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 $10^{-5}$ contrast planets, from $100$ to $300$ mas separations, under poor and highly variable seeing conditions ($0.8$ to $1.5$ arcsec FWHM) and in only $20$ min of acquisition. A comparison with a standard ADI approach shows that the contrast limit is improved by a factor of $5$. 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$_{\star}$ ~ 0.1 - 0.2 M$_{\odot}$. We obtained spectroscopic and photometric data with VLT/ISAAC and VLT/SINFONI in the near-infrared ($J$, $H$, and $K$ bands) across four epochs (2005, 2010, 2013, and 2014). We used accretion-tracing lines (Pa$\beta$ and Br$\gamma$) and outflow-tracing lines (H$_2$ 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 $K$ band, followed in 2014 by a steep decrease. Consistently, the mass accretion rate ($\dot{M}_{acc}$) rose by an order of magnitude from ~ 10$^{-8}$ M$_{\odot}$ yr$^{-1}$ to ~ $10^{-7}$ M$_{\odot}$ yr$^{-1}$ between the two early epochs. The visual extinction ($A_V$) has also increased from ~ 15 mag in 2005 to ~ 24 mag in 2013. This rise in $A_V$ in tandem with the increase in $\dot{M}_{acc}$ 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

The Relation between the Mass Accretion Rate and the Disk Mass in Class I Protostars

Evidence of a relation between the mass accretion rate and the disk mass is established for young, Class II pre-main-sequence stars. This observational result opened an avenue to test theoretical models and constrain the initial conditions of disk formation, fundamental in the understanding of the emergence of planetary systems. However, it is becoming clear that planet formation starts even before the Class II stage, in disks around Class 0 and I protostars. We show for the first time evidence for a correlation between the mass accretion rate and the disk mass for a large sample of Class I young stars located in nearby (<500 pc) star-forming regions. We fit our sample, finding that the Class I object relation has a slope flatter than Class II stars, and the former have higher mass accretion rates and disk masses. The results are put in context of disk evolution models