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Giannini T.,National institute for astrophysics | Lorenzetti D.,National institute for astrophysics | Antoniucci S.,National institute for astrophysics | Arkharov A.A.,Central Astronomical Observatory of Pulkovo | And 4 more authors.
Astrophysical Journal Letters | Year: 2016

After a long-lasting period of quiescence of about a decade, the source V1118 Ori, one of the most representative members of the EXor variables, is now outbursting. Since the initial increase of the near-infrared flux of about 1 mag (JHK bands) registered on 2015 September 22, the source brightness has remained fairly stable. We estimate ΔV ∼ 3 mag, with respect to the quiescence phase. An optical/near-IR low-resolution spectrum has been obtained with the Large Binocular Telescope instruments MODS and LUCI2 and compared with a spectrum of a similar spectral resolution and sensitivity level taken during quiescence. Together with the enhancement of the continuum, the outburst spectrum presents a definitely higher number of emission lines, in particular H i recombination lines of the Balmer, Paschen, and Brackett series, along with bright permitted lines of several species, forbidden atomic lines, and CO ro-vibrational lines. Both mass accretion and mass-loss rates have significantly increased (by about an order of magnitude: Ṁacc = 1.2-4.8 10-8 M⊙ yr-1, Ṁloss= 0.8-2 10-9 M⊙ yr-1), with respect to the quiescence phase. If compared with previous outbursts, the present one appears less energetic. Alternatively, it could already be in the fading phase (with the maximum brightness level reached when the source was not visible), or, viceversa, still in the rising phase. © 2016. The American Astronomical Society. All rights reserved.


Lorenzetti D.,National institute for astrophysics | Giannini T.,National institute for astrophysics | Larionov V.M.,Central University of Chile | Arkharov A.A.,Central Astronomical Observatory of Pulkovo | And 6 more authors.
Astrophysical Journal | Year: 2011

We present the results of a simultaneous monitoring, lasting more than two years, of the optical and near-infrared photometric and polarimetric activity of the variable protostar PV Cep. During the monitoring period, an outburst with declining phase (ΔJ ≈ 3mag) lasting about 120days occurred in all the photometric bands. A time lag of 30days between optical and infrared light curves has been measured and interpreted in the framework of an accretion event. This latter is directly recognizable in the significant variations of the near-infrared colors, which appear bluer in the outburst phase, when the star dominates the emission, and redder in the declining phase, when the disk emission prevails. All the observational data have been combined to derive a coherent picture of the complex morphology of the whole PV Cep system that is composed of, in addition to the star and the accretion disk, a variable bi-conical nebula. The mutual interaction between all these components is the cause of the high value of the polarization (≈ 20%) and of its fluctuations. The observational data concur to indicate that PV Cep is not a genuine EXor star, but rather a more complex object; moreover, the case of PV Cep leads to arguments about the classification of other recently discovered young sources in outburst that have been considered, perhaps oversimplifying, as EXor. © 2011. The American Astronomical Society. All rights reserved.


Antoniucci S.,National institute for astrophysics | Arkharov A.A.,Central Astronomical Observatory of Pulkovo | Di Paola A.,National institute for astrophysics | Giannini T.,National institute for astrophysics | And 8 more authors.
Astronomy and Astrophysics | Year: 2014

Aims. We report on the ongoing outburst of the young variable V1180 Cas, which is known to display characteristics in common with EXor eruptive variables. We present results that support the scenario of an accretion-driven nature of the brightness variations of the object and provide the first evidence of jet structures around the source. Methods. We monitored the recent flux variations of the target in the RC, J, H, and K bands. New optical and near-IR spectra taken during the current high state of V1180 Cas are presented, in conjunction with H2 narrow-band imaging of the source. Results. Observed near-IR colour variations are analogous to those observed in EXors and consistent with excess emission originating from an accretion event. The spectra show numerous emission lines, which indicates accretion, ejection of matter, and an active disc. Using optical and near-IR emission features we derive a mass accretion rate of ~3 × 10-8M⊙ yr-1, which is an order of magnitude lower than previous estimates. In addition, a mass loss rate of ~4 × 10-9 and ~4 × 10-10M⊙ yr-1 are estimated from atomic forbidden lines and H2, respectively. Our H2 imaging reveals two bright knots of emission around the source and the nearby optically invisible star V1180 Cas B, clearly indicative of mass-loss phenomena. Higher resolution observations of the detected jet will help to clarify whether V1180 Cas is the driving source and to determine the relation between the observed knots. © ESO, 2014.


