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Register a new userHighly Variable Extinction and Accretion in the Jet-driving Class I Type Young Star PTF 10nvg (V2492 Cyg, IRAS 20496+4354)
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We report extensive new photometry and spectroscopy of the highly variable young stellar object PTF 10nvg including optical and near-infrared time series data as well as mid-infrared and millimeter data. Following the previously reported 2010 rise, during 2011 and 2012 the source underwent additional episodes of brightening and dimming events including prolonged faint states. The observed high-amplitude variations are largely consistent with extinction changes having a 220 day quasi-periodic signal. Spectral evolution includes not only changes in the spectral slope but correlated variation in the prominence of TiO/VO/CO bands and atomic line emission, as well as anticorrelated variation in forbidden line emission which, along with H_2, dominates optical and infrared spectra at faint epochs. Neutral and singly-ionized atomic species are likely formed in an accretion flow and/or impact while the origin of zero-velocity atomic LiI 6707 in emission is unknown. Forbidden lines, including several rare species, exhibit blueshifted emission profiles and likely arise from an outflow/jet. Several of these lines are also seen spatially offset from the continuum source position, presumably in a shocked region of an extended jet. CARMA maps resolve on larger scales a spatially extended outflow in mm-wavelength CO. We attribute the observed photometric and spectroscopic behavior in terms of occultation of the central star as well as the bright inner disk and the accretion/outflow zones that renders shocked gas in the inner part of the jet amenable to observation at the faint epochs. We discuss PTF 10nvg as a source exhibiting both accretion-driven (perhaps analogous to V1647 Ori) and extinction-driven (perhaps analogous to UX Ori or GM Cep) high-amplitude variability phenomena.
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Context. V2492 Cyg is a young eruptive star that went into outburst in 2010. The near-infrared color changes observed since the outburst peak suggest that the source belongs to a newly defined sub-class of young eruptive stars, where time-dependent accretion and variable line-of-sight extinction play a combined role in the flux changes. Aims. In order to learn about the origin of the light variations and to explore the circumstellar and interstellar environment of V2492 Cyg, we monitored the source at ten different wavelengths, between 0.55 mu m and 2.2 mu m from the ground and between 3.6 mu m and 160 mu m from space. Methods. We analyze the light curves and study the color-color diagrams via comparison with the standard reddening path. We examine the structure of the molecular cloud hosting V2492 Cyg by computing temperature and optical depth maps from the far-infrared data. Results. We find that the shapes of the light curves at different wavelengths are strictly self-similar and that the observed variability is related to a single physical process, most likely variable extinction. We suggest that the central source is episodically occulted by a dense dust cloud in the inner disk, and, based on the invariability of the far-infrared fluxes, we propose that it is a long-lived rather than a transient structure. In some respects, V2492 Cyg can be regarded as a young, embedded analog of UX Orionis-type stars. Conclusions. The example of V2492 Cyg demonstrates that the light variations of young eruptive stars are not exclusively related to changing accretion. The variability provided information on an azimuthally asymmetric structural element in the inner disk. Such an asymmetric density distribution in the terrestrial zone may also have consequences for the initial conditions of planet formation.
V2492 Cyg is a young pre-main sequence star presenting repetitive brightness variations of significant amplitude (Delta R > 5 mag) whose physical origin has been ascribed to both extinction (UXor-type) and accretion (EXor-type) variability, although their mutual proportion has not been clarified yet. Recently, V2492 Cyg has reached a level of brightness ever registered in the period of its documented activity. Optical and near-infrared photometry and spectroscopy have been obtained in October 2016 and between March and July 2017. The source has remained bright until the end of May 2017, then it started to rapidly fade since the beginning of June at a rate of about 0.08 mag/day. On mid-July 2017 the source has reached the same low-brightness level as two years before. Extinction and mass accretion rate were derived by means of the luminosity of the brightest lines, in particular Halpha and Hbeta. A couple of optical high-resolution spectra are also presented to derive information on the gas kinematics. Visual extinction variations do not exceed a few magnitudes, while the mass accretion rate is estimated to vary from less than 10^-8 up to a few 10^-7 M_sun/yr. This latter is comparable to that estimated on the previous high-state in 2010, likely occurred under more severe extinction conditions. The combined analysis of the optical and near-infrared (NIR) observations extends to the present event the original suggestion that the V2492 Cyg variability is a combination of changing extinction and accretion.
