129 research outputs found
The multiplicity of massive stars in the Orion Nebula Cluster as seen with long-baseline interferometry
This is the final version of the article. Available from EDP Sciences via the DOI in this record.Context. The characterization of multiple stellar systems is an important ingredient for testing current star formation models. Stars are more often found in multiple systems, the more massive they are. A complete knowledge of the multiplicity of high-mass stars over the full range of orbit separations is thus essential to understand their still debated formation process.
Aims. Infrared long baseline interferometry is very well suited to close the gap between spectroscopic and adaptive optics searches. Observations of the Orion Nebula Cluster (ONC) in general and the Trapezium Cluster in particular can help to answer the question about the origin and evolution of multiple stars. Earlier studies provide a good knowledge about the multiplicity of the stars at very small (spectroscopic companions) and large separations (AO, speckle companions) and thus make the ONC a good target for such a project.
Methods. We used the near infrared interferometric instrument AMBER at ESOs Very Large Telescope Interferometer to observe a sample of bright stars in the ONC. We complement our data set by archival NACO observations of θ1 Ori A to obtain more information about the orbit of the close visual companion.
Results. Our observations resolve the known multiple systems θ1 Ori C and θ1 Ori A and provide new orbit points, which confirm the predicted orbit and the determined stellar parameters for θ1 Ori C. Combining AMBER and NACO data for θ1 Ori A we were able to follow the (orbital) motion of the companion from 2003 to 2011. We furthermore find hints for a companion around θ1 Ori D, whose existence has been suggested already before, and a previously unknown companion to NU Ori. With a probability of ~90% we can exclude further companions with masses of ≥ 3 M⊙ around our sample stars for separations between ~2 mas and ~110 mas.
Conclusions. We conclude that the companion around θ1 Ori A is most likely physically related to the primary star and not only a chance projected star. The newly discovered possible companions further increase the multiplicity in the ONC. For our sample of two O and three B-type stars we find on average 2.5 known companions per primary, which is around five times more than for low-mass stars.
Key words: binaries:We gratefully acknowledge funding of this work by the German Deutsche Forschungsgemeinschaft, DFG project number PR 569/8-1. K.G.H. acknowledges support provided by the Proyecto FONDECYT Postdoctoral No. 3120153 and the Polish Nacional Science Center grant No. 2011/03/N/ST9/01819. This research has made use of the Jean-Marie Mariotti Center AMBER data reduction package3 and LITpro4 service co-developed by CRAL, LAOG and FIZEAU. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. We thank the referee Douglas Gies for the helpful und constructive comments, which helped to improve the paper
Mid-infrared interferometry of the massive young stellar object NGC 2264 IRS 1
The optically invisible infrared-source NGC 2264 IRS 1 is thought to be a
massive young stellar object (~10 Msun). Although strong infrared excess
clearly shows that the central object is surrounded by large amounts of
circumstellar material, no information about the spatial distribution of this
circumstellar material has been available until now. We used the ESO Very Large
Telescope Interferometer to perform long-baseline interferometric observations
of NGC 2264 IRS 1 in the mid-infrared regime. Our observations resolve the
circumstellar material around NGC 2264 IRS 1, provide the first direct
measurement of the angular size of the mid-infrared emission, and yield direct
constraints on the spatial distribution of the dust. We use different
approaches (a geometrical model, a temperature-gradient model, and radiative
transfer models) to jointly model the observed interferometric visibilities and
the spectral energy distribution. The derived visibility values between ~0.02
and ~0.3 show that the mid-infrared emission is clearly resolved. The
characteristic size of the MIR-emission region is ~30-60 AU; this value is
typical for other YSOs with similar or somewhat lower luminosities. A
comparison of the sizes for the two position angles shows a significant
elongation of the dust distribution. Simple spherical envelope models are
therefore inconsistent with the data. The radiative transfer modeling of our
data suggests that we observe a geometrically thin and optically thick
circumstellar disk with a mass of about 0.1 Msun. Our modeling indicates that
NGC 2264 IRS 1 is surrounded by a flat circumstellar disk that has properties
similar to disks typically found around lower-mass young stellar objects. This
result supports the assumption that massive young stellar objects form via
accretion from circumstellar disks.Comment: 11 pages, accepted for publication in A&
Masses of the components of SB2 binaries observed with Gaia. II. Masses derived from PIONIER interferometric observations for Gaia validation
In anticipation of the Gaia astrometric mission, a sample of spectroscopic
binaries is being observed since 2010 with the Sophie spectrograph at the
Haute--Provence Observatory. Our aim is to derive the orbital elements of
double-lined spectroscopic binaries (SB2s) with an accuracy sufficient to
finally obtain the masses of the components with relative errors as small as 1
% when combined with Gaia astrometric measurements. In order to validate the
masses derived from Gaia, interferometric observations are obtained for three
SB2s in our sample with F-K components: HIP 14157, HIP 20601 and HIP 117186.
