arXiv subjects

Simone Antoniucci

Publications and source records attributed to Simone Antoniucci.

28 records · Page 2Linked to original sources

arXiv2020

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$β$ and Br$γ$) 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.

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arXiv2018

Young Stars and their Variability with LSST

Young stars exhibit short-term photometric variability caused by mass accretion events from circumstellar disks, the presence of dusty warps within the inner disks, starspots that rotate across the stellar surfaces, and flares. Long-term variability also occurs owing to starspot longevity and cycles, and from changes in stellar angular momenta and activity as the stars age. We propose to observe the Carina star-forming region in different bands with a cadence of 30 minutes every night for one week per year to clarify the nature of both the short-term and long-term variability of the thousands of young stars in this region. By obtaining well-sampled multicolor lightcurves of this dense young cluster, LSST would acquire the first statistically significant data on how these objects vary on both short and long timescales. This information will allow us to relate the observed variability to stellar properties such as mass, age, binarity, and to environmental properties such as location within or exterior to the H II region, and to the presence or absence of a circumstellar disk.

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arXiv2017

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.

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arXiv2014

On-Sky Tests of an A/R Coated Silicon Grism on board NICS@TNG

We present the results of our project for the design and construction and on-sky test of silicon grisms. The fabrication of such devices is a complex and critical process involving litho-masking, anisotropic etching and direct bonding techniques. After the successful fabrication of the silicon grating, we have optimized the bonding of the grating onto the hypotenuse of a silicon prism to get the final prototype. After some critical phases during the experimentation a silicon grism with 363 grooves/mm and a blaze angle of 14 degrees has been eventually fabricated. The application of an A/R coating on both the surfaces has been the last step: this procedure is critical because of the groove geometry of the diffraction grating, whose performace might be compromised by the coating. Then, the grism was inserted in the filter wheel of the Near Infrared camera NICS, at the focal plane of the National Galileo Telescope (TNG), the 3.5 m Italian facility in the Canary Islands (E). The result of the on-sky tests are given in detail.

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arXiv2014

Mid- and Far-Infrared Variability of PV Cep

We present the collection of all the mid- and far-IR observations (3-170 um) of the young eruptive variable PV Cep available so far in the literature. These data allow us to confirm that flux variability is a prominent feature at mid-IR wavelength (3-25 um). Color-magnitude plots clearly indicate that the observed variability is not extinction-driven, but mainly influenced by fluctuations of the mass accretion rate. We interpret such variability as due to a hot spot created onto the stellar surface by the column of accreting matter, which heats the inner parts of the disk and determines the observed increase of the near- mid-IR luminosity. A quantitative characterization is given for both the spot itself and the additional thermal component created by it. Far-IR data (60-170 um) are consistent with the presence of a temperature stratification in a massive and quite un-evolved circumstellar disk.

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arXiv2013

WISE photometry of EXor sources and candidates

We present a collection of WISE photometry of EXor sources and candidates (more recently identified). This represents the first complete survey of such objects in the mid-IR (3.4 - 22 um) that was carried out with the same instrumentation. Two-color diagrams constructed with WISE data evidence a clear segregation between classical and newly identified sources, being these latter characterized by colder (and less evolved) circumstellar disks. By combining 2MASS and WISE data, we obtain spectral energy distributions (SED's) that are compatible with the existence of an inner hole in the circumstellar disk. A compilation of all EXor observations given in the literature at wavelengths very similar to those of WISE is also provided. This allows us to study their mid-IR variability, which has been poorly investigated so far and without any coordination with shorter wavelengths surveys. The presented it WISE photometry and the compilation of the literature data are intended as a first step toward the construction of a significant database in this spectral regime. Preliminary indications on the mechanisms responsible for the luminosity fluctuations are provided.

