43 research outputs found
VINCI-VLTI measurements of HR 4049: the physical size of the circumbinary envelope
We present the first detection of the envelope which surrounds the post-AGB
binary source HR 4049. VINCI-VLTI K-band interferometric observations of this
source imply the existence of a large structure with a Gaussian angular FWHM
22.4 +/- 1.4 mas or uniform disk diameter of 34.9 +/- 1.9 mas. With the
Hipparcos parallax of 1.50 +/- 0.64 mas these values correspond to a physical
size of 14.9 (+11.1,-4.4) AU and 23.3 (+17.3,-7.0) AU, respectively. Our
measurements, covering an azimuth range of about 60 degrees, for the
sky-projected baseline, provide information on the geometry of the emitting
region and show that there is only a slight variation of the measured angular
values along the different directions sampled. Thus, our results are consistent
with a spherical geometry of the envelope. However, we cannot completely rule
out the existence of an asymmetric envelope (like the circumbinary disk
envisaged by some recent models) because of the limited spatial frequency and
azimuth range covered by the observations.Comment: 4 pages, 4 postscript figures, accepted letter for A&
Star-disk interaction in young stars with E-ELT/HIRES. A preview from X-Shooter spectra
High-resolution multi-band spectra are a fundamental tool to investigate the inner regions of young stellar objects (YSOs) and thus probe the processes that involve interaction between star and disk, like the accretion and ejection of matter. Recent works with VLT/X-Shooter have shown the potential of wide-band simultaneous spectroscopic observations to provide a full characterization of both the stellar and accretion properties of YSOs. X-Shooter results give us an exciting preview of what E-ELT and HIRES will achieve with a much more increased spatial/spectral resolution and sensitivity. Exploiting HIRES high-resolution coupled with spatially-resolved information at the diffraction limit of the E-ELT will reveal the dynamics, chemistry, and physical conditions of the innermost regions of the disks, thus providing unprecedented constraints on the physics of the accretion, on the jet-launching mechanism and on the planetary formation. A few scientific cases for which HIRES is expected to provide breakthrough results are presented
SFADI: the Speckle-Free Angular Differential Imaging method
We present a new processing technique aimed at significantly improving the
angular differential imaging method (ADI) in the context of high-contrast
imaging of faint objects nearby bright stars in observations obtained with
extreme adaptive optics (EXAO) systems. This technique, named "SFADI" for
"Speckle-Free ADI", allows to improve the achievable contrast by means of
speckles identification and suppression. This is possible in very high cadence
data, which freeze the atmospheric evolution. Here we present simulations in
which synthetic planets are injected into a real millisecond frame rate
sequence, acquired at the LBT telescope at visible wavelength, and show that
this technique can deliver low and uniform background, allowing unambiguous
detection of contrast planets, from to mas separations,
under poor and highly variable seeing conditions ( to arcsec FWHM)
and in only min of acquisition. A comparison with a standard ADI approach
shows that the contrast limit is improved by a factor of . We extensively
discuss the SFADI dependence on the various parameters like speckle
identification threshold, frame integration time, and number of frames, as well
as its ability to provide high-contrast imaging for extended sources, and also
to work with fast acquisitions.Comment: Accepted for publication in Ap
Investigating episodic accretion in a very low-mass young stellar object
Very low-mass Class I protostars have been investigated very little thus far.
Variability of these young stellar objects (YSOs) and whether or not they are
capable of strong episodic accretion is also left relatively unstudied. We
investigate accretion variability in IRS54, a Class I very low-mass protostar
with a mass of M ~ 0.1 - 0.2 M. We obtained spectroscopic
and photometric data with VLT/ISAAC and VLT/SINFONI in the near-infrared (,
, and bands) across four epochs (2005, 2010, 2013, and 2014). We used
accretion-tracing lines (Pa and Br) and outflow-tracing lines
(H and [FeII] to examine physical properties and kinematics of the object.
A large increase in luminosity was found between the 2005 and 2013 epochs of
more than 1 magnitude in the band, followed in 2014 by a steep decrease.
