156 research outputs found
Chandra X-ray observation of the HII region Gum 31 in the Carina Nebula complex
(abridged) We used the Chandra observatory to perform a deep (70 ksec) X-ray
observation of the Gum 31 region and detected 679 X-ray point sources. This
extends and complements the X-ray survey of the central Carina nebula regions
performed in the Chandra Carina Complex Project. Using deep near-infrared
images from our recent VISTA survey of the Carina nebula complex, our Spitzer
point-source catalog, and optical archive data, we identify counterparts for
75% of these X-ray sources. Their spatial distribution shows two major
concentrations, the central cluster NGC 3324 and a partly embedded cluster in
the southern rim of the HII region, but majority of X-ray sources constitute a
rather homogeneously distributed population of young stars. Our color-magnitude
diagram analysis suggests ages of ~1-2 Myr for the two clusters, whereas the
distributed population shows a wider age range up to ~10 Myr. We also identify
previously unknown companions to two of the three O-type members of NGC 3324
and detect diffuse X-ray emission in the region. Our results suggests that the
observed region contains about 4000 young stars in total. The distributed
population is probably part of the widely distributed population of ~ 1-10 Myr
old stars, that was identified in the CCCP area. This implies that the global
stellar configuration of the Carina nebula complex is a very extended stellar
association, in which the (optically prominent) clusters contain only a
minority of the stellar population.Comment: Accepted for publication in Astronomy & Astrophysics. A high quality
preprint is available at
http://www.usm.uni-muenchen.de/people/preibisch/publications.htm
Disc Clearing of Young Stellar Objects: Evidence for Fast Inside-out Dispersal
The time-scale over which and the modality by which young stellar objects
(YSOs) disperse their circumstellar discs dramatically influences the eventual
formation and evolution of planetary systems. By means of extensive radiative
transfer (RT) modelling, we have developed a new set of diagnostic diagrams in
the infrared colour-colour plane (K-[24] vs. K-[8]), to aid with the
classification of the evolutionary stage of YSOs from photometric observations.
Our diagrams allow the differentiation of sources with unevolved (primordial)
discs from those evolving according to different clearing scenarios (e.g.
homologous depletion vs. inside-out dispersal), as well as from sources that
have already lost their disc. Classification of over 1500 sources in 15 nearby
star-forming regions reveals that approximately 39 % of the sources lie in the
primordial disc region, whereas between 31 % and 32 % disperse from the
inside-out and up to 22 % of the sources have already lost their disc. Less
than 2 % of the objects in our sample lie in the homogeneous draining regime.
Time-scales for the transition phase are estimated to be typically a few 10^5
years independent of stellar mass. Therefore, regardless of spectral type, we
conclude that currently available infrared photometric surveys point to fast
(of order 10 % of the global disc lifetime) inside-out clearing as the
preferred mode of disc dispersal.Comment: 31 pages, 21 figures, 6 tables, accepted for publication in MNRA
High spatial resolution mid-infrared observations of the low-mass young star TW Hya
We want to improve knowledge of the structure of the inner few AU of the
circumstellar disk around the nearby T Tauri star TW Hya. Earlier studies have
suggested the existence of a large inner hole, possibly caused by interactions
with a growing protoplanet. We used interferometric observations in the N-band
obtained with the MIDI instrument on the Very Large Telescope Interferometer,
together with 10 micron spectra recorded by the infrared satellite Spitzer. The
fact that we were able to determine N-band correlated fluxes and visibilities
for this comparatively faint source shows that MIR interferometry can be
applied to a large number of low-mass young stellar objects.
The MIR spectra obtained with Spitzer reveal emission lines from HI (6-5), HI
(7-6), and [Ne II] and show that over 90% of the dust we see in this wavelength
regime is amorphous. According to the correlated flux measured with MIDI, most
of the crystalline material is in the inner, unresolved part of the disk, about
1 AU in radius. The visibilities exclude the existence of a very large (3-4 AU
radius) inner hole in the circumstellar disk of TW Hya, which was required in
earlier models. We propose instead a geometry of the inner disk where an inner
hole still exists, but at a much reduced radius, with the transition from zero
to full disk height between 0.5 and 0.8 AU, and with an optically thin
distribution of dust inside. Such a model can comply with SED and MIR
visibilities, as well as with visibility and extended emission observed in the
NIR at 2 micron. If a massive planet was the reason for this inner hole, as has
been speculated, its orbit would have to be closer to the star than 0.3 AU.
