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Register a new userThe diagnostic potential of Fe lines applied to protostellar jets
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We investigate the diagnostic capabilities of the iron lines for tracing the physical conditions of the shock-excited gas in jets driven by pre-main sequence stars. We have analyzed the 300-2500 nm X-shooter spectra of two jets driven by the pre-main sequence stars ESO-Halpha 574 and Par-Lup 3-4. Both spectra are very rich in [FeII] lines over the whole spectral range; in addition, lines from [FeIII] are detected in the ESO-Halpha 574 spectrum. NLTE codes along with codes for the ionization equilibrium are used to derive the gas excitation conditions of electron temperature and density, and fractional ionization. The iron gas-phase abundance is provided by comparing the iron lines emissivity with that of [OI] 630 nm. The [FeII] lines indicate ESO-Halpha 574 jet is, on average, colder (T_e = 9000 K), less dense (n_e = 2 10^4 cm^-3) and more ionized (x_e = 0.7) than the Par-Lup 3-4 jet (T_e = 13000 K, n_e = 6 10^4 cm^-3, x_e < 0.4), even if the existence of a higher density component (n_e = 2 10^5 cm^-3) is probed by the [FeIII] and [FeII] ultra-violet lines. Theoretical models suggest that the shock at work in ESO-Halpha 574 is faster and likely more energetic than the Par-Lup 3-4 shock. This latter feature is confirmed by the high percentage of gas-phase iron measured in ESO-Halpha 574 (50-60% of its solar abundance in comparison with less than 30% in Par-Lup 3-4), which testifies that the ESO-Halpha 574 shock is powerful enough to partially destroy the dust present inside the jet. This work demonstrates that a multiline Fe analysis can be effectively used to probe the excitation and ionization conditions of the gas in a jet without any assumption on ionic abundances. The main limitation on the diagnostics resides in the large uncertainties of the atomic data, which, however, can be overcome through a statistical approach involving many lines.
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Using three-dimensional magnetohydrodynamics simulations, the driving of protostellar jets is investigated in different star-forming cores with the parameters of magnetic field strength and mass accretion rate. Powerful high-velocity jets appear in strongly magnetized clouds when the mass accretion rate onto the protostellar system is lower than $dot{M} lesssim 10^{-3},{rm M}_odot$ yr$^{-1}$. On the other hand, even at this mass accretion rate range, no jets appear for magnetic fields of prestellar clouds as weak as $mu_0 gtrsim 5$--$10$, where $mu_0$ is the mass-to-flux ratio normalized by the critical value $(2pi G^{1/2})^{-1}$. For $dot{M}gtrsim 10^{-3},{rm M}_odot$ yr$^{-1}$, although jets usually appear just after protostar formation independent of the magnetic field strength, they soon weaken and finally disappear. Thus, they cannot help drive the low-velocity outflow when there is no low-velocity flow just before protostar formation. As a result, no significant mass ejection occurs during the early mass accretion phase either when the prestellar cloud is weaky magnetized or when the mass accretion rate is very high. Thus, protostars formed in such environments would trace different evolutionary paths from the normal star formation process.
In the collapsar scenario, gamma ray bursts are caused by relativistic jets expelled along the rotation axis of a collapsing stellar core. We discuss how the structure and time-dependence of such jets depends on the stellar envelope and central engine properties, assuming a steady jet injection. It takes a few seconds for the jet to bore its way through the stellar core; most of the energy output during that period goes into a cocoon of relativistic plasma surrounding the jet. This material subsequently forms a bubble of magnetized plasma that takes several hours to expand, subrelativistically, through the envelope of a high-mass supergiant. Jet break-through and a conventional burst would be expected not only in He stars but possibly also in blue supergiants. Shock waves and magnetic dissipation in the escaping bubble can contribute a non thermal UV/X-ray afterglow, and also excite Fe line emission from thermal gas, in addition to the standard jet deceleration power-law afterglow.
