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Register a new userEvidence for T Tauri-like emission in the EXor V1118 Ori from near-IR and X-ray data
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D. Lorenzetti
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We present a near-IR study of the EXor variable V1118 Ori, performed by following a slightly declining phase after a recent outburst. In particular, the near-IR (0.8 - 2.3 micron) spectrum, obtained for the first time, shows a large variety of emission features of the HI and HeI recombination and CO overtone. By comparing the observed spectrum with a wind model, a mass loss rate value is derived along with other parameters whose values are typical of an accreting T Tauri star. In addition, we have used X-ray data from the XMM archive, taken in two different epochs during the declining phase monitored in IR. X-ray emission (in the range 0.5 - 10 keV) permits to derive several parameters which confirm the T Tauri nature of the source. In the near-IR the object maintains a low visual extinction during all the activity phases, confirming that variable extinction does not contribute to brightness variations. The lack of both a significant amount of circumstellar material and any evidence of IR cooling from collimated jet/outflow driven by the source, indicates that, at least this member of the EXor class, is in a late stage of the Pre-Main Sequence evolution. In the X-ray regime, an evident fading is present, detected in the post-outburst phase, that cannot be reconciled with the presence of any absorbing material. This circumstance, combined with the persistence (in the pre- and post-outburst phases) of a temperature component at about 10 MK, suggests that accretion has some influence in regulating the coronal activity.
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After a quiescence period of about 10 years, the classical EXor source V1118 Ori has undergone an accretion outburst in 2015 September. The maximum brightness (DV > 4 mag) was reached in 2015 December and was maintained for several months. Since 2016 September, the source is in a declining phase. Photometry and low/ high-resolution spectroscopy were obtained with MODS and LUCI2 at the {it Large Binocular Telescope}, with the facilities at the Asiago 1.22 and 1.82 m telescopes, and with GIANO at the {it Telescopio Nazionale Galileo}. The spectra are dominated by emission lines of hi and neutral metallic species. From line and continuum analysis we derive the mass accretion rate and its evolution during the outburst. Considering that extinction may vary between 1.5 and 2.9 mag, we obtain m_acc= 0.3$-$2.0 10$^{-8}$ m_sun/yr, in quiescence and m_acc= 0.2$-$1.9 10$^{-6}$ m_sun/yr, at the outburst peak. The Balmer decrement shape has been interpreted by means of line excitation models, finding that from quiescence to outburst peak, the electron density has increased from $sim$ 2 10$^9$ cm$^{-3}$ to $sim$ 4 10$^{11}$ cm$^{-3}$. The profiles of the metallic lines are symmetric and narrower than 100 km s$^{-1}$, while hi, and hei,,lines show prominent wings extending up to $pm$ 500 km s$^{-1}$. The metallic lines likely originate at the base of the accretion columns, where neutrals are efficiently shielded against the ionizing photons, while faster ionized gas is closer to the star. Outflowing activity is testified by the detection of a variable P Cyg-like profile of the H$alpha$ and hei, 1.08,$mu$m lines.
We investigate the relationship between the IR observed properties of the EXor variables and the mechanisms active during their evolutionary stage. To this aim, we have constructed a catalog of all the IR (1-100 micron) photometric and spectroscopic observations appearing during the last 30 years in the literature. New results of our monitoring program based on near- and mid-IR photometry and near-IR spectroscopy and polarimetry of one object (V1118 Ori) are presented, complementing those given in a previous paper and related to a different activity period. Our catalog indicates how the database accumulated so far is inadequate for any statistical study of the EXor events. Nevertheless, all the observational evidence can be interpreted into a coherent scheme. The sources that present the largest brightness variations tend to become bluer while brightening. The scenario of disk accretion based on viscous friction between particles agrees with the observations. The new results on V1118 Ori confirm such a general view. The striking novelty is represented by a near-IR spectrum of V1118 Ori taken 1 yr after the last monitored outburst: any emission line previously detected has now totally disappeared at our sensitivity. For the same source, mid-IR photometry is provided here for the first time and allows us to construct a meaningful SED. The first polarimetric data show that V1118 Ori is intrinsically polarized and its spotted, magnetized surface becomes recognizable during the less active phases.
