V899 Mon: An Outbursting Protostar with a Peculiar Light Curve, and Its Transition Phases (original) (raw)
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A multi-wavelength study of the young star V1118 Orionis in outburst
Astronomy and Astrophysics, 2010
Context. The accretion history of low-mass young stars is not smooth but shows spikes of accretion that can last from months and years to decades and centuries. Aims. Observations of young stars in outbursts can help us understand the temporal evolution of accreting stars and the interplay between the accretion disk and the stellar magnetosphere. Methods. The young late-type star V1118 Orionis was in outburst from 2005 to 2006. We followed the outburst with optical and near-infrared photometry; the X-ray emission was further probed with observations taken with XMM-Newton and Chandra during and after the outburst. In addition, we obtained mid-infrared photometry and spectroscopy with Spitzer at the peak of the outburst and in the post-outburst phase.
Outflows from Young Stars: Theory and Observation
Symposium - International Astronomical Union, 2004
Recent observations have revealed that young stellar objects are associated with jet-like structures and Herbig-Haro objects emitting at wavelengths ranging from optical lines to radio continua. These phenomena are similar in morphologies, and have mostly comparable energetics, dynamics, and kinematics. Probing such phenomena observed at various wavelengths with self-consistent physical and radiative processes arising within an inner disk-wind driven magnetocentrifugally from the circumstellar accretion disk is a challenge for confronting theory and observation of outflows. How such early outflow phase may play a role in forming planetary materials may help solve puzzles posed by meteorites. We will discuss the relevant observations, theoretical foundations for modelling approaches, magnetic structures and dynamical effects, and the connection to the early solar system.
The 2008 outburst in the young stellar system Z CMa
Astronomy and Astrophysics, 2010
Context. Accretion is a fundamental process in star formation. Although the time evolution of accretion remains a matter of debate, observations and modelling studies suggest that episodic outbursts of strong accretion may dominate the formation of the central protostar. Observing young stellar objects during these elevated accretion states is crucial to understanding the origin of unsteady accretion. Aims. Z CMa is a pre-main-sequence binary system composed of an embedded Herbig Be star, undergoing photometric outbursts, and a FU Orionis star. This system therefore provides a unique opportunity to study unsteady accretion processes. The Herbig Be component recently underwent its largest optical photometric outburst detected so far. We aim to constrain the origin of this outburst by studying the emission region of the HI Br γ line, a powerful tracer of accretion/ejection processes on the AU-scale in young stars. Methods. Using the AMBER/VLTI instrument at spectral resolutions of 1500 and 12 000, we performed spatially and spectrally resolved interferometric observations of the hot gas emitting across the Br γ emission line, during and after the outburst. From the visibilities and differential phases, we derive characteristic sizes for the Br γ emission and spectro-astrometric measurements across the line, with respect to the continuum. Results. We find that the line profile, the astrometric signal, and the visibilities are inconsistent with the signature of either a Keplerian disk or infall of matter. They are, instead, evidence of a bipolar wind, maybe partly seen through a disk hole inside the dust sublimation radius. The disappearance of the Br γ emission line after the outburst suggests that the outburst is related to a period of strong mass loss rather than a change of the extinction along the line of sight. Conclusions. Apart from the photometric increase of the system, the main consequence of the outburst is to trigger a massive bipolar outflow from the Herbig Be component. Based on these conclusions, we speculate that the origin of the outburst is an event of enhanced mass accretion, similar to those occuring in EX Ors and FU Ors.
Changes in the structure of the accretion disc of EX Draconis through the outburst cycle
Monthly Notices of the Royal Astronomical Society, 2001
We report on the analysis of high-speed photometry of the dwarf nova EX Dra through its outburst cycle with eclipse mapping techniques. The eclipse maps show evidence of the formation of a one-armed spiral structure in the disc at the early stages of the outburst and reveal how the disc expands during the rise until it fills most of the primary Roche lobe at maximum light. During the decline phase the disc becomes progressively fainter until only a small bright region around the white dwarf is left at minimum light. The eclipse maps also suggest the presence of an inward and an outward-moving heating wave during the rise and an inward-moving cooling wave in the decline. The inferred speed of the outward-moving heating wave is of the order of 1 km s −1 , while the speed of the cooling wave is a fraction of that. Our results suggest a systematic deceleration of both the heating and the cooling waves as they travel across the disc, in agreement with predictions of the disc instability model. The analysis of the brightness temperature profiles indicates that most of the disc appears to be in steady-state during quiescence and at outburst maximum, but not during the intermediate stages. As a general trend, the mass accretion rate in the outer regions is larger than in the inner disc on the rising branch, while the opposite holds during the decline branch. We estimate a mass accretion rate ofṀ=10 −8 M ⊙ yr −1 at outburst maximum andṀ=10 −9.1 M ⊙ yr −1 in quiescence. The brightness temperature profile in quiescence also suggests that the viscosity parameter is high at this stage, α cool > ∼ 0.25, which favours the mass-transfer instability model. The uneclipsed light has a steady component, understood in terms of emission from the red secondary star, and a variable component that is proportional to the out of eclipse flux and corresponds to about 3 per cent of the total brightness of the system. The variable component is interpreted as arising in a disc wind.
