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Register a new userInvestigating episodic accretion in a very low-mass young stellar object
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Very low-mass Class I protostars have been investigated very little thus far. Variability of these young stellar objects (YSOs) and whether or not they are capable of strong episodic accretion is also left relatively unstudied. We investigate accretion variability in IRS54, a Class I very low-mass protostar with a mass of M$_{star}$ ~ 0.1 - 0.2 M$_{odot}$. We obtained spectroscopic and photometric data with VLT/ISAAC and VLT/SINFONI in the near-infrared ($J$, $H$, and $K$ bands) across four epochs (2005, 2010, 2013, and 2014). We used accretion-tracing lines (Pa$beta$ and Br$gamma$) and outflow-tracing lines (H$_2$ and [FeII] to examine physical properties and kinematics of the object. A large increase in luminosity was found between the 2005 and 2013 epochs of more than 1 magnitude in the $K$ band, followed in 2014 by a steep decrease. Consistently, the mass accretion rate ($dot{M}_{acc}$) rose by an order of magnitude from ~ 10$^{-8}$ M$_{odot}$ yr$^{-1}$ to ~ $10^{-7}$ M$_{odot}$ yr$^{-1}$ between the two early epochs. The visual extinction ($A_V$) has also increased from ~ 15 mag in 2005 to ~ 24 mag in 2013. This rise in $A_V$ in tandem with the increase in $dot{M}_{acc}$ is explained by the lifting up of a large amount of dust from the disc of IRS54, following the augmented accretion and ejection activity in the YSO, which intersects our line of sight due to the almost edge-on geometry of the disc. Because of the strength and timescales involved in this dramatic increase, this event is believed to have been an accretion burst possibly similar to bursts of EXor-type objects. IRS54 is the lowest mass Class I source observed to have an accretion burst of this type, and therefore potentially one of the lowest mass EXor-type objects known so far.
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Solar-mass stars form via circumstellar disk accretion (disk-mediated accretion). Recent findings indicate that this process is likely episodic in the form of accretion bursts, possibly caused by disk fragmentation. Although it cannot be ruled out that high-mass young stellar objects (HMYSOs; $M>$8 M$_odot$, $L_{bol}>$5$times$10$^3$ L$_odot$) arise from the coalescence of their low-mass brethren, latest results suggest that they more likely form via disks. Accordingly, disk-mediated accretion bursts should occur. Here we report on the discovery of the first disk-mediated accretion burst from a $sim$20 M$_odot$ HMYSO. Our near-infrared images show the brightening of the central source and its outflow cavities. Near-infrared spectroscopy reveals emission lines typical of accretion bursts in low-mass protostars, but orders of magnitude more luminous. Moreover, the energy released and the inferred mass-accretion rate are also orders of magnitude larger. Our results identify disk accretion as the common mechanism of star formation across the entire stellar mass spectrum.
In the last twenty years, the topic of episodic accretion has gained significant interest in the star formation community. It is now viewed as a common, though still poorly understood, phenomenon in low-mass star formation. The FU Orionis objects (FUors) are long-studied examples of this phenomenon. FUors are believed to undergo accretion outbursts during which the accretion rate rapidly increases from typically $10^{-7}$ to a few $10^{-4}$ $M_odot$ yr$^{-1}$, and remains elevated over several decades or more. EXors, a loosely defined class of pre-main sequence stars, exhibit shorter and repetitive outbursts, associated with lower accretion rates. The relationship between the two classes, and their connection to the standard pre-main sequence evolutionary sequence, is an open question: do they represent two distinct classes, are they triggered by the same physical mechanism, and do they occur in the same evolutionary phases? Over the past couple of decades, many theoretical and numerical models have been developed to explain the origin of FUor and EXor outbursts. In parallel, such accretion bursts have been detected at an increasing rate, and as observing techniques improve each individual outburst is studied in increasing detail. We summarize key observations of pre-main sequence star outbursts, and review the latest thinking on outburst triggering mechanisms, the propagation of outbursts from star/disk to disk/jet systems, the relation between classical EXors and FUors, and newly discovered outbursting sources -- all of which shed new light on episodic accretion. We finally highlight some of the most promising directions for this field in the near- and long-term.