Alternatively, we may be witnessing the end of the accretion phase and an early
phase of an inward-out dispersal of the circumstellar disk.Comment: 13 pages, 9 figures, accepted by A&
Multiwavelength interferometric observations and modeling of circumstellar disks
We investigate the structure of the innermost region of three circumstellar
disks around pre-main sequence stars HD 142666, AS 205 N, and AS 205 S. We
determine the inner radii of the dust disks and, in particular, search for
transition objects where dust has been depleted and inner disk gaps have formed
at radii of a few tenths of AU up to several AU. We performed interferometric
observations with IOTA, AMBER, and MIDI in the infrared wavelength ranges
1.6-2.5um and 8-13um with projected baseline lengths between 25m and 102m. The
data analysis was based on radiative transfer simulations in 3D models of young
stellar objects (YSOs) to reproduce the spectral energy distribution and the
interferometric visibilities simultaneously. Accretion effects and disk gaps
could be considered in the modeling approach. Results from previous studies
restricted the parameter space. The objects of this study were spatially
resolved in the infrared wavelength range using the interferometers. Based on
these observations, a disk gap could be found for the source HD 142666 that
classifies it as transition object. There is a disk hole up to a radius of
R_in=0.30AU and a (dust-free) ring between 0.35AU and 0.80AU in the disk of HD
142666. The classification of AS 205 as a system of classical T Tauri stars
could be confirmed using the canonical model approach, i. e., there are no
hints of disk gaps in our observations.Comment: accepted by Astronomy & Astrophysic
Mid-infrared interferometry of massive young stellar objects. I. VLTI and Subaru observations of the enigmatic object M8E-IR
[abridged] Our knowledge of the inner structure of embedded massive young
stellar objects is still quite limited. We attempt here to overcome the spatial
resolution limitations of conventional thermal infrared imaging. We employed
mid-infrared interferometry using the MIDI instrument on the ESO/VLTI facility
to investigate M8E-IR, a well-known massive young stellar object suspected of
containing a circumstellar disk. Spectrally dispersed visibilities in the 8-13
micron range were obtained at seven interferometric baselines. We resolve the
mid-infrared emission of M8E-IR and find typical sizes of the emission regions
of the order of 30 milli-arcseconds (~45 AU). Radiative transfer simulations
have been performed to interpret the data. The fitting of the spectral energy
distribution, in combination with the measured visibilities, does not provide
evidence for an extended circumstellar disk with sizes > 100 AU but requires
the presence of an extended envelope. The data are not able to constrain the
presence of a small-scale disk in addition to an envelope. In either case, the
interferometry measurements indicate the existence of a strongly bloated,
relatively cool central object, possibly tracing the recent accretion history
of M8E-IR. In addition, we present 24.5 micron images that clearly distinguish
between M8E-IR and the neighbouring ultracompact HII region and which show the
cometary-shaped infrared morphology of the latter source. Our results show that
IR interferometry, combined with radiative transfer modelling, can be a viable
tool to reveal crucial structure information on embedded massive young stellar
objects and to resolve ambiguities arising from fitting the SED.Comment: 7 pages, 5 figures, accepted for publication in A&A, new version
after language editing, one important reference added, conclusions unchange
Mid-infrared interferometric variability of DG Tau: implications for the inner-disk structure
Context. DG Tau is a low-mass pre-main sequence star, whose strongly
accreting protoplanetary disk exhibits a so-far enigmatic behavior: its
mid-infrared thermal emission is strongly time-variable, even turning the 10
m silicate feature from emission to absorption temporarily. Aims. We look
for the reason for the spectral variability at high spatial resolution and at
multiple epochs. Methods. We study the temporal variability of the mid-infrared
interferometric signal, observed with the VLTI/MIDI instrument at six epochs
between 2011 and 2014. We fit a geometric disk model to the observed
interferometric signal to obtain spatial information about the disk. We also
model the mid-infrared spectra by template fitting to characterize the profile
and time dependence of the silicate emission. We use physically motivated
radiative transfer modeling to interpret the mid-infrared interferometric
spectra. Results. The inner disk (r<1-3 au) spectra exhibit a 10 m
absorption feature related to amorphous silicate grains. The outer disk (r>1-3
au) spectra show a crystalline silicate feature in emission, similar to the
spectra of comet Hale-Bopp. The striking difference between the inner and outer
disk spectral feature is highly unusual among T Tauri stars. The mid-infrared
variability is dominated by the outer disk. The strength of the silicate
feature changed by more than a factor of two. Between 2011 and 2014 the
half-light radius of the mid-infrared-emitting region decreased from 1.15 to
0.7 au. Conclusions. For the origin of the absorption we discuss four possible
explanations: a cold obscuring envelope, an accretion heated inner disk, a
temperature inversion on the disk surface and a misaligned inner geometry. The
silicate emission in the outer disk can be explained by dusty material high
above the disk plane, whose mass can change with time, possibly due to
turbulence in the disk.Comment: 16 pages, 13 figure
FU Orionis - The MIDI/VLTI Perspective
We present the first mid-infrared interferometric measurements of FU Orionis.
We clearly resolve structures that are best explained with an optically thick
accretion disk. A simple accretion disk model fits the observed SED and
visibilities reasonably well and does not require the presence of any
additional structure such as a dusty envelope. The inclination and also the
position angle of the disk can be constrained from the multibaseline
interferometric observations. Our disk model is in general agreement with most
published near-infrared interferometric measurements. From the shape and
strength of the 8-13 micrometer spectrum the dust composition of the accretion
disk is derived for the first time. We conclude that most dust particles are
amorphous and already much larger than those typically observed in the ISM.