Consistently, the mass accretion rate () rose by an order of
magnitude from ~ 10 M yr to ~ M
yr between the two early epochs. The visual extinction () has also
increased from ~ 15 mag in 2005 to ~ 24 mag in 2013. This rise in in
tandem with the increase in is explained by the lifting up of a
large amount of dust from the disc of IRS54, following the augmented accretion
and ejection activity in the YSO, which intersects our line of sight due to the
almost edge-on geometry of the disc. Because of the strength and timescales
involved in this dramatic increase, this event is believed to have been an
accretion burst possibly similar to bursts of EXor-type objects. IRS54 is the
lowest mass Class I source observed to have an accretion burst of this type,
and therefore potentially one of the lowest mass EXor-type objects known so
far
Spitzer spectral line mapping of protostellar outflows: II H2 emission in L1157
We present an analysis of Spitzer-IRS spectroscopic maps of the L1157
protostellar outflow in the H2 pure-rotational lines from S(0) to S(7). The aim
of this work is to derive the physical conditions pertaining to the warm
molecular gas and study their variations within the flow. The mid-IR H2
emission follows the morphology of the precessing flow, with peaks correlated
with individual CO clumps and H2 2.12{\mu}m ro-vibrational emission. More
diffuse emission delineating the CO cavities is detected only in the low-laying
transitions, with J(lower) less or equal to 2. The H2 line images have been
used to construct 2D maps of N(H2), H2 ortho-to-para ratio and temperature
spectral index beta, in the assumption of a gas temperature stratification
where the H2 column density varies as T^(beta). Variations of these parameters
are observed along the flow. In particular, the ortho-to-para ratio ranges from
0.6 to 2.8, highlighting the presence of regions subject to recent shocks where
the ortho-to-para ratio has not had time yet to reach the equilibrium value.
Near-IR spectroscopic data on ro-vibrational H2 emission have been combined
with the mid-IR data and used to derive additional shock parameters in the
brightest blue- and red-shifted emission knots. A high abundance of atomic
hydrogen (H/H2 about 0.1-0.3) is implied by the observed H2 column densities,
assuming n(H2) values as derived by independent SiO observations. The presence
of a high fraction of atomic hydrogen, indicates that a partially-dissociative
shock component should be considered for the H2 excitation in these localized
regions. However, planar shock models, either of C- or J-type, are not able to
consistently reproduce all the physical parameters derived from our analysis of
the H2 emission. Globally, H2 emission contributes to about 50% of the total
shock radiated energy in the L1157 outflow.Comment: 31 pages, 9 figure, Accepted for publication on Ap
Fast cadence speckle-free high-contrast imaging: SFADI and SFI
We present the research and developement status of the Speckle-Free Angular Differential Imaging method (SFADI), that we developed for the SHARK-VIS high-contrast imager for the LBT telescope. The technique bases on the acquisition of kHz frame-rate image sequences, which we combine in post-processing after speckle identification and suppression in each frame. With respect to the standard angular differential imaging, this method reaches a much smoother residual background and hence higher detection contrast at a given signal-to-noise ratio. Furthermore, it can reveal faint extended sources around bright central stars, and can use de-rotated images as well as quick second-lasting sequences. We reached a contrast of around 1e-5 for integration times of the order of tens of minutes at 100 mas for a 5.7 magnitude star, as we demonstrated on both a real-sky acquisition and at the SHARK-VIS laboratory test bench. Such long sequences though produces a large amount of data (around a million frames every 15 minutes) that we manage to processed in a reasonable computation time with the described implementation scheme
High-resolution TNG spectra of T Tauri stars. Near-IR GIANO observations of the young variables XZ Tauri and DR Tauri
Aims: We aim to characterise the star-disk interaction region in T Tauri stars that show photometric and spectroscopic variability. Methods: We used the GIANO instrument at the Telescopio Nazionale Galileo to obtain near-infrared high-resolution spectra (R 50 000) of XZ Tau and DR Tau, which are two actively accreting T Tauri stars classified as EXors. Equivalent widths and profiles of the observed features are used to derive information on the properties of the inner disk, the accretion columns, and the winds. Results: Both sources display composite H I line profiles, where contributions from both accreting gas and high-velocity winds can be recognised. These lines are progressively more symmetric and narrower with increasing upper energy which may be interpreted in terms of two components with different decrements or imputed to self-absorption effects. XZ Tau is observed in a relatively high state of activity with respect to literature observations. The variation of the He I 1.08 ÎŒm line blue-shifted absorption, in particular, suggests that the inner wind has undergone a dramatic change in its velocity structure, connected with a recent accretion event. DR Tau has a more stable wind as its He I 1.08 ÎŒm absorption does not show variations with time in spite of strong variability of the emission component. The IR veiling in the two sources can be interpreted as due to blackbody emission at temperatures of 1600 K and 2300 K for XZ Tau and DR Tau, respectively, with emitting areas 30 times larger than the central star. While for XZ Tau these conditions are consistent with emission from the inner rim of the dusty disk, the fairly high temperature inferred for DR Tau might suggest that its veiling originates from a thick gaseous disk located within the dust sublimation radius. Strong and broad metallic lines, mainly from C I and Fe I, are detected in XZ Tau, similar to those observed in other EXor sources during burst phases. At variance, DR Tau shows weaker and narrower metallic lines, despite its larger accretion luminosity. This suggests that accretion is not the only driver of metallic line excitation. Conclusions: The presented observations demonstrate the potential of wide-band, high-resolution near-IR spectroscopy to simultaneously probe the different phenomena that occur in the interaction region between the stellar magnetosphere and the accretion disk, thus providing hints on how these two structures are linked to each other