We present new K-band spectroscopy of the UY Aur binary star system. Our data are the first to show H$_{2}$ emission in the spectrum of UY Aur A and the first to spectrally resolve the Br{gamma} line in the spectrum of UY Aur B. We see an increase in the strength of the Br{gamma} line in UY Aur A and a decrease in Br{gamma} and H$_{2}$ line luminosity for UY Aur B compared to previous studies. Converting Br{gamma} line luminosity to accretion rate, we infer that the accretion rate onto UY Aur A has increased by $2 times 10^{-9}$ M$_{odot}$ yr$^{-1}$ per year since a rate of zero was observed in 1994. The Br{gamma} line strength for UY Aur B has decreased by a factor of 0.54 since 1994, but the K-band flux has increased by 0.9 mags since 1998. The veiling of UY Aur B has also increased significantly. These data evince a much more luminous disk around UY Aur B. If the lower Br{gamma} luminosity observed in the spectrum of UY Aur B indicates an intrinsically smaller accretion rate onto the star, then UY Aur A now accretes at a higher rate than UY Aur B. However, extinction at small radii or mass pile-up in the circumstellar disk could explain decreased Br{gamma} emission around UY Aur B even when the disk luminosity implies an increased accretion rate. In addition to our scientific results for the UY Aur system, we discuss a dedicated pipeline we have developed for the reduction of echelle-mode data from the ARIES spectrograph.
We report on the source Gaia 19ajj, identifying it as a young star associated with a little-studied star-forming region seen along a complex line-of-sight through the Gum Nebula. The optical lightcurve recently recorded by Gaia exhibits a slow and unsteady 5.5 mag rise over about 3 years, while the mid-infrared lightcurve from NEOWISE over the same time period shows a 1.2 mag rise having similar structure. Available color information is inconsistent with pure extinction reduction as the cause for the photometric brightening. Optical spectroscopic characteristics in the current bright phase include: little in the way of absorption except for the hallmark Li I 6707 A signature of youth plus weak e.g. Ca I and notably Ba II; strong wind/outflow in Ca II, Mg I b, Na I D, Halpha, K I, O I; jet signatures in [O I], [S II], [Ca II], [Fe II], and [Ni II]; and narrow rest-velocity emission in neutral species such as Fe I, Ni I, and Mg I. The infrared spectrum is also characterized by outflow and emission, including: a hot He I wind, jet lines such as [Fe II] and H2; and weak narrow rest-velocity atomic line emission. The 12CO bandheads are weakly in emission, but there is also broad H2O absorption. Gaia 19ajj exhibited a previous bright state in the 2010-2012 time frame. The body of photometric and spectroscopic evidence suggests that the source bears resemblance to V2492 Cyg (PTF 10nvg) and PV Cep, both of which similarly experience bright phases that recur on long timescales, with large-amplitude photometric variations and emission-dominated spectra. We interpret the behavior of Gaia 19ajj as caused by cycles of enhanced disk accretion accompanied by reduced extinction.
We analyze the variability in accretion-related emission lines for 40 Classical T Tauri stars to probe the extent of accretion variations in young stellar objects. Our analysis is based on multi-epoch high-resolution spectra for young stars in Tau-Aur and Cha I. For all stars, we obtain typically four spectra, covering timescales from hours to months. As proxies for the accretion rate, we use the H-alpha 10% width and the CaII-8662 line flux. We find that while the two quantities are correlated, their variability amplitude is not. Converted to accretion rates, the CaII fluxes indicate typical accretion rate changes of 0.35 dex, with 32% exceeding 0.5 dex, while H-alpha 10% width suggests changes of 0.65 dex, with 66% exceeding 0.5 dex. We conclude that CaII fluxes are a more robust quantitative indicator of accretion than H-alpha 10% width, and that intrinsic accretion rate changes typically do not exceed 0.5 dex on timescales of days to months. The maximum extent of the variability is reached after a few days, suggesting that rotation is the dominant cause of variability. We see a decline of the inferred accretion rates towards later spectral types, reflecting the dM/dt vs. M relationship. There is a gap between accretors and non-accretors, pointing to a rapid shutdown of accretion. We conclude that the ~2 orders of magnitude scatter in the dM/dt vs. M relationship is dominated by object-to-object scatter instead of intrinsic source variability.
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