63 research outputs found
An unusual very low-mass high-amplitude pre-main sequence periodic variable
We have investigated the nature of the variability of CHS7797, an unusual
periodic variable in the Orion Nebula Cluster. An extensive I-band photometric
data set of CHS7797 was compiled between 2004-2010 using various telescopes.
Further optical data have been collected in R and z' bands. In addition,
simultaneous observations of the ONC region including CHS7797 were performed in
the I, J, Ks and IRAC [3.6] and [4.5] bands over a time interval of about 40d.
CHS7797 shows an unusual large-amplitude variation of about 1.7 mag in the R,
I, and z' bands with a period 17.786. The amplitude of the brightness
modulation decreases only slightly at longer wavelengths. The star is faint
during 2/3 of the period and the shape of the phased light-curves for seven
different observing seasons shows minor changes and small-amplitude variations.
Interestingly, there are no significant colour-flux correlations for
wavelengths smaller than 2microns, while the object becomes redder when fainter
at longer wavelengths. CHS7797 has a spectral type of M6 and an estimated mass
between 0.04-0.1Msun. The analysis of the data suggests that the periodic
variability of CHS7797 is most probably caused by an orbital motion.
Variability as a result of rotational brightness modulation by spots is
excluded by the lack of any color-brightness correlation in the optical. The
latter indicates that CHS7797 is most probably occulted by circumstellar matter
in which grains have grown from typical 0.1 microns to 1-2 micron sizes. We
discuss two possible scenarios in which CHS7797 is periodically eclipsed by
structures in a disc, namely that CHS7797 is a single object with a
circumstellar disc, or that CHS7797 is a binary system, similar to KH15D, in
which an inclined circumbinary disc is responsible of the variability. Possible
reasons for the typical 0.3mag variations in I-band at a given phase are
discussed.Comment: 11 pages, 9 figures, accepted for publication A&
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
The origin of mid-infrared emission in massive young stellar objects: multi-baseline VLTI observations of W33A
The circumstellar structure on 100 AU scales of the massive young stellar
object W33A is probed using the VLTI and the MIDI instrument. N-band
visibilities on 4 baselines are presented which are inconsistent with a
spherically symmetric geometry. The visibility spectra and SED are
simultaneously compared to 2D axi-symmetric dust radiative transfer models with
a geometry including a rotationally flattened envelope and outflow cavities. We
assume an O7.5 ZAMS star as the central source, consistent with the observed
bolometric luminosity. The observations are also compared to models with and
without (dusty and gaseous) accretion disks. A satisfactory model is
constructed which reproduces the visibility spectra for each (u,v) point. It
fits the silicate absorption, the mid-IR slope, the far-infrared peak, and the
(sub)mm of the SED. It produces a 350 micron morphology consistent with
observations. The 10 micron emission on 100 AU scales is dominated by the
irradiated walls of the cavity sculpted by the outflow. The visibilities rule
out the presence of dust disks with total (gas and dust) masses more than 0.01
Msun. However, optically thick accretion disks, interior to the dust
sublimation radius, are allowed to accrete at rates equalling the envelope's
mass infall rate (up to 10^(-3) Msun/yr) without substantially affecting the
visibilities due to the extinction by the extremely massive envelope of W33A.Comment: Accepted for publication in A&
The Mid-IR spatially resolved environment of OH26.5+0.6 at maximum luminosity
We present observations of the famous OH/IR star OH26.5+0.6 obtained using
the Mid-Infrared Interferometric Instrument MIDI at the European Southern
Observatory (ESO) Very Large Telescope Interferometer VLTI. The emission of the
dusty envelope, spectrally dispersed at a resolution of 30 from 8 to 13.5
micron, appears resolved by a single dish UT telescope. In particular the
angular diameter increases strongly within the silicate absorption band.