We present results from a multi-wavelength campaign to monitor the 2005 outburst of the low-mass young star V1118 Ori. Although our campaign covers the X-ray, optical, infrared, and radio regimes, we focus in this Letter on the properties of the X-ray emission in V1118 Ori during the first few months after the optical outburst. Chandra and XMM-Newton detected V1118 Ori at three epochs in early 2005. The X-ray flux and luminosity stayed similar within a factor of two, and at the same level as in a pre-outburst observation in 2002. The hydrogen column density showed no evidence for variation from its modest pre-outburst value of $N_mathrm{H} sim 3 times 10^{21}$ cm$^{-2}$. However, a spectral change occurred from a dominant hot plasma ($sim 25$ MK) in 2002 and in January 2005 to a cooler plasma ($sim 8$ MK) in February 2005 and in March 2005. We hypothesize that the hot magnetic loops high in the corona were disrupted by the closing in of the accretion disk due to the increased accretion rate during the outburst, whereas the lower cooler loops were probably less affected and became the dominant coronal component.
We study the properties of X-ray emitting plasma of MP Mus, an old classical T Tauri star. We aim at checking whether an accretion process produces the observed X-ray emission and at deriving the accretion parameters and the characteristics of the shock-heated plasma. We compare the properties of MP Mus with those of younger classical T Tauri stars to test whether age is related to the properties of the X-ray emission plasma. XMM-Newton X-ray spectra allows us to measure plasma temperatures, abundances, and electron density. In particular the density of cool plasma probes whether X-ray emission is produced by plasma heated in the accretion process. X-ray emission from MP Mus originates from high density cool plasma but a hot flaring component is also present, suggesting that both coronal magnetic activity and accretion contribute to the observed X-ray emission. We find a Ne/O ratio similar to that observed in the much younger classical T Tauri star BP Tau. From the soft part of the X-ray emission, mostly produced by plasma heated in the accretion shock, we derive a mass accretion rate of 5x10^{-11} M_{sun} yr^{-1}.
We present pre- and post-outburst observations of the new FU Orionis-like young stellar object PTF 10qpf (also known as LkHa 188-G4 and HBC 722). Prior to this outburst, LkHa 188-G4 was classified as a classical T Tauri star on the basis of its optical emission-line spectrum superposed on a K8-type photosphere, and its photometric variability. The mid-infrared spectral index of LkHa 188-G4 indicates a Class II-type object. LkHa 188-G4 exhibited a steady rise by ~1 mag over ~11 months starting in Aug. 2009, before a subsequent more abrupt rise of > 3 mag on a time scale of ~2 months. Observations taken during the eruption exhibit the defining characteristics of FU Orionis variables: (i) an increase in brightness by > 4 mag, (ii) a bright optical/near-infrared reflection nebula appeared, (iii) optical spectra are consistent with a G supergiant and dominated by absorption lines, the only exception being Halpha which is characterized by a P Cygni profile, (iv) near-infrared spectra resemble those of late K--M giants/supergiants with enhanced absorption seen in the molecular bands of CO and H_2O, and (v) outflow signatures in H and He are seen in the form of blueshifted absorption profiles. LkHa 188-G4 is the first member of the FU Orionis-like class with a well-sampled optical to mid-infrared spectral energy distribution in the pre-outburst phase. The association of the PTF 10qpf outburst with the previously identified classical T Tauri star LkHa 188-G4 (HBC 722) provides strong evidence that FU Orionis-like eruptions represent periods of enhanced disk accretion and outflow, likely triggered by instabilities in the disk. The early identification of PTF 10qpf as an FU Orionis-like variable will enable detailed photometric and spectroscopic observations during its post-outburst evolution for comparison with other known outbursting objects.
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