228 research outputs found
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
A network of filaments detected by Herschel in the Serpens core : a laboratory to test simulations of low-mass star formation
V.R. was partly supported by the DLR grant number 50 OR 1109 and by the Bayerische Gleichstellungsförderung (BGF). This research was partly supported by the Priority Programme 1573 âPhysics of the Interstellar Mediumâ of the German Science Foundation (DFG), the DFG cluster of excellence âOrigin and Structure of the Universeâ and by the Italian Ministero dellâIstruzione, UniversitĂ e Ricerca through the grant Progetti Premiali 2012 -iALMA (CUP C52I13000140001). C.E. is partly supported by Spanish Grants AYA 2011-26202 and AYA 2014-55840-P.Context. Filaments represent a key structure during the early stages of the star formation process. Simulations show that filamentary structures commonly formed before and during the formation of cores. Aims. The Serpens core is an ideal laboratory for testing the state of the art of simulations of turbulent giant molecular clouds. Methods. We used Herschel observations of the Serpens core to compute temperatureand column density maps of the region. We selected the early stages of are cent simulation of star-formation, before stellar feedback was initiated, with similar total mass and physical size as the Serpens core. We also derived temperature and column density maps from the simulations. The observed distribution of column densities of the filaments was analyzed, first including and then masking the cores. The same analysis was performed on the simulations as well. Results. A radial network of filaments was detected in the Serpens core. The analyzed simulation shows a striking morphological resemblance to the observed structures. The column density distribution of simulated filaments without cores shows only a log-normal distribution, while the observed filaments show a power-law tail. The power-law tail becomes evident in the simulation if the focus is only the column density distribution of the cores. In contrast, the observed cores show a flat distribution. Conclusions. Even though the simulated and observed filaments are subjectively similar-looking, we find that they behave in very different ways. The simulated filaments are turbulence-dominated regions; the observed filaments are instead self-gravitating structures that will probably fragment into cores.Publisher PDFPeer reviewe
Stellar and circumstellar properties of visual binaries in the Orion Nebula Cluster
Our general understanding of multiple star and planet formation is primarily
based on observations of young multiple systems in low density regions like
Tau-Aur and Oph. Since many, if not most, of the stars are born in clusters,
observational constraints from young binaries in those environments are
fundamental for understanding both the formation of multiple systems and
planets in multiple systems throughout the Galaxy. We build upon the largest
survey for young binaries in the Orion Nebula Cluster (ONC) which is based on
Hubble Space Telescope observations to derive both stellar and circumstellar
properties of newborn binary systems in this cluster environment. We present
Adaptive Optics spatially-resolved JHKL'-band photometry and K-band
R\,5000 spectra for a sample of 8 ONC binary systems from this database.
We characterize the stellar properties of binary components and obtain a census
of protoplanetary disks through K-L' color excess. For a combined sample of ONC
binaries including 7 additional systems with NIR spectroscopy from the
literature, we derive mass ratio and relative age distributions. We compare the
stellar and circumstellar properties of binaries in ONC with those in Tau-Aur
and Oph from samples of binaries with stellar properties derived for each
component from spectra and/or visual photometry and with a disk census obtained
through K-L color excess. The mass ratio distribution of ONC binaries is found
to be indistinguishable from that of Tau-Aur and, to some extent, to that of
Oph in the separation range 85-560\,AU and for primary mass in the range 0.15
to 0.8\,M_{\sun}.A trend toward a lower mass ratio with larger separation is
suggested in ONC binaries which is not seen in Tau-Aur binaries.The components
of ONC binaries are found to be significantly more coeval than the overall ONC
population and as coeval as components of binaries in Tau-Aur and Oph[...]Comment: Accepted for publication in 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
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
Nanotechnology versus stem cell engineering: in vitro comparison of neurite inductive potentials.
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
Sculpting the disk around T Cha: an interferometric view
(Abridged) Circumstellar disks are believed to be the birthplace of planets
and are expected to dissipate on a timescale of a few Myr. The processes
responsible for the removal of the dust and gas will strongly modify the radial
distribution of the dust and consequently the SED. In particular, a young
planet will open a gap, resulting in an inner disk dominating the near-IR
emission and an outer disk emitting mostly in the far-IR. We analyze a full set
of data (including VLTI/Pionier, VLTI/Midi, and VLT/NaCo/Sam) to constrain the
structure of the transition disk around TCha. We used the Mcfost radiative
transfer code to simultaneously model the SED and the interferometric
observations. We find that the dust responsible for the emission in excess in
the near-IR must have a narrow temperature distribution with a maximum close to
the silicate sublimation temperature. This translates into a narrow inner dusty
disk (0.07-0.11 AU). We find that the outer disk starts at about 12 AU and is
partially resolved by the Pionier, Sam, and Midi instruments. We show that the
Sam closure phases, interpreted as the signature of a candidate companion, may
actually trace the asymmetry generated by forward scattering by dust grains in
the upper layers of the outer disk. These observations help constrain the
inclination and position angle of the outer disk. The presence of matter inside
the gap is difficult to assess with present-day observations. Our model
suggests the outer disk contaminates the interferometric signature of any
potential companion that could be responsible for the gap opening, and such a
companion still has to be unambiguously detected. We stress the difficulty to
observe point sources in bright massive disks, and the consequent need to
account for disk asymmetries (e.g. anisotropic scattering) in model-dependent
search for companions.Comment: Removed the word "first" in the abstract of the paper: "obtained with
the first 4-telescope combiner (VLTI/Pionier)
Variable accretion as a mechanism for brightness variations in T Tau S
(Note: this is a shortened version of the original A&A-style structured
abstract). The physical nature of the strong photometric variability of T Tau
Sa, the more massive member of the Southern "infrared companion" to T Tau, has
long been debated. Intrinsic luminosity variations due to variable accretion
were originally proposed but later challenged in favor of apparent fluctuations
due to time-variable foreground extinction. In this paper we use the timescale
of the variability as a diagnostic for the underlying physical mechanism.
Because the IR emission emerging from Sa is dominantly thermal emission from
circumstellar dust at <=1500K, we can derive a minimum size of the region
responsible for the time-variable emission. In the context of the variable
foreground extinction scenario, this region must be (un-) covered within the
variability timescale, which implies a minimum velocity for the obscuring
foreground material. If this velocity supercedes the local Kepler velocity we
can reject foreground extinction as a valid variability mechanism. The variable
accretion scenario allows for shorter variability timescales since the
variations in luminosity occur on much smaller scales, essentially at the
surface of the star, and the disk surface can react almost instantly on the
changing irradiation with a higher or lower dust temperature and according
brightness. We have detected substantial variations at long wavelengths in T
Tau S: +26% within four days at 12.8 micron. We show that this short-term
variability cannot be due to variable extinction and instead must be due to
variable accretion. Using a radiative transfer model of the Sa disk we show
that variable accretion can in principle also account for the much larger
(several magnitude) variations observed on timescales of several years. For the
long-term variability, however, also variable foreground extinction is a viable
mechanism.Comment: 15 pages, 8 figures, Accepted for publication in Astronomy and
Astrophysic
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