The masses of the six stellar components are derived. Due to its edge-on
orientation, HIP 14157 is probably an eclipsing binary. We note that almost all
the derived masses are a few percent larger than the expectations from the
standard spectral-type-mass calibration and mass-luminosity relation. Our
calculation also leads to accurate parallaxes for the three binaries, and the
Hipparcos parallaxes are confirmed.Comment: 10 pages, 3 figures, accepted by MNRA
Probing the envelopes of massive young stellar objects with diffraction limited mid-infrared imaging
Massive stars form whilst they are still embedded in dense envelopes. As a
result, the roles of rotation, mass loss and accretion in massive star
formation are not well understood. This study evaluates the source of the
Q-band, lambda=19.5 microns, emission of massive young stellar objects (MYSOs).
This allows us to determine the relative importance of rotation and outflow
activity in shaping the circumstellar environments of MYSOs on 1000 AU scales.
We obtained diffraction limited mid-infrared images of a sample of 20 MYSOs
using the VLT/VISIR and Subaru/COMICS instruments. For these 8 m class
telescopes and the sample selected, the diffraction limit, ~0.6", corresponds
to approximately 1000 AU. We compare the images and the spectral energy
distributions (SEDs) observed to a 2D, axis-symmetric dust radiative transfer
model that reproduces VLTI/MIDI observations of the MYSO W33A. We vary the
inclination, mass infall rate, and outflow opening angle to simultaneously
recreate the behaviour of the sample of MYSOs in the spatial and spectral
domains. The mid-IR emission of 70 percent of the MYSOs is spatially resolved.
In the majority of cases, the spatial extent of their emission and their SEDs
can be reproduced by the W33A model featuring an in-falling, rotating dusty
envelope with outflow cavities. There is independent evidence that most of the
sources which are not fit by the model are associated with ultracompact HII
regions and are thus more evolved. We find that, in general, the diverse 20
micron morphology of MYSOs can be attributed to warm dust in the walls of
outflow cavities seen at different inclinations. This implies that the warm
dust in the outflow cavity walls dominates the Q-band emission of MYSOs. In
turn, this emphasises that outflows are an ubiquitous feature of massive star
formation.Comment: Accepted for publication in A&A. The images in this version have been
compressed. A high resolution version is available on reques
Monitoring the Dusty S-Cluster Object (DSO/G2) on its Orbit towards the Galactic Center Black Hole
We analyse and report in detail new near-infrared (1.45 - 2.45 microns)
observations of the Dusty S-cluster Object (DSO/G2) during its approach to the
black hole at the center of the Galaxy that were carried out with ESO
VLT/SINFONI between February and September 2014. Before May 2014 we detect
spatially compact Br-gamma and Pa-alpha line emission from the DSO at about
40mas east of SgrA*. The velocity of the source, measured from the red-shifted
emission, is 2700+-60 km/s. No blue-shifted emission above the noise level is
detected at the position of SgrA* or upstream the presumed orbit. After May we
find spatially compact Br-gamma blue-shifted line emission from the DSO at
about 30mas west of SgrA* at a velocity of -3320+-60 km/s and no indication for
significant red-shifted emission. We do not detect any significant extension of
velocity gradient across the source. We find a Br-gamma-line full width at half
maximum of 50+-10 Angstroem before and 15+-10 Angstroem after the peribothron
transit, i.e. no significant line broadening with respect to last year is
observed. Br-gamma line maps show that the bulk of the line emission originates
from a region of less than 20mas diameter. This is consistent with a very
compact source on an elliptical orbit with a peribothron time passage in
2014.39+-0.14. For the moment, the flaring activity of the black hole in the
near-infrared regime has not shown any statistically significant increment.