Lorenzetti D.,National institute for astrophysics | Antoniucci S.,National institute for astrophysics | Giannini T.,National institute for astrophysics | Harutyunyan A.,Fundacion Galileo Galilei INAF | And 8 more authors.
Astrophysical Journal | Year: 2015

V1118 Ori is an eruptive variable belonging to the EXor class of pre-main-sequence stars whose episodic outbursts are attributed to disk accretion events. Since 2006, V1118 Ori has been in the longest quiescence stage ever observed between two subsequent outbursts in its recent history. We present near-infrared photometry of V1118 Ori carried out over the last eight years, along with complete spectroscopic coverage from 0.35 to 2.5 μm. Long term sampling of V1118 Ori in quiescence has never been performed, and hence we can benefit from the current circumstance by determining the lowest values (i.e., the zeroes) of the parameters to be used as reference for evaluating the physical changes typical of more active phases. A quiescence mass accretion rate of 1-3 × 10-9 yr-1 can be derived and the difference from previous determinations is discussed. Based on line emission and IR color analysis, a visual extinction of 1-2 mag is consistently derived, confirming that V1118 Ori (at least in quiescence) is a low-extinction T Tauri star with a bolometric luminosity of about 2.1 . An anti-correlation exists between the equivalent width of the emission lines and the underlying continuum. We searched the literature to evaluate whether or not such behavior is a common feature for the whole class. The anti-correlation is clearly recognizable for all of the available EXors in the optical range (Hβ and H lines); however, this is not as evident in the infrared (Paβ and Brγ lines). The observed anti-correlation supports the accretion-driven mechanism as the most likely to account for continuum variations. © 2015. The American Astronomical Society. All rights reserved.


Lorenzetti D.,National institute for astrophysics | Antoniucci S.,National institute for astrophysics | Giannini T.,National institute for astrophysics | Li Causi G.,National institute for astrophysics | And 6 more authors.
Astrophysical Journal | Year: 2012

We present the results of a comparison between classical and newly identified EXor based on literature data and aimed at recognizing possible differences or similarities between the categories. Optical and near-IR two-color diagrams, modalities of fluctuations, and derived values of the mass accretion rates are indicative of strong similarities between the two samples. We demonstrate how the difference between the outburst and the quiescence spectral energy distribution of all EXor can be well fitted with a single blackbody, as if an additional thermal component appears during the outbursting phase. Temperatures of this additional component span between 1000 and 4500 K, while the radii of the emitting regions (assumed to be a uniform disk) span between 0.01 and 0.1 AU, sizes typical of the inner portions of the circumstellar disk. Spots persisting up to 50% of the outburst duration, not exceeding 10% of the stellar surface, and with temperatures compatible with the EXor mass accretion rates, are able to account for both the appearance of the additional thermal component and the dust sublimation in the inner structures of the disk. We also compare the EXor events with the most significant color and magnitude fluctuations of active T Tauri stars finding that (1) burst accretion phenomena should also be important for this latter class and (2) EXor events could be more frequent than those accidentally discovered. A remarkable case is that of the source V2493 Cyg, a T Tauri star recently identified as a strong outbursting object: New optical and near-IR photometric and spectroscopic data are presented in an attempt to clarify its EXor or FUor nature. © 2012. The American Astronomical Society. All rights reserved.

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