Although the high accretion rate of the system provides both, high temperatures
out to large radii and an effective transport mechanism to distribute
crystalline grains, we do not see any evidence for crystalline silicates
neither in the total spectrum nor in the correlated flux spectra from the inner
disk regions. Possible reasons for this non-detection are mentioned. All
results are discussed in context with other high-spatial resolution
observations of FU Ori and other FU Ori objects. We also address the question
whether FU Ori is in a younger evolutionary stage than a classical TTauri star.Comment: 41 pages (aastex style), 11 figures, 8 tables, accepted by Ap
The environment of the fast rotating star Achernar - Thermal infrared interferometry with VLTI/MIDI and SIMECA modeling
Context: As is the case of several other Be stars, Achernar is surrounded by
an envelope, recently detected by near-IR interferometry.
Aims: We search for the signature of circumstellar emission at distances of a
few stellar radii from Achernar, in the thermal IR domain.
Methods: We obtained interferometric observations on three VLTI baselines in
the N band (8-13 mic), using the MIDI instrument.
Results: From the measured visibilities, we derive the angular extension and
flux contribution of the N band circumstellar emission in the polar direction
of Achernar. The interferometrically resolved polar envelope contributes 13.4
+/- 2.5 % of the photospheric flux in the N band, with a full width at half
maximum of 9.9 +/- 2.3 mas (~ 6 Rstar). This flux contribution is in good
agreement with the photometric IR excess of 10-20% measured by fitting the
spectral energy distribution. Due to our limited azimuth coverage, we can only
establish an upper limit of 5-10% for the equatorial envelope. We compare the
observed properties of the envelope with an existing model of this star
computed with the SIMECA code.
Conclusions: The observed extended emission in the thermal IR along the polar
direction of Achernar is well reproduced by the existing SIMECA model. Already
detected at 2.2mic, this polar envelope is most probably an observational
signature of the fast wind ejected by the hot polar caps of the star.Comment: A&A Letter, in pres
The 2008 outburst in the young stellar system ZCMa: I. Evidence of an enhanced bipolar wind on the AU-scale
Accretion is a fundamental process in star formation. Although the time
evolution of accretion remains a matter of debate, observations and modelling
studies suggest that episodic outbursts of strong accretion may dominate the
formation of the protostar. Observing young stellar objects during these
elevated accretion states is crucial to understanding the origin of unsteady
accretion. ZCMa is a pre-main-sequence binary system composed of an embedded
Herbig Be star, undergoing photometric outbursts, and a FU Orionis star. The
Herbig Be component recently underwent its largest optical photometric outburst
detected so far. We aim to constrain the origin of this outburst by studying
the emission region of the HI Brackett gamma line, a powerful tracer of
accretion/ejection processes on the AU-scale in young stars. Using the
AMBER/VLTI instrument at spectral resolutions of 1500 and 12 000, we performed
spatially and spectrally resolved interferometric observations of the hot gas
emitting across the Brackett gamma emission line, during and after the
outburst. From the visibilities and differential phases, we derive
characteristic sizes for the Brackett gamma emission and spectro-astrometric
measurements across the line, with respect to the continuum. We find that the
line profile, the astrometric signal, and the visibilities are inconsistent
with the signature of either a Keplerian disk or infall of matter. They are,
instead, evidence of a bipolar wind, maybe partly seen through a disk hole
inside the dust sublimation radius. The disappearance of the Brackett gamma
emission line after the outburst suggests that the outburst is related to a
period of strong mass loss rather than a change of the extinction along the
line of sight. Based on these conclusions, we speculate that the origin of the
outburst is an event of enhanced mass accretion, similar to those occuring in
EX Ors and FU Ors.Comment: Accepted for publication in Astronomy and Astrophysics Letter
Spatially resolved mid-infrared observations of the triple system T Tauri
To enhance our knowledge of the characteristics and distribution of the
circumstellar dust associated with the individual components of the young
hierarchical triple system T Tau, observations in the N-band with MIDI at the
VLTI were performed. Our study is based on both the interferometric and the
spectrophotometric measurements and is supplemented by new visual and infrared
photometry. Also, the phases were investigated to determine the dominating
mid-infrared source in the close southern binary. The data were fit with the
help of a sophisticated physical disc model. This model utilises the radiative
transfer code MC3D that is based on the Monte-Carlo method. Extended
mid-infrared emission is found around all three components of the system.
Simultaneous fits to the photometric and interferometric data confirm the
picture of an almost face-on circumstellar disc around T Tau N. Towards this
star, the silicate band is seen in emission. This emission feature is used to
model the dust content of the circumstellar disc. Clear signs of dust
processing are found. Towards T Tau S, the silicate band is seen in absorption.
This absorption is strongly pronounced towards the infrared companion T Tau Sa
as can be seen from the first individual N-band spectra for the two southern
components. Our fits support the previous suggestion that an almost edge-on
disc is present around T Tau Sa. This disc is thus misaligned with respect to
the circumstellar disc around T Tau N. The interferometric data indicate that
the disc around T Tau Sa is oriented in the north-south direction, which
favours this source as launching site for the east-western jet. We further
determine from the interferometric data the relative positions of the
components of the southern binary.Comment: 24 pages, 19 figures, accepted for publication in A&
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