We report the results of VERA multi-epoch VLBI 22 GHz water maser observations of S255IR-SMA1, a massive young stellar object located in the S255 star forming region. By annual parallax the source distance was measured as D = 1.78 +-0.12 kpc and the source systemic motion was (u alpha cos d, u d) = (-0.13 +- 0.20, -0.06 +- 0.27) mas yr-1. Masers appear to trace a U-shaped bow shock whose morphology and proper motions are well reproduced by a jet-driven outflow model with a jet radius of about 6 AU. The maser data, in the context of other works in the literature, reveal ejections from S255IR-SMA1 to be episodic, operating on timescales of ~1000 years.
Aims: Accretion rates in low-mass protostars can be highly variable in time. Each accretion burst is accompanied by a temporary increase in luminosity, heating up the circumstellar envelope and altering the chemical composition of the gas and dust. This paper aims to study such chemical effects and discusses the feasibility of using molecular spectroscopy as a tracer of episodic accretion rates and timescales. Methods: We simulate a strong accretion burst in a diverse sample of 25 spherical envelope models by increasing the luminosity to 100 times the observed value. Using a comprehensive gas-grain network, we follow the chemical evolution during the burst and for up to 10^5 yr after the system returns to quiescence. The resulting abundance profiles are fed into a line radiative transfer code to simulate rotational spectra of C18O, HCO+, H13CO+, and N2H+ at a series of time steps. We compare these spectra to observations taken from the literature and to previously unpublished data of HCO+ and N2H+ 6-5 from the Herschel Space Observatory. Results: The bursts are strong enough to evaporate CO throughout the envelope, which in turn enhances the abundance of HCO+ and reduces that of N2H+. After the burst, it takes 10^3-10^4 yr for CO to refreeze and for HCO+ and N2H+ to return to normal. The chemical effects of the burst remain visible in the rotational spectra for as long as 10^5 yr after the burst has ended, highlighting the importance of considering luminosity variations when analyzing molecular line observations in protostars. The spherical models are currently not accurate enough to derive robust timescales from single-dish observations. As follow-up work, we suggest that the models be calibrated against spatially resolved observations in order to identify the best tracers to be used for statistically significant source samples.
The dependence of the mass accretion rate on the stellar properties is a key constraint for star formation and disk evolution studies. Here we present a study of a sample of stars in the Chamaeleon I star forming region carried out using the VLT/X-Shooter spectrograph. The sample is nearly complete down to M~0.1Msun for the young stars still harboring a disk in this region. We derive the stellar and accretion parameters using a self-consistent method to fit the broad-band flux-calibrated medium resolution spectrum. The correlation between the accretion luminosity to the stellar luminosity, and of the mass accretion rate to the stellar mass in the logarithmic plane yields slopes of 1.9 and 2.3, respectively. These slopes and the accretion rates are consistent with previous results in various star forming regions and with different theoretical frameworks. However, we find that a broken power-law fit, with a steeper slope for stellar luminosity smaller than ~0.45 Lsun and for stellar masses smaller than ~ 0.3 Msun, is slightly preferred according to different statistical tests, but the single power-law model is not excluded. The steeper relation for lower mass stars can be interpreted as a faster evolution in the past for accretion in disks around these objects, or as different accretion regimes in different stellar mass ranges. Finally, we find two regions on the mass accretion versus stellar mass plane empty of objects. One at high mass accretion rates and low stellar masses, which is related to the steeper dependence of the two parameters we derived. The second one is just above the observational limits imposed by chromospheric emission. This empty region is located at M~0.3-0.4Msun, typical masses where photoevaporation is known to be effective, and at mass accretion rates ~10^-10 Msun/yr, a value compatible with the one expected for photoevaporation to rapidly dissipate the inner disk.
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