables, accepted for publication in A&
Optical Method for Determination of Carrier Density in Modulation Doped Quantum Wells
An optical method is suggested to determine the concentration of
two-dimensional electrons in modulation-doped quantum wells at low and moderate
electron densities between 10^{9} and 2x10^{11} cm^{-2}. The method is based on
an analysis of magneto-reflectivity spectra of charged excitons (trions). The
circular polarization degree and the oscillator strength of the charged
excitons contain information about the density and spin polarization of
two-dimensional electron gas. The method is applied to CdTe/(Cd,Mg)Te and
ZnSe/Zn,Mg)(S,Se) heterostructures
On the infant weight loss of low- to intermediate-mass star clusters
Star clusters are born in a highly compact configuration, typically with
radii of less than about 1 pc roughly independently of mass. Since the
star-formation efficiency is less than 50 per cent by observation and because
the residual gas is removed from the embedded cluster, the cluster must expand.
In the process of doing so it only retains a fraction f_st of its stars. To
date there are no observational constrains for f_st, although Nbody
calculations by Kroupa et al. (2001) suggest it to be about 20-30 per cent for
Orion-type clusters. Here we use the data compiled by Testi et al. (1997, 1998,
1999) for clusters around young Ae/Be stars and by de Wit et al. (2004, 2005)
around young O stars and the study of de Zeeuw et al. (1999) of OB associations
and combine these measurements with the expected number of stars in clusters
with primary Ae/Be and O stars, respectively, using the empirical correlation
between maximal-stellar-mass and star-cluster mass of Weidner & Kroupa (2006).
We find that f_st < 50 per cent with a decrease to higher cluster
masses/more-massive primaries. The interpretation would be that cluster
formation is very disruptive. It appears that clusters with a birth stellar
mass in the range 10 to 10^3 Msun keep at most 50 per cent of their stars.Comment: 17 pages, 5 figures, to be published in MNRA
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