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Register a new userMaser Flares Driven by Variations in Pumping and Background Radiation
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We simulate maser flares by varying either the pump rate or the background level of radiation in a 3D model of a maser cloud. We investigate the effect of different cloud shapes, saturation levels and viewpoints. Results are considered for clouds with both uniform and internally variable unsaturated inversion. Pumping and background variations are represented by several different driving functions, some of which are light curves drawn from observations. We summarise the pumping variability results in terms of three observable parameters, the maximum flux density achieved, a variability index and duty cycle. We demonstrate typical ranges of the flux density that may result from viewing an aspherical object from random viewpoints. The best object for a flare is a prolate cloud, viewed close to its long axis and driven from unsaturated conditions to at least modest saturation. Results for variation of the background level are qualitatively different from the variable pumping results in that they tend to produce short intervals of low flux density under conditions of moderate saturation and sufficient variability to be consistent with strong flaring. Variable background models typically have a significantly higher duty cycle than those with variable pumping.
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A 3D maser model has been used to perform an inverse problem on the light curves from three high-amplitude maser flares, selected on the basis of contemporaneous infra-red observations. Plots derived from the model recover the size of the maser cloud, and two parameters linked to saturation, from three observational properties of the light curve. Recovered sizes are consistent with independent interferometric measurements. Maser objects transition between weak and moderate saturation during a flare.
High resolution maps of maser emission provide very detailed information on processes occurring in circumstellar envelopes of late-type stars. A particularly detailed picture of the innermost shells around AGB stars is provided by SiO masers. Considerable progress is being made to provide astrometrically aligned multi-transition simultaneous observations of these masers, which are needed to better constrain the models. In view of the large amount of high quality data available, models should now be developed to fully explain all maser characteristics together (spatial distribution, variability, etc). New generation instruments (VERA, VSOP-2), new observational techniques (frequency-phase transfer), and new models promise important improvements of our knowledge on this topic.
Context. Stars on the asymptotic giant branch (AGB) are long-period variables that present strong flux variations at almost all wavelengths, including the SiO maser lines. The periods of these variations are of 300-500 days in Mira-type stars and somewhat shorter in semi-regular variables. The variability of the SiO lines on short timescales has been investigated, but the data are inconclusive. Aims. We aim to study the time evolution of the SiO maser lines in Mira-type and semi-regular variables at short timescales. We also discuss the origin of the observed fast variations. Methods. We observed the SiO maser lines at 7 mm (28SiO v=1,2 J=1-0) and 3 mm (28SiO v=1 J=2-1) using the 40 m Yebes antenna and the 30 m IRAM telescope, respectively, with a minimum spacing of 1 day. We studied the semi-regular variables RX Boo and RT Vir and the Mira-type variables U Her, R LMi, R Leo, and $chi$ Cyg. We performed a detailed statistical analysis of the variations on different timescales. Results. RX Boo shows strong and fast variations in the intensity of the different spectral features of the SiO lines at 7 mm and 3 mm. On a timescale of one day, we find variations of >10% in 25% of the cases. Variations of greater than $sim$50% are often found when the observations are separated by 2 or 3 days. A similar variation rate of the SiO lines at 7 mm is found for RT Vir, but the observations of this object are less complete. On the contrary, the variations of the SiO maser line intensity in the Mira-type variables are moderate, with typical variation rates around <10% in 7 days. This phenomenon can be explained by the presence of particularly small maser-emitting clumps in semi-regular variables, which would lead to a strong dependence of the intensity on the density variations due to the passage of shocks.
Protostellar flares are rapid magnetic energy release events associated with formation of hot plasma in protostars. In the previous models of protostellar flares, the interaction between a protostellar magnetosphere with the surrounding disk plays crucial roles in building-up and releasing the magnetic energy. However, it remains unclear if protostars indeed have magnetospheres because vigorous disk accretion and strong disk magnetic fields in the protostellar phase may destroy the magnetosphere. Considering this possibility, we investigate the energy accumulation and release processes in the absence of a magnetosphere using a three-dimensional magnetohydrodynamic simulation. Our simulation reveals that protostellar flares are repeatedly produced even in such a case. Unlike in the magnetospheric models, the protostar accumulates magnetic energy by acquiring large-scale magnetic fields from the disk by accretion. Protostellar flares occur when a portion of the large-scale magnetic fields are removed from the protostar as a result of magnetic reconnection. Protostellar flares in the simulation are consistent with observations; the released magnetic energy (up to $sim 3times 10^{38}$ erg) is large enough to drive observed flares, and the flares produce hot ejecta. The expelled magnetic fields enhance accretion, and the energy build-up and release processes are repeated as a result. The magnetic flux removal via reconnection leads to redistribution of magnetic fields in the inner disk. We therefore consider that protostellar flares will play an important role in the evolution of the disk magnetic fields in the vicinity of protostars.
This paper presents an overview of some recent observational and theoretical results on solar flares, with an emphasis on flare impulsive-phase chromospheric properties, including: electron diagnostics, optical and UV emission, and discoveries made by the Hinode mission, especially in the EUV. A brief perspective on future observations and theoretical requirements is also given
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