Sporadic Accretion Research Articles

The CO Outflow Components Ejected by a Recent Accretion Event in B335

Protostellar outflows often present a knotty appearance, providing evidence of sporadic accretion in stellar mass growth. To understand the direct relation between mass accretion and ejection, we analyze the contemporaneous accretion activity and associated ejection components in B335. B335 has brightened in the mid-IR by 2.5 mag since 2010, indicating increased luminosity, presumably due to an increased mass accretion rate onto the protostar. Atacama Large Millimeter/submillimeter Array (ALMA) observations of 12CO emission in the outflow reveal high-velocity emission, estimated to have been ejected 4.6–2 yr before the ALMA observations and consistent with the jump in mid-IR brightness. The consistency in timing suggests that the detected high-velocity ejection components are directly linked to the most recent accretion activity. We calculated the kinetic energy, momentum, and force for the ejection component associated with the most recent accretion activity and found that, at least, about 1.0% of the accreted mass has been ejected. More accurate information on the jet inclination and the temperature of the ejected gas components will better constrain the ejected mass induced by the recently enhanced accretion event.

Three-dimensional, Time-dependent MHD Simulation of Disk–Magnetosphere–Stellar Wind Interaction in a T Tauri, Protoplanetary System

We present a three-dimensional, time-dependent MHD simulation of the short-term interaction between a protoplanetary disk and the stellar corona in a T Tauri system. The simulation includes the stellar magnetic field, self-consistent coronal heating and stellar wind acceleration, and a disk rotating at sub-Keplerian velocity to induce accretion. We find that, initially, as the system relaxes from the assumed initial conditions, the inner part of the disk winds around and moves inward and close to the star as expected. However, the self-consistent coronal heating and stellar wind acceleration build up the original state after some time, significantly pushing the disk out beyond 10R ⋆. After this initial relaxation period, we do not find clear evidence of a strong, steady accretion flow funneled along coronal field lines, but only weak, sporadic accretion. We produce synthetic coronal X-ray line emission light curves, which show flare-like increases that are not correlated with accretion events nor with heating events. These variations in the line emission flux are the result of compression and expansion due to disk–corona pressure variations. Vertical disk evaporation evolves above and below the disk. However, the disk–stellar wind boundary stays quite stable, and any disk material that reaches the stellar wind region is advected out by the stellar wind.

Sporadic and intense accretion in a 1 Myr-old brown dwarf candidate

Context. Studying the accretion process in very low-mass objects has important implications for understanding their formation mechanism. Many nearby late-M dwarfs that have previously been identified in the field are in fact young brown dwarf members of nearby young associations. Some of them are still accreting. They are therefore excellent targets for further studies of the accretion process in the very low-mass regime at different stages. Aims. We aim to search for accreting young brown dwarf candidates in a sample of 85 nearby late-M dwarfs. Methods. Using photometric data from DENIS, 2MASS, and WISE, we constructed the spectral energy distribution of the late- M dwarfs based on BT-Settl models to detect infrared excesses. We then searched for lithium and Hα emission in candidates that exhibit infrared excesses to confirm their youth and the presence of accretion. Results. Among the 85 late-M dwarfs, only DENIS-P J1538317−103850 (M5.5) shows strong infrared excesses in WISE bands. The detection of lithium absorption in the M5.5 dwarf and its Gaia trigonometric parallax indicate an age of ~1 Myr and a mass of 47 MJ. The Hα emission line in the brown dwarf shows significant variability that indicates sporadic accretion. This 1 Myr-old brown dwarf also exhibits intense accretion bursts with accretion rates of up to 10−7.9 M⊙ yr−1. Conclusions. Our detection of sporadic accretion in one of the youngest brown dwarfs might imply that sporadic accretion at early stages could play an important role in the formation of brown dwarfs. Very low-mass cores would not be able to accrete enough material to become stars, and thus they end up as brown dwarfs.

Supergiant Fast X-ray Transients uncovered by the EXTraS project: flares reveal the development of magnetospheric instability in accreting neutron stars

ABSTRACT The low luminosity, X-ray flaring activity, of the sub-class of high-mass X-ray binaries called Supergiant Fast X-ray Transients, has been investigated using XMM–Newton public observations, taking advantage of the products made publicly available by the EXTraS project. One of the goals of EXTraS was to extract from the XMM–Newton public archive information on the aperiodic variability of all sources observed in the soft X-ray range with EPIC (0.2–12 keV). Adopting a Bayesian block decomposition of the X-ray light curves of a sample of SFXTs, we picked out 144 X-ray flares, covering a large range of soft X-ray luminosities (1032–1036 erg s−1). We measured temporal quantities, like the rise time to and the decay time from the peak of the flares, their duration and the time interval between adjacent flares. We also estimated the peak luminosity, average accretion rate, and energy release in the flares. The observed soft X-ray properties of low-luminosity flaring activity from SFXTs is in qualitative agreement with what is expected by the application of the Rayleigh–Taylor instability model in accreting plasma near the neutron star magnetosphere. In the case of rapidly rotating neutron stars, sporadic accretion from temporary discs cannot be excluded.

LONG FADING MID-INFRARED EMISSION IN TRANSIENT CORONAL LINE EMITTERS: DUST ECHO OF A TIDAL DISRUPTION FLARE

ABSTRACT The sporadic accretion following the tidal disruption of a star by a super-massive black hole (TDE) leads to a bright UV and soft X-ray flare in the galactic nucleus. The gas and dust surrounding the black hole responses to such a flare with an echo in emission lines and infrared emission. In this paper, we report the detection of long fading mid-IR emission lasting up to 14 years after the flare in four TDE candidates with transient coronal lines using the WISE public data release. We estimate that the reprocessed mid-IR luminosities are in the range between and erg s−1 and dust temperature in the range of 570–800 K when WISE first detected these sources three to five years after the flare. Both luminosity and dust temperature decrease with time. We interpret the mid-IR emission as the infrared echo of the tidal disruption flare. We estimate the UV luminosity at the peak flare to be 1 to 30 times 1044 erg s−1 and that for warm dust masses to be in the range of 0.05–1.3 within a few parsecs. Our results suggest that the mid-infrared echo is a general signature of TDE in the gas-rich environment.

The Magellanic squall: gas replenishment from the Small to the Large Magellanic Cloud

Abstract We first show that a large amount of metal-poor gas is stripped from the Small Magellanic Cloud (SMC) and falls into the Large Magellanic Cloud (LMC) during the tidal interaction between the SMC, the LMC and the Galaxy over the last 2 Gyr. We propose that this metal-poor gas can closely be associated with the origin of the LMC's young and intermediate-age stars and star clusters with distinctively low metallicities with [Fe/H]< −0.6. We numerically investigate whether gas initially in the outer part of the SMC's gas disc can be stripped during the LMC–SMC–Galaxy interaction and consequently can pass through the central region (R < 7.5 kpc) of the LMC. We find that about 0.7 and 18 per cent of the SMC's gas could have passed through the central region of the LMC about 1.3 Gyr ago and 0.2 Gyr ago, respectively. The possible mean metallicity of the replenished gas from the SMC to LMC is about [Fe/H]=−0.9 to −1.0 for the two interacting phases, if a steep metallicity gradient of the SMC's gas disc is assumed. These results imply that the LMC can temporarily replenish gas supplies through the sporadic accretion and infall of metal-poor gas from the SMC. They furthermore imply that if this gas from the SMC can collide with gas in the LMC to form new stars in the LMC, the metallicities of the stars can be significantly lower than those of stars formed from gas initially within the LMC.