EPISODIC HIGH-VELOCITY OUTFLOWS FROM V899 Mon: A CONSTRAINT ON THE OUTFLOW MECHANISMS
The Astrophysical Journal, 2016
We report the detection of large variations in the outflow wind velocity from a young eruptive star, V899 Mon during its ongoing high accretion outburst phase. Such large variations in the outflow velocity (from-722 km s −1 to-425 km s −1) have never been reported previously in this family of objects. Our continuous monitoring of this source shows that the multi-component, clumpy, and episodic high velocity outflows are stable in the time scale of a few days, and vary over the time scale of a few weeks to months. We detect significant decoupling in the instantaneous outflow strength to accretion rate. From the comparison of various possible outflow mechanisms in magnetospheric accretion of young stellar objects, we conclude magnetically driven polar winds to be the most consistent mechanism for the outflows seen in V899 Mon. The large scale fluctuations in outflow over the short period makes V899 Mon the most ideal source to constrain various magnetohydrodynamics (MHD) simulations of magnetospheric accretion.
V346 Normae: first post-outburst observations of an FU Orionis star
Monthly Notices of the Royal Astronomical Society: Letters, 2016
During their formation phase stars gain most of their mass in violent episodic accretion events, such as observed in FU Orionis (FUor) and EXor stars. V346 Normae is a wellstudied FUor that underwent a strong outburst beginning in ∼ 1980. Here, we report photometric and spectroscopic observations which show that the visual/near-infrared brightness has decreased dramatically between the 1990s and 2010 (∆R ≈ 10.9 m , ∆J ≈ 7.8 m , ∆K ≈ 5.8 m). The spectral properties of this fading event cannot be explained with variable extinction alone, but indicate a drop in accretion rate by 2-3 orders of magnitude, marking the first time that a member of the FUor class has been observed to switch to a very low accretion phase. Remarkably, in the last few years (2011-2015) V346 Nor has brightened again at all near-infrared wavelengths, indicating the onset of a new outburst event. The observed behaviour might be consistent with the clustered luminosity bursts that have been predicted by recent gravitational instability and fragmentation models for the early stages of protostellar evolution. Given V346 Nor's unique characteristics (concerning outburst duration, repetition frequency, and spectroscopic diagnostics), our results also highlight the need for revisiting the FUor/EXor classification scheme.
The Formation of High-Mass Stars: from High-Mass Clumps to Accretion Discs and Molecular Outflows
High-mass stars play a significant role in the evolution of the Universe and the process that leads to the formation of such objects is still an open question in Astrophysics. The details of the structures connected to the central sources, such as the circumstellar discs and the morphology of the jets at their launching points, still lack of observational evidence. In this thesis, the high-mass star forming process is investigated in terms of the evolution of high-mass clumps selected from the ATLASGAL survey based on their 12 CO emission in the sub-millimetre. While single-dish sub-millimetre observations provide a large-scale view of the high-mass star formation process, higher angular resolution observations are required to disentangle the details of the protostars within the clumps. For this, threedimensional infrared spectroscopy was obtained for a group of RMS sources to characterise the circumstellar environment of high-mass YSOs in linear scales of ∼100-1000 AU. The ATLASGAL TOP100 sample offers a unique opportunity to analyse a statistically complete sample of high-mass clumps at different evolutionary stages. APEX data of three rotational J transitions of the CO (the CO (4-3), CO (6-5) and CO (7-6)) were used to characterise the properties of their warm gas (155 K) content and to derive the relations between the CO and the clump properties. The CO line luminosities were derived and the analysis indicated that the CO emission increases as a function of the evolutionary stage of the clumps (from infrared-weak to H ii regions) and as a function of the bolometric luminosity (L bol) and mass of the sources (M clump). The comparison of the TOP100 with low-mass objects observed in the CO (6-5) and CO (7-6), together with CO (10-9) data observed for a complementary sample of objects indicated that the dependency of the CO luminosity (L CO) with the bolometric luminosity of the sources gets steeper towards higher-J transitions. Although the CO luminosity of more luminous clumps are systematically larger than the values obtained for the less luminous sources, the individual analysis of each subsample suggests a similar dependency of L CO versus L bol for each luminosity regime. Finally, the presence of high-velocity CO emission observed for the TOP100 suggests that ∼85% of the sources are driving molecular outflows. The selection of isolated high-mass objects undergoing mass accretion is fundamental to investigate if these objects are formed through an accretion disc or if they are formed by merging of low-mass YSOs. The near-infrared window provides one of the best opportunities to investigate the interior of the sub-mm clumps and study in details their individual members. Thanks to the relatively high-resolution obtained in the K-band and the moderate reddening effectsin the K-band, a sample of eight (8) HMYSOs exhibiting large-scale H 2 outflows were selected to follow-up K-band spectroscopic observations using the NIFS spectrometer (Gemini North). All sources exhibit extended continuum emission and exhibit atomic and molecular transitions typical of embedded objects, such as Brγ, H 2 and the CO lines. The H 2 lines are tracing the launching point of the large-scale jets in scales of ∼ 100 AU in five of eight sources (63%). The identification of jets at such small scales indicates that these objects are still undergoing mass accretion. The Brγ emission probes the ionised gas around the HMYSOs. The analysis of the Brγ spectro-astrometry at sub-pixel scales suggests that the line arises from the cavity of the outflows or from rotating structures perpendicular to the H 2 jets (i.e., disc). Five sources also exhibit CO emission features (63%), and three HMYSOs display CO absorption features (38%), indicating that they are likely associated with circumstellar discs. By further investigating the kinematics of the spatially resolved CO absorption features, the Keplerian mass of three sources was estimated in 5±3, 8±5 and 30±10 M ⊙. These results support that high-mass stars are formed through discs, similarly as observed towards low-mass stars. The comparison between the collimation degree of the molecular jets or outflows detected in the NIFS data with their large-scale counterparts indicate that these structures present a relatively wide range of collimation degrees.