Increased accretion activity of SgrA* may still be upcoming. We discuss details
of a source model according to which the DSO is rather a young accreting star
than a coreless gas and dust cloud.Comment: 32 pages - 3 tables - 17 figure - accepted by Ap
The 2014 interferometric imaging beauty contest
This is the final version of the article. Available from SPIE via the DOI in this record.From Conference Volume 9146: Optical and Infrared Interferometry IV. Jayadev K. Rajagopal; Michelle J. Creech-Eakman; Fabien Malbet. Montréal, Quebec, Canada, June 22, 2014Here we present the results of the 6th biennial optical interferometry imaging beauty contest. Taking advantage of a unique opportunity, the red supergiant VY CMa and the Mira variable R Car were observed in the astronomical H-band with three 4-telescope configurations of the VLTI-AT array using the PIONIER instrument. The community was invited to participate in the subsequent image reconstruction and interpretation phases of the project. Ten groups submitted entries to the beauty contest, and we found reasonable consistency between images obtained from independent workers using quite different algorithms. We also found that significant differences existed between the submitted images, much greater than in past beauty contests that were all based on simulated data. A novel crowd-sourcing" method allowed consensus median images to be constructed, filtering likely artifacts and retaining real features." We definitively detect strong spots on the surfaces of both stars as well as distinct circumstellar shells of emission (likely water/CO) around R Car. In a close contest, Joel Sanchez (IAA-CSIC/Spain) was named the winner of the 2014 interferometric imaging beauty contest. This process has shown that new comers" can use publicly-available imaging software to interpret VLTI/PIONIER imaging data, as long as sufficient observations are taken to have complete uv coverage { a luxury that is often missing. We urge proposers to request adequate observing nights to collect sufficient data for imaging and for time allocation committees to recognise the importance of uv coverage for reliable interpretation of interferometric data. We believe that the result of the proposed broad international project will contribute to inspiring trust in the image reconstruction processes in optical interferometry
Nutrients cause grassland biomass to outpace herbivory
Human activities are transforming grassland biomass via changing climate, elemental nutrients, and herbivory. Theory predicts that food-limited herbivores will consume any additional biomass stimulated by nutrient inputs ('consumer-controlled'). Alternatively, nutrient supply is predicted to increase biomass where herbivores alter community composition or are limited by factors other than food ('resource-controlled'). Using an experiment replicated in 58 grasslands spanning six continents, we show that nutrient addition and vertebrate herbivore exclusion each caused sustained increases in aboveground live biomass over a decade, but consumer control was weak. However, at sites with high vertebrate grazing intensity or domestic livestock, herbivores consumed the additional fertilization-induced biomass, supporting the consumer-controlled prediction. Herbivores most effectively reduced the additional live biomass at sites with low precipitation or high ambient soil nitrogen. Overall, these experimental results suggest that grassland biomass will outstrip wild herbivore control as human activities increase elemental nutrient supply, with widespread consequences for grazing and fire risk
Blinded by the light: on the relationship between CO first overtone emission and mass accretion rate in massive young stellar objects
To date, there is no explanation as to why disc-tracing CO first overtone (or ‘bandhead’) emission is not a ubiquitous feature in low- to medium-resolution spectra of massive young stellar objects (MYSOs), but instead is only detected towards approximately 25 per cent of their spectra. In this paper, we investigate the hypothesis that only certain mass accretion rates result in detectable bandhead emission in the near-infrared spectra of MYSOs. Using an analytic disc model combined with an LTE model of the CO emission, we find that high accretion rates (≳10⁻⁴ M⊙ yr⁻¹) result in large dust sublimation radii, a larger contribution to the K-band continuum from hot dust at the dust sublimation radius, and therefore correspondingly lower CO emission with respect to the continuum. On the other hand, low accretion rates (≲10⁻⁶ M⊙ yr⁻¹) result in smaller dust sublimation radii, a correspondingly smaller emitting area of CO, and thus also lower CO emission with respect to the continuum. In general, moderate accretion rates produce the most prominent, and therefore detectable, CO first overtone emission. We compare our findings to a recent near-infrared spectroscopic survey of MYSOs, finding results consistent with our hypothesis. We conclude that the detection rate of CO bandhead emission in the spectra of MYSOs could be the result of MYSOs exhibiting a range of mass accretion rates, perhaps due to the variable accretion suggested by recent multi-epoch observations of these objects
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