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arXiv2012

Image reconstruction for observations with a high dynamic range: LINC-NIRVANA simulations of a stellar jet

We report the results of a simulation and reconstruction of observations of a young stellar object (YSO) jet with the LINC-NIRVANA (LN) interferometric instrument, which will be mounted on the Large Binocular Telescope (LBT). This simulation has been performed in order to investigate the ability of observing the weak diffuse jet line emission against the strong IR stellar continuum through narrow band images in the H and K atmospheric windows. In general, this simulation provides clues on the image quality that could be achieved in observations with a high dynamic range. In these cases, standard deconvolution methods, such as Richardson-Lucy, do not provide satisfactory results: we therefore propose here a new method of image reconstruction. It consists in considering the image to be reconstructed as the sum of two terms: one corresponding to the star (whose position is assumed to be known) and the other to the jet. A regularization term is introduced for this second component and the reconstruction is obtained with an iterative method alternating between the two components. An analysis of the results shows that the image quality obtainable with this method is significantly improved with respect to standard deconvolution methods, reducing the number of artifacts and allowing us to reconstruct the original jet intensity distribution with an error smaller than 10%.

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arXiv2011

The nature of the embedded intermediate-mass T Tauri star DK Cha

Most of our knowledge about star formation is based on studies of low-mass stars, whereas very little is known about the properties of the circumstellar material around young and embedded intermidiate-mass T Tauri stars (IMTTSs). We present an analysis of the excitation and accretion properties of the young IMTTS DK Cha. The nearly face-on configuration of this source allows us to have direct access to the star-disk system through the excavated envelope and outflow cavity. Based on LR optical and IR spectroscopy obtained with SofI/EFOSC2 on the NTT we derive the spectrum of DK Cha from ~0.6 to 2.5 μm. From the detected lines we probe the conditions of the gas that emits the HI IR emission lines. In addition, we derive the mass accretion rate (Macc) from the relationships that connect the luminosity of the Brγ and Paβ lines with the accretion luminosity (Lacc). The observed optical/IR spectrum is extremely rich in forbidden and permitted atomic and molecular emission lines, which makes this source similar to very active low-mass T Tauri stars. Some of the permitted emission lines are identified as being excited by fluorescence. We derive Brackett decrements and compare them with different excitation mechanisms. The Paβ/Brγ ratio is consistent with optically thick emission in LTE at a temperature of ~3500 K, originated from a compact region of ~5 Rsun in size: but the line opacity decreases in the Br lines for high quantum numbers n_{up}. A good fit to the data is obtained assuming an expanding gas in LTE, with an electron density at the wind base of ~10^13 cm-3. In addition, we find that the observed Brackett ratios are very similar to those reported in previous studies of low-mass CTTSs and Class I sources, indicating that these ratios are not dependent on masses and ages. Finally, Lacc~9 Lsun and Macc~3x10-7 Msun/yr values were found.

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arXiv2004

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.

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arXiv2004

Observations of the Brackett decrement in the Class I source HH100 IR

The Brackett decrement in the Class I source HH100 IR has been observed and analyzed to set constraints on the origin of the IR HI emission in this young object. We have used both low resolution (R ~ 800) observations of the Brackett lines from Brg, to Br24, and medium resolution (R~9000) spectra of the Brg, Br12 and Br13 lines. The dereddened fluxes indicates that the lines remain moderately thick up to high quantum numbers. Moreover, the profiles of the three lines observed in medium resolution are all broad and nearly symmetric, with a trend for the lines at high n-number to be narrower than the Brg, line. With the assumption that the three lines have different optical depths and consequently trace zones at different physical depths, we interprete the observed profiles as evidence that the ionized gas velocity in the HI emitting region is increasing as we move outwards, as expected in an accelerating wind more than in an infalling gas. We have modelled the observed line ratios and velocities with a simplified model for the HI excitation in an expanding circumstellar gas. Such a comparison indicates that the observations are consistent with the emission coming from a very compact region of 4-6 solar radii, where the gas has been already accelerated to velocities of the order of 200 \kms, with an associated mass flow rate of the ionized component of the order of 10^{-7} solar masses per year. This implies that the observed lines should originate either from a stellar wind or from the inner part of a disk wind, providing that the disk inner truncation radius is close to the stellar surface.

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