The outburst and nature of young eruptive low mass stars in dark clouds
2012
The FU Orionis (FUor) or EX Orionis (EXor) phenomenon has attracted increasing attention in recent years and is now accepted as a crucial element in the early evolution of low-mass stars. FUor and EXor eruptions of young stellar objects (YSOs) are caused by strongly enhanced accretion from the surrounding disk. FUors display optical outbursts of ∼ 4 mag or more and last for several decades, whereas EXors show smaller outbursts (∆m ∼ 2-3 mag) that last from a few months to a few years and may occur repeatedly. Therefore, FUor/EXor eruptions represent a rare but very important phenomenon in early stellar evolution, during which a young low-mass YSO brightens by up to several optical magnitudes. Hence, longterm observations of this class of eruptive variable are important to design theoretical models of low-mass star formation. In this paper, we present recent results from our long-term monitoring observations of three rare types of eruptive young variables with the 2-m Himalayan Chandra Telescope (HCT) and the 2-m IUCAA Girawali Observatory (IGO) telescope.
The spectroscopic evolution of the recurrent nova T Pyxidis during its 2011 outburst (Corrigendum)
Astronomy & Astrophysics, 2012
We aim to derive the physical properties of the recurrent nova T Pyx and the structure of the ejecta during the early stages of expansion of the 2011 outburst. The nova was observed with high resolution spectroscopy (R ≈ 65000 spectroscopy, beginning 1 day after discovery of the outburst and continuing through the last visibility of the star at the end of May 2011. The interstellar absorption lines of Na I, Ca II, CH, CH + , and archival H I 21 cm emission line observations have been used to determine a kinematic distance. Interstellar diffuse absorption features have been used to determine the extinction independent of previous assumptions. Sample Fe-peak line profiles show the optical depth and radial velocity evolution of the discrete components. We propose a distance to T Pyx ≥4.5kpc, with a strict lower limit of 3.5 kpc (the previously accepted distance). We derive an extinction, E(B-V)≈0.5±0.1, that is higher than previous estimates. The first observation, Apr. 15, displayed He I, He II, C III, and N III emission lines and a maximum velocity on P Cyg profiles of the Balmer and He I lines of ≈2500 km s −1 characteristic of the fireball stage. These ions were undetectable in the second spectrum, Apr. 23, and we use the recombination time to estimate the mass of the ejecta, 10 −5 f M ⊙ for a filling factor f . Numerous absorption line systems were detected on the Balmer, Fe-peak, Ca II, and Na I lines, mirrored in broader emission line components, that showed an "accelerated" displacement in velocity. We also show that the time sequence of these absorptions, which are common to all lines and arise only in the ejecta, can be described by recombination front moving outward in the expanding gas without either a stellar wind or circumstellar collisions. By the end of May, the ejecta were showing signs of turning optically thin in the ultraviolet.
Subsequent outbursts of the same EXor source possibly present similar features
Astronomy & Astrophysics, 2020
Context. V1118 Ori is a classical EXor source whose light curve has been monitored, although not continuously, over the past 30 years. This source underwent a powerful outburst in 2005, followed by 10 years of quiescence and a less intense outburst in 2015. In 2019, a new intense brightness increase was observed (Δg ∼ 3 mag). Aims. This new accretion episode offers the opportunity to compare the photometric and spectroscopic properties of multiple outbursts of the same source. This allows us to highlight the differences and similarities among various events by removing any possible bias related to the intrinsic properties of the star-disk system. Methods. We discovered the 2019 V1118 Ori outburst by examining the g-band light curve acquired by the Zwicky Transient Facility and followed the declining phase with the Rapid Eye Mount telescope in the griz bands. Two near-infrared spectra were also acquired at different brightness stages with the Large Binocular Telescope. Results. The l...