Moreover an acquisition image taken at 8.7 micron exhibits, after
deconvolution, a strong asymmetry. The axis ratio is 0.75+/-0.07 with the FWHM
of the major and minor axis which are 286mas and 214mas respectively. The
measured PA angle, 95 degrees +/-6 degrees is reminiscent of the asymmetry in
the OH maser emission detected at 1612MHz by Bowers & Johnston (1990) for this
star. In interferometric mode the UT1-UT3 102m baseline was employed to detect
the presence of the star. No fringes have been found with a detection threshold
estimated to be of the order of 1% of the total flux of the source, i.e. 5-8
Jy. These observations were carried out during the phase of maximum luminosity
of the star, when the dust shell is more diluted and therefore the chance to
detect the central source maximized. We modeled the dusty environment based on
the work of Justannont et al. (1996). In particular, the failure to detect
fringes provides strong constraints on the opacities in the inner regions of
the dust shell or in the close vicinity of the star.Comment: Accepted in A&
Mid-infrared sizes of circumstellar disks around Herbig Ae/Be stars measured with MIDI on the VLTI
We present the first long baseline mid-infrared interferometric observations of the circumstellar disks surrounding Herbig Ae/Be stars. The observations were obtained using the mid-infrared interferometric instrument MIDI at the European Southern Observatory (ESO) Very Large Telescope Interferometer VLTI on Cerro Paranal. The 102 m baseline given by the telescopes UT1 and UT3 was employed, which provides a maximum full spatial resolution of 20 milli-arcsec (mas) at a wavelength of 10 μm. The interferometric signal was spectrally dispersed at a resolution of 30, giving spectrally resolved visibility information from 8 μm to 13.5 μm. We observed seven nearby Herbig Ae/Be stars and resolved all objects. The warm dust disk of HD 100546 could even be resolved in single-telescope imaging. Characteristic dimensions of the emitting regions at 10 μm are found to be from 1 AU to 10 AU. The 10 μm sizes of our sample stars correlate with the slope of the 10–25 μm infrared spectrum in the sense that the reddest objects are the largest ones. Such a correlation would be consistent with a different geometry in terms of flaring or flat (self-shadowed) disks for sources with strong or moderate mid-infrared excess, respectively. We compare the observed spectrally resolved visibilities with predictions based on existing models of passive centrally irradiated hydrostatic disks made to fit the SEDs of the observed stars. We find broad qualitative agreement of the spectral shape of visibilities corresponding to these models with our observations. Quantitatively, there are discrepancies that show the need for a next step in modelling of circumstellar disks, satisfying both the spatial constraints such as are now available from the MIDI observations and the flux constraints from the SEDs in a consistent way
The sub-arcsecond dusty environment of Eta Carinae
The core of the nebula surrounding Eta Carinae has been observed with the VLT
Adaptive Optics system NACO and with the interferometer VLTI/MIDI to constrain
spatially and spectrally the warm dusty environment and the central object. In
particular, narrow-band images at 3.74 and 4.05 micron reveal the butterfly
shaped dusty environment close to the central star with unprecedented spatial
resolution. A void whose radius corresponds to the expected sublimation radius
has been discovered around the central source. Fringes have been obtained in
the Mid-IR which reveal a correlated flux of about 100Jy situated 0.3"
south-east of the photocenter of the nebula at 8.7 micron, which corresponds
with the location of the star as seen in other wavelengths. This correlated
flux is partly attributed to the central object, and these observations provide
an upper limit for the SED of the central source from 2.2 to 13.5 micron.
Moreover, we have been able to spectrally disperse the signal from the nebula
itself at PA=318 degree, i.e. in the direction of the bipolar nebula 310
degree) within the MIDI field of view of 3". A large amount of corundum (Al2O3)
is discovered, peaking at 0.6-1.2" south-east from the star, whereas the dust
content of the Weigelt blobs is dominated b silicates. We discuss the
mechanisms of dust formation which are closely related to the geometry of this
Butterfly nebulae
Zur systematischen Stellung der Tannenh\ue4her ( Nucifraga caryocatactes) des Balkans und der sowjetischen Karpaten
Volume: 20Start Page: 69End Page: 7
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