We aim at studying the causal link between the knotty jet structure in CARMA 7, a young Class 0 protostar in the Serpens South cluster, and episodic accretion in young protostellar disks. We used numerical hydrodynamics simulations to derive the protostellar accretion history in gravitationally unstable disks around solar-mass protostars. We compared the time spacing between luminosity bursts Deltatau_mod, caused by dense clumps spiralling on the protostar, with the differences of dynamical timescales between the knots Deltatau_obs in CARMA 7. We found that the time spacing between the bursts have a bi-modal distribution caused by isolated and clustered luminosity bursts. The former are characterized by long quiescent periods between the bursts with Deltatau_mod = a few * (10^3-10^4) yr, whereas the latter occur in small groups with time spacing between the bursts Deltatau_mod= a few * (10-10^2) yr. For the clustered bursts, the distribution of Deltatau_mod in our models can be fit reasonably well to the distribution of Deltatau_obs in the protostellar jet of CARMA 7, if a certain correction for the (yet unknown) inclination angle with respect to the line of sight is applied. The K-S test on the model and observational data sets suggests the best-fit values for the inclination angles of 55-80 deg., which become narrower (75-80 deg.) if only strong luminosity bursts are considered. The dynamical timescales of the knots in the jet of CARMA 7 are too short for a meaningful comparison with the long time spacings between isolated bursts in our models. The exact sequences of time spacings between the luminosity bursts in our models and knots in the jet of CARMA 7 were found difficult to match. (abridged)
Polarized dust continuum emission has been observed with ALMA in an increasing number of deeply embedded protostellar systems. It generally shows a sharp transition going from the protostellar envelope to the disk scale, with the polarization fraction typically dropping from ${sim} 5%$ to ${sim} 1%$ and the inferred magnetic field orientations becoming more aligned with the major axis of the system. We quantitatively investigate these observational trends using a sample of protostars in the Perseus molecular cloud and compare these features with a non-ideal MHD disk formation simulation. We find that the gas density increases faster than the magnetic field strength in the transition from the envelope to the disk scale, which makes it more difficult to magnetically align the grains on the disk scale. Specifically, to produce the observed ${sim} 1%$ polarization at ${sim} 100,mathrm{au}$ scale via grains aligned with the B-field, even relatively small grains of $1,mathrm{mu m}$ in size need to have their magnetic susceptibilities significantly enhanced (by a factor of ${sim} 20$) over the standard value, potentially through superparamagnetic inclusions. This requirement is more stringent for larger grains, with the enhancement factor increasing linearly with the grain size, reaching ${sim} 2times 10^4$ for millimeter-sized grains. Even if the required enhancement can be achieved, the resulting inferred magnetic field orientation in the simulation does not show a preference for the major axis, which is inconsistent with the observed pattern. We thus conclude that the observed trends are best described by the model where the polarization on the envelope scale is dominated by magnetically aligned grains and that on the disk scale by scattering.
We present 2.5-D global, ideal MHD simulations of magnetically and rotationally driven protostellar jets from Keplerian accretion discs, wherein only the initial magnetic field strength at the inner radius of the disc, $B_{rm i}$, is varied. Using the AMR-MHD code AZEUS, we self-consistently follow the jet evolution into the observational regime ($>10^3,mathrm{AU}$) with a spatial dynamic range of $sim6.5times10^5$. The simulations reveal a three-component outflow: 1) A hot, dense, super-fast and highly magnetised jet core; 2) a cold, rarefied, trans-fast and highly magnetised sheath surrounding the jet core and extending to a tangential discontinuity; and 3) a warm, dense, trans-slow and weakly magnetised shocked ambient medium entrained by the advancing bow shock. The simulations reveal power-law relationships between $B_{rm i}$ and the jet advance speed, $v_{rm jet}$, the average jet rotation speed, $langle v_varphirangle$, as well as fluxes of mass, momentum, and kinetic energy. Quantities that do not depend on $B_{rm i}$ include the plasma-$beta$ of the transported material which, in all cases, seems to asymptote to order unity. Jets are launched by a combination of the magnetic tower and bead-on-a-wire mechanisms, with the former accounting for most of the jet acceleration---even for strong fields---and continuing well beyond the fast magnetosonic point. At no time does the leading bow shock leave the domain and, as such, these simulations generate large-scale jets that reproduce many of the observed properties of protostellar jets including their characteristic speeds and